Title:
After Two Hundred Years of Estimating Evaporation, It Is Still a Mystery
Author(s):
Kenneth C. Hover
Publication:
Symposium Paper
Volume:
338
Issue:
Appears on pages(s):
10-28
Keywords:
Bleeding, Cracking, Curing, Drying, Evaporation, Finishing, Shrinkage, Weather
DOI:
10.14359/51724723
Date:
3/1/2020
Abstract:
PCA researchers interested in the problem of evaporation of bleed water from concrete surfaces borrowed
an equation developed by hydrologists to predict evaporation from Lake Hefner in Oklahoma. PCA’s graphical
representation of that equation, subsequently modified to its present form by NRMCA, was later incorporated into
multiple ACI documents, and is known by concrete technologists world-wide as the “Evaporation Rate
Nomograph.” The most appropriate use of this formulation in concrete construction is to estimate the evaporative
potential of atmospheric conditions (known as “evaporativity”). Since the difference between actual and estimated
evaporation rate can be in the range of ± 40% of the estimate, best use of the equation as routinely applied is as a
semi-quantitative guide to estimate risk of early drying and inform decisions about timing and conduct of concrete
placing and finishing operations. Use of the “Nomograph” and related “Apps” in specifications is more problematic, however, given: 1.) the inherent uncertainty in its underlying equation, 2.) the difficulty in obtaining input data that appropriately characterize jobsite microclimate, and 3.) establishing a mixture-specific criterion for tolerable evaporation rate.