FORMWORK (SHUTTERING AND CENTERING)

A correctly constructed formwork paves the foundation of a durable concrete structure. Make sure that the formwork is checked and accepted by a competent structural designer before the initial stage of pouring concrete on it.

Following are the guidelines that can help you build quality Formworks:

• It is always wise to use steel supports
• The props should lay on hard surface for the formwork of ground floor slabs; stay dry and well compacted. Spreaders should be used to keep away from a contact amid them.
• Props should never be inclined as it decreases their load-carrying ability
• Bracings should be used on both the sides and on the runners at the centre to keep the props fixed
• Avoid of leakage of cement slurry by sealing gaps in shutter joints
• Avoid the use of engine oil or other lubricants for oiling the interior of the form as these lubricants may stain the concrete surface
• After concreting, a structural designer should be consulted before removal of the formwork.

REINFORCEMENT

These are crucial steps during construction and it largely contributes to the longevity of the structure being constructed.

Following are the guidelines that can help build quality Formworks:

• Minimize rough handling, shock loading and dropping of steel from a height
• Prefer steel supporters instead of wood
• Keep the reinforcement free of rust, scales. Coats of mud, oil or any other substances should be avoided to ensure a direct bond between steel and concrete
• Avoid rebending and straightening of torsteel
• Get the reinforcement checked by the structural designer at every step
• 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’
• Use concrete or mortar to make cover blocks

CONCRETE PLACEMENT

Even a minute mistake in placing the concrete may induce disastrous results. From costly repairs to structural anomaly, it can induce several kinds of harms. Therefore, at the time of placing concrete, be double sure that:

• The concrete deposited is very near to its finishing position
• Do not place the concrete in huge heaps on the formwork. Rather you should avoid moving heaped concrete either by manual effort or with machines
• To avoid separation, pour it perpendicularly from a height of one meter or less than that.
• Conventionally, the permissible width of each concrete layer ranges from 150 mm (6 inches) to 300 mm (1 foot). When thickness of structure is more than 300 mm (1 foot), ideally, concrete is placed in two or more layers with equal thickness/depth
• In order to avoid cold joints in layered construction, you need to put the fresh layer of concrete once the lower layer is correctly compressed.
• Sufficient visibility has to be ensured while driving in deep sections with congested reinforcement. insufficient care in placing may lead to separation, poor finish and honeycombing effect

COMPACTION OF CONCRETE

Compacting concrete is a typical step. It removes the trick from air and voids to thus increase the power and durability of the structure.

Follow these rules to achieve time after time compacted dense concrete:

• Use of needle vibrators of 50 mm diameter size for the construction of slabs, beams, and columns is better.
• It is recommended to use a 30mm diameter needle vibrator for thin sections, confined locations, and structures with crowded reinforcement
• Ensure you can adequately provider of spare vibrators and sufficient fuel at the site
• Now you need to insert the needle rapidly and permit it to wholly enter the concrete
• The needle should not be left in the concrete for more than 10 seconds
• Employ experienced operators to run vibrators to achieve adequate compaction
• Withdraw the needle from concrete slowly to make sure that the space is filled
• The needle should enter the bottom layer by 80-100 mm in a layered placement
• Plough up and mix crude aggregates with the aid of forks before concluding to avoid gathering on concrete surface
• If construction, joint has to be provided, stop-ends should be determinedly fixed to stay away from joint movement. Also, you need to ensure proper compaction of the concrete near stop-ends.

CURING OF CONCRETE

Curing of concrete is one of the most significant steps in concrete construction. It assists in retaining the water in concrete which may get evaporated due to existing high ambient temperatures or windy circumstances. Lack of durability is the special effects of improper curing.

• It is usually done by sprinkling as well as spraying of water on concrete
• For sloped roofs, columns, staircases, and other perpendicular surfaces, wrap the entire surface with hessian or jute cloth saturated with water
• Continuous and adequate supply of water should be available at the site all through the curing period
• Ideally, curing for fourteen days is the best. But concrete surfaces should be kept uncovered under moist and damp condition at least for 10 days since the date of placing of concrete
• In hot and humid weather, curing by sprinkling of water gets started within 4 to 5 hours of batching of concrete and it continues till all bunds are prepared
• Trowel the broken concrete surface to stay away from plastic decrease cracks
• Keep the curing water free from all smelly substances
• Prefer to use cement mortar for preparing bunds
• In order to avoid dry spots in bunds, pound for at least 20 mm on top of the highest point of the concerning concrete surface