The chart you wish you had on the truck dash. How fast a slab gains strength changes a lot with temperature — here's a foreman's reference for the ranges you'll actually see.
This chart assumes adequate moisture (good curing) and a typical 4,000 psi mix. Numbers are rough averages — your mix design, supplementary cementitious materials, and admixtures will shift these by a few percent. Use this for planning, not for final stressing decisions.
| Avg. Curing Temp | 1 day | 3 days | 7 days | 14 days | 28 days |
|---|---|---|---|---|---|
| 50°F | ~8% | ~25% | ~50% | ~70% | ~90% |
| 60°F | ~12% | ~35% | ~60% | ~80% | ~95% |
| 70°F | ~18% | ~40% | ~70% | ~88% | 100% |
| 80°F | ~25% | ~50% | ~75% | ~90% | 100% |
| 90°F+ | ~30% | ~55% | ~78% | ~88% | ~95%* |
*Hot-weather concrete sets faster but can finish lower at 28 days due to moisture loss during placement. The early gain looks great. The final number disappoints. This is the trap.
50°F is borderline. The slab's gaining strength — but slowly. If you placed at 50°F and it dropped to 40°F overnight, those numbers slide down further. This is why ACI specifies maintaining concrete above 50°F for at least 3 days when ambient is cold.
70°F is the sweet spot. The whole industry's strength curve assumes 70°F. Your spec sheet, your mix design, your cylinder breaks — all calibrated to this temperature. When you're close to 70°F you can trust the numbers.
90°F+ is the trap. The slab gains strength fast in the first few days, which makes everything look great. But hot weather pulls moisture out before hydration finishes, so the final 28-day strength can come in lower than spec. The 7-day cylinder break looks fine. The 28-day break is where you find out.
Wind and humidity. Not on the chart but huge in practice. A 95°F day with 60% humidity and no wind is workable. The same temperature with 20% humidity and a 15 mph wind is dangerous — evaporation rate spikes and the surface loses moisture before hydration starts. Evaporation rate ties it all together.
Mix design. Type 1L blended cement (the new normal) hydrates a little slower than the old Type 1. Slag and fly ash slow it down further. High-early mixes speed it up. Add 10% to early-age strength for high-early; subtract 10% for slag-heavy mixes.
Curing method. The numbers above assume good curing. Skip the curing compound, leave the slab uncovered, let the wind dry it out — knock 15-20% off the 28-day number.
The temp on the chart isn't the temp on the slab. PourDay tracks ambient AND concrete temperature for your job site so you know what's actually happening at placement. Free on iOS.
Walk on it: 24 hours at 70°F+. Add half a day per 10°F below that.
Drive a passenger car on it: 7 days at 70°F+. Add 2-3 days at 50°F.
Drive a loaded truck on it: 28 days, regardless of temperature. Don't cheat this one.
Strip forms (slabs/beams): 3 days minimum, 7 days for anything significant. Engineer's call on big stuff.
Apply load (sealers, coatings, mortar): 28 days for moisture-sensitive products. Read the product spec — some require 60 days.
The 24-hour, 7-day, 28-day milestones in plain English.
Read articleThe temperature where curing stops, and how to keep heat in the slab.
Read articleThe 0.15 evaporation rule and how to protect the surface.
Read articleReal-time pour conditions. 16-day forecast. Pour logging. Free for contractors.