English-1 New Final · TENSILE CRACK IN MASONARY WALL Shear crack at corner of Wall due to expansion SHEAR CRACK IN MASONARY WALL SHEAR CRACK IN MASONARY PILLAR AT BEAM SUPPORT Shear

English-1 New Final.indd 8English-1 New Final.indd 8 12/16/13 10:31 AM12/16/13 10:31 AM

CONTENTS Design Life Extension of RC Structure

I. INTRODUCTION 7 II. OBJECTIVES OF RESEARCH 11 III. CLASSIFICATIONS OF CRACKS 15

IV. ASSESSMENT OF CONCRETE STRUCTURES 37 V. METHODS OF CRACKS REPAIR & CLASSIFICATION OF REPAIR OPTIONS 51 VI. METHODS FOR ESTIMATING AND EVALUATING CORROSION RATE 65 VII. ESTIMATING SERVICE-LIFE 77 VIII. IMPROVING CORROSION RESISTANCE CHARACTERISTICS 99 IX. REPAIR AND STRENGTHENING OF STRCUCTURES 121 X. REPAIR OF POST-TENSION STRUCTURES 137 APPENDIX A – PHOTOS OF TYPICAL CONCRETE CRACKS 151 APPENDIX B – LOCAL REPAIR MATERIALS FOR DIFFERENT PROBLEMS 155 APPENDIX C – VISUAL INSPECTION SHEET 159

APPENDIX D – SITE VISITS 163

APPENDIX E – STRENGTHENING METHOD STATEMENTS 169

APPENDIX F – PREVENTIVE MAINTENANCE 183



I - INTRODUCTION Design Life Extension of RC Structure

I - INTRODUCTION

07


08 I - INTRODUCTION

A large number of existing reinforced concrete (RC) structures in Dubai, and the UAE at large, are suffering from durability problems, leading to accelerated deterioration and cracking, thus reducing the functional service life. The public funds are not generally available for the replacement of existing deteriorated structures, and hence there is a need to establish preventive maintenance and corrective actions to extend the service life in a cost-effective manner.

Design service life of structures depends primarily on concrete quality and surrounding environmental conditions. Measures needed to extend the functional service life of RC structures depends on whether the aim of the protection is to control corrosion damage rate, to upgrade the structural capacity of the damaged structure, or both. The role of post-repair performance monitoring of the structure on the design life, as well as the estimated extension of the design life needs to be evaluated.Therefore, this research aims to develop a Repair Manual that summarizes causes of concrete deterioration and cracking, different repair methods and prediction of service life. The manual also includes guidelines for durable concrete structures.

How to use this manual

Figure 1 is for a flow chart that summarizes typical strategy to be followed for new and existing reinforced concrete structures. This is linked to this manual so that each step is associated with relevant chapters or appendices (shown between parentheses in this flowchart).


Design Life Extension of RC Structure09



II - OBJECTIVES Design Life Extension of RC Structure

II - OBJECTIVES

11



II - OBJECTIVES Design Life Extension of RC Structure 13

The main goal of this research is to develop a “Repair Manual” for deteriorated reinforced concrete structures and guidelines for durable concrete. The main objectives are to:

• Classify cracks in the concrete structures.

• Provide methods for repairing of cracks.

• Provide methods for estimating and evaluating the corrosion rate.

• Provide methods for estimating the remaining design life of the structures

• Provide recommendations for the selection of the type of corrosion inhibitor and methods of application.

• Provide methods for the rehabilitation of corrosion-damaged RC structures.

• Provide methods for the structural assessment after repair.


CL


III - CLASSIFICATIONS OF CRACKS Design Life Extension of RC Structure 15

III - CLASSIFICATIONS OF CRACKS

CLASSIFICATIONS OF CRACKS


16 III -CLASSIFICATIONS OF CRACKS

1. INTRODUCTION The cracks in buildings can be classified into two main types: cracks in block walls and cracks in concrete elements.

2. CRACKS IN BLOCK WALLS2.1 GeneralMost of the building materials that are subject to cracking, namely masonry, mortar etc are weak in tension and shear and thus forces of even small magnitude are able to cause cracking. It is possible to distinguish between tensile and shear cracks by closely examining their physical characteristics.Figure 1shows the differences between tensile and shear cracks.

Figure 1 Tension cracks and Shear cracks in bricks (Bureau of Indian Standards)

Cracks may vary appreciably in width from very thin hair cracks barely visible to naked eye (about 0.01 mm in width) to gaping cracks 5 mm or more in width. A commonly known classification of cracks, based on their width is:a) Thin - less than 1mm in widthb) Medium - 1 to 2 mm in width c) Wide - more than 2 mm in width

TENSILE CRACK IN MASONARY WALL

Shear crack at corner of Wall due to expansion

SHEAR CRACK IN MASONARY WALL

SHEAR CRACK IN MASONARY PILLAR

AT BEAM SUPPORT

Shear crack in masonry Pillar due to expansion

Of R.C.C. beam



Cracks may be of uniform width or may be narrow at one end. Cracks could be straight, toothed, stepped, map pattern or random and may be vertical, horizontal or diagonal. Cracks may be only at the surface or may extend to more than one layer of materials. Occurrence of closely spaced fine cracks at the surface of a material is sometimes called crazing. Cracks from different causes have varying characteristics and it is by careful observation of these characteristics that one can correctly diagnose the cause or causes of cracking and adopt appropriate remedial measures.

Depending on certain properties of building materials, shrinkage cracks may be wider but further apart, or may be thin but more closely spaced. As a general rule, thin cracks, even though closely spaced and greater in number, are less damaging to the structure and are not so objectionable from aesthetic and other considerations as a fewer number of wide cracks.

Modern structures are comparatively tall and slender, have thin walls are designed for higher stresses and are built at a fast pace. These structures are therefore, more prone to cracks as compared with old structures, which used to be low, had thick walls, were lightly stressed and were built at a slow pace. Moreover moisture can easily reach the inside of the modern buildings due to the usage of thin walls. Thus measures for control of cracks in buildings assume much greater importance than ever before.

Structural cracks mainly occur due to: a) Defective design and defective load assumptions and perception of behavior of the structure.b) In correct assessment of bearing capacity of foundation soil and soil properly.c) Defective detailing of joints of components like roof with brick wall corner joints of wallsd) Defective detailing of structural detailing of steel reinforcement.e) Lack of quality control during construction.

The cracks in block walls can be classified into two categories: non-structural cracks including expansion and shrinkage cracks, and structural cracks including settlement cracks and loading cracks.

2.2 Non-structural Cracks in Block Walls

2.2.1 Expansion cracks Expansion cracks can be initial expansion of the walls or the elements attached to the walls as shown in Figures 2 and 3.

Figure 2 Cracks due to initial expansion of bricks (Bureau of Indian Standards)

PLAN

This end of wall A has due to expansion oversailed at DPC level and caused rotation of return wall B.Resulting in cracks at X.



Figure 3 Cracks of due to thermal movement of the top most story beams and slabs above a load bearing masonry walls structure (Bureau of Indian Standards)

In case of framed buildings due to thermal movement frames are distorted and cracks may appear as shown in figure 4.

ENLARGED DETAIL AT AFigure 4 Cracking in cladding and cross walls of a frame structure(Bureau of Indian Standards)


2.2.2 Shrinkage cracksShrinkage cracks show up in two basic locations in most walls; the approximate mid-point of a long section of wall, and the narrowed section of the wall such as across a door or window head. Shrinkage cracks are virtually uniform in width from top to bottom and typically extend from the top of the wall to within a couple of feet of the foundation.

Shrinkage cracks are usually caused by an inadequate number of control or contraction joints within the constructed wall. A common rule of thumb for the placement of these joints would have them spaced no further apart than three times the height of the wall, or forty feet. There are several conditions under which this spacing would be too great. One of those is an extremely irregular shaped wall. This type of wall will require twice as many control joints as a normal flat, one-directional wall.Another common cause for shrinkage cracks in concrete walls would be excessive water content within the concrete. In general terms, higher water content within a concrete mix will result in a greater amount of shrinkage. This is quite evident in some concrete walls where there are an excessive number of cracks.

A common cause of shrinkage cracks in masonry walls is using uncured masonry units. When green, or uncured units are used to construct a masonry wall, they will continue to cure once placed resulting in excessive shrinkage of the wall. Masonry units experience a significant amount of shrinkage when curing. In If this curing shrinkage is not virtually complete when the unit is placed within the wall; it will result in an increased number of shrinkage cracks within the finished product. In a concrete wall, a shrinkage crack will typically fall within the middle section of the wall length and run virtually vertical in that wall. In a masonry wall, the shrinkage cracks will usually find a weakened point in the wall such as a location of several penetrations for piping or conduit. From this point, they will run vertically most often in a stair-stepping fashion following the mortar joints. If however, there is an exceptionally good bond between the mortar joints and masonry units, the shrinkage crack may extend through the masonry units themselves thereby making a vertical crack.

The cracks between masonry walls and concrete elements such as columns and beams are common due to the different coefficient of expansion for both materials and due to the change in temperature. The typical cracks are shown in figure 5.

Figure 5 Cracks due to differential temperature of masonry walls and concrete elements




2.3 Structural Cracks in Block Walls 2.3.1 Settlement cracks Settlement cracks, as the name implies, result from a settlement of a support condition. This support condition can either be the soils on which a wall rests and depends for its support, or it could be the crushing of a column or other support element which supports one end of the wall. In either case, settlement cracks are caused by a change in the vertical location of one section of the wall relative to the remainder of the wall.

Settlement cracks typically appear in two different locations. The first and primary location would be at wall corners and wall ends. This type cracking is the result of settlement of the wall corner or wall end and manifests itself as a diagonal crack beginning near the corner or end of the wall at the top and progressing downward away from the corner. This type of cracking is illustrated in Figure 6. Another characteristic of a normal settlement crack in this location is that it will be significantly wider at the top, tapering to closure at the bottom in a uniform fashion.

Figure 6 settlement of wall end (Cracks in Concrete)

There are two types of problems which create the majority of the settlement cracks described above. The first is erosion which seems to occur more quickly at the end of a wall or the corner of a building. This erosion will remove the supporting soils from underneath this corner and allow settlement to occur.

The second most common cause is that of a concentrated load being applied to the wall at the corner or wall end. This load can come from a column above or from a beam bearing condition.When this additional concentrated load is not accounted for in the foundation design, a much higher stress will exist under the footing at the wall end or corner than along the remainder of the wall. This will often result in settlement of the corner related to the remainder of the wall.Another type of settlement crack which occurs at the corner or end of the wall will have the completely opposite orientation than the one just described. This would be a crack which originates close to the corner at the bottom of the wall, proceeds upward and away from the corner, and would be wider at the bottom than it is at the top. These crack characteristics indicate a condition where the end or corner of the wall has a significantly stronger support condition than the remaining portion of the wall. This allows the central portion of the wall to settle further into the ground or into its support while the corner portion remains supported and held at a rigid location. Such a condition can occur when a wall is supported by a beam and column system if the column is coincident with the end or corner of the wall and the central portion of the wall is supported by a beam which has inadequate stiffness and deflects enough to allow wall cracking. This cracking pattern is illustrated in Figure 7 and photo 1.

Figure 6 settltlltltllltltlllllltlltltltlttltltltltlllttlllllllemeeeemeeememeeeeemmmemmmeeeeemmmmmeeemeeeemeenennnnnnnennnennnntt ttttttttttttt ofoooffoffoffoffofffffofffffofofffofoofoffooffoooofffoof wwwwwwwwwwwwwwwwwwwwwwwwwwwalaaalalalalalalallllllalalllllllallalalaalaaaala ll llllllllllllllllllllllllllllllllllllll eneeeeeeeeeeee d (Cracks in Concrete)


Design Life Extension of RC Structure

Figure 7 differential settlements of supports(Cracks in Concrete)

Photo 1 Differential settlement of boundary wall in UAE

Another location of commonly seen settlement cracking is within the central portion of a length of a wall. This crack will normally appear as a vertical crack in a concrete wall and a stair-stepping diagonal crack in a masonry wall. The width of this crack will be much larger at the base of the wall than it is near the top when it reaches closure.

This type of cracking will most often occur when the central section of the wall is supported on a flexible element such as interior floor framing which will allow too much flexing of the support thereby generating a settlement type condition for the central portion of the wall resulting in the cracks. See Figures 8 and 9 for an illustration of this type of cracking.




Figure 8 Flexural cracks due to settlement at mid span of the wall(Cracks in Concrete)

Figure 9 Cracks in walls due to deflection of concrete beam.

Buildings on expansion clays are extremely crack prone. The soil movement in such clay is more appreciable up to a depth of 1.5 to 2m and this cause swelling and shrinkage and results in crack in the structure. The cracks due to settlement are usually diagonal in shape. Crack appearing due to swelling is vertical figure 10.


Figure 10 Thermo-osmotic heaving of buildings on desiccated clay soils(Bureau of Indian Standards)

all and a stair-stepping diagonal crack in a masonry wall. The width of this crack will be much larger at the base of the wall than it is near the top when it reaches closure.This type of cracking will most often occur when the central section of the wall is supported on a flexible element such as interior floor framing which will allow too much flexing of the support thereby generating a settlement type condition for the central portion of the wall resulting in the cracks. See Figures 8 and 9 for an illustration of this type of cracking.

2.3.2 Loading cracksA loading crack is a result of the loading to which the wall is subjected. A properly designed wall would not exhibit these cracks, but an improperly designed wall is very susceptible to this damage. A loading crack is found more often in residential structures than in commercial or industrial structures primarily due to the design effort that is put forth in commercial and industrial projects. There are three basic types of loading cracks that occur repeatedly; vertical cracking at the end of a wall; vertical cracking in the center of the wall and horizontal cracking in the center of the wall.

Vertical cracking at the end of a wall is typically due to a concentrated force being applied at the top of a wall which exceeds the shear capacity within the end section of the wall. This results in a minute amount of compression occurring within this end section which does not occur within the adjacent section of the wall thus causing a vertical crack at the interface between the two segments. This type crack typically maintains a tight appearance at the top and at the bottom but may show a wider gap at approximately mid-height of the wall. This would tend to indicate a bulging effect of the end segment of the wall away from the remainder of the wall. This crack is illustrated in Figure 11.

Figure 11 Cracks in wall due to heavy concentrated load at the end of the wall(Cracks in Concrete)




Horizontal cracking within the center portion of the wall is typically caused by lateral pressures on the wall which exceed the flexural capacity of the wall. These pressures are normally generated by saturated soil conditions being applied to a basement type wall. When the pressures exerted by the soils retained behind this wall exceed the flexural capacity of the wall, a crack is generated. Observing within the crack, one will note that the crack on the exposed face of the wall is considerably wider than the crack on the concealed face of the wall. Accompanying this crack, one will find a measurable amount of bowing within the wall. This will exhibit itself as a bulge at mid-height into the basement area. This type cracking is illustrated in Figure 12.

Figure 12 cracks due to lateral pressure on the wall(Cracks in Concrete)

A vertical loading crack within the center section of a wall is again, typically the result of lateral pressures exceeding the flexural capacity of the wall. However, in this case the wall typically has insufficient support at the top as compared to the wall discussed above. This condition generates a bowing inward of the wall near the top of the wall. When the pressures exerted by the material retained behind the wall generate stresses within the wall that are in excess of the capacity of the wall, the vertical crack results. This crack will be much wider near the top than it is near the bottom of the wall. Accompanying this crack will be the noticeable bowing of the upper section of the wall inward at the location of the crack.

The repair for cracks may be undertaken after ascertaining the reasons for the appearance of the crack. A few basic principles if followed will be more effective:1. Rendering of minor crack less that 1mm wide may be done after observing the crack for some

time and then sealing it with weak mortar of cement and sand.2. Cracks where width change with season should be filled up with elastic fillers like silicon or

polyurethene compound.3. Where shear crack are observed shear keys made of RCC concrete with at least 1.5 percent steel

reinforcement may be provided at 1 to 1.5m intervals.4. If cracks are due to movement of soil in black cotton once, prevention of moisture penetration in

the surrounding areas has to be ensured by providing a waterproof blanket around the plinth. The masonry wall below ground level should also be separated from the adjoining soil by replacing the existing soil with coarse grain material.

3. CRACKS IN CONCRETE ELEMENTSThe following table shows the general classification of the different types of cracks in concrete together with the symptoms/diagnosis for each type. Also, the table includes the main causes and effects for each type. The right column of the table includes the protective and preventive measures to reduce cracks, and/or to prevent the cracks from occurring.


TABL

E (1

): CO

NCR

ETE

CRA

CKS

CLA

SSIF

ICAT

ION

Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Earl

y ag

ePl

astic

sh

rink

age

• Sh

allo

w c

rack

s of

var

ying

dep

ths

in a

form

of a

rand

om m

ap p

at-

tern

or p

aral

lel t

o ea

ch o

ther

.

• Cr

acks

are

typi

cally

1 –

2 m

m

wid

e, 3

00 –

500

mm

long

, and

20

– 50

mm

dee

p.

• In

som

e ci

rcum

stan

ces

they

ex

tend

thro

ugh

the

full

dept

h of

th

e m

embe

r.

• Sh

rinka

ge o

f the

sur

face

laye

r due

to

evap

orat

ion

of m

oist

ure

from

the

surf

ace

of fr

eshl

y pl

aced

con

cret

e fa

ster

than

it is

re

plac

ed b

y bl

eed

wat

er (r

ate

of e

vapo

ra-

tion

exce

eds

the

rate

of b

leed

ing)

.

• D

ue to

the

rest

rain

t pro

vide

d by

the

conc

rete

bel

ow th

e dr

ying

sur

face

laye

r, te

nsile

str

esse

s de

velo

p in

the

wea

k, s

tiff-

enin

g pl

astic

con

cret

e. T

he ra

pid

loss

of

moi

stur

e fr

om th

e su

rfac

e la

yer i

s ca

used

by

: hig

h ai

r and

con

cret

e te

mpe

ratu

re;

low

rela

tive

hum

idity

; hig

h w

ind

velo

city

at

the

surf

ace

of th

e co

ncre

te.

• Co

ncre

te w

ith lo

wer

am

ount

s of b

leed

w

ater

, suc

h as

thos

e co

ntai

ning

min

eral

ad

mix

ture

s (es

peci

ally

silic

a fu

me)

hav

e a

grea

ter t

ende

ncy

to u

nder

go p

last

ic sh

rink-

age

crac

king

than

con

cret

e w

ith a

gre

ater

te

nden

cy to

ble

ed.

• Pr

ovid

es

rout

e in

to th

e co

ver f

or

moi

stur

e,

oxyg

en a

nd

chlo

ride

• Re

duce

rate

of e

vapo

ratio

n by

ear

ly c

urin

g

• Us

e of

fog

nozz

les t

o sa

tura

te th

e ai

r abo

ve th

e su

rfac

e

• U

se o

f pla

stic

she

etin

g to

cov

er th

e su

rfac

e be

-tw

een

finis

hing

ope

ratio

ns.

• Us

e of

win

dbre

aks t

o re

duce

the

win

d ve

locit

y and

sun-

shad

es to

redu

ce th

e su

rface

tem

pera

ture

are

also

hel

pful

.

• Av

oid

conc

rete

pla

cem

ent d

urin

g ho

t, w

indy

w

eath

er w

ith lo

w h

umid

ity.

• G

ood

and

corre

ct tr

owel

fini

shin

g of

con

cret

e su

rfac

e

• A

pply

nec

essa

ry p

reca

utio

ns to

pre

vent

rapi

d m

oist

ure

loss

due

to h

ot w

eath

er a

nd d

ry w

inds

(A

CI 2

24R,

302

.1R,

and

305

R).

Plas

ticse

ttle

men

t•

Crac

ks, v

oids

or b

oth

appe

ar in

pl

astic

con

cret

e ad

jace

nt to

an

ele-

men

t res

trai

ning

con

solid

atio

n of

co

ncre

te (e

.g. s

teel

bar

, sub

-gra

de

hard

ened

con

cret

e, a

nd fo

rmw

ork)

.

• Cr

acks

are

typi

cally

1 m

m w

ide

at

the

surf

ace

and

usua

lly ru

n fro

m

the

surf

ace

to th

e ba

rs

• Cr

acks

typi

cally

occ

ur o

n th

e to

p su

rfac

e an

d us

ually

follo

w th

e lin

e of

the

uppe

rmos

t bar

s, gi

ving

a

serie

s of p

aral

lel c

rack

s.

• Sh

orte

r cra

cks m

ay a

lso a

ppea

r ov

er th

e ba

rs ru

nnin

g tr

ansv

erse

ly

• H

igh

amou

nt o

f ble

edin

g an

d se

ttle

men

t in

the

pres

ence

of a

form

of r

estr

aint

to

the

sett

lem

ent.

• Co

nsol

idat

ion

of c

oncr

ete

(loca

lly re

-st

rain

ed b

y re

info

rcin

g st

eel o

r for

mw

ork)

af

ter i

nitia

l pla

cem

ent,

vibr

atio

n, a

nd

finis

hing

.

• In

suffi

cien

t vib

ratio

n.

• U

se o

f lea

king

or h

ighl

y fle

xibl

e fo

rms.

• Se

ttle

men

t cra

ckin

g in

crea

ses

with

in

crea

sing

bar

siz

e, in

crea

sing

slu

mp,

and

de

crea

sing

cov

er.

• Lo

ss o

f bo

nd b

e-tw

een

top

bars

and

co

ncre

te

• Pr

ovid

es

rout

e in

to th

e co

ver f

or

moi

stur

e,

oxyg

en a

nd

chlo

ride

• Re

duce

ble

edin

g an

d se

ttle

men

t by

the

addi

tion

of fi

bers

or a

ir-en

trai

ning

adm

ixtu

res.

• Pr

oper

des

ign

of th

e fo

rmw

ork

follo

win

g AC

I 347

• Pr

oper

con

cret

e vi

brat

ion

(ACI

309

R).

• U

se o

f the

low

est p

ossi

ble

slum

p.

• In

crea

sing

the

conc

rete

cov

er.

• U

se o

f ste

el b

ars

with

a s

mal

ler d

iam

eter

.

• Pr

ovis

ion

of a

tim

e in

terv

al b

etw

een

the

plac

e-m

ent o

f con

cret

e in

col

umns

or d

eep

beam

s an

d th

e pl

acem

ent o

f con

cret

e in

sla

bs a

nd b

eam

s (A

CI

309.

2R)




Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Earl

y ag

eEa

rly

age

ther

mal

m

ovem

ent

• Su

rfac

e cr

acks

par

alle

l to

each

ot

her.

• Co

ncre

te c

antil

ever

wal

ls a

re

very

vul

nera

ble

to e

arly

ther

-m

al c

rack

s (s

ervi

ce re

serv

oirs

, re

tain

ing

wal

ls, b

ridge

abu

t-m

ents

and

bas

emen

ts)

• Cl

assi

cal c

ase:

Ver

tical

wal

l cas

t on

stiff

str

ip fo

otin

g: c

rack

s de

velo

p at

the

base

and

run

vert

ical

ly

• Cr

acks

nea

r the

end

of b

ays

may

be

incl

ined

at 4

5o

• H

eat o

f hyd

ratio

n w

ill in

itial

ly

rise

the

conc

rete

tem

pera

ture

• A

fter

few

day

s th

e co

ncre

te

will

coo

l cau

sing

con

trac

tion

of th

e el

emen

t.

• Co

ntra

ctio

n, w

hen

rest

rain

ed,

will

cau

se c

rack

ing.

Typ

es o

f re

stra

ints

incl

ude:

Exte

rnal

re

stra

int:

conc

rete

is c

ast o

nto

prev

ious

ly h

arde

ned

base

or

betw

een

hard

ened

ele

men

ts

with

out m

ovem

ent j

oint

s;

and

Inte

rnal

rest

rain

t: in

thic

k se

ctio

ns, t

he s

urfa

ce w

ill c

ool

quic

ker c

ausi

ng d

iffer

entia

l te

mpe

ratu

re a

nd s

trai

ns a

cros

s th

e se

ctio

n an

d th

us c

rack

ing

of th

e su

rfac

e la

yer.

• Pr

ovid

es ro

ute

into

the

cove

r fo

r moi

stur

e,

oxyg

en a

nd

chlo

ride

• Pr

ovid

e m

ovem

ent j

oint

s to

avo

id e

xter

nal r

estr

aint

• Li

miti

ng th

e tim

e in

terv

al b

etw

een

pour

s to

avo

id e

xces

-si

ve d

issi

mila

rity

betw

een

adja

cent

pou

rs a

nd e

xter

nal

rest

rain

t

• D

elay

rem

oval

of f

orm

wor

k to

con

trol

rate

of s

urfa

ce c

ool-

ing

and

avoi

d in

tern

al re

stra

int

• U

se o

f ins

ulat

ion

to k

eep

the

surf

ace

war

m, d

elay

the

onse

t of c

oolin

g an

d av

oid

inte

rnal

rest

rain

t

• U

se o

f sm

alle

r bar

dia

met

er, d

ecre

ase

bar s

paci

ng, r

educ

e co

ncre

te c

over

to th

e m

inim

um a

llow

able

val

ues

• Th

e le

ss m

assi

ve th

e st

ruct

ure,

the

less

the

pote

ntia

l for

te

mpe

ratu

re d

iffer

entia

l and

rest

rain

t.

• Li

mes

tone

and

gra

nite

agg

rega

te c

oncr

ete

have

low

er c

oef-

ficie

nt o

f the

rmal

exp

ansi

on th

an d

ense

agg

rega

te a

nd

henc

e th

ey w

ould

resu

lt in

less

ther

mal

cra

cks.

• Re

duce

the

inte

rnal

tem

pera

ture

by

keep

ing

the

core

co

ncre

te c

ool (

e.g.

inje

ctio

n of

liqu

id n

itrog

en)

• In

crea

se te

nsile

str

engt

h of

con

cret

e by

usi

ng s

teel

fibe

rs

in c

oncr

ete

floor

s to

hel

p in

con

trol

ling

crac

ks.

• U

se o

f ble

nded

cem

ent (

GG

BS &

PFA

) to

redu

ce th

e he

at

deve

lopm

ent a

nd th

eref

ore

the

rise

in te

mpe

ratu

re in

co

ncre

te (t

his

will

how

ever

redu

ce th

e te

nsile

str

engt

h an

d st

rain

cap

acity

of c

oncr

ete

at e

arly

age

s).

• O

ther

met

hods

use

d to

redu

ce c

rack

ing

in m

assi

ve c

on-

cret

e ar

e pr

esen

ted

in A

CI 2

07.1

R, 2

07.2

R,20

7.4R

, and

224

R


Design Life Extension of RC StructureTy

peD

iagn

oses

Caus

eEff

ect

Prot

ectiv

e / P

reve

ntio

n M

easu

res

Envi

ron-

men

tal

Dry

ing

shri

nkag

e &

Cr

azin

g

Shri

nkag

e:

• Sh

allo

w, c

lose

ly s

pace

d, fi

ne

crac

ks o

n su

rfac

e of

wal

ls o

r sl

abs

• Cr

acki

ng o

f the

sur

face

laye

r of

conc

rete

into

sm

alle

r irr

egu-

larly

sha

ped

cont

iguo

us a

reas

(s

urfa

ce c

razi

ng –

alli

gato

r pa

tter

n) o

n su

rfac

e of

wal

ls.

Crac

ks a

re ty

pica

lly a

t rig

ht

angl

e to

dire

ctio

n of

rest

rain

ts

Craz

ing:

• Cl

ose

patt

ern

of n

arro

w

(abo

ut 0

.1 m

m w

ide)

sha

llow

in

terc

onne

cted

cra

cks

usua

lly

form

clo

sed

poly

gona

l loo

ps.

The

poly

gona

l are

as a

re ty

pi-

cally

10-

75 m

m a

cros

s an

d th

e cr

acks

are

usu

ally

onl

y fe

w

mill

imet

ers

deep

.

• Dr

ying

shrin

kage

occ

urs d

ue to

re

duct

ion

in vo

lum

e of

conc

rete

ca

used

by l

oss o

f wat

er d

urin

g th

e ha

rden

ing

proc

ess a

nd su

bseq

uent

ex

posu

re to

uns

atur

ated

air. T

his

will

resu

lt in

crac

king

of c

oncr

ete

if re

stra

ined

in so

me

way

.

• Lo

ss o

f moi

stur

e fro

m re

stra

ined

ce

men

t pas

te co

nstit

uent

, whi

ch

can

shrin

k by

as m

uch

as 1

%.

• In

mas

sive

conc

rete

ele

men

ts,

tens

ile st

ress

es in

duci

ng cr

acks

ar

e ca

used

by

diffe

rent

ial s

hrin

k-ag

e be

twee

n th

e su

rface

and

th

e in

terio

r con

cret

e. Th

e hi

gher

sh

rinka

ge a

t the

surfa

ce ca

uses

cr

acks

to d

evel

op th

at m

ay, w

ith

time,

pen

etra

te d

eepe

r int

o th

e co

ncre

te.

• Cr

acks

may

pro

paga

te a

t muc

h lo

wer

stre

sses

than

are

requ

ired

to

caus

e cr

ack

initi

atio

n (A

CI 4

46.1

R).

• Th

e am

ount

of d

ryin

g sh

rinka

ge is

in

fluen

ced

mai

nly b

y the

am

ount

an

d ty

pe o

f agg

rega

te a

nd th

e ce

men

t pas

te (c

emen

t and

wat

er)

cont

ents

of t

he m

ixtu

re.

• Cr

azin

g us

ually

occ

urs w

hen

the

surfa

ce la

yer o

f the

conc

rete

has

hi

gher

wat

er co

nten

t tha

n th

e in

terio

r con

cret

e due

to o

ver t

row

el-in

g fo

r exa

mpl

e. Di

scon

tinui

ty in

co

mpo

sitio

n ne

ar ex

pose

d su

rface

.

• Co

ntra

ctio

n an

d/or

defl

ec-

tion

• Re

duce

the

wat

er c

onte

nt (n

ot th

e w

/cm

ratio

)

• Pr

ovid

e ad

equa

te re

info

rcem

ent a

nd s

uffici

ent j

oint

s

• A

s th

e qu

antit

y an

d si

ze o

f agg

rega

te in

crea

ses,

the

dryi

ng

shrin

kage

dec

reas

es.

