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Courtesy of NPCA; Precast Inc. Magazine May-June 2008 -
"Hot Weather Concreting"
How to keep on pouring when the temperature’s soaring.
By Evan Gurley
Hot 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.
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).
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).
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 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.
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 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.
Preventing moisture loss
Any combination of high ambient temperature, high concrete temperature, low relative humidity, solar radiation and wind can 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 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 temperature will increase the concrete temperature by about 1 F (0.5 C).
Aggregates. If you consider that most mix designs include 60 percent to 80 percent aggregates, then the temperature and moisture content of aggregates should have the most significant impact on the concrete. A 1 F decrease in concrete temperature can be obtained by lowering the aggregate temperature 2 F, for example. Consequently, extra efforts should be taken to keep aggregates cool during hot weather.
Aggregate factors such as shape, size and grading all affect the amount of water needed in a mix to produce the required slump. Crushed coarse aggregates provide better resistance to cracking than round aggregates, but they also require additional water. Blending two or more sized aggregates can reduce the mixing water demand and increase workability.
Cementitious Materials. Adding supplementary cementitious materials (fly ash, slag, etc.) should be considered when it is necessary to delay the setting time or lessen the temperature rise from hydration.
Formwork/Reinforcement. Misting the forms and reinforcement immediately before placement can help cool them and prevent unwanted temperature increases. However, ensure that form release agents are not adversely affected, and always avoid pooling water anywhere within the forms.
Post-Pour. After placing and finishing concrete, you can prevent moisture loss by immediately covering fresh concrete with any moisture-retaining material such as burlap or a curing compound described in ACI 306. Retention of moisture will optimize the cement hydration process and allow the concrete to develop its full strength potential. Failure to keep exposed surfaces from drying excessively fast may result in cracking and shrinking, and jeopardizes the integrity of product.
Air content and temperature
As concrete temperature increases, entrained air decreases. Reduction in entrained air content is typically a result of slump loss. An increase in concrete temperature will require additional air entraining admixtures in order to maintain the air content of the mix.
In general, it is impractical to establish a maximum ambient temperature as your upper limit for production practices because of the other factors affecting the mix – concrete temperature, solar radiation, relative humidity and wind. Plants in climate controlled facilities, obviously, are not seriously affected by these factors. If your plant is not climate controlled, ACI 305 advises creating a set of measures that would include all the factors and testing your limits.