Roof and floor slab concreting is a major construction activity in a building, whether it is an individual house or a flat. Often errors occur because the operations involved in slab concreting are not organised properly. Important steps that help in proper execution are listed below:

Formwork and scaffolding: The formwork should be strong and the struts erected braced in both directions. All joints in formwork should be properly sealed. It is not proper to use mud, jute or paper to seal the openings in the formwork. Proper putty with grease and cement will seal the joint perfectly. Strips of steel sheet may be used to seal large size openings.

Preparation of grill: The reinforcement grill should be made ready as per the designer's requirement and checked by him before concreting. The reinforcements have to be supported by the cover blocks of proper size and quality. The density and impermeability of the cover block should match the surrounding concrete. Poor mortar cover blocks should not be used as they get crushed under the weight of the grill. The rebars in the top layer is likely to be bent down during the concreting operation. Therefore, they should be provided with `chairs' made up of steel and supported at the correct level above the centring sheet.

Inserts in slab: A number of functional inserts, such as electrical conduits, M.S. boxes and fan hooks are embedded in the slab. The location and positioning of these should be based on electrical wiring diagram or light and fan points. The inserts should be positioned near the mid depth of the slab so that they do not reduce the flexural rigidity of the slab. Positioning them very near the surface should be avoided.

Collection of material: In case of site-mixed concrete, adequate quantity of cement, sand, water and metal required for casting should be collected at site. In case of ready mixed concrete, the time of arrival, machinery required for pumping and adequacy of formwork should be ensured.

Construction joints: The work has to stop at the construction joint. The joint location and the type of treatment to be given at the joints should be pre-planned. It may be necessary to increase the steel percentage crossing the joint to avoid premature opening of the same during service.

Machinery and labour: The mixer, vibrator concrete pump as per requirement should be readily available. For critical machinery, spares should also be made ready. Adequate labour for concreting should be mobilised.

Special provisions: The movement of labour will depend on the sequence of concreting. Once this is finalised, arrangements for wooden planks to serve as bridge for the labour movement without affecting the rebar grill must be got ready. Labour should never be allowed to walk on the reinforcement directly.

Deployment of checking personnel: During concreting one supervisor should be positioned at the mixer end to check the proper batching of ingredients. One person is required to see that the reinforcements are not displaced during concreting and proper compaction is being effected at the location of concreting. Preferably a carpenter is required to check the formwork, scaffolding system and the props to ensure that they are not disturbed due to wet weight of concrete.

Location of steel grill: Cover blocks and chairs should be checked to ensure tolerances for the steel grill. Top steel should not get dislodged due to either weight of concrete, construction operations or labour.

Methods to avoid cracks: The concrete should not be finished smooth or done up with a trowel as it will encourage the laitance to come to the surface, thereby forming a weak top layer. No dry cement should be sprinkled to absorb moisture. These will encourage micro-shrinkage cracks. All cold joints should be avoided. It is also necessary to moisten the formwork before starting the concreting work.

Test cubes: It is always advisable to prepare test cubes of required number depending on the volume of concrete executed. The tests on these will also decide when the shuttering can be released. This will act as a check on the final resulting strength of the concrete.

Special weather precautions: During concreting in summer season, it is advisable to cover the freshly laid wet concrete by moist gunny bag. Drastic drying of green concrete should be prevented in the interest of avoiding plastic shrinkage cracks. In case of rain or heavy winds concrete should be protected with a tarpaulin.

Post-construction measures: De-shuttering of vertical sides of the slab can be done after 24 hours. For curing, 2mx2m ponds made with lean mortar on the surface of concrete may be used.

Curing of the slab should start before the top surface starts drying and should be continued for 10 to 20 days depending on the type of cement used. De-shuttering of slabs can be done after 7, 10 or 15 days depending on the type of cement and the span of the slab.

Hacking the bottom of the slab for making it rough is not permitted as this act causes micro cracking.

Instead, a good bonding agent will have to be applied before plastering to ensure good bond between the plaster and concrete.

However, it is best to leave the concrete as `form finished'. If meticulous care is taken during formwork preparation, then there will be no need for plastering for hiding the defects in concrete!