• Th

e hi

gher

the

stiff

ness

of t

he a

ggre

gate

, the

mor

e eff

ec-

tive

it is

in re

duci

ng th

e sh

rinka

ge o

f the

con

cret

e

• Th

e sh

rinka

ge o

f con

cret

e co

ntai

ning

san

dsto

ne a

ggre

gate

m

ay b

e m

ore

than

twic

e th

at o

f con

cret

e w

ith g

rani

te,

basa

lt, o

r hig

h-qu

ality

lim

esto

ne.

• Th

e lo

wer

the

wat

er a

nd c

emen

t con

tent

s, th

e le

ss th

e am

ount

of d

ryin

g.

• Sh

rinka

ge c

rack

ing

can

be c

ontr

olle

d by

usi

ng c

ontr

actio

n jo

ints

and

pro

per d

etai

ling

of th

e re

info

rcem

ent.

• Sh

rinka

ge c

rack

ing

may

als

o be

redu

ced

or e

ven

elim

inat

ed

by u

sing

shr

inka

ge-c

ompe

nsat

ing

cem

ent o

r a s

hrin

kage

-co

mpe

nsat

ing

adm

ixtu

re.

• Ad

equa

te c

urin

g to

exp

osed

sur

face

s

• Pr

ovid

e m

ovem

ent j

oint

s to

elim

inat

e ex

tern

al re

stra

ints

w

here

app

licab

le

• Su

ffici

ent c

rack

con

trol

and

ste

el d

istr

ibut

ion.

• U

se G

GBS

or P

FA to

redu

ce th

e w

ater

dem

and

for a

giv

en

wor

kabi

lity.




Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Chem

ical

Corr

osio

n of

st

eel

- Car

bona

tion-

in

duce

d

-Chl

orid

e-in

duce

d

• Ru

st s

tain

s

• Lo

ngitu

dina

l cr

acks

in a

di

rect

ion

para

llel t

o th

e st

eel

rein

forc

ing

bars

.

• Sp

allin

g of

co

ncre

te c

over

• A

bro

ad c

rack

m

ay fo

rm a

t a

plan

e of

ba

rs p

aral

lel

to a

con

cret

e su

rfac

e,

resu

lting

in

dela

min

atio

n,

whi

ch is

a

wel

l-kno

wn

prob

lem

in

brid

ge d

ecks

• Ex

posu

re

to c

hlor

ides

de

stro

ying

the

pass

ivity

of t

he

stee

l

• Re

duct

ion

in

the

alka

linity

of

the

conc

rete

th

roug

h ca

rbon

atio

n

• Lo

cal c

rack

ing

due

to h

igh

bond

str

esse

s, tr

ansv

erse

te

nsio

n,

shrin

kage

, and

se

ttle

men

t, ca

n in

itiat

e co

rros

ion.

• Sp

allin

g of

co

ncre

te c

over

• Lo

ss o

f eff

ectiv

e cr

oss

sect

iona

l are

a of

con

cret

e

• Lo

ss o

f re

info

rcem

ent

cros

s-se

ctio

n

• Lo

ss o

f bon

d at

ste

el/

conc

rete

in

terf

ace

• Lo

ss o

f loa

d ca

rryi

ng

capa

city

• Th

e ke

y fo

r pro

tect

ing

met

al fr

om c

orro

sion

is to

sto

p or

reve

rse

the

chem

ical

re

actio

ns. T

his

may

be

done

by:

• Cu

ttin

g off

the

supp

lies

of o

xyge

n or

moi

stur

e

• Su

pply

ing

exce

ss e

lect

rons

at t

he a

node

s to

pre

vent

the

form

atio

n of

the

met

al io

ns

(cat

hodi

c pr

otec

tion)

.

For g

ener

al co

ncre

te co

nstr

uctio

n, th

e be

st p

rote

ctio

n ag

ains

t cor

rosi

on-in

duce

d sp

littin

g is

the

use

of:

• Co

ncre

te w

ith lo

w p

erm

eabi

lity

and

adeq

uate

cov

er.

• In

crea

sed

conc

rete

cov

er o

ver t

he re

info

rcem

ent i

s eff

ectiv

e in

del

ayin

g th

e co

rros

ion

proc

ess

by li

miti

ng c

arbo

natio

n, a

s w

ell a

s ac

cess

by

oxyg

en, m

oist

ure,

and

chl

orid

es,

and

also

in re

sist

ing

the

split

ting

and

spal

ling

caus

ed b

y co

rros

ion.

• In

the

case

of l

arge

bar

s an

d th

ick

cove

rs, i

t may

be

nece

ssar

y to

add

sm

all t

rans

vers

e re

info

rcem

ent (

whi

le m

aint

aini

ng th

e m

inim

um c

over

requ

irem

ents

) to

limit

split

ting

and

to re

duce

the

surf

ace

crac

k w

idth

(ACI

345

R).

• Co

ncre

te s

ubje

cted

to w

ater

-sol

uble

sal

ts s

houl

d be

am

ply

air e

ntra

ined

• U

se o

f ade

quat

e co

ver o

f the

rein

forc

ing

stee

l.

• Th

e us

e of

hig

h-qu

ality

and

low

-per

mea

bilit

y co

ncre

te.

In v

ery

seve

re e

xpos

ure

cond

ition

s, ad

ditio

nal p

rote

ctiv

e m

easu

res m

ay b

e re

quire

d. A

num

ber

of o

ptio

ns a

re a

vaila

ble,

such

as:

• Ep

oxy-

coat

ed re

info

rcem

ent (

shou

ld b

e us

ed w

ith c

autio

n an

d hi

gh q

ualit

y co

ntro

l).

• Se

aler

s or

ove

rlays

on

the

conc

rete

.

• Co

rros

ion-

inhi

bitin

g ad

mix

ture

s.

• Ca

thod

ic p

rote

ctio

n.

• In

mos

t cas

es, c

oncr

ete

mus

t be

allo

wed

to b

reat

he; t

hat i

s, w

ater

mus

t be

allo

wed

to

evap

orat

e fr

om th

e co

ncre

te.


Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Chem

ical

Alk

ali-s

ilica

re

actio

n (A

SR)

• N

etw

ork

pat-

tern

of c

rack

s (m

ap c

rack

ing)

• Co

ncre

te m

ay c

rack

with

tim

e as

the

resu

lt of

slo

wly

dev

elop

ing

expa

nsiv

e re

actio

ns b

etw

een

aggr

egat

e co

ntai

n-in

g ac

tive

silic

a an

d al

kalis

der

ived

fr

om c

emen

t hyd

ratio

n, a

dmix

ture

s, or

ex

tern

al s

ourc

es (s

uch

as c

urin

g w

ater

, gr

ound

wat

er, d

eici

ng c

hem

ical

s, an

d al

kalin

e so

lutio

ns s

tore

d or

use

d in

the

finis

hed

stru

ctur

e). W

hen

the

alka

lis

in c

emen

t rea

ct w

ith s

usce

ptib

le

aggr

egat

e pa

rtic

les,

a re

actio

n rim

of

alk

ali-s

ilica

gel

is fo

rmed

aro

und

the

aggr

egat

e. If

this

gel

is e

xpos

ed

to m

oist

ure,

it e

xpan

ds, c

ausi

ng a

n in

crea

se in

vol

ume

of th

e co

ncre

te

mas

s th

at w

ill re

sult

in c

rack

ing

and

may

eve

ntua

lly re

sult

in th

e co

mpl

ete

dete

riora

tion

of th

e st

ruct

ure.

• Ce

rtai

n ca

rbon

ate

rock

s pa

rtic

ipat

e in

reac

tions

with

alk

alis

that

, in

som

e in

stan

ces,

prod

uce

detr

imen

tal e

xpan

-si

on a

nd c

rack

ing.

The

se d

etrim

enta

l al

kali-

carb

onat

e re

actio

ns a

re u

sual

ly

asso

ciat

ed w

ith a

rgill

aceo

us d

olo-

miti

c lim

esto

ne th

at h

ave

a ve

ry fi

ne-

grai

ned

(cry

ptoc

ryst

allin

e) s

truc

ture

(A

CI 2

01.2

R). T

he re

actio

n is

dis

tin-

guis

hed

from

the

alka

li-si

lica

reac

tion

by th

e ge

nera

l abs

ence

of s

ilica

gel

su

rfac

e de

posi

ts a

t the

cra

ck.

• Lo

ss o

f com

pres

sive

str

engt

h of

con

cret

e

• Lo

ss o

f ten

sile

str

engt

h of

co

ncre

te

• Lo

ss o

f effe

ctiv

e cr

oss

sec-

tiona

l are

a of

con

cret

e

• Th

e us

e of

non

-rea

ctiv

e ag

greg

ates

, low

-alk

ali

cem

ent,

and

pozz

olan

s th

at c

onsi

st p

rinci

pally

of

ver

y fin

e, h

ighl

y ac

tive

silic

a. T

he fi

rst m

ea-

sure

may

pre

clud

e th

e pr

oble

m fr

om o

ccur

ring,

w

hile

the

othe

r tw

o m

easu

res

have

the

effec

t of

decr

easi

ng th

e al

kali-

reac

tive

silic

a ra

tio, r

esul

t-in

g in

the

form

atio

n of

a n

onex

pand

ing

calc

ium

al

kali

silic

ate

hydr

ate.

• Av

oidi

ng re

activ

e ag

greg

ates

, dilu

tion

with

non

-re

activ

e ag

greg

ates

,

• Th

e us

e of

a s

mal

ler m

axim

um s

ize

aggr

egat

e,

• Th

e us

e of

low

-alk

ali c

emen

t (AC

I 201

.2R)

.




Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Chem

ical

Sulfa

te A

ttac

k•

Soft

enin

g of

co

ncre

te s

ur-

face

laye

r

• Cl

osel

y sp

aced

cr

acks

• Ce

rtai

n Su

lfate

s in

soi

l and

w

ater

are

a s

peci

al d

urab

ility

pr

oble

m fo

r con

cret

e. W

hen

sulfa

te p

enet

rate

s hy

drat

ed

cem

ent p

aste

, it c

omes

in

cont

act w

ith h

ydra

ted

calc

ium

al

umin

ate.

Cal

cium

sul

foal

u-m

inat

e is

form

ed, w

hich

may

re

sult

in a

n in

crea

se in

vol

ume.

• Fo

rmat

ion

of e

xpan

sive

et

trin

gite

or g

ypsu

m in

the

hard

ened

con

cret

e ca

usin

g cr

acki

ng a

nd e

xfol

iatio

n

• So

ften

ing

and

diss

olut

ion

of th

e hy

drat

ed c

emen

ting

com

poun

ds d

ue to

dire

ct a

t-ta

ck o

n th

ese

com

poun

ds b

y su

lfate

s or

by

thei

r dec

ompo

si-

tion

whe

n ca

lciu

m h

ydro

xide

re

acts

with

the

sulfa

tes

and

is

rem

oved

• Lo

ss o

f con

cret

e co

ver

• Lo

ss o

f effe

ctiv

e cr

oss

sec-

tiona

l are

a of

con

cret

e

• Re

duct

ion

in m

echa

nica

l pro

p-er

ties

(stiff

ness

, com

pres

sive

&

tens

ile s

tren

gths

)

• Th

e us

e of

AST

M C

150

Typ

es II

and

V P

ortla

nd c

emen

t, w

hich

are

low

in tr

ical

cium

alu

min

ate,

will

min

imiz

e th

e fo

rmat

ion

of c

alci

um s

ulfo

alum

inat

e.

• Th

e us

e of

Sulfa

te-r

esis

tant

cem

ents

spe

cifie

d in

AST

M

C 59

5 an

d C

1157

are

als

o us

eful

in im

prov

ing

sulfa

te

resi

stan

ce.

• Th

e us

eofP

ozzo

lans

that

hav

e be

en te

sted

and

sho

wn

to

impa

rt a

dditi

onal

resi

stan

ce to

sul

fate

att

ack

are

bene

fi-ci

al.

• Th

e us

e of

con

cret

e w

ith a

low

w/c

m is

impo

rtan

t to

prov

idin

g pr

otec

tion

agai

nst s

ever

e su

lfate

att

ack.


Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Stru

ctur

al

Flex

ural

•

Clos

ely

spac

ed c

rack

s ru

nnin

g pe

rpen

dicu

lar t

o th

e lo

ngitu

di-

nal m

ain

stee

l rei

nfor

cem

ent.

• U

nifo

rmly

dis

trib

uted

in

regi

ons

of m

axim

um m

omen

ts

• Er

ror i

n de

sign

• Ex

cess

ive

load

ing

• Lo

wer

load

car

ryin

g ca

paci

ty

than

des

igne

d •

Prop

er d

esig

n

• Th

orou

gh u

nder

stan

ding

of s

truc

tura

l be

havi

or

• Pe

er re

view

of d

esig

n

Shea

r / to

rsio

n •

Crac

ks n

ear s

uppo

rts

incl

ined

at

30

o to

45o

.

• Sh

ear c

rack

s ar

e in

clin

ed in

the

sam

e di

rect

ion

on b

oth

side

s of

the

beam

.

• To

rsio

nal c

rack

s ar

e in

clin

ed

and

para

llel t

o on

e an

othe

r bu

t run

in o

ppos

ite d

irect

ions

on

the

beam

sid

es fo

rmin

g a

spira

l pat

tern

• Er

ror i

n de

sign

• Ex

cess

ive

load

ing

• Lo

wer

load

car

ryin

g ca

paci

ty

than

des

igne

d •

Prop

er d

esig

n

• Th

orou

gh u

nder

stan

ding

of s

truc

tura

l be

havi

or

• Pe

er re

view

of d

esig

n

Punc

hing

she

ar•

Crac

ks fo

rms

at te

nsio

n si

de o

f th

e fla

t sla

b ra

diat

ing

out f

rom

th

e co

lum

n. T

hese

may

be

join

ed to

a s

erie

s of

circ

um-

fere

ntia

l cra

cks

at a

dis

tanc

e ab

out 3

tim

es th

e sl

ab th

ick-

ness

from

the

colu

mn

face

• Er

ror i

n de

sign

• Ex

cess

ive

load

ing

• Lo

wer

load

car

ryin

g ca

paci

ty

than

des

igne

d•

Prop

er d

esig

n

• Th

orou

gh u

nder

stan

ding

of s

truc

tura

l be

havi

or

• Pe

er re

view

of d

esig

n

crac

king

in m

assi

ve co

ncre

te a

re p

rese

nted

in

ACI 2

07.1

R, 2

07.2

R,20

7.4R

, and

224

R

Axi

al•

Long

itudi

nal c

rack

s pa

ralle

l to

the

stee

l rei

nfor

cem

ent i

n co

lum

ns

• Er

ror i

n de

sign

• Ex

cess

ive

load

ing

• Lo

wer

load

car

ryin

g ca

paci

ty

than

des

igne

d•

Prop

er d

esig

n

• Th

orou

gh u

nder

stan

ding

of s

truc

tura

l be

havi

or

• Pe

er re

view

of d

esig

n



Type

Dia

gnos

esCa

use

Effec

tPr

otec

tive

/ Pre

vent

ion

Mea

sure

s

Chem

ical

Foun

datio

n se

ttle

men

t D

iffer

entia

l set

tlem

ent:

• D

ifficu

lty in

ope

ratin

g do

ors

or

win

dow

s

• Cr

acki

ng o

f pla

ster

or g

ypsu

m

wal

l boa

rd in

clin

ed a

t app

roxi

-m

atel

y 45

o

• Cr

acki

ng in

the

mas

onry

fa

çade

at a

ppro

xim

atel

y 45

o on

the

diag

onal

in a

ste

pwis

e fa

shio

n al

ong

the

bric

k-m

orta

r jo

ints

.

• Se

ries

of d

iago

nal c

rack

s be

twee

n w

indo

ws

that

are

st

acke

d ve

rtic

ally

• In

clin

ed c

rack

s at

the

corn

er o

f w

indo

ws

and

door

s

Uni

form

sett

lem

ent:

• D

amag

e of

ser

vice

s co

nnec

ted

to th

e st

ruct

ure

• Er

ror i

n de

sign

of f

ound

atio

n

• In

corr

ect a

ssum

ptio

n ab

out

prop

ertie

s an

d di

strib

utio

n of

th

e so

il be

low

the

stru

ctur

e

• Er

ror i

n st

ruct

ural

des

ign

of

elem

ents

suc

h as

pile

cap

s

• Co

nsol

idat

ion

of a

sof

t and

/or

orga

nic

soil

• Pr

esen

ce o

f exp

ansi

ve s

oil.

• Se

ttle

men

t fro

m u

ncon

trol

led

deep

fill

• D

evel

opm

ent o

f lim

esto

ne

cavi

ties

or s

ink

hole

s

• So

il su

bsid

ence

Ext

ract

ion

of

oil o

r gro

und

wat

er

• W

ater

infil

trat

ion

into

the

grou

nd th

at m

ay c

ause

un-

stab

le s

oil t

o co

llaps

e

• Yi

eldi

ng o

f adj

acen

t exc

ava-

tions

or c

olla

pse

of li

mes

tone

ca

vitie

s an

d un

derg

roun

d m

ines

and

tunn

els

• D

iffer

entia

l set

tlem

ent:

• Se

rvic

eabi

lity

and

func

tiona

l da

mag

e

• Im

pose

sig

nific

ant s

tres

ses

and

alte

ratio

n of

the

cond

ition

s on

whi

ch th

e ba

sic

stru

ctur

al

assu

mpt

ions

wer

e m

ade

(e.g

. re

gion

s of

hog

ging

mom

ent

may

be

subj

ecte

d to

sag

-gi

ng m

omen

t ins

tead

). Th

ese

regi

ons

will

be

sign

ifica

ntly

un

der-

stre

ngth

• D

amag

e m

ay a

ffect

the

stab

il-ity

of t

he b

uild

ing

such

as

crac

king

and

dis

tort

ions

to

supp

ort m

embe

rs w

hich

may

le

ad to

com

plet

e co

llaps

e of

th

e bu

ildin

g.

Uni

form

sett

lem

ent:

• It

will

not

nor

mal

ly c

ause

st

ruct

ural

dis

tres

s bu

t it m

ay

dam

age

serv

ices

con

nect

ed to

th

e st

ruct

ure

• Pr

oper

soi

l inv

estig

atio

n an

d su

bsur

-fa

ce e

xplo

ratio

n

• Th

orou

gh u

nder

stan

ding

of s

truc

-tu

ral b

ehav

ior

• Pr

oper

des

ign

of fo

unda

tion

• Pe

er re

view

of d

esig

n



4. PRECAUTIONS TO AVOID FOUNDATION SETTLEMENT • Proper soil investigation and subsurface exploration• Proper design of foundation and tie beams• Proper compaction of soil layers as per the soil investigation report• Avoid excessive irrigation near foundation to minimize water infiltration into the ground• Avoid soil subsidence extraction of oil or ground water beside foundation • Avoid deep excavation beside foundation of existing building unless proper earth retaining walls

are provided by means of Contiguous Bored Piled Wall, Secant Piled Wall, or Diaphragm (D-wall).• Proper isolation of foundation to avoid deterioration due to presence of water table or aggressive

agents. • Proper drainage system to avoid leaking and/or water infiltration into the ground.

5. REFERENCES• ACI 201.1RGuide for Making a Condition Survey of Concrete in Service• ACI 201.2RGuide to Durable Concrete• ACI 201.3RGuide for Making a Condition Survey of Concrete Pavements• ACI 207.1RGuide to Mass Concrete• ACI 207.2REffect of Restraint, Volume Change, and Reinforcement on Cracking of Mass Concrete• ACI 207.3RPractices for Evaluation of Concrete in Existing Massive Structures for Service Conditions• ACI 207.4RCooling and Insulating Systems for Mass Concrete• ACI 222RProtection in Metals in Concrete Against Corrosion• ACI 224RControl of Cracking in Concrete Structures• ACI 224.2RCracking of Concrete Members in Direct Tension• ACI 224.3RJoints in Concrete Construction• ACI 228.2RNondestructive Test Methods for Evaluation of Concrete in Structures• ACI 302.1RGuide for Concrete Floor and Slab Construction• ACI 304RGuide for Measuring, Mixing, Transporting, and Placing Concrete• ACI 305RHot Weather Concreting• ACI 308RGuide to Curing Concrete• ACI 309RGuide for Consolidation of Concrete• ACI 309.2RIdentification and Control of Visual Effects of Consolidation Formed Concrete Surfaces• ACI 318Building Code Requirements for Structural Concrete343RAnalysis and Design of Reinforced

ConcreteBridge Structures• ACI 345RGuide for Concrete Highway Bridge DeckConstruction• ACI 345.1RGuide for Maintenance of Concrete Bridge Members• ACI 347Guide to Formwork for Concrete• ACI 446.1RFracture Mechanics of Concrete: Concepts,Models, and Determination of Material

Properties • “Cracks in Concrete”,Au Yong TheanSeng , E-book, www.madisonvelocity.blogspot.com.




• ACI 503RUse of Epoxy Compounds with Concrete• ACI 504RGuide to Sealing Joints in Concrete Structures• ACI 517.2RAccelerated Curing of Concrete at AtmosphericPressure• ACI 546RConcrete Repair Guide• ACI 546.1RGuide for Repair of Concrete Bridge Superstructures• ACI 546.2RGuide to Underwater Repair of Concrete548.1RGuide for the Use of Polymers in Concrete• Bureau of Indian standards, “Handbook on Causes and Prevention of Cracks in Buildings”, SP

25:1984, pp68.





IV - ASSESSMENT OF CONCRETE STRUCTURES Design Life Extension of RC Structure 37

IV - ASSESSMENT OFCONCRETE STRUCTURES


38 IV - ASSESSMENT OF CONCRETE STRUCTURES

1. INTRODUCTIONStructural assessment can be initiated, when there has been a change in resistance. Such as structural deterioration due to time-depending processes (e.g. corrosion, fatigue) or structural damage by accidental actions. Also when there will be a change in loading (e.g. increased traffic load) or an extension of the design working life. Assessment can also be carried out to analyze the current structural reliability (e.g. for environmental hazards like earthquake or extreme winds and/or waves).

This section of this report presents a methodological framework of the assessment of existing structures and a summarization of the manifold methods developed in recent years for structural assessment. It is intended to describe the coherency and difference between methods and to provide an understanding and to help practicing engineers finding the suitable assessment procedure depending on the assessment objectives as well as on different boundary conditions.

Within management of groups of structures it is necessary to assessment unify, so that different structures are assessed in the same way and results are comparable between authorities, regions or countries. The guideline is meant to provide a framework to achieve that goal. It is intended to explain the principles of structural assessment and to feature the several levels of structural assessment, starting with simple but conservative methods and progressing to more refined but also more expensive methods.

The guideline can be applied to all kind of existing structures (e.g. bridges and tunnels, buildings, industrial structures on- and offshore) of any type of structural material (concrete, steel, timber, masonry, composite material). The structures to be assessed can be designed based on accepted engineering principles or design rules as well as on good workmanship, historic experience and accepted professional practice. Since fire resistance requires properties different from those of structural safety and integrity, the assessment of fire resistance is not part of the guideline.

2. PRINCIPLES OF STRUCTURAL ASSESSMENT2.1. ObjectivesIn general structural assessment is a process to determine, how reliable the existing structure is able to carry current and future loads and to fulfill its task for a given time period. The first step of the assessment process must always be the clear specification of the assessment objective. This is essential to identify the most significant limit states. Associated with the limit states are the structural variables to be investigated and with those the assessment procedure to be applied.

A wide range of different assessment procedures exists with varying complexity and the choice of the appropriate procedure depends highly on the specified requirements of assessment. There are two main objectives to conduct assessment of existing structures, the assurance of structural safety and serviceability and the minimization of costs.

2.2. Structural safety and serviceabilityThe main task of assessment is to ensure that the structure or parts of the structure do not fail under loading. The assessment is carried out for ultimate limit states, which are:• Loss of equilibrium of the structure or parts of it as a rigid body (e.g. overturning)• attainment of the maximum resistance capacity transformation of the structure or part of it into a mechanism• instability of the structure of part of it• sudden change of the assumed structural system to a new system (e.g. snap through)

A reduction of serviceability may lead to a limitation of use and therefore serviceability assessment might become necessary. Serviceability limit states include:• local damage which may reduce the working life of the structure


• unacceptable deformations which affect the efficient use• excessive vibrations which cause discomfort to people

Safety and serviceability can be evaluated for a variety of reasons, among others for changes in use or increase of loads, effects of deterioration, damage as result of extreme loading events and concern about design and construction errors and about the quality of building material and workmanship. Increases of the maximum live load limits and changes of use are probably the main reasons for structural assessment. For buildings such changes could result in the need to support higher floor loadings. For bridges there is a worldwide demand to raise the limits for traffic loads.

Any structure is undergoing some degree of deterioration. The effects of deterioration are structure and site specific. Concerning structural strength, corrosion and fatigue are the main deterioration processes. Spalling, cracking, and degraded surface conditions are typical indications of deterioration. Impact, earthquake or wind storms can result in structural damage. The remaining load carrying capacity needs to be analyzed after such events. It may be necessary to assess an existing structure, after concerns about the correct design and constructions arise, including low quality building material or workmanship.

2.3. MethodologyThe assessment of existing structures can be carried out with methods of varying sophistication and effort. The core objectives, as described above, are to analyze the current load carrying capacity and to predict the future performance with a maximum of accuracy and a minimum of effort. In most cases it judiciously to start with simple conservative routines and use more sophisticated routines only when the evaluated load carrying capacity is insufficient. Generally structural assessment should be carried out using limit state principles with characteristic values and partial safety factors. If more refined methods are necessary, the probabilistic approach has to be applied, if economic. If structures have failed assessment to an acceptable capacity, the engineer can make a recommendation, but the technical authority is likely to be ultimately responsible for public safety and therefore has to do the final decision. A structure, failed in assessment, may remain in service if it presents a low risk, subject to monitoring.

2.4. Measurement based serviceability assessment Focusing on concrete structures, an important range of test methods is available for use during inspection, or on samples extracted from the structure for laboratory investigation. For existing structures, the need for testing may arise from a variety of causes which can include:a) change of usage or extension of a structureb) acceptability of a structure for purchase or insurancec) assessment of structural integrity or safety following material deterioration, or structural damage

such as caused by fire, blast or overload

d) serviceability or adequacy of members known or suspended to contain material which does not meet specifications, or with design faults

e) monitoring of strength development in relation to framework stripping, prestressing or load application

The tests for concrete structures can also be classified as non-destructive (NDT), which are not harmful, semi-destructive or minor destructive (MDT), which usually cause minor, but reversible, damage to the structure, and destructive (D), which are the most invasive. In concrete structures, there is usally a need for better understanding or assessing the properties of materials for restoration and rehabilitation purposes, especially when not enough information, such as documentation, is available. In such cases, only non-destructive tests should be regarded as suitable, in order not to harm the material and not to cause any alteration to parts of the structure. However, NDT can give qualitative results in most cases, and their calibration can be rather difficult. As a result of this, MDT can also be used to form a basis for the calibration of NDT.




According to the necessity of information, there is a variety of tests that are available. The principle options can be presented in the following Table 1. These methods can generally be performed as nondestructive, apart from chemical and petrographic methods which require cutting or drilling small samples from the concrete. Many of these methods require expensive equipment, with extensive safety precautions in some cases. Cost consideration, as well as the aspect of damage, time and reliability can strongly influence the choice of the method to be executed. Table 2 indicates the damage resulting from strength tests, along with the main restrictions in each case. While, Table 3 gives information about the relative cost, the speed, the damage and the reliability of calibration for each test.

Table 1- In-situ tests on concrete

Information required Methods available

• Member behavior and strength Load tests with deflections and strain measurements

• Concrete strength Cores Rebound hammer Pull-out and internal fracture Break-off and pull-off Penetration resistance Ultrasonic pulse velocity

• Cracking Ultrasonic pulse velocity Acoustic emission and hologra-phy

• Honeycombing and compaction Ultrasonic pulse velocity γ-radiography Cores Pulse echo techniques

• Density γ-radiometry

• Permeability Absorption, flow tests and capillary rise

• Moisture content Nuclear methods Electrical resistivity Microwave absorp-tion

• Cement content Chemical analysis Nuclear methods

• Mix properties and constituents Chemical analysis Cores Micrometric methods

• Reinforcement detection Magnetic methods X-and γ-radiography

• Chemical analysis

• Thermo-luminescence

• Ultrasonic pulse velocity

• Micrometric methods

Concrete deterioration

• Rebound hammer

• Wear tests

• Physical methods

• Infrared thermo-graphy

Abrasion resistance and soundness


Table 2- Strength tests: damage and restrictions

Test method Probable damage Major restrictionsCollapse load test Member destroyed • Member must be isolated, and

preferably removed from rest of structure before the test

Overload test Possible loss of member • Member must be isolated, or allowance made for load distri-bution to adjacent parts of the structure

Cores Holes to be made good • Limitation of cores size and pro-portion

• Safety precautions for critical members

Penetration resis-tance

(Windsor probe)

Cone approx. 50 mm dia. To be

made good

• Minimum edge distance

• Minimum member thickness

Pull-out Bolt hole remains • PreplannedInternal fracture Bolt to be cropped or cone

approx. 75 mm dia. To be made

good

• Drilling difficulties

Ultrasonics None • Two smooth surfaces necessaryRebound hammer None (for mature concrete) • Smooth surface necessary

Table 3- Strength tests: relative merits

Test method Cost Test speed

Damage to concrete

Representa-tiveness

Reliability of strength calibra-tions

Collapse load test High Slow Total Good Good Overload test High Slow Variable Good Good Cores High Slow Moderate Moderate Good Penetration resis-tance Moderate Fast Minor

Near surface only Moderate

Pull-out/ Internal fracture Moderate Fast Minor

Near surface only Moderate

Ultrasonic Low Fast None Good Moderate Rebound hammer Very low Fast Unlikely Surface only Poor




3. CODES, STANDARDS, SPECIFICATIONS AND PROCEDURESTo provide guidance on test methods, a national standards organization may develop Guidelines or Recommendations to assist users of a specific technology. These types of documents are not usually called up in contracts between organizations.

