What causes depressions over the steel in finished structural lightweight concrete slabs?
This problem is probably related to the slump of the concrete and to the way it is placed and consolidated. Lightweight concrete, because the aggregate is lighter, slumps less than a normal weight concrete of equivalent workability. Accordingly, structural lightweight concrete slabs should never be placed at slumps of more than three inches, but usually in the range of 1 1/2 to 3 inches. When high slump structural lightweight concrete is vibrated, some of the mortar in the slab is likely to separate and move toward the bottom of the slab; this separation will be worse if the vibration is excessive. Reinforcing steel is apt to transmit vibration, particularly if the vibrator happens to be laid directly on the steel mat. This causes coarse aggregate to bridge over the steel and leave voids into which mortar can later settle during the finishing operations, thus causing surface depressions. During the work of subsequent finishing operations, some mortar may be moved back into the depressions from the nearby surface but often not enough to make the surface plane. It is best to keep slump low.