Lecture #13

1. Lecture #13 Properties of Hardening Concrete

2. Curing

3. Cracking Factors

4. Temperature and Evaporation

5. Thermal Stress Temperature change Coefficient of thermal expansion Concrete stiffness Cracking stress

6. Concrete Thermal Contraction = = Coefficient of thermal expansion ~ 5*10-6 /oF = Difference in concrete temperature (T) and the concrete setting temperature (T set) = T set - T T = Variation of the average concrete temperature after placement. Assume this variation tracks closely to the 24-hour ambient air temperature cycle (after a 72 hour period). T set = 0.95(T conc + TH)

7. Concrete Thermal Contraction (con’t) T conc = Concrete placement temperature at construction (oF). Assume this value (approx. 80 oF) = Change in concrete temperature due to heat of hydration = Hu = Total heat of hydration per gram (kJ/g) = 0.007 (Tconc) – 3x10-5 (Tconc)2 –0.0787 C = amount of cement (grams) per m3 = Degree of hydration (estimate to be approximately 0.15-0.2) cp = Specific heat of cement = 1.044 kJ/g = Density of concrete ~ 2400 kg/m3

9. Strength(ft) vs. Time ft= a log (t) + b

10. 40 30 20 10 TEMPERATURE (C) 0 -10 -20 0 12 24 36 48 60 72 84 96 TIME (hours) FIGURE 1. The Nurse-Saul Maturity Function

11. S M, te

12. Maturity

13. Maturity Concepts Nurse - Saul Equation (units: Temp – Time) Maturity: Product of time & temperature To = Datum Temperature T = Average Concrete Temperature over Time “t” M = Maturity

14. ARRHENIUS EQUATIONS E = Activation Energy R = Gas Constant te = Equivalent Age or Time

15. LABORATORY TESTING FIELD MEASUREMENT Procedures for using maturity method involve laboratory testing and field measurements.

16. Elastic Modulus

17. Sawcut Timing and Depth

18. Curing

19. Strength Factors

20. Relative Humidity at ¾ inch

21. Effective Curing Thickness Effective Curing Thickness

22. Curing Quality Wind Wind No Wind No Wind

23. Structure of CSH Model of CSH

24. Nature of Concrete Creep and Shrinkage

25. a Elastic recovery Creep strain Creep recovery Microstrain Irreversible creep Elastic strain Concrete unloaded Time after loading Typical creep curve for cement paste.

26. Burger Model Constant Stress (Creep) Strain time

27. b Free shrinkage (no load) sh Basic creep (no drying) bc Microstrain Loading and drying dc cr bc tot sh Time Creep of cement under simultaneous loading & drying. 62sh=free shrinkage; bc=basic creep (specimen loaded but not drying); dc=drying creep; cr=total creep strain; tot=total strain (simultaneous loading & drying)

28. UPPER JACK PLATE LOAD BARS LOWER JACK PLATE UPPER LOAD PLATE 6 X 3 IN. PLUG (CONCRETE) C C C C C C = 6 X 12 IN. TEST CYLINDERS 6 X 3 IN. PLUG (CONCRETE) LOWER LOAD PLATE UPPER BASE PLATE SPRINGS LOWER BASE PLATE Spring-Loaded Creep Frame

29. Horizontal Mold for Creep Specimens

30. Cracking Frame

31. specimen strain gauge T = 1.010-6K-1 T = 1210-6K-1 The Cracking Frame Test

32. Crack in Specimen

33. Preparation of Fracture Specimens

34. Determination of Creep where crp = Creep strain v = Shrinkage strain (ASTM C 157) e = Frame strain Fs= Force in concrete (F) Ec = Modulus of elasticity of concrete (F/L-2) Ac = Specimen cross sectional area (L2)

35. Accumulative vs. Time

36. Burger Model Constant Stress (Creep) Strain time

37. Aggregate Effects

38. Effects of Paste Properties Effect of age of loading on the creep strain. Effect of w/c ratio on the shrinkage strain.

39. Mechanisms of Creep and Shrinkage • Creep • It is a complex process involving slipping of surfaces • past one another within the structure of C-S-H. It is a function • of pore structure and ease of slippage of C-S-H particles. As • the space between particles becomes less and less the degree • of creep becomes less and less. • Drying Shrinkage • Moisture loss is driven by the ambient relative humidity. • As moisture escapes from the capillaries, menisci are created and • capillary stresses are developed. As more moisture is evaporated, • smaller and smaller menisci are created. This action creates stress • and causes slippage between C-S-H particles.

40. ACI Committee 209 Method This method is based upon a method proposed by Branson and Christiason (2.3) and was developed by ACI Committee 209 (2.4) In 1982, ACI Special Publication 76 (2.5) gives an updated but not significantly changed version of this method. This method uses the as the creep coefficient.

41. Shrinkage – ACI The shrinkage strain at t days after the end of initial curing is where = ultimate shrinkage strain = 415 to 1070 micro-strain = 0.9 to 1.10 and f = 20 to 130 days In the absence of specific data for local aggregate and conditions Committee 209 suggests that With = product of applicable correction factors The equations for the correction factors are given in Table A

42. Creep – ACI 209 The creep coefficient at t days after loading is given by where = ultimate creep coefficient = 1.30 to 4.15 = 0.40 to 0.80 d = 6 to 30 days In the absence of specific data for local aggregates and conditions Committee 209 suggests that where = product of applicable correction factors The equations for the correction factors are given in Table A

43. Strength and Modulus of Elasticity The concrete strength at t days is given by with suggested values of a = 4.0 days = 0.85 for most cured ordinary Portland cement concrete. The modulus of elasticity Ec at t days is given by which is often taken as when E and f’c are in MPa

44. Notes Correction Factors Creep Shrinkage Loading Age Relative Humidity Average h=average 1.14 - 0.00092 h 1.23 - 0.0015 h during 1st year Thickness thickness in mm for 1st yr. of 150 < h < 300 loading 1.10 - 0.00067 h 1.17 - 0.0014 h ultimate value ultimate value Concrete s= slump in mm 0.82 + 0.00264s 0.89 + .00161s Composition c= cement content - kg/m3 0.75 + .00061c Table A ACI Creep and Shrinkage Correction Factors