Crack the concrete

To avoid cracks in concrete, avoid using high slump concrete. Concrete of moderate slump less than 5 inches results in less cracking. High slump concrete can only be used if it is controlled by concrete admixtures or designed such that segregation and bleeding of concrete is controlled. Concrete admixtures such as water reducers or super plasticizers can be used to increase slump while reducing addition of water content.

Air entertained concrete can also be produced to increase slump especially when concrete is to be subjected to freeze and thaw cycles. Bull Floating. Finishing operations are very important in concrete.

Finishing operations must not started until bleeding of concrete is completed or there is water present on concrete surface. Overworking concrete surface finish also results in cracking of concrete surface. Concrete should be bull floated or broom finished if adhesion is required with other surface. In hot weather, arrangements should be made to avoid rapid drying of concrete. These arrangements can include covering the concrete with polythene sheet or burlap.

Fog sprays or wind breaks may also be used for the same purpose. Rapid drying results in plastic shrinkage cracks on concrete surface. Curing of concrete is very important in controlling concrete cracks. Curing of concrete must be started as soon as possible. There are many ways to cure the concrete. Concrete can be cured by spraying the surface with curing compound or covering it with moist burlap ho hessian cloth. Curing of concrete should be done for at least 7 days or as directed by the engineer.

Changes in concrete volume due to temperature or moisture should be controlled by providing contraction joints in concrete slabs. Spacing between contraction joints should be about 24 to 36 times the thickness of concrete slab with maximum spacing of 15 feet. Concrete Contraction Joint. Isolation joints are another type of joints which are used when movement of slab is restricted by some other element such as columns, walls or footings.

These joints are constructed by making full depth groove across thickness of the slab and then adding some bond preventing material to avoid bond of concrete slab with other elements. Large tree roots can have the same effect on a slab. If a tree is located too close to a slab, the growing roots can lift and crack the concrete surface.

Always consider this when laying a slab. On the other hand, ground settling below a concrete slab can also cause cracking. Settling cracks typically occur in situations where a void is created in the ground below the concrete surface.

Think about when a large tree is removed from nearby and the roots begin to decompose or when a utility company digs a trench for their lines, pipes, etc. Although concrete is a very strong building material, it does have its limits. Placing excessive amounts of weight on top of a concrete slab can cause cracking. Instead, what is more likely to occur is excess overload on the ground below the slab. After a heavy rain or snowmelt when the ground below is soft and wet, excessive weight on the slab can press the concrete down and result in cracks.

Residential homeowners who place large recreational vehicles or dumpsters on their driveways are more likely to see this type of cracking. There are two common types of cracks brought on by premature drying. Crazing cracks are very fine, surface cracks that resemble spider webs or shattered glass. When the top of a concrete slab loses moisture too quickly, crazing cracks will likely appear. While unsightly, crazing cracks are not a structural concern.

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 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.