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The sample of concrete to be tested is placed gently
in the upper hopper. The hopper is filled level with
its brim and the trap-door is opened to allow the
concrete to fall into the lower hopper. Certain mixes
have a tendency to stick in one or both of the hoppers.
If this occurs, the concrete may be helped through by
pushing the rod gently into the concrete from the
top. During this process, the cylinder should be
covered by the trowels. Immediately after the concrete
has come to rest, the cylinder is uncovered, the trap-
door of the lower hopper is opened, and the concrete
is allowed to fall into the cylinder. The excess of
concrete remaining above the level of the top of the
cylinder is then cut-off. The weight of the concrete in
the cylinder is then determined to the nearest 10 g as
the weight or partially compacted concrete. The
cylinder is refilled with concrete from the same
sample in layers of approximately 50 mm, the layers
being heavily rammed or preferably vibrated so as
to obtain full compaction. The top surface of the fully
compacted concrete is carefully struck off level with
the top of the cylinder. Compacting factor is defined as the ratio of the weight of partially
compacted concrete to the weight of fully compacted concrete. It is normally stated to the
nearest second decimal place.
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X The test determines the time required for transforming, by
vibration, a concrete specimen in the shape of a conical frustum into a cylinder.
The apparatus (Fig. 10.34) consists of a vibrator table resting upon elastic supports, a metal
pot, a sheet metal cone, open at both ends, and a standard iron rod.
A slump test as described earlier is performed in the cylindrical pot of the consistometer. The
glass disc attached to the swivel arm is moved and placed just on the top of the slump cone in
the pot and before the cone is lifted up, the position of the concrete cone is noted by adjusting
the glass disc attached to the swivel arm. The cone is then lifted up and the slump noted on the
graduated rod by lowering the glