holland cracking

8/10/2019 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!

holland cracking

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