Michigan Concrete Association

SUCCESSFUL HOT WEATHER CONCRETING
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.

RECOGNIZING HOT WEATHER
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 CRACKING
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 bleedwater 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.

CURING
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.

RECOMMENDED CONSTRUCTION PRACTICES
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:

1. Prior to placement, dampen forms and subgrade materials.
2. Consult your ready mix supplier for hot weather mix design alternatives.
3. Schedule prompt transport, placement, and finishing of concrete. Whenever possible, schedule early morning placements.
4. Erect temporary windbreaks to reduce wind velocity over the concrete surface.
5. Erect temporary sunshades to reduce concrete surface temperatures.
6. To minimize surface evaporation, incorporate fog misting or evaporation retarders. Continue fog misting until curing is initiated. Evaporation retarders (usually polymers) can be spray applied following screeding with little or no affect on finishing or adhesion of membrane curing compounds.
7. Incorporate synthetic fibers (polypropylene) to resist shrinkage cracking.
8. The follwing chart provides a graphic method of estimating the loss of surface moisture for various weather conditions. To use the chart follow the four steps outlined above. If the rate of evaporation approaches 0.2 lb/ft²/hr (1kg/m²/hr), precautions against plastic shrinkage cracking are necessary (Lerch 1957).

Figure. 1 - Effect of concrete and air temperatures, relative humidity, and wind velocity on the rate of evaporation of surface moisture from concrete.