Floor Cracking: How, What, Where?

Fred Goodwin, FICRIFellow Scientist

BASF Construction ChemicalsBeachwood OH

OutlineHow, Why, Where, and When Does Concrete

Crack• Tensile failure

– Restraint of internal and external volume changes• Plastic Cracking• Hardened Cracking• Cracking Potential• Deterioration Cracking• Avoiding Cracking• Crack Repair

Does Concrete Crack?

How does concrete crack?

The Tensile Strength is Exceeded

The Simple Answer Is:

TIME

Stress (i.e.,Shrinkage)

Start of Crack = Stress + Strain

Relief

TE

NS

ILE

ST

RE

SS

CRACKING TENDENCY

Tensile Stress Capacity

(i.e. Tensile Strength)

TE

NS

ILE

ST

RE

GT

H

Why does concrete crack?

RESTRAINT

The Simple Answer Is:

Internal RestraintExternal Restraint

Where does concrete crack?

Through th

e weakest p

artThe Simple Answer Is:

Micro CRACKS

TRANSITION ZONES

VOIDS

PORES

DefectDefects Control or Contraction Joints:

If it’s gonna crack, then at least we can compromise with the concrete as to where (usually).

Early Cracks Caused by

• Setting shrinkage– Plastic shrinkage– Drying shrinkage

• Construction movement– Sub grade movement– Form movement or premature form removal

• Settlement– Such as when rebar too close to surface

Early Cracking• Plastic Shrinkage

Dampen Base if No Vapor RetarderAvoid Use of Under Slab Vapor Retarder

Use Moisture Retaining Coverings/Evaporation RetardersWind, Sun, Temperature, RH, Mix DesignPostpone Finishing Steps

H2OH2O

Early Cracking• Plastic Shrinkage

Settlement Shrinkage

• Cracks may form over areas of restraint (i.e., rebar)

• Occurs within the concrete paste itself as air voids collapse and aggregates wet out

• Settlement may also create pockets under rebar and aggegates.

Settlement Shrinkage

Areas of stress concentration are prone toCracking

• Reentrant corners• Sudden change in placement depth

Movement of Formwork

sub-gradeSettlement of theMovement of the Sub-grade

Surrounding structures and conditions

From Structural Condition Assessment, Robert Ratay, Wiley & Sons, 2005

Thermal Cracking

Crazing Cracking•Caused by Minor Surface Shrinkage

•Surface Effect Mostly Cosmetic

To Avoid:Cure Immediately After FinishingAvoid Water >20F Cooler Than SlabAvoid Wetting/Drying CyclesDo Not Over-ConsolidateDo Not Over-FinishDo Not Dust With CementDo Not Finish With WaterUse Clean Aggregates Avoid Excessive Fines

• Drying shrinkage• Curling• Applied loads

– Too early– Impact– Earth movements

• Deterioration

Drying Shrinkage

Hardened Cracking

Premature Loading

Drying Shrinkage

• Decrease in volume due to the loss of free moisture from concrete through evaporation

• Stresses caused by volume differences from variations in moisture loss and restraint

Drying Shrinkage Cracking:

Reducing Drying Shrinkage Cracking• Low Water to Cement Ratio

– Less Water to Evaporate, Usually Excess for HydrationOR ACTUALLY

– Less Paste (cementitious and water)• Avoid:

– Restraint– High Early Mixes, – High Cement Fineness, – High Cement Factors– High Alkali Cement– Dirty & high fines in aggregate

• Use Shrinkage Reducing Admixtures• Slow & Thorough Curing

– Controlled Uniform Water EvaporationTwo Methods for NO DRYING SHRINKAGE CRACKING• Place Underwater or Keep Wet Forever

– No Drying = No Drying Shrinkage• Post Tensioning and Shrinkage Compensating Concrete

– Always Under Compression

Post- Tensioning

Example

Post- Tensioning

Example

Post TensioningShrinkage Compensating Concrete

Drying from TWO sides

No external humidity

Drying from ONE side

Bottom side moist

Drying of 4” Slabs to MVTR = 3 Lb/1000 sq. ft.

Kanare, H. Concrete Floors & Moisture, Eng. Bulletin #119 PCA/NRMCA, 2005

Higher W/C dry slower.

If bottom of slab is wet, harder to dry.

Drying Shrinkage

Drying & Curling of Concrete Floor

Time→

Dry

ing

Rat

e → Stage 1 Bleed water on surface evaporates

Stage 2 Water evaporates from pores refilled from within concrete = settlement

Stage 3 Water evaporates from within as vapor= dryingStage 4 Top drys & shrinks more than bottom

Curling occurs lifting edges of slab.

