PowerPoint 프레젠테이션 - SNU OPEN COURSEWARE

Contents:

• Permeability & Diffusivity

• Chemical attack

• Leaching and efflorescence

• Sulfate attack

• Attack by acids and bases

• Corrosion of sewer pipes

• Alkali-aggregate reaction

• Corrosion of steel reinforcement

• Physical attack

• Freezing and thawing

Delayed Ettringite Formation (DEF) in concrete box-beam (Texas, USA)

Durability in general

• Durability depends on

• concrete quality

• service environment

• design service life

• Premature failure

• ignorance in design

• poor specification

• poor workmanship

• Quality of concrete related to

• permeability, diffusivity, absorption

• strength

• type of cementitious material: OPC, SRC, blended cements

• aggregate: reactivity

Permeability & Diffusivity

Permeability

• Play an important role in durability

• Essential for water-retaining structures and construction below grade

– water tightness

• Flow of water through cement paste obeys D’Arcy’s law

= rate of flow of water

= head of water (hydraulic pressure)

= thickness of specimen

= permeability coefficient, depend on capillary porosity which

is affected by w/c and degree of cement hydration

p

hK

x

hx

pK

Permeability

• Depends on• porosity• pore size distribution – dominated by large capillary pores• continuity of pores

• W/C (w/cm) has the most significant influence on permeability and durability• w/c , porosity , permeability

• Lower w/c reduces capillary porosity, reduce penetration of water and harmful substances

• Lower w/c increases strength, improve concrete resistance to cracking from internal stresses

• Permeability is also reduced by• use of mineral admixtures• increase in curing

Permeability

Composition of sealed & fully hydrated Portland

cement paste

Volume of capillary pores markedly for w/c > 0.42

Effect of ITZ in conc

rete on permeability

Permeability

(Mindess et al 2003)

Chemical transport

• Most concrete contains capillary water

• Chemical transport will be affected by interaction between pore

solution and chemicals

• Moisture content largely determine the nature and speed of

penetration of chemicals into concrete

• Movement of dissolved ions with water under pressure head

• For dry or semi-dry concrete exposed to water, capillary suction

pressure has the same effect on flow as a pressure head of 2.4

MPa in saturated concrete

• In near-saturated concrete, diffusion under a concentration

gradient provides the principal method of transport

Diffusion

• Diffusion can be described by Fick’s second law

• C=concentration, t=time,• Kd =diffusion coefficient, x=depth

• Penetration of Cl follows Fick’s Law very closely

• Factors affecting diffusion

• Pore structure

• Relative humidity of concrete

• Ion diffusion (chlorides, sulphates) is most effective

• when the pores in the cement paste are saturated

• Age

2

2d

C CK

t x

Diffusion

• Tortuosity

(Provis et al.

CCR 2012)

2

0

2&D

D aas t

D t d r

d

Self-diffusivity of a random walker (free space)0D

( )D t

2

r

Self-diffusivity of a random walker (porous medium)

Mean-squared displacement a.f.o. time steps

Permeability

• Water Permeability Test

• Results exhibit significant variability

• Typical tests involve movement of water through concrete specimens under high water pressures

• Permeability coefficient

• calculated according to D’Arcy’s Law

• calculated based on water penetration depth

Where KP = coefficient of water permeability (m/s);

d = water penetration depth (m);v = porosity of concrete [m = gain in mass (g);A = cross-sectional area of specimen (mm2) , = density of water]h = water hydraulic head (m); t = time under pressure (s).

• Difficulty

• permeability coefficient as the pressure

2

2p

d vK

h t

mv

Ad Specimen

Water under pressure

Permeability

• Permeability test

• Tested according to ASTM C 1585• mass increase resulted from absorption of water as a

function of time• 3 discs ø100× 50 mm

• Sorptivity S (kg/m2 h0.5): slope of regression curve of water absorbed by a unit surface area vs SQRT of time from 1 to 24 hrs

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6

Ma

ss

in

cre

as

e, k

g/m

2

√time, h0.5

Permeability

Charge passed,Coulombs

Chloride ion penetrability

> 4,000 High

2,000-4,000 Moderate

1,000-2,000 Low

100-1,000 Very Low

< 100 Negligible

• Rapid Cl Penetrability Test (RCPT)

