What is the importance of the Vebe Time test of concrete?

A Vebe test is routinely done on concrete for machine-placed pavement slabs. Ie slip-formed roads. It gives an indication of the ease of compaction of the concrete by vibration.

A Vebe consistometer is merely a small and chunky vibrating table on which is mounted a container with 240 mm diameter, and 200 mm height. Inside that is placed a slump cone, and the latter is filled as per any other concrete slump cone.

The cone is removed in the usual manner, and a specified transparent plastic disk is placed on top of the slumped concrete, and the vibrator is set running at the same time as a stop watch. The number of seconds until the plastic disk is fully covered with cement grout is the Vebe time.