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The purpose of curing is to reduce the rate of heat loss of freshly placed concrete to the atmosphere and to minimize the temperature gradient across concrete cross section. Moreover, curing serves to reduce of the loss water from freshly placed concrete to the atmosphere.
Ponding: This method of thermal curing is readily affected by weather condition (cold wind). Moreover, a large amount of water used has to be disposed off the construction sites after curing.
Plastic sheets are commonly used in curing to prevent moisture loss from concrete surface. However, it is not uncommon that discolouration occurs on the concrete surface.
After concreting, the time at which striking of formworks should not be too long, otherwise it would affect the colour of concreted structures. For long span concrete structures, when they have attained sufficient strength to support their self-weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork. Therefore, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them.
When a rectangular panel is subject to an eccentric load or a lateral load, it tends to deform into a parallelogram with one diagonal shortening and the other elongating. Theoretically, it is sufficient to brace along one of the diagonals (the one in tension). If one diagonal is only allowed to brace inside the rectangular panel, it should be not braced in the diagonal in compression because under severe lateral loading the diagonal may buckle leading to failure of structure.
Let us take a circular column as an example to illustrate effect of internal restraint to thick sections.
When the temperature is rising, temperature in the core is higher than that at outer zone. The inner core will have a higher expansion and exert pressure to the outside. The induced compressive stress will result in the formation of radial cracks near the surface of concrete.
Freshly placed concrete exerts pressure on formwork during the placing operation. It is influenced by the rate of placing and the air temperature. For instance, if the concrete pouring rate is too slow, setting of concrete starts to take place. As a result, the concrete at the bottom of the formwork sets prior to the placing of fresh concrete at the top and the maximum pressure will be reduced.
Permeable formwork serves as a filter that allows excess water and trapped air to escape from concrete surface. During compaction by vibrators, the fluid movement through permeable formwork drives out air and water, leaving behind a denser and stronger concrete. The movement of water results in a decrease in water in fresh concrete and fines cement particles from interior concrete shall be carried towards the formwork. Hence, it lowers the water cement ratio at concrete surface and enhances a higher strength near concrete surface.
The size of concrete is affected by changes in atmospheric humidity: moisture causes expansion while drying causes shrinkage. Such moisture movement is reversible. This is totally different from drying shrinkage in which concrete slowly loses moisture during hardening, thus causing irreversible shrinkage.
External waterstops are applied externally on the structures and they proved to be effective when installed on the face with a net clamping pressure. For instance, external waterstops can be placed on the outer face of a basement to guard against water entry into the basement.