Cracking as slab no longer supported by subbase

Swedish Concrete Association, 1997

Thickness Drying Factors4” Thick 0.5 W/CM 64oF RH 60% 2 weeks rain, 2 weeks moistDry to 90% RH Two Side Drying

Thickness4” = 16” = Twice as Long7” = 2 ½ Times as Long8” = 2.8 Times Longer than 4”10” = 3 ½ Times Longer

Thinner Sections Dry Faster than Thicker

Avoid Restraint• Subbase Friction or Unevenness• Doweling• Reentrant Corners• Lack of / Or Improper Joints

External RestraintPermaban Floor

Solutions

Recommended layout

CO

LUM

NS

WALLS

Dissimilar Materials or Placement Sections

•Reinforcement Tie In to Columns, Walls, Etc.

Avoid Restraint

•Reinforcement Continuing Through Joints

TIME

Shrinkage

NO Cracking if Shrinkage is Low Enough

Tensile Capacity

TE

NS

ILE

ST

RE

SS

Reducing Drying Cracking

TIME

Shrinkage

Tensile Capacity

TE

NS

ILE

ST

RE

SS

NO Cracking if Tensile Capacity is High Enough to Overcome Shrinkage Stress

Extremely Strong

?

Reducing Drying Cracking

TENSILE STRAIN/Time

TE

NS

ILE

ST

RE

NG

TH

/Tim

e

Hig

h M

od

ulu

sLo

w M

odul

usModulus = dy/dx= slope in linear portion

MODULUS EFFECTS Reducing Drying Cracking

TENSILE STRAIN/Time

TE

NS

ILE

ST

RE

NG

TH

/Tim

e

Hig

h M

od

ulu

sLo

w M

odul

usModulus = dy/dx= slope in linear portion

Shrinkage stress

Crack Occurs

Lower Modulus Shifts the Intersection of Shrinkage Stress and Tensile Capacity Where Cracking Occurs.

But a Low Modulus is Like “Bubblegum”

Reducing Drying Cracking

TIME

Tensile Stress From Restrained Shrinkage

CREEP

TE

NS

ILE

ST

RE

SS

CREEP EFFECTS

INTERNAL ABSORPTION OF SHRINKAGE STRESS = “COLD FLOW"

Reducing Drying Cracking

Or at 10000F

Combined Material Properties

If only

we had a test method

for all these properties

simultaneously.

TensileTensileStrengthStrength

ShrinkageShrinkage

Cracking Cracking

PotentialPotential

ModulusModulus

TensileTensileCreepCreep

ASTM C1581

Cracking

Resistance

Inner and Outer Steel Ring for MoldCast Repair DonutStrip off Outer Steel RingWax Top SurfaceShrinkage HappensCompresses Steel RingSteel Ring ResistsSpecimen Cracks

Steel Ring & Strain Gauges

Master Bui ldersTechnologies

®

23 ± 2 °C (73.4 ± 3 °F) 50 ± 4% RH

Volume Stability

√ Shrinkage√ Tensile

Strength

√ Tensile Creep & Tensile Modulus

Ring Test Graph Example

Ring Test Graph Example

ASTM C1581 Cracking Resistance

Volume Stability

HIGH Cracking Potential

Moderate Cracking Potential

LOW Cracking Potential

Deterioration

• Interior Restraint– AAR– Sulfate Expansion– Reinforcement Corrosion– F/T Cycle Deterioration

Some aggregates react with alkali

(Na, K) causing expansion

AAR=Alkali Aggregate Reaction

a.k.a ASR or ACR

Reacting Aggregate

Select non-reactive aggregates, low alkali cement,

mitigating admixtures

Sulfate Attack• Sulfates react with aluminates in the

cement to form ettringite

• Some shrinkage compensating concretes use the same reaction

• Use sulfate resistant cements and pozzolan admixtures

Steel Reinforcement Corrosion

Corrosion Cracks

Steel

Concrete

• The carbonation reaction lowers the pH• If pH of concrete surrounding steel falls

below 8.5, corrosion will occur• Cl- ion accelerates corrosion• Steel must be properly embedded

Cl-

O2

No Corrosion

Corrosion

Air Entraining Agents

• Provide small, correctly sized & uniformly distributed air bubbles that provide the freezing water a place to expand into.

Frost damage, concrete not air entrained Air entrained concrete

Detecting Cracks

• Visually – dampening substrate helps

• Magnification

• Pulse velocity devices – measure cracks’ effect of the velocity of sound waves

• Impact echo – short duration pulse is reflected by a flaw

Classification of CracksDirectional cracks indicate restraint

perpendicular to the crack direction – propagate from reentrant corners – parallel companion cracks– penetrations through the concrete

Classification of Cracks

• Classified by direction, width & depth• Hexagonal pattern of short cracks -

Surface had more restraint than the concrete interior or substrate

Active and Dormant Cracks

• Active cracks continue to grow after the concrete has hardened.

