does concrete crack? -...
TRANSCRIPT
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To Crack or Not to Crack?To Crack or Not to Crack?That is the QuestionThat is the Question
Causes and PreventionCauses and PreventionPresented by: Dave FrentressPresented by: Dave Frentress
Does Concrete Crack?Does Concrete Crack?
Is it Dark at Night?
CrackingCrackingA recent NRMCA survey identified A recent NRMCA survey identified cracking as the number one cracking as the number one complaint in the concrete industry.complaint in the concrete industry.
How many types of cracks are there?How many types of cracks are there?
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20 Causes of Cracks20 Causes of Cracks
Physical Chemical Thermal Structural
After Hardening
Plastic Construction Thermal
Before Hardening
Types of Cracks
D Cracking
Crazing
DryingShrinkage
ShrinkableAggregates
Sulfate
Carbonation
AlkaliAggregate
RebarCorrosion
Early ThermalContraction
SeasonalVariations
Freeze Thaw
SettlementMovement
Creep
Loads
PlasticSettlement
PlasticShrinkage
TrafficVibration
Sub gradeMovement
FormworkMovement
Early FrostDamage
Identifying Causes of CracksIdentifying Causes of CracksIdentify crack typeIdentify crack type•• When did they show up?When did they show up?•• What type of pattern?What type of pattern?
L k tL k tLook atLook at•• Mix designMix design•• Ambient conditionsAmbient conditions•• Structure design & dimensionsStructure design & dimensions•• Placing, finishing, curingPlacing, finishing, curing
Plastic Shrinkage CracksPlastic Shrinkage CracksShow up before concrete is setShow up before concrete is setTypically disconnected Typically disconnected -- not through full not through full depth or thickness of structuredepth or thickness of structureRapid evaporation Rapid evaporation -- slow bleedingslow bleedingWeatherWeather•• Hot Dry WindyHot Dry Windy•• Cold Cold -- slow set Temp Dropslow set Temp DropLow bleed concreteLow bleed concrete•• Low w/cLow w/c HRWRHRWR•• High airHigh air Silica fumeSilica fume
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Plastic Shrinkage
Plastic ShrinkagePlastic Shrinkage
Plastic Shrinkage CracksPlastic Shrinkage CracksPreventionPrevention
Fog mistFog mistEvaporation retarderEvaporation retarder•• “Man in a Can”“Man in a Can”
B i TiB i Ti•• Buying TimeBuying Time
Polypropylene FiberPolypropylene FiberWind or sun screensWind or sun screensFaster setFaster set
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Man in a can…Man in a can…
Plastic Shrinkage CracksPlastic Shrinkage CracksPreventionPrevention
Faster setFaster setHigher early strengthHigher early strengthCover/protect concreteCover/protect concrete
Drying Shrinkage CracksDrying Shrinkage CracksShow up 4 days to 8 weeksShow up 4 days to 8 weeksThrough entire structure at weak Through entire structure at weak pointpointN l h i k N l h i k 1/2” 100’1/2” 100’Normal shrinkage Normal shrinkage -- 1/2” per 100’1/2” per 100’Aggravated byAggravated by•• High water contentHigh water content•• Insufficient control jointsInsufficient control joints•• Inadequate curingInadequate curing
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Factors Which Affect Factors Which Affect Shrinkage CrackingShrinkage CrackingPaste to aggregate ratioPaste to aggregate ratio
W/C ratioW/C ratio
Aggregate type and Aggregate type and Aggregate type and Aggregate type and gradationgradation
CementCement
WeatherWeather
Finishing practicesFinishing practices
Drying Shrinkage CracksDrying Shrinkage CracksPreventionPrevention
Lower shrinkage mixLower shrinkage mixLarger Coarse Agg. sizeLarger Coarse Agg. size
Lower water contentLower water contentMax .45 w/cMax .45 w/c
Proper control joints Proper control joints Psst Psst -- they they actually are cracks actually are cracks -- planned onesplanned ones
Per ACI Per ACI On the Plans!On the Plans!
