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Dusting refers to the formation of loose powder arising from the disintegration of concrete surface. For dusty concrete surface, they can be easily scratched by nails. The cause of dusting can be related to finishing works carried out before completion of concrete bleeding. The working back of bleeding water to concrete surface produces a low strength layer with high water cement ratio. It may also arise owing to inadequate curing and inadequate protection of freshly placed concrete against rain, wind and snow.
Isolation joints isolate slabs or concrete structure from other parts of structure. The presence of isolation joints allows independent vertical or horizontal movement between adjoining parts of the structure.
Plastic shrinkage cracks are caused by a rapid loss of water from concrete surface before setting of concrete such that the rate of evaporation of surface water is higher than the rate of replacement of upward rising water. Tensile force is developed at concrete surface which forms plastic shrinkage cracks when the concrete starts to stiffen. Plastic shrinkage cracks appear to be parallel to each other with spacing of about 300mm to 1m. The cracks are shallow and generally do not intersect the perimeter of concrete slab.
Curing compound is primarily used for reducing the loss of moisture from freshly-placed concrete and it is applied once after concrete finishing is completed.
In watertightness test of water retaining structures, it normally requires the filling of water at a uniform rate or letting the water pool stand alone for some time before actual measurement is carried out. The reason for such provision in watertightness test is to allow sufficient time for water absorption to take place in concrete.
While concrete sets, it gains hardness and strength as the process of hydration slowly spreads the entire body of material. Curing should be allowed to continue for several days before subjecting the new concrete to significant stress.
The problems of excessive application of form oils are:
(i) It stains the surface of hardened concrete.
There are generally two main types of form releaser: barrier type or chemically active type.
For barrier type (e.g. form oil), it creates a barrier between the form and the fresh concrete. However, the quick evaporation of diesel oils affects clean air.
