Evaporation Rate Calculator — Menzel / ACI 305R Significance

Why this calc exists

Plastic-shrinkage cracking happens within the first few hours after placement, before the concrete has set. The crack pattern is distinctive: parallel cracks across the slab, perpendicular to the wind direction, 0.3–1 m apart, 25–50 mm deep. The risk is governed by the rate at which bleed water evaporates from the surface vs the rate at which it rises from below.

Menzel's 1954 formula and its metric restatement by Uno (1998) compute this rate from concrete T, air T, humidity, and wind speed. ACI 305R plots the result on a nomograph with bands: < 0.5 kg/m²/h normal, 0.5–1.0 precautions, > 1.0 plastic-shrinkage danger. The evaporation rate calculator implements the metric formula with the band flags.

Code references (with provenance)

• ACI 305R-10 — Guide to Hot Weather Concreting. Includes the nomograph and band definitions.
• Menzel (1954) — Original English-units formula, ACI Journal.
• Uno (1998) — Metric restatement of Menzel, ACI Materials Journal.
• FHWA HRT-05-038 — Cautions that the nomograph overestimates above ~0.5 kg/m²/h (~50 % at 1.8 kg/m²/h).

Worked: Pune summer pour, Tc = 35, Ta = 38, RH = 35 %, wind = 18 km/h

Formula (metric, °C, wind in km/h): E = 5.275e-6 × [(Tc+17.8)^2.5 − RH·(Ta+17.8)^2.5] × (v+4.023).

Intermediate: [(35+17.8)^2.5 − 0.35·(38+17.8)^2.5] × (18+4.023) = [20250 − 8132] × 22.023 ≈ 12118 × 22.023 ≈ 266,840.

Result: 5.275e-6 × 266,840 ≈ 1.41 kg/m²/h.

Band: > 1.0 = plastic-shrinkage danger. Mitigate: windbreaks, sun-shades, fog misting, or evaporation retarder.

Field notes (what trips people up)

• Wind unit is km/h, not m/s. The Uno (1998) metric formula is dimensionally consistent only when v is in km/h. Using m/s gives a reading 2.24× off. The calculator's input field is labelled accordingly.
• The formula overestimates at high evaporation. FHWA HRT-05-038 shows the nomograph reads ~50 % too high at 1.8 kg/m²/h. For high-risk pours, treat 1.0 kg/m²/h as the danger threshold, not 1.2.
• Low-bleed mixes crack at lower evaporation. Modified concretes with low water content and high SCM may crack at 0.5 kg/m²/h or below. Use the band as a guide, not a guarantee.
• Wind at the slab, not at the site gate. Wind speed at 2 m height in open ground is half what's measured at the met station 10 m above. Reduce your measured wind by 50 % if the met reading is the only input you have.

Worked example

Site: highway pavement pour, 35 °C ambient, 50 % RH, 25 km/h wind.

Result: ~0.7 kg/m²/h — precautions band.

Action: erect windbreaks on the windward edge, schedule the pour for early morning, apply a single aliphatic-alcohol evaporation retarder after the bull-float pass, start fog-misting within 30 min of placement.

FAQ

Related reading & tools

• ../calculators/evaporation-rate.html — Evaporation Rate calculator
• hot-weather-concreting.html — Hot Weather Concreting (blog)
• calc-concrete-temperature-significance.html — Concrete Temperature Significance