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Floor Cracking: How, What, Where?. Fred Goodwin, FICRI Fellow Scientist BASF Construction Chemicals Beachwood OH. How, Why, Where, and When Does Concrete Crack Tensile failure Restraint of internal and external volume changes Plastic Cracking Hardened Cracking Cracking Potential

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Floor cracking how what where

Floor Cracking: How, What, Where?

Fred Goodwin, FICRI

Fellow Scientist

BASF Construction Chemicals

Beachwood OH


Outline

How, 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

Outline


Does concrete crack

Why

Where

When

How

Does Concrete Crack?

YES!

?


How does concrete crack
How does concrete crack?

The Simple Answer Is:

The Tensile Strength is Exceeded


CRACKING TENDENCY

Stress (i.e.,Shrinkage)

Tensile Stress Capacity

(i.e. Tensile Strength)

TENSILE STRESS

TENSILE STREGTH

Start of Crack = Stress + Strain Relief

TIME


Why does concrete crack
Why does concrete crack?

The Simple Answer Is:

RESTRAINT

Internal Restraint

External Restraint


Where does concrete crack

Micro CRACKS

PORES

TRANSITION ZONES

VOIDS

Where does concrete crack?

The Simple Answer Is:

Through the weakest part

Defects

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
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

Dampen Base if No Vapor Retarder

  • Plastic Shrinkage

Avoid Use of Under Slab Vapor Retarder

Use Moisture Retaining Coverings/Evaporation Retarders

Wind, Sun, Temperature, RH, Mix Design

Postpone Finishing Steps

H2O

H2O


Early cracking
Early Cracking

  • Plastic Shrinkage


Settlement 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 shrinkage1

Settlement of the

sub-grade

Settlement Shrinkage

Areas of stress concentration are prone to

Cracking

  • Reentrant corners

  • Sudden change in placement depth

Movement of

Formwork

Movement of the Sub-grade


Surrounding structures and conditions

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



Crazing cracking
Crazing Cracking

  • Caused by Minor Surface Shrinkage

  • Surface Effect Mostly Cosmetic

  • To Avoid:

  • Cure Immediately After Finishing

  • Avoid Water >20F Cooler Than Slab

  • Avoid Wetting/Drying Cycles

  • Do Not Over-Consolidate

  • Do Not Over-Finish

  • Do Not Dust With Cement

  • Do Not Finish With Water

  • Use Clean Aggregates

  • Avoid Excessive Fines


Hardened cracking
Hardened Cracking

  • Drying shrinkage

  • Curling

  • Applied loads

    • Too early

    • Impact

    • Earth movements

  • Deterioration

Premature

Loading

Drying Shrinkage


Drying shrinkage
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



Reducing drying shrinkage cracking
Reducing Drying Shrinkage Cracking

  • Low Water to Cement Ratio

    • Less Water to Evaporate, Usually Excess for Hydration

      OR 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 Evaporation

      Two 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
Post Tensioning

Shrinkage Compensating Concrete


Drying Shrinkage

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

Drying from ONE side

Bottom side moist

Drying from TWO sides

No external humidity

Higher W/C dry slower.

If bottom of slab is wet, harder to dry.

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


Drying & Curling of Concrete Floor

Drying Rate →

Time→

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 = drying

Stage 4 Top drys & shrinks more than bottom

Curling occurs lifting edges of slab.

Cracking as slab no longer supported by subbase


Thickness DryingFactors

4” Thick 0.5 W/CM 64oF RH 60%

2 weeks rain, 2 weeks moist

Dry to 90% RH

Two Side Drying

Thickness

4” = 1

6” = Twice as Long

7” = 2 ½ Times as Long

8” = 2.8 Times Longer than 4”

10” = 3 ½ Times Longer

Thinner Sections Dry Faster than Thicker

Swedish Concrete Association, 1997


Avoid restraint

Recommended layout

Avoid Restraint

  • Subbase Friction or Unevenness

  • Doweling

  • Reentrant Corners

  • Lack of / Or Improper Joints

External Restraint

Permaban Floor Solutions


Avoid Restraint

WALLS

  • Reinforcement Tie In to Columns, Walls, Etc.

COLUMNS

  • Reinforcement Continuing Through Joints

Dissimilar Materials or Placement Sections


Shrinkage

Reducing Drying Cracking

Tensile Capacity

TENSILE STRESS

NO Cracking if Shrinkage is Low Enough

TIME


Shrinkage

Extremely Strong

Reducing Drying Cracking

Tensile Capacity

TENSILE STRESS

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

?

