Technical Services

A properly constructed formwork is the cornerstone of a reliable concrete structure. Do have the formwork inspected and approved by a qualified structural designer before pouring the concrete. Here are a few guidelines that will help you build quality formworks:

1. For formwork of ground floor slabs, the props should rest on firm surface, preferably PCC.
2. When the formwork is rested on earth, ensure that the ground is dry and well compacted. Use spreaders for props resting on ground.
3. Avoid the use of wooden props if possible; steel supports are preferred.
4. For slabs, ensure that props are vertical. Load carrying capacity reduces significantly when props are inclined.
5. To fix props, use bracings on both sides and runners at the center.
   Seal any gaps in shutter joints to avoid leakage of cement slurry.
6. Before concrete is poured, ensure that interiors of the forms (shutters) are cleaned and shuttering oil (form release agent) is applied. Avoid the use of engine oil or other such lubricants as they may stain the concrete surface.
7. During concreting, forms, props, and all other supports must be continuously checked.
8. After concreting, do consult your structural designer before removing the formwork.
9. Listed in the table below are the recommended time limits as defined in IS 456:2000, ‘Plain & Reinforced Concrete – Code of Practice’.

1. Rough handling, shock loading and dropping of steel from a height should be avoided.
2. Avoid the use of wooden props if possible; steel supports are preferred.
3. Reinforcement should be free of rust, scales and coats of mud, oil or any other substances that may reduce or destroy the bond between steel and concrete.
4. Rebending and straightening of torsteel should preferably be avoided.
5. Do have the reinforcement checked by the structural designer after it is placed and secured.
6. The cover is the distance between the reinforcement and the concrete surface and is vital in ensuring the durability of the structure. Listed in the table below are the nominal minimum cover recommendations as defined in IS 456:2000, ‘Plain & Reinforced Concrete – Code of Practice’.
7. Cover blocks should be of concrete or mortar of same strength or of PVC.

Improper placing of concrete could result in a defective structure that may require expensive repairs. While placing concrete, do ensure that:

1. The concrete is deposited in or very near to its final position.
2. The concrete is not placed in large heaps on the formwork. Moving heaped concrete manually or with a vibrator should also be avoided.
3. The concrete is poured vertically from a height of less than one meter; otherwise it may lead to segregation.
4. In mass concrete works, the concrete is placed in uniform layers; sloped layers may lead to segregation.
5. When thickness/depth of structure is more than 300 mm (1 foot), concrete is placed in two or more layers of equal thickness/depth. In general, the allowable thickness of each concrete layer ranges from 150 mm (6 inches) to 300 mm (1 foot).
6. In layered construction, fresh layer of concrete is placed only after the lower layer is properly compacted, but before that layer has set. If the continuity in placement is not maintained, it will lead to the formation of cold joints that will weaken the structure.
7. For slabs, concrete placement starts from the corners and ends, and then continues inwards.
   There is no displacement of reinforcement and/or damage to formworks.
8. For large concrete pours, proper planning and co-ordination are crucial in avoiding delays, cold joints, or segregation.
9. When concrete is being poured into deep sections with heavy/congested reinforcement, ensure sufficient visibility so as to ensure proper placement of concrete. Inadequate care in placing may lead to segregation, poor finish and honeycombing effect, particularly in the lower portions of the structure.