Cracks in concrete are quite common phenomena. But unforeseen cracking in concrete that are not anticipated in design stage, affects the durability and integrity of structure. This in-turn results in poor performance on the structure over its service life.
The nature and pattern of cracks will vary based on the root cause of cracking. Hence, it is important to understand the major types of cracks that can occur in concrete. Primarily, cracks occur due to environmental exposure, improper construction methodology, design inadequacy and incorrect detailing.
Types of cracking
Cracks in concrete can be classified into different types. Main types are discussed briefly in this article.
These cracks may occur in reinforced concrete elements after the hardening state. Flexural cracks are vertical cracks that develop at the tension zone of the member up to their neutral axis. Width of these cracks are usually wider at the mid-span of the member. This is because the maximum strain in reinforcement occurs at this zone.
Flexural cracks will form when the tensile stress caused by bending exceeds the tensile strength of the concrete. This is purely a design issue, and the responsibility of limiting flexural crack width rests with the designer.
Providing the adequate reinforcing steel area based on the load effects on concrete member controls cracks due to flexure. Eurocode provides simplified and with actual calculations for limiting crack width in concrete.
Drying Shrinkage Cracks:
Drying shrinkage is also a crack that occurs after hardening of concrete. It happens due to the reduction of concrete volume when the water content in the concrete evaporates. As the water content increases, the rate of shrinkage rises equivalently. Drying shrinkage may be a full-length cracking. Usually contraction joints are placed to predetermine the drying shrinkage. This cracking will be in the form of transverse, pattern or map cracking.
The designer should control the shrinkage effects by providing adequate rebar and predetermined contraction joints. During the time of construction provide good curing to allow the concrete to gain sufficient tensile strength before significant shrinking forces develop. Use of shrinkage compensating concrete and admixtures prevent shrinkage cracks .
Concrete will contract and expands with changes in moisture and temperature. As long as these contraction movement is free to take place tensile stress will not develop in concrete members. But in reality, movement of this contraction will be restrained due to internal or external restraints, which causes development of tensile stress, leading to cracking.
Thermal cracking occurs during the hardening state. When cement reacts with water, heat of hydration is released from the cement, as a result thermal expansion occurs in the concrete member. Thermal crack possibly will be a full depth crack.
Control of thermal crack width also relies on temperature drop of concrete from the Peak to normal temperature. Avoiding placing of concrete in peak temperature conditions will help reduce thermal cracks. Provision of adequate rebars is also necessary to avoid thermal cracks. Early thermal cracking is also a phenomenon associated with thermal movements in concrete.
Plastic Shrinkage Cracks:
Plastic-shrinkage cracks are common in slabs. These cracks are comparatively short cracks. When the evaporation of the moisture at the surface of concrete is more than the availability of the rising bleed to fill the surface moisture, if this process happens before concrete attaining its tensile strength, the volume change may cause cracks.
The key to controlling plastics shrinkage cracks is to find ways to decrease the rate of drying of the concrete, includes do not place the concrete in adverse conditions, erect wind screens, properly use evaporation retardants and curing must be done with proper care.
Other types of Cracks in Concrete
Cracks can also be caused by alkali- aggregate reaction, sulfate attack, settlement cracking, freezing and thawing (D-cracking) of saturated concrete, or corrosion of reinforcing steel.
Design Limit of Cracks in Concrete
The objective of calculating crack width is purely to provide guidance to the structural designer in making suitable structural arrangements and in avoiding gross errors in design, which might result in
concentration and excessive width of flexural cracks. Limits of cracking differ with the category of structure and its environment.
The maximum limit of 0.3 mm on the surface width of cracks for concrete structures subject to mild exposure is acceptable. This limit of 0.3 mm is based basically on aesthetic considerations, but this limit is also considered to be satisfactory for the purpose of durability when the member is completely protected against weather or aggressive conditions.
In the case of members when they are exposed to the effects of weather or continuously exposed to moisture or in contact with soil or ground water called moderate exposure category, the crack-width limit is restricted to 0.2mm.
Under more aggressive environments that is ‘severe’, ‘very severe’ and ‘extreme’ exposure category, a more stringent limit of 0.1 mm is suggested. It is believed that cracks less than 0.2mm will heal naturally; as water percolates through the crack and dissolves calcium salts in the cement, preventing subsequent leakage.