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Floor Cracking: How, What, Where?
Fred Goodwin, FICRI Fellow Scientist BASF Construction Chemicals Beachwood OH
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Outline Plastic shrinkage Restraint of Volume Change Factors Crazing
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 >20F 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 Non-rigid formwork Early thermal cracking Form removal damage Placing concrete around preformed joint filler 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 Sawn early enough Slab not strongly restrained at perimeter Bond of slab to foindation Tyiing in reinforcement to foindation, docks, tilet up walls Placing isolation joints around columns Diagonal reinforcement or joint at reentrant conrners Discontinuous reinforcement at joints Mix design Low W/C Necessary strength Low shrinkage materials AAR Reinforcing corrosion Freezing/thawing D Cracking Proper curing Design features Smooth base Constant slab depth Low coefficient of friction
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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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Why How Where When Does Concrete Crack? YES! ?
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How does concrete crack?
The Simple Answer Is: The Tensile Strength is Exceeded
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Tensile Stress Capacity Start of Crack = Stress + Strain Relief
CRACKING TENDENCY Stress (i.e.,Shrinkage) Tensile Stress Capacity (i.e. Tensile Strength) TENSILE STREGTH TENSILE STRESS 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: PORES Micro CRACKS Through the weakest part Defects TRANSITION ZONES VOIDS Control or Contraction Joints: If it’s 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 Construction movement
Plastic shrinkage Drying shrinkage Construction movement Sub grade movement Form movement or premature form removal Settlement Such as when rebar too close to surface Cracks form in plastic (or prehardened) concrete due to movement of the subgrade or forms, settlement, consolidation of voids, plastic shrinkage and drying shrinkage.
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Early Cracking Plastic Shrinkage H2O H2O
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 Aesthetic Surface Texture cOLOR
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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. Settlement shrinkage also causes cracks to form in the prehardened phase. As air voids collapse, aggregates wet out and the concrete consolidates, the paste settles, which can cause cracks to form in areas of restraint such as over a rebar or in areas where there is a sudden change in placement depth.
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Movement of the Sub-grade
Settlement Shrinkage Areas of stress concentration are prone to Cracking Reentrant corners Sudden change in placement depth Movement of Formwork Settlement shrinkage cracking can be reduced by vibrating the concrete paste during placement, by using low slump concrete or viscosity modifying admixtures. It is important to provide sufficient cover over reinforcement. Areas of stress concentrations such as at reentrant corners or areas of sudden change of placement depth will be prone to cracking. Movement of the Sub-grade sub-grade Settlement of the
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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 To Avoid: Cure Immediately After Finishing
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 Aesthetic Surface Texture cOLOR
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Hardened Cracking Drying shrinkage Curling Applied loads Deterioration
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 In concrete paste, there is more water present than is necessary for the hydration. Water is lost from the hardened concrete due to evaporation. Water loss is often not uniform resulting in volume differentials resulting in stress. For example slab on grade concrete will sometimes curl because the top dries out while remians moist.
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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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Shrinkage Compensating Concrete
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
Time→ Drying Rate → 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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Permaban Floor Solutions
Avoid Restraint Subbase Friction or Unevenness Doweling Reentrant Corners Lack of / Or Improper Joints Recommended layout External Restraint Permaban Floor Solutions
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Avoid Restraint Reinforcement Tie In to Columns, Walls, Etc.
Reinforcement Continuing Through Joints Dissimilar Materials or Placement Sections
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Reducing Drying Cracking NO Cracking if Shrinkage is Low Enough
Tensile Capacity TENSILE STRESS NO Cracking if Shrinkage is Low Enough TIME
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Reducing Drying Cracking
Shrinkage Tensile Capacity TENSILE STRESS NO Cracking if Tensile Capacity is High Enough to Overcome Shrinkage Stress Extremely Strong ? 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
Modulus If only we had a test method for all these properties simultaneously. Tensile Strength Tensile Creep Cracking Potential Shrinkage
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Volume Stability ASTM C1581 Cracking Resistance √ Shrinkage
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 √ Shrinkage Wax Top Surface √ Tensile Strength Shrinkage Happens Compresses Steel Ring Steel Ring Resists √ Tensile Creep & Tensile Modulus Specimen Cracks
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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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AAR=Alkali Aggregate Reaction a.k.a ASR or ACR
Some aggregates react with alkali (Na, K) causing expansion Reacting Aggregate 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 Picture from DRP Consulting, Inc. Web Site The red arrows highlight areas where gypsum and ettringite fill fractures, air voids, and the interfacial transition zone (ITZ) between aggregate particles and the cement paste. The green arrows highlight empty microfractures that formed orthogonally to the filled fractures.
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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 Cl- O2 pH No Corrosion Corrosion 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. Picture on left from Picture on right from 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
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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 <0.035” (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
Cracks must be clean and free from debris Install entry ports Install cap seal Continue injection until refusal Remove cap seal & ports Start injection at widest segment of crack
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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
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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. 67
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Crack Sealer Crack pre-treatment Resin is mixed & poured into crack
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
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Crack Sealer Vacuum Injection
Vacuum pump and plastic tube circuitry used to inject resin into cable sheathing. 69
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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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Thanks for listening.
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? Questions? THANK YOU ! Fred Goodwin Fellow Scientist
BASF Construction Chemicals Beachwood, Ohio
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