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Curing is the last and an important activity for concrete. Once placing and finishing stage of fresh concrete is finished, it gets converted from plastic to hard state. Significant water losses occur due to evaporation from the surface of concrete. When evaporation occurs, it results in slowing down or stopping of hydration process and development of shrinkage cracks may occur. Consequently, it reduces the strength and durability of concrete and makes it weak and porous.To keep the hydration process continuous, it is essential to maintain humid and warm environment around the freshly placed concrete or mortar until it achieves desired strength. This process is called curing of concrete.
The water content of fresh concrete is considerably more than enough for hydration of the cement. During making and placing, it is basically required for workability. However, an appreciable loss of this water by evaporation. The object of curing is to prevent or replenish the loss of moisture.
There is reaction between cement and water and because of hydration, gel is formed in the concrete. In early stages, this action is very rapid and hardening and evolution of heat are pronounced. All the cement particles dissolves in the presence of continuously available fresh water. But the outer layer of the cement particles set and get more hardened than inner mass of the cement/concrete. The cement rarely gets fully hydrated and the full benefit of the total strength of cement is never obtained.
The concrete attains a major portion of its strength in about 21 days. The chemical reaction takes place over a long period of time in the presence of moisture at a favorable temperature. Therefore, it is necessary that sufficient quantity of water is available for curing before it attains design strength. Thus 21 day is considered to be enough for wet curing.
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Curing can begin when the surface of freshly laid concrete is hard enough for a person to work over. While walking on freshly laid concrete,a person should not damage the surface. Sometimes the surface moisture can be maintained by splashing or spraying water without pressure.
Things to keep in mind while curing
Following things to be keep in mind while curing.
- It is advisable to start curing operations as soon as possible after concrete gets initial set. Usually, initial setting starts within 3 – 7 hours after casting.
- For proper curing concrete needs moisture. Sufficient amount of water should be available for curing.
- The water to be used for curing should be clean and free from oils, acids, alkalis, salts, organic materials or other substances. Use potable water for curing.
- Hydration process of cement slows down as concrete dries and finally stops at some degree of dryness. On again wetting the hydration is resumed at a steady but reduced rate as now the moisture cannot penetrate the mass as effectively as it did before drying. Therefore, continuity in curing is necessary because alterations of wetting and drying may develop cracking and crazing on the concrete surface.
- The ideal temperature for curing is 27°C.
- The curing period of concrete is very important. It is essential for continuing the hydration process of cement with water until concrete attains the maximum compressive strength. Curing period should not be less than 10 days for concrete exposed to dry and hot weather conditions. If mineral admixtures or blended cement are used, it is recommended that minimum curing period extended to 14 days.