holland cracking
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International Conference on Advances inConcrete Technology and SustainabilityIssues
Volume Change and Cracking in High-Performance Concrete
Dr. Terry Holland
Quito
11 January 2012
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High-Performance
Concrete is not a preciselydefined term!
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Assume for HPC
High cementitious materials content Multiple cementitious materials
Low w/cm
Not necessarily low total water
Multiple admixtures
In general, all may contribute tovolume change and cracking
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Cracking in General
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Structural Versus Non-Structural Cracking
Load or not load inducedNon-structural cracking includes:
CorrosionAlkali-aggregate reactions
Freezing and thawingRestrained volume change
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Without restraint, there wouldbe no cracking!
Impossible to build structureswithout some degree of
restraint
Restraint can be internal orexternal
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Primary Types ofRestrained Volume Change
Cracking
Plastic shrinkage cracking Thermal contraction/thermal differences
Drying shrinkage (early and long-term)
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How to Identify
AppearanceSome types are easyto identify visually
Timing
Hours
Days
Weeks
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Plastic ShrinkageCracking
Rapid loss of water from flatwork
Loss not made up by bleeding
Differential moisture contents result indifferential volumes
Cracking is typically wide but notthrough the concrete
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Dont mistake surface
drying for initialsetting!
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Finishing Concrete
Place -- Screed -- Bull Float
Wait -- Danger!
Float -- TrowelWait -- Less Danger!
Cure
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Plastic shrinkagecracking
Air temperature
Relative humidity
Concrete temperature
Wind speed
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Where to Measure?
Air Temp: 1.2-1.8 m abovesurface, in shade
Water Temp: Equals concretetemp
RH: 1.2-1.8 m above surface, in
shade, upwindWind Speed: 0.5 m above
surface
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Dont Forget: All of
this works only if theconcrete surface is
wet!
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Protecting againstDrying
Fog the concrete
Erect wind breaks
Finish in the shade Use plastic between finishing
operations
Use evaporation retarder
Work at night
Use synthetic Fibers
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Temperature-Related
Cracking
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NOT limited to massive structures!
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Temperature-RelatedCracking
Internal (heat of hydration)
Internal restraintExternal restraint
External (daily variations)
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Internally generated
temperature changes=
Heat of hydration
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Typical thermal expansion
10 x 10-6 /deg C
approx. 17 mm/30 m/55 C
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ccHOTCENTER
Cold exterior
Internal thermal restraint
(Conc movement)
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TemperatureDifference -- Rule of
Thumb
Maximum difference:
20 deg C
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concrete hydrates and expands
while weak
External thermal restraint
Base restraint minimal
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External thermal restraint
concrete cools and cracks
after hardening
Base restraint significant
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MitigationLower the initial concrete
temperatureUse a low-heat mixture
Spec appropriate strength and ageControl rate of concrete cooling
Instrument the concrete to knowwhat is happening
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Recommended Cooling
< 300 mm 28 C
300-900 mm 22 C
900-1,800 mm 17 C > 1,800 mm 11 C
Maximum Temp drop in first 24 hours
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Dont forget Springand Fall
Nice/hot days -- cool evenings
and nightsSignificant potential for thermal
cracking
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Externally generated
temperature changes
Daily or seasonalvariations
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You must take into
account unequalheating of a structure.
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Mitigation
Plan for temperature exposure ofstructure during construction andin service
Allow for appropriate movement
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Moisture-RelatedCracking
(Shrinkage)
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Types of shrinkage
Plastic
Drying
Autogeneous
Carbonation
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After shrinkage
Original size
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Originalsize
Ends
restrained
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Originalsize
Restrained
shrinka e
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Originalsize
Base restrained
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Original size
Restrained shrinkage
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Drying Shrinkage
Typical drying
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Typical dryingshrinkage
400 to 800 x 10-6
approx. 18 mm/30 m
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Shrinkage: Rules ofThumb
For drying shrinkage at 20 years:
50 percent at 28 days
80 percent at 1 year
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Early Drying Shrinkage
=
Crazing
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Differential DryingShrinkage
=
Curling
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Curling
more shrinkage
less shrinkage
joint joint
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Mitigating DryingShrinkage
Reduce total water content
Reduce paste content
Increase MSA Look at shrinkage potential of
all concrete materials
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Shrinkage ReducingAdmixtures
Expensive, $$$
Generally, cannot offsetthe cost
Use only on criticalstructures
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Reducing Curling
Use low shrinkage concreteBe careful with all admixtures
Use minimum strength concreteReduce joint spacing
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Controlling ShrinkageCracking
Minimize shrinkage!
Proper amount of mild reinforcing
Prestress flatwork
Joint and more Joints
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What About Curing?
Probably delay cracking, butnot eliminate it
Will eliminate crazing if done
promptly after placement
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Autogenous Shrinkage
Volume loss without loss ofmoisture from the concrete
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Carbonation Shrinkage
Shrinkage as a result ofcarbonation
Will not be a problem for HPCwith low w/cm
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Summary
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HPC is more susceptible tocracking
More care is requiredRules of physics cannot be ignored
Volume reduction must beaccounted for in design
Some cracking may be inevitable
Consider testing versus real world-- scale effects
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Joints or Cracks --
Your Choice!
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