What are the RCC Concrete Slab Cracking Causes?
The cracks in a RCC slab can be categorized as :
- Internal cracks . Not appearing on the surface. This category of cracks is the most threatening since there could not be any prior detection possible. It could be quite late when we discover the presence of such cracks. In case , the slab is prestressed ( and it is pre-tensioned ), there could be a slippage between the tendon surface and the concrete. This could result in a crack in the direction of the tendon.
- Visually apparent cracks .These could be either on the top of concrete or on bottom portion of the slab. The cracks on the top surface if penetrating the slab depth could be serious to the structural safety . Specially when we know that this is the compression zone and hence surface should normally contract rather expand through cracks. The reasons need to be properly investigated and mitigation action must be initiated. Cracks appearing from below are also serious since this is the tension zone . the cracks here might signify that the tension in the concrete has not been properly transferred to the rebars.
- Hairline cracks appearing on the top of the slab. Normally seen on the next day of casting of the slab. these are found whenever a higher grade of cement is used . ( OPC Grade 53 ) . In my view , there is no need to be concerned about these types of hair line cracks provided the slab under question is not the terrace slab.
Having said this , let me summarise the causes for cracks in RCC slab :
- Inadequate quality control in accepting the ingredients.
- Improper control on material proportions especially the silt content , water cement ratio.
- Excessive vibrations during casting resulting in opening up of the binding wires ( stirrups opening )
- Inadequate curing of the slab.
- Insufficient precautions during striking off. ( may be early opening of the slab before it attains the strength )
- Excessive deformation of the cantilever slab at the ends (or in case of simply supported slabs in the mid span )due to settlement of any of the staging props .
Hope this gives you an initial insight into the causes of the cracks. This is the main reason for concrete slab cracking causes.