1. 2009 Quality Concrete School IT’S HOT!! IT’S CHILLY!! SHRINKAGE!!

2. Detrimental Hot Weather Conditions High ambient temperature High concrete temperature Low relative humidity High wind speed Solar radiation

3. Hot Weather Effects on Concrete Increased water demand Accelerated slump loss Faster set Increased tendency for plastic cracking Difficulties controlling entrained air Increased potential for thermal cracking

4. Precautions Cool concrete Cool concrete ingredients Reduce the time of transport, placing and finishing Use sunshades, windscreens, fogging, or spraying to limit moisture loss during placing and finishing

5. Rule of Thumb For every 10º F increase in concrete temperature: –You need 7 lbs (~ 1 gallon) of water per yd 3 to maintain a given slump –You lose about 1 hr of setting time –Oh yah, if you add that 1 gallon of water you will lose about 150 psi of compressive strength

6. Effect of Concrete Temperatures on Strength

7. BE COOL! More Rules of Thumb –Drop the water temp by 4º, concrete temp drops 1º –Drop the aggregate temp by 2º, concrete temp drops 1º

9. Evaporation of Surface Moisture from Concrete

10. Precautions to Minimize Plastic Shrinkage Cracking Moisten aggregates Cool aggregates and mixing water Dampen subgrade Erect temporary windbreaks and sunshades Cover concrete Fog slab immediately after placing Add plastic fibers

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12. Cold Weather (ACI 306) Average daily temperature <5ºC (40ºF) for 3 successive days Stays <10ºC (50ºF) for more than ½ of any 24h period

13. Effect of Freezing Fresh Concrete Up to 50% reduction of ultimate strength can occur if frozen — –Within a few hours –Before reaching a strength of 3.5 MPa (500 psi) Frozen only once at an early age — –With curing nearly all strength can be restored –Less resistance to weathering –More permeable

14. Setting Times at Different Temperatures

15. Rule of Thumb For every 10°C (18°F) reduction in concrete temperature, the times of setting of the concrete double - thus increasing the amount of time that the concrete is vulnerable to damage due to freezing

16. Effect of Temperature on Strength Development

17. Effect of Casting Temperature on Slump

18. Relationship Between Temperature, Slump and Air Content

19. Retaining Heat of Hydration

20. Cold-Weather Concreting Type III or HE high- early-strength cement Additional portland cement (60 to 120 kg/m 3 or 100 to 200 lb/yd 3 ) Chemical accelerators Methods to accelerate strength gain:

22. Recommended Concrete Temperatures—Air-Entrained Concrete LineCondition Thickness of sections, mm (in.) Less than 300 (12) 300 to 900 (12 to 36) 900 to 1800 (36 to 72) 1 Minimum temperature of fresh concrete as mixed for weather indicated. Above -1°C (30°F) 16°C (60°F)13°C (55°F) 2 -18°C to -1°C (0°F to 30°F) 18°C (65°F)16°C (60°F) 3 Below -18°C (0°F) 21°C (70°F)18°C (65°F) 4 Minimum temperature of fresh concrete as placed and maintained. 13°C (55°F)10°C (50°F)

24. NRMCA Heating Criteria For plants seeking certification to supply concrete in subfreezing weather i.e., where concrete is placed regularly during sub-freezing weather, minimum heating capacity for water and/or aggregate of 15 boiler output horsepower per 100 cubic yard average daily cold weather production. (May be reduced to 10 bph if storage capacity permits round-the-clock operation of heating equipment.) One boiler horsepower = 33,500 BTU per hour transferred to the water.

25. Protect from freezing Agitate if necessary Admixtures

26. Recommended Duration of Temperature Air-entrained concrete Service category Protection from early-age freezing For safe stripping strength Convent. concrete, days High-early strength concrete, days Convent. concrete, days High-early- strength concrete, days No load, not exposed, favorable moist-curing 2121 No load, exposed, but later has favorable moist- curing 32 32 Partial load, exposed64 Fully stressed, exposedSee next slide

27. Thermal Resistance, R, for (10- mm [1-in.] Thick) Insulating Materials Board and Slabs (m 2 ·K)/W (°F·hr·ft 2 )/Btu Expanded polyurethane 0.4386.25 Expanded polystyrene 0.2774.0 Mineral fiberboard 0.2042.94 Plywood 0.0871.24 Loose fill Wood fiber, soft woods 0.2313.33 Vermiculite0.1482.13

28. Cold-Weather Insulation, Aboveground

29. Insulating Blankets Thermal resistance of mineral fiber blanket (50 to 70-mm [2 to 2.75-in.] thick) 1.2 (m 2 ·K)/W 7 (°F·hr·ft 2 )/Btu

30. Insulating Concrete Forms (ICF)

31. Insulated Column Forms High-density plywood Rigid polystyrene Rough plywood

32. Cooling After Protection Section size, minimum dimensions, mm (in.) Less than 300 (12) 300 to 900 (12 to 36) 900 to 1800 (36 to 72) (36 to 72)Over 1800 (72) 28°C (50°F) 22°C (40°F) 17°C (30°F) 11°C (20°F) Maximum Temperature Drop in 24 Hours

33. Enclosures Wood Canvas Tarpaulins Polyethylene Film

34. Heated Enclosure

35. Indirect-Fired Heater

36. Hydronic Systems

37. Video Concreting on Ground

38. SHRINKAGE!!!

39. Chemical and Autogenous Shrinkage

40. Plastic Shrinkage

41. Volume Changes

42. Shrinkage and Cracking

43. Minimize Shrinkage Cracking

44. LUNCHTIME