Curing is the set of measures that prevent the surface of freshly placed concrete from drying out too early. Cement needs water to harden. If that water evaporates from the top layer before hydration has progressed far enough, the cover concrete stays porous, weak and dusty — and the cover is precisely the layer that has to protect the reinforcement from corrosion.
Why it matters
The heart of the problem is that the damage is invisible. A poorly cured slab looks normal after a week, but its skin has become permeable to water, chlorides and CO₂. The results are dusting, early carbonation, plastic shrinkage cracking and, in time, corrosion of the reinforcement. Research into curing consistently shows that the expected service life of a structure drops sharply when concrete is cured for only one to three days instead of the specified period.
Methods
- Plastic sheeting — cheap and effective, provided it is lapped, sealed at the edges and cannot blow off; watch for mottling from condensation drips
- Wet hessian or fleece — must be kept continuously damp; alternate wetting and drying is worse than nothing
- Ponding or fogging — suitable for slabs, but never apply cold water to warm concrete: the thermal shock causes cracking
- Curing compound — a sprayed liquid membrane that slows evaporation. Check compatibility with any later screed, coating or adhesive
- Leaving the formwork in place — formwork is itself good curing; do not strike earlier than necessary, and cover the surface immediately after striking
How long?
The European execution standard EN 13670 uses curing classes 1 to 4. Class 1 is a fixed period of twelve hours; classes 2, 3 and 4 are tied to reaching 35%, 50% and 70% respectively of the characteristic 28-day compressive strength at the surface. The time needed depends on cement type, mix composition and temperature: in cold weather curing takes longer, not less.
Points to watch
- Start immediately. Curing begins as soon as the surface stops bleeding water, not the following morning
- Wind is worse than sun. Evaporation is driven mainly by wind speed and low humidity; put up windbreaks on exposed sites
- Frost. Below roughly 5 °C strength development largely stops; young concrete then needs thermal protection as well
- Power-floated slabs. Curing follows immediately after the final float — do not let programme pressure skip this step
Related terms
- Hydration
- Curing compound
- Shrinkage crack
- Concrete cover
- Formwork
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