Concrete will set in
three days but the chemical reaction between water and cement continues much
longer. If the water disappears through evaporation, the chemical reaction will
stop. It is therefore very important to keep the concrete wet (damp) for at
least 7 days.
Premature drying out may
also result in cracking due to shrinkage. During curing the strength and
impermeability increases and the surface hardens against abrasion. Watering of
the concrete should start as soon as the surface is hard enough to avoid
damage, but not later than 10 to 12 hours after casting. Covering the concrete
with sacks, grass, hessian, a layer of sand or polythene helps to retain the
moisture and protects the surface from dry winds. This is particularly
important in tropical climates.
Temperature is also an
important factor in curing. For temperatures above 0° C and below 40° C
strength development is a function of temperature and time. At temperatures
above 40°C the stiffening and hardening may be faster than desired and result
in lower strength.
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