Hot Weather & Concrete
Weather extremes can be a challenge when finishing concrete. The key to hot weather concrete is allowing the cement to properly hydrate and cure. When the ambient temperature goes beyond 80º, special precautions need to be made to ensure there is enough moisture in the concrete during in the curing process. If the water in the concrete evaporates too early, you will likely have plastic shrinkage cracking and scaling problems.

During hot weather, we use several techniques to ensure proper hydration of the cement paste. This includes lowering the temperature of the concrete mix ingredients at our plants and adding retardants. On the job site, there are several keys that will help you with the finishing of hot weather concrete.

Five factors to watch in hot weather
Weather circumstances have the greatest affect on concrete on hot days, Here are the five that will cause you the most trouble.

1. Hot and dry conditions
As we get into the dog days of summer, watch the relative humidity. The higher the humidity, the slower water will evaporate from the concrete surface. With low humidity, evaporation speeds up. Also watch the dew point. This is the temperature when water vapor turns to liquid, or dew. A low dew point means that moisture from any exposed material will quickly turn into vapor. On hot and dry days, make sure you are spraying cold water on your forms and your aggregate sub-base before pouring concrete. This keeps the moisture from being pulled into the sub-base. Make sure to get the concrete off of the truck as soon as you can. Shade any trucks in waiting. Heat is the enemy here.
2. Wind
Gusty winds will rob the surface of the concrete of its water. This leads to uneven water to cement ratios and, eventual scaling. You can combat the wind with a wind barrier. If you notice cracks appearing that are perpendicular to the wind, you are dealing with an evaporation problem. Mist the surface. You also should cover the concrete with a monomolecular film to retain surface water and recoat the surface after every finishing process. (These are sold at our plants, ask your IMI rep.)
3. Direct sunlight
On a cloudless, dry day, try to pour slabs on the west side of a building in the morning and slabs on the east side in the late afternoon or evening hours. If the sun dries out the surface quicker than the rest of the slab, bleed water can get trapped just under the surface and cause crazing. If you are placing concrete in an area where there is partial direct sunlight and shade, take extra precautions to block the sunlight from the solar exposed areas. Keep your tools in the shade. Mist the sun-exposed areas, but do not over-water the shaded areas. Ask your IMI rep about the use of retardants in your concrete batch. This will slow down the hydration process and buy your crews time on a hot, sunny day.
4. Quick temperature drops
Watch for quick temperature shifts in the forecast. In hot conditions, pop up storms can occur and greatly lower the air temperature. You will need to cover your concrete to ensure consistent curing. Do not add accelerators to the mix in the summer.
5. Rain, hail or anything else that falls from the sky
If cold precipitation falls on concrete that has just been finished, it may damage the surface. Cover curing concrete with plastic sheeting if there is a threat of storms. Never finish concrete in the rain.

Understanding hot weather concrete

Portland Cement Association
Precautions for hot weather concrete