Proper curingProper curingIf you want Curing, you MUST specify it.If you want Curing, you MUST specify it.Bid ItemBid Item
Batched into the load?Batched into the load?
At the Plant…At the Plant…
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Added to the load!Added to the load!
On the jobsite…
What happens?What happens?
i l li l lMoisture loss creates volume Moisture loss creates volume reduction and tensile forces, these reduction and tensile forces, these exceed the concretes ability to exceed the concretes ability to resist shrinkage and the concrete resist shrinkage and the concrete cracks.cracks.
Strength Results using W/C Values…………Strength Results using W/C Values…………
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Cracking of Concrete Due to ShrinkageCracking of Concrete Due to Shrinkage
Original LengthOriginal Length
Unrestrained ShrinkageUnrestrained ShrinkageNo Stress DevelopedNo Stress Developed
Restraints CauseRestraints CauseTensile StressTensile Stress
When Tensile StressWhen Tensile Stressis Greater Thanis Greater Than
Tensile Strength...Tensile Strength...Concrete Cracks.
200 to 600 psi Tensile Stress Developed200 to 600 psi Tensile Stress Developed
27 cubic feet 2 cubic feet are coming back out!
200250300350400
of W
ater
EXCESS WATER IN ONE CUBIC YARD OF CONCRETE WITH 500 POUNDS OF CEMENT
1
4
32
050
100150200
Poun
ds o
.25 .30 .35 .40 .45 .50 .55 .60 .65 .70 .75Water/Cement Ratio
1
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Understanding ShrinkageUnderstanding Shrinkage
Even after hydration, concrete is a porous material
Why does concrete shrink Why does concrete shrink as it dries? as it dries?
Pores lose water due to Pores lose water due to hydration and evaporationhydration and evaporationAs pores become less than As pores become less than fully saturated, meniscus fully saturated, meniscus forms at the airforms at the air--water water interface due to surface interface due to surface tensiontensionThe surface tension of pore The surface tension of pore solution which forms solution which forms meniscus also exerts inward meniscus also exerts inward pulling force on the side walls pulling force on the side walls of the poreof the poreThese forces in all pores in These forces in all pores in range of 2.5 to 50 nm is range of 2.5 to 50 nm is primary cause of shrinkageprimary cause of shrinkage
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Thermal Shrinkage CracksThermal Shrinkage CracksShow up 1st or 2nd nightShow up 1st or 2nd nightSeasonal temperature variationsSeasonal temperature variationsHigh cement mass concreteHigh cement mass concreteMay or may not go through entire May or may not go through entire May or may not go through entire May or may not go through entire structurestructureShatter or three point are frequentShatter or three point are frequentCaused byCaused by•• Rapid cooling of entire structureRapid cooling of entire structure•• Differential coolingDifferential cooling
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Thermal Shrinkage CracksThermal Shrinkage CracksPreventionPrevention
Reduce temperature riseReduce temperature rise•• Less cement Less cement -- more pozzolanmore pozzolan
Increase early age strengthIncrease early age strengthA l tA l t•• AcceleratorsAccelerators
Reduce temperature loss or gradientReduce temperature loss or gradient•• Cover or insulateCover or insulate•• Beware Thermal shock!Beware Thermal shock!
CrazingCrazing
Crazing CracksCrazing CracksMap cracking 1 to 7 days of ageMap cracking 1 to 7 days of ageShrinkage of surface pasteShrinkage of surface pasteVery fine Very fine -- depth of < 1/8”depth of < 1/8”No impact on durabilityNo impact on durabilityp yp yRapid loss of surface moistureRapid loss of surface moisture•• Lack of curingLack of curing•• High W/C ratio (over a .45)High W/C ratio (over a .45)•• Finishing bleed water into surfaceFinishing bleed water into surface•• “Blessing” the slab surface with water“Blessing” the slab surface with water•• “Jitterbugging”“Jitterbugging”
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What happens?What happens?
