Influence of mortar rheology on aggregate settlement

The influence of the rheology of fresh concrete on the settlement of aggreg ate is examined. Fresh concrete exhibits a yield stress that, under certain conditions, prevents the settlement of coarse aggregate, although its dens ity is larger than that of the suspending mortar. Calculations, based on es timates of the yield stress obtained from slump tests, predict that aggrega te normally used in concrete should not sink. To test this prediction, the settlement of a stone in fresh mortar is monitored. The stone does not sink in the undisturbed mortar (which has a high yield stress), but sinks when the mortar is vibrated, presumably due to a large reduction in its yield st ress. This implies that during placement of concrete, the aggregate settles only while the concrete is being vibrated. A unique experimental method fo r measuring aggregate settlement is also introduced and demonstrated.