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Non-Structural Crackscauses and control
Ian Gibb
Principal Materials Engineer
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Learning points
• Causes of cracking
• Mitigation methods
• Design / construction
guidance
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Causes of cracking
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Some people want cracks!
Tate modern, London, 2007
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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
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Autogenous healing (EN1992-3)
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Non-structural cracks
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Non-structural cracks
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Non-structural cracks
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Non-structural cracks
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Non-structural cracks
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Non-structural cracks
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Non-structural cracks
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Causes of cracking
BEFORE HARDENING
• Early-age settlement /
shrinkage
AFTER HARDENING
• Early thermal contraction
• Drying shrinkage
• Corrosion of reinforcement
• Sulfate attack
• Alkali-aggregate reaction
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Early ageplastic settlement / shrinkage
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Time to appearance
Alkali-
aggregate
Sulfate attack
Corrosion
Drying
Shrinkage
Early thermal
Plastic
Settlement
Plastic
Shrinkage
YearsMonthsDaysHours
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Plastic settlement
Specific Gravities
Cement:
Aggregate:
GGBS:
Fly ash:
Water:
3.2
2.5 - 3.0
2.9
2.3
1.0
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Plastic settlement
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Plastic settlement
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Plastic settlement
• Occurs in first few hours
• Restraint to movement
causes cracking
• Cracks tend to follow
reinforcement
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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
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Plastic settlement
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Plastic settlement
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Prevention
• Re-vibration
• Mix design (e.g. polypropylene fibres, air, VMA)
• Increase cover to top steel
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Plastic shrinkage
Evaporation
Reduction in volume
Bleeding
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Plastic shrinkage
Evaporation
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Plastic shrinkage
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Plastic shrinkage
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Plastic shrinkage
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Plastic shrinkage
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Plastic shrinkage
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Plastic shrinkage
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Mix Design
• air entrainment (reduces surface tension forces)
• polypropylene fibres
Prevention
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Curing
• resin-based / silicate based (too late / too slow)
• polythene sheet on light wooden frame
Prevention
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Non-structural cracks
Alkali-
aggregate
Sulfate attack
Corrosion
Drying
Shrinkage
Early thermal
Plastic
Settlement
Plastic
Shrinkage
YearsMonthsDaysHours
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Early agethermal
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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
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Stresses and strains due to thermal effects
Measured
Free
Restrained
Time
The
rma
l S
tra
inT
em
pe
ratu
re
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Stress induced by thermal effects
TimeS
tre
ss
With creep
No creep
Design assumption
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Crack formation (Internal Restraint)
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External Restraint
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External Restraint
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External Restraint
Heating -
expansion
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External Restraint
Cooling -
contraction
Restraint
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External Restraint
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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
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Factors Influencing Heat Generation
• Section thickness
• Cement type
• Concrete mixture proportions
• Ambient & placing temperatures
• Formwork & insulation
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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)
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Tensile Strain Capacity
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Example limiting temperature change
Assumes
fck
C30/37
K1
= 0.65
εca = 5µε
20 28 35 53
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Early thermal
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Early thermal
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CIRIA C660
Design procedure
• Temperature
differentials
• Internal / external
restraint
• Tensile strain capacity
• Area of reinforcement
• Crack spacing
• Crack width
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Drying shrinkage
Long termdrying shrinkage
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• 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
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Drying shrinkage
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Drying shrinkage
Alkali-
aggregate
Sulfate attack
Corrosion
Drying
Shrinkage
Early thermal
Plastic
Settlement
Plastic
Shrinkage
YearsMonthsDaysHours
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Drying shrinkage
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Drying Shrinkage
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Drying shrinkage
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How to reduce shrinkage
• Aggregate content
(71 to 74% ~ 20%
reduction!)
• Aggregate size
(impacts paste volume)
• Aggregate type
• Admixtures
(water / shrinkage reducing)
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• Aggregate content
(71 to 74% ~ 20%
reduction!)
• Aggregate size
(impacts paste volume)
• Aggregate type
• Admixtures
(water / shrinkage reducing)
How to reduce shrinkage
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• Aggregate content
(71 to 74% ~ 20%
reduction!)
• Aggregate size
(impacts paste volume)
• Aggregate type
• Admixtures
(water / shrinkage
reducing)
How to reduce shrinkage
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• Aggregate content
(71 to 74% ~ 20%
reduction!)
• Aggregate size
(impacts paste volume)
• Aggregate type
• Admixtures
(water / shrinkage
reducing)
How to reduce shrinkage
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• Aggregate content
(71 to 74% ~ 20%
reduction!)
• Aggregate size
(impacts paste volume)
• Aggregate type
• Admixtures
(water / shrinkage reducing)
-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 (
%)
How to reduce shrinkage
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Drying shrinkage
• To reduce drying shrinkage
cracking:
• Adequate curing
(increases tensile strain
capacity)
• Reduce internal restraint
(movement joints)
• Crack control reinforcement
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Technical Report 67
• Types of movement
• Magnitudes of movement
• Restraint (internal / surface /
end / edge)
• Crack width calculation
• Mitigation measures
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Long termdurability issue
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Reinforcement corrosion
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Phenolphthalein reaction
pH 9.0–9.5
Reinforcement corrosion
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Sulfate attack
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End
Thank you for your attentionQuestions?