Concrete doesn't dry. It hardens because the cement reacts with water, and that reaction keeps going for weeks. It needs moisture and warmth to keep going. Take away either one early and the slab never reaches the strength on the ticket.
The numbers below are rules of thumb for an ordinary portland cement mix, moist-cured at around 70°F. Your ready-mix supplier's data for the mix you're pouring wins. High-early mixes gain strength faster, and mixes with a lot of fly ash or slag gain it slower.
The chart
| Age | Rough share of 28-day strength |
|---|---|
| 1 day | 15–25% |
| 3 days | 40–50% |
| 7 days | 65–70% |
| 14 days | 85–90% |
| 28 days | 100% (the design strength) |
28 days is the benchmark by definition. The strength on the spec, written f'c, is the 28-day strength. A 4,000 psi mix is expected to test at 4,000 psi or better at 28 days. Concrete keeps gaining a little after that, but nobody counts on it.
7-day cylinders are an early warning. If the 7-day breaks come in well under two-thirds of the design strength, find out why before the next pour: water added at the truck, cylinders left in the sun, or the wrong mix.
What the customer can do, and when
Put a schedule like this in the proposal, in writing, adjusted for your mix and your weather:
- Foot traffic: after about 24 to 48 hours.
- Cars: after about 7 days.
- Heavy trucks, like a moving truck, dumpster or delivery: hold off to 28 days, or until your supplier says the mix is there.
The day-three phone call about a car parked on the new driveway is easier when the homeowner signed a page that said seven days.
On flatwork, forms usually come off the next day. Driveway edges are more easily chipped early, so keep the equipment and the wheelbarrows off them.
Weather changes the clock
Cold slows everything. Below about 50°F strength gain slows noticeably. At or near freezing it nearly stops. Fresh concrete that freezes before it reaches about 500 psi can be permanently damaged. The industry guidance on cold-weather concreting (ACI 306) treats roughly 40°F and falling as cold weather and calls for protecting the pour.
In practice: check the overnight lows for the first few days, not just pour day. Have insulated blankets on the truck. Ask your supplier about hot water, accelerators or a high-early mix. Never pour on frozen ground.
Heat and wind dry the surface too fast. That causes plastic shrinkage cracks and a weak, dusty surface. Pour early in the day, keep the surface from drying out, and get curing started as soon as finishing is done.
Curing is part of the job
Curing means keeping the slab moist and at a reasonable temperature long enough for it to gain strength. The usual ways:
- Curing compound sprayed on right after finishing
- Wet burlap or curing blankets kept damp
- Plastic sheeting laid flat and weighted down
Write down which one you used and when on each job, with the weather. If a slab scales or cracks and the customer calls, that note is what shows the work was done right.
Keep the supplier's delivery tickets and any cylinder test reports with the job file. On commercial work they're usually required anyway.
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- Change orders: get the extra work signed before you do it
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