Concrete placing, compacting and curing


Concrete shall be transported from the mixer to the formwork as rapidly as possible by methods which will prevent the segregation or loss of any of the ingredients and maintaining the required workability. During hot or cold weather, concrete shall be transported in deep containers. Other suitable methods to reduce the loss of water by evaporation in hot weather and heat loss in cold weather may also be adopted.

The concrete shall be deposited as nearly as practicable in its final position to avoid rehandling. The concrete shall be placed and compacted before setting commences and should not be subsequently disturbed. Methods of placing should be such as to preclude segregation. Care should be taken to avoid displacement of reinforcement or movement of formwork.

Concrete should be thoroughly compacted and fully worked around the reinforcement, around embedded fixtures and into corners of the formwork. The mechanical vibrators is use for compacting concrete. Over-vibration or vibration of very wet mixes is harmful and should  be avoided under vibration is also harmful. Whenever vibration has to be applied externally, the design of formwork and the disposition of vibrators should receive special consideration to ensure efficient compaction and to avoid surface blemishes.


Construction Joints

Construction joints are the temporary joints left between subsequent concreting operations.

When the work has to be resumed on a surface which has hardened, such surface shall be roughened. It shall then be swept clean and thoroughly wetted. For vertical joints neat cement slurry shall be applied on the surface before it is dry. For horizontal joints  the surface shall be covered with a layer of mortar about 10 to 15 mm thick composed of cement and sand in the same ratio as the cement and sand in concrete mix. This layer of cement slurry or mortar shall be freshly mixed and applied immediately before placing of the concrete.

Where the concrete has not fully hardened, all laitance shall be removed by scrubbing in the wet surface with wire or bristle brushes, care being taken to avoid dislodgement of particles of aggregate. The surface shall be thoroughly wetted and all free water removed. The surface shall then be coated with neat cement slurry. On this surface, a layer of concrete not exceeding 150 mm in thickness shall first be well rammed against old work, particular attention being paid to corners and close spots; work thereafter shall proceed in the normal way.


Curing

Moist Curing – Exposed surfaces of concrete shall be kept continuously in a damp or wet condition by ponding or by covering with layer of sacking, canvas, hessian or similar materials and kept constantly wet for at least seven days from the date of placing of concrete.

Membrane Curing – Approved curing compounds may be used in lieu of moist curing. Such compounds shall be applied to all exposed surfaces of the concrete  as soon as possible after the concrete has set.