In Search of Crack-Free Concrete: Current Research on Volume Stability and Microstructure

1. In Search of Crack-Free Concrete:Current Research on Volume Stability and Microstructure David A. Lange University of Illinois at Urbana-Champaign Department of Civil & Environmental Engineering

2. Motivation: Early slab cracks • Early age pavement cracking is a persistent problem • Runway at Willard Airport (7/21/98) • Early cracking within 18 hrs and additional cracking at 3-8 days

3. Motivation: Slab curling SLAB CURLING P HIGH STRESS Material (I) Material (II)

4. Material properties are key • Properties are time-dependent • Stiffness develops sooner than strength Ref: After Olken and Rostasy, 1994

5. A “materials” approach • Understand… • Cement • Microstructure • Source of stress • Nature of restraint • Structural response

6. Overview Early Age Volume Change Thermal Shrinkage Creep Swelling External Influences Heat release from hydration Autogenous shrinkage External drying shrinkage Basic creep Drying creep Redistribution of bleed water or water from aggregate Early hydration Chemical shrinkage Cement hydration

7. Now put them all together… • …and you have a very complex problem • All of the possible types of volume change are interrelated. For example: • Temperature change affects shrinkage, hydration reaction (i.e. crystallization, chemical shrinkage, pore structure) • Even worse, the mechanisms for each type often share the same stimuli. For example: • Drying effects shrinkage and creep

8. The goal: optimization • A challenging problem • Methods that improve performance in regard to one issue may exacerbate another. For example: • Lowering w/c is known to reduce drying shrinkage and increase strength, but… • Creep is reduced, autogenous shrinkage is increased, and material is more brittle. All BAD.

9. Applying knowledge to potential materials Methods for quantifying material properties that affect volume change and thus cracking potential

10. Methods of measurement • Volume change: • Embedded strain gages • LVDT • Dial gage • Environmental stimuli • Temperature • Thermocouple or thermistor • Internal or external RH • Embeddable RH sensor Field ready!

11. Measurements (cont’d) • Creep • Tensile – uniaxial loading frames • Compressive – creep frames

12. Examples of field instrumentation

13. Bridge Deck Temperatures – 1st week

14. Strain in bridge deck

15. Summary • The primary causes of volume change have been discussed • Along with ideas for minimization and optimization • The goal of our research is to provide info that aids in the development of specs that minimize problems due to concrete volume change • Ultimate goal: crack free concrete • Immediate goal: maximizing joint spacing and minimizing random cracking

16. In search of crack free concrete: Basic principles • Limit paste content • Aggregates usually are volume stable • Use moderate w/c • Limits overall shrinkage (autogenous + drying) • Avoids overly brittle material • Use larger, high quality aggregates • Improves fracture toughness

17. In search of crack free concrete: Emerging approaches • Shrinkage reducing admixtures • Reduces drying or autogenous shrinkage • Saturated light-weight aggregate • Reduces autogenous shrinkage • Fibers • Reduces drying or autogenous shrinkage

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19. Thermal dilation • Some sources of thermal change: • Ambient temperature change • Solar radiation • Hydration (exothermic reaction)

20. Heat of hydration Hardening Setting Dormant

21. Mechanisms of thermal dilation • 3 components: • Solid dilation – same as dilation of any solid • Hygrothermal dilation – change in pore fluid pressure with temperature • Delayed dilation (relaxation of stress) • Linked to moisture content, but dominated by aggregate CTD • CTD of concrete ~10 x 10-6/C

22. Timing of set & early heat

23. Thermal problems • Hydration heat  early age cracking on cool-down • Thermal gradients • High restraint stresses at top of pavement  cracking • Low restraint curling  cracking under wheel loading • Buckling