3.1. American Society for Testing and Materials (ASTM)(1) ASTM C 42 - 87, Standard Test Method (STM) for obtaining and testing Drilled Cores and

Sawed Beams of Concrete, Annual Book of ASTM Standards, 1988, ASTM, Philadelphia, USA

(2) ASTM C85 - 66, “Cement content of hardened Portland cement concrete”, ASTM, Philadelphia, USA

(3) ASTM C457 - 80 “Air void content in hardened concrete”, ASTM, Philadelphia, USA

(4) ASTM C823 - 75 “Examining and sampling of hardened concrete in constructions”

(5) ASTM C779- 76 “Abrasion resistance of horizontal concrete surfaces”

(6) ASTM C944 -80 “Abrasion resistance of concrete or mortar surfaces by the rotating cutter method”

(7) ASTM C856 -77 “Petrographic examination of hardened concrete”

(8) ASTM D4788 - 88 Standard Test Method for detecting Delamination in Bridge Decks using Infrared Thermography

(9) ASTM D6087 - 97 STM for Evaluating Asphalt covered Concrete Bridge Decks using Ground Penetrating Radar

(10) ASTM D4580 - 86 (1997) Standard Practice for measuring Delamination in Concrete Bridge Decks by Sounding

(11) ASTM D2950 - 91 (1997) STM for Density of Bituminous Concrete in place by Nuclear Methods

(12) ASTM C1383 - 98 a STM for measuring P wave Speed and the Thickness of Concrete Plates using the Impact-Echo Method

(13) ASTM C1150 - 96 STM for the Break off Number of Concrete

(14) ASTM C1040- 93 STM for Density of Unhardened and Hardened Concrete in place by Nuclear Methods

(15) ASTM C900- 94 STM for Pullout Strength of Hardened Concrete

(16) ASTM C876 -91 STM for Half-cell Potentials of Uncoated Reinforcing Steel in Concrete

(17) ASTM C805 - 97 STM for Rebound Number of Hardened Concrete

(18) ASTM C 803 - 82 STM for Penetration Resistance of Hardened Concrete

(19) ASTM C801- 98 STM for Determining the Mechanical Properties of Hardened Concrete under Triaxial Load

(20) ASTM C597- 97 STM for the Pulse Velocity through Concrete


3.1.1. Tests to evaluate existing concrete structures according to the ASTM StandardsTable 4 Nondestructive test to determine materials properties of hardened concrete (ACI 228.2R-98

PropertyPossible Methods

CommentPrimary Secondary

Compressive Strength Cores for compression testing (ASTM C42 and C39)

Penetration resistance (ASTM C803; pullout test-

ing drilled in)

Strength of in-place concrete; comparison of strength in different locations; and drilled-in pullout test

not standardized

Relative Compressive StrengthRebound number (ASTM

C805); Ultrasonic pulse veloc-ity (ASTM C597)

ــــــــــــــــ

Rebound number influenced by near surface properties; ultrasonic pulse

velocity gives average result through thickness

Tensile Strength Splitting –tensile strength of core (ASTM C496)

In-place pulloff test (ACI 503R; BS 1881:Part207) Assess tensile strength of concrete

Density Specific gravity of samples (ASTM C642) Nuclear gage ــــــــــــــــ

Moisture Content Moisture meter Nuclear gage ــــــــــــــــ

Static Modulus of Elasticity Compression test of cores (ASTM C469) ــــــــــــــــ ــــــــــــــــ

Dynamic Modulus of Elasticity

Resonant frequency testing of sawed specimens (ASTM

C215)

Ultrasonic pulse velocity (ASTM C597); Impact echo spectral analysis of surface

waves (SAWS)

Requires knowledge of density and Poisson’s ratio (except ASTM C215);

dynamic elastic modulus is typi-cally greater than the static elastic

modulus

Shrinkage/ExpansionLength change of drilled or sawed specimens (ASTM C

341)ــــــــــــــــ

Measure of incremental potential length change

Resistance to Chloride Penetra-tion

90-day ponding test (AASH-TO-T_259)

Electric indication of concrete’s ability to resist chloride ion penetration

(ASTM C1202)

Establishes relative susceptibility of concrete to chloride ion intrusion;

assess effectiveness of chemical seal-ers, membranes and overlays

Air Content; Cement Content; and Aggregate Properties (scal-ing alkali-aggregate reactivity, freezing and thawing suscep-

tibility)

Petrographic examination of concrete samples removed from structure (ASTM C856,

ASTM C457); Cement content (ASTM C1084)

Petrographic examina-tion of aggregates (ASTM

C294, ASTM 295)

Assist in determination of cause(s) of distress; degree of damage; quality

of concrete when originally cast and current

Alkali-Silica Reactivity Cornell/SHRP rapid test (SHRP-C-315) ــــــــــــــــ

Establish in field if observed deterio-ration is due to alkali-silica reactivity

Carbonation, pH Phenolphthalein (qualitative indication); pH meter

Other pH indicators (e.g. litmus paper)

Assess corrosion protection value of concrete with depth and susceptibil-

ity of steel reinforcement to corro-sion; depth of carbonation

Fire Damage Petrography; rebound num-ber (ASTM C805)

SAES; Ultrasonic pulse velocity; impact-echo;

Impulse-response

Rebound number permits demarca-tion of damaged concrete

Freezing and Thawing Damage Petrography SAWS; Impulse response ــــــــــــــــ

Chloride ContentAcid-soluble (ASTM C1152) and Water soluble (ASTM

C1218)

Specific ion probe (SHRP-S-328)

Chloride ingress increases suscep-tibility of steel reinforcement to

corrosion

Air Permeability SHRP Surface air flow method (SHRP-S-329) ــــــــــــــــ

Measures in-place permeability in-dex of near surface concrete (15mm)

Electrical Resistance of Concrete AC resistance using four probe resistance meter

SHRP surface resistance test (SHRP-S-327)

AC resistance useful for evaluating effectiveness of admixtures and

cementing additions; SHRP method useful for evaluating effectiveness

of sealers




Table 5 Nondestructive test methods to determine structural properties and assess conditions of concrete (ACI 228.2R-98)

Prop

erty

Met

hods

Com

men

tPr

imar

ySe

cond

ary

Rein

forc

emen

t Loc

atio

nCo

verm

eter

; Gro

und

pene

trat

ing

rada

r (G

PR) (

AST

M D

4748

)X-

ray

and

γ-ra

y ra

diog

raph

ySt

eel l

ocat

ion

and

dist

ribut

ion;

con

cret

e co

ver

Conc

rete

Com

pone

nt T

hick

ness

Impa

ct-e

cho

(I-E)

; GPR

(AST

M D

474

8)In

trus

ive

prob

ing

Verif

y th

ickn

ess

of c

oncr

ete;

pro

vide

mor

e ce

rtai

nty

in

stru

ctur

al c

apac

ity c

alcu

latio

ns; I

-E re

quire

s kn

owle

dge

of

wav

e sp

eed,

and

GPR

of d

iele

ctric

con

stan

t

Stee

l Are

a Re

duct

ion

Ultr

ason

ic th

ickn

ess

gage

(req

uire

s di

rect

co

ntac

t with

ste

el)

Intr

usiv

e pr

obin

g; ra

diog

ra-

phy

Obs

erve

and

mea

sure

rust

and

are

a re

duct

ion

in s

teel

; O

bser

ve c

orro

sion

of e

mbe

dded

pos

t-te

nsio

ning

com

po-

nent

s; V

erify

loca

tion

and

exte

nt o

f det

erio

ratio

n; p

rovi

de

mor

e ce

rtai

nty

in s

truc

tura

l cap

acity

cal

cula

tions

Loca

l or G

loba

l Str

engt

h an

d Be

havi

orLo

ad te

st, d

eflec

tion

or s

trai

n m

easu

re-

men

tsAc

cele

ratio

n, s

trai

n, a

nd d

is-

plac

emen

t mea

sure

men

tA

scer

tain

acc

epta

bilit

y w

ithou

t rep

air o

r str

engt

heni

ng;

Det

erm

ine

accu

rate

load

ratin

g

Corr

osio

n Po

tent

ials

Hal

f-ce

ll po

tent

ial (

AST

M C

876)

ــــــــــــــــ

Iden

tifica

tion

of lo

catio

n of

act

ive

rein

forc

emen

t cor

ro-

sion

Corr

osio

n Ra

teLi

near

pol

ariz

atio

n (S

HRP

-S-3

24 a

nd

S-33

0)ــــــــــــــــ

Corr

osio

n ra

te o

f em

bedd

ed s

teel

; rat

e in

fluen

ced

by

envi

ronm

enta

l con

ditio

ns

Loca

tions

of D

elam

inat

ion,

Vo

ids,

and

othe

r hid

den

defe

cts

Impa

ct-e

cho;

Infr

ared

ther

mog

raph

y (A

STM

D47

88);

Impu

lse-

resp

onse

; Rad

i-og

raph

y; G

PR

Soun

ding

(AST

M D

4580

); Pu

lse-

echo

; SAW

S; In

trus

ive

drill

ing

and

bore

scop

e

Ass

essm

ent o

f red

uced

str

uctu

ral p

rope

rtie

s; e

xten

t and

lo

catio

n of

inte

rnal

dam

age

and

defe

cts;

Sou

ndin

g lim

-ite

d to

sha

llow

del

amin

atio

n


3.2. British Standards Institution (BSI)(1) BS 1881: Part 102: 1983 - Method for Determination of Slump

(2) BS 1881 Part 5:1970 - Testing Concrete. Methods of testing hardened concrete for other than strength. Determination of dynamic modulus of elasticity by electromagnetic method

(3) BS 1881 Part 205:1970 - Testing Concrete. Recommendations for radiography of concrete

(4) BS 1881 Part 206:1986 - Testing Concrete. Recommendations for determination of strain in concrete. Advice on the use of mechanical, electrical resistance and vibrating wire gauges and electrical displacement gauges

(5) BS 1881 Part 202:1986 - Testing Concrete. Recommendations for surface hardness testing by rebound hammer

(6) BS 1881: Part 114: 1983 - Methods for Determination of Density of Hardened Concrete

(7) BS 1881: Part 116: 1983 - Method for Determination of Compressive Strength of Concrete Cubes

(8) BS 1881: Part 117: 1983 - Method for Determination of Tensile Splitting Strength

(9) BS 1881: Part 118: 1983 - Method for Determination of Flexural Strength

(10) BS 1881: Part 120: 1983 - Method for Determination of Compressive Strength of Concrete Cores

(11) BS 1881: Part 121: 1983 - Method for Determination of Static Modulus of Elasticity in Compression

(12) BS 1881: Part 122: 1983 - Method for Determination of Water Absorption

(13) BS 1881: Part 201: 1986 - Guide to the Use of Non-Destructive Methods of Test for Hardened Concrete

(14) BS 1881: Part 202: 1986 - Recommendations for Surface Hardness Testing by Rebound Hammer

(15) BS 1881: Part 203: 1986 - Measurement of the Velocity of Ultrasonic Pulses in Concrete

(16) BS 1881: Part 204: 1986 - Recommendations on the Use of Electromagnetic Covermeters

(17) BS 1881: Part 207 1992 - Recommendations for the Assessment of Concrete Strength by Near-to-Surface Tests

(18) BS 8110: Part 1: 1985 - Structural Use of Concrete: Code of Practice for Design and Construction

(19) BS 8110: part 2: 1985 - Structural Use of Concrete: Code of Practice for Special Circumstances

(20) BS 4408: pt. 4, “Non-destructive methods of test for concrete - surface hardness methods” British Standards Institution, London

(21) BS 1881: pt. 4, “Methods of testing concrete for strength”, British Standards Institution, London

(22) BS 4408: pt. 5, “Non-destructive methods of test for concrete - Measurement of the velocity of ultrasonic pulses in concrete”, British Standards Institution, London

(23) BS 4408: pt. 2, “Recommendations for non-destructive methods of test for concrete – strain gauges for concrete investigations”, British Standards Institution, London, 1969,(83)

(24) BS 4408: pt. 1, “Non-destructive methods of test for concrete-electromagnetic cover measuring devices”, British Standards Institution, London

(25) BS 4408: pt. 3, 1970 “Non-destructive methods of test for concrete-gamma radiography of concrete”, British Standards Institution, London

(26) BS 1881: pt. 6, “Methods of testing concrete: analysis of hardened concrete”, British Standards Institution, London

(27) BS4551, “Methods of testing mortars, screeds and plasters”, British Standards Institution, London

(28) BS 812: pt. 1, “Methods for sampling and testing of mineral aggregates, sands and fillers”, British Standards Institution, London.




4. METHODS OF STRUCTURAL ANALYSISIn this category fall all those assessment routines, where the load effects are determined by model based structural analysis. Using this method Ultimate Limit State and Serviceability State can be modeled and therefore assessed. It is a three component-procedure where the components are the follow:• acquisition of data of loading and resistance• calculation of load effects on structural models• safety and serviceability verification

Structural performance shall be analyzed using models that reliably represent the loading on the structure, the behavior of the structure and the resistance of its components. The analytical model should reflect the actual condition of the existing structure.

1.1. Simple analysis methodsHere it is often effective to calculate load effects with basic conservative methods with simple structural models, provided that the approximately large uncertainty is regarded with an adequate safety measure. Typical simple analysis methods are among others space frame and grillage analysis combined with a simple load distribution and linear elastic material behavior, which result in a lower bound equilibrium solution.

1.2. Complex analysis methodsHere refined load effect calculation methods need to be accomplished. Refined methods include mainly finite element analysis and non-linear methods such as yield line analysis, where these may lead to higher capacities. Particularly a specified modelling of the material behavior such as time-variant behavior (e.g. shrinkage and creeping of RC structures) and the consideration of interactions between material components (e.g. bond, tension stiffening in RC) will uncover hidden capacity reserves and reduce conservatism.

Applying full probability safety verification, stochastic finite elements can be used to model the structure. The difference to conventional finite element models is that the stochastic elements take the spatial correlation of the random variables into account. Figure 1 shows examples of modelling a slab of an existing reinforced concrete structure. This slab is strengthened using FRP materials. In such a complex analysis the bond between the FRP and the concrete is accounted for using special finite element elements.

In Figure 2, an example of finite element model of a beam component of a structure that is also strengthened using FRP to resist shear deficiency.

Figure 1 Finite element model of a slab (Elsayed et al 2009a)


Figure 2 Finite element model of a beam (Elsayed et al 2009b)

1.3. ACI Recommended mechanism for strength assessment of existing concrete buildings Once the critical structural components have been identified through the condition assessment, a structural assessment can be required to determine the current condition, to form the basis for estimating future performance or service life, or both. As part of the assessment it is important to note irregularities or inconsistencies in properties of materials, in design, in construction and maintenance practices, and the presence and effects of environmental factors. Although the assessment of a structure involves more than its load-carrying ability (for example, the permeability of hydraulic structures), an assessment of structural demand versus capacity is the first step. Performance requirements other than structural capacity are then addressed through supplementary tests to establish characteristics, such as leakage rate or permeability.

Procedures to evaluate the strength of existing structures have been published (ACI 437R). The recommendations developed are intended to establish the loads that can be sustained safely and serviceably by an existing building under several conditions:

There is evidence of possible structural weakness (for example, excessive cracking or spalling);• The building or a portion of it has undergone general or local damage (for example, environmental or earthquake effects);• There is doubt concerning the structure’s capacity; and• Portions of a building are suspected to be deficient in design, detail, material, or construction.

Methods for strength evaluation of existing concrete structures include either an analytical assessment or a load test as shown in Figure 3. An analytical assessment is recommended when sufficient background information is not available (for example, sectional characteristics, material properties, and construction quality), a static load test is impractical because of the test complexity or magnitude of the load required, sudden failure during a static load test can endanger the integrity of the member or the entire structure, or it is required by an authority.

Some supplemental destructive or nondestructive tests described previously can be required to obtain this information. For the evaluation it is recommended that the theoretical analyses follow principles of strength design and that a structure be considered satisfactory if capacity, deformation, and other serviceability criteria satisfy the requirements and intent of the ACI 318.




Static-load tests should be utilized only when the analytical method is impractical or otherwise unsatisfactory. Situations where a static load test of a bridge or building component is recommended include those where at least one of the following cases and all of the following conditions apply (ACI 437R). Cases include incidences where structural element details are not readily available; deficiencies in details, materials, or construction are best evaluated by a load test; and the design is extremely complex with limited prior experience for a structure of this type. Conditions include: 1) results of a static load test permit a reasonable interpretation of structural adequacy; 2) principal structural elements under investigation are primarily flexural members; and 3) adjacent structure’s effects can be accounted for in the evaluation of the load test results. Before conduct of a load test, some repair actions can be required and an approximate analysis should be conducted. After establishing the magnitude of the test load, the load is applied incrementally with deflections measured. The structure is considered to have passed the load test if it shows no visible evidence of failure, such as excessive cracking or spalling, and it meets requirements for deflection. In certain applications, serviceability requirements, such as allowable leakage at maximum load, can also be a criterion.

Figure 3: Recommended procedure for strength evaluation of existing concrete buildings (ACI 437).

PROCEDURE APPLIESTO ALL OR PART OF

EXISTINGCONCRETE BUILDINGS

EVALUATION BY

STATICLOAD TEST

EVALUATION BY

THEORETICAL STRESSANALYSIS

• CONDUCT A PRELIMINARYSTRUCTURAL ANALYSIS

• PERFORM REPAIRS NECESSARYFOR THE STATIC LOAD TEST

• ESTABLISH THE MAGHNITUDE OF THE TEST LOADS

• INSTALL DISPLACEMENT AND STRAIN INSTRUMENTATION

• PROVIDE SCAFFOLDING TO SUPPORT THE STRUCTURE IN CASE OF FAILURE

• APPLY THE TEST LOADS WITH OUT SHOCK OR VIBRATION AND RECORD THE DISPLACEMENT AND STRAIN DATA

• AFTER 24 HOURS. RECORD THE DISPLACEMENT AND STRAIN DATA

• REMOVE THE TEST LOADS AND AFTER 24 HOURS. RECORD THE DISPLACEMENT AND STRAIN DATA

• IF THE PORTIONS OF THE STRUCTURE THAT WERE TESTED SHOW NO SIGNS OF FAILURE. THE STRUCTURE PASSES THE TEST

• OBTAIN SECTIONAL CHARAC.TERISTICS AND DETAILS OF THESTRUCTURAL MEMBERS ANDCONNECTIONS

• IDENTIFY PROPERTES OF THESTRUCTURAL MATERIALSUSING DESTRUCTIVE OR ACOMBINATII OF DESTRUCTIVEAND NONDESTRUCTIVETESTING METHODS

• ASSESS THE QUALITY OF THECONSTRUCTION AND THEPRESENT CONDITION OF THESTRUCTURE

• CONDUCT A THEORETICALSTRESS ANALYSIS TODETERMINE THE SAFE SERVICELOAD CARRYING CAPACITY OFTHE STRUCTURE FOLLOWINGRECOGNIZED PRINCIPLES FORSTRENGTH DESIGH IN ACI 318


2. REFERENCES• ACI 437.1R-07, “Load Tests of Concrete Structures: Methods, Magnitude, Protocols, and Acceptance Criteria”• ACI 228.2R-98 (Reapproved 2004), “Nondestructive Test Methods for Evaluation of Concrete in

Structures”, ACI Manual of Concrete Practice 2008.• Elsayed, W., Ebead, U.A. and Neale, K.W., “Mechanically fastened FRP-strengthened two-way concrete slabs with and without cut-outs” Journal of Composites for Construction, ASCE, Vol. 13, No. 3, pp. 198–207, May/June 2009a. • Elsayed, W., Ebead, U.A. and Neale, K.W., “Studies on mechanically fastened fiber-reinforced polymer strengthening systems” ACI Structural Journal, Vol. 106, No. 1, pp. 49–59, January/February 2009b.• fib 2000, ‘’Federation International du Beton (fib) (2000). Bond of Reinforcement in Concrete’’




V -METHODS OF CRACKS REPAIR & CLASSIFICATION OF REPAIR OPTIONSDesign Life Extension of RC Structure

51

V - METHODS OF CRACKS REPAIR& CLASSIFICATION OF REPAIR OPTIONS


52 V -METHODS OF CRACKS REPAIR & CLASSIFICATION OF REPAIR OPTIONS

The repair of cracks can be classified into repair of cracks in block walls and repair of cracks in concrete elements.

1. REPAIR OF CRACKS IN BLOCK WALLS1.1 General Consideration - Remove existing render/plaster in strips of 150 mm width, on either side where cracks or openings occur in block wall and clean the surface from dust, loose materials etc.- Chase out the crack in a V- shape to the full depth of the crack or minimum of 30 mm whichever is greater. The V notes shall be of equal depth and width. Chased V- section shall be thoroughly cleaned from any contamination, oil, grease, dirt, dust or fire particles of concrete. Air compressor and mechanical water jet shall be used for cleaning. Wall surface shall be allowed to dry thoroughly before proceeding with the works.- Fill the chased V-shape with a thixotropic epoxy resin fairing coat (CONCRESSIVE 2200 from BASF or Nitomartar FC from Fosroc, or similar approved. All works shall be as per manufacturer instructions.- 24 hours allowance shall be made before applying subsequent work.- Fix galvanized expanded metal mesh (minimum 7.5 Kg/m2) bridging the crack location and covering 100mm on each side, as manufactured by EXPAMET or similar approved with galvanized nails at intervals not exceeding 300mm on both sides.- Reinstate render/plaster with RENDEROC HB cementitious mortar as manufactured by FOSROC or EMACO R303 from BASF or similar approved.- Paint completed repairs to match existing.

1.2 Repair of Cracks Between Block Walls and Concrete Elements- Remove existing plaster and painting along the column height covering 750mm on each side of the crack (i.e. 150mm in total).- Open the crack between the block wall and the concrete member (beam or column).- Install galvanized steel mesh at the distance between the wall and the concrete member.- Fill the distance between the block wall and the concrete member with cementitious base mortar such as EMACO S22NB or Sikagrout 114.- Reinstate plastering and painting.

1.3 Repair of Damaged Existing Joints Between Block Walls And Concrete Elements- Make good edge of block wall as applicable.- Repair the joint using cement base materials such as Emaco S22NB.- Fix 2 nos. plaster beads at junction between column and walls to form movement joint in the plaster layer.- Reinstate plastering and painting.- Apply approved sealant at two movement joints. Two part Polyurethane NB2 based sealants shall be used to BS4254. MASTERFLEX 700 PG from BASF or Thioflex 600 from FOSROC, or \ approved similar shall be used.


2. REPAIR OF CRACKS IN CONCRETE ELEMENTS2.1 General Consideration2.1.1 Actions to be Taken Before Repairs are Specified• Identify the location and extent of cracking. • Determine whether the cracks are from structural problems or not.• Take into consideration both present and anticipated loading conditions. • Review drawings, specifications, and construction and maintenance records. • Establish the cause of the cracking. • If these documents, along with field observations, do not provide the needed information, afield

investigation and structural analysis should be completed before proceeding with repairs.

2.1.2 Why Cracks Shall Be Repaired?Cracks need to be repaired if:• They cause any reduction in the strength, stiffness, or durability of the structure.• The function of the structure is seriously impaired.• In some cases, such as cracking in water-retaining structures, the function of the structure will dictate the need for repair, even if strength, stiffness, and appearance are not seriously impaired. • Cracks in pavements and slabs-on-ground may require repair to prevent edge spalls or the

migration of water to the subgrade, or to maintain intended load-carrying capacity. • Generally, repairs that improve the appearance

2.1.3 Field Observation Information from field observations shall include:• The locations and widths of cracks should be noted on a sketch of the structure.• Crack widths can be measured to an accuracy of approximately 0.025 mm using a crack comparator or hand-held microscope.• Any displacement of the surface (change in elevation) across the crack should also be documented. • Observations such as spalling, exposed reinforcement, surface deterioration, and rust staining

should be noted on the sketch. • Internal conditions at specific crack locations can be observed with the use of flexible shaft

fiberscopes or rigid borescopes.• Crack movement can be monitored with mechanical movement indicators.• If more detailed time histories are desired, a wide range of transducers (most notably, linear

variable differential transformers [LVDTs]) and data-acquisition systems (ranging from strip chart recorders to computer-based systems) could be used.

• Sketches can be supplemented by photographs documenting the condition of the structure at the time of investigation. • Guidance for making a condition survey of concrete in service is given in ACI 201.1R, 201.3R,

207.3R, 345.1R,and 546.1R.

Depending on the nature of the damage, one or more repair methods may be selected.




3. REPAIR METHODS3.1 Epoxy Injection• Cracks as narrow as 0.05mm can be bonded by the injection of epoxy.• Epoxy injection has been successfully used in the repair of cracks in buildings, bridges, dams, and other types of concrete structures (ACI 503R). • With the exception of certain moisture-tolerant epoxies, this technique is not applicable if the cracks are actively leaking and cannot be dried out. • Wet cracks can be injected using moisture-tolerant materials that will cure and bond in the presence of moisture, but contaminants in the cracks (including silt and water) can reduce the effectiveness of the epoxy to structurally repair the cracks. • Unless the cause of the cracking has been corrected, however, new cracks will probably form near the original crack. If the cause of the cracks cannot be removed, then three options are available: o Rout and seal the crack, thus treating it as a joint. o Establish a joint that will accommodate the movement and then inject the crack with epoxy or other suitable material o Install additional support or reinforcement at the crack location to minimize movement.

Procedure:• The technique generally consists of (Figure 1): o Cleaning the cracks. o Sealing the crack on exposed surfaces o Installing entry and venting ports at close intervals along the cracks o Mixing the epoxy. o Injecting the epoxy under pressure. o Removing the surface seal.

Surface seal to contain epoxy adhesive

Figure 1Repair of crack by epoxy injection (Emmons 1993)


3.2 Routing and Sealing• Routing and sealing of cracks can be used in conditions requiring repair where structural repair is not necessary (Figure 2) • Routing and sealing is used to treat both narrow and wide cracks. • This is a common technique for crack treatment and is relatively simple compared with the procedures and the training required for epoxy injection. • The method is most applicable to approximately flat horizontal surfaces such as floors and pavements. • Routing and sealing can be accomplished on vertical surfaces (with a non-sag sealant) as well as on curved surfaces (pipes, piles, and poles). • A common and effective use is for waterproofing by sealing cracks on the concrete surface where water stands or where hydrostatic pressure is applied. • This treatment reduces the ability of moisture to reach the reinforcing steel or pass through the concrete, causing surface stains or other problems. • The sealants may be any of several materials, including epoxies, urethanes, silicones, polysulfides, asphaltic materials, or polymer mortars. Cement grouts should be avoided due to the likelihood of cracking.• For floors, the sealant should be sufficiently rigid to support the anticipated traffic. Load transfer

at the floor crack should be provided by aggregate interlock or dowels; otherwise, traffic loads moving across the crack may cause rigid sealants to de-bond.

Procedure:• This method involves: o Enlarging the crack along its exposed face by preparing a vertical walled groove at the surface typically ranging in depth from 6 to 25 mm. A concrete saw or right-angle grinder may be used. o The groove is then cleaned by air blasting, sandblasting, or water blasting, and dried. o A sealant is placed into the dry groove and allowed to cure. o The final step is to fill the vertical-walled groove with a high-viscosity, rigid epoxy. o In some cases, over-banding (strip coating) is used independently of or in conjunction with

routing and sealing to ensure a waterproof repair. A typical procedure for over-banding is:• Prepare an area approximately 25 to 75 mm on each side of the crack by sandblasting or other means of surface preparation• Applying a coating (such as urethane) 1 to 2 mm thick in a band over the crack. • Before over-banding in non-traffic areas, a bond breaker is sometimes used over a crack that has not been routed or over a crack previously routed and sealed. In traffic areas, a bond breaker is not recommended. Cracks subject to minimal movement may be over-banded, but if significant movements can take place, routing and sealing should be used in conjunction with overbanding to ensure a waterproof repair.• Active cracks should be repaired using a bond breaker at the base of the routed channel. A flexible sealant is then placed in the routed channel. It is important that the width-to-depth ratio of the channel is usually 2 or more. This permits the sealant to respond to movement of the crack with high extensibility. This method is used to enhance protection from edge spalling and, for aesthetic reasons, to create a more uniform-appearing treatment.




Figure 2 Repair of crack by routing and sealing (Johnson1965).

3.3 Near-surface Reinforcing Near-surface reinforcing (NSR) is a method used to add tensile reinforcement perpendicular to the line of the crack (Figure 3).

Procedure:• This method involves: o Slot is saw-cut across the crack, and the slot is then cleaned. o Typically, an epoxy resin is placed in the slot to act as a bonding agent and protective barrier to the bar that is subsequently placed. o Both deformed steel reinforcing bars and pre-cured fiber-reinforced polymer (FRP) bars are placed in the slot that is cut to approximately 3 mm wider and deeper than the diameter of the reinforcement to be installed. o The reinforcing needs to be designed to increase the capacity beyond the tensile forces at the

crack location.

NSR method is typically used when it is necessary to restrict widening of existing cracks. Engineering judgment shall be exercised.

Figure 3 Repair of crack by near surface mounted reinforcement (Stratton et al. 1978)


3.4 Additional Reinforcement3.4. 1 Internally placed conventional reinforcementCracked reinforced concrete bridge girders have been successfully repaired by inserting reinforcing bars and bonding them in place with epoxy (Figure 4).

Procedure:• This method involves: o Sealing the crack, drilling internal holes that intersect the crack plane at approximately 9 degrees. o Filling the hole and crack with injected epoxy, and placing a reinforcing bar into the drilled hole. o Typically the additional bars extend at least 500mm each side of the crack. o The reinforcing bars can be spaced to suit the needs of the repair. They can be placed in any desired pattern, depending on the design criteria and the location of the in-place reinforcement. o The epoxy bonds the bar to the walls of the hole, fills the crack plane, bonds the cracked concrete

surfaces back together in one monolithic form, and thus reinforces the section. The epoxy used to rebond the crack should have a low viscosity and conform to ASTM C881 Type IV.

Figure 4 Crack repair by additional conventional reinforcement (Emmons 1993)

3.4.2 Prestressing steelPost-tensioning is often the desirable solution when a major portion of a member must be strengthened or when the cracks that have formed must be closed(Figure 5).Procedure:• Prestressing strands or bars are used to apply a compressive force. • Adequate anchorage should be provided for the prestressing steel, and care is needed so that the problem will not merely migrate to another part of the structure. • The effects of the tensioning force (including eccentricity) on the stress within the structure should be carefully analyzed.• For indeterminate structures post-tensioned using this procedure, the effects of secondary moments and induced reactions should be considered.




