Concrete Curing and Maturity
Curing time by method and cement type, the maturation factor at your temperature and days, the percent of 28-day strength you have now, and the evaporation risk that decides the method.
A slab held at 70°F for 7 days has a maturation factor near 27 and about 92% of its 28-day strength, and a 4,000 psi mix is at 3,680 psi. Curing at 50°F for the same 7 days lands at 63%, which is why a cold slab forms slowly.
How the math works
- Curing time (ACI 308.1R): 3 days of continuous moisture at 50 to 70°F for a conventional mix, 7 days where slag, fly ash, or a Type IL cement is in the mix, and 7 days for a mass pour. Hot, dry, and windy takes 7 days on any mix, and 12 hr of a membrane compound at finishing counts as the initial cure.
- Maturation factor (ACI 503): MF = hours of curing × (temperature − 32°F), divided by 1,000, counting 32 to 100°F, and 28 days at 70°F is a maturation factor of 26.8. Strength tracks the maturation factor, not the calendar.
- Strength curve (ACI 503): percent of 28-day strength = 17.5 × MF^0.51 for a maturation factor of about 3 to 30, which puts 26.8 at 90% and 13.4 at 63%. Type III and a hot mix climb faster on the same curve shape.
- Form removal (ACI 308.1R and ACI 347): 12 hr for a vertical wall form at 1,000 psi, 36 hr for a beam bottom at 1,000 psi with the shoring left in place, and 3,500 psi for a beam bottom with a span over 20 ft. The maturity meter is the honest answer, not the hour clock.
- Evaporation (ACI 305R): the plastic-shrinkage threshold is about 1.0 lb of evaporation per 1,000 sq ft per hour, and hot, dry, and windy pushes past it. Fogging, an evaporation retarder at finishing, and a cure applied at screed are the four controls, and a fogger at finishing is the cheapest.
How long should you cure a concrete slab? Three days of continuous moisture at 50 to 70°F on a conventional mix, and seven days where the mix has slag, fly ash, or a Type IL cement, with seven days in hot, dry, and windy weather. A membrane compound is a one-spray cure at finishing, and water and burlap beat film on a hot slab.
Hosing a slab twice a day is not continuous curing, and a poly film applied wet and sealed at the edges is a better cure than a sprinkler that runs dry at noon.
What is a maturation factor in concrete?
The time and temperature the concrete has seen, added up: hours times the degrees above 32°F, divided by 1,000. 28 days at 70°F is 26.8, and strength follows 17.5 × MF^0.51 on that scale, so a maturation factor of 13 is about 63% of the 28-day strength. It is the ACI 503 maturity method, and a maturity sensor in the slab reads it live.
Can you cure concrete with a hose? Yes, and continuous water is the best cure there is: a sprinkler that runs all day, a ponded slab, or a fog. Twice-a-day hosing is not continuous, and a wet burlap that dries out at 3 pm is a wet-dry cycle, which is the cure that produces craze cracking.
Water curing on a hot, dry slab is the one case where the cure has to start at finishing, not the next morning, because plastic shrinkage cracks are set in the first two hours.