Concrete is prone to developing shrinkage cracks due to its tendency to change its moisture levels. The structure of concrete and mortar is permeable, containing inter-molecular spaces. As a result, these materials expand when they absorb moisture and contract when they dry out. This natural behavior is the primary cause of shrinkage cracks in concrete that occur during the drying process. It is essential to note that the shrinkage of concrete is an irreversible process, meaning that once it occurs, it cannot be undone. Therefore, it is crucial to understand the reasons behind concrete shrinkage and take measures to minimize the occurrence of these cracks.
Types of Shrinkage in Concrete
In concrete, there are two distinct forms of shrinkage that occur during the curing process: initial shrinkage and plastic shrinkage. Initial shrinkage is a type of shrinkage that occurs when the concrete is first poured and begins to set. As the water in the mix evaporates, the concrete begins to shrink. This type of shrinkage can cause cracking and other issues if not managed properly.
On the other hand, plastic shrinkage is a type of shrinkage that occurs when the concrete is still in its plastic state. This means that it has not fully hardened or set yet, and is still relatively wet. Plastic shrinkage occurs when the surface of the concrete dries out too quickly due to high temperatures, low humidity, or wind. This can cause the surface of the concrete to shrink and crack, which can be a major issue for the overall durability and strength of the structure.
Initial Shrinkage Cracks in Concrete
Shrinkage cracks are a common occurrence in cement-based building materials such as concrete, mortar, masonry units, masonry, and plaster. These cracks are mainly caused by the initial shrinkage of the material due to the drying out of moisture during construction. While some of the initial shrinkage is reversible with the maintenance of moisture, it becomes irreversible once the material becomes dry. Further wetting and drying during the curing process can exacerbate the shrinkage, leading to the development of cracks in the concrete.
The extent of initial shrinkage in cement-based materials depends on several factors. Firstly, the cement content of the mix has a direct impact on the level of shrinkage. A higher cement content leads to greater shrinkage. Secondly, the amount of water used in the mix also affects shrinkage, with a greater quantity of water resulting in greater shrinkage.
Additionally, the maximum size, grading, and quality of aggregate used in the mix play a role in determining the level of shrinkage. The use of larger maximum size aggregate and good grading can reduce the water requirement for the desired workability, thereby reducing shrinkage. Moreover, curing the concrete as soon as the initial set takes place and continuing the process for at least 7 to 10 days can also reduce the extent of initial shrinkage.
The presence of excessive fines in aggregates can increase specific surface area, requiring more water for the desired workability and increasing initial shrinkage. The chemical composition of the cement also plays a role, with greater proportions of tri-calcium silicate and lower proportions of alkalis leading to less shrinkage. The temperature of the fresh concrete and relative humidity of the surroundings can also impact shrinkage, with lower surrounding temperatures requiring less water and resulting in less shrinkage.
Shrinkage in concrete occurs over a period of time, with 1/3rd of the shrinkage taking place in the first 10 days, 1/2 within one month, and the remaining 1/2 within a year. Therefore, shrinkage cracks can continue to occur and widen for up to a year after the initial construction.
Plastic Shrinkage Cracks in Concrete
Concrete undergoes plastic shrinkage right after it has been poured. This is caused by the settling of large solid particles due to the gravitational force, which results in water rising to the surface. This phenomenon is commonly referred to as bleeding of concrete. Bleeding of concrete can persist until the layer of water on the surface has set. If the rate of evaporation is lower than the rate of bleeding, there will be a constant layer of water on the surface called a “water sheen.” In such cases, no shrinkage occurs.
However, if the surface loses water at a faster rate than the bleeding action can replenish it, the top layer of concrete will undergo shrinkage. The concrete in its plastic state is unable to resist any tension, and this results in cracks on the surface. These cracks are commonly observed in slabs. The extent of plastic shrinkage is affected by several factors, including the temperature of the concrete, exposure to heat from solar radiation, relative humidity of the ambient air, and wind velocity.