Understanding the Causes of Concrete Cracking
It is however necessary to know some of the causes of the cracks before embarking on taking any preventive measures. There are various causes of cracks and they require different solutions.
- Shrinkage: The concrete contracts as it hardens because of the evaporation of water. The process of drying and setting results in reduction in volume of water and hence reduction in volume of concrete. If this shrinkage is not controlled, it is likely to crack, and this is something that needs to be avoided at all costs.
- Temperature Changes: Concrete is sensitive to temperatures and undergoes changes in size depending on the temperature of its surrounding environment. The effects of heat are that it causes the material to expand while the effects of cold are that it causes the material to contract. Such expansion and contraction create pressure on the surface of the material, which may result in cracks.
- Improper Mixing: This is when the concrete is not mixed properly and this may result to the concrete having a poor strength and therefore the weak areas that are prone to crack. The proportion of water to cement also influences the strength of concrete and the use of low-quality materials will also impact the strength of the concrete.
- Excessive Load: This is a factor that can lead to cracking of a concrete surface especially if it is overloaded beyond its capacity. This is especially so with driveway, walkway and floor surfaces that are likely to be used by heavy vehicles or equipment.
- Poor subgrade preparation: The ground on which concrete is laid is known as the subgrade and this should be well prepared. A weak or poorly compacted subgrade means that the ground can subside or shift and this will result to formation of cracks on the concrete surface.
- Moisture Issues: Water is one of the most important elements in the construction process of concrete, but it is also one of the most dangerous elements that can cause more harm to the concrete. Whereas for curing it is desirable, presence of moisture under or on the concrete surface creates pressure which leads to formation of cracks. Water can also degrade concrete over time and thus leads to deterioration of the material.
Proper Concrete Mix and Placement
To avoid formation of cracks, it is important to do the following while handling concrete; mixing and placing. The ratio of cement, water and aggregates (fine aggregate-sand, coarse aggregate-gravel or crushed stone) should be proper. It is a ratio of water to cement and the problem is that if the ratio of water is high, the mixture becomes weak and if the ratio of water is low, it becomes difficult to work with. Compact the concrete uniformly to prevent the formation of cracks as well as to allow uniform curing of the concrete. Ensure that the aggregates used are clean and free from contaminants and do not tamper with the mixture after it has been poured. It should be allowed to settle on its own and de-aired so that a strong and crack free surface is formed.
Reinforcement Techniques
Preventing the cracks is another major factor where one can employ the right reinforcement of the concrete used in construction. Reinforcement assists in spreading of stresses on its surface and minimizes crack formation resulting from shrinkage, thermal expansion and other forces.
- Rebar and Wire Mesh: The steel used for reinforcement comes in the form of rebar or reinforcing bars or wire mesh in the construction of concrete structures. This makes these materials impart tensile strength to the concrete and prevent it from cracking under pressure. Rebar should be placed in the middle of the slab while the wire mesh can be used in small projects such as parking lots or pathways.
- Fibers in Concrete: The addition of synthetic or steel fibers in concrete mix can reduce the cracking. These fibers improve the tensile strength of the concrete and hence minimize the possibility of cracks developing in the concrete. This is particularly useful when dealing with surfaces that are under heavy pressure or those that are subjected to high temperatures.
Control Joints and Expansion Joints
For purposes of controlling shrinkage and expansion of concrete surfaces it is necessary to incorporate control joints and expansion joints respectively. These joints are made in the concrete structure to accommodate expansion and contraction of the concrete without developing cracks.
- Control Joints: These are designed cracks that are provided in the concrete to control the direction and location of cracking. These joints are normally placed at certain distances and can be created by using a saw or trowel to scratch the surface of the concrete before it hardens. Control joints are used to prevent and contain random cracking since the concrete will crack at the joints intentionally.
- Expansion Joints: Expansion joints are provided in concrete structures to permit free movement of concrete due to thermal movements. These joints are normally employed in large slabs like highways, bridges, and large foundations of buildings and structures. Expansion joints are also sealed with a compressible material to ensure that pressure exerted on them causes no strain on the concrete and consequently formation of cracks.
Curing and Moisture Control
Curing of concrete is one of the critical steps that help in preventing cracks in concrete. Curing enables the concrete to gain strength over time since it is protected from drying up quickly by placing it in a wet condition.
- Water curing: After placing the concrete, it is advisable to cover it with water for at least 7 days particularly if the weather is hot. This can be achieved by using water sprays on the surface or placing wet burlap or plastic sheet on the concrete surface to retain moisture.
- Curing Compounds: In the instances where water curing is inconvenient, curing compounds can be used to cover the concrete surface to retain water. These compounds create a barrier that assists the concrete to have the right moisture content in the curing process.
- Over wetting: Although, water is essential in the curing process, excessive water can cause the surface to crack. Prevent water on the surface that can dry up and cause shrinkage cracks on the surface of the wall.
Proper Subgrade Preparation
The surface in which the concrete is poured is just as crucial as the material itself. A good subgrade is required to bear the load of concrete and to avoid any movement that may cause cracking of the concrete surface.
- Compaction: The ground on which the concrete is to be placed should be compacted to ensure that it can support the structure well. So if the soil is loose or uncompacted then it is likely to settle unevenly and this will cause the formation of cracks. Among them, the subgrade must be compacted well to provide a good base for the concrete.
- Leveling: The subgrade should be leveled before laying concrete. If the surface is uneven, then there will be stress points that lead to the cracking of the concrete. Level the area with leveling tools to ensure that the ground is flat and properly leveled for the construction of the building.
Conclusion
To avoid cracks on concrete surfaces, proper workmanship should be observed right from the mixing process, placing, reinforcement, and curing of the concrete. It is important to know the causes of cracking and prevent them so that you can avoid damaging the concrete surfaces and make them last longer. Some of the factors that are vital in subgrade preparation, jointing, and curing are as follows. By following the above preventive measures, you will be able to protect the concrete surfaces from cracking and ensure they stay strong and durable for a long duration.