Concrete poured in hot weather, low humidity, or high wind can suffer adverse effects if proper adjustments are not made to the process. Any time hot and/or dry conditions are present when pouring concrete, it is important to schedule the work during the coolest part of the day, if possible, and to have plans in place to keep the concrete cool. Among other things, this can include using shade to block direct sunlight or spraying aggregates with cold water to keep them cool.
How Concrete Sets
Concrete sets by hydrating. In other words, it sucks up water and forms crystals around the particles in the concrete. The cooler the concrete, the longer this process takes and the more time the crystals have to strengthen. When the concrete is hotter, the crystallization process happens more quickly, giving the crystals less time to strengthen. Evaporation also can have a negative effect on the surface layer of the concrete. The lack of water there will lead to weaker concrete at the top of the slab, which means the concrete will be more susceptible to cracking.
Concrete vs. Cement
These terms sometimes are used interchangeably, but cement is an ingredient in concrete. Cement is made up of water and Portland cement, which is primarily limestone in the form of fine powder. The paste it forms is combined with larger aggregates to make concrete.
While hot weather is an important and notable obstacle, it's necessary to understand that maintaining an appropriate level of moisture in the concrete is the highest priority. While temperatures in excess of about 85 degrees can make this difficult, low humidity and high wind speeds also can increase the rate of evaporation, even at lower temperatures. So, what often is described as "hot" weather when pouring concrete is often a combination of factors that combine to make it more challenging to retain moisture in the concrete.
Setting and Curing Times
According to the Penn State University College of Engineering, concrete will set in anywhere from about two to 19 hours, depending on the temperature. It will set in slightly less than two hours at 100 degrees Fahrenheit, but at 30 degrees Fahrenheit, it will take 19 hours. It will not set in temperatures as cold as 20 degrees Fahrenheit.
Keep in mind that the setting is not the same thing as curing. Setting simply means the concrete has reached a completely solid-state, but it still needs additional time to reach its full strength. According to the American Concrete Institute, concrete typically reaches 70 percent of its strength within seven days, and after 28 days concrete is generally considered to be fully cured and at 100 percent strength.
Tips for Placing Concrete in Hot Weather
Concrete placed in hot weather sets more quickly and produces higher early strength, but its ultimate strength through the curing process will be lower than expected. Proper mix design can compensate for these conditions, and in combination with protective measures to prevent rapid evaporation, quality concrete can be poured in hot temperatures with consideration of additional factors.
- Have sufficient manpower to manage the concrete when it is being poured and for the finishing process.
- Use a large size and amount of coarse aggregate particles if hot weather is likely to occur during the concrete placement. Larger aggregates will minimize the probability of having concrete shrink due to environmental conditions.
- If possible, avoid placing concrete at noon or during the afternoon.
- Plan with the batch plant an acceptable delivery temperature so that materials can be cooled by the supplier as needed. Aggregates can be cooled down by spraying water over the stockpile.
- Consult with the structural engineer or designer to maximize and implement an effective plan to properly space control joints. When placing concrete in hot weather, control joints should be spaced at smaller intervals than cold weather concrete joints.
- Use sunshades or windbreaks to reduce possible harsh conditions.
- Plan to have indoor slabs poured after all walls and roofs are built.
- Keep an evaporative retarder ready on-site in case the temperature gets hotter and water is rapidly evaporating.
- Use ice as part of the concrete water mix, or use liquid nitrogen to cool the concrete.
- Reduce the mixing time once the water has been added to the mix.
- Consider batching and mixing at a job site plant.
- Do not add water to the premixed concrete unless it is part of the design.
- All necessary equipment should remain covered until the last moment before using. Keep chutes, conveyors, and accessories under a roof if possible and spray some water over them regularly.
- When placing concrete for a slab, first dampen the subgrade.
- Use cool water to dampen side forms for slabs or walls.
- Do not begin finishing concrete while water is still on the surface.
- Implement the correct curing method to allow the concrete to set uniformly.
- Be ready to receive and place concrete.
Prepare for These Hot Weather Problems
When pouring concrete in hot weather, you must be prepared for some possible problems. Due to the temperatures during the pour, shrinkage cracks can be quite deep. This deep cracking is because the concrete has little capacity to resist shrinkage stresses. The cracks can continue to widen and propagate until the shrinkage stresses are relieved. Other problems may include:
- Difficulties in finishing the concrete could increase.
- Cold joints could be formed due to hot weather decreasing the setting time.
- Strength and durability characteristics may be reduced.
- Concrete compression tests could yield lower strength results.
- The drying shrinkage of the hardened concrete could increase.
- Placing concrete in hot weather produces an increased rate of the slump. This means a slab of concrete will not be consistent throughout.
- The risk of thermal cracking could increase.
- The heat of hydration raises the temperature of the interior of the concrete.