• According to ASTM C 1202• 6 h charges passed (C)• RCPT ratings

• Used as construction acceptance test

Permeability

• Rapid Migration Test

• According to NT Build 492

• 3 discs ø100×50 mm

• External potential: 30 V

• Duration: 24 hours

• Specimen split into two halves

• Open interior surface sprayed with 0.1N AgNO3 to determine chloride penetration depth

• AgNO3 + Cl- AgCl (white) + NO3-

Leaching & Efflorescence


• Efflorescence

• Occur frequently on the surface of concrete when water can percolate through the concrete

• Major constituent is calcium carbonate

Ca(OH)2+ CO2 = white crusts of CaCO3 on the surface

• Among hydration

products, Ca(OH)2 is

most susceptible to

leaching due to its

relatively high solubility

(1230 mg/liter)


• Efflorescence

• An aesthetic problem in itself

• Extensive leaching of Ca(OH)2 exposes other cementitious

constituents to chemical decomposition, results in

• in porosity and permeability

• strength

• Rate of leaching

• Depends on the amount of dissolved salts in percolating water

• Soft waters (rain water) – most aggressive

• Hard water (containing large amounts of Ca++) – less dangerous

• Temperature: Ca(OH)2 is more soluble in cold water than in

warm water

Sulfate Attack

Sulfate Attack

• Most widespread & common form of chemical attack

• Sulfates sources

• Natural origins: ground water, sea water

• Industrial sources: mine tailings

(D. Hooton)

Sulfate Attack

• Mechanism of sulfate attack

• Dissolved sulphate penetrate into concrete

• Gypsum corrosion

• Accompanied by an expansion in volume by~120%

• Sulfoaluminate corrosion

• Accompanied by 55% increase in solid volume

• Volume expansion may be due to water absorption when

ettringite is in microcrystalline form

2

4 2 2 ( )CH SO aq CSH OH aq

34 12 2 6 322 16C ASH CSH H C AS H

Sulfate Attack

• Crystal structure of Ettringite

34 12 2 6 322 16C ASH CSH H C AS H

(Manzano et al. J. Phy. Chem. C, 2012)

Sulfate Attack

• Consequence • expansion• cracking• loss of strength due to the loss

of cohesion in the cement paste and its bond with aggregate

• damage usually starts at edges and corners followed by progressive cracking and spalling which reduce the concrete to a friable or even soft state

• Field observation shows• Sulfate attack is not always

accompanied by expansion

Underground pile subjected to sulfate attack

(Scrivener 2012)

Sulfate Attack

• Effect of different sulfates

• Type of sulfate commonly encountered: Na, K, Ca, Mg• Mg sulfates can be more aggressive because of possible

additional reactions which decompose C-S-H and calcium sulfoaluminates

• SHx may react with MH to form crystalline magnesium silicate, no cementing property

• MH often forms in pores, reduce porosity of concrete, and hinders penetration of sulfates

• The reactions pH of pore solution

4 12 2 33 ( ) 4 3C ASH M S aq CSH MH AH

2( )CH M S aq CSH MH

3 2 3 23 ( ) 3 3 2 xC S H M S aq CSH MH SH

Sulfate Attack (Sea water)

• Action of sea water

MgSO4+ CaOH MgOH + CaSO4

• Formation of MgOH in pores is one reason why seawater is less

corrosive than might be expected

• Gypsum and ettringite are more soluble in solutions containing Cl

ion; this reduces deleterious expansions(Mindess et al 2003)

Sulfate Attack

• Control of sulfate attack

• Make dense concrete

• Reduce w/c

• Use mineral admixtures (GGBFS, Class F FA, etc) or blended cements

• Reduce Ca(OH)2

• Use mineral admixtures (SF, GGBFS, Class F FA) or blended cements

• Minimise C3A content

• Use sulphate resistant cements(Verbeck 1969)

Sulfate Attack (DEF)

• Delayed Ettringite Formation (DEF)

• Occur in concrete cured at high temperatures (>70 oC) in many precast plants

• A special case of sulphate attack where the source of sulphate ions happens to be internal (within concrete)

• Sulfate in cement

• Gypsum-contaminated aggregates

• At high T, redistribution of aluminate and sulphate in other phases, e.g. C-S-H, AFm phase.

