The current definition, ACI 306R-16 §2.2
Cold weather is when the air temperature has fallen to, or is expected to fall below, 40°F (4°C) during the protection period.
Three things to read closely:
- The threshold is 40°F, not 32°F. 32°F is the freezing point.
- "Expected to fall" means a forecast counts. You do not wait for the drop to happen.
- The window is the whole protection period, not just the time of placement.
Applying it
A slab is placed on a 58°F afternoon. The forecast calls for 36°F two nights later, inside the protection period, and the last three days averaged 55°F. That is cold weather under ACI 306R-16: the air is expected to fall below 40°F during the protection period.
- The warm placement temperature does not control; the definition looks ahead.
- The three-day average does not control; that test belongs to the old standard.
The superseded definition, ACI 306.1-90
The older standard defined cold weather as the average daily temperature below 40°F for more than three successive days. Applying this three-day test instead of the 306R-16 protection-period definition is the common error.
Under 306.1-90, the average daily temperature is the mean of the midnight-to-midnight high and low:
- High 46°F, low 30°F: (46 + 30) ÷ 2 = 76 ÷ 2 = 38°F.
- 38°F is below 40°F, so the day counts toward the three-successive-days test.
Use both the high and the low. Taking only the high (46°F) or only the low (30°F) is wrong.
When the old cold period ends
ACI 306.1-90 says that when temperatures above 50°F occur during more than half of any 24-hour duration, the period is no longer regarded as cold weather.
- One reading above 40°F does not end it; 40°F is the start threshold.
- Three successive days above 40°F mirrors the start test and is not the end rule.
- 500 psi is a concrete strength, the freeze-protection figure, not a weather definition.