Curing

Frequently asked questions

Is curing the same as drying?

It is the opposite, and treating them as the same is how a slab is permanently weakened on its first day. Concrete gains strength by HYDRATION — cement reacting with water — and that reaction continues only while water is present. A slab that is allowed to dry stops hydrating, and the strength it has not gained by then it never gains: the reaction does not resume when it rains next week, because the microstructure has already formed around the water that was there. So curing means keeping water IN, by ponding, by covering with polythene or wet hessian, by fogging, or by spraying a curing compound that forms a film to stop evaporation. The confusion is reinforced by everyday language — nobody says a floor is curing, they say it is drying before the tiles go down — and that later drying, of a hardened slab losing its residual moisture before a floor finish, is a genuinely different process happening weeks afterwards.

Why does concrete crack before it has set?

Because water leaves the surface faster than bleed water rises to replace it, and PLASTIC SHRINKAGE cracks open in concrete that is still soft. The rate depends on four things together — air temperature, concrete temperature, relative humidity and wind speed — and wind is the one people underestimate: a moderate breeze can multiply the evaporation rate several times over on an otherwise unremarkable day. Above roughly a litre per square metre per hour the risk is high, which is the figure the standard evaporation nomograph exists to estimate. The cracks are typically shallow, roughly parallel and open within the first few hours, and they are entirely preventable by the measures that reduce evaporation: wind breaks, fogging the air above the surface, covering between operations, and not pouring a large flat area into a drying wind at all. They are also not structural cracks, which is why they are so often dismissed — and why they later become a durability problem in a slab exposed to salt.

What does the maturity method actually let you do?

Stop guessing when concrete is strong enough, and stop breaking cubes to find out. Strength gain depends on temperature as well as time, so a cylinder cured in a laboratory tells you little about a slab that spent its first night at four degrees. The maturity method instead logs the concrete's own temperature history, converts it into a maturity index, and reads strength off a curve established for that specific mix by breaking specimens at known maturities. Because the sensor is in the element itself, it reports what that element has actually achieved — so formwork can be struck, a slab loaded, or post-tensioning stressed as soon as the element is ready rather than on a calendar. The two conditions people forget are that the calibration curve belongs to ONE mix — change the cement source and it is void — and that it measures strength, not durability, so it cannot shorten a curing period specified for cover protection rather than for load.