Learn How to Pour Concrete in Hot Weather
Placing concrete in hot weather is defined as the moment when the following conditions occur: High ambient temperature, low relative humidity and/or high wind speed. These conditions are further described and explained under ACI 305. Placing concrete in hot weather affects laboratory test results, showing that higher temperatures affect the compressive strength gain of hardened concrete. It is important that field test cylinders be protected by shading and to prevent evaporation.
When placing concrete in hot weather it is important to protect the top layer of concrete, which tends to dry faster than the bottom part. Concrete placed in hot weather will produce higher early strength but as time goes by, the ultimate strength 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.
Tips for Placing Concrete in Hot Weather
If you are planning to place concrete in hot weather consider these recommendations:
- 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 aggregates if hot weather is likely to occur during the concrete placement. Larger size 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 concrete temperature so 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.
- When pouring concrete in hot weather 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 water has been added to the mix.
- Consider batching and mixing at a job site plant.
- Do not add water to the pre-mixed concrete unless it is part of the design.
- All equipment needed to place concrete in hot weather 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 sub-grade.
- 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.
Placing Concrete in Hot Weather - Problems
When pouring concrete in hot weather, you must be aware of some issues if you don't follow the right steps.
Placing concrete in hot weather could affect the following:
- Placing concrete in hot weather could increase difficulties in finishing concrete
- Cold joints could be formed due to hot weather decreasing the setting time
- It is expected that strength and durability characteristics may be reduced
- Concrete compression tests could yield lower strength results
- Placing concrete in hot weather could increase the drying shrinkage of the hardened concrete
- It will produce an increased rate of slump
- High heat or hot weather could increase the risk of thermal cracking
- The heat of hydration raises the temperature of the interior of the concrete.
- Plastic shrinkage cracks can be quite deep as the plastic concrete has little capacity to resist shrinkage stresses, and cracks continue to widen and propagate until the shrinkage stresses are relieved