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Floor Cracking: How, What, Where? Fred Goodwin, FICRI Fellow Scientist BASF Construction Chemicals Beachwood OH.

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Presentation on theme: "Floor Cracking: How, What, Where? Fred Goodwin, FICRI Fellow Scientist BASF Construction Chemicals Beachwood OH."— Presentation transcript:

1 Floor Cracking: How, What, Where? Fred Goodwin, FICRI Fellow Scientist BASF Construction Chemicals Beachwood OH

2 Outline 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 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

3 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

4 Does Concrete Crack?

5 How does concrete crack? The Tensile Strength is Exceeded The Simple Answer Is:

6 TIME Stress (i.e.,Shrinkage) Start of Crack = Stress + Strain Relief TENSILE STRESS CRACKING TENDENCY Tensile Stress Capacity (i.e. Tensile Strength) TENSILE STREGTH

7 Why does concrete crack? RESTRAINT The Simple Answer Is: Internal Restraint External Restraint

8 Where does concrete crack? Through the weakest part The Simple Answer Is: Micro CRACKS TRANSITION ZONES VOIDS PORES Defect Defects Control or Contraction Joints: If its gonna crack, then at least we can compromise with the concrete as to where (usually).

9 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

10 Early Cracking Plastic Shrinkage Dampen Base if No Vapor Retarder Avoid Use of Under Slab Vapor Retarder Use Moisture Retaining Coverings/Evaporation Retarders Wind, Sun, Temperature, RH, Mix Design Postpone Finishing Steps H2OH2OH2OH2O

11 Early Cracking Plastic Shrinkage

12 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.

13 Settlement Shrinkage Areas of stress concentration are prone to Cracking Reentrant corners Sudden change in placement depth Movement of Formwork sub-grade Settlement of the Movement of the Sub-grade

14 Surrounding structures and conditions From Structural Condition Assessment, Robert Ratay, Wiley & Sons, 2005

15 Thermal Cracking

16 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

17 Drying shrinkage Curling Applied loads –Too early –Impact –Earth movements Deterioration Drying Shrinkage Hardened Cracking Premature Loading

18 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

19 Drying Shrinkage Cracking:

20 Reducing Drying Shrinkage Cracking Low Water to Cement Ratio –L–Less Water to Evaporate, Usually Excess for Hydration OR ACTUALLY –L–Less Paste (cementitious and water) Avoid: –R–Restraint –H–High Early Mixes, –H–High Cement Fineness, –H–High Cement Factors –H–High Alkali Cement –D–Dirty & high fines in aggregate Use Shrinkage Reducing Admixtures Slow & Thorough Curing –C–Controlled Uniform Water Evaporation Two Methods for NO DRYING SHRINKAGE CRACKING Place Underwater or Keep Wet Forever –N–No Drying = No Drying Shrinkage Post Tensioning and Shrinkage Compensating Concrete –A–Always Under Compression

21 Post- Tensioning Example

22 Post Tensioning Shrinkage Compensating Concrete

23 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

24 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

25 Swedish Concrete Association, 1997 Thickness Drying Factors 4 Thick 0.5 W/CM 64 o F 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

26 Avoid Restraint Subbase Friction or Unevenness Doweling Reentrant Corners Lack of / Or Improper Joints External Restraint Permaban Floor Solutions Recommended layout

27 COLUMNS WALLS Dissimilar Materials or Placement Sections Reinforcement Tie In to Columns, Walls, Etc. Avoid Restraint Reinforcement Continuing Through Joints

28 TIME Shrinkage NO Cracking if Shrinkage is Low Enough Tensile Capacity TENSILE STRESS Reducing Drying Cracking

29 TIME Shrinkage Tensile Capacity TENSILE STRESS NO Cracking if Tensile Capacity is High Enough to Overcome Shrinkage Stress Extremely Strong ? Reducing Drying Cracking

30 TENSILE STRAIN/Time TENSILE STRENGTH/Time High Modulus Low Modulus Modulus = dy / dx = slope in linear portion MODULUS EFFECTS Reducing Drying Cracking

31 TENSILE STRAIN/Time TENSILE STRENGTH/Time High Modulus Low Modulus Modulus = 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

32 TIME Tensile Stress From Restrained Shrinkage C REEP TENSILE STRESS CREEP EFFECTS INTERNAL ABSORPTION OF SHRINKAGE STRESS = COLD FLOW" Reducing Drying Cracking Or at 1000 0 F

33 Combined Material Properties If only we had a test method for all these properties simultaneously. TensileStrength Shrinkage CrackingPotential Modulus TensileCreep

34 ASTM C1581 Cracking Resistance Inner and Outer Steel Ring for Mold Cast Repair Donut Strip off Outer Steel Ring Wax Top Surface Shrinkage Happens Compresses Steel Ring Steel Ring Resists Specimen Cracks Steel Ring & Strain Gauges 23 ± 2 °C (73.4 ± 3 °F) 50 ± 4% RH Volume Stability Shrinkage Tensile Strength Tensile Creep & Tensile Modulus

35 Ring Test Graph Example


37 ASTM C1581 Cracking Resistance Volume Stability HIGH Cracking Potential Moderate Cracking Potential LOW Cracking Potential

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

39 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


41 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

42 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 - O2O2 No Corrosion Corrosion

43 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 entrainedAir entrained concrete


45 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

46 Classification of Cracks Directional cracks indicate restraint perpendicular to the crack direction –propagate from reentrant corners –parallel companion cracks –penetrations through the concrete

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

48 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

49 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

50 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

51 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

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

53 Structural Crack Repair Techniques Epoxy Resin Injection Stitching & Doweling Bandaging Post Tensioning

54 Structural Repair with Epoxy Injection Cracks must be clean and free from debris Install entry portsInstall cap seal Start injection at widest segment of crack Continue injection until refusal Remove cap seal & ports

55 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

56 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

57 Steel Dowel Reinforcement Steel reinforcement bars or dowels are embedded across crack Number and location as determined by engineering requirements

58 Cross-Stitching Method Holes drilled ~35 o 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

59 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

60 Structural Strengthening with FRP Epoxy primer/putty/adhesive/fiber/adhesive/ topcoat composite Carbon/Aramid/Glass Fibers

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

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

63 Routing and Sealing Groove routed and filled with sealant Crack Crack routed Sealant

64 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

65 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

66 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

67 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.

68 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.

69 04.11.2004 69 Crack Sealer Vacuum Injection Vacuum pump and plastic tube circuitry used to inject resin into cable sheathing.

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


72 Questions? THANK YOU ! ? Fred Goodwin Fellow Scientist BASF Construction Chemicals Beachwood, Ohio

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