The paste shrinks more than the The paste shrinks more than the concrete, so when the concrete is concrete, so when the concrete is over worked the aggregate gets over worked the aggregate gets pushed down leaving only paste near pushed down leaving only paste near pushed down leaving only paste near pushed down leaving only paste near the surface. It shrinks more than the surface. It shrinks more than the concrete below it, leaving a fine the concrete below it, leaving a fine map pattern of cracking.map pattern of cracking.
Crazing CracksCrazing CracksPreventionPrevention
Proper curingProper curingModerate slump or use admixturesModerate slump or use admixturesDon’t finish bleed water into concreteDon’t finish bleed water into concreteDon’t sprinkle or spray water on the Don’t sprinkle or spray water on the slabslabUse surface retarders whenever it is Use surface retarders whenever it is hot, dry and windy. Especially cool hot, dry and windy. Especially cool dry and windydry and windyWindy means +5 MPH!Windy means +5 MPH!
CurlingCurling
MoistureMoistureProfileProfile
Differential Differential ShrinkageShrinkage
Concrete slabs curl due to differential moisture Concrete slabs curl due to differential moisture through the thickness, and the differing through the thickness, and the differing shrinkage response.shrinkage response.
ShrinkageShrinkage
Curling Curling ResponseResponse
CrackingCrackingUpon LoadingUpon Loading
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Curling in SlabsCurling in SlabsDifferential volume change fromDifferential volume change from•• Differential moisture lossDifferential moisture loss•• Temperature gradientTemperature gradientPreventionPrevention•• Proper curingProper curing•• Proper jointingProper jointing•• Low water mixLow water mix-- reasonable Cement content, max Agg. reasonable Cement content, max Agg.
sizessizes•• No poly vapor barriersNo poly vapor barriers•• Prevent excessive bleedingPrevent excessive bleeding•• Shrinkage reducing admixtureShrinkage reducing admixture•• Evaporation Control/retarderEvaporation Control/retarder
Jointing…Those Planned CracksJointing…Those Planned Cracks
Do not extend joint spacings over 1 1/2 times length versus the width.
2 to 3 times the depth
¼ the depth minimum¼ the depth - minimum
Example: A four foot wide slab the joints should not extend past 6 feet.
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Compacting the SubgradeCompacting the Subgrade
Base/subgradeBase/subgrade
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Base/subgradeBase/subgrade
Joint PlanningJoint Planning
Watch those Watch those cornerscorners
Plan Joints Plan Joints --
-- prior to prior to placementplacement
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Joint Minimization,Joint Minimization,Crack “Elimination”Crack “Elimination”
Shrinkage Reducing AdmixesShrinkage Reducing AdmixesWarranty SlabsWarranty SlabsCost savings on Joint stretching, Cost savings on Joint stretching, maintenance.maintenance.40 X 40 Portland, 50 X 50 in Seattle40 X 40 Portland, 50 X 50 in Seattle
Performance of SRAsPerformance of SRAsReduce shrinkage by as much as 80% at Reduce shrinkage by as much as 80% at 28 days.28 days.
Reduce ultimate shrinkage on the order of Reduce ultimate shrinkage on the order of Reduce ultimate shrinkage on the order of Reduce ultimate shrinkage on the order of 25 to 50%. Reduce25 to 50%. Reduce--not eliminate…not eliminate…
Allow up to 50 ft joint spacing with no Allow up to 50 ft joint spacing with no cracking cracking –– Glacier NorthwestGlacier Northwest(6” thick, proper practices, 3 day wet cure, (6” thick, proper practices, 3 day wet cure, etc.)etc.)
