Concrete operations

Mixing, Placing, Compacting, and Curing of Concrete

Mixing of Concrete:

The mixing of constituents materials for concrete should ensure that the mass becomes homogeneous, uniform in color and consistency. Hand mixing of concrete is not desirable, machine mixing should be adopted for better quality.

If there is segregation after unloading from the mixer, the concrete should be remixed

While using conventional tilting type drum mixture, the mixing time should be at least two minutes and the mixer should be operated at a speed recommended by the manufacturer (normal speeds are 15-20 revolutions per minute.

Transportation of Concrete:

Concrete can be transported from the mixer to the formwork by a variety of methods and equipment such as mortar pans, wheel barrows, belt conveyors, truck mixer mounted conveyer belts etc.
As there is  a risk of segregation during transportation, care should be taken to avoid it.

Placing of Concrete:

It is also important that the concrete is placed in the formwork properly to yield optimum results. Prior to placing, reinforcements must be checked for their location and size, splices and anchorage requirements and any loose rust must be removed.

The formwork must be cleaned, adequately supported, joints plugged and the inside of the formwork applied with mold releasing agents for easy stripping.

Concrete should be placed in thicker members in horizontal layers of uniform thickness (about The concrete should be deposited and 150 mm for reinforced concrete members). Each layer should be thoroughly consolidated before the next is placed.

Compacting of Concrete:

Right after placement, concrete contains about 20 % entrapped air. The concrete should be deposited and compacted before the initial setting of concrete and should not be disturbed subsequently.

Curing of Concrete:

All newly placed and finished concrete members should be cured and protected from drying and from extreme changes in temperature.
Wet curing should be started as soon as the final set occurs and should be continued for a minimum of 7-15 days.

Removal of Forms:

It is advantageous to leave forms in place as long as possible to continue the curing period.

The vertical supporting members of formwork (shoring) should not be removed until the concrete is strong enough to carry at least twice the stresses to which the concrete may be loaded at the instant of  removal of formwork.