Figure 5 Repair of crack by external prestressing (Johnson 1965)

3.5 Drilling and Plugging• This method is most often used to repair vertical cracks in retaining walls (Figure 6)• This technique is only applicable when cracks run in reasonably straight lines and are accessible at one end.

Drilling and plugging a crack consists:• Drilling down the length of the crack and grouting it to form a key.• A hole 50 to 75 mm in diameter should be drilled, centered on and following the crack. The hole

should be large enough to intersect the crack along its full length and provide enough repair material to structurally take the loads exerted on the key.

• The drilled hole should then be cleaned, made tight, and filled with grout. The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall.• If water tightness is essential and structural load transfer is not, the drilled hole should be filled with a resilient material of low modulus instead of grout. If the keying effect is essential, the resilient material can be placed in a second hole, with the first being grouted.


Figure 6 Repair of crack by plugging and drilling (ACI Concrete Repair Manual)

3.6 Gravity Filling and/or Polymer Impregnation • Low-viscosity monomers and resins to seal cracks with surface widths of 0.03 to 2 mm by gravity filling (ACI RAP-2). • High-molecular-weight methacrylate, urethanes, and some low-viscosity epoxies have been used successfully. • The lower the viscosity, the finer the cracks that can be filled.

Procedure:• This method involves: o Clean the surface by air-blasting, water-blasting, or both. o Wet surfaces should be permitted to dry for several days to obtain the best crack filling. o Pour the monomer or resin onto the surface and spread them with brooms, rollers, or squeegees. o The material should be worked back and forth over the cracks to obtain maximum filling because the monomer or resin recedes slowly into the cracks. o The use of this method on elevated slabs will require sealing of the cracks on the bottom of the

slab to contain material from leaking through the crack. o Excess material should be broomed off the surface to prevent slick, shining areas after curing. o If surface friction is important, sand should be broadcast over the surface before the monomer or resin cures. o If the cracks contain significant amounts of silt, moisture, or other contaminants, the sealant cannot fill them. o Water-blasting followed by a drying time may be effective in cleaning and preparing these cracks. o Cores may be taken to verify crack filling and the depth of penetration measured. o Caution should be employed to avoid cutting existing reinforcement during the coring process. o Cores can be tested to give an indication of the effectiveness of the repair method. o The accuracy of the results may be limited, however, as a function of the crack orientation or due to the presence of reinforcing steel in the core. o For some polymers, the failure crack will occur outside the repaired crack.




3.7 GROUTING3.7.1 Cement-based groutingThis method is effective in repairing wide cracks and stopping water leaks, but it will not structurally bond cracked sections. Epoxy-based materials (recall Section 3.1) can be used in case cracked sections need to be structurally bonded.

Procedure:• This method involves: o Clean the surface by cleaning the concrete along the crack o Installing built-up seats (grout nipples) at intervals astride the crack (to provide a pressure-tight

connection with the injection apparatus) o Sealing the crack between the seats with a cement paint, sealant, or grout o Flushing the crack to clean it and test the seal o Grouting the whole area. o Grout mixtures may contain cement and water or cement plus sand and water, depending on the

width of the crack. The w/cm, however, should be kept as low as practical to maximize the strength and minimize shrinkage. Water reducers or other admixtures may be used to improve the properties of the grout.

o For small volumes, a manual injection gun may be used; for larger volumes, a pump may be used. After the crack is filled, the pressure should be maintained for several minutes to ensure good penetration.

3.7.2 Chemical grouting• Cracks in concrete as narrow as 0.05 mm have been filled with chemical grout.• Chemical grouts, such as urethanes, are activated by catalysts or water to form a gel, a solid precipitate, or foam that will fill void space within concrete. The materials are primarily used for sealing cracks from water penetration. Advantages: • Applicability in moist environments (excess moisture available), • Wide limits of control of gel time, and their ability to be applied in very fine fractures. Disadvantages• High degree of skill needed for satisfactory use• Lack of strength.

3.8 DRYPACKING• The use of drypack is not advisable for filling or repairing active cracks.• Hand placement of a low water content mortar followed by tamping or ramming of the mortar into place, producing intimate contact between the mortar and the existing concrete .• Because of the low w/cm of the material, there is little shrinkage, and the patch remains tight and can have good quality with respect to durability, strength, and water tightness. • Drypack can be used for filling narrow slots cut for the repair of dormant cracks.

Procedure:• This method involves: o Before a crack is repaired by drypacking, the portion adjacent to the surface should be widened to a slot about 25 mm wide and 25 mm deep. o The slot should be undercut so that the base width is slightly greater than the surface width. o After the slot is thoroughly cleaned and dried, a bond coat, consisting of cement slurry or equal

quantities of cement and fine sand mixed with water to a fluid paste consistency, or an appropriate latex bonding compound, should be applied.


o Placing of the drypack mortar should begin immediately. o The mortar consists of one part cement, one to three parts sand passing a No. 1.18 mm sieve, and just enough water so that the mortar will stick together when molded into a ball by hand. o If the patch must match the color of the surrounding concrete, a blend of gray Portland cement and white Portland cement may be used. Normally, about 13/ white cement is adequate, but the precise proportions can be determined only by trial. o To minimize shrinkage in place, the mortar should stand for 12/ hour after mixing, and then be remixed before use. The mortar should be placed in layers about 10 mm thick. Each layer should be thoroughly compacted over the surface using a blunt stick or hammer and each underlying layer scratched to facilitate bonding with the next layer. There need be no time delays between layers. The repair should be cured by using either water or a curing compound. The simplest method of moist curing is to support a strip of folded wet burlap along the length of the crack.

4. Recommendations for Selection of Repair Option • General classification of repair options is highlighted in the Table below. More details on repair

and strengthening methodology of structures are given in chaptersIX and X.

Repair Option Recommendation

Structural Repairs

(a) Cutting out contaminated, cracked or defec-tive concrete and replacing it

(b) Cutting out and replacing corroded rein-forcement

(c) Cutting out concrete, adding protection to the reinforcement and replacing the concrete

(a) Typically used when the damage has been caused by chemical or physical actions

(b) Typically used when corrosion resulted in more than 10% reduc-tion in the cross sectional area of the steel rebar.

(c) Typically used in aggressive environment. Caution should be taken to avoid damage of the protection layer to the steel during application or handling.

External Strengthening

(a) Replacing complete element

(b) Plate of composite bonding

(c) External prestressing

(a) Typically used when deterioration is extensive and disruption to operations is minimal

(b) Typically used when additional strength is required. Calculation is needed. Bond and adhesion are important to ensure compos-ite action.

(c) Typically used when additional strength is required. Calculation is needed.

Surface Coatings and Impregnations

(a) Surface treatment

(b) Coatings

(c) Impregnations

Typically used to prevent ingress of adverse agents or to control moisture penetration Surface treatment

(a) Involves making good minor defects followed by a coating

(b) Smooth surface is needed to ensure effectiveness and to have the ability to accommodate movement without fracture to the protective film or barrier

(c) Must be able to penetrate the concrete to a minimum specified depth

Filling Cracks and Voids Surface voids or defects can be treated as patch repairs. For the effective filling of cracks injection is often used. The technique is often comple-mented by surface coating or treatment

Electrochemical Techniques

(a) Re-alkalization

(b) Chloride extraction

(c) Cathodic protection

These techniques require Performance Standards unlike other repair options. Cathodic protection is typically the most common and successful technique.

Table (1): General Classification of Repair Options:




5. REFERENCES

• ACI 201.1R Guide for Making a Condition Survey of Concrete in Service

• ACI 201.2R Guide to Durable Concrete

• ACI 201.3R Guide for Making a Condition Survey of Concrete Pavements

• ACI 207.1R Guide to Mass Concrete

• ACI 207.2R Effect of Restraint, Volume Change, and Reinforcement on Cracking of Mass Concrete

• ACI 207.3R Practices for Evaluation of Concrete in Existing Massive Structures for Service Conditions

• ACI 207.4R Cooling and Insulating Systems for Mass Concrete

• ACI 222R Protection in Metals in Concrete Against Corrosion

• ACI 224R Control of Cracking in Concrete Structures

• ACI 224.2R Cracking of Concrete Members in Direct Tension

• ACI 224.3R Joints in Concrete Construction

• ACI 228.2R Nondestructive Test Methods for Evaluation of Concrete in Structures

• ACI 302.1R Guide for Concrete Floor and Slab Construction

• ACI 304R Guide for Measuring, Mixing, Transporting, and Placing Concrete

• ACI 305R Hot Weather Concreting

• ACI 308R Guide to Curing Concrete

• ACI 309R Guide for Consolidation of Concrete

• ACI 309.2R Identification and Control of Visual Effects of Consolidation Formed Concrete Surfaces

• ACI 318 Building Code Requirements for Structural Concrete343RAnalysis and Design of Reinforced Concrete Bridge Structures

• ACI 345R Guide for Concrete Highway Bridge Deck Construction

• ACI 345.1R Guide for Maintenance of Concrete Bridge Members

• ACI 347 Guide to Formwork for Concrete

• ACI 446.1R Fracture Mechanics of Concrete: Concepts, Models, and Determination of Material Properties

• ACI 503R Use of Epoxy Compounds with Concrete

• ACI 504R Guide to Sealing Joints in Concrete Structures

• ACI 517.2R Accelerated Curing of Concrete at Atmospheric Pressure

• ACI 546R Concrete Repair Guide

• ACI 546.1R Guide for Repair of Concrete Bridge Superstructures

• ACI 546.2R Guide to Underwater Repair of Concrete548.1RGuide for the Use of Polymers in Concrete• ACI RAP-1 Structural Crack Repair by Epoxy Injection, http://www.concrete.org/general/RAP-1. pdf


• ACI RAP-2 Crack Repair by Gravity Feed by Resin, http://www.concrete.org/general/RAP-2.pdf

• Concrete RepairManual, 3rd edition, ACI.

• Emmons, P. H, 1993, Concrete Repair and Maintenance Illustrated, R.S. Means CMD Group.

• Johnson, S. M., 1965, Deterioration, Maintenance, and Repair of Structures, McGraw-Hill Book Co., New York,373 pp.

• Stratton, F. W.; Alexander, R.; and Nolting, W., 1978, “Cracked Structural Concrete Repair through Epoxy Injection and Rebar Insertion,” ReportNo. FHWA-KS-RD.783-,Kansas Department of Transportation, Topeka, Kans., Nov.,56 pp.




VI -METHODS FOR ESTIMATING AND EVALUATING CORROSION RATEDesign Life Extension of RC Structure 65

VI - METHODS FOR ESTIMATING& EVALUATING CORROSION RATE


66 VI -METHODS FOR ESTIMATING AND EVALUATING CORROSION RATE

1. INTRODUCTIONThe corrosion of steel is an electrochemical process that produces an electric current, measurable as an electric field on the surface of the concrete. Most detection techniques currently used rely on the electrochemical nature of corrosion for their data collection. A wide variety of instruments produced by different manufacturers exists for this purpose. They may vary in size, cost, application methods, underlying theories, and information given. In addition, visual inspections should complement any monitoring program, but they may not detect corrosion early enough to prevent serious damage. This section provides a review and detailed information of existing corrosion detection methods and monitoring for future data collection that can be used in reinforced concrete structures to assess the corrosion condition of the reinforcement.

These techniques can be used to detect electrochemical corrosion activity of metallic reinforcements. The aim of their applications may be one of the following:1. Quality control of new constructions2. Condition evaluation of existing structures for: - Identification of steel de-passivation - Detecting corroding areas for rehabilitation purposes - Prediction of the damage evolution - Determination of the optimum time for repair.3. Evaluation of repair technique

2. CORROSION MONITORINGVisual observation has been the common technique used to detect corrosion due to induced rust, which in turn produces the cracking and spalling of cover. These damages are an indication of the corrosion activity developed underneath the cover. However, due to the different degrees of aggressive environments, of concrete quality and of structural geometry, the conclusions taken from visual inspection may be misleading.Corrosion monitoring is the practice of measuring the degree of corrosion of reinforcement in concrete structure under actual conditions. There are different methods used to quantify the process of corrosion in concrete environments. The different methods used in corrosion monitoring can be classified into two main categories; non electrochemical methods and electrochemical methods. Table (1) shows the different methods for corrosion monitoring in reinforced concrete. The shaded methods in Table (1) are the most widely used methods and will be described in this report.All these techniques can be used individually or combined to provide an “instantaneous” condition of a structure. However, if one is interested in the rate of deterioration of a structure, it is useful to monitor the condition change with time. This should be done on structures with long lifetime requirements and older structures when corrosion damage has been found and repair is being deferred to cost, logistical or other reasons.

Table 1: Methods for corrosion monitoring in reinforced concrete

Non Electrochemical Electrochemical

Visual InspectionAcoustic EmissionInfrared ThermographyRadiography and Radiometry RadarSeismic MethodElectric ResistivityOptical Fiber Sensors Microwave Thermoreflectometry

Static Measurements- Half Cell Potential (handheld & em-

bedded devices)- Macrocell Current (embedded de-

vices)- Electrochemical Noise

Polarization Measurements- Linear Polarization (handheld & em-

bedded devices)- Impedance Spectroscopy- Glavanostatic Pulse- Scanning Reference Electrode


The portable or handheld field test devices are extremely useful and convenient; however in some applications it may be preferable to have a permanent, nondestructive testing procedure. Embedded devices are designed to be cast into the concrete or repair material matrix to allow repeated testing at probe locations. The wires from the probe may be combined in a cable extending out of the concrete member for ease of measurement. The devices will be categorized into two sections:- External monitoring devices- Embedded continued monitoring devices

3. EXTERNAL MONITORING DEVICES3.1 Electrical ResistivityThe electrical resistivity is defined as the ratio between the applied potential and the current circulating between two electrodes providing the arrangement enables the calculation of the geometrical characteristics. The electrical resistivity is an indirect measurement of the porosity and the connectivity of the pores. It is used to detect wet areas in the concrete and therefore the measurement provides information about the risk of corrosion. Although it is inversely related to the corrosion rate, results cannot be used to calculate corrosion rates due to the big scatter detected in the measurements.

The four point method does not need a direct contact with the reinforcement. Concrete resistivity can be measured directly on the surface of the structure by means of Wenner technique. This method uses four equally spaced point electrodes in contact with the concrete surface, as shown in Figure 1. Figure 2 shows a photo of the equipment and the equipment in use.

Figure 1 Schematic diagram for Wenner technique for resistivity measurement

The resistivity is calculated as follows:




Where; ρconcrete is the resistivity of the concrete (in Ωm), Rmeasured the resistance from the four-electrode measurement (the voltage divided by the current, in Ω), and (a) is the distance between the measuring electrodes.

Figure 2 Wenner resistivity meter

The measurement on-site of the resistivity is carried out directly on the surface of the structure. First it is necessary to moisten the electrode tips with a conducting liquid in order to provide a good contact with concrete. An alternating current with a frequency between 50 and 1000 Hz is passed between the outer electrodes and the potential difference is measured between the inner ones. Resistivity is obtained as a function of voltage, current and distance between tips (usually 50 mm).

One of the most important problems arising for the measurement of concrete resistivity is its variability with changes in the environment. The factors which influence the resistivity are:a) Humidity content: The ρ decreases when concrete moisture increases and vice versa. The ρ is an

indirect measure of saturation of the concrete.b) Temperature: The effect of temperature is controversial, as its effect on ρ depends on whether

the concrete is shielded or not. That is, whether the water can evaporate or condensate. c) Chloride content: The presence of chlorides or any other inorganic compound induces a certain

decrease of the ρ.d) Carbonation: It aims into an increase in ρ due the densification that the formation of calcium

carbonates usually induces.e) Type of cement: Blending agents (fly ash, slag or silica fume) in general induce an increase of ρ

when compared with ordinary Portland cement.f ) Porosity: The porosity is a consequence of the w/c ratio and to the compaction and curing. An

increase in w/c leads into a decrease of ρ.

Resistivity does not show, by itself, whether or not steel in concrete is in an active state of corrosion. It informs on the risk of corrosion due to the humidity content in the concrete. Table 2 shows the ranges of resistivity that can be related to the corrosion rate.

Table 2: Resistivity ranges related to the risk of corrosion

Resistivity (kW.cm) Corrosion Risk

> 100Nil

50 – 100 Low

10 – 50 Moderate

< 10 High


3.2. Half Cell PotentialThe corrosion or half cell potential, Ecorr, is defined as the voltage difference between the reinforcement and a reference electrode (RE). Figure 3 shows a schematic diagram for the half cell potential measurement. It characterizes the state of the metal in its environment. Figure 4 shows a photo of the equipment and the equipment in use. Different devices are commercially available for single electrode or multiple (wheel) electrodes.

The potential Ecorr can only qualitatively inform on the risk of corrosion. It cannot account for it. The main objective of potential measurements on a structure is to locate areas in which reinforcement has become depassivated and hence, is able to corrode if appropriate oxygen and moisture conditions occur.

Figure 3 Schematic diagram for half cell potential measurement

Figure 4 Half cell potential device

The most common procedure involves the mapping of the structure. The standard test method is given in ASTM C87691-. As mentioned, the objective of potential measurement is to determine those areas of corroding reinforcement. To achieve this goal, first it is completely necessary to define a work strategy that provides a fast and economical overview on the state of the structure. This strategy must involve the definition of a co-ordinate system to correlate readings and measuring points. A grid usually makes it with a cell size that varies from 15 square centimeters to 2 square meters, depending on the type of the structure, its characteristics and the scope of the work. The size of this co-ordinate system will determine the accuracy of the measurements. Measurements made with a big grid cell size could not detect corrosion activity whereas minimum spacing generally should provide high differences between readings. The spacing must be adequate to the type of structure surveyed (cover and moisture content) and the expected use of measurements.



A good electrical connection to the reinforcement has to be made. It could be made by means of a compression-type ground clamp or by brazing or welding a protruding rod, but a direct contact should not be made if reinforcement steel is connected to an exposed steel member. The other input of the high impedance voltmeter must be a suitable external reference electrode placed on the concrete surface by a wet sponge in order to provide a good electrolytic contact between them. The sponge should be always wetted with a diluted solution of detergent. Copper/Copper sulfate (Cu/CuSO4) electrode is the most used reference for in-situ potential measurement, whereas silver/silver chloride (Ag/AgCl) electrodes are used more in lab works.

It is completely necessary to assure the electrical continuity of the reinforcement steel. Measuring the resistance between separated areas checks it. If resistance values are less or equal than 0.3 Ω, electrical continuity is indicated.

Potential measurements can be performed with a single electrode or with one or several wheel electrodes. Once the data are obtained, the best way to their representation depends on their number and the type of the structure. So it varies from tables to a colored grid map of the potential field (i.e. contour line map). The color gradation step should not be greater than 50 mV in order to provide a clearest way to result interpretation. A 3-D surface can also be represented both by measured and interpolated values.

The interpretation of the potential readings, according to ASTM C87691- standard, a threshold potential value of –350 mV CSE was established. Lower values of potential suggested corrosion with 95% probability; if potentials are more positive than -200 mV CSE, there is a greater than 90% probability that no reinforcement steel corrosion occurs, and for those potentials between -200 mV and - 350 mV corrosion activity is uncertain. Table 3 shows the interpretation of the corrosion potential measurements.

Table 3: Interpretation of corrosion potential measurement for half cell device (ASTM C876 -91)

Practical experiences have shown that different potential values indicate corrosion for different conditions so absolute values cannot be taken into account to indicate corrosion hazard, that is, the relationship between concrete condition and potential values is not well-defined enough, with the exception of those potentials at extreme ends. The statistical representation of the data by means of cumulative frequency plots as indicated in the ASTM standard gives a better indication of the boundary potential between active and passive zones and of their percentage in the tested area. A wide range of factors influences the corrosion potentials as:a) Concrete moisture content: Changes in moisture content may lead to a difference of potentials up to 200

mV. It is important to consider not only different moisture conditions in a determined point but changes along the whole structure. Potential values become more negative as concrete moisture increases.

b) Cover thickness: As concrete cover increases, the difference between active and passive potential values diminishes, resulting on a uniform potential value at infinite. Thus, the location of small corrosion spots gets more difficult with increasing cover depth.

c) Concrete carbonation: As carbonation process leads to an increase of concrete resistivity, potential measurements show more positive values on both passive and corroding rebars.

d) Polarization effects: The corroding zones polarize the passive rebars in their vicinity to more negative potentials. This shift is higher in lower resistivity concrete.

Reference ElectrodeInterpretationCu/CuSO4 (CSE)

Ag/AgCl

More positive than -200mV More positive than -119mV More than 90% probability that no corrosion is occurring

Between -200mV & -350mV Between -119mV & -269mV Corrosion activity is uncertain

More negative than -350mV More negative than -269mV More than 90% probability that corro-sion is occurring



e) Oxygen content: Conditions of aeration, i.e. oxygen access, strongly determine rest potential values of passive steel in concrete. Low oxygen content leads to a pronounced decrease of the rest potential. In wet concrete due to very low oxygen diffusivity coefficient, conditions may arise in a shift of potential to comparably negative values so passive steel may show negative potentials similar to those of corroding steel. This leads to the risk that passive areas under low aeration conditions could be considered as corroding areas.

f) Chloride content: Field experience on a large number of bridge decks has shown a certain correlation between concrete chloride content and the potential values. The most negative values coincide with the areas of higher chloride content.

3.3. Linear PolarizationThe linear polarization method, or polarization resistance method, is an electrochemical method for determining corrosion rate. Polarization is defined as the potential change in a metal due to a change in electron flow, and therefore a change in the reaction rates, at the corroding surface. Linear polarization has become a rather widely used method because of the ease and efficiency of the testing. Linear polarization takes a short time and is both nondestructive and repeatable. This allows more versatile and continuous use in engineering and/or quality control applications.

A number of portable devices use the concept of linear polarization for in-service testing. There are two common probe types for linear polarization measurement: two-electrode probes and three-electrode probes.

Polarization resistance techniques have other inherent limitations that must be considered. The corrosion rate determination is an instantaneous test and gives only the value of the rate at that particular time. For an accurate measurement of the deterioration caused by corrosion, the rate should be taken at intervals over a period of time.

There are two distinct instrumentation methods for polarization resistance: glavanostatic and potentiodynamic. Glavanostatic methods involve the application of polarizing currents in a step-by-step fashion. Potentiodynamic testing is similar, except over voltages are applied in steps. Both are steady state methods. Glavanostatic instrumentation has wider use in conventional corrosion rate measurement equipment. Polarization resistance, or linear polarization, appears to be the preferred method for measuring the corrosion rate of steel reinforcement.

Corrosion current (Icorr) represents the instantaneous value when referred to the corroding area and its units are μA/cm2. It is converted into corrosion rate (Vcorr) when measured periodically and refers to a particular period in order to characterize the development of the process. The Vcorr units are mm/year.

The main precaution which has to be taken into consideration when measuring Rp in the case of the steel embedded in concrete are:- Achievement of a quasi steady-state response. This requires waiting times between 30 an 100 seconds during measurements.The shorter waiting times or faster scan rates are suitable for the case of active corrosion, while if the steel is passive, the quasi steady state is achieved at longer times or slower scans.The main aims of the measurement of corrosion current through the Rp technique are:a) The identification of corroding zones. Corrosion maps enable to identify the corrosion zones in the same

manner than the potential mapping.b) The evaluation of the efficiency of repair techniques.

The measurement of the corrosion current is made by means of a reference electrode, which indicates the electrical potential, and an auxiliary electrode, which gives the current. In site measurements, a second auxiliary electrode (guard ring) is being used in order to confine the current into a limited reinforcement surface. Figure 5 shows the photo of the PR-monitor device. The instrumentation needed for measuring linear polarization is now relatively simple, and devices such as Gecor 6 and Gecor 8 are commercially available devices for PR-measurement.




Figure 5 The PR-monitor device

With some small variants among the different devices, the procedure for measuring the corrosion rate is as follows:• Locate the reinforcing steel grid with a cover meter and mark it on the concrete surface.• Selection of the measurement locations. Record the cover depth and bar diameters in these locations.• Proper connection to the rebar and between the concrete surface and the measuring probe.• Measure the corrosion potential.• Application of the electrical current, and recording of the response.The execution of the Rp measurement may take from few seconds to 5 minutes, depending upon the used equipment. Repetition of the measurement in the same location is recommended in order to check on the reproducibility of the method. Table 4 gives the range of values for the interpretation of the measured corrosion currents (Icorr).

Table 4 Correlation between measured corrosion current and corrosion level

Other influencing parameters related to the environment of the concrete itself are:a) Moisture content: The moisture or liquid water in the concrete pores is the most relevant parameter

influencing the corrosion current. It is responsible of the electrolyte continuity (pore connectivity) and of the oxygen availability at the steel surface. Moisture fixes the electrical resistivity, which is the most comprehensive parameter determining the corrosion current. Oxygen content is more secondary unless below a certain level.

b) Temperature: It has an opposite effect on the corrosion rate. When the temperature increases, the moisture evaporates, which may counter-balance the trend to increase the corrosion rate. Only in water saturated structures, the temperature may present a direct relationship with the corrosion rate.

c) Chloride content: The chlorides not only act by depassivating the steel, but also enhance the corrosion rate.

Icorr (mA/cm2) Corrosion level

< 0.1 Negligible

0.1 – 0.5 Low

0.5 – 1.0 Moderate

> 1.0 High


4. EMBEDDED CONTINUED MONITORING DEVICESAny external protection application will prevent contact with the concrete surface. Contact is required for all of the field corrosion rate measurements, as is electrical contact with the reinforcement. Therefore, the performance of field corrosion rate tests on wrapped structures would necessitate removal of the external protection system at test locations. The tests would also demand either the installation of lead-wire connections to the reinforcement during the repair process, or core drilling to achieve electrical contact during the test. In effect, portable field corrosion rate tests would be destructive and could compromise the integrity of the externally applied protective systems at locations where the environment was most corrosive. A nondestructive corrosion rate measurement technique is required.

Embedded corrosion rate probes and sensors could be installed during the construction or repair processes. Embedded probes are considered to be desirable devices for use in structures repaired using externally protective systems, such as surface treatments, because it is important to keep the system intact.

4.1. Half Cell PotentialHalf cells can be embedded in concrete close to the reinforcing steel. Different reference electrodes are commercially available. Figure 6 shows a schematic diagram for a reference electrode consisting of mixed metal oxide activated titanium which is cast in specially developed cementitious body. The probe is wrapped around the reinforcing steel to be monitored. Figure 7 shows a photo of the probe installed beside a reinforcing steel bar. The wires from the probe can be wrapped and extended outside the concrete surface. The potential between the electrode and the reinforcing steel can be measured by using a half cell.

Figure 6 Schematic diagram for the reference electrode Figure 7 The probe installed beside a reinforcing bar

4.2. Macrocell MeasurementDuring the corrosion process, corrosion macrocells are formed with a distribution of anodic and cathodic areas. Voltage in a macrocell element equal to potential difference between active and passive steel gives the corrosion current. A macrocell current device can be set up using a configuration with a series of mild steel working electrodes and a stainless steel cathode instead of a half cell. The sensor can be embedded inside the concrete during new construction or repair. The sensor is fixed in such a way the first working electrode is near the concrete surface, and the last at the rebar depth. As each working electrode is depassivated with incoming chlorides or carbonation, there is a step up in the current flow between the depassivated working electrodes as it becomes an anode, and the stainless steel cathode. Figure 8 shows a different macrocell embedded sensors (i.e. sometimes known as the corrosion ladder meter).

Figure 8 Macrocell embedded sensors (ladder meter)

Cementitious backfill

CableProbe

Heat shrinkable cover

Cementitious backfill

CableProbe

Heat shrinkable cover




4.3. Linear PolarizationBased on the linear polarization principle, embedded sensors are developed. Different types of sensors are commercially available. Figure 9 shows typical sensors installed besides reinforcing steel. The embedded linear polarization sensors can measure corrosion potentials, polarization resistance and electrolyte resistance. They can also monitor the advance of the carbonation or the chloride front and the availability of oxygen at the sensor level by using special types of sensors. These sensors have possibility of automated measurements using computer controlled equipment.

Figure 9 Typical embedded linear polarization sensors installed in concrete

The embedded probes should be placed and oriented to allow current flow from the counter electrode to the reinforcement under evaluation. Generally probes are placed next to the longitudinal reinforcement and are connected electrically to the reinforcement cage, as shown in Figure 9. Electrical connection should be established by means of silver soldering the copper ground wire from the probe to the reinforcement. The steel surface should be prepared for the soldering procedure. The steel surface should be ground for a length of about 2.5 to 5 cm until bright steel is produced, a cleaning agent (flux) is applied to all surfaces and the copper ground wire is placed in contact with the steel reinforcement. A connection of about 2.5 cm should be provided to prevent detachment during subsequent construction. The probe is installed some distance away from the solder point to prevent the erroneous measurement of corrosion due to the other metals and the heat-treated steel.

The area of steel polarized by the probes is essentially the surface area of the portion of the bar receiving current from the probe. This area is calculated as the circumference of each reinforcing bar facing the counter electrode times the length of the probe. The connector on the end of the connection cable could be used with any portable linear polarization monitor corrosion rate device.

The electrodes are encased in a mortar block. The mortar block protects the electrodes and houses them in a consistent cementitious environment. The wires are wrapped together and extended outside the concrete for access to the measurement device.

5. LONG-TERM EVALUATIONThe measurements taken with the corrosion meter give information on the corrosion potential and corrosion rate. These values are only valid for the time of the test, requiring a repeated test schedule for accurate long-term evaluation. Depending to the life expectancy of the repaired structures the time interval should be determined. However, a six-month test interval would be appropriate.

6. SELECTION OF MONITORING LOCATIONSThe selection of monitoring points is of significant importance as corrosion factors to be considered are often related to the geometry of systems and components. Selection of these points should be based on a thorough knowledge of process conditions, materials of construction, geometrical details of the system, external factors and historical records. The number of points will depend upon:a) The amount of time available.b) Access and size of structure.c) The aim of the inspection.