SRA job: Layout & PlanningSRA job: Layout & Planning
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Greg’s baby
No RestraintNo Restraint
DetailsDetails
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EIC WinnerEIC Winner
SRA Impact on CurlingSRA Impact on Curling
Joint Minimization,Joint Minimization,Crack “Elimination”Crack “Elimination”
TYPE K Komponent Cement TYPE K Komponent Cement Shrinkage compensating CementShrinkage compensating Cement15% Replacement of regular cement15% Replacement of regular cement
Bi B Fl F d M Old NBi B Fl F d M Old N•• Big Box Floors Fred Meyer Old NavyBig Box Floors Fred Meyer Old Navy•• Parr LumberParr Lumber
Additional Curing RequirementsAdditional Curing Requirements
•• Operational ChallengesOperational Challenges--Silo or BagsSilo or Bags
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ORIGINAL LENGTH
Potential Expansion
Actual Expansion
K FIBER REBAR
FINAL LENGTH
CONCRETE K-FIBER or REBAR
Figure 1
System-K with Fibers
Type K with Rebar (#4 @ 24”ocew)
Traditional Portland Concrete with Rebar (#4 @ 24”ocew)
Red = Portland Cement & RebarsBlue = Type K & RebarsGreen= Type K & K Fibers-no Rebars
Test slabs were:50’ X 6’ by 6” thick
System-K Concrete has only 210 lf of joints
50,000 square foot slabsPortland Concrete has 6270 lf of joints
OVER A MILE OF JOINTS ARE
ELIMINATED
Figure 3
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How Type K WorksHow Type K Works
Komponet System
w/c 0.45 HydrationConventional ODOT DECK
Place w/c 0.40
Hydration w/c 0.25
Current DOT’s Using Type KCurrent DOT’s Using Type K
California DOTCalifornia DOTMichigan DOTMichigan DOTOhio DOTOhio DOT
Mix DesignMix Design
CementCement15 Percent type K15 Percent type KStandard AggregatesStandard AggregatesW/C ratio of 0.45 to 0.50W/C ratio of 0.45 to 0.50Slump of 5 to 6 inchSlump of 5 to 6 inchStandard admixtures Standard admixtures
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Fred Meyers Distribution Center
Chehalis
Fred Meyer Distribution Center
Chehalis, WA
60,000 square foot expansion
125’-157’ x 55’ joint spacing
Costco Distribution Center
Salt Lake City, UT
300,000 square feet
100’ x 200’ joint spacing
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Old NavyOld Navy
Foam around ColumnsFoam around Columns
Bars at CornerBars at Corner
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Parr LumberParr Lumber
FlatnessFlatness
Water CureWater Cure
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Sofa Express70,000 square feetMax Joint Spacing 130’No RebarAfter 5 weeks
FF = 98.6FL = 61.0
No cracks
Cracks Normally Occur at:1. Mid-Panel 2. Columns in the Slab (notice no boxouts)3. Perimeter Columns4. Bollards5. Re-entrant Corner6. Plastic and Crazing
Type K Advantages:Type K Advantages:No Drying Shrinkage No Drying Shrinkage Easily PlacedEasily PlacedEasily FinishedEasily FinishedLower maintenance CostsLower maintenance CostsNo need for joints, dowels, etc. No need for joints, dowels, etc. Up to 50,000 square feet without jointsUp to 50,000 square feet without jointsLength to width ratios increaseLength to width ratios increaseNo CurlingNo CurlingMoisture is used Moisture is used -- Vapor tests fasterVapor tests faster
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Crack Control Crack Control --SummarySummary
Prepare subgrade properlyPrepare subgrade properly--watch restraintswatch restraintsSpecify and Order correct mixSpecify and Order correct mix--don’t add water!don’t add water!Plan and cut proper control jointsPlan and cut proper control jointsP t i tiP t i tiPrevent excessive evaporationPrevent excessive evaporationStart curing earlyStart curing earlySynthetic Fiber ReinforcementSynthetic Fiber Reinforcement
Shrinkage Reducing AdmixturesShrinkage Reducing AdmixturesType K CementType K Cement
Smokey the BearSmokey the Bear
Only YOU can prevent Cracks…Only YOU can prevent Cracks…