Control of ns cracks (conc soc 070411)

www.urs-scottwilson.com

Non-Structural Crackscauses and control

Ian Gibb

Principal Materials Engineer


Learning points

• Causes of cracking

• Mitigation methods

• Design / construction

guidance


Causes of cracking


Some people want cracks!

Tate modern, London, 2007


Crack width, mm

Vie

win

gd

ista

nc

e,

m

Durability

BS EN 1992-1-1 (table NA.4)

Serviceability (water retaining)

BSEN 1992-3 (cl.7.3.1)

Non-structural cracks


Autogenous healing (EN1992-3)
















Causes of cracking

BEFORE HARDENING

• Early-age settlement /

shrinkage

AFTER HARDENING

• Early thermal contraction

• Drying shrinkage

• Corrosion of reinforcement

• Sulfate attack

• Alkali-aggregate reaction


Early ageplastic settlement / shrinkage


Time to appearance

Alkali-

aggregate

Sulfate attack

Corrosion

Drying

Shrinkage

Early thermal

Plastic

Settlement

Plastic

Shrinkage

YearsMonthsDaysHours


Plastic settlement

Specific Gravities

Cement:

Aggregate:

GGBS:

Fly ash:

Water:

3.2

2.5 - 3.0

2.9

2.3

1.0


Plastic settlement


Plastic settlement


Plastic settlement

• Occurs in first few hours

• Restraint to movement

causes cracking

• Cracks tend to follow

reinforcement


Secondary influences

• Slag cement – may increase bleeding

• Fly ash cement - likely to reduce bleeding

• Slow setting rates - increase potential for

bleeding

• Depth of pour

• Ambient Temperature

• Aggregate grading


Plastic settlement


Plastic settlement



Prevention

• Re-vibration

• Mix design (e.g. polypropylene fibres, air, VMA)

• Increase cover to top steel


Plastic shrinkage

Evaporation

Reduction in volume

Bleeding


Plastic shrinkage

Evaporation


Plastic shrinkage


Plastic shrinkage


Plastic shrinkage


Plastic shrinkage


Plastic shrinkage


Plastic shrinkage


Mix Design

• air entrainment (reduces surface tension forces)

• polypropylene fibres

Prevention


Curing

• resin-based / silicate based (too late / too slow)

• polythene sheet on light wooden frame

Prevention



Alkali-

aggregate

Sulfate attack

Corrosion

Drying

Shrinkage

Early thermal

Plastic

Settlement

Plastic

Shrinkage



Early agethermal



Heat of hydration

50% Tricalcium silicate 3CaO.SiO2

25% Dicalcium silicate 2CaO.SiO2

10% Tricalcium aluminate 3CaO.Al2O3

10% Tetracalcium aluminoferrite 4CaO.Al2O3.Fe2O3

5% Gypsum CaSO4.2H2O

minutes hours

Rate of heat

evolution

days


Stresses and strains due to thermal effects

Measured

Free

Restrained

Time

The

rma

l S

tra

inT

em

pe

ratu

re


Stress induced by thermal effects

TimeS

tre

ss

With creep

No creep

Design assumption


Crack formation (Internal Restraint)


External Restraint


External Restraint


External Restraint

Heating -

expansion


External Restraint

Cooling -

contraction

Restraint


External Restraint


Magnitudes of free movement

• Early age issues

• ~130C per 100kg/m3 of cement (CEM I)

• >500C for typical structures

• Unrestrained early age contractions ~ 300µε

2036530% fly ash

2135550% ggbs

31340CEM I

410

Cementitious content(kg/m3)

Cement type

70% ggbs 18

Temperature rise(0C)

300mm slab

Cast in summer (20oC)

19mm plywood formwork

C32/40 concrete


Factors Influencing Heat Generation

• Section thickness

• Cement type

• Concrete mixture proportions

• Ambient & placing temperatures

• Formwork & insulation


Factors Affecting Early Thermal Cracking

• Aggregate type

(7–14 µε / oC)

• Tensile strain capacity

(may be less with slag and fly ash)

• External restraint

(previous pours)

• Internal restraint

(temp. profiles in large members)

• Stress raisers

(changes in section)

• Reinforcement

(controls, but does not eliminate)


Tensile Strain Capacity


Example limiting temperature change

Assumes

fck

C30/37

K1

= 0.65

εca = 5µε

20 28 35 53


Early thermal


Early thermal


CIRIA C660

Design procedure

• Temperature

differentials

• Internal / external

restraint

• Tensile strain capacity

• Area of reinforcement

• Crack spacing

• Crack width


Drying shrinkage

Long termdrying shrinkage


• Cement = 320 kg/m3

• Water = 90 litres/m3

• W/C = 0.28

Water needed:

• Cement = 320 kg/m3

• Water = 160 litres/m3

• W/C = 0.50

Water added:

?

Drying shrinkage


Drying shrinkage


Drying shrinkage

Alkali-

aggregate

Sulfate attack

Corrosion

Drying

Shrinkage

Early thermal

Plastic

Settlement

Plastic

Shrinkage



Drying shrinkage


Drying Shrinkage


Drying shrinkage


How to reduce shrinkage

• Aggregate content

(71 to 74% ~ 20%

reduction!)

• Aggregate size

(impacts paste volume)

• Aggregate type

• Admixtures

(water / shrinkage reducing)



(71 to 74% ~ 20%

reduction!)

• Aggregate size


• Aggregate type

• Admixtures





(71 to 74% ~ 20%

reduction!)

• Aggregate size


• Aggregate type

• Admixtures

(water / shrinkage

reducing)




(71 to 74% ~ 20%

reduction!)

• Aggregate size


• Aggregate type

• Admixtures

(water / shrinkage

reducing)




(71 to 74% ~ 20%

reduction!)

• Aggregate size


• Aggregate type

• Admixtures


-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0 10 20 30 40 50 60 70

Days

Le

ng

th c

ha

ng

e (

%)



Drying shrinkage

• To reduce drying shrinkage

cracking:

• Adequate curing

(increases tensile strain

capacity)

• Reduce internal restraint

(movement joints)

• Crack control reinforcement


Technical Report 67

• Types of movement

• Magnitudes of movement

• Restraint (internal / surface /

end / edge)

• Crack width calculation

• Mitigation measures


Long termdurability issue


Reinforcement corrosion


Phenolphthalein reaction

pH 9.0–9.5

Reinforcement corrosion


Sulfate attack


End

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Control of ns cracks (conc soc 070411)