Why Does Concrete Crack?

Concrete Defects in Civil Engineering

Cracks are one of the most common defects in concrete. This article will discuss the main reasons for concrete cracking and how concrete surface cracking can be avoided. Various types of cracks are quite common in civil construction. Most of the time cracks could be avoided by using proper concrete mix proportions. Following are the four major reasons why cracks develop in concrete:

•Excess water quantity: Most often time Portland cement is used for construction. Portland cement, being a hydraulic type of cement, needs water for developing strength. Also, water increases the flowability of the cement concrete and thus increases the ease of its use. But when the concrete dries and hardens, the excess water from the concrete evaporates and the concrete shrinks. As a result of that, cracks develop in the concrete.

•Too dry weather: After the wet concrete mix is spread, it takes a few weeks to complete the whole hydration process. This hydration process gives strength to the concrete but also needs sufficient water. If the concrete does not get sufficient water for hydration, it tends to develop cracks.

•Incorrect selection of cement: Cement comes in different grades as per the strength requirement of the concrete. In case you have selected wrong or weak cement, your concrete won’t have enough strength and will develop cracks.

•Control Joints: Control joints are planned cracks. Control joints give room for shrinkage and thus control joints help in avoiding random cracks. Typically a 4” thick concrete slab should have the concrete joints around 8-12 feet apart.

Common Factors in Concrete Surface Cracks

1. Shrinkage. The most common cause of cracks in concrete is the shrinkage, that occurs as the material dries. As concrete cures, the water, that has been mixed with the cement and sand, begins to evaporate. This causes the concrete to shrink slightly. Because the concrete surface is restrained by formwork or surrounding objects, it tries to resist the effects of shrinkage. The additional tension on the material, that occurs when the concrete shrinks, leads to cracks. This is often addressed through the use of control joints, which are joints placed in the concrete, that cause it to crack in a straight, even line. These joints are later filled with grout or caulk to mask their appearance.

2. Temperature Changes. Temperature change is a major cause of cracks in large concrete applications. This is because of the temperature variations between concrete, that is poured first, and subsequent layers of concrete. These temperature variations can cause some portions of the slab to expand, while others are shrinking, which leads to cracks. The more extreme the difference in temperature, the more dramatic and deep the cracking will be.

3. Lack of Support. Some of the most severe cracks in concrete are caused by inadequate structural support, which occurs when the soil beneath a concrete slab shifts position or is eroded by wind and rain. It can also be caused by poor installation, including improper use of rebar or insufficient form work. Concrete, that lacks structural support, can suffer catastrophic failure, which can cause injury to occupants as well as a great deal of property damage.

4. Rusting. Many items made from concrete contain metal reinforcement, which helps increase the material's tensile strength. These reinforcements include wire mesh, rebar or steel beams. If these materials start to rust, the rust will cause the concrete to expand. This expansion can cause hairline cracks and small surface cracks. After the metal has started to rust, the resulting cracks will allow more water into the concrete, which will result in even more rust. To solve this problem, galvanized metals should be used for their ability to resist corrosion and rust.

5. Spalling. Spalling is the term given to concrete that has a rough, flaky surface. This condition is the result of hundreds of tiny cracks, which usually appear only on the surface of the concrete. Spalling typically occurs when too much water is added to the concrete mixture, or when the surface layer dries faster than interior layers. If the water evaporates too quickly, the resulting increase in tension will be too much for the concrete, causing it to crack. This type of surface problem can sometimes be fixed with topping layers or seal coats.

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