Plastic Shrinkage in Concrete
Plastic shrinkage in concrete is a result of rapid evaporation of water from newly placed concrete, and is a common problem. It results when bleed water evaporates faster than the amount of water that can reach the surface. Most concrete mixes contain a small percentage of bleed moisture, and the bleeding rate is less than 0.2 pounds per square foot per hour. It is a common cause of surface crusting and cracking, and it is best to prevent it by carefully planning the concrete mix.
One of the most important factors in controlling plastic shrinkage is the amount of water in concrete pores. The proportion of water in concrete pores decreases with time. When this happens, plastic shrinkage occurs and the concrete fails. The amount of water is reduced to zero, and the cement can be used as-is. This phenomenon occurs in a variety of concrete materials, and is the reason for the shrinkage seen in many types of materials.
While plastic shrinkage in concrete rarely affects the structural strength of concrete structures, it should not be overlooked. A crack of this type can range from a few millimeters to two meters long. Usually, the plastic shrinkage cracks appear as parallel lines, spaced three or four meters apart. Nevertheless, if the cracks are not attended to soon, they can spread out and become deeper. Fortunately, this is not always a major problem, and they are easily remedied with a wooden float.
Besides cracks caused by shrinkage, the concrete is susceptible to other factors that can affect its quality. In case of a weak concrete, plastic shrinkage cracks will begin to appear. For example, the concrete will lose strength when it becomes brittle. Therefore, it is crucial to avoid any damage to the structure. The shrinkage will result in a lower-quality structure and could even result in a collapse.
In some cases, cracks in concrete are caused by evaporation of moisture from the surrounding air. A concrete nomograph is a simple method to predict the occurrence of plastic shrinkage. It is an indicator of evaporation of water from the concrete, and can be applied to cracks. It is also useful for estimating the time required for a structure to cure. There are many other methods that can prevent the onset of cracks due to plastic shrinkage.
There are several causes of plastic shrinkage in concrete. Some are the result of a high water-cement ratio, which affects the shrinkage of concrete. Others are caused by the evaporation of moisture. A higher water-cement ratio increases the risk of autogenous shrinkage in concrete. While both of these factors can cause cracks, the presence of superplasticizer and silica fume in a concrete mixture can increase the chances of the latter.