Plastic Shrinkage Cracks in Concrete and Its Prevention
Plastic shrinkage cracking is a common issue in freshly placed concrete floors, slabs, and other large surface elements. This type of cracking typically occurs when the concrete is exposed to rapid moisture loss due to low humidity and wind or high temperatures, or a combination of both. As the concrete dries out, it contracts, leading to stress buildup and ultimately cracking on the surface.
The cracking is caused by the tensile stresses that develop in the concrete as it shrinks. These stresses are particularly high on the surface of the concrete, which is exposed to the greatest amount of moisture loss. The cracking may appear as a network of fine cracks or larger, more significant cracks, depending on the severity of the shrinkage.
One of the main factors that contribute to plastic shrinkage cracking is the rate at which the concrete dries out. When the drying occurs too quickly, the tensile stresses in the concrete increase rapidly, leading to cracking. This can be exacerbated by factors such as low humidity, high temperatures, and wind, which can all cause moisture to evaporate from the surface more quickly.
To prevent plastic shrinkage cracking, it is important to take steps to slow down the rate of moisture loss in the concrete. This may include applying a curing compound to the surface of the concrete or using a plastic sheeting to cover the surface and protect it from wind and direct sunlight. Additionally, controlling the temperature and humidity of the surrounding environment can also help to minimize the risk of cracking.
Plastic shrinkage is a common phenomenon that occurs in freshly placed concrete before it begins to cure. This happens when the moisture on the surface of the concrete evaporates faster than it can be replenished by the curing water. As a result, the surface layer of the concrete undergoes shrinkage due to the restraint provided by the concrete. This shrinkage creates tensile stresses in the plastic concrete that is still in the process of stiffening.
The tensile stresses on the surface layer of the concrete cause it to develop shallow cracks that run in various directions. These cracks are typically not very short and can range in width from a few millimeters to several meters. The cracks are spaced out at distances that can vary from a few centimeters to as much as 3 meters apart. Usually, the cracks are wider at the surface and appear to be running in all directions. This is a common occurrence in concrete construction that takes place prior to final finishing.
Preventing Plastic Shrinkage Cracks in Concrete
Plastic shrinkage cracks are a common issue with elevated structural slabs, where the cracks may extend the full depth of the slab. This occurs due to a differential volume change in the plastic concrete, which causes cracking. To prevent such cracks, it is important to reduce the relative volume change between the surface and other portions of the concrete.
There are several methods and techniques to prevent plastic shrinkage cracks, especially during hot weather and dry winds. One such method is to use fog nozzles to saturate the air above the surface. Another technique is to use plastic sheeting to cover the surface between the final finishing operations. These methods help to reduce the rapid loss of moisture and prevent the formation of cracks.
During construction, it is preferable to use wind breakers to reduce the wind velocity and sunshades to reduce the surface temperature. This helps to minimize the likelihood of plastic shrinkage cracks. It is also good practice to schedule flat work after the walls have been erected.
Overall, it is important to take preventive measures to control plastic shrinkage cracks, as they can compromise the structural integrity of the elevated structural slabs. By reducing the relative volume change and minimizing rapid moisture loss, these measures can help to ensure a strong and durable concrete structure.