Placing concrete in hot weather might mean cooling materials or placing concrete at nighttime to avoid daytime extremes.
Throughout summer, difficulties in mixing, placing and curing concrete often arise. High ambient/concrete temperatures, low relative humidity, and moderately high wind velocities contribute to rapid evaporation of moisture from the surface and accelerated set characteristics (hydration). By reviewing the guidelines presented in this bulletin, you will recognize and be prepared for hot weather conditions.
The evaporation chart located below can determine when precautions need to be exercised during hot weather conditions. Any combination of air temperature, relative humidity, concrete temperature, and wind velocity that results in an evaporation rate exceeding 0.10 lbs/ft2/hr will increase the probability of plastic shrinkage crack development and related finishing concerns. Precautions to minimize the rate of surface evaporation are recommended at this point. When the evaporation rate exceeds 0.20 lbs/ft2/hr, the recommended precautions become mandatory.
Plastic shrinkage cracks are defined as a network of discontinuous cracks that appear on the surface of a freshly cast concrete slab during or shortly after finishing. The cracks typically form parallel to each other at intervals up to 3 feet apart, penetrating the surface to a depth of 1-2 inches. Plastic shrinkage cracks develop when the rate of evaporation exceeds the rate of bleed-water migration to the surface. During placement, concrete is unable to withstand the tensile force resulting from the rapid evaporation of surface moisture (and associated linear shrinkage) eventually leading to plastic shrinkage crack formation.
Immediately following placement, cure the concrete with one of the noted techniques: membrane curing compound application, continuous seven-day soaking, wet burlap/polyethylene cover, curing paper.
To minimize the potential for plastic shrinkage crack development and related finishing concerns, the following recommendations are listed for planning your next hot-weather related construction project:
Michigan Weather is always changing, check out our recommendations on placing Cold Weather Concrete!