Pouring Concrete When It's Hot

WHY IS HOT WEATHER A PROBLEM?

Air temperature, relative humidity, and concrete temperature, determine the evaporation rate. If the evaporation rate is greater than 0.1, plastic shrinkage cracking is likely.

Concrete sets as the cement hydrates. Hydration is a chemical reaction that generates heat, and that reaction moves faster when the concrete is hot. So the main concern with the concrete’s strength and set time isn’t really the air temperature but the concrete temperature. When cement hydrates it absorbs water and grows crystals around the particles. When it’s hot and that reaction is sped up, the crystals grow quickly but don’t have time to grow strong. Early strength will be not be effected, but the strength a month later will suffer. If the concrete is about 18° hotter than normal (for example, 86° instead of 68°), the eventual concrete strength will be approximately 10% lower. This is a substantial decrease, and will effect the structural integrity of the project.

Hot weather can cause increased cracking and plastic shrinkage cracking, if the concrete temperature is high and the humidity is low. Another potential problem in hot weather is surface drying. If the concrete is warm and the sun is shining and there is a hot dry wind across the concrete, you are likely to get more drying and surface shrinkage. When one part of the concrete mass is warmer than another part, and the differential is greater than about 20°F then it is likely the concrete will crack. This tends to be a chronic problem in thick concrete (thicker than 18 inches), but frequently happen in smaller pours as well.

STIFFENING, CRUSTING AND CRACKING

In hot weather, protection is required to stop concrete from stiffening or crusting and to help minimize cracking, crazing and plastic shrinkage. When temperatures are above 70 degrees, concrete set times accelerate much faster than normal. This can cause a chain reaction of ill-effects, as the drying process becomes inconsistent. In an attempt to offset effects of high temperatures, low humidity or winds, contractors often erect sun shades and wind barriers. Other best practices are to fog or sprinkle concrete with cool water; forms, reinforcing steel and sub-grade, the evening before, and to ensure there are no puddles on forms or sub-grade.

PITFALLS POURING CONCRETE IN HOT WEATHER

Typical mistakes contractors make when pouring concrete in hot weather is adding excessive water to the concrete mix. In an attempt to keep the concrete cool, and hydrated, so it doesn’t dry too rapidly, water is added throughout the process. Water weakens concrete and can cause scaling and/or excessive cracking.

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