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CONCRETE CRACKING –WHO IS TO BLAME?
ByChristopher Stanley
TECHNICAL DIRECTORUNIBETON READY MIX
CRACKING FACTS(Concrete Society Technical Report Number 22, non-structural cracks in concrete)
“If a concrete is either cooled or dried, then provided it is free from restraint, it will reduce in length and no cracks will
develop”
“Cracks will not form unless there is some form of restraint”
“Because it acts as a form of internal restraint, reinforcement governs the spacing and width of cracks in hardened concrete
but reinforcement does not have the same effect in plastic concrete”
CRACK ASSESSMENTUsually based on:
Critical viewing distance and personal viewpoint
Type of structure, often using an arbitrary or “prestige”scale (e.g monumental or public buildings, commercial buildings and car parks, public paving, driveways, private housing)
Cracks can be classified thus
Fine cracks - up to 1mm wideWide cracks - from 1mm to 6mm wideFractures - over 6mm wide
It is sometimes specified or implied that crack widths of up to 0.3mm are aesthetically acceptable
All concrete cracks but some can be prevented
•
CRACKS OCCURINGBEFORE HARDENING
CONSTRUCTION MOVEMENT
SUB-GRADE MOVEMENT
FORMWORK MOVEMENT
CRACKS OCCURING AFTER HARDENING
STRUCTURAL CRACKS
DESIGN LOADS
ACCIDENTAL OVERLOAD
CREEP
EXTERNAL SEASONAL TEMPERATURE VARIATIONS
FREEZE/THAW CYCLES
CHEMICAL
CEMENT CARBONATION
ALKALI-AGGREGATE REACTIONS
CORROSION OF REINFORCEMENT
SHRINKABLE AGGREGATE
CRACK CLASSIFICATION
PLASTIC CRACKS
PLASTIC SHRINKAGEPLASTIC SETTLEMENT
CRAZING
DRYING SHRINKAGE
PHYSICALCRACKS
EXTERNAL RESTRAINT
INTERNAL TEMPERATURE
GRADIENTS
EARLY AGE THERMAL
CONTRACTION
THERMAL CRACKS
TYPE OF CRACK -time of appearance REF * FORM, LOCATION, etc. PRIMARY/SECONDARY
CAUSES REMEDY * *
PLASTIC SETTLEMENT TYPE A Cracks over reinforcement
in deep sections Excess bleeding (PC) Reduce bleeding
10 minutes to three hours
TYPE B “Arching” cracks in columns Re-vibrate
TYPE C Cracks at change of depth in slab/beam sections
Rapid early drying conditions
Add Air entrainment
PLASTIC SHRINKAGE TYPE D Diagonal cracks in roads
and slabs Rapid early drying (PC)
30 minutes to six hours
TYPE E Random cracks in reinforced slabs Low rate of bleeding Improve early
curing
TYPE F Cracks over reinforcement in slabs
Ditto and steel near surface
EARLY THERMAL CONTRACTION TYPE G External restraint cracks
in thick walls or columnsExcess heat generation
(PC) rapid coolingReduce heat
and/or insulateOne day to three
weeks TYPE H Internal restraint cracks in thick slabs
Excess temperature gradients, rapid cooling
LONG TERM DRYING SHRINKAGE
weeks - monthsTYPE I
Cracking in thin slabsand walls
Inefficient joints (PC) Excess shrinkage and
inefficient curing
Reduce water content
Improve curing
CRAZINGTYPE J Cracks “off the form” in
fair-faced concreteImpermeable formwork, rich mixes, poor curing
Improve curing and finishing
1 - 7 days sometimes much later
TYPE K Cracks in power- floated slabs
over-trowelling.
SIMPLE CRACK MODEL
Initial state after pouring
r est r ai nt
If dried out or cooled with partial
or no restraint
rest
rain
tRestraint - shortterm effects
RestraintMedium/long term effects
Crack relieves tension
Contraction without stress
n o re
stra
int re s tra in t
Tension!
