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.
Example
You enter
- Air temperature (°F) 90
- Concrete temperature (°F) 90
- Relative humidity (%) 40
- Wind speed (mph) 15
You get
- Metric rate 1.509 kg/m^2/hr
- Evaporation rate 0.309 lb/ft^2/hr
- Plastic-shrinkage risk TAKE PRECAUTIONS
Details, formula, and sources
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).
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.
The evaporation-rate equation and the plastic-shrinkage threshold are published in ACI 305; the arithmetic is public.
Estimate. AHJ and licensed professional govern.
Field names used by the API: air_temp_f, concrete_temp_f, rh_pct, wind_mph, E_metric, E_us, flag
- Driving term the concrete temperature drives the vapor coming off the surface; the air term is what the air can reabsorbACI 305 / Menzel
- Threshold 0.2 lb/ft^2/hr is the precaution line; low-bleed mixes crack at the lower 0.1 cautionACI 305
- Screen only a go / no-go on precautions, not a cure certification; curing follows ACI 308ACI 308