Floor Cracking: How, What, Where?

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

2. 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

3. Why Where When How Does Concrete Crack? YES! ?

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

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

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

7. 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).

8. 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

9. 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

10. Early Cracking • Plastic Shrinkage

11. 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.

12. 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

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

14. Thermal Cracking

15. 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

16. Hardened Cracking • Drying shrinkage • Curling • Applied loads • Too early • Impact • Earth movements • Deterioration Premature Loading Drying Shrinkage

17. 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

18. Drying Shrinkage Cracking:

19. 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

20. Post- Tensioning Example

21. Post Tensioning Shrinkage Compensating Concrete

22. 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

23. 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

24. 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

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

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

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

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

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

30. 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

31. Reducing Drying Cracking CREEP EFFECTS Tensile Stress From Restrained Shrinkage CREEP TENSILE STRESS Or at 10000F INTERNAL ABSORPTION OF SHRINKAGE STRESS = “COLD FLOW" TIME

32. Modulus Tensile Strength Tensile Creep Shrinkage CombinedMaterial Properties If only we had a test method for all these properties simultaneously. Cracking Potential

33. 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

34. Ring Test Graph Example

35. Ring Test Graph Example

36. Volume Stability ASTM C1581 Cracking Resistance LOW Cracking Potential Moderate Cracking Potential HIGH Cracking Potential

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

38. 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

40. 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

41. 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

42. 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

44. 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

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

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

47. 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

48. 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

49. 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

50. 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?