Concrete does not dry, it cures
Fresh concrete hardens through hydration, a chemical reaction between cement and water. That is why concrete keeps gaining strength even under water, and why keeping it moist during the first days makes it stronger. Strength grows fast at first and then slowly for months. By convention, concrete is specified by its 28-day compressive strength, for example 4,000 psi for a typical slab, or class C25/30 under the European standards.
Two standard curves
ACI 209 (American Concrete Institute) for moist-cured concrete:
f(t) = f₂₈ × t ÷ (a + b × t)
with a = 4 and b = 0.85 for ordinary Type I cement, and a = 2.3 and b = 0.92 for high-early-strength Type III cement.
Eurocode 2, used in Europe:
f(t) = f₂₈ × exp(s × (1 − √(28 ÷ t)))
with s = 0.20 for rapid-hardening cement (class R), 0.25 for normal cement (class N) and 0.38 for slow cement (class S).
Both assume a temperature of about 68 to 73 °F and moist curing.
Worked example
A slab of 4,000 psi concrete made with Type I cement, at 7 days:
- Strength: 4,000 × 7 ÷ (4 + 0.85 × 7) = 2,814 psi.
- Share of the 28-day strength: 70.4%.
- Days to reach 70%: about 6.9 days.
- Strength at 3 days: about 1,832 psi (46%).
Typical strength gain
| Age | ACI Type I | ACI Type III | Eurocode class N |
|---|---|---|---|
| 1 day | 21% | 31% | 34% |
| 3 days | 46% | 59% | 60% |
| 7 days | 70% | 80% | 78% |
| 14 days | 88% | 92% | 90% |
| 28 days | 100% | 100% | 100% |
Common milestones
- Walking: usually after 24 to 48 hours.
- Side forms of footings and walls: often after 1 to 2 days, once the edges will not be damaged.
- Cars on a driveway: about 7 days. Heavy trucks: 28 days.
- Slab and beam forms and shoring: typically when the concrete reaches about 70% of its design strength, often 7 days or more, or as the engineer specifies.
Temperature and curing
Cold slows hydration dramatically: below about 50 °F strength gain is much slower, and near freezing it almost stops. Fresh concrete must be protected from freezing until it reaches about 500 psi, or it can be permanently damaged. In hot, dry or windy weather, the surface can dry out before the cement has hydrated, leading to cracks and a weak, dusty surface.
Good curing means keeping the concrete moist and warm for at least the first 7 days: cover it with plastic sheeting or wet burlap, spray a curing compound, or mist it regularly. Properly cured concrete can be noticeably stronger and more durable than concrete left to dry out.
Limits
These curves are averages. The actual strength depends on the mix, the water-to-cement ratio, admixtures and the temperature history. For structural decisions, test cylinders or cores broken at specific ages, or maturity sensors embedded in the concrete, give the real answer.