Floor Cracking: How, What, Where

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Floor Cracking How, What, Where?

• Fred Goodwin, FICRI
• Fellow Scientist
• BASF Construction Chemicals
• Beachwood OH

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Outline

• Early cracking
• Plastic shrinkage
• Reduce wind, raise humidity, lower temperatures
  (concrete ambient)
• Narrow with depth, go around aggregate
• Dampen base if no vapor retarder
• Avoid use of vapor retarder
• Moistue retaining coverings
• Postpone finishing steps
• Crazing
• Due to minor surface shrinkage / shallow mud
  cracking
• Cure immediately after finishing
• Curing water gt20F cooler than concrete
• Avoid alternate wetting / drying cycles
• Do not overuse consolidation or finishing
• Do not prematurely float of finish
• Do not dust with cement
• Dirty aggregates
• Blessing during finishing
• Settlement around reinforcement or embedment

• Tensile failure
• Restraint of Volume Change
• Internal
• External
• Factors
• Drying shrinkage
• Thermal contraction
• Rapid change is worse
• Curling
• Settlement of soil support system
• High clay or sulfate content in subgrade
• Applied loads
• Too early
• Impact
• Earth movements
• Prevention
• Joints
• Contraction joint Spacing
• Sawn Deep enough

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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

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Does Concrete Crack?
Where
When
Why
How
YES!
?
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How does concrete crack?
The Simple Answer Is
The Tensile Strength is Exceeded
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CRACKING TENDENCY
Stress (i.e.,Shrinkage)
Tensile Stress Capacity (i.e. Tensile Strength)
TENSILE STRESS
TENSILE STREGTH
Start of Crack Stress Strain Relief
TIME
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Why does concrete crack?
The Simple Answer Is
RESTRAINT
Internal Restraint
External Restraint
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Where does concrete crack?
The Simple Answer Is
Through the weakest part
Defects
Control or Contraction Joints If its gonna
crack, then at least we can compromise with the
concrete as to where (usually).
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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

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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
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Early Cracking

• Plastic Shrinkage

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Settlement Shrinkage

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

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

• Settlement may also create pockets under rebar
  and aggegates.

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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
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Surrounding structures and conditions
From Structural Condition Assessment, Robert
Ratay, Wiley Sons, 2005
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Thermal Cracking
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Crazing Cracking

• Caused by Minor Surface Shrinkage
• Surface Effect Mostly Cosmetic

To Avoid Cure Immediately After Finishing Avoid
Water gt20F 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
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Hardened Cracking

• Drying shrinkage
• Curling
• Applied loads
• Too early
• Impact
• Earth movements
• Deterioration

Premature Loading
Drying Shrinkage
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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

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• Drying Shrinkage Cracking

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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

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Post- Tensioning Example
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Post Tensioning
Shrinkage Compensating Concrete
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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
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Drying Curling of Concrete Floor
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
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Thickness Drying Factors 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
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Avoid Restraint

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

External Restraint
Permaban Floor Solutions
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Avoid Restraint
WALLS

• Reinforcement Tie In to Columns, Walls, Etc.

COLUMNS

• Reinforcement Continuing Through Joints

Dissimilar Materials or Placement Sections
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Reducing Drying Cracking
Tensile Capacity
TENSILE STRESS
NO Cracking if Shrinkage is Low Enough
TIME
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Reducing Drying Cracking
Tensile Capacity
TENSILE STRESS
NO Cracking if Tensile Capacity is High Enough to
Overcome Shrinkage Stress
?
TIME
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Reducing Drying Cracking
MODULUS EFFECTS
Modulus dy/dx slope in linear portion
High Modulus
TENSILE STRENGTH/Time
Low Modulus
TENSILE STRAIN/Time
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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
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Reducing Drying Cracking
CREEP EFFECTS
Tensile Stress From Restrained Shrinkage
CREEP
TENSILE STRESS
Or at 10000F
INTERNAL ABSORPTION OF SHRINKAGE STRESS COLD
FLOW"
TIME
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Combined Material Properties
If only we had a test method for all these
properties simultaneously.
Cracking Potential
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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
v Shrinkage
Shrinkage Happens
v Tensile Strength
Compresses Steel Ring
Steel Ring Resists
Specimen Cracks
v Tensile Creep Tensile Modulus
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Ring Test Graph Example
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Ring Test Graph Example
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Volume Stability
ASTM C1581 Cracking Resistance
LOW Cracking Potential
Moderate Cracking Potential
HIGH Cracking Potential
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Deterioration

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

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AARAlkali Aggregate Reaction a.k.a ASR or ACR
Some aggregates react with alkali (Na, K)
causing expansion
Select non-reactive aggregates, low alkali
cement, mitigating admixtures
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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

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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
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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
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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

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Classification of Cracks

• Directional cracks indicate restraint
  perpendicular to the crack direction
• propagate from reentrant corners
• parallel companion cracks
• penetrations through the concrete

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Classification of Cracks

• Classified by direction, width depth
• Hexagonal pattern of short cracks -
• Surface had more restraint than the concrete
  interior or substrate

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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
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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 lt0.035
  (0.89mm) PCA Concrete Floors on Ground
• Bridging with elastomers
• Bridging and distribution with fibers

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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

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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?

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Structural Crack Repair

• Repair the cause not the symptoms
• Structural integrity must be maintained!
• Anticipate crack propagation movement
• Expansion joints may be necessary

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Structural Crack Repair Techniques

• Epoxy Resin Injection
• Stitching Doweling
• Bandaging
• Post Tensioning

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Structural Repair with Epoxy Injection
http//www.concrete.org/COMMITTEES/CommitteeDocume
nts.Asp?Committee_Code000E706-00
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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
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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

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Steel Dowel Reinforcement

• Steel reinforcement bars or dowels are embedded
  across crack

• Number and location as determined by engineering
  requirements

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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

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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/t
winseam_data.pdf
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Structural Strengthening with FRP

• Epoxy primer/putty/adhesive/fiber/adhesive/
  topcoat composite
• Carbon/Aramid/Glass Fibers

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Post Tensioning

• A compressive force is applied across the crack
  using reinforcing tendons.
• External
• Internal
• Bonded
• Unbonded

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Non-structural Repair

• Routing and Sealing
• Injection and Vacuum Sealant Application
• Gravity-Soak Technique
• Overlays and Toppings
• Hydraulic Cement Based Crack Repair
• Autogenous Healing

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Routing and Sealing

• Groove routed and filled with sealant

Crack
Crack routed
Sealant
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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
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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

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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

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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.

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04.11.2004
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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.

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04.11.2004
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Crack Sealer Vacuum Injection

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

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04.11.2004
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Outline

• How, Why, Where, and When Does Concrete Crack
• Tensile failure
• Plastic Cracking
• Hardened Cracking
• Cracking Potential
• Deterioration Cracking
• Avoiding Cracking
• Crack Repair

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Questions?
?
THANK YOU !
Fred GoodwinFellow Scientist BASF Construction
Chemicals Beachwood, Ohio