TIME


Reducing Drying Cracking

MODULUS EFFECTS

Modulus = dy/dx= slope in linear portion

High Modulus

TENSILE STRENGTH/Time

Low Modulus

TENSILE STRAIN/Time


Reducing Drying Cracking

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

Modulus = dy/dx= slope in linear portion

High Modulus

TENSILE STRENGTH/Time

Low Modulus

Shrinkage stress

But a Low Modulus is Like “Bubblegum”

Crack Occurs

TENSILE STRAIN/Time


Reducing Drying Cracking

CREEP EFFECTS

Tensile Stress From Restrained Shrinkage

CREEP

TENSILE STRESS

Or at 10000F

INTERNAL ABSORPTION OF SHRINKAGE STRESS = “COLD FLOW"

TIME


Modulus

Tensile

Strength

Tensile

Creep

Shrinkage

CombinedMaterial Properties

If only

we had a test method

for all these properties

simultaneously.

Cracking

Potential


Volume Stability

ASTM C1581

Cracking

Resistance

23 ± 2 °C (73.4 ± 3 °F) 50 ± 4% RH

Steel Ring & Strain Gauges

Inner and Outer Steel Ring for Mold

Cast Repair Donut

Strip off Outer Steel Ring

Wax Top Surface

√ Shrinkage

√ Tensile Strength

Shrinkage Happens

Compresses Steel Ring

Steel Ring Resists

√ Tensile Creep & Tensile Modulus

Specimen Cracks




Volume Stability

ASTM C1581 Cracking Resistance

LOW Cracking Potential

Moderate Cracking Potential

HIGH Cracking Potential


Deterioration
Deterioration

  • Interior Restraint

    • AAR

    • Sulfate Expansion

    • Reinforcement Corrosion

    • F/T Cycle Deterioration


Reacting Aggregate

AAR=Alkali Aggregate Reaction a.k.aASR or ACR

Some aggregates react with alkali

(Na, K) causing expansion

Select non-reactive aggregates, low alkali cement, mitigating admixtures


Sulfate attack
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

Cl-

Corrosion

No Corrosion

O2

pH

Steel Reinforcement Corrosion

  • 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

Cracks

Corrosion

Steel

Concrete


Air entraining agents
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
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 cracks
Classification of Cracks

Directional cracks indicate restraint perpendicular to the crack direction

  • propagate from reentrant corners

  • parallel companion cracks

  • penetrations through the concrete


Classification of cracks1
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 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
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
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


Crack repair selection1
Crack Repair Selection

  • 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?


Structural crack repair
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
Structural Crack Repair Techniques

  • Epoxy Resin Injection

  • Stitching & Doweling

  • Bandaging

  • Post Tensioning


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

Structural Repair with Epoxy Injection

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


Epoxy resin injection
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


Structural repair with stitching doweling
Structural Repair with Stitching & Doweling

  • 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


Steel dowel reinforcement
Steel Dowel Reinforcement

  • Steel reinforcement bars or dowels are embedded across crack

  • Number and location as determined by engineering requirements


Cross stitching method
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
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
Structural Strengthening with FRP

  • Epoxy primer/putty/adhesive/fiber/adhesive/ topcoat composite

  • Carbon/Aramid/Glass Fibers


Post tensioning1
Post Tensioning

  • A compressive force is applied across the crack using reinforcing tendons.

  • External

  • Internal

  • Bonded

  • Unbonded


Non structural repair
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
Routing and Sealing

  • Groove routed and filled with sealant

Crack

Crack routed

Sealant


Routing and sealing1
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 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
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


Healer sealer application
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.

67

04.11.2004


Crack Sealer

Crack pre-treatment

  • Surface preparation removes contaminants that inhibit penetration

  • Also exposes additional cracks that were not previously visible.

Resin is mixed & poured into crack

Distributed by brush or roller.

68

04.11.2004


Crack sealer vacuum injection
Crack Sealer Vacuum Injection

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

69

04.11.2004


Outline1

How, Why, Where, and When Does Concrete Crack

Tensile failure

Plastic Cracking

Hardened Cracking

Cracking Potential

Deterioration Cracking

Avoiding Cracking

Crack Repair

Outline


Questions
Questions?

?

THANK

YOU !

Fred GoodwinFellow Scientist

BASF Construction Chemicals

Beachwood, Ohio


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