Precast Inc. - May/June 2008 - (Page 14) Table 1 – Potential problems during hot weather concreting (ACI 305) Potential Problems for concrete in the Freshly Mixed States • Increased water demand • Increased rate of slump loss • Increased rate of setting o Greater difficulty with handling, compacting and finishing o Greater risk of cold joints • Increased tendency for plasticshrinkage cracking • Increased difficulty in controlling entrained air content Potential Problems for concrete in the Hardened State • Decreased 28-day and later strengths (resulting from either higher water demand, higher concrete temperature or both at time of placement or during the first several days) • Increased tendency for drying shrinkage and differential thermal cracking (resulting from cooling of the overall structure, or from temperature differentials within the cross section of the member) • Decreased durability (resulting from cracking) • Greater variability of surface appearance, such as cold joints or color difference (resulting from different rates of hydration or different w/c material ratios • Increased potential for reinforcing steel corrosion • Increased permeability (resulting from high water content, inadequate curing, carbonation, lightweight aggregates, or improper matrix-aggregate proportions) Potential Problems related to other factors • Use of cements with increased rate of hydration • Use of higher-compressive strength concrete • Design of thin concrete sections with correspondingly greater percentages of steel • Economic necessity to continue work in hot weather • Use of shrinkage-compensating cement ratio of the mix will be compromised, resulting in a decrease in watertightness, strength and durability of the final product. The bottom line is that if extra water is needed for a given mix design, this water must be accounted for during mix proportioning. In hot weather, a mix will tend to set sooner than expected. There will be about a 30 percent decrease in set time for each 10 F (5.5 C) increase in concrete temperature, as shown in Figure 2. This decrease in set time can make handling, consolidating and finishing the concrete very difficult. When a decrease in initial set time is correlated with the decrease in slump, slump loss is taking place. As stated in ACI 305, there is about 1 inch (25 millimeters) of change in slump for every 20 F (11 C) increase in concrete temperature. increase in the concrete temperature in larger members. In larger precast concrete members, there is an increased rate of hydration and heat evolution that will increase the range of temperatures between the interior and exterior concrete, increasing the chances for thermal cracking. Plastic Cracking. Plastic shrinkage cracking is typically considered an arid-climate problem, but humidity is not the only determining factor. Low relative humidity in combination with high wind speed and/or high concrete temperatures can cause problems in freshly mixed/placed concrete members. These factors cause accelerated evaporation of surface moisture and become a problem when the evaporation rate exceeds the bleeding rate (rate at which water rises to the surface from within the concrete mix). The most commonly used bleeding rate value is 0.2 pounds per square foot (ACI 305). The potential for shrinkage increases when the evaporation rate exceeds the bleeding rate. Incorporating additional materials incorrectly into the concrete mix, along with hot weather conditions, increases the possibility for plastic shrinkage cracking and drying cracking. Fly ash, silica fume and fine cements have a low bleeding rate. This makes the concrete mixture very sensitive even in moderately arid conditions, increasing the possibility of plastic shrinkage. Extra precautions should be taken when incorporating these materials into the mix, given that plastic-shrinkage cracks are hard to repair. Cracking and shrinkage Even if you plan ahead and maintain the water-cement (w/c) ratio at an acceptable level, cracking in hot weather conditions may still occur, which further emphasizes the need to maintain complete control over your mixing, placing and curing practices. Three types of cracking are most common: Drying Cracking. Drying shrinkage typically occurs when water content in a mix is increased without adjusting the amount of cementitious material, altering the w/c ratio. Thermal Cracking. Thermal cracking may occur when fluctuations in ambient temperatures (such as a hot day followed by a cool night) cause a rapid drop in concrete temperature during initial strength gain. Thermal cracking can also be caused by an 14 MAY/JUNE 2008 | PRECAST INC. Preventing moisture loss Any combination of high ambient temperature, high concrete temperature, low relative humidity, solar radiation and wind can
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