• After cooling and during service life, sulphate and aluminate are desorbed to form ettringite, in volume

• The expansion results in formation of microcrystalline ettringite adjacent to aggregate particles

• Cracking

Sulfate Attack (DEF)

• Delayed Ettringite Formation (DEF)

Alleged DEF in concrete box-beam

(Texas, D. Hooton) 3-year old highway viaduct (Malaysia)

(Verbeck 1969)

Sulfate Attack

• Thaumasite formation from sulfate attack• Primary risk factors

• presence of sulphate and/or sulphide • presence of carbonate ions, from concrete aggregate, or in

cement as filler, or due to carbonation of concrete• low temperatures (generally below 15 C)• presence of water, or very wet

C-S-H + H2O/CO3-2/SO4

-2

• Thaumasite• soft white powder, no binding power

• Sulphate resisting cement will not be immune to this type of attack although the formation of thaumasite is decreased with decreasing C3A content

• Concrete with 70-90% OPC replaced by slag performed well under conditions in which concrete with OPC alone performed poorly

23 15C SCS H

Crystallization of salts

• Salts can also cause damage to concrete through the development

of crystal growth pressure that arise through physical causes

• Penetration of water containing considerable quantities of dissolved

salts into concrete

• Salt crystallization in pores when water evaporates

• Repeated or continued evaporation cause salts built up

• Cracking

• Where?

• Fluctuating water levels

• Concrete is in contact with ground water rich in salts

• Control

• Use low w/c, low permeability concrete

• For existing concrete, seal concrete to prevent the ingress of moisture

and evaporation


• Changes in temperature and RH can cause alternate cycles of dissolution and crystallization of Na2SO4 salts, resulting in phase changes between Na2SO4 (thenardite) and Na2SO4 · 10H2O (mirabilite).

(R. Flatt 2002)

(D. Hooton)


- Current standards deal

with evaporative

transport and sulfate

salt crystallization by

limiting the W/CM of

concrete

- At W/CM < 0.45, the

rate of evaporative

transport rapidly

diminishes.

Acid Attack

Acid Attack

• Naturally occurring acidic ground water are not common

• Acidic waters may occur in landfilled areas or mining operations

• Highly acidic conditions may exist in industrial wastes


Acid Attack

• Concrete made with Portland cement is not resistant to attack by

strong acids or compounds which may convert to acids.

• pH < 6.5, concrete can be attacked

• 4.5 < pH < 5.5, severe attack

• pH < 4.5, very severe attack

• Most vulnerable hydration products is Ca(OH)2

• If the acid is highly concentrated, C-S-H may also be attacked,

forming silica gel

• The nature of the anion that accompanies the hydrogen ion may

further aggravate the situation.

2

2 2( ) 2 2Ca OH H Ca H O

2

2 2 2 2 23 2 3 6 3 2 6CaO SiO H O H Ca SiO nH O H O

2H O

Acid Attack

HCl(from ACI Committee 201 report)

• Examples: sulfuric acid, sulfate attack, expansion

• If the reactions of an acid and cement paste form

• Soluble products: leaching, losing binding capacity

• Expansive products: expansion and cracking

• Insoluble products: fill voids, reduce rate of deterioration

• Limestone and dolomite aggregates are also subjected to attack by acids.

Corrosion of sewer pipes


• Domestic sewage itself is usually harmless

• Problem: corrosion caused by bacteria

• Anaerobic bacteria reduces sulphur compounds to H2S

• H2S dissolves in water film in upper part of the pipe

• Aerobic bacteria

oxidises H2S, and

produce sulphuric

acid

• sulphuric acid attacks

concrete both above

and at the level of

flow of sewage, but

the attack is more severe

above the flow level


• Service life of concrete with limestone aggregate > that of concrete with siliceous aggregate

• Protective treatment of surface

• Coatings

• Bitumens, resins

• Calcium aluminate cement concrete

• Epoxy mortars

• Treatment

• Sodium silicate (water glass)

• Lining of polyvinyl chloride (PVC) sheets

(ACI Committee 201 report)

Alkali Aggregate Reaction (AAR)


