Why curing matters: protecting concrete in the first 7 days
Concrete does not "dry" into strength. It gains strength through a chemical reaction between cement and water, and that reaction needs moisture. If the surface dries out too early, the concrete can end up weaker and more prone to cracks.
Start early
Begin curing as soon as the surface is firm enough not to be marred, usually within hours of finishing. In hot, dry or windy weather, protect the surface even sooner with shade or wind breaks.
Common curing methods
- Ponding: for flat slabs, build small mud or sand bunds and keep a shallow layer of water on the surface.
- Wet coverings: wet gunny bags or cotton sheets, kept damp, work well for columns, walls and slabs.
- Curing compounds: a liquid membrane sprayed on the surface slows moisture loss, which helps on large areas or where water is limited.
- Plastic sheets: they reduce evaporation, but make sure the edges are weighed down and the concrete does not heat up under direct sun.
How long?
A common rule of thumb is at least 7 days for ordinary Portland cement concrete and longer, often 10 to 14 days, for blended cements such as PPC or in hot weather. Your engineer or the project specification gives the final number.
Mistakes to avoid
- Splashing water on a hot surface once a day and calling it curing. The surface should stay continuously moist.
- Removing formwork early and leaving the concrete exposed.
- Loading slabs with bricks or sand before the concrete has gained adequate strength.
Good curing costs very little compared with the cost of repairing cracks later.