As only a finite number of points can be considered, it is usually desirable to monitor the «worst-case» conditions, at points where corrosion damage is expected to be most severe. Often, such locations can be identified by reasoning with basic corrosion principles, analysis of in-service failure records and in consultation with operational personnel. With regard to the duration of each measurement, each reading may take from less than 1 minute to about 5 minutes depending upon the actual corrosion conditions and the method of measurement. There may also be a set up time of 25- minutes due to the physical processing of placing the sensor. So the operator must allow 5 to 15 minutes per location. The operator must also account for time taken to get access to each location, other measurements taken and other logistical factors associated with site work. This will control the total number of readings that can be taken.

The previous measurement of chloride concentration, rust staining, cover, carbonation depths, etc. can also be used as indications for selecting the measurement points. With regard to the spatial frequency, the structure has to be divided in sections attending:a) The severity of the ambientb) The structural typologyc) The aim of the inspection.

The groups of structural elements will be then statistically treated in order to obtain a representation value of each elements group or on the opposite; they will be mapped to obtain a pattern of the damage.

The lack of measurements along the time is recommended to be balanced by taking the larger possible number of readings in different locations of the structure, selected regarding several degrees of apparent damage or exposure severity.When the appraisal of the load-bearing capacity of the structure is required, then, it is necessary to obtain an accurate enough representative value of the corrosion rate (Icorr), by performing several measurements along a whole year or a suitable period of time following the seasonal changes.

7. SUMMARYThe use of performance monitoring equipment is essential in understanding the corrosion process in concrete structures in aggressive environment. The tests should be conducted during the assessment of concrete structures in service and/or after the rehabilitation of concrete structures to assess the effectiveness of the method and materials used in rehabilitation.

Portable devices are especially useful and allow for extensive and repeatable testing. It is important that the equipment is well maintained and working properly. Embedded sensors are for continued in-service evaluation. The following observations are based on the purpose and applicability of the described devices:• Corrosion rate values from field testing are qualitative, but not quantitative.• Embedded corrosion rate probes are convenient for in-service testing.• A continuous monitoring is essential to quantify the corrosion process of reinforcing steel.• Determination of reinforcement steel corrosion with nondestructive methods is complex and may lead to wrong interpretation of results. To avoid misinterpretation it is recommended to combine several nondestructive testing methods, before making any conclusion about reinforcement corrosion.

8. REFERENCES• Polder, R.B., Bamforth, P.B., Basheen, M., Chapmas-Andrews J. et al- «Reinforcement Corrosion and Concrete Resistivity-State of the Art Laboratory and Field Results», Int. Conf. on Corrosion and Corrosion Protection of steel in concrete. Ed. R.N. Swamy, Sheffield, July (1994), pp.571- 580.

• ASTM C876- 91 «Standard test method for half-cell potentials of uncoated reinforcing steel in concrete».• González, J.A., Molina, A., Escudero, M.L. & Andrade, C. «Errors in the Electrochemical Evaluation of Very Small Corrosion Rates. Part I. Polarization Resistance Method Applied to Corrosion of Steel in Concrete», Corrosion Science (UK), Vol.25, 1985, pp.917- 930.




VII - ESTIMATING SERVICE-LIFE Design Life Extension of RC Structure77

VII - ESTIMATING SERVICE-LIFE


78 VII - ESTIMATING SERVICE-LIFE

1. INTRODUCTIONConcrete is the primary construction and repair material of many structural systems in the UAE, such as bridges and high-rise buildings. Today, many of the concrete structures, which have been exposed to aggressive environments, suffer from durability problems and fail to fulfill their design service life requirements. The problem is particularly serious in reinforced concrete structures where corrosion of reinforcing steel can impair their safety. Carbonation and chloride-induced corrosion are two major causes of deterioration of concrete structures in Dubai. The limited knowledge of the field performance of corrosion-damaged structures and the lack of systematic approaches for their inspection, maintenance and repair contribute to the increase of their life-cycle costs, and result in the loss of functionality and safety. There is a need for the implementation of a systematic approach for inspection and rehabilitation of concrete structures in order to ensure their safety and durability and minimize their life cycle costs.

The corrosion of embedded steel reinforcement in concrete due to the penetration of chlorides from groundwater or seawater is the most prevalent form of premature concrete deterioration in the UAE. There are currently numerous strategies available for increasing the service life of reinforced structures exposed to chloride salts, including the use of:• low-permeability (high-performance) concrete,• chemical corrosion inhibitors,• protective coatings on steel reinforcement (e.g. epoxy-coated or galvanized steel),• corrosion-resistant steel (e.g. stainless steel),• non-ferrous reinforcement (e.g. fiber-reinforced plastics),• waterproofing membranes or sealants applied to the exposed surface of the concrete,• cathodic protection (applied at the time of construction), and• combinations of the above.

Each of these strategies has different technical merits and costs associated with their use. Selecting the optimum strategy requires the means to weigh all associated costs against the potential extension to the life of the structure.

2. LIFE-365 SERVICE LIFE PREDICTION MODELThe Life-365™ v1.0 program and manual were written by E. C. Bentz and M. D. A. Thomas. The Life-365™ v2.0 program and manual are adaptations of these documents, and were written by M. A. Ehlen under contract to the Life-365 Consortium II, which consists of the Concrete Corrosion Inhibitors Association, the National Ready Mix Concrete Association, the Slag Cement Association, and the Silica Fume Association.

Life-cycle cost analysis (LCCA) is being used more and more frequently for this purpose. Life-365 LCCA uses estimated initial construction costs, protection costs, and future repair costs to compute the costs over the design life of the structure. Many concrete protection strategies may reduce future repair costs by reducing the extent of future repairs or by extending the time between repairs. Thus, even though the implementation of a protection strategy may increase initial construction costs, it may still reduce life-cycle cost by lowering future repair costs.

A number of models for predicting the service life of concrete structures exposed to chloride environments or for estimating life-cycle cost of different corrosion protection strategies have been developed recently and some of these are available on a commercial basis. The approaches adopted by the different models vary considerably and consequently there can be significant variances between the solutions produced by individual models.


The current version of the software has many limitations in that a number of assumptions or simplifications have been made to deal with some of the more complex phenomena or areas where there is insufficient knowledge to permit a more rigorous analysis. Users are encouraged to run their own user-defined scenarios in tandem with minor adjustments to the values (e.g. D28, m, Ct, Cs, tp) selected by Life-365. This will aid in the development of an understanding of the roles of these parameters and the sensitivity of the solution to the values.

This version of Life-365 as all software is a closed book and therefore cannot be modified to includes parameters that are not originally included by the developers of the software. Yet, Life-365 is deemed the most comprehensive Service Life and Life Cycle Cost Analysis software available in the market. It can be used to determine the service life and the life cycle cost of a building, part of a building, or a structural member of the building.

Figure 1 shows a schematic of conceptual model of corrosion of steel reinforcement in concrete. Based on this graph, the program evaluates two key periods, the initiation period and the propagation period.

Figure 1: Schematic of conceptual model of corrosion of steel reinforcement in concrete

2.1. Using the Life 365 softwareLife 365 is easy to use software for predicting the service life and life cycle cost for:1- An entire structure where its structural elements are all subjected to the same environmental

conditions2- Part of the structure that may be subjected to different environmental conditions and is expected

to be severely affected more than the rest of the structure3- One element of the structure that is service subjected to different environmental conditions and

is expected to be severely affected more than the rest of the structure.The estimation of the service life can be done to one of the three cases mentioned above. Therefore the estimation of the entire service life of the entire structure can only be made using option 1 above.

2.2. Predicting the Initiation PeriodThe initiation period, ti, defines the time it takes for sufficient chlorides to penetrate the concrete cover and accumulate in sufficient quantity at the depth of the embedded steel to initiate corrosion of the steel. Specifically, it represents the time taken for the critical threshold concentration of chlorides, Ct, to reach the depth of cover, xd. Life-365 uses a simplified approach based on Fickian diffusion that requires only simple inputs from the user.




2.3. Predicting Chloride Ingress due to DiffusionThe model predicts the initiation period assuming diffusion to be the dominant mechanism. Fick’s second law is the governing differential equation: Eq. 1 whereC= the chloride content, D = the apparent diffusion coefficient, x = the depth from the exposed surface, and t = time.The chloride diffusion coefficient is a function of both time and temperature, and Life-365 uses the following relationship to account for time-dependent changes in diffusion: Eq. 2 whereD(t) = diffusion coefficient at time t, Dref= diffusion coefficient at time tref(= 28 days in Life- 365), and m = diffusion decay index, a constant.

Life-365 selects values of Drefandmbased on the mixture design details (i.e., water-cementitious material ratio, w/cm, and the type and proportion of cementitious materials) input by the user. In order to prevent the diffusion coefficient decreasing with time indefinitely, the relationship shown in Eq. 2 is assumed to be only valid up to 25 years, beyond which D(t) stays constant at the D(25 years) value.

Life-365 uses the following relationship to account for temperature-dependent changes in diffusion: Eq. 3 WhereD(T) = diffusion coefficient at time t and temperature T, Dref= diffusion coefficient at time trefand temperature Tref, U = activation energy of the diffusion process (35000 J/ mol), R = gas constant, and T = absolute temperature.

The temperature T of the concrete varies with time according to the geographic location selected by the user. If the required location cannot be found in model database, the user can input one’s own temperature data.

The chloride exposure conditions (e.g., rate of chloride build up at the surface and maximum chloride content) are selected by the model based on the type of structure (e.g., bridge deck, parking structure), the type of exposure (e.g., to marine or deicing salts) and the geographic location.

The solution for time to initiation of corrosion is carried out using a finite difference implementation of Eq. 1 where the value of D is modified at every time step using Eqs. 2 and 3.

2.4. Input Parameters for Predicting the Initiation PeriodThe following inputs are required to predict the initiation period:• Geographic location;• Type of structure and nature of exposure;• Depth of clear concrete cover to the reinforcing steel (xd), and

Details of each protection strategy scenario, such as water-cement ratio, type and quantity of mineral admixtures and corrosion inhibitors, type of steel and coatings, and type and properties of membranes or sealers. From these input parameters the model selects the necessary coefficients for calculating the time to corrosion, as detailed above.


2.5. Surface Chloride Build UpThe model determines a maximum surface chloride concentration, Cs, and the time taken to reach that maximum, tmax, based on the type of structure, its geographic location, and exposure, as input by the user. For example, if the user selects a bridge deck in an urban area, the model will use the surface concentration profile shown in the left panel of Figure 2. The user can input his own profile, in terms of maximum surface concentration and the time (in years) to reach that maximum as shown in Figure 1.

Figure 2: Examples of Concrete Surface History and Environmental Temperatures (Life 365v2.0)

2.6. Temperature ProfileThe model determines yearly temperature profiles based on the user’s input for geographical location using a database compiled from meteorological data. For example, if the user selects Moline, Illinois, the model will use the temperature profile in the right panel in Figure 1. The user can input one’s own temperature profile, in terms of monthly average temperatures in Celsius degrees.

2.7. Base Case Concrete MixtureThe base case concrete mixture assumed by the model is plain Portland cement concrete with no special corrosion protection strategy. For the base case, the following values are assumed:D28 = 1 x 10(-12.06 + 2.40w/cm) meters-squared per second (m2/s)m = 0.20Ct = 0.05 percent (% wt. of concrete)

The relationship between D28 and the water-cementitious materials ratio (w/cm) is based on a large database of bulk diffusion tests. The nature of the relationship is shown in Figure 3 (corrected to 20οC). The value of m is based on data from the University of Toronto and other published data and decreases the diffusion coefficient over the course of 25 years, after which point Life-365 holds it constant at the 25-year value, to reflect the assumption that hydration is complete. The value of Ct is commonly used for service-life prediction purposes (and is close to a value of 0.40 percent chloride based on the mass of cementitious materials for a typical structural concrete mixture).

It should be noted that these relationships pertain to concrete produced with aggregates of normal density and may not be appropriate for lightweight concrete.

Figure 3: Relationship between D28 and w/cm (Life 365v2.0)




2.8. Effect of Silica FumeThe addition of silica fume is known to produce significant reductions in the permeability and diffusivity of concrete. Life-365 applies a reduction factor to the value calculated for Portland cement, DPC, based on the level of silica fume (%SF) in the concrete. The following relationship, which is again based on bulk diffusion data, is used: DSF = DPC ·e-0.165·SF.

Eq. 4The relationship is only valid up to replacement levels of 15 percent silica fume. The model will not compute diffusion values (or make service life predictions) for higher levels of silica fume. Life-365 assumes that silica fume has no effect on either Ct or m.

Figure 4: Effect of Silica Fume on DSF (Life 365v2.0)

2.9. Effect of Fly Ash and SlagNeither fly ash nor slag are assumed to effect the early-age diffusion coefficient, D28, or the chloride threshold, Ct. However, both materials impact the rate of reduction in diffusivity and hence the value of m. The following equation is used to modify m based on the level of fly ash (%FA) or slag (%SG) in the mixture: M = 0.2 + 0.4(%FA/50 + %SG/70) Eq. 5

The relationship is only valid up to replacement levels of 50 percent fly ash or 70 percent slag and m itself cannot exceed 0.60 (which would occur if fly ash and slag were used at these maximum levels), that is, m must satisfy m ≤0.60�. Life-365 will not compute diffusion values (or make service life predictions) for higher levels of these materials, and after 25 years holds the diffusion constant at the 25-year value to reflect that hydration is complete.

Figure 5 shows the effect of m for three mixtures with w/cm = 0.40 and with plain Portland cement (PC), 30 percent slag, and 40 percent fly ash. Table 1 lists these mixture proportions and their computed the diffusion coefficients, for 28 days, 10 years, and 25 years. For years greater than 25, Life-365 uses the computed 25-year diffusion coefficient.

Figure 5: Effects of Fly Ash and Slag on Dt (Life 365v2.0)


2.10. Effect of Corrosion InhibitorsThe model accounts for two chemical corrosion inhibitors with documented performance: calcium nitrite inhibitor (CNI) and Rheocrete 222+ (a proprietary product from Master Builders; in the Life-365 software, it is referred to as “A&E,” for “amines and esters”). It is intended that more inhibitors be included when appropriate documentation of their performance becomes available.

Ten dosage levels of 30 percent solution calcium nitrite are permitted in Life-365. The inclusion of CNI is assumed to have no effect on the diffusion coefficient, D28, or the diffusion decay coefficient, m. The effect of CNI on the chloride threshold, Ct, varies with dose as shown in Table 2.

In addition, a single dose of Rheocrete 222+ (or amines and esters, as it is referred to in the software) is permitted in the model; the dose is 5 litres/m3 concrete. This dose of the admixture is assumed to modify the corrosion threshold to Ct = 0.12 percent (by mass of concrete). Furthermore, it is also assumed that the initial diffusion coefficient is reduced to 90 percent of the value predicted for the concrete without the admixture and that the rate of chloride build up at the surface is decreased by half (in other words it takes twice as long for Cs to reach its maximum value). These modifications are made to take account of the pore modifications induced by Rheocrete 222+ (or amines and esters), which tend to reduce capillary effects (i.e. sorptivity) and diffusivity.

2.11. Effect of Membranes and SealersMembranes and sealers are dealt with in a simplified manner: Life-365 assumes that both membranes and sealers only impact the rate of chloride build-up, and can only be reapplied up to the time of the first repair.

Table (1): Effects of Slag and Fly Ash on Diffusion Coefficients




Membranes start with an efficiency of 100 percent, which deteriorates over the lifetime of the membrane, a lifetime of 20 years, and no re-applications. This means that the rate of build-up starts at zero and increases linearly to the same rate as that for an unprotected concrete at 20 years. As shown in the left panel of Figure 6, surface chlorides for unprotected concrete (labeled “PC”) increases at a rate of 0.04 percent per annum and reaches a maximum concentration of 0.60 percent at 15 years. In the right panel, surface chlorides for concrete protected by a default membrane increase at a lower rate, but then reach the same rate after 20 years. The user can also set his own values for initial efficiency, lifetime of the membrane, and re-applications.

Sealers are dealt with in the same way, except that the default lifetime is only 5 years. The example in Figure 6 shows the effect of reapplying the sealer every 5 years. Each time the sealer is applied, the build-up rate is reset to zero and then allowed to build up back to the unprotected rate (0.04 percent per annum in the example) at the selected lifetime of the sealer (5 years in the example).

Figure 6: Effects of Membranes and Sealers (Life 365v2.0)

2.12. Effect of Epoxy-Coated SteelEpoxy-coated reinforcing bars have been widely used in aggressive environments since about 1973 and have generally met with success in delaying corrosion due to the ingress of chlorides. ASTM A 775 standard specifications were developed that outlined coating application and testing. The standard has the following requirements (ASTM A 775): 1) the coating thickness should be in the range of 130 to 300 microns; 2) bending of the coated bar around a standard mandrel should not lead to formation of cracks; 3) the number of pinhole defects should be no more than six per meter; and 4) the damage area on the bar should not exceed 2%.

According to the ASTM A775, Standard Specification for Epoxy-Coated Steel Reinforcing Bars, the epoxy coatings with thicknesses in the range of 160 to 510 μm reduce the bond strength of deformed reinforcing bars to concrete. Epoxy coatings with a thickness between 160 and 420 μm differences in coating thickness have little effect on the amount of bond strength reduction and, by extension, larger bars. Coatings thicker than 420 μm cause an additional drop in bond strength relative to the bond strength obtained with thinner coatings. The maximum allowable coating thickness should be increased from 300 μm to 420 μm to meet the requirements of ASTM A 775. It should be emphasized here that the aforementioned Standards should be used in the structural design when the epoxy coating is to be used for protecting the steel reinforcement from corrosion. This is particularly important to avoid over-coating the bars so that bond with concrete will not be affected.

The main disadvantage of using epoxy is that it has to be done very carefully to assure that the coating will not be scratched or damaged. Perhaps the best-known instance of poor field performance of epoxy-coated bars was in several of the rebuilt bridges in the Florida Keys. Florida researchers established that the primary causes of corrosion were inattention to preparation of the bars before coating and debonding of the coating before placement in the structures.

Age year


Since 1991, a substantial improvement in the quality of epoxy-coated bars and understanding of adhesion of coatings to steel has developed, primarily as a result of additional research and plant certification programs. Considerable research has been conducted on epoxy-coated reinforcing bars over the last 10 years, and field investigations have been conducted by many state agencies all over the world. These studies have found that structures containing epoxy-coated bars are more durable than structures with uncoated bars. Laboratory research has shown that new coating products and test methods may improve the long-term durability of concrete structures. To assess the long-term durability of epoxy coating products, these new test methods should be put in the form of consensus standards. The Life 365 software increases accordingly the service life due to coating steel reinforcement.

2.13. Effect of Stainless SteelIn the current version of Life-365 it is assumed that grade 316 stainless steel has a corrosion threshold of Ct = 0.50 percent (i.e., ten times the black steel Ct of 0.05 percent).

2.14. Predicting the Propagation PeriodThe propagation period, tp, is fixed at 6 years. In other words, the time to repair, tr, is simply given by tr= ti+ 6 years. The only protection strategy that influences the duration of the propagation period is the use of epoxy-coated steel, which increases the period to tp= 20 years. The propagation period can be changed by the user.

3. ESTIMATING LIFE-CYCLE COSTTo estimate life-cycle cost, Life-365 follows the guidance and terminology in ASTM E-917 Standard Practice for Estimating the Life-Cycle Cost of Building Systems. This includes the process of 1. Defining a base year, study period, rates of inflation and discount and project requirements;2. Calculating the present value of future costs;3. Reporting results in present value (constant dollar) and current dollar terms; and4. Conducting uncertainty and sensitivity analysis.

3.1. User Input ParametersThe user is responsible for providing the following cost information needed for the lifecycle cost analysis:• Cost of concrete mixtures (including corrosion inhibitors) for the various corrosion protection strategies under consideration,• Cost, coverage, and timing of repairs,• Inflation rate, i, and• Real discount rate, r.• Life-365 provides the following default costs for the included rebars:• Black steel = $1.00/kg ($0.45/lb)• Epoxy-coated rebar = $1.33/kg ($0.60/lb)• Stainless steel = $6.60/kg ($2.99/lb)

The user should review and if necessary change the costs of these materials to better reflect actual project costs in his area. Here the currency is expressed in US dollars. The software allows only the use of the US currency in its calculations of the LCC. In fact the user may just enter the data in AED and the evaluated cost will be expressed using the $ sign, which then will mean AED. For example, a user wants to use this software to evaluate the service life cost needs first to enter the cost of the aforementioned cost information of which for instance for the cost of the black steel that happens to be 4 AED per kg in the local market at the time the analysis was done.




Then the user will input it in the field for the black steel as 4. The software understands it as $4; but the user is putting it and keeping in mind that the sign for a dirham is now $. Therefore the total cost will be in dirham. It is important to know that the user cannot add any additional parameters as the software does not allow such addition since it is a “closed book” to the user.

4. CALCULATING LIFE-CYCLE COST4.1. Present Worth CalculationsLife-cycle cost is calculated as the sum of the initial construction costs and the discounted future repair costs over the life of the structure. The initial construction costs are calculated as the sum of concrete costs, steel (or other reinforcement) costs, and any surface protection (membrane or sealer) costs. Life-cycle cost is expressed in either total dollars or dollars per unit area of the structure (e.g. dollars per square meter). Future repair costs are calculated on a “present worth” basis using the inflation rate, i, and the real discount rate, r, both provided by the user. The present worth, PW, of a future cost c in year t is calculated asEq. 6

All future repair costs over the entire design life of the structure are calculated in this manner.

5. EXAMPLEA hypothetical example of a building located in Dubai will be presented to introduce the features of the program using a step-by-step approach.

5.1. Step 1- Project TabIn this tab, the user defines the following parameters:• Identify Project• Select Structure Type and Dimensions (Figures 7 through 9) • Define Economic Parameters (Figure 10) o Base year o Analysis period o Inflation rate (%) o Real discount rate (%)• Define Alternatives

Figure 7: Defining the type of structure as Slabs and Walls (Life 365v2.0)


Figure 8: Defining the type of structure as rectangular columns (Life 365v2.0)

Figure 9: Defining the type of structure as circular columns (Life 365v2.0)

Figure 10: Defining the base year, analysis period, inflation rate and real discount rate (Life 365v2.0)




5.2. Step-2: Exposure TabIn this tab, the user defines the following parameters:• Select Location• Define Chloride Exposure (Figure 11) o Max surface conc.• Time to build to max (yrs)• Define Temperature Cycle (Figure 12)

Figure 11: Defining maximum surface chloride concentration and time to buildto maximum (Life 365v2.0)

Figure 12: Defining the temperature history in Dubai (Life 365v2.0)

5.3. Step 3: Concrete MixesIn this tab, the user defines the following parameters for a new structure, part of a structure, or a structural element (Figures 13, 14): For repaired structures, Life 365 allows including the repair effect only when section enlargement is used using additional concrete or cementitious material. The Life 365 software does not support any other materials for repair. It is believed that newer version of the software can take into consideration the life cycle prediction for repaired structures using other materials.


• Define Concrete Mixes and Steel used (Figures 13, 14):• Selected mix and corrosion inhibitors or external barrier (Figures 15, 16):• Service Life Graphs (Figures 17):

Figure 13: Defining W/C ratio, Slab %, Fly ash % and Silica fume % (Life 365v2.0)

Figure 14: Defining % of steel in section and type of used steel (Life 365v2.0)




Figure 15: Defining dosage of corrosion inhibitor (Life 365v2.0)

Figure 16: Defining external barrier type (Life 365v2.0)


Figure 17: Obtaining the initiation period, propagation period and total service life (Life 365v2.0)

5.4. Individual Costs tabThe Individual Costs tab (Figures 18, 19) allows you to edit the different constituent concrete costs, and view the effects they have on the constituent costs that make up life-cycle cost. The following parameters can be defined:• Set Concrete Costs• Default Concrete and Repair Costs• Costs for Each Mix Design• Cost Timeline

Figure 18: Default Concrete and Repair Costs (Life 365v2.0)




Figure 19: Defining individual costs (Life 365v2.0)

5.5. Life-Cycle Costs tabOnce the general, environmental, concrete mix, and individual costs have been entered, the resulting life-cycle costs of the mixes can be compared in the Life-Cycle Costs tab, Figure 20.

Figure 20: Life cycle cost (Life 365v2.0)


5.6. Service Life and Life-Cycle Cost ReportsFinally, Life-365 provides two pre-defined reports of your project: a SL Report (for «Service Life Report»; Figure 20) and an LCC Report (or «Life-Cycle Cost Report»; Figure 21). These two reports list many but not all of the parameters used in your analysis. Each report can be printed by pressing the Print button at the bottom of the window. (If you do not have a printer installed, then Life-365 will ask you to install one first.)

Figure 21: Service life report (Life 365v2.0)

Figure 22: Life cycle cost report (Life 365v2.0)




5.7. Effect of the different parameters on the Service lifeHere, a hypothetical example is presented for the sake of running the Life 365 software. The software is easy to use; just follow the procedure presented in the previous sections of this chapter. Such a procedure can be followed for the entire concrete structure, part of it, or a structural member, i.e., slab, beam column, wall, etc.). We used here the data for the same building presented for the demonstration of the procedure in Section 5. Therefore, for the same building locations and condition, the effect of different parameters on the service life can be obtained. Table 3 list the service life estimated for 36 alternatives. The user can certainly prepare a different list of alternatives that suit the actual building structure. It should be emphasized here that this is a demonstration of the capability of the software that the research team see as the most appropriate and comprehensive tool available for the evaluation of the service life and the life cycle cost. In these alternatives the effect of the following parameters has been obtained:• W/C ratio, Slag %, Fly ash %, Silica fume %• Rebar type• Barrier type, Inhibitor type• Diffusion decay index, Chloride concentration


Table 3: Service life estimated for 36 alternatives

* The alternative with highest service life prediction

Alt. W/C Slag %

Fly Ash %

Silica Fume

%Rebar type Barrier Inhibitor,

L/m3 m Ct

Ser-vice Life (yrs)

Alt. 0 0.35 0 0 10 black steel non non 0.2 1.175 15.3


Alt. 2 0.45 0 0 10 black steel non non 0.2 1.175 12




Alt. 6 0.35 0 0 10 black steel non 15 0.2 5.64 27.1






Alt. 12 0.35 0 0 10 black steel membrane non 0.2 1.175 23

Alt. 13 0.4 0 10 0 black steel membrane non 0.28 1.175 13.9





Alt. 18 0.35 0 0 10 Epoxy coated non non 0.2 1.175 29.3


Alt. 20 0.45 0 0 10 Epoxy coated non non 0.2 1.175 26




Alt. 24 0.35 0 0 10 Epoxy coated sealer non 0.2 1.175 31.8






Alt. 30 0.35 0 0 10 Epoxy coated sealer 15 0.2 5.64 43.9




Alt. 34* 0.4 10 0 10 Epoxy coated sealer 15 0.26 5.64 44.2





6. REFERENCES• A 775/A 775 M Standard Specification for Epoxy- Coated Reinforcing Steel Bars Steel Corrosion in

Concrete, E & FN Spon, London, 1997, pp. 136 -139.

• Bamforth, P.B. 1998. “Spreadsheet model for reinforcement corrosion in structures exposed to chlorides.” In Concrete Under Severe Conditions 2 (Ed. O.E. Gjørv, K. Sakai and N. Banthia), E&FN Spon, London, pp. 64- 75.

• Bentz, E.C. 2003. “Probabilistic modeling of service life for structures subjected to chlorides,” Materials Journal, Vol. 100 (5), pp. 391 -397.

• Berke, N.S. and Rosenberg, A. 1989. «Technical Review of Calcium Nitrite Corrosion Inhibitor in Concrete», Transportation Research Record 1211, Concrete Bridge Design and Maintenance, Steel Corrosion in Concrete, Transportation Research Board, National Research Council, Washington D.C.

• Boddy, A., Bentz, E., Thomas, M.D.A. and Hooton, R.D. 1999. “An overview and sensitivity study of a multi-mechanistic chloride transport model.” Cement and Concrete Research, Vol. 29, pp. 827- 837.

• Concrete Reinforcing Steel Institute. 1998. “Life-cycle costs reinforce epoxy-coated bar use,” Concrete Products, Penton Media, Inc., p. 82.

• Frederiksen, J.M., Sorensen, H.E., Andersen, A., and Klinghoffer, O. 1997. HETEK, The Effect of the w/c ration on Chloride Transport into Concrete -Immersion, Migration and Resistivity Tests, Report No. 54.

• Frohnsdorff, G., 1999, Modeling Service Life and Life-Cycle Cost of Steel Reinforced Concrete, Report from the NIST/ACI/ASTM Workshop, November 91998 ,10-, National Institute of Standards and Technology Report NISTIR 6327, 43 p.

• Gjorv, O.E., Tan, K., and Zhang, M-H. 1994. «Diffusivity of Chlorides from Seawater into High-Strength Lightweight Concrete» ACI Materials Journal, Vol. 91 (5), pp. 447- 452.

• Glass, G.K. and Buenfeld, N.R. 1995. “Chloride threshold levels for corrosion induced deterioration of steel in concrete.” Chloride Penetration into Concrete, (Ed. L.-O. Nilsson and J. Ollivier), pp. 429 -440.

• Life-365 v2.0: Service Life Prediction Model and Computer Program for Predicting the Service Life and Life-Cycle Costs of Reinforced Concrete Exposed to Chlorides, written by E. C. Bentz and M. D. A. Thomas, 2010.

• McDonald, D. B., and Pfeifer, D. W., “Epoxy-Coated Bars—State-of-the-Art,” Proceedings of the Second Regional Conference and Exhibition, ASTM, Saudi Arabian Section, V. 2, Nov. 1995.

• Maage, M., Helland, S. and Carlsen, J.E. 1995. “Practical non-steady state chloride transport as a part of a model for predicting the initiation period.” Chloride Penetration into Concrete, (Ed. L.-O. Nilsson and J. Ollivier), pp. 398 -406.


• MacDonald, D., Pfeiffer, D. and Sherman, M. 1998. “Corrosion evaluation of epoxy coated, metallic-clad, and solid metallic reinforcing bars in concrete.” FHWA-RD-98153-, Federal Highways Administration, Washington, D.C.

