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Curing concrete surfaces
How proper curing affects the strength, durability, and long-term performance of concrete.
Curing has a strong influence on the properties of hardened concrete, including durability, strength, watertightness, wear resistance, volume stability and resistance to freezing and thawing.
When Portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed determines the strength, durability and density of the concrete. Since hydration is relatively rapid in the first few days, it is important to retain moisture during this period to prevent or reduce evaporation.
The objects of curing are to prevent (or replenish) the loss of moisture, and to control the concrete temperature for a definite time.
With proper curing, concrete becomes stronger and more resistant to stress, abrasion and frost. When moist curing is interrupted, strength development continues briefly then stops. Loss of water also causes the concrete to shrink, creating tensile stresses at the drying surface. If these develop before the concrete has attained adequate tensile strength, surface cracking can result.
Hydration proceeds at a much slower rate when the concrete temperature is low. Temperatures below 10°C are unfavourable for the development of early strength; below 4.5°C early strength is greatly retarded.
There are five main curing methods:
Ponding: build a dike and fill with water to cover the entire slab. Use curing water within 20°F of the concrete temperature. Avoid premature or sudden release of ponded water.
Sprinkling or fog spraying: keep the surface continuously wet. Alternate wetting and drying causes craze cracking. Use low water pressure to avoid washing away the fresh concrete surface.
Using wet materials: cover with wet hessian, straw, sawdust or sand. Wet continuously or cover with plastic sheets and wet frequently.
Using plastic sheets or waterproof paper: use flat sheets with lapped edges of 6 inches and cover exposed concrete edges. Use minimum 4 mil thick plastic (white in hot weather, black in cold weather).
Using curing compounds: apply as soon as bleed water has gone and the new concrete has hardened sufficiently so as not to be marked by the application process.