ACI 305R-10 Guide to Hot Weather Concreting

ACI 305R-10 Guide to Hot Weather Concreting. 3.1—Potential problems in hot weather Potential problems for concrete in the freshly mixed state include: • Increased water demand; • Increased rate of slump loss and corresponding tendency to add water at the job site; • Increased rate of setting, resulting in greater difficulty with handling, compacting, and finishing, and a greater risk of cold joints; • Increased tendency for plastic shrinkage and thermal cracking; and • Increased difficulty in controlling entrained air content. Damage to concrete caused by hot weather can never be fully alleviated. Potential deficiencies to concrete in the hardened state can include: • Decreased strengths resulting from higher water demand; • Increased tendency for drying shrinkage and differential thermal cracking from either cooling of the overall structure, or from temperature differentials within the cross section of the member; • Decreased durability resulting from cracking; and • Greater variability of surface appearance, such as cold joints or color difference, due to different rates of hydration or different water-cementitious material ratios (w/cm). 3.3—Practices for hot weather concreting Good judgment is necessary to select procedures that appropriately blend quality, economy, and practicability. The procedures selected will depend on type of construction, characteristics of the materials being used, and the experience of the local industry in dealing with high ambient temperature, high concrete temperatures, low relative humidity, and high wind speed. The most serious difficulties occur when personnel placing the concrete lack experience in constructing under hot weather conditions or in doing the particular type of construction. Last-minute improvisations are rarely successful. Early preventive measures should be applied with the emphasis on materials evaluation, advanced planning and purchasing, and coordination of all phases of work. Planning in advance for hot weather involves detailed procedures for mixing, placing, protecting, curing, and testing of concrete. Precautions to avoid plastic shrinkage cracking are important. The potential for thermal cracking, either from overall volume changes or from internal restraint, should be anticipated to be properly assessed. Typical methods to minimize and to limit crack size and spacing include: proper use and timely installation of joints, increased amounts of reinforcing steel, practical limits on concrete temperature, reduced cement content, low-heat-of- hydration cement, and selection and dosage of appropriate chemical and mineral admixtures.ACI 305R pdf download.