When concrete is compacted with vibrators the following precautions may be taken to produce a concrete of good quality.
The formwork should be strong and tight so that there is no leakage of mortar. The leakage of mortar results in honey combing. A lining of plywood may be provided to prevent the leakage if the surface finish is important.
The concrete should be laid in layers of thickness 15 cm to 60 cm. in order to avoid trapping of air in the concrete the thickness should be as less as practicable.
To regulate the degree of compaction in all portions of the concrete the vibrators should be inserted vertically.
The vibrations must be carefully controlled. The internal vibrators may be inserted at a spacing of 45 cm to 60 cm (or 8 to 10 times the diameter of needle) for a period ranging from 30 sec to 2 min. the vibrator should be withdrawn when the mortar just appears at the surface. The surface should neither be honey combed nor it should contain excess mortar.
Over vibration of concrete, particularly if the slump exceeds 100 mm should be avoided as it causes settling of the coarse aggregate at the bottom and the accumulation of the mortar or water at the top.
The vibrators should not be used to move the concrete laterally in the forms as it leads to segregation.
The vibrator should be immersed into the entire depth of the freshly laid concrete or even into the layer below, if it is still plastic or can be brought into the plastic condition. This will avoid the formation of a weak joint between the two layers.
The vibrator should not touch the surface of the formwork so that it may not be damaged.
The vibrator should be withdrawn slowly at each location and should be operated continuously while being withdrawn so that no hole will remain in the concrete.
Re vibration of concrete is not harmful provided the concrete has not set so much that it will not become plastic again by vibration.