• Dormant cracks remain unchanged– Plastic cracks– Cracks formed by temporary overloading of

the concrete

• Crack movement monitored by glued-in-place crack gauges, optical comparators

http://www.avongard.com/whatisit.htm

Crack Width

• Smaller cracks less problematic than wide– Autogenous healing

• Requires moisture and continued cement hydration

– Aggregate Interlock • Load transfer can occur at crack widths <0.035” (0.89mm)

[PCA Concrete Floors on Ground]

– Bridging with elastomers– Bridging and distribution with fibers

Crack Repair Selection

• Purpose of the structure

• Active or dormant

• Structural or non-structural concrete

• Number of cracks

• Isolated crack or part of a pattern

• Crack depth

• Location of the crack– On the surface, underneath, or near a joint

• Crack orientation relative to the structure – transverse or longitudinal

• Is weather resistance required?

• Is chemical resistance required?

• Must the repair be waterproof?

Crack Repair Selection

Structural Crack Repair

• Repair the cause not the symptoms

• Structural integrity must be maintained!

• Anticipate crack propagation & movement

• Expansion joints may be necessary

Structural Crack Repair Techniques

• Epoxy Resin Injection

• Stitching & Doweling

• Bandaging

• Post Tensioning

Structural Repair with Epoxy Injection

http://www.concrete.org/COMMITTEES/CommitteeDocuments.Asp?Committee_Code=000E706-00

Cracks must be clean and free from debris

Install entry ports Install cap seal

Start injection at widest segment of crack

Continue injection until refusal

Remove cap seal & ports

Epoxy Resin Injection

• ASTM C 881 2-K epoxy injected through plugs

• Excellent cohesive strength

• Not successful if movement occurs

• Not practical if cracks are wet or too numerous

Crack filled using epoxy injection process

• Steel reinforcement to restore strength

• Metal staples or ‘stitching dogs’ across cracks, legs anchored in epoxy-filled holes

• Number, size & spacing of staples determined by necessities of tensile strength restoration

• Cracks will occur elsewhere if movement continues

Structural Repair with Stitching & Doweling

Steel Dowel Reinforcement

• Steel reinforcement bars or dowels are embedded across crack

• Number and location as determined by engineering requirements

Cross-Stitching Method• Holes drilled ~35o angles through the crack

• Steel bars embedded into holes with epoxy.

• Used in roadways and airport runways

• No Joint Movement – Similar to cracking pattern of misaligned dowels

Bandaging• Surface seal or bandage is used when the

crack will remain active

• Flexible strip placed across crack with edges attached

• Wearing course or aggregate broadcast in traffic areas

• Movement in more than one plane

http://www.wbacorp.com/downloads/DataSheets/Arch/twinseam_data.pdf

Structural Strengthening with FRP• Epoxy primer/putty/adhesive/fiber/adhesive/

topcoat composite

• Carbon/Aramid/Glass Fibers

Post Tensioning• A compressive force is applied across the

crack using reinforcing tendons. • External• Internal • Bonded• Unbonded

Non-structural Repair

• Routing and Sealing

• Injection and Vacuum Sealant Application

• Gravity-Soak Technique

• Overlays and Toppings

• Hydraulic Cement Based Crack Repair

• Autogenous Healing

Routing and Sealing

• Groove routed and filled with sealantCrack Crack routed

Sealant

Routing and Sealing

• Not dynamic cracks – Epoxy compounds

• Active cracks – Elastomeric polysulphide & polyurethane sealants

• Flexible sealant repair should use bond breaker at bottom of routed groove

Routed and sealed crack

Bond breaker, backer rod

Vacuum Sealant Application

• Vacuum pulled through ports, pulls sealant into concrete

• Viscosity of sealant depends on cracks– Microcracks require low viscosity– Gel or foam required for larger cracks

• Higher pressure injection allows deeper penetration but can widen cracks

Gravity Soak

• Polymers applied onto horizontal surface

• Squeegeed on, allowed to soak in

• Easier and cheaper than injection and vacuum, but limited depth of penetration

• Epoxy, MMA, HMWM, & urethane used

• Unsuitable if crack runs to underside

04.11.2004 67

Healer Sealer Application

• Crack Sealer poured onto concrete• Workers moved material around deck with solvent

resistant rollers on extension polls. • This material applied at ~100 square feet per gallon.

04.11.2004 68

Crack Sealer Crack pre-treatment

Resin is mixed & poured into crack

Distributed by brush or roller.

•Surface preparation removes contaminants that inhibit penetration

•Also exposes additional cracks that were not previously visible.

04.11.2004 69

Crack SealerVacuum Injection

• Vacuum pump and plastic tube circuitry used to inject resin into cable sheathing.

OutlineHow, Why, Where, and When Does

Concrete Crack• Tensile failure

– Plastic Cracking• Hardened Cracking• Cracking Potential• Deterioration Cracking• Avoiding Cracking• Crack Repair

Questions?

THANK

YOU !

?

Fred GoodwinFellow ScientistBASF Construction ChemicalsBeachwood, Ohio