Evaporation Rate Forecast
Plastic-shrinkage cracking is frequently associated with hot weather concreting in arid climates. It occurs in exposed concrete, primarily in flatwork, but also in beams and footings.
Surface drying is initiated whenever the evaporation rate is greater than the rate at which water rises to the surface of recently placed concrete by bleeding.
High concrete temperatures, high wind speed, and low humidity, alone or in combination, cause rapid evaporation of surface water.
Plastic shrinkage cracking is seldom a problem in hot-humid climates where relative humidity is rarely less than 80%.
Early in the bleeding process, however, and at rates of evaporation less than or equal to 0.2 lb/ft2/h, the method has been shown to be in good agreement with water
loss measurements, as long as the temperature, humidity, and wind speed have been measured. Because bleeding rates vary from zero to over 0.2 lb/ft2/h, over time,
and are not normally measured, it is common to assume a value for the critical rate of evaporation. The most commonly quoted value is 0.2 lb/ft2/h.
The probability for plastic-shrinkage cracks to occur may be increased if the setting time of the concrete is delayed due to the use of slow-setting cement, an excessive dosage of retarding admixture, fly ash as a cement replacement, or cooled concrete.
(ACI 305 R-99: Hot Weather Concreting)
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Evaporation Rate Calculator
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