Free to shrink
Common crack types – Plastic shrinkage cracking (Type A)
Caused by BLEEDING (“A special case of sedimentation” - T.C.Powers, 1939)
The phenomenon of water rising to the surface of plasticconcrete, caused by gravity pulling heavier particlesdownward, the latter being known as sedimentation
Bleeding is not a result of poor compaction, and it cannot be eliminated by improved compaction
Sedimentation
bleed water evaporates - volume change - shrinkage - tension - restraint - crack?
PLASTIC SETTLEMENT CRACKSType A cracks
Water void formed under steel – desiccation - water
evaporates leaving dry void
Settlement cracks occurring at changes in slab depth
Type C cracks
PLASTIC SETTLEMENT CRACKS
Tension
Item cast into slab acts as crack
inducement - crack follows line of least
resistance
Uneven sub base acts as crack
inducement - crack follows line of least
resistance
PLASTIC SHRINKAGE CRACKS
Type E - random
Type F – over reinforcement
PLASTIC SHRINKAGE CRACKS
Insufficient reinforcement cover ?(cracks induced by steel proximity
to surface which is in tension due to rapid drying)
Differentiated from plastic settlementcracks because plastic shrinkage cracks
tend to pass through slab depth
Tension
Plastic Shrinkage Cracking
Plastic Shrinkage Cracking
Plastic Shrinkage Cracking
Plastic Shrinkage Cracking
Plastic Shrinkage Cracking
Thermal cracking - specificationQuestion - Why 70°C max. temperature?Past experience with mass concrete and accelerated curing e.g. in precasting suggests that the quality of the cement hydrate at elevated temperatures >70°C is inferior to that in a normally cured concrete so mechanical strength tends to be lower - in addition a phenomenon known as Delayed Ettringite Formation (DEF) may affect durability
Question - why require a maximum temperature differential of 20°C?
Raw materials for concrete expand at different rates when heated up - this may lead to “micro-cracking” when the respective coefficients of expansion of cement paste and aggregates are significantly different in some cases
Most aggregates can absorb a degree of strain from temperature movements (“tensile strain capacity”) therefore it does not automatically follow that aggregates with significantly different E. coef.are going to cause or influence cracking.
20°C taken as a conservative limit on differentials due to lack of knowledge of local aggregate expansion coefficients
20°C
EARLY THERMAL CONTRACTION CRACKS
Core temperature
Surfacetemperature
Higher Temperature differential -Possibility of
cracks?
Time after casting - hours
Tem
pera
ture
°C
>20°C?≤20°C?
Temperature differential
EARLY THERMAL CHANGES - 24 HOURS
Heat and Expansion
75°C?
Sections > 0.5m thick considered “Self-Insulating”
Insu
latio
n pr
ovid
ed b
y fo
rmw
ork?
Ambient temperature say 35°C?
Formwork temperature say 55°CCover to Insulate?
Insu
latio
n pr
ovid
ed b
y fo
rmw
ork?
Maybe base restraint from
mature concrete or sub base material
No edge restraint
Free to shrink?
EARLY REMOVAL OF FORMWORK - THERMAL SHOCK
Core Heat say 75°C
BASE RESTRAINT
Remove forms early?
Remove forms early?Expansion
Rap
id c
oolin
g to
am
bien
t te
mpe
ratu
re –
say
35°C
?
Rap
id c
oolin
g to
am
bien
t te
mpe
ratu
re –
say
35°C
?
Tension - Plastic shrinkage
Tension!Tens
ion!
METHOD CONTRIBUTION DISADVANTAGEPour concrete continuously
Improves uniformity of pouring temperatures
Places additional demands on production and
handling
Pour concrete at nightNegligible.Depends on
speed and volume placed and nature of hydration of
cement
Extra logistical considerations for night
working
Delay removal of formworkSignificant contributionPrevents thermal shock
Allows uniform controlled cooling to take place
Formwork re-use delayed
Use insulated curing methods
Significant reduction in temperature differentials.
