Concrete Pour Weather Limits
A cancelled pour costs a truck, a crew, and a day. A pour you should have cancelled costs a slab. Here are the weather limits that matter — and why the ambient temperature on your phone is the least useful of them.
Cold weather: below 40°F
ACI 306 defines cold-weather concreting as beginning when the average daily air temperature drops below about 40°F for three consecutive days. The practical rules:
| Condition | Effect | Response |
|---|---|---|
| Below 50°F | Hydration slows; set times stretch | Expect longer finishing windows |
| Below 40°F | Strength gain badly retarded | Heated enclosures, insulating blankets |
| Freezing before ~500 psi | Permanent damage — expansion disrupts the paste | Do not place unprotected |
| Frozen subgrade | Settlement when it thaws under the slab | Never place on frozen ground |
The overnight low is the number that matters, not the daytime high. A pleasant 55°F afternoon that drops to 28°F at 2 a.m. will freeze concrete that has not reached strength. Check the low for the full protection period, not the pour window.
Hot weather: it is not the temperature, it is the evaporation rate
ACI 305 flags hot-weather concreting above roughly 90°F — but temperature alone is a poor predictor. What cracks a slab is evaporation rate, driven by four variables together: air temperature, concrete temperature, relative humidity, and wind speed.
The threshold is about 0.2 lb/ft²/hr. Above it, plastic shrinkage cracking becomes likely — the surface loses water faster than bleed water can replace it, and it tears while still plastic.
The trap that catches people: 70°F, 30% humidity, 20 mph wind will exceed the evaporation threshold. That is a beautiful spring day on any forecast app, and it will crack your slab. Meanwhile 95°F at 80% humidity with no wind may be fine. Wind is the variable nobody checks.
Rain: timing is everything
| When | Risk | Verdict |
|---|---|---|
| Before placement | Standing water in forms, soft subgrade | Remove water; check subgrade |
| During placement | Raises surface water-cement ratio | Stop — this is the danger zone |
| During finishing | Scaling, dusting, weak surface | Stop; do not finish rain into the surface |
| After set (surface unmarred) | Minimal | Often beneficial — free curing |
The cardinal sin is working rainwater into the surface while finishing. You are locally raising the water-cement ratio in exactly the layer that needs to be strongest. The result shows up months later as scaling and dusting — long after anyone connects it to that Tuesday.
Documenting a weather-cancelled pour
A cancelled pour is a schedule impact, and it is claimable if the weather was abnormal and it hit critical-path work. But "it looked like rain" will not survive a GC's scheduler. Record, on the day:
- The forecast you relied on, captured at decision time — with a timestamp
- Measured conditions: temperature, humidity, wind, precipitation
- The specific limit exceeded — cite it: "ACI 305 evaporation rate exceeded 0.2 lb/ft²/hr" is a technical finding; "too windy" is an opinion
- Cost of the cancellation: trucks ordered, crew hours idled
- Whether it was on the critical path — if it had float, there is no claim
Then compare against the baseline: were these conditions abnormal for that month at that site? That is the test A201 §8.3 actually applies, and it is the leg most claims are missing. Details in documenting a weather delay claim.
Planning around it
Concrete scopes have a different weather profile than earthwork. Earthwork loses days to soil that stays wet long after the rain stops; concrete loses days to narrow placement windows — any rain during the pour, cold nights during cure, high evaporation during finishing. A contingency built from rain days alone will be wrong for both, in opposite directions.
Build your weather contingency from data
Ilystics generates an Anticipated Weather Exhibit for your jobsite: 10-year NOAA baseline with month-by-month precipitation and temperature. Free, no account required.
Generate a Free ExhibitACI references are provided for general orientation only and are not a substitute for the standards themselves, your project specifications, or your mix design. Placement decisions are the responsibility of the contractor and engineer of record. Ilystics provides weather and soil data, not engineering judgement.