Precast Inc. - May/June 2008 - (Page 13) Figure 1 – Effects of humidity and temperature on curing Courtesy of Portland Cement Association. Effects of hot weather on concrete properties Hot weather conditions can lead to problems in mixing, placing and curing hydraulic cement concrete that can adversely affect the properties and serviceability of the concrete. If precautions are not effectively implemented during hot weather, the concrete may be damaged through plastic-shrinkage cracking, thermal cracking and decreased 28-day strengths. Once damaged, the concrete can never be entirely restored. Increased rate of cement hydration at elevated temperatures and the increased evaporation rate of moisture from the freshly mixed concrete are the causes of most of the problems associated with hot-weather concreting. The ability of a mix to reach its design strength is determined by the efficiency of the chemical reaction that takes place between water and cement. That reaction is responsible for solidifying the entire concrete mass. As concrete hardens, cement is said to be hydrating and the concrete is said to be curing. In principle, curing refers to the concrete’s gain in strength, but technically speaking, the rate of cement hydration is what can be adversely affected during hot weather. Potential problems associated with hot weather can be categorized into three different groups: problems for concrete in a freshly mixed state; problems for concrete in the hardened state; and problems related to other factors (See Table 1). Figure 2 – Set time as a function of air temperature Temperature Degrees F 100 90 80 70 60 50 40 Degrees C 37.8 32.2 26.7 21.1 15.6 10.0 4.4 Approximate Set Time (For a sample mix) Hours 1 2/3 2 2/3 4 6 8 10 2/3 14 2/3 Temperature, water and slump While increased concrete temperatures produce higher earlier strengths, the concrete’s 28-day strengths are lower and the final product may never reach its optimal design strength, as seen in Figure 2. We know that when concrete cures, the hydration process creates added heat and raises the temperature of the concrete, but excessively high ambient temperatures and solar radiation also contribute to the heating effect. Water is obviously a crucial component that must be carefully regulated in any precast mix design, but this is especially true for hot weather conditions. The higher the temperature of the concrete, the more water needed for the required slump (increases with time). If water is not added to the mix, placing and handling operations may be negatively affected. An increase in water should be offset by a proportional increase in the quantity of cementitious material, which will increase production costs. If water is simply added to the mix without the addition of cementitious material, the water/cement MAY/JUNE 2008 | WWW.PRECAST.ORG 13 http://www.precast.org
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