• Types of AAR

• alkali-silica reaction (ASR) (most common)

• alkali-carbonate reaction (ACR)

• Alkali-silica reaction

• Certain types of aggregates contain reactive silica which may react withalkalis from cement and cause damage

• symptoms

• pop-outs and map cracking with gel

coming through cracks to form jelly like

or hard beads on surface

• Consequences of ASR

• appearance

• serviceability

• severe cracking can facilitate

other damages (ACI Committee 201 report)


• Mechanism of damage


• Rate of reaction depends on the size of the aggregate particles and the

form of silica

• Fine particles (20 to 30 m) lead to expansion within a few months

• Large ones lead to expansion only after many years

• High temperature accelerates the reaction

• ASR takes places at high concentration of OH- in pore water

• Cracking pattern

• irregular, spider's web

• Reaction rim formed on affected aggregate particles, destroy the bond

between the aggregate and hydrated cement paste

• Diagnose

• Visual inspection

• Optical microscopy

• Scanning electron microscopy

• energy dispersive X-ray analysis


• Factors affecting expansion

• Nature of reactive silica• Amount of reactive silica

• The pessimum % depends on • form of reactive silica• degree of alkalinity• w/c

• Typical range 2-10%

• Particle size of reactive material

• Amount of available alkali

• Amount of available moisture Pessimum %


• Pessimum amount in reactive silica

• Low SiO2/Na2O, high pH, low solubility of CH and low Ca2+ in pore solution• form Na(K)-Si-H gel• expansion

• Increase SiO2/Na2O, • reaction product Na(K)-Si-H • More expansion

• High SiO2/Na2O, low pH, high solubility of CH and high Ca2+ in pore solution• Non-swelling Ca-Na(K)-Si-H

• Expansion


• Factors affecting expansion

Particle size , expansion

Na2O eq. <0.6% in cement, del

eterious expansion usually do

not occur


• Control alkali concentration (use low alkali cement)

• Control pH in pore solution

• Control the amount of reactive silica

• Avoid susceptible aggregate based on petrographic analyses and service records

• Control moisture

• reduce permeability of concrete

• reduce the water supply, reduce swelling

• Alteration of alkali-silica gel

• Use lithium & barium salts as additive (mechanism: preferential formation of non-swelling lithium & barium silicate hydrates)

• Use mineral admixtures (fly ash, slag, silica fume)

• Reduce cement content

• Reduce pH of pore solution

• Reduce permeability

• Alkali content in mineral admixtures need to be checked before being used to control ASR

Carbonation

Carbonation

• CO2 concentrations:

• rural air 0.03%

• unventilated lab. >0.1%

• large cities 0.3% on average

Ca(OH)2 + CO2 → CaCO3 +H2O

pH >12.5 ~9

• When pH reduced to a level <~11.5, passive film is destroyed

• Depth of carbonation

d = k t 0.5

t - time (year)

k - relates to the permeability of concrete, cement type, T, RH,

micro- and macro climatic conditions (frequency and duration

of wetting and drying)

Carbonation

- The depth of carbonation is determined by spraying phenolphthalein solution on a freshly broken concrete surface

- Noncarbonated areas turn red or purple, carbonated areas stay colorless.

Corrosion of steel embedded concrete


• In high alkaline environments, ordinary steel products are covered by a thin iron-oxide film -FeOOH (passive film)

• impermeable

• strongly adhere to the steel surface

• make the steel passive to corrosion

• metallic iron will not corrode until the passive film is destroyed

• The passive film may be destroyed by

• Carbonation

• Chloride ions


• Sources of Cl

• sea water

• de-icing salt

• salt contaminated aggregate

• contaminated water

• admixtures

• Three forms of chlorides in concrete

• Chemically bound

• reaction of Cl with C3A calcium chloroaluminate

• Physically bound

• adsorbed on the surface of gel pores

• Free chlorides

• water soluble

• responsible for the initiation of steel corrosion


• Equilibrium of the three forms of Cl

• The distribution of the chlorides among the three forms is

not permanent as there is an equilibrium such that some

free chloride ions are always present in the pore solution.

