Concrete, unless it is specifically designed for self-compaction, must be compacted. Compacting concrete removes the entrapped air and voids, and consequently increases the strength and durability of the concrete. To achieve uniformly compacted dense concrete:
In the general construction of slabs, beams and columns, use needle vibrators of 50 mm diameter. For thin sections, in confined locations, and in structures with congested reinforcement, use a 30 mm diameter needle vibrator.
Ensure adequate supply of spare vibrators and adequate fuel at the site.
The concrete surface being compacted must be visible.
The needle should be inserted quickly and allowed to quickly and completely penetrate the concrete on its own weight.
The needle should not be left in the concrete for more than 10 seconds. If the concrete is being pumped, it will have a slump of 80-120 mm; in this case, the needle should not be left in the concrete for more than 5 seconds.
Over vibration may cause mortar to accumulate on the top with the resultant thin layer having poor strength.
Use experienced operators to man vibrators; they can determine when adequate compaction is achieved. Typical signs of sufficient compaction are air bubbles
on the surface and the constant drone of the needle vibrator.
Withdraw the needle from concrete slowly to ensure that the hole left by the needle is filled.
Insert needles in staggered fashion to ensure uniform compaction for the concrete.
In a layered placement, the needle should penetrate the bottom layer by 80-100 mm.
For compaction in hot weather (for concreting of slabs), use a light initial vibration and then another round of vibration after about 30 minutes; this helps in significantly reducing shrinkage cracks.
If concrete segregates due to over vibration, fine aggregates tend to accumulate on concrete surface and there is a tendency to shrink and crack.
To avoid this, plough up and mix coarse aggregates with the help of forks (punja) before finishing.
Wherever construction joint is to be provided, stop-ends should be firmly fixed to avoid movement of joints and ensure proper compaction of the concrete near stop-ends.