Q. What is Over vibration of fresh concrete?

For proper compaction of concrete by immersion vibrators, vibrating part of the vibrators must be completely inserted into the concrete. The action of compaction is enhanced by providing a sufficient head of concrete above the vibrating part of vibrators. This serves to push down and subject the fresh concrete to confinement within the zone of vibrating action. 

Over-vibration must generally be avoided at the time of the compaction of concrete. If concrete mix is designed with low workability,over-vibration basically consumes extra power of vibration andleads to wastage of energy. For most of concrete mixes, over-vibration causes the problem of segregation in that denser aggregates settle to the bottom whereas the lighter cement paste is likely to move upwards. If concrete structure is cast by successive lifts of concrete pour, upper weaker layer (or laitance) caused by segregation forms potential plane of weakness resulting in possible failure of concrete structure during operation. If concrete is located in a single lift for road works, resistance to abrasion is poor for laitance surface of the carriageway. This becomes a serious problem to concrete carriageway where its surface is constantly subject to tearing and traction forces exerted by vehicular traffic.