Concrete is defined as the composite material that consists of a mixture of fine aggregate, coarse aggregate and binding material that has hardened over time. The binding material used maybe hydraulic cements such as Portland cement or Calcium Aluminate cement, lime, putty etc. Concrete is an important type of construction material and is widely used. Despite its immense importance, in the absence of proper technique and maintenance and several external factors may pose a great threat of cracking in the concrete. Cracks in concrete is a very common defect and are almost inevitable. However, the extent or degree to which cracks are formed can be controlled by various techniques.
The common type of cracks in concrete can be summarized as follows:
i. PLASTIC SHRINKAGE CRACK
Plastic Shrinkage Crack is the type of crack that develops during the initial stage itself i.e. before hardening. It is formed due to the shrinkage of concrete. When the concrete is in the plastic state, before hardening, it is full of water. Eventually, the water collected before evaporates leaving behind voids between the aggregate particles. The voids or the empty spaces thus formed weaken the concrete increasing the possibility of cracking. Such crack originates from the corner of slabs and spread throughout the thickness of the slab.
The main causes of plastic shrinkage can be listed as follows:
1. Addition of too much water.
2. Low relative humidity.
3. High ambient temperature.
4. High wind velocity.
The various precautions that must be taken for preventing plastic shrinkage crack are mentioned below:
1. In case harsh climatic condition exists at the site such as high wind velocity, the placement must be done at an appropriate time such as late evening or early night.
2. In very hot or dry conditions, fog sprays must be used. Fog sprays reduce the rate of evaporation of water.
3. Proper workmanship, equipment and materials must be ensured at all times.
4. The concrete must be covered with wet burlap or polyethene sheets between finishing operations.
5. Curing must be done properly.
ii. EXPANSION CONCRETE CRACK
The cracks that are associated with the expansion of concrete are known as expansion concrete cracks. These are the type of cracks that are formed as a result of the expansion of concrete, particularly at the hot climatic condition. Due to the extreme heat, the concrete begins to expand and it pushes away anything present on its way such as a brick. Since neither of it can flex, the resulting expanding force is so strong that it can cause cracking of the concrete.
Some of the causes of expansion concrete cracks can be listed as follows:
1. Wrong choice of materials.
2. Excessive temperature fluctuations.
3. Alternate hot and cold cycle.
Some of the preventive measures that must be adopted to prevent the cracking are as follows:
1. Application of expansion joints made up of asphalt, rubber etc.
2. Appropriate placement and curing methods.
iii. HEAVING CONCRETE CRACKS
Due to the alternate freezing and thawing cycle, the ground rises slightly. Normally, the ground rises when it freezes and settles back down during the thawing action. In such condition, if the concrete associated with the soil such as a slab does not rise and subsequently settle down with the soil then cracks may develop in it. Such cracks are known as heaving concrete cracks. Moreover, the deep spreading roots of trees may also cause a similar rise of the ground thereby forming heave cracks.
Some of the causes of heaving concrete cracks can be listed as follows:
1. Alternate freezing and thawing cycle.
2. Roots of the trees.
3. Abnormal soil condition causing uplift.
Some of the preventive measures to prevent such cracks are as follows:
1. Adequate planning to minimize such causes.
2. Placing of concrete in a suitable location as far as possible.
iv. SETTLING CONCRETE CRACKS
Settling concrete cracks are the cracks that are associated with the settling of the soil. It is the type of crack that is caused by the reverse action than heave cracks i.e. due to settling of the soil. As mentioned earlier, the roots of trees can be a prime source of the crack in the concrete. When a tree is uprooted, voids are formed between the concrete particles thereby forming cracks.
v. CONCRETE CRACKS DUE TO OVERLOADING
Although concrete gains adequate strength after curing, cracks may develop on it if subjected to excessive load for a long time. Such cracks on concrete that are formed due to the inability of concrete to withstand the excessive stress imposed are known as a concrete crack due to overloading. This type of concrete generally occurs on concrete that is placed on the ground.
Some of the causes of such cracks are listed as follows:
1. Overloading of concrete.
2. Softening of concrete after heavy rainfall or snow.
3. Insufficient curing of concrete.
Some of the preventive measures that can be adopted to avoid such cracks are as follows:
1. Adequate curing of concrete.
2. Proper maintenance of concrete particularly in harsh weather.
3. Avoiding continuous overloading of concrete.
vi. CONCRETE CRACK DUE TO PREMATURE DRYING
There are two types of crack associated with premature drying i.e. crusting cracks and crazing cracks. Crazing cracks are the type of cracks that appear as fine lines and mostly resemble a web of spider. On the other hand, crusting cracks are the type of cracks that occur due to the stamping process. The stamping process is the process of giving a certain texture to the concrete.
Some of the causes of cracks due to premature death can be listed as follows:
1. Improper mixing of concrete.
2. Excessive evaporation of moisture from the surface of the concrete.
3. The sunny or windy climatic condition that causes premature drying of concrete.
4. Improper stamping technique.
Some of the preventive measures that may be adopted to prevent such type of cracking can be listed as below:
1. Use of a quality mix.
2. Proper workmanship.
3. Application of appropriate concrete finishing technique.