早龄期混凝土.ppt

Chapter 10 concrete at early ages (continued) Retempering of fresh concrete is frequently necessary in hot weather. At times, this causes adverse effects on strength, durability, dimensional stability, and appearance of the hardened concrete. Also, concrete placed and cured at higher than moderate ambient temperatures normally develops high early strength but at 28 days and later ages the strength is usually lower than the same concrete placed and cured at a relatively lower temperature. Chapter 10 concrete at early ages Figure 10-11 (a) Effect of concrete temperature on the slump and the water requirement to the change slump; (b) Effect of ambient temperature on the water requirement of concrete. (a)—left, (b)---right Chapter 10 concrete at early ages Control of concrete temperature. The use of cooled mixing water and/or ice offers the best way of lowering the temperature of concrete. The use of shaved or chipped ice as a substitute for all or part of the required mixing water is the most effective way of reducing the concrete temperature because ice absorbs 80 cal/g (144 Btu/lb) on melting. Chapter 10 concrete at early ages Figure 10-12 Determination of reduction in concrete temperature: (a) by adding cooled water; (b) by adding ice. a) b) Chapter 10 concrete at early ages 10.11. Testing and Control of Concrete Quality 10.11.1. Methods and Their Significance Engineers representing the owners, designers, and builder of structures are frequently required to develop or approve a quality assurance program that, among other things, involves the selection of test methods, statistical analysis of the test results, and follow-up procedures. The objective of such a program is to assure that a finished concrete element is structurally adequate for the purpose for which it was designed. Chapter 10 concrete at early ages Accelerated strength testing offers one solution to the problem. Increasingly, large projects are using procedur