Careful supervision during concrete manufacture is necessary for all concreting operations such as batching, mixing, transporting, laying, compacting and curing. While inspecting concreting work following points should be remembered for effective quality control.
Inspection during Concreting
The concrete mix should be designed in the laboratory with the materials to be used in the field.
Weight batching should be adopted as far as possible. Where facilities for weight batching are not available the engineer-in-charge may permit volume batching.
During mixing, the mixer should be charged to its full rated capacity. The materials should be fed in proper sequence. The speed of the mixer should range from 15 to 20 r.p.m. The mixing time should not be less than 2 minutes in any case. Segregation should be avoided while unloading the concrete from the mixer.
In the plastic state, workability of concrete is an important property. It should be checked by slump test or compaction factor test. About three tests should be carried out daily for every 25m3 of concrete.
No segregation should take place during transporting of concrete.
Concrete should not be dropped from a height of more than 1 m. If the drop height exceeds 1 m, chutes should be used.
To avoid re-handling of concrete it should be placed at its final position as far as possible.
For compaction, vibrators should be used. The insertion spacing of internal vibrators should not be more than 0.6 m. It should be drawn out slowly so that no holes remain in the concrete. The frequency of vibrator should not be less than 7000 cycles/minute.
Curing should be done for a specified period so that concrete develops requisite strength. Concrete should be covered with hessian as soon as it becomes hard.
The form work should correspond to final form of the structure. It should be checked before concreting is started. The inside of the forms should be cleaned and oiled. The forms should be removed after the specified period.
Concrete should be protected from hot and cold weather at early ages. Concreting should not be done at temperature below 4.50C and above 400C. In very hot weather water and aggregates should be cooled. Retarders of approved quality can be used.
In very cold weather water and aggregates should be heated. Accelerators of approved quality can also be used.