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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  • Keep an eye on the weather to avoid low relative humidity and high winds.
  • Plan for placing and finishing flatwork earlier in the day to avoid these low humidity afternoon conditions when needed.
  • Use wind breaks to protect from rapid evaporation.
  • Use foggers to increase the relative humidity above the slab.
  • Wet the subgrade to avoid evaporation from below the slab.
  • Maximize the water available per design for your flatwork mixes.
  • Order monofiliment (M100) fibers to minimize the surface cracking.
  • Order viscosity modifying admixture (VMA) to increase resistance to segregation, enhance finishing and control bleeding (we allow up to 6 oz/cwt if needed).
  • Order an accelerators to minimize the differential set times between the concrete core and the surface.
  • When needed, use Confilm after initial finishing to reduce surface evaporation.
  • Cure the concrete immediately after final finishing or brooming.
  • Installation of saw cut joints may need to occur sooner than usual.