Enables more uniform temperature rise and fall
Large scale use of insulation materials may be
expensive
METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
METHOD CONTRIBUTION DISADVANTAGEInstall sacrificial cooling system in the concrete in
the concrete massDepends on efficiency of
cooling system
Very wasteful.Durability problems from
embedded pipes
Reduce design strength margin or observe 60 or 90
day compliance
Significance based on amount of cement reduced
28 day requirements?Durability may be
compromised
Use superplastisizing admixtures
Can significantly reduce cement content.
Significance dependant on amount of reduction
achieved
Extra cost of superplasticizer over
normal admixture cost
Use admixtures formulated for hydration control and/or significantly
increase dosage
Can significantly reduce cement content ,delay
hydration and reduce peak temperatures
Extra cost of admixture over normal admixtureSetting times extended
Formwork removal may be delayed
METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
METHOD CONTRIBUTION DISADVANTAGE
Use chilled water to partly or wholly replace mixing
water
Significant- 4oC water temperature =
Approx - 1oC concrete temperature
Plant equipment expensive. Consumption usually
exceeds supply capacity. Storage tanks usually
required
Use ice to partly or wholly replace mixing water
Significant50% ice = approximately
- 10oC concrete temperature100% ice =
approximately - 17oC
Usually expensive. Handling difficult, must be
weighed.May not be available in
remote areas.
Nitrogen gas injection Depends on quantity of Nitrogen gas injected
ExpensiveHandling difficult in remote areas. Difficult to control.Efficiency lost due to gas
leakage to atmosphere during injection
METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
METHOD CONTRIBUTION DISADVANTAGE
Shade stockpiles Significant
Exposed stockpile temperatures can reach >50°C in mid day direct
sunlight
Extensive shading required - combination of shading
and fresh deliveries of aggregates to manage
temperatures effectively
Water spraying of stockpiles
Depends on requirements of pour and aggregate
demand
Some difficulty in maintaining uniform, effective, large scale
spraying for mass pours -moisture uniformity can be
compromised
Control temperature of fresh cement
Influence of cement temperature not significant as volume of cement is only
about 12% of concrete mass
Logistical difficulties as large pour cement demand maximizes available silage
STOCKPILE TEMPERATURE MEASUREMENT
Base restraint from mature
concrete
No edge restraintFree to shrink?
Less “Self-insulation”from lower section
thickness
Sections > 0.5m thick considered “Self-Insulating”
Thermal contraction of concrete after
hardening
EARLY THERMAL CONTRACTION CRACKS
Internal restraintType H cracks
Type I cracks
LONG TERM DRYING SHRINKAGE CRACKS
Primary causes - Impermeable form-face materials
- Over-trowelling
Secondary causes
- Rich, pasty mixes
- Poor curing
- Thermal shock (application of cool water on hot surfaces)
Time of appearance - 1 - 7 days, sometimes much later
Remedial measures
Improve curing
Avoid over-trowelling
CRAZING
Type J cracks (crazing)
CRAZING
Heat of hydration
Heat of hydration
SIMPLE INSULATION OF FORMWORK for CONCRETE BREAKWATERS – GUAM 1998
Plastic cling film wrap
creates cells
Plastic cling film
wrap
Cell heats up to higher
temperature
Site LocationDate concrete placedGrade/type of concrete/slumpWeather/site conditions at time of pouring e.g dry, wet, sunny,changeable,cloudy, sunlight,
shaded, exposed
Temperature range ºC, (check met.reports?)Wind conditions/speed (check met.reports?)Relative humidity (check met.reports?)Curing system usedType of structureApproximate dimensionsImmediate sub-baseDetails of reinforcement especially top steel
When was cracking first noted – hours/days
Cement type Coarse aggregate Fine aggregate Admixture
kg/m³ 20mm kg/m³ (1) kg/m³ (1) mls/100kgFly Ash kg/m³ 10mm kg/m³ (2) kg/m³ (2) mls/100kg
CRACK FIELD REPORT