Precast Inc. - May/June 2008 - (Page 12) TECHNICAL Hot-Weather Concreting How to keep on pouring when the temperature’s soaring. By Evan Gurley H ot weather creates special challenges for precasters, and technically speaking, there are more obstacles to overcome when placing concrete in hot weather than in the cooler seasons. By understanding how heat, humidity and wind affect the curing of concrete, you can adjust your mix and compensate in a variety of other ways to maintain high quality standards. While you will not likely need to take all of the recommended precautions stated below, each hot-weather scenario should be analyzed individually by qualified personnel, who should find the optimum mix of quality, practicability and economy. What’s hot? To an inexperienced precaster, “hot-weather concreting” can be a misleading label. If the ambient temperature outside isn’t “hot” then why should we be concerned about hot-weather concreting problems? The fact is that adjustments may need to be made to your mix as the weather becomes just slightly warmer, since your everyday mix can begin to perform differently as temperatures rise above 75 F (23.9 C). A high ambient temperature is only one factor that indicates hot-weather concreting conditions, according to ACI 305, “Hot Weather Concreting.” Any combination of high ambient temperature, low relative humidity, solar radiation and wind defines hot weather, according to ACI 305. Wind is not customarily associated with hot weather, for example, but it is an important factor because the wind accelerates the curing process in combination with temperature, humidity and solar radiation. Efforts to preserve concrete quality on windy, sunny days are more critical than those required on calm, humid days, even if ambient air temperatures are identical (see Figure 1). Selecting the Correct Cement and Admixtures The selection of the type of cement in a concrete mix, especially during hot weather, will play a major role in the overall performance of the concrete. The use of a slower-setting Type II portland cement or a blended cement (Type IP or IS) will typically result in improved handling in hot weather. Slower hydrating cements have a slower rate of heat development. Slower heat development, along with the dissipation of heat from the concrete, will result in an overall lower peak temperature. Slower-setting cement will also be less prone to thermal expansion cracking upon cooling due to lower peak temperatures. Note that the temperature increase from hydration is directly proportional to the cement content. The more cement in the concrete mix, the higher the temperature increase from hydration. Therefore the amount of cement used in your mix design should be limited to strength and durability requirements. While working in hot weather conditions, adjustments may need to be implemented in the mix design proportioning. Adjustments should be based in accordance with ACI 318/318R. Proper selection of materials and their proportions should be based on how the concrete will perform in current hot weather conditions, and proper adjustments should be made to prevent the possibility of cracking. Proportioning varies case by case, but there are some general guidelines to use to your advantage. Admixtures that reduce the bleeding rate help to prevent the surface from drying and cracking. These admixtures are of the hydroxylated carboxylic acid type (ACI 212.3R, Class 3) and meet ASTM C494, Type D requirements. Work with your supplier to optimize the dosage of any admixture you choose, including air-entraining admixtures, which may need to be increased depending on the combination of hot-weather precautions you take. Other admixtures such as fiber reinforcement can also help prevent cracking by helping to prevent drying shrinkage cracking. Water-reducing admixtures can help offset problems such as lower strength, higher water demand and slump loss. ASTM C 494 water-reducing admixtures can help keep the water demand down and design strength consistent while maintaining the design slump. Since admixture performance is correlated with cement type, you should work with your admixture supplier for proper admixture selection and dosage rate. 12 MAY/JUNE 2008 | PRECAST INC.
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