Concrete Surface Evaporation Rate and Plastic-Shrinkage Risk (ACI 305)
The finisher's go / no-go on plastic-shrinkage precautions, read off the day's weather: the ACI 305 / Menzel evaporation rate from the concrete and air temperatures, the humidity, and the wind. Above about 0.2 lb/ft^2/hr the surface cracks before it sets unless you fog, screen, retard, or cover. A 90 F, 40% RH, 15 mph pour evaporates several times faster than a calm humid one; the concrete temperature, not the air, drives it. A field screen, not a curing specification (curing follows ACI 308).
Formula and source
Tc = (concrete_temp_f - 32) / 1.8; Ta = (air_temp_f - 32) / 1.8; V = wind_mph x 1.609; E_metric = 5 x [(Tc + 18)^2.5 - (rh/100)(Ta + 18)^2.5](V + 4) x 1e-6; E_us = E_metric x 0.2048.
ACI 305 (Hot Weather Concreting) nomograph and the Menzel / NRMCA evaporation equation, by name; the F-to-C and mph-to-km/h conversions are exact.
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