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Causes or reasons for cracks in concrete slab./ Why concrete slab cracks?

 Now, let us go through different reasons or causes that create a crack in a concrete slab.

1. Improper concrete mix:

While concreting a slab, we should strictly adhere to the specified mix design for that particular slab. To maintain the quality of the concrete mix, it is advisable to prepare the concrete with weigh batching, rather than volumetric batching. Using ghamela (mortar pan ) instead of the batch box in volumetric batching results in the concrete of inferior grade.

2. Rebar cover:

We have to use spacer blocks or cover blocks to provide the specified covers to the slab reinforcement at a required spacing ( as per the IS code ). Improper & inadequate covering of the rebar leads to the sagging of the rebars resulting in crack development when the concrete hardens.

Note: I have seen some contractors using waste tile pieces as slab covers. Don't do such mistakes as it is improper & non-technical.   

3. Excess vibration:

Concrete vibration always needs a skilled worker to run the process. We have to use a specified immersion needle for the concrete work, as the needles are available in different sizes targeting different types of concrete structures. 

Excess vibration surpassing its time limit leads to segregation & concrete bleeding. The weak pockets of concrete created due to excess vibration may cause cracks in the slabs at a later stage.

4. Improper w/c ratio:

Maintaining a  specified w/c ratio is a must for all types of concrete works. Improper w/c ratio is also one of the main culprits in concrete segregation and bleeding.

5. Excess heat:

Excess atmospheric heat evaporates the water within the concrete too quickly, resulting in dehydration.  Water evaporation helps in the development of surface cracks in the slab due to shrinkage. The temperature should be within 35°c. & not less than 5°c to avoid such cracks. 

6. Bar spacing:

 Spacing in main & distribution bars has certain limitations as mentioned in the IS CODES. Excess spacing between the two rebars creates micro-cracks in the concrete surface.

7. Low compaction:

At the junction point of the column, beam, and slab, it becomes difficult to run the vibrator due to overcrowded bars. Special care should be taken in such junction points, so that concrete poured is workable. Low compaction creates voids in the concrete body, that may develop cracks in the near future. 

8. Formwork settlement:

If the formworks of the particular slab part settle down due to the dead load of the concrete, it may create line cracks in such junctions due to shear. Formwork props and supports should be rested over the hard surface area to avoid such settlement.

9. Early de-shuttering:

All type of concrete structures has their own formwork de-shuttering period to follow with. Premature de-shuttering may cause cracks in slabs, as the concrete lacks the final setting strength to sustain the load.

10. Improper curing:

Curing is an essential part of concrete work to complete the hydration process in a proper way. Improper curing may create concrete having below normal design strength. This may cause cracks when the slab is ponded with an extra live load.

11. Grade of cement:

The grade and type of cement should be known before using them in the slab concreting. The cement older than 3 months should be avoided and it is advisable to use a truckload of fresh cement directly transported from the factory to the slab concreting site.

12. Raw material quality:

Lab & field test is a must for the sand, coarse aggregates, and cement to know their quality beforehand. Using sand of heavy silt content or improper-sized coarse aggregate may cause cracks in the slab sooner or later.

13. Rebar quality:

TMT or TMX bars of specified grade are preferred over other types of bars as they are more construction-friendly in all aspects. Rebars should be free of rust or surface flakes to avoid any cracks in the slab structure.

14. Structural design:

We design the concrete structures for the specific load conditions with safety factors. But any mistakes in the calculation of the number of rebars and their spacing while designing may lead to cracks at such slab parts.

15. Binding wire tye:

It is advisable to cut the leftover extra binding wire after the tying process. Protrusion of such binding wire end above the concrete surface may cause water seepage and may help in the formation of micro-cracks in concrete.

16. Concealed pipes:

Sometimes, the electrical conduits used in the concealed wiring invade the concrete cover area in slabs. Concealed materials should be checked before pouring the concrete, to avoid the formation of cracks in line with conduits.

17. Loose formworks:

Care should be taken to plug all the minute openings in the formworks before slab concreting. The openings may leak cement slurry while vibrating the concrete, resulting in lower-grade concrete. This inferior concrete surface area then becomes a hotspot to invite cracks.

18. Gaping in concrete timing:

All the slab concrete works should be completed on that particular day to have a solid one-piece seamless concrete mass. The concrete should be poured continuously, before the initial setting time of prior concrete joints. Always you have to start the work from one end and care should be taken that, concrete should not be partially filled in any formwork to exceed the initial setting time.  

To get the answer to more such construction-related FAQs, click here.

Thank you for going through this article. Have a good day😄


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