• Gerald G. Miller, Jennifer L. Kepler, and David Darwin, 2003. “ Effect of Epoxy Coating Thickness on Bond Strength of Reinforcing Bars”, ACI Structural Journal, V. 100, No. 3, 1 -7.

• Samples, L. M., and Ramirez, J. A., Field Investigations of Existing and New Construction Concrete Bridge Decks, Research Series 6, Concrete Reinforcing Steel Institute, Schaumburg, Ill., 1999.

• Virmani, P., “Corrosion Evaluation of Epoxy Coated, Metallic Clad, and Solid Metallic Reinforcing Bars in Concrete”, FHWA-RD-98- 153.




VIII - IMPROVING CORROSION RESISTANCE CHARACTERISTICSDesign Life Extension of RC Structure

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VIII - IMPROVING CORROSIONRESISTANCE CHARACTERISTICS


100 VIII - IMPROVING CORROSION RESISTANCE CHARACTERISTICS

1. INTRODUCTIONIn recent years, more emphasis has been given to creating more durable and cost effective concrete structures. This is due to the increase cost of repair and maintenance, which may be similar to the cost of building a new structure in many cases. “Cost optimal design” of new concrete structures is no longer a strange terminology to owners, specifiers and engineers.

Ensuring durability of concrete structure is definitely not a simple and static process. It requires clear definitions, understanding of responsibilities and collaborations. Design for durability of concrete structure needs to be considered on case-by-case basis depending on the exposure environment and intended structure service life.

Corrosion-induced deterioration of reinforced concrete structures occurs when the environmental loading on the structure is greater than the ability of the structure to resist the environmental loading (environmental resistance). One can either decrease the loading or increase the resistance or do a combination of both.

Corrosion can also occur as a result of other deterioration processes: expansive reactions such as; sulfate attack, thermal cracking, etc., excessive deflections, etc. These processes cause the concrete to crack, which subsequently allows water and chlorides easy access to the interior of the concrete and the steel reinforcing bars. These other deterioration mechanisms create conditions more conducive to the corrosion of the embedded steel reinforcing bars, which leads to further deterioration of the concrete.

The factors that influence the corrosion of steel reinforcing bars embedded in concrete are the amount of chloride ions at the steel level, the resistivity of the concrete, temperature, relative humidity (both internal and external), and the concrete microstructure. In general, by controlling these factors to an acceptable level, the corrosion of the steel reinforcing bars and resulting concrete deterioration can be minimized. This is the first step in most corrosion-control strategies in addition to other suitable corrosion-protection systems. Corrosion-control methods or systems are classified as mechanical or electrochemical.

Mechanical methods are physical barriers that prevent or delay the ingress of chlorides, oxygen, and moisture through the concrete cover to the reinforcing steel. They include admixtures, sealers and membranes, overlays, and coatings on steel reinforcing bars. Sealers and membranes made with materials such as resins, epoxies, emulsions, etc. are used to reduce the ingress of deleterious species. Portland cement concrete, low-slump dense concrete, latex-modified concrete, silica fume-modified concrete, and polymer concrete overlays are commonly used. Coatings used on steel reinforcing bars are either organic or metallic. Organic coatings include the non-metallic and epoxy coatings. Metallic coatings include materials such as nickel, stainless steel, and zinc. The nickel and stainless steel coatings protect steel by being a barrier system and more noble, i.e., have a lower potential than iron to corrode. The zinc coating protects steel by being sacrificial or more active (i.e., it has a greater potential than iron to corrode). Corrosion-resistant materials include fiber-reinforced polymer (FRP) rebars.

Electrochemical methods force the steel reinforcing bars to be cathodic. They include chloride extraction and cathodic protection. Chloride ion extraction and cathodic protection are typically used in the rehabilitation of reinforced concrete structures and not as a corrosion-control measure for new construction.

There are three categories of variables that influence the corrosion and deterioration processes and the extent of the corrosion-induced deterioration of reinforced and prestressed concrete members:- Environmental variables.- Materials.- Design.

Material variables for making durable concrete include cement type, supplementary cementing materials (SCM) such as silica fume, slag and fly ash, admixtures, aggregate quality, and the water-cement ratio.

Design variables include the depth of concrete cover, properties of the hardened concrete; mainly compressive strength, the size and spacing of the steel reinforcing bars, and the efficiency of drainage from the structure.


Environmental variables include the source of chloride ions; sulfate ions, temperature extremes; wet-dry cycles; relative humidity; and, to a certain extent, applied live loading. Although little can be done to control environmental variables, material and design variables can be adjusted to build durable concrete structures that can resist corrosion-induced deterioration in environments conducive to the initiation and sustenance of the corrosion process.

The durability of concrete structure especially in aggressive environments has been the focus for almost the last two decades. The deterioration of concrete structures due to induced reinforcement corrosion is classified as the number one cause worldwide. The main issues that should be taken into consideration to improve the protection of concrete to the reinforcement corrosion includes design, construction practice and materials.

The following sections give a review for the design provisions and construction provisions. Also, a review of the concrete materials selection is included. The report will provide information on corrosion protection systems such as corrosion inhibitors, external protection barriers and cathodic protection.

2. SELECTION OF PROTECTION SYSTEMSThe proper corrosion-protection strategy will vary from structure to structure. Some factors to be considered during the design of a structure include:• Intended design life of the structure.• Effects of corrosion and corrosion induced deterioration (i.e. including cost due to closure either permanent or temporary repair).• Quality of workmanship in construction (i.e. including good consolidation, proper rebar placement, sufficient concrete cover over the steel reinforcing bars, and other measures).• Possible rehabilitation methods (i.e. the design of structures should include provisions for the possible future rehabilitation of corrosion-induced deterioration).• Initial costs (i.e. life-cycle costs).

As the rehabilitation and replacement costs increase, corrosion-control measures become more cost effective. Multiple protection strategies may be cost-effective for long-term corrosion protection. One such strategy is the use of galvanized steel reinforcement or FRP rebar or the use of surface corrosion inhibitors or surface sealants in combination with durable concrete containing having low permeability, and maintaining adequate concrete cover during construction. However, there is a need to balance the costs of the additional control measures against how much additional service life can be expected as a result of the added control measures.

The additional costs can usually be justified based on a life-cycle cost analysis. Some factors to be considered when choosing a corrosion-control measure include:• How aggressive the environment is where the structure will be located.• Effectiveness of the protective system(s).• Possibility of future installation of other control measures.• Life expectancy of the measure.• Any incremental costs over the “do nothing” option• Any impacts on the cost of other elements in the structure.

Corrosion-protection strategies for steel reinforcing bars embedded in concrete can be grouped into the following categories:• Design.• Concrete.• Corrosion inhibitors.• Surface sealants.• Reinforcement type.




The design category includes:• Compressive strength.• Concrete cover.• Maximum allowable crack widths in service.• Reinforcement distribution (crack control provisions).

The concrete category includes:• Water-cement ratio.• SCM; such as silica fume, fly ash, slag.• Latex, epoxy, and polymer admixtures.• Cement type.• Aggregate quality.

The inhibitor category and surface treatment includes:• Corrosion inhibitors systems (organic-inorganic).• Surface sealants.

The reinforcement category includes such items as:• Galvanized bars.• Stainless steel bars.• Non-metallic bars (FRP rebars).

3. GENERAL DESIGN PROVISIONSIt is generally the design details that influence the overall performance and durability of concrete structures. Some design factors that affect the durability of concrete structures include:• Environmental exposure.• Tendency of concrete to crack.• Compressive Strength.• Concrete cover.• Architectural and structural details (i.e. joints (if any), drainage, access for inspection & maintenance, etc.).

The environmental exposure to which the structure will be exposed to should be carefully identified in order to identify the severity of exposure to different aggressive species. This step is very important at the beginning of the design process in order to establish the concrete grade suitable for such exposure. In some cases, the strength needed for certain exposure level will override that required for structural design. Also, minimum concrete quality, expressed in the form of maximum w/c ratio and/or minimum cementitious content and/or cementitious type, will be specified to be considered in the project specification and during construction. Table 1, from BS 81101:1997- and BS 5328-1:1997, lists the different exposure conditions for RC structures. Table 2, from BS 81101:1997-, lists the minimum concrete quality needed for each exposure condition. Similar tables are listed in EN 2061:2000-. Similarly, the different exposure conditions and minimum concrete quality, according to CIRIA C5772002-, are listed in Tables 4 and 5. While, requirements for special exposure conditions according to ACI 318M-05 are listed in Tables 5, 6, 7 and 8. While, the minimum concrete cover for reinforcement protection according to ACI 318M-05 is listed in Table 9.


Environmental conditions in Dubai are characterized with the following:• High average temperature and relative humidity around the year as shown in Figure 1.• High ground water table (12- meters below ground level) which is high in sulfates and/or chlorides

concentration.• High windborne salts.• Gulf water is considered to be one of the highest in the world with respect to chloride concentration.

Therefore, exterior parts of, and underground structures in Dubai could be classified to be exposed to severe to very severe exposure conditions.

Figure 1. Annual average weather conditions in Dubai

Table 1: Classification of exposure condition (BS 81101:1997- and BS 53281:1997-)

0

5

10

15

20

25

30

35

40

45

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Tem

pera

ture

o C

Monthly Average Tempearure in Dubai, UAE

Average High

Average Low

78

80

82

84

86

88

90

92

94

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Rela

tive

Hum

idit

y (%

)

Monthy Average Relative Humidity in Dubai, UAE



Table 2: Minimum concrete quality for different exposure conditions (BS 53281:1997-)

Table 3: Classification of exposure condition (CIRIA C557- 2002)


Exposurecondition Location

a Superstructure iniand with no risk ofwindbome stats

b Superstructures in areas of salt flats, inlandor near the coast, exposed to windbomesalts

c Parts of structures in contact with soil, wellabove capillary rise zone, with no risk ofwater introduced at the surface by imigation,faulty drainage systems, washing down ets.

d Parts of structures in contact with the soil,whin the capilary rise zone, belowgroundwater level, or where water may may beintroduced at the surface by irigation,discharge of wastes, wasting down etc.These situations all lead to a potential forthe concentration of aggressive salts byevaporation(i) Sigrificant sulfate contamination only (ii) Sigrificant chloride contamination only (iii) Sigrificantcontamination with both sulfates and chorides

e Marine structures (splash zone)

f Waterretaining structures (inctudingsewage treatment plants)


Table 4: Minimum concrete quality for different exposure conditions (CIRIA C577 -2002)

Table 5: Requirements for special exposure conditions (ACI 318M-05)

Table 6: Requirements for concrete exposed to deicing chemicals (ACI 318M-05)




Table 7: Requirements for concrete exposed to sulfate containing solutions (ACI 318M-05 & ACI 201.2R-01)

Table 8: Requirements for minimum concrete quality exposed to special exposure conditions(ACI 318M-05)

Table 9: Minimum concrete cover for reinforcement protection (ACI 318M-05)

Exposure Condition Max w/c Min. Strength (MPa)

Concrete intended to be watertight:•Exposed to fresh water•Exposed to sea water

0.500.45

2530

Concrete exposed to F/T in moist conditions:•Thin sections•Other elements•Presence of de-icing salt

0.450.500.45

302530

For corrosion protection of RC structures exposed to de-icing salts, or sea water

0.40 33

Exposure ConditionMin. Cover (mm)

Cast in Situ

Precast Prestressed

Concrete cast against or permanently exposed to earth 70 --- 70

Concrete exposed to earth or weather:•Wall panels•Slabs•Other members

40-5040-5040-50

20-40----

30-50

303040

Concrete NOT exposed to weather or in contact with earth:•Slabs•Beams, columns•Shells, folded plates•Non prestressed reinforcement

20-4040

15-20----

15-3010-4010-15

----

2020-40

1020

Concrete exposed to de-icing salt or sea water:•Walls and slabs•Other elements

5060

4050

--------


Cracks may be thermal or shrinkage. Proper care in the layout and sequencing of concrete pours should be exercised to minimize the risk of cracking. Several design parameters can be adjusted as cost-effective corrosion-control measures. These include the use of adequate concrete cover, reinforcement distribution, the size and spacing of reinforcing steel for crack control. Care should be taken to minimize any cracks that may help in transmitting water and other aggressive species (BS 81101:1997-).

The width of cracks in concrete is more of a concern than the number of cracks. The use of an increased number of well-distributed reinforcing steel bars is more effective in controlling crack widths than a smaller number of larger bars. Maximum crack width is set to 0.3mm according to BS 81101:1997- and BS 81102:1997- in aggressive environment for corrosion protection. In the assessment of the behavior of prestressed concrete members (Class 3 according to BS 81101:1997-) in very severe environment the crack width is limited to 0.1mm. For Class 1 no flexural tensile cracks are allowed while for Class 2 no visible cracks are allowed. According to ACI 224R-01, the crack width ranges from 0.10 to 0.40mm depending on the exposure condition, for seawater and wetting and drying condition it is set to 0.15mm. Meanwhile, according to CIRIA C5772002-, the maximum crack width ranges from 0.10 to 0.30mm according to the exposure condition. It should be highlighted that the maximum crack width could be increased in cases where external finishing layers or barriers are used.

The use of well-consolidated, low-permeability adequate concrete cover is a cost-effective corrosion control measure. The amount of concrete cover significantly influences the time-to-corrosion of the steel reinforcing bars and its quality influences the diffusion rate of chloride ions through the concrete. Since the diffusion of chloride ions in concrete is non-linear with increasing cover thickness, there is a significant increase in the time required for the chloride ions to reach the steel reinforcing bars. As a rule of thumb the cover to a main bar should not be less than the main bar size or where bars are in pairs or bundles the size of a single bars of cross-sectional area equal to the sum of their cross-sectional areas (BS 81101:1997-).

However, with increased concrete cover, there is an increase in the potential for concrete cracking from shrinkage and thermal effects. The reinforcing steel bars become less effective for crack control with increasing cover thickness. Chloride concentrations in the top 10 mm of a concrete element can be very high when compared to the concentrations at depths of 25 to 50 mm. A concrete cover of 25 mm has been shown to be inadequate in severe environments, even with a water-cement ratio as low as 0.30. For moderate to severe environments, the amount of concrete cover should be at least 40 mm and, preferably, 50 mm (ACI 201.2R-01). The minimum cover for the main reinforcing steel with no positive corrosion protection in concrete structures frequently exposed to chlorides is 65 mm (ACI 201.2R-01). The minimum concrete cover for reinforcing steel embedded in concrete with direct exposure to saltwater is 100 mm; and 75 mm for concrete cast against earth (ACI 201.2R-01).

There are some precautions that can be taken during the design of a structure to help minimize the potential for corrosion. The number of joints should be as few as possible and unnecessary joints should be eliminated. Open joints should be located as far as is practical from critical structural components.The coupling of dissimilar metals should be avoided to minimize galvanic corrosion. High performance concrete has been used to help prevent the penetration of salt-contaminated water into structural components. This will extend the repair-free life of the concrete structure (i.e., the time period before the structure needs to be repaired).

4. GENERAL CONSTRUCTION PROVISIONSThere are several construction variables that influence the durability of concrete structures. These include concrete materials, placing, consolidating, curing and rebar placement. Poor construction practices can easily negate the best design provisions taken to produce a durable concrete structure. Good consolidation practices help to avoid segregation and honeycombing, while yielding a uniform concrete with low permeability. A well-consolidated concrete can be achieved through the use of proper construction techniques and equipment. Poor consolidation results in concrete with higher permeability and voids, cavities, and poor bonding. Voids, cavities, and areas of poor bonding aid in the corrosion process.




Proper consolidation of the concrete ensures that it is in intimate contact with the steel reinforcing bars. A good bond between the steel reinforcing bars and the surrounding concrete is critical for corrosion control. As a result of the intimate contact between the steel reinforcing bars and the concrete, the steel will be in the high-alkaline environment, necessary for the formation and maintenance of the passive oxide film. Exercise extra care when placing and consolidating concrete around embedded or partially embedded items so that water and chlorides do not have easy access to the steel reinforcing bars.

Concrete curing procedures are an important part of workmanship. Proper and adequate curing provides durable concrete through increased cement hydration. A minimum of 7 days of uninterrupted moist cure is recommended. Whatever the curing method used, the surface of the concrete must be kept wet. Alternating wet-dry cycles promotes cracking in the concrete. There are three general categories of curing methods. A continuous water cure is done by a continuous spray, ponded water, or saturated surface coverings (burlap). Curing compounds seal the surface of the concrete. Moisture barrier materials, such as plastic sheets or waterproof paper, cover the surface of the concrete. A continuous water cure supplies sufficient water to prevent the surface of the concrete from drying. Both membranes and moisture barriers work by preventing evaporation of the mix water from the surface of the concrete (ACI 222R-01).

The accurate placement of steel reinforcing bars ensures that an adequate concrete cover over the bars will be obtained. Methods for placing and tying bars to ensure proper cover include the use of chairs, spacers, and form ties. Allowances for tolerances in bending bars may also be needed. Reinforcing steel should be adequately tied to prevent it from moving from the desired location during concrete placement and consolidation. Reinforcement support and ties should have adequate strength to carry construction loading before and during concrete placement and to avoid excessive deflection of the reinforcing steel. All intersections around the perimeter of the reinforcing steel mat should be tied. Elsewhere within the reinforcing steel mat, the tie spacing should be not less than 0.6-m centers or every intersection, whichever is greater. Work platforms should be supported on the forms and not the reinforcing steel.

Mechanical finishing machines (screeds) are used to strike off the concrete to the desired profile grades. In order to not reduce the amount of concrete cover over the reinforcing steel bars, allowances for deflection, settlement, and camber need to be made. When the finishing machine is supported on rails, the rail supports need to be placed to minimize or eliminate any deflection of the rail between rail supports due to the weight of the finishing machine.

4.1. ConcreteIn new structures with good-quality concrete, the concrete can protect the steel reinforcing bars from corrosion for the service life of the structure. For steel in good-quality sound concrete –»uncontaminated» (little or no chlorides), un-carbonated, and un-cracked – the steel is passivated and no corrosion, or a corrosion rate that is very low, can be expected. Any corrosion-induced concrete deterioration is not likely to reach a point where repair or rehabilitation will be required during the expected service life of the structure.However, the concrete quality can be violated by either chemical or mechanical means. Chemical means are chloride diffusion, carbonation, sulfate attack, and alkali aggregate reaction, while the primary mechanical means is cracking. Cracks in concrete allow water, oxygen, and chlorides to enter the concrete at a faster rate and reach the reinforcing steel sooner than by the diffusion process alone. The material variables included:- Cementitious materials; mainly type, content and inclusion of SCM.- Aggregate type; mainly quality and chloride ion content.- Water-cement ratio.

The key to long-term durability of reinforced concrete structures is the use of Portland cement concrete with low permeability and adequate concrete cover. A concrete with low permeability has an improved resistance to chloride ion penetration or diffusion. This keeps chlorides, as well as water and oxygen, from reaching the steel reinforcing bars. An adequate concrete cover increases the amount of time required for any chlorides to reach the steel reinforcing bars.


A lower water-cement ratio generally makes concrete less permeable. Although a low water-cement ratio does not ensure that the concrete will have a low permeability. Concretes with the proper gradation and type of fine and coarse aggregates, supplementary cementing materials and low water to cement ratio will have a higher resistance to chloride penetration compared with those with low water-cement ratio alone. Concrete also needs to be properly proportioned and well-consolidated. A decrease in the water-cement ratio results in concrete with a reduced porosity and a reduced permeability. A reduction in water-cement ratio and the use of supplementary cementing materials, especially silica fume, are very effective strategies for reducing the permeability of the hardened concrete. With adequate cover, concrete with lower water-cement ratios perform better than those with higher water-cement ratios.Changes in the water-cement ratio do not significantly influence resistivity at an earlier age but significantly affect it at later age. Table 10 show the risk of corrosion as related to concrete resistivity. The improved performance of concretes with lower water-cement ratios is due to a reduction in concrete permeability and an increase in resistivity.

Table 10: Resistivity ranges related to the risk of corrosion

Supplementary cementing materials (SCM) can be used to enhance the corrosion-control potential of the concrete by reducing permeability. Some common admixtures used are fly ash, blast-furnace slag, and silica fume. SCM contribute to a reduction in permeability and a reduced chloride diffusion rate. The sources of ground granulated blast-furnace slag and fly ash should be evaluated for changes in their chemistry. Any changes can significantly affect the characteristics of the concrete and ultimately its performance. Concrete mixes containing silica fume are highly impermeable to chloride penetration and are resistant to the flow of corrosion currents due to their high electrical resistivity. Compressive strengths are also higher. Silica fume has been shown to offer the largest and most consistent reduction in penetration rates for chloride ions in concrete. However, these mixes are more susceptible to cracking. For silica fume mixes, a superplasticizer is needed to improve workability without increasing the water demand of the mix. Also, the use of ternary blends of mineral admixtures with Portland cement, such as slag and silica fume, proved to be very efficient in very aggressive environments.

Cement type appears to influence the diffusion of chloride ions through concrete. This is achieved by binding chloride ions and reducing its concentration from the pore water and reduces the amount of free chlorides available to participate in the depassivation and corrosion processes. The amount of free chloride ions in the pore water is more important than the amount of total chloride ions. Concrete mixes containing cements with high C3A contents and ground granulated blast-furnace slag exhibit a significantly greater ability to bind chlorides. Table 11 shows the limits set for chlorides in the ACI 318M-05. When concrete members are expected to be exposed to chlorides, it is advisable to keep any chlorides added to the concrete from the mix ingredients to a minimum. The chloride limits in concrete for new construction given by ACI 318M-05, are intended to minimize the risk of chloride-induced corrosion.

Table 11 Recommended chloride limits for new construction (ACI 318M-05)

Resistivity (kW.cm) Corrosion Risk

> 100 Nil

50 – 100 Low

10 – 50 Moderate

< 10 High




Corrosion can initiate at chloride concentrations as low as 0.71 kg/m3, approximately 0.15% for 400 kg of cement mix in dry conditions. For wet conditions and 400 kg of cement mix, the ACI criteria allows for chloride concentrations up to 0.40 kg/m3, which leaves little room for the ingress of additional chlorides from the surrounding environment.

4.2. Concrete AdmixturesLowering water-cement ratio and the inclusion of supplementary cementing materials generally makes concrete less permeable. All these will have a significant impact on the concrete’s workability. Reduced workability and the inability to produced well compacted concrete will negate the effect desired by lowering water-cement ratio and the inclusion of supplementary cementing materials. As partial compaction increases the porosity and reduced the concrete quality. Therefore, the use of concrete admixtures such as normal range water reducers (Type A, ASTM C494) and high range water reducers (Type F, ASTM C494) and superplasticizer (Type I, ASTM C1017) will help regain the concrete workability. Also, it is frequently the use of retarders in combination with workability enhancing admixtures, such as Type D admixtures (ASTM C494) and Type II (ASTM C1017). It should be emphasized that the dosage of the admixture to be used should be determined in accordance of the concrete mixture proportions and the manufacturer data sheet of the used admixture. Manufacturers are to be consulted regarding the type and dosage of each admixture type with respect to the concrete mixture proportions and the required property improvement. Also, manufacturers and admixtures data sheet are to be consulted regarding overdose of the admixture and the expected side effects. The side effect of the overdose differs from one type of admixture to other and for different manufacturer as well.In case of combining more than one admixture in a concrete mixture, manufacturer should be consulted as for the compatibility of proposed admixtures and also according to recommendations of ASTM C494.

Also, waterproofing admixtures could be used to improve the concrete impermeability characteristics. It is highly recommended to consult manufacturers for the compatibility of such admixtures with cement type and other admixtures used in the mix. Improper selection with respect to compatibility or over dosage of such admixtures usually results in unrecoverable side effects.

4.3. Corrosion InhibitorsCorrosion inhibitors are chemical admixtures added to Portland cement concrete mixes during batching, usually in very small concentrations, as a corrosion-protection measure. Corrosion inhibitors are a viable corrosion-protection measure for the long-term durability of both conventionally reinforced and prestressed concrete bridge structures. When used as part of a multiple-strategy corrosion-protection system, they are promising materials to delay the onset of steel corrosion.

Inhibitors are often used in combination with low-permeability concrete and usually they have the effect of increasing the threshold chloride concentration needed to initiate corrosion. Inhibitors may also reduce the subsequent corrosion rate after the initiation of corrosion, which ultimately leads to less corrosion-induced concrete deterioration.

Inhibitors may have an effect on the corrosion process after corrosion initiation. An insufficient dosage will have a negative impact on corrosion progression. Some inhibitors will have an effect on chloride transport and can reduce the rate of chloride ion migration.

Corrosion inhibitors are either inorganic or organic and, in general, are classified based on their protection mechanism. They can protect by affecting the anodic reaction, the cathodic reaction, or both reactions (mixed). An active type of inhibitor (anodic) facilitates the formation of an oxide film on the surface of the steel reinforcing bars.


There are different major commercially available corrosion inhibitors: • Rheocrete 222+ • PROTECTOSIL CIT • Ferrogard 901 • Sika CNI • Ferrogard 903 • Conplast CN • Conplast CNI

Also, there is imbedded corrosion inhibiting systems such as; Vector Galvashield XP (from Vector Corrosion Technologies – www.vector-corrosion.com), Mapeshield I (from MAPEI – ww.mapei.com), and SENTINEL-GL (from The Euclid Chemical Company – www.euclidchemical.com), which could be embedded in the concrete or the repaired portions besides the reinforcing steel to prevent corrosion of the reinforcement. These systems could be used in new construction and repaired structures.

• Rheocrete 222+Rheocrete 222+ is an organic corrosion-inhibiting concrete admixture. It is manufactured and marketed by BASF. The recommended dosage rate Rheocrete 222+ is 5 liters/m3. The dosage rate is not adjusted for the anticipated corrosiveness of the expected service environment. The dosage rate is designed to provide optimum corrosion protection of reinforced concrete structures in severe corrosion environments. It is typically added to the concrete batch water.

• PROTECTOSIL CITPROTECTOSIL CIT is an advanced organo functional silane based corrosion inhibitor. It is manufactured and marketed by BASF. It is low viscosity clear liquid, designed to be applied to the surface of reinforced concrete structures. The recommended dosage rate of PROTECTOSIL CIT is 600ml/m² applied in two or three coats Horizontal surfaces: 2 coats @300ml/m² Vertical or overhead surfaces: 3 coats @ 200ml/m².

• Ferrogard 901Ferrogard 901 is manufactured and marketed by Sika Corporation. It is combination of organic and inorganic inhibitors. It protects the steel reinforcing bars by forming a continuous monomolecular film on the steel surface and covers both the anodic and cathodic sites (mixed type). This film consists of an adsorbed layer of amino-alcohol that leads to the formation of insoluble iron oxide complexes. These stabilize the oxide surface and inhibit further corrosion. The film is typically 108- μm thick and also acts as a barrier to aggressive ions migrating through the concrete.

The recommended dosage rate for Ferrogard 901 is 12 kg/m3 of concrete. The admixture is typically added with the mixing water or added at the same time into the concrete mixer.

• Sika CNISika CNI is manufactured and marketed by Sika Corporation. It is a calcium nitrite-based admixture. The recommended dosage rate for Sika CNI is 10 to 30 lit/m3 of concrete depending on the severity of the corrosion environment. The admixture is typically added with the mixing water or added at the same time into the concrete mixer.

• Ferrogard 903Ferrogard 903 is manufactured and marketed by Sika Corporation. It is a surface applied corrosion inhibitor. It is based on organic and inorganic compounds. Its use both delays the start of the corrosion and reduces the corrosion rate. It is suitable for use in hot and tropical climates. The recommended dosage rate is 0.50 kg/m², and for very dense low permeability concrete lower dosage could be used but should not be less than 0.30 kg/m².

• ConplastCN & Conplast CNIConplast CN and Conplast CNI are manufactured and marketed by FOSROC. Both are inorganic inhibitors based on calcium nitrite. Both are supplied in solution form that could be dissolved in the mixing water. Conplast CNI is modified for good slump retention and retardation to offset the acceleration setting normally associated with calcium nitrite. Typical dosage levels range from 7.5 to 22.5 lit/m3 of concrete. Because of the high dosage level, the w/c ratio should be adjusted to include the water added to the mix in the admixture.




4.4. Externally Applied Protective BarriersThe application of external coating, surface treatment or barrier on the existing concrete provides a barrier that prevents the intrusion of chloride ions, moisture, and oxygen that are necessary for corrosion to continue. Coatings and surface treatments are effective means of achieving the design life and controlling concrete degradation (ACI 201.2R-01).

In selection of surface treatments/coatings for concrete durability, focus should be paid to field data/experience. The protection strategy will influence the selection of the concrete surface treatment systems. With a strategy where additional protection measures are used as “insurance”, the permanence of the surface treatments/coating is important but not critical. With a strategy where additional protection measures are integral parts in durability risk evaluation, the surface treatments/coatings may require to last the design life of the structure.

To be effective in protecting concrete, a barrier material should have certain basic properties as follows:• When the barrier material is exposed to chemicals from the environment, the chemicals should not cause swelling, dissolution, cracking, or embrittlement of the barrier material.• The chemicals should not permeate or diffuse through the barrier to destroy the adhesion between it and concrete.• The abrasion resistance should be adequate to prevent the barrier material from being removed during normal service.• The adhesive bond strength of a non-bituminous barrier to the concrete should be at least equal to the

tensile strength of the concrete at the surface; this bond is affected by the cleanliness of the interface when the barrier material is being applied.

There is no guarantee that materials made by different manufacturers will perform similarly, even when classified as the same generic type. In addition, the application characteristics, such as ease of applying the material to concrete, sensitivity to moisture on a concrete surface, or limited temperature application range, will affect performance.

The thickness of the barrier required will depend on the severity of the environment. Table 12 shows the general categories for protective barrier systems (ACI 201.2R-01). Barrier selection should be based on testing or past experience. If tests are to be conducted, the entire barrier system should be applied to concrete specimens before exposing them to the actual environment or one that simulates as closely as possible this environment. If a selection must be made before tests of sufficient duration (as agreed between manufacturer and user) can be conducted, the barrier supplier should be asked to supply fully documented case histories where his or her system has protected concrete under the same or similar environmental conditions. The selection of a reliable barrier manufacturer and applicator is as important as the selection of the barrier itself.

