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.