24. Thermal gradient issues • Highly restrained slab  Cracking • Low restraint in slab  Curling + Wheel Load  Cracking

25. Can construction practices counteract thermal stress? • Construct during low ambient heat • Morning hours, moderate seasons • Use wet curing • Use low fresh concrete temperatures • Use blankets or formwork that reduce RATE of cooling • Reduce joint spacing in pavements and reduce restraint of structure • Avoid early thermal shock upon form removal

26. Shrinkage • Usually divided into components: • Chemical shrinkage • Internal drying shrinkage • Known as Autogenous Shrinkage • External drying shrinkage

27. Chemical shrinkage Typical values for PC: 7-10% Ref: Neville, 1995

28. Autogenous shrinkage: Particularly a problem of HPC • Internal drying (self-desiccation) associated with hydration • Only occurs with w/c below ~ 0.42 • Same mechanism as drying shrinkage • Reason to place LOWER limit on w/c • Traditional curing NOT very effective

29. Autogenous Shrinkage

30. Autogenous shrinkage: why only low w/c?

31. The “traditional” shrinkage: external drying shrinkage • Occurs when pore water diffuses to surface • Risk increases as diffusivity (porosity) goes up • Reason to place UPPER limit on w/c (or have minimum strength requirement)

32. Hydration product Hydration product Mechanism of shrinkage • Both autogenous and drying shrinkage dominated by capillary surface tension mechanism • As water leaves pore system, curved menisci develop, creating reduction in RH and “vacuum” (underpressure) within the pore fluid

33. Internal Drying Shrinkage Red. External Drying Adm. (SRA) Hydration Surface tension Temperature Pore Radius Radius of meniscus Salt Concentration curvature Mechanical Physicochemical equilibrium Equilibrium g - g 2 ln( RH ) RT 2 = - = p " p ' r v ' r Underpressure in Internal Relative Kelvin - Laplace pore fluid Humidity Change Equation - ln( RH ) RT - = p " p ' v ' RH-stress relationship • Kelvin-Laplace equation allows us to relate RH directly to capillary stress development • Drying shrinkage • Autogenous shrinkage p” = vapor pressure p’ = pore fluid pressure RH = internal relative humidity R = Universal gas constant v’ = molar volume of water T = temperature in kelvins

34. Visualize scale of mechanism Capillary stresses present in pores with radius between 2-50 nm Note the dimensions • C-S-H makes up ~70% of hydration product • Majority of capillary stresses likely present within C-S-H network *Micrograph take from Taylor “Cement Chemistry” (originally taken by S. Diamond 1976)

35. Shrinkage problems • Like thermal dilation… • Shrinkage gradients • High restraint  tensile stresses on top of pavement  micro and macrocracking • Low restraint  curling  cracking under wheel loading • Bulk (uniform) shrinkage  cracking under restraint

36. Evidence of surface drying damage Hwang & Young ’84 Bisshop ‘02

37. Overall stress gradient in restrained concrete Free shrinkage drying stresses Applied restraint stress T=0 ft + + + + + - External restraint stress superposed

38. Time to fracture (under full restraint) related to gradient severity Failed at 7.9 days Failed at 3.3 days

39. Fracture related to gradient severity Load removed from B-44 prior to failure Grasley, Z.C., Lange, D.A., D’Ambrosia, M.D., Internal Relative Humidity and Drying Stress Gradients in Concrete, Engineering Conferences International, Advances in Cement and Concrete IX(2003).

40. Creep: our friend? • In restrained concrete, creep alleviates tensile stresses • Reduces tendency to crack • Many possible mechanisms including moisture movement, microscale particle “sliding”, microcracking • Difficult to measure, quantify, and account for in pavement and mixture design

41. Creep comes in two flavors • Basic creep • Time-dependent deformation that occurs in all loaded concrete • Drying creep • Additional creep that occurs when load is present during drying • Occurs for both tensile and compressive loads

42. Swelling • Bleed water readsorption • As water is consumed during hydration, bleed water may be sucked back in • Crystallization pressure • Certain hydration products force expansion during formation