Vibration of the concrete sample is not allowed when the slump exceeds what measurement?

When considering the measurement at which vibration of concrete is not allowed, a slump exceeding three inches is critical. At this level of slump, the mixture has reached a consistency that is relatively fluid, and further vibration can lead to segregation of the aggregates within the mix. This segregation can adversely affect the cohesiveness and structural integrity of the concrete, as the heavier particles may settle while the finer particles are displaced, leading to inconsistent strength and durability.

It is important to maintain proper workability of the concrete without compromising its properties. A slump that goes beyond three inches typically indicates that the concrete is already workable enough to fill forms and support rebar without additional vibration. Beyond this point, the potential negative impacts of vibration on quality and performance become significant, emphasizing the importance of controlling consistency during placement.