• Passive film maybe destroyed even at pH >11.5

• Formation of soluble iron-chloride complex which results in

deposition of loose porous rust

• The amount of Cl required to initiate corrosion depends on

pH of pore solution

• Cl/OH 0.6, corrosion

2Fe Cl FeCl complex

2

2FeCl OH Fe OH Cl


• Mechanism of corrosion

• Electrochemical process

• Anode reaction

(oxidation)

• Cathode reaction

(reduction)

22Fe e Fe

2 20.5 2 2O H O e OH



• Mechanism of corrosion

• At the anode

(1)

(2)

• Formation of hydrated ferric oxide (rust),volume increase that cause

cracking and spalling

• When O2 is limited, reaction

limited to Step (1), steel bars

can be corroded without

cracking the concrete cover

2

22Fe OH Fe OH

2 3 22 32 2Fe OH Fe OH Fe O nH O

2 2,O H O

(Broomfield, 2007)


• Development of anode & cathode areas due to different electr

ochemical potentials

• Different impurity levels in steel bars

• Different residue strains

• Different concentration of oxygen or electrolyte in contact with the metal

• Rate of corrosion depends on

• Availability of O2 and H2O

• Resistivity of concrete

• Consequence of corrosion of embedded steel

• Stain

• Reduce cross sectional area of the steel bars, and reduce load carrying ca

pacity

• Loss bond between steel and concrete

• Cracking and spalling of concrete cover


• Material selection

• Reduce the porosity of concrete

• reduce w/c

• use mineral admixtures

• Corrosion inhibitors

• Calcium nitrite: Fe2+ + OH- + NO2- NO + -FeOOH

• -FeOOH redeposit on the surface of steel reinforcing bars to maintain

the passivity of steel

• For long-term protection, high doses of calcium nitrite are needed

• Stainless steel, or other reinforcing materials

• Design: Sufficient thickness of concrete cover

• Coating and membrane

• Epoxy coating of steel reinforcing bars

• Protective layer for concrete

• Cathodic protection

Deterioration of concretein marine environment

Freezing/Thawing (Metha and Monteiro 2006)

Freeze/Thawing

• Freezing of cement paste

• Freezing of most H2O in saturated cement paste does not occur immediately

when the concrete is cooled below 0 oC

• Freezing point depends on the size of pores

• Salt in pore solution also reduce freezing point

• Mechanism of Frost Attack

• Generation of hydraulic pressure

• water ice: volume increase ~9%

• generate hydraulic pressure

• with the increase of saturation degree of concrete, the volume

increase upon freezing causes damage, cracking

• cycles of freezing and thawing: cumulative effect

Freeze/Thawing

• Mechanism of Frost Attack

- Freezing damage is observed with liquids

that do not expand upon freezing

- Generation of osmotic pressure

solute concentration in pore water adjacent to freezing sites draw water from the more dilute pore solution in surrounding unfrozen paste

movement of water create osmotic pressure

cause the surrounding paste to crack.

Chemical potential of water in the form of ice < that of water in unfrozen pores, effective RH at the freezing sites is lowered, water moves towards the freezing sites desorption of water from C-S-H

Freeze/Thawing

• Protection of frost attack

• Use Air Entrainment to protection

• Provide empty space so the excess water

can move and freeze without causing

damage

• Spacing factor determines the average

distance water must travel to reach the

free space

• Design requirements

• air content

• spacing factor (characteristic space

between air bubbles) <0.2mm

(Cordon 1967)

Freeze/Thawing

• Resistance of concrete to freezing/thawing cycling depends on

• Permeability of cement paste

• Degree of saturation of the cement paste and amount of freezable

water

• Below some critical value of saturation, concrete is highly

resistant to frost

• Fully saturated concrete will be damaged even if it is properly

air-entrained

• Rate of freezing

• Average distance from any point in the paste to a free surface

• Strength of hardened cement paste

Freeze/Thawing

• Improving concrete sustainability by designing and specifying for durability

• Making durable concrete structures has a large impact on sustainability

• the elapsed time, from construction to rehabilitation and replacement, can be extended by increase the service life.

• Improvements in concrete mixture design

• Optimize of combined aggregate gradation

• Use of WRA and superplasticizers

• Use of supplementary cementing materials

• Use of recycled aggregates

Documents

PowerPoint 프레젠테이션 - SNU OPEN COURSEWARE