Precast Inc. - May/June 2008 - (Page 16) Planning for the Heat A little planning can go a long way if your plant traditionally faces extreme weather conditions. If you do not have a hot-weather concreting plan, now is the perfect time to gather your technical and production managers and create one. Mixing, protection, placing, temperature monitoring and testing of concrete are issues that must be addressed, in addition to defining the combination of factors (air temperature, wind, sun, humidity) that would set the plan into motion. Once the plan has been created, it should be written and communicated to all employees involved in mix design and production. Factors to consider include: • Proper selection of concrete materials and proportions • Proper cooling procedures • Implementing a concrete with a consistency that permits rapid placement and rapid consolidation • Maximizing efficiency when placing, consolidating and finishing the concrete (reduce time) • Hours of operation during extreme weather conditions (if possible) • Protection of the concrete from moisture loss during curing precautions, based on local conditions and the upcoming weather forecast: Water. While water seems to cause most of the problems in hot-weather concreting, controlling the water temperature is easier to execute and has the greatest effect per unit weight on the temperature of concrete. This is because water has a specific gravity that is four to five times that of aggregates or cement. In general, adding cool water to the mix will reduce the overall concrete temperature, but typically not more than 8 F (4.4 C). The ACI 305 document estimates that lowering the temperature of the batch water by 3.5 F to 4 F (1.9 C to 2.2 C) will reduce the concrete temperature approximately 1 F (0.5 C). Ice. Adding ice chips to the concrete mix must be done properly to be effective. Ice must be crushed, chipped or shaved before it is added into the mixer. For maximum efficiency, the ice should not melt before it is placed into the mixer, but the ice should be fully melted before the mixing of the concrete is complete. As ice melts, it absorbs the heat from the concrete at an estimated rate of 144 Btu per pound and lowers the overall concrete temperature. The ice should not comprise more than 75 percent of the batch water. If proper procedures are followed, ice can potentially lower the concrete temperature as much as 20 F (11 C). If a 20 F reduction in temperature is still not enough, injecting liquid nitrogen into the mixer is another option. Cement. 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 that which meets strength and durability requirements. Also consider that if newly manufactured cement is delivered to your plant, its temperature may be elevated. According to ACI 305, concrete mixtures consist of approximately 10 percent to 15 percent cement. Using that estimate, each 8 F (4.4 C) increase in cement cause moisture loss. In windy, dry climates, moisture loss in freshly placed concrete can be accelerated and cause evaporation of water from the concrete member. This leaves less water in the concrete mix than was called for by design. Without proper precautions, water remaining in the mix cannot completely hydrate the cement, resulting in less than optimal economic efficiency and a decrease in strength and durability in the final product. Here are a few precautions that help prevent moisture loss; most precasters will choose some combination of these BOOTH 1344 16 MAY/JUNE 2008 | PRECAST INC. http://www.eagleeyeproducts.com http://www.eagleeyeproducts.com
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