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3.23.1.15 Curing
Concrete shall be adequately cured to achieve full design strength.
Concrete performance relies on the curing process. The design should clearly indicate where there are any special requirements for curing concrete.
Freshly poured concrete should be kept moist by covering as soon as the surface is hard enough to resist damage. This is particularly important in hot, windy or cold weather to prevent the surface drying out too rapidly, or freezing. Damp hessian, damp sharp sand or an impervious sheet (such as polyethylene) are acceptable as surface coverings. Alternatively, a curing agent can be applied to the surface.
In addition when concreting in hot weather (air temperature above 30°C), account should be taken of avoiding reductions in the working life of fresh concrete due to loss of mix water by accelerated hydration and/or evaporation and preventing high temperature rise within the concrete element that could lead to unacceptable levels of early-age thermal cracking, reduction in the ultimate quality of the concrete and delayed ettringite formation (ie, crystallisation that occurs within setting concrete).
However, there are ways in which the concrete can be modified to help counter hot weather effects, such as:
- increasing the initial workability and/or retarding the hydration process using admixtures. Note: a retarder does not compensate for stiffening by moisture loss
- spraying the formwork/base with water before concreting
- using a cement or combination cement that has low heat evolution
- avoiding concreting during the middle of the day when temperatures peak
- specifying a maximum temperature of fresh concrete less than 35°C – see BS 8500-2. Note: temperature of fresh concrete is measured as described in BS EN 12350-1.
Other methods are available, for example pre-cooling the mix constituents and/or the use of embedded cooling pipes (latter only likely to be considered for larger/deeper concrete pours), but these should only be used following specialist’s advice.
No load should be applied to the work until the concrete has cured sufficiently. It is recommended that plain unreinforced concrete made with ordinary Portland cement is left for at least four days to cure.
It is possible to proceed with substructure masonry above strip or trench fill foundations on unreinforced ordinary Portland cement concrete at an early stage, provided that care is taken to protect the surface from damage.
Reinforced concrete or concrete containing cement replacements, such as PFA, will require a longer curing period. This will normally take seven days, during which the concrete structure should not be loaded.
Any curing agents should comply with Technical Requirement R3 and should be applied strictly in accordance with the manufacturer’s instructions. Curing agents should never be used on floors which are to receive either a topping or a screed, as it could affect the future bond. Curing periods may be extended at low temperatures.
Last updated: 2nd January 2025