Most barrier materials are formulated for use over concrete will develop and maintain adhesive bond strength greater than the tensile strength of the concrete, provided that the surface is properly prepared. The surface should be free of loose particles, dirt, dust, oil, waxes, and other chemicals that prevent adhesion. Moisture within the concrete can affect the ability of a barrier system to adhere to the surface if water vapor diffusing out of the concrete condenses at the concrete-barrier interface before the barrier has had an opportunity to cure. Concrete should be dry before the barrier material is applied. Not only is surface moisture, but moisture within the concrete can also affect the ability of a coating to adhere to the surface. For some barrier systems, a qualitative moisture test for concrete is recommended and helpful.

Moisture content is considered excessive if moisture collects at the bond line between the concrete and the barrier material before the barrier has cured. This is evaluated by taping a 1.2 x 1.2 m clear polyethylene sheet to the concrete surface and determining the time required for moisture to collect on the underside of the sheet. The time for moisture to collect should be compared with the time required for the barrier material to cure, a value that should be supplied by the material manufacturer. If it cures in a time that is less than that required for moisture to collect, it can be concluded that the concrete is adequately dry. Also the ambient conditions, that is sunlight, temperature, and humidity, during the test should simulate, as much as practicable, the conditions existing during application and curing of the barrier.


Table 12: General categories for protective barrier systems (ACI 201.2R-01)

4.5. Corrosion-Resistant ReinforcementSteel reinforcing bars are subject to various severe environments before being placed in concrete (during transportation, storage, and installation) and in service while embedded in the concrete. During storage, reinforcing bars can be exposed to condensation, rain, harmful chemicals, and seawater. After the reinforcing bars are embedded in concrete, they are initially exposed to a high pH and moist environment, which, over time, is changed to high and low pH areas, high chloride, and moist environment.

The use of corrosion-resistant reinforcing bars can provide an additional layer of corrosion protection for reinforced concrete structures. Although there is a significant initial cost for corrosion resistant reinforcing bars, the increase in overall initial structure cost may be justified. An extended service life decreases life-cycle costs. When considering the consequences of unintended low concrete cover, poor curing, permeable concrete, concrete cracking, and harsh service environments, the use of corrosion-resistant materials may be very cost-effective, especially when repair of corrosion-induced deterioration is costly and/or hard to do. A 75 to 100 years design life can be achieved by extending the corrosion initiation period and reducing corrosion rates. The use of reinforcing material that is less sensitive to depassivation can extend the corrosion initiation period. A reduced corrosion rate results in a decreased amount of metal loss and extends the time period until subsequent cracking. The corrosion resistant reinforcing bars include galvanized steel bars, stainless steel bars and non-metallic reinforcing bars (FRP).

• Stainless steel barsStainless steel (eg. type 304 and 316) bar is undoubtedly more resistance than normal black steel bar against chloride induced corrosion of reinforcement (ACI 201.2R-01, ACI 222R-01, ACI 362.1R-00, ACI 365.1R-00 & ACI 301M-05). The selection of stainless steel reinforcement or normal steel reinforcement depends on cost and protection strategy. It is wasteful to consider stainless steel reinforcement as an insurance type protection.

Stainless steel reinforcement can be used in conjunction with black steel reinforcement. Practically there has been no reported problem of galvanic corrosion due to stainless steel/black steel connection in chloride-free concrete. In the presence of chloride, this can be a serious issue. Therefore, specifiers must ensure that in mixed usage mode, the black steel is situated beyond the estimated zone of chloride affected area in concrete during the design life.

severity of chemicalenvironment

Total nominal thicknessrange

Typical protective barrier systems Typical but not exclusive uses of protective systemsin order of severity

Mild Under 40 mil (1mm)

Polyvinyly butyral, polyurethane, epoxy, acrylic, styrene-acrylic copolymer asphalt, coal tar, chlorinated rubber, vinyl, neoprene, coal-tar epoxy, coal-tar urethane

• Protection against deicing salts• Improve freezing-thawing resisance

• Prevent staining of concrete• Use for high-purity water service• Protect concrete in contact with chemical

solutions haveing a pH as low as ٤, depending on the chernical

Intermediate 125 to 375 mil (3 to 9 mm)Sand-filled epoxy, sand-filled polyester, sand-filled polyurethane, bituminous materials

• Protect concrete from abrasion and intermittens exposure to dilute acids in chemical, dairy,and food-processing plants

Severe 20 to 350 mil (1 /2 to 6mm)Glass-reinforced epoxy, glass-reinforced polyester, procured neoprene sheet, plasticized polyvinyl chloride sheet

• Protect concrete tanks and floors during continuous exposure to dilute material (PH is bleow 3) organic acids, salt solutions, strong askalies

Severe20 to 280 mil (1 /2 to 7mm)

Over 250 mil (6mm)

Composite systems:(a) Sand-filled epoxy system topcoated with a pigmented but unfilied epoxy(b) Asphalt membrane covered with acid-proof

brick using chemical-resistant mortar

• Protect concrete tanks during continuous or

intermittent immersion, exposure to water, dilute acids, strong alkalies, and salt solutions

• Protect concrete from concentrated acids or combinations of acids and solvents.




• Galvanized Reinforcing Steel BarsHot-dipped galvanized coatings for reinforcing steel in concrete have been used since the 1940s. ASTM A767, “Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement” specifies the requirements for the galvanized coating. A Class I coating has a zinc coating weight of approximately 1,070 g/m2 and a Class II zinc coating has a coating weight of approximately 610 g/m2.

Both laboratory experiments and ongoing field surveys of existing structures has demonstrated that the galvanizing of steel rebar extends the service life of many types of reinforced concrete structures (ACI 201.2R-01, ACI 222R-01, ACI 362.1R-00, ACI 301M-05 & ACI 222.2R-01).

Though the cost of galvanizing may double the price of the reinforcing steel itself, the overall premium for the use of galvanized rebar in mass concrete is often less than 10% of total concreting costs. This premium reduces considerably in buildings and large constructions.

Hot-dip galvanizing produces a tough and adherent coating on steel which resists abrasion and fairly heavy handling, and which can be stored, handled and transported in much the same way as black steel. Other than following general guidelines when bending and fabricating galvanized steel, no special precautions are required to protect the coating against light mechanical damage. In the design and construction of reinforced concrete utilizing galvanized rebar, the same design parameters and construction practices are used that apply to conventional black steel reinforcement.

Though zinc can be applied to steel by a number of commercial processes, each producing a characteristic range of thickness and coating structure, hot dipping should always be specified. Hot dipping involves the immersion of the steel bars in molten zinc at about 450°C and holding for a sufficient period to allow the development of a metallurgically bonded coating of zinc and zinc-iron alloys on the base steel. The minimum specified coating thickness on steel products greater than 5 mm thick should be 84 microns, which is equivalent to a coating mass of 600g/m2 per surface. In routine processing, hot dipping results in coatings that would generally be at least 100120- microns thick. Each successive layer of the coating from the steel substrate outwards contains a higher proportion of zinc and the layers are harder than ordinary steels. This feature, combined with the good adherence of the coating, gives the coating its good abrasion and impact resistance.

It is generally most economical to process straight lengths of reinforcing bar with all fabrication being done after galvanizing. During fabrication, the tendency for cracking and flaking of the coating in the area of the bend increases with bar diameter and the severity and rate of bend. The use of large bend diameters, typically 58-X the bar diameter, can minimize damage to the coating. Some cracking and flaking of the coating at the bend is common, and shall not be the cause for rejection. Should repairs be required, an organic zinc rich paint containing a high proportion of metallic zinc is generally used. As an alternative, to post-galvanizing fabricating, bars bent to special shapes (ties, stirrups etc) or complete sections such as prefabricated column forms or precast cages can be galvanized. This offers the distinct advantage of little or no fabrication-related damage to the coating. Difficulties in the marking, handing and transportation of prefabricated pieces or sections, and possible scheduling delays, make this option somewhat more expensive.

• Non-Metallic Reinforcing BarsRecently, composite materials made of fibers embedded in a polymeric resin, also known as fiber-reinforced polymers (FRP), have become an alternative to steel reinforcement for concrete structures. Non-metallic reinforcements are alternative solution to solving the problem of managing chloride attack. These include glass, carbon or aramid set in a suitable resin to form a rod or a grid. The use of these non-metallic reinforcements has shown good performance in several projects worldwide (ACI 440R-07 & ACI 440.1R-06).

Fiber reinforced polymer (FRP) composite rebar have the potential to address the corrosion deficiency. FRP rebar can be used as non-prestressed reinforcement in concrete for members subjected to flexure, shear, and compression loadings. FRP composite rebars are totally resistant to chloride ion attack, offer a tensile strength of 1½ - 2 times that of steel, and weigh only 25% of the weight of equivalent size steel rebar. The following are the main features and benefits for FRP reinforcing rebars:


• Non-Corrosive: will not corrode when exposed to a wide variety of corrosive elements including chloride ions. • High Strength-to-Weight Ratio: provides good reinforcement in weight sensitive applications. • Non-Conductive: provide excellent electrical and thermal insulation. • Excellent Fatigue Resistance: performs very well in cyclic loading situations. • Good Impact Resistance: resists sudden and severe point loading. • Magnetic Transparency: not affected by electromagnetic fields; excellent for use in MRI and other

types of electronic testing facilities. • Lightweight: easily transported in the field without need for expensive heavy lifting equipment.

Commercially available FRP reinforcing materials are made of continuous aramid (AFRP), carbon (CFRP), or glass (GFRP) fibers embedded in a resin matrix. Typical FRP reinforcement products are grids, bars, fabrics, and ropes. The bars have various types of cross-sectional shapes (square, round, solid, and hollow) and deformation systems (exterior wound fibers, sand coatings, and separately formed deformations). Figure 2 shows samples of different GFRP reinforcing bars.

The physical characteristic of the surface of the FRP bar is an important property for mechanical bond with concrete. Three types of surface deformation patterns for FRP bars commercially available are shown in Figure 3.

Bending FRP rebars made of thermoset resin should be carried out before the resin is fully cured. After the bars have cured, bending or alteration is not possible due to the inflexibility or rigid nature of a cured FRP bar. Because thermoset polymers are highly cross-linked, heating the bar is not allowed as it would lead to a decomposition of the resin, thus a loss of strength in the FRP.

The mechanical behavior of FRP reinforcement differs from the behavior of steel reinforcement. Therefore, changes in the design philosophy of concrete structures using FRP reinforcement are needed. FRP materials are anisotropic and are characterized by high tensile strength only in the direction of the reinforcing fibers. This anisotropic behavior affects the shear strength and dowel action of FRP bars, as well as the bond performance of FRP bars to concrete. Furthermore, FRP materials do not exhibit yielding; rather, they are elastic until failure. Design procedures should account for a lack of ductility in concrete reinforced with FRP bars.

Figure 2. Commercially available GFRP

Figure 3. Surface deformation patterns for commerciallyFRP rebar (ribbed-sand coated-ribbed and sand coated)




4.6. Cathodic prevention and protectionCathodic protection (CP) is a corrosion control method that imposes an external voltage on the steel surface in a manner that forces the steel to become cathodic (reduction reactions are favored and anodic reactions, which result in metal loss, are decreased), thereby mitigating corrosion. In simple terms, CP transfers the oxidation (anodic) reactions, which result in metal loss (and thereby corrosion) of the rebar, over to the anode of the CP system. Therefore, selection of the proper anode material for the application is critical, since anode failure results in CP system failure (ACI 201.2R-01, ACI 362.1R-00, ACI 365.1R-00 & ACI 222.2R-01).

However, while maintenance cost can be low, installation of this measure needs high capital investment. The high capital cost may be “justifiable” for new structures of high importance (disruption in service can result in high economic loss) or repair and maintenance is difficult or impossible due to limited access.

The primary strength of CP is that it can mitigate corrosion after it has been initiated. Although CP is often placed on pipelines, underground storage tanks, and other structures during construction, it is generally installed on bridge members only after corrosion has initiated and some amount of deterioration has occurred. The primary reason for not installing CP systems on bridge components during construction is that corrosion often does not initiate for 10 to 20 years following construction; therefore, the CP system maintenance and a large portion of the CP system design life would be used on a structure that is not corroding. When properly applied and maintained, CP mitigates corrosion of the reinforcing steel and extends the performance life of concrete structures.

CP systems are characterized by the source of the driving voltage that forces the rebar to become cathodic with respect to the anode. The two principal methods for applying CP are impressed-current CP and sacrificial (galvanic) anode CP. In an impressed-current CP system, an external power source is used to apply the proper driving voltage between the rebar and the anode. For impressed-current systems, the anode can be a wide range of materials since the driving voltage can be adjusted to suit the application and anode material selected. For a sacrificial anode CP system, the driving voltage is created by the electrochemical potential difference between the anode and the rebar. Therefore, selection of the anode material is more limited.

Two features of cathodic protection, which can be usefully adopted in formulating protection measures for concrete structures, are continuous monitoring of steel condition and preparation of reinforcement for electrical continuity. These are simple procedures involving embedding reference electrode (or corrosion sensor or even some indirect means such as embedded studs for resistivity measurement) in concrete at areas of high corrosion risk of reinforcement. Continuous/regular monitoring of the corrosion condition of the reinforcement can provide critical inputs into the decision making process.

5. SUMMARYTo enhance the durability of existing and future concrete structures can be summarized in the following:• Exposure conditions for exterior and underground structures in Dubai are classified as severe to

very severe depending on the availability of moisture.• Specification of minimum concrete quality (i.e. minimum strength, or max w/c ratio, or minimum

cement content, or cement type or minimum concrete cover or maximum crack width) for expected exposure conditions.

• Guideline for mix design of minimum concrete quality for Dubai exposure conditions are: o Minimum cement content of 350 kg/m3, maximum w/c ratio of 0.45, and minimum compressive strength of 30 MPa are considered the minimum concrete quality for severe exposure conditions in Dubai. o Minimum cement content of 375 kg/m3, maximum w/c ratio of 0.40 and minimum compressive

strength of 35 MPa are considered the minimum concrete quality for very severe exposure conditions in Dubai.


o Implementation of supplementary cementing materials (SCM) such as slag, silica fume and fly ash is highly recommended in very severe exposure conditions and for desired extended service life.

• Guideline for structural design for Dubai exposure conditions are: o A minimum concrete cover of 30 – 50 mm is required for exterior, underground and unprotected structures. For Protected Structures a minimum concrete cover of 20 – 30 mm is required. o For interior air conditioned sealed environment the minimum concrete cover is 15 – 20 mm. o A maximum crack width of 0.20 mm is allowed for underground structures without water table. In case of surface applied protection such as finishes and surface sealants and barriers the crack width could be increased to 0.30 mm. o A maximum crack width of 0.20 mm is allowed for underground structures with water table

and water retaining structures provided the application of surface protection such as finishes, surface sealants and barriers.

o A maximum crack width of 0.10 mm is allowed for prestressed concrete structures. o For un-exposed superstructure elements the maximum crack width allowed is 0.30 mm. o For surface finished or protected concrete structures, it is required the removal of the surface finish/protection (i.e. window opening) in order to assess the crack width at the concrete surface.• Poor quality construction is a frequent source of inadequacies in most of concrete structures. • Improvements in construction practice and supervision. Priority should be given to quality control during construction.• Durability tests should be specified for concrete acceptance besides strength tests, such as

Rapid Chloride Permeability Test (ASTM C1202), Rate of Water Absorption (ASTM C1585), Water Absorption Test (ASTM C642) and Water Penetration Test (DIN 1048).

• Concrete resistivity is a reasonable test to evaluate reinforcement corrosion protection for the concrete.• Implementation of corrosion protection system(s).• A multiple corrosion protection systems incorporating more than one protection system in

conjunction with each other can be implemented in concrete structures in extremely severe environment or for desired extended service life.




6. REFERENCES

• ACI 201.2R-01, «Guide to Durable Concrete», ACI Manual of Concrete Practice 2008.

• ACI 222R-01, «Protection of Metals in Concrete against Corrosion», ACI Manual of Concrete Practice 2008.

• ACI 222.2R-01, «Corrosion of Prestressing Steel», ACI Manual of Concrete Practice 2008.

• ACI 224R-01, “Control of Cracking in Concrete Structures”, ACI Manual of Concrete Practice 2008.

• ACI 301M-05, «Specifications for Structural Concrete», ACI Manual of Concrete Practice 2008.

• ACI 318M-05, “Building Code Requirements for Structural Concrete & Commentary”, ACI Manual of Concrete Practice 2008.

• ACI 362.1R-00, «Guide for the Design of Durable Parking Structures», ACI Manual of Concrete Practice 2008.

• ACI 365.1R-00, «Service-Life Prediction – State-of-the-Art Report», ACI Manual of Concrete Practice 2008.

• ACI 440R-07, «State-of-the-Art Report on Fiber Reinforced Plastic (FRP) Reinforcement for Concrete», ACI Manual of Concrete Practice 2008.

• ACI 440.1R-06, «Guide for the Design and Construction of Concrete Reinforced with FRP Bars», ACI Manual of Concrete Practice 2008.

• BS 53281:1997-, “Concrete-Part 1: Guide to Specifying Concrete”.

• BS 81101:1997-, “Structural Use of Concrete-Part 1: Code Practice for Design and Construction”.

• BS 81102:1997-, “Structural Use of Concrete-Part 1: Code Practice for Special Circumstances”.

• BS-EN 2061:2000-, “Concrete-Part 1: Specification, Performance, Production and Conformity”.

• CIRIA C5772002-, “Guide to the Construction of Reinforced Concrete in the Arabian Peninsula”, the Concrete Society Special Publication CS 136, Editor Mike Walker.





IX –REPAIR AND STRENGTHENING OF STRCUCTURESDesign Life Extension of RC Structure

121

IX - REPAIR AND STRENGTHENINGOF STRCUCTURES


122 IX –REPAIR AND STRENGTHENING OF STRCUCTURES

3. INTRODUCTIONAlthough durable, buildings constructed using reinforced and prestressed concrete have a finite service life. When exposed to harsh environments and chemicals, these structures may experience significant deterioration, which typically occurs in the form of steel corrosion, concrete spalls, delamination and cracks. Some of the reinforced or prestressed concrete structures in Dubai have reached the end of their planned service life and shown deterioration in the form of steel corrosion, concrete cracking and spalling. In addition, much of the concrete is custom made for almost every job, using local materials of varying quality, some designs that are not standard and accelerated construction processes that sometimes sacrifice quality in the interest of meeting a schedule. In addition, many of these structures were built to carry loads that are significantly smaller than the current needs. These factors indeed leave Dubai Municipality with the challenge of the assessment of and adopting effective and economical repair and strengthening techniques and systems. Implementing such systems and techniques is rather complicated as most of the affected structures are occupied. However, success can be obtained if the strengthening techniques are tailored to serve the structural use without interfering with its occupants. The concrete repair, protection and strengthening industry is driven by deterioration of, damage to, and defects in concrete structures.

A typical value of corrosion related loss in the area of the reinforcement of 10% is the trigger for repair of the reinforced concrete structure, otherwise adding reinforcement will be required as part of the repair. Therefore it is a wise decision to start repair before reaching such corrosion related steel reinforcement area loss. Most of the repair strategies presented here are for such a case where corrosion related loss is less than 10%.

4. STRATEGY OF STRUCTURAL REPAIR AND REHABILITATIONFlow chart in Figure 1 provides useful guidance for the strategy that can be followed for structural repair. Interestingly, one of the most severe and widespread problems in concrete is the internal damage caused by the corrosive action of external chlorides on reinforcing or prestressing steel embedded in concrete. Corrosion problems are basically caused by corrosion-process by-product (rust) that expands up to eight times its original volume, thus creating internal pressure, which causes the concrete to crack and spall. If not addressed at early stages, corrosion will continue to grow rapidly, ultimately creating a safety issue due to falling concrete and loss of strength.

The assessment, design and implementation of a durable repair to an existing structure are indeed more complex than for new construction. In addition to the unknown state of existing structural materials, the degree to which repair materials and the existing material will act as a composite and share loads must be addressed. Below are the steps for the repair, protection and strengthening of a reinforced concrete structure.

Step 2: Management Strategy Options • Do nothing and monitor• Re-analyse the structural capacity and downgrade as necessary• Prevent or reduce further deterioration• Strengthen, repair or protect all or pat of the structure• Reconstruct all or part of the structure• Demolish all or part of the structure

Step 1: Assess damage• Health and Safety risks• Present condition• Original design approach• Environment and contamination• Conditions during construction and service• Conditions of use• Future use


Step 3: Factors affecting Management Strategy • Consider intended use, design life, service life, appearance and performance• Compare with service life of the protection & repair works and repeat cycles• Whole life cost of the strategies, including future inspection & maintenance• Condition of substrate and methods of preparation• Effect on structural load paths during and after repair• Health & Safety considerations for occupiers, users & third parties• Future environmental exposure of the structure and its mitigation

Figure 1. Strategy of Structural Repair




Step 4: Choose Repair Principle(s) Defects in Concrete • Protection against ingress • Moisture control • Concrete restoration • Structural strengthening • Physical resistance • Resistance to chemicals

Reinforcement corrosion • Preserving or restoring passivity • Increasing resistivity • Cathodic control • Cathodic protection • Control of anodic areas

Step 5: Choose Method(s) • Choose separate principles or in combination• Appropriate to type and cause (or combinations) and extent of defects• Appropriate to future service conditions• Appropriate to protection or repair option chosen• Availability of products and systems

Step 6: Choose Materials • Minimum performance characteristics for all intended uses• Minimum performance characteristics for certain intended uses• Performance characteristics for specific applications• Health & Safety of materials and compliance with fire regulations• Competence of personnel in using materials

Step 7: Specify ongoing requirements • Record of protection or repair works carried out including any testing• Instructions on inspection and maintenance to be undertaken during the remaining design life of

the repaired part of the concrete structure

4.1. Concrete Repair StepsThe steps for repair can be summarized to three main steps, these are:• Concrete surface preparation• Application of a suitable repair system• After repair

4.2. Concrete Surface PreparationSurface preparation provides a reasonable strong concrete surface, which is free from harmful substances. The repair surface must be free of loose, weak, cracked or otherwise damaged concrete.


Mechanical techniques for surface treatment of concrete:• Chipping: Jackhammer, hammer and chisel• Hammering: Needle gun, hammering machine• Milling: Miller• Grinding: Grinder• Brushing: Wire brushing, brushing machine• Shotblasting: Shotblasting machine• Free blasting: Gritblasting, sandblasting• Vacu-blasting: Gritblasting with suction unit attached• Water-abrasive blasting: Water-grit/sand blasting• Pressure water blasting: < 40 MPa• High pressure water blasting: 40 – 120 MPa• Ultra high pressure water blast.: 150 – 300 MPa• Water-vacu blasting

The figures below show demonstrative photos of some of the aforementioned techniques.

• Grinding (Figure 2)

Figure 2: Grinding

• Shot-blasting (Figure 3)

Figure 3: Shot-blasting




• Injection with Epoxy systems (EP) or injection with Microcement (CEM) (Figure 5)

Figure 5: Epoxy injection

• Sealing crack repair: Injection with Polyurethane Systems or Injection with Acrylic Systems (Figure 6)

Figure 6: Crack sealing

• Re-bonding crack repairs: Stitch pinning with Epoxy Systems (EP) (Figure 7)

Figure 7: Crack re-bonding

• Ultra high pressure water blasting (Figure 4

Figure 4: Ultra high pressure water blasting


After repairAll cement based repair systems require after treatment by means of keeping the surface moist for a certain time after application. If after treatment is not carried out properly this can jeopardize the success of the repair.

b. Structural RepairRepairing the damaged surfaces of concrete can restore the structural function; protect the surface itself or the underlying concrete and reinforcement from aggressive environments, or restore any lost performance requirements including drainage and abrasive resistance. All repairs require initial surface preparation, which might include abrasive or hydro blasting, chipping, milling, sanding or chemical treatments. Systems for repairing surfaces include overlaying, resurfacing, formed repairs, hand-troweled mortars, cast-in-place repairs, shotcrete and, in some cases, full section replacement in case of member collapse as shown in Figure 8.

4.3. Application of a suitable repair systemIt is found convenient here to categorize repair into two types; namely crack repair and structural repair. Crack repair is used when the defected structural member can be treated using crack injection, sealing, or re-bonding. If the cracks are severe or as a result of structural damage where the aforementioned crack repairs will not be effective; then we can use structural repair. In structural repair, additional strengthening materials are usually applied to the defected structural member to restore its capacity.

a. Crack repairThe choice of a crack repair system depends on the overall requirements regarding the concrete crack repair. The following table provides different alternatives for crack repair:

Repair with Concrete Repair with Shotcrete

Repair with Polymer Cement Concrete (PCC) Repair with Polymer Concrete (PC)

Table (1): Alternatives for Crack Repair




Figure 8: Cloumn Failure

4.4. ProtectionProtection techniques are designed to extend the life of the structure by protecting it from the attack of an aggressive environment. Systems are available in the form of coatings, sealers, membranes, liners, cathodic protection and overlays.

4.5. WaterproofingWaterproofing techniques prevent water from entering or exiting structures through cracks, joints or failed water stops (Figure 9 left). Systems include replacement joints and sealants, waterproofing membranes and crack grouting (Figure 9 right).

Figure 9: Water proofing and crack grouting (Vision 2020)

4.6. Common Structural Strengthening TechniquesStrengthening is the process of adding or restoring capacity to a member or structure (Figure 10). Techniques include the addition of steel, FRP composite systems, concrete or other special materials to existing members providing for additional strength and capacity of the structure.

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Figure 10: Structural repair example (Vision 2020)

Structural strengthening can be subcategorized to two main cases:A) Repair: where there is• Insufficient reinforcement• Corrosion of reinforcement• Structural/fire damage

B) Adjustment to changing service conditions such as:• Excessive deflections• Change in use• Seismic upgrade• Design mistakes

Many school buildings that were originally constructed for a specific use now are being renovated or upgraded for a different application that may require higher load-carrying capacity. Typical examples of changing uses include the upgrade of parking garages and access ramps to carry the heavier loads of fire trucks and emergency vehicles; the conversion of administrative buildings to storage areas or classes with heavier load demands; and the installation of high-density filing systems in schools and education administrative offices. As a result of these higher load demands, existing structures need to be reassessed and may require strengthening to meet heavier load requirements.

The structural upgrade of concrete structures can be achieved using one of many different upgrading methods such as span shortening, externally bonded steel, fiber-reinforced polymer (FRP) composites, external or internal post-tensioning systems, section enlargement, or a combination of these techniques. Similar to concrete repair, strengthening systems must perform in a composite manner with the existing structure to be effective and share the applied loads. The following gives a brief description of these methods and case-study applications. The following table includes demonstrative photos of structural repairs.

4.6.1. Strengthening with FRP compositesFiber-reinforced polymer (FRP) systems are paper-thin fabric sheets bonded to concrete members with epoxy adhesive to increase their load-carrying capacity significantly. Usually carbon-based, these systems have been used extensively in the aerospace, automotive and sport-equipment industries, and are now becoming a mainstream technology for the structural upgrade of concrete structures. Important characteristic of FRPs for structural repair and strengthening applications include their non-corrosive properties, speed and ease of installation, lower cost, and aesthetic appeal. Examples of FRP strengthening are demonstrated in figure (11) below.



Figure 11: FRP Strengthening

• Column Strengthening • Beam strengthening • Slab strengthening

As with any other externally bonded system, the bond between the FRP system and the existing concrete is critical, and surface preparation is very important. Typically, installation is achieved by applying an epoxy adhesive to the prepared surface, installing the FRP fabric into the epoxy and then applying a second layer of the epoxy adhesive. After curing, the FRP composite will add considerable capacity to the element despite the fact that it is a very thin laminate. This is because the carbon FRP has tensile strength approximately 10 times that of steel. Figure 12 shows a schematic for the structural strengthening of a utility tunnel. The utility tunnel roof originally functioned as a pedestrian walkway.

Figure 12: Tunnel slab FRP strengthening (Vision 2020)

A new dormitory structure required the walkway to be the primary access for emergency vehicles. Analysis of the tunnel›s top slab revealed it did not have adequate strength to carry loads from fire trucks and other emergency vehicles. A structurally efficient, easy to install and cost-effective strengthening option was achieved by using externally bonded FRP sheets. The strengthening solution consisted of carbon FRP sheets bonded to the bottom of the slab, serving as additional bottom tension reinforcement, as shown in Figure 13.

Figure 13: Carbon FRP fabric on slab underside (Vision 2020)



In addition, the overhanging portions of a slab were strengthened using carbon FRP bars epoxy-bonded in grooves made on the slab›s top side. This technique is more appropriate than FRP sheets, because the bars were bonded below the surface, thereby avoiding traffic damage to the externally bonded reinforcement (see Figure 14).

Figure 14: Installation of FRP rod (Vision 2020)

4.6.2. Span shorteningSpan shortening is accomplished by installing additional supports underneath existing members to reduce the span length. Materials used for span-shortening applications include structural steel members and cast-in-place reinforced concrete members, which are quick to install. Connections can be designed easily using bolts and adhesive anchors. Span shortening may result in loss of space and reduced headroom. An example of this upgrading method is shown in Figure 15. The structural steel system shown was installed on a parking deck to shorten the span and carry part of the load, transferring it to the existing supporting system.

Figure 15: Parking garage using span shortening (Vision 2020)

4.6.3. Bonded steel elements Strengthening concrete members by using bonded steel plates was developed in the 1960s in Switzerland and Germany. In this method, steel elements are glued to the concrete surface by a two-component epoxy adhesive to create a composite system. The steel elements can be steel plates, channels, angles or built-up members. Steel elements bonded to the sides or bottom of a structural member can improve its shear or flexural strength.




In addition to epoxy adhesive, mechanical anchors typically are used to ensure the steel element will share external loads in case of adhesive failure. The exposed steel elements must be protected with a suitable system immediately following installation. Regardless of the specified corrosion protection system, its long-term durability properties and maintenance requirements must be fully considered.

Figure 16 illustrates a schematic for the strengthening of a roof system of an elementary school in New Jersey. The school administration wanted to install skylights on the existing roof. The roof consisted of prestressed concrete hollow planks. Installation of the skylights required cutting openings in the planks that would reduce their load-carrying capacity. This issue was resolved by designing a hybrid strengthening system composed of FRP fabric and steel elements. The externally bonded FRP strengthened the planks to be cut, while the steel elements tied the plank to the adjacent ones, thus creating a new unit consisting of three planks with adequate capacity. In addition to the fast application of this system, this was a less expensive solution that was also aesthetically pleasing.

