Before the time of a 1938 paper in overvibration and revibration, few were aware of the considerable benefits that result from late revibration. Many had serious reservations about it, fearing that such disturbance of the early paste changes that progress to hardening would somehow impair the quality of the concrete. But first, just what is late revibration? It is exactly that. It is a revibration delayed until all slump is gone and after a time the concrete is approaching initial set, but the running vibrator will slowly sink of its own weight into the concrete and close the concrete behind it as it is carefully withdrawn. No matter how late revibration is attempted, if the running vibrator can be made to penetrate and the concrete again becomes plastic, no harm will be done and results will be beneficial. The latest practical time is the most rewarding. This time may vary from 1 to 4 hours after placement depending on temperature of concrete and the ambient, as well as the type. The benefits of late revibration primarily result from its removal of the effects of settlement. Due to the extra water in concrete mixes- more than is needed for cement hydration- solids will settle downward before hardening and some of the surplus water will move upward. An indication of this settlement is the crack that often forms over a thinly covered reinforcing bar near the top surface of a concrete placement. Sometimes settlement cracks and checks can be seen opposite horizontal bars near formed surfaces. Depending on the presence of restrictive factors, such as low slump, more fines and higher temperature, the amount of rising water will vary. Whatever the amount, films and thin layers of it will accumulate under pieces of aggregate and under horizontal surfaces of reinforcing bars, form bolts and other embedments. This obviously prevents mortar contact and consequent bond in these areas, with consequent loss of otherwise potential strength. Watertightness is also lost. There are a number of benefits that make the investment in delayed revibration worth considering. Better watertightness, when important for the sake of appearance or to meet test requirements, alone can be worth the cost of a late revibration of the upper layers of placement. Is also could be less expensive than exterior waterproofing treatments; with or without them, the primary source of leakage is poor joints or cracks, which neither the coating nor revibration will correct. It has been observed that here were considerably fewer bugholes in the upper areas of wall surfaces where the upper layers had been revibrated.