Figure 16: Schematic of the hybrid strengthening system (Vision 2020)

4.6.4. External post-tensioningThe external post-tensioning technique has been effectively used to increase the flexural and shear capacity of both reinforced and prestressed concrete members since the 1950s. With this type of upgrading, active external forces are applied to the structural member using post-tensioned (stressed) cables to resist new loads. Because of the minimal additional weight of the repair system, this technique is effective and economical, and has been employed with great success to correct excessive deflections and cracking in beams and slabs, parking structures and cantilevered members. The post-tensioning forces are delivered by means of standard prestressing tendons or high-strength steel rods, usually located outside the original section. The tendons are connected to the structure at anchor points, typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the structural member, or reinforced concrete blocks that are cast in-situ. The desired uplift force is provided by deviation blocks, fastened at the high or low points of the structural element. Prior to external prestressing, all existing cracks are epoxy-injected and spalls are patched to ensure prestressing forces are distributed uniformly across the section of the member.

Figure 17 illustrates an external post-tensioning system used to strengthen prestressed double tees damaged by vehicular impact. Four double-tee stems on an overpass located on the premises of a university in Washington, D.C., were damaged when the driver of an over-height truck failed to observe the posted height restriction.


Figure 17: Schematic for external post-tensioning system (Vision 2020)

The four stems suffered excessive concrete cracking and spalling, and damage occurred to some of the internal prestressing steel.

Proposed solutions included replacing the damaged double tees with new ones and installing a steel frame underneath for support. Both options would render the overpass out of service for a longer-than-desired period. The option of an external post-tensioning system was more economical, required less time to complete, and allowed for a strengthening system that provided active forces and therefore was more compatible with the existing construction.

After all cracks were injected, the sides of the stems were formed and new concrete was cast to restore the integrity of the stems. The strengthening system was then installed, and - after the concrete cured - the external strands were stressed according to the engineer-specified forces. This structural-strengthening option was fast and effective, saving the owner a considerable amount in construction and operation costs.

4.6.5. Section enlargementThis method of strengthening involves placing additional «bonded» reinforced concrete to an existing structural member in the form of an overlay or a jacket. With section enlargement, columns, beams, slabs and walls can be enlarged to increase their load-carrying capacity or stiffness. A typical enlargement is approximately 50 to 75mm for slabs and 75 to 150mm for beams and columns.

Figure 18 depicts details of a section enlargement used to increase the capacity of a main girder in a university parking garage. The girder was re-evaluated because of a change in the required loading and found to be deficient in flexure and shear. To correct the deficiency, additional flexural and shear steel were added. The entire beam was then formed and a 4-inch jacket of concrete was cast to enlarge the section.

Figure 18: Beam strengthening using section enlargement (Vision 2020)




Caution The effects of strengthening or removing part or all of a structural element - such as penetrations or deteriorated materials - must be carefully analyzed to determine its influence on the global behavior of the structure. Failure to do so may overstress the structural element surrounding the affected area, which can lead to a bigger problem and even localized failure. With upgrade projects, contractors also must deal with critical issues related to access to the work area, constructability of the repair, noise and dust control, and type of construction materials that may not be as critical for new construction projects.

4.7. Effect of Corrosion on Bond CharacteristicsThe bond strength typically increases with an increase in corrosion level up to a certain level prior to cracking of the concrete cover after which the bond strength took place, Figure 19. This reduction is significant in the absence of a confining reinforcement. The point where the trend of the bond strength changes from an increase to a decrease with an increase in corrosion depends on the cover-to-bar diameter (c/d) ratio. The change typically occurs at almost half of the amount of corrosion required to cause cracking of the concrete cover.

Figure 19: Effect of corrosion on bond strength (fib 2000)


5. REFERENCES

• fib 2000, ‘’Federation International du Beton (fib) (2000). Bond of Reinforcement in Concrete’’

• International Code Council, 2003 International Building Code, International Code Council, Country Club Hills, IL, 2002, 656pp.

• International Code Council, 2000 International Building Code, International Code Council, Falls Church, VA, 2000, 756 pp.

• International Code Council, 2006 International Existing Building Code, International Code Council, Country Club Hills, IL, 2006, 288pp.

• MacGregor, D.C., and Riley, G.J., «New Requirements for Old Buildings,» Structure, V.13, No.9, Sept. 2006, pp.4447-.

• Vision 2020: A Vision for the Concrete Repair, Protection, and Strengthening Industry, ACI Strategic Development Council, Farmington Hills, MI, 2006, p.17.




X - REPAIR OF POST-TENSION STRUCTURESDesign Life Extension of RC Structure

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X - REPAIR OF POST-TENSIONSTRUCTURES


138 X - REPAIR OF POST-TENSION STRUCTURES

1. INTRODUCTIONEvaluation of grouted post-tensioned tendons is a difficult undertaking and no single method can be used to fully describe the damage or life expectancy of a tendon.

Corrosion of unbonded and even the bonded tendons is common for post-tensioned structures exposed to aggressive environments. Insufficient cover over the tendons and lack of corrosion protection at anchorages combined with exposure to chlorides through deicing materials and coastal air lead to the corrosion of post-tensioning systems.

Barrier cables used in parking garages are similar to post-tension cables in concrete slabs and are subject to the similar deterioration mechanisms. In some cases the failures occur more quickly since the cables are directly exposed to the elements and do not have the added protection provided by concrete cover. Replacing barrier cables and anchors requires the same expertise as repairing post-tensioning tendons.

2. TYPES OF CRACKS IN POST-TENSIONED STRUCTURESCrack types presented in section 3 of chapter III may occur also in post-tensioned structures. Post-tensioned structures are also susceptible to the following additional crack types:

• Cracks due to Corrosion of the Post-tensioning Tendons: o Post-tensioning tendons may corroded due to penetration of water and/or chlorides through broken sheathing at locations where the concrete cover is eroded or broken as shown in figure 1. Parking garages and waterfront buildings are very vulnerable to corrosion of tendons. Figures2 and 3 show concrete spalling and the tendons corrosion.

• Cracks due to Incorrect Placement of the Post-tensioning Cables o Concrete cracking along the lines of tendons may occur due to incorrect placing of the post- tensioning cables as shown in figure 4. Improper placement of the cables can damage the member by magnifying the stresses from service loads rather than counteracting these stresses.

• Cracks due to Elastic & Plastic Shortening o Lack of design consideration of volume changes in members caused by elastic and plastic (creep) shortening would also result in cracking of the post-tensioned members. Cast-in- place post-tensioned construction that does not permit shortening of the prestressed member is susceptible to cracking in both the member and the supporting structure as shown in figure 5. The columns supporting the post-tensioned element are subjected to horizontal pulling in opposite directions thereby causing severe shear cracks. The shear is also aggravated by the daytime solar heating if the structure is directly exposed to sun.

• Cracks due to Improper Design o Concentration of high compressive stresses at the end anchorage block due to improper design would cause crushing of concrete (figure 6). o Insufficient amount of post-tensioned cables or insufficient stressing of cable would cause excessive deflection and/or flexural cracking at tension side o Over stressing of cables or unnecessary increase in the amount of tendons would result in crushing of concrete at compression side. o Insufficient stirrups would result in shear cracks

• Accidental Damage o Careless floor coring during utility installation or maintenance would cause damage to the post-tensioned cables.


Figure 1: Corrosion of unbounded post-tensioning strands (Emmons 1993)

Figure 2: Spalling of concrete (Bondy 2006)

Unprotected strand without Protective Sheathing




Figure 3: Corroded tendons (Bondy 2006)

Figure 4: Concrete cracking caused by improper draping of post tensioning cables (Emmons 1993)

Figure 5: Shear cracking in concrete columns due to horizontal pulling caused by pre-stressing (Emmons 1993)

3. REQUIREMENTS FOR NEW POST TENSION CONSTRUCTION3.1. Grouting requirementsThe PTI Specification (PTI Committee on Grouting Specifications 2003) classified the requirements into four requirements:

1. Materials requirements— Grout consists of cement and water and may also include chemical or mineral admixtures as well as fine sand. Pozzolans used in concrete may also be used in grout. Silica fume and fly ash, both Class C and Class F, are the most commonly used. Because fly ash may vary considerably from different sources, a consistent source should be used to achieve similar field properties to the ones found during mixture development in the lab. Fly ash can potentially reduce bleed, increase workability, and reduce permeability.


Silica fume has been shown to improve corrosion protection in concrete by reducing the permeability of the concrete. In grouts, however, the particles may agglomerate because the coarse aggregate that aids in mixing is absent. Recent testing has shown better performance with interground silica fume cements.

Figure 6: Crushing of concrete due to concentration of stresses at the end anchorage of tendons

Chemical admixtures used in grout have included high range water-reducing admixtures, anti-bleed admixtures, expansive admixtures, and corrosion inhibitors. High-range water-reducing admixtures help achieve workability at low water-cementitious material ratios. For constant water content, adding only water reducers or high-range water reducing admixtures usually increases bleed, so the admixture should not be added beyond the mixture design amount.

Anti-bleed admixtures, also called gelling agents or stabilizers, are necessary for applications requiring strong bleed resistance or water retention in grout. Silica fume and fly ash mildly improve bleed resistance, but are not adequate alone for the high water-retentive properties needed for many applications.

Expansive agents were used in grouts for many years with the premise of counteracting bleed and voids with expansion of the grout. A small amount of expansion to offset settlement may be desirable, but the use of foaming-type expansion admixtures should be avoided. These admixtures may have widely varying levels of expansion when combined with different cements. They also may cause an interconnected air void system, leading to higher permeability. The perceived benefit of counteracting bleeding does not occur because bleeding continues until the grout is set.

Corrosion inhibitors have not been shown to be effective in grouts. Accelerated testing has indicated a reduction in corrosion protection. A well-designed anti-bleed grout with low water content is preferable over the addition of corrosion inhibitors in grout.

For any grout design, care should be taken to use consistent materials between lab testing, mixture development, and field mixing. Materials should be stored per manufacturer’s directions and care should be taken to make sure materials are not past their expiration date.

Grout material properties and grouting practices should be in conformance with PTI specifications to ensure a successful grouting operation. The grout must have appropriate fluidity to pump and retain this fluidity in a wide range of temperatures over the time frame of pumping. Bleed resistance, is critical in preventing voided areas. Set time, chloride penetration resistance, and strength are also important properties and are covered by the PTI Specification (PTI Committee on Grouting Specifications 2003).




Mixing procedures and equipment can have a significant effect on grout behavior. Thixotropic grouts tend to be mixed much more effectively with a high shear or colloidal mixer. A holding tank is then needed to keep the grout agitated. All grouts will have different properties when mixed with various types of mixers and different mixing times. Both over-mixing and under-mixing can be problematic. The same equipment that will be used in the field should be used for property testing and any field mockup tests. Backup equipment should be on hand in case of equipment malfunction during pumping.

Proper inlets, outlets, and vents are necessary to facilitate complete filling of the duct. Vents are placed at intermediate high points and at locations slightly downstream of the high points to ensure that air can be expelled, particularly in a backflow situation. A vent should also be included in the end cap at both the fixed and stressing ends of the tendon. Vents included at low points will aid in draining out moisture. This is particularly critical in situations where entrapped water may freeze. All inlets, outlets, and vents should have a positive shut-off valve. Past practice of bending over vent tubes and tying with tie wire is not sufficient. Vents should be shut off in successive downstream order, with the exception of the vents located slightly downstream of the intermediate high points, which should be closed prior to their associated high point vent. Grout should be allowed to flow from the vents until all visible air and water pockets are gone from the flow stream. At least 1 gal. (3.8 L) of grout should be expelled from each vent and 5 gal. (19 L) from the outlet. The vents should not be reopened prior to grout set. Vents should be cut flush with the concrete surface and sealed to make sure that they do not provide a method of ingress for chloride and moisture. If grouting of a tendon is interrupted, the continuation of the grouting process should be done in a manner that ensures complete filling of the duct. This may require injecting grout into the last vent from which grout has flowed, tapping a new inlet adjacent to the blockage, and/or vacuum grouting.

2. Design requirements — The design section of the PTI Specification (PTI Committee on Grouting Specifications 2003) covers requirements for grouts divided by classes. Class A grouts are for nonaggressive exposure applications, Class B grouts are for aggressive exposure applications, Class C grouts are prepackaged grouts suitable for aggressive environments, and Class D grouts are for special applications. For example, a stay cable application with grouting lifts greater than 100 ft (30.5 m) would be considered a Class D grout. The PTI Specification (PTI Committee on Grouting Specifications 2003) provides performance requirements for each grout class.

3. Testing requirements — Laboratory testing must be performed at a constant water content for a given grout design. Prepackaged materials must state a maximum and minimum water-to cementitious material ratio on the bag and laboratory testing must be performed at both water contents. Property testing includes: set time, strength, permeability, volume change, fluidity, bleed resistance, corrosion resistance, and wet density. Fluidity testing gives an indication of the pumpability of the grout. A flow cone test is used to estimate fluidity. A modification is made for thixotropic grouts because these grouts require additional energy to flow. Bleed testing includes a wick-induced bleed test that indicates a grout’s ability to resist the filtering effects of the strand in low vertical rise bleed situations. For more severe bleed situations, the “Test Method for Bleed Stability of Cementitious Post-Tensioning Tendon Grout (ACI 423.9M-10)” is more appropriate (Joint ACI-ASCE Committee 423 2010). This test requires pressurization of a cylinder of grout against a filter to separate the water. Various pressure levels are used to give an indication of the grout’s bleed resistance at different vertical heights (columns of grout). Field testing may include fluidity, wet density, sampling for strength, and bleed resistance. Field mock-up tests may be required by the engineer.

4. Construction requirements — The construction section of the PTI Specification (PTI Committee on Grouting Specifications 2003) covers material storage, preparation for grouting, the grouting operation, and post-grouting procedures.


3.2. Ducts RequirementsTraditionally, post-tensioning ducts have been fabricated of galvanized steel. Often the seams in the metal duct are easily opened, particularly when bent. This type of duct has not proven to be an effective barrier. In addition, once the sacrificial zinc layer is gone, the underlying steel duct has no corrosion protection and may corrode rapidly. The corrosion product from the duct may also cause cracking in the surrounding concrete, providing an additional direct path for moisture and chlorides. In an aggressive environment, a ribbed plastic duct is preferred for internal tendons. The duct itself can provide an effective barrier against ingress of aggressive agents to the grout and strand. Support of the duct prior to concreting should be adequate to ensure the duct holds the proper profile and concrete cover.

External ducts are typically smooth black plastic. Care must be taken that the material used in these ducts meets specific environmental stress ratings. Poor lots of plastic duct may be prone to splitting, effectively removing the duct as a protection barrier. Care must be taken with duct splicing to ensure that the splice does not become an entrance point for corrodents. A splice made up of ethylene propylene diene monomer (EPDM) marine exhaust hose with stainless steel bands used to clamp the hose to the duct has been used as an effective splice for plastic duct. Duct tape wrapping is not an adequate splice method for plastic duct systems.

Although galvanized steel duct is not recommended in aggressive environments, it is often used in non-aggressive environments. The typical splice for metallic duct is a piece of oversized duct over the splice area with each end of the splice wrapped in duct tape. Another option is a heat-shrink splice. Both splice types are shown in Figure 7. Research has shown problems with the heat-shrink splices on metallic duct (Salas et al. 2004) due to trapped moisture causing corrosion of the splice.

1. Duct testing—Ducts should be leak tested with compressed air prior to grouting. This testing is done with oil-free air to assure the continuity and pressure tightness of the duct system. Guidance for pressure testing of ducts can be found in the FDOT Specification 2007 and the fib’s Bulletin 7 article 2000. Note that methods for leak testing of assembled components for system approval differ from methods used in field testing on in-place ducts.

Primarily, the difference relates to the air pressure used and the acceptable values for pressure loss. If it is determined that cross-grouting is likely between two tendons and it cannot be repaired, the affected tendons should be grouted in a single operation.

Figure 7: Galvanized duct. Top: heat-shrink splice; and bottom: duct-type splice




2. Flushing of ducts—Flushing of ducts is not recommended. Ducts should be sealed from debris prior to tendon stressing and then again after stressing, prior to grouting. Water flushing is not effective in removing debris and often results in pockets of entrapped water in the strand bundle. While flushing may help increase grout flow, it is an inappropriate way to do so. The grout will often mix with the remaining water, except where strong thixotropic grouts are used. The resulting grout can have a significantly higher water-cement ratio and, thus, higher bleed than intended. Pockets of water may also be trapped in the duct. Flushing of ducts is sometimes used as a means to remove water-soluble oils applied to the strand for temporary corrosion protection. Studies have proved this not to be an effective means of removing water-soluble oils, and this practice should be avoided. Past practice included flushing ducts with water to remove debris, removing temporary corrosion protection products, or cooling or lubricating the ducts prior to pumping.

4. POTENTIAL PROBLEM AREAS DUE TO GROUTING IN EXISTING POST TENSION STRUCTURESGrout properties such as fluidity, bleed resistance, low permeability, reasonable set time, stability, and others, are all desirable in a quality grout for post-tensioning. The property that has caused the most problems and has the biggest potential for allowing corrosion in the tendon is bleed resistance. Bleed water from the grout is trapped in the duct and tends to rise to the highest vertical point. In a draped tendon, this may be at a high-end anchor or an intermediate crest. In a vertical tendon, the bleed water may only migrate to a certain intermediate height prior to grout set and thus bleed lenses may form at several locations along the height of the tendon in addition to the top of the tendon. In a horizontal tendon, the bleed lens may be shallow and extend over a long horizontal distance. The tendency of a grout to bleed is accentuated by the filtering effect of seven-wire strand, particularly in large, multi-strand tendons. The pressure head developed over vertical distances further accentuates the bleed. Bleed should be a consideration in all grouted tendons, but the most severe bleed situations arise from large multi-strand tendons and tall vertical tendons (such as in bridge piers).

After the grout sets, the bleed lenses leave an ungrouted area full of bleed water. This water may reabsorb into the grout in some cases, or may partially remain for long periods of time in the void. The strand in the void has no protection from the high-pH grout environment and may corrode even without outside ingress of chlorides. If chlorides or outside moisture are able to reach the duct, the voided area provides a collection point and a potential hot spot for corrosion.

To combat the bleed problem, an anti-bleed admixture, also known as a gelling agent or stabilizer, is often added to the grout formulation. Fly ash and silica fume can reduce bleed to some extent, but typically a chemical anti-bleed agent is needed for multi-strand tendons and tendons with vertical rises (including draped tendons). The amount of anti-bleed admixture required is dependent on the vertical pressure head and number of strands in the duct. Prepackaged grouts that have bleed-resistant properties typically contain some form of anti-bleed agent.

Many anti-bleed or water-retentive grouts also exhibit thixotropic properties. Thixotropic grouts will be fluid when agitated, but will appear gel-like at rest. These grouts may appear too stiff to those unaccustomed to their behavior, but a good thixotropic grout will have stable fluid behavior when mixed or agitated. Thixotropic grouts tend to flow through the duct with a steep front, which should fill all cavities as the grout front advances (Figure 8(a)). This type of flow can push out air and moisture as the front passes. In contrast, water cement grout is generally more fluid and self-leveling than thixotropic grout during injection. Consequently, water cement grout may fill the bottom of the duct first as illustrated in Figure 8(b). In some cases, backflow may result, which is when the grout front turns and flows back on top of the grout layer in the bottom of the duct. Backflow can entrap air in the duct, causing voids if it is not expelled through proper venting. Draped tendon configurations are particularly susceptible to backflow.

Voids may also result from poor construction practices and lack of quality assurance. Examples of poor construction practices that may result in voids are listed in the following:


• Lack of high point vents in ducts, or improper placement of the high point vents;• Use of improper mixing equipment for the grout leading to poor grout quality and fluidity;• Inaccurate measurement of water for mixing of the grout;• Lack of shut-off valves that can hold pressure on the grout when grouting is complete; and• Blockages within the duct or grout that leaks out of the duct.

Other potential problem areas are related primarily to providing multiple robust layers of protection between the environment and the strand. These barriers include concrete cover, membranes, anchor protection, and the duct.

Figure 8: Typical flow patterns for: (a) thixotropic grout in high viscosity state; and (b)plain water-cement grout

5. EVALUATING CORROSION DAMAGEThe multiple levels of protection in a grouted post tensioning system can make inspection difficult. While duct, grout caps at anchorages, and concrete cover represent physical barriers to visual inspection of the prestressing steel, they can also hinder the performance of non-destructive evaluation (NDE) methods. Visual inspection may reveal corrosion, cracking, or spalling of concrete. Substantial corrosion may be present in the duct, even without visual signs. Visual inspection should include a focus on potential problem areas, such as run off from a roadway onto the anchorage area, or locations where voids may exist such as tendon high points.

Visual inspection of external tendons can reveal split duct or some indication of voided areas through hammer sounding. It is difficult to determine the difference between a small area where the duct has separated from the grout, versus a voided area with hammer sounding. Breaching the duct with nails to determine whether a void exists is counterproductive, as it allows oxygen into the duct and provides a path for ingress of contaminants. If the duct is opened or breached in any way, it should be inspected and then repaired immediately.

5.1. MonitoringElectrically isolated systems have been implemented in Europe for enhanced corrosion protection and for monitoring corrosion in post-tensioned structures. In this type of system, an electrical contact is made with the tendon and used to monitor changes in potential (voltage), indicating corrosion activity. An isolated system requires a nonmetallic duct as well as special anchorage details that electrically insulate the anchorage from the surrounding concrete. Corrosion measurements can be made at any time during the life of the structure and can be valuable even without continuous monitoring. Acoustic emission has been used in some cases to detect prestressing steel breakage in post-tensioning tendons. While these systems can be effective, they entail continuous monitoring of the structure to provide beneficial information.




5.2. Nondestructive methodsAlthough many nondestructive evaluation (NDE) methods have been successful on reinforced concrete structures, none have proven to be comprehensive for locating voids and corrosion in post-tensioned structures. Post-tensioned structures are typically very complex with congested reinforcement and multi-strand tendons encased in concrete. Evaluation at the critical anchorage zones is further complicated by congested spiral reinforcement and often large cover concrete. NDE methods can be divided into several categories, although many permutations of these exist. Ultrasonic methods have been successful in the medical industry as well as in the aerospace industry. A large number of methods are included under the heading of ultrasonic, including both two-dimensional and now three-dimensional imaging. All methods are based on high-frequency sound waves and their echoes. Ultrasonic techniques have been evaluated in laboratory testing for use in void detection in post-tensioning tendons, including C-Scan imaging. Although the method shows promise in preliminary investigations, many items need to be addressed, such as transducer type and compensation for various tendon duct cross-section curvatures, transducing medium, limitations on depth, use in multi-strand systems, signal processing, and advanced pattern recognition. The impact-echo technique has been useful in flaw detection in concrete including promising results for detecting voids in post-tensioned structures. A major drawback of the impact-echo method for use in post-tensioned structures is that it cannot detect voids inside plastic ducts. Radiography or x-ray has been effective at detecting voids in both concrete and grouted tendons, but radiation safety concerns and expense have limited its use. The equipment also needs access from both sides of the tested area. Ground-penetrating radar is effective in locating embedded objects in concrete, including non-prestressed reinforcement, tendons, and voids. Although promising for detecting voids in plastic duct, it cannot penetrate steel duct to detect voids. Duct detection is also complicated by the presence of non-prestressed reinforcement. Remnant magnetism has met success in detecting fractures in prestressing steel strands, while the magnetic flux method has been used to detect small flaws and corrosion in post-tensioning tendons. The magnetic flux equipment must run along the duct itself, so it is limited to use in the free length of external tendons. Both methods have limitations for potential use in internal tendons due to difficulties with interference from non-prestressed reinforcement and tie wire. Reflectometrical impulse measurement (RIMT) and time domain reflectometry (TDR) methods send an electrical signal along the tendon from the anchor to detect flaws in the tendon. The anchor must be exposed to use these methods.

TDR has been used successfully for many years to detect defects in transmission lines, but only recently evaluated for post-tensioning applications. Lab testing indicated promise for detecting both corrosion and voids, but testing focused on specimens with a sensor wire running alongside the steel strand. The method shows promise, but at this time requires that the sensor wire be in place at the time of construction or destructive methods be used to place the sensor and make contact with the tendon. Electrochemical methods, such as half-cell potential monitoring and linear polarization for corrosion rate measurement, have been used successfully to help understand the corrosion state of the reinforcement within a concrete structure. Other methods common in the corrosion industry, such as electrochemical impedance spectroscopy (EIS) also have been considered for reinforced concrete. Due to the many variables affecting each of these methods, they must be used in coordination with other data such as visual inspection, chloride penetration samples, and concrete resistivity to establish the likelihood and severity of corrosion. Temperature, humidity, concrete type, bar coatings, duct type and other factors can significantly affect readings.

5.3. Exploratory methodsBecause NDE methods have met only limited success in evaluation of post-tensioning tendons, exploratory methods are often necessary. The NDE methods may be useful for focusing the invasive methods in areas where corrosion is most likely.

Drilling may be required to determine where voids exist in the tendon and to evaluate the extent of corrosion. A less invasive procedure involves drilling into the anchorage area through the grout port (not into the anchorage itself ) and inserting a borescope to investigate the status of the tendon (photo 1). Post-tensioning anchorage manufacturers have recently adapted their anchor ages so that the grout port provides easier access for the borescope into the tendon. The borescope allows the user to see inside the tendon with a video camera. From this perspective, even small amounts of corrosion may appear disproportionately large, so an operator experienced with the procedure is needed.


Photo 1: inspection of tendons

6. REPAIR SCHEMES AND METHODSMethods of cracks repair and strengthening systems for conventional RC structures presented earlier in chapters V and IX can be used to repair cracks and/or strengthening of post-tensioned structures. Additional repair schemes and methods for post-tensioned structures are given in the following sections. Because grouted post-tensioned structures have generally performed well, there is limited experience with their repair. The most comprehensive document in this area is available from the Florida DOT.

6.1. Grouting of voidsRepair of grout voids is the most common type of repair encountered in grouted post-tensioned structures. Even if no corrosion is evident, eliminating voids that expose strand is desirable. The voids can either be filled with a cement-based grout or a resin type of product. Although a cement-based product can restore the alkaline environment, a resin product can also be effective if it fully coats the strand. One method of filling the void is gravity filling with a tube and funnel. This method can effectively cover the strand bundle depending on the location of the void, but may not be able to completely fill the void due to trapped air.

Another method is vacuum grouting. Special equipment is needed for this method. A vacuum measuring technique is used to determine the volume of the void after which a vacuum is pulled on the tendon and the same volume of grout is injected into the void. This method is effective in filling the void. It does require that the void is sealed off so that the vacuum is effective.

A third method for repairing a grout void is pressure grouting. This method is effective when the void is very large and an inlet and outlet can be drilled into the duct. This method allows the use of conventional pressure grouting methods for the repair.

6.2. Tendon repairSplit or breached ducts should be repaired to restore the protective envelope for the tendon. In case of heavy corrosion resulting in strand failure, the entire tendon may need to be replaced. This is a very specialized operation that has only been completed in a few instances. The FDOT has performed tendon replacements and has detailed information on this process. Strengthening options may include external post-tensioning to counteract cracking, deflections, and to increase capacity. Detailed tendon replacement and strengthening strategies are beyond the scope of this report as they tend to be project specific. Repair design should not be undertaken without significant knowledge and experience with grouted multistrand and bar tendon systems.

7. STRUCTURAL REPAIRSThese repairs address system failures. Signs of structural post-tensioning system distress disclosed in the inspection can include the following and will require immediate repairs to restore strength and integrity to the structure:




• Broken strands which can be seen coming out of the structure • End anchorage which has lifted out of the concrete • Cracking concrete which is running parallel to the strands • Excessive deflections • Punching shear cracks in slabs • Diagonal shear / flexure cracks in beams and joists

Methods of repairing post-tensioning tendons depend upon the type of problem the system is experiencing, as well as the type of post-tension system utilized in the original construction. The options to be considered are:• Full-strand replacement - Feeding new strand from one anchorage location to another existing

anchorage location. The type of sheathing used in original construction may affect the use of full-strand replacement. For example, if a paper wrap system was used in the original construction, it will be difficult to push the strand the full length required for the repair.

• Partial-strand replacement utilizing a variety of splicing techniques: - Using an existing anchorage in case of good condition, splicing and stressing coupler. The

anchors may need sandblasting, painting and waterproofing. - Installing a new anchorage in case of corroded anchors and splicing coupler (stressing at the

new anchorage location) - Installing a new stressing coupler while utilizing a dead-end anchor location

• Replacement of broken strands with external strengthening systems, such as new post-tensioning, CFRP, steel or concrete. The external post-tensioning system consisted of external tendons placed on both sides of the beams with anchors located in heavily reinforced cast-in-place concrete column collars. In addition, composite strengthening with CFRP was installed to further strengthen the slab.

8. PREVENTATIVE MAINTENANCERegular inspection of strand systems for signs of distress is one of the best ways for a building owner to prevent costly repairs. Items to be considered in the inspection of post-tensioned structures:• Appropriate concrete cover over strands exposed to aggressive environments • Condition of protective grease and sheathing • Cracking near strands (P-T systems are designed to reduce or eliminate cracking, so any excess

cracking may indicate a problem) • Condition of concrete at anchorage pockets • Rust staining on the concrete near strands • Grease staining on slab soffits


9. REFERENCES• ACI 423.8R-10 Report on Corrosion and Repair of Grouted Multistrand and Bar Tendon Systems

American Concrete Institute, Farmington Hills, MI, 10 pp.

• PTI Committee on Grouting Specifications, 2003, “Specification for Grouting of Post-Tensioned Structures,” second edition, Post-Tensioning Institute, Farmington Hills, MI, 60 pp.

• Emmons, concrete Repair and Maintenance Illustrated, 1993

• Bondy, K. Dirk, Evaluation and repair of existing post tensioned buildings with paper wrapped tendons experiencing corrosion damage” PCI Journal, Vol. 4, No. 2, December 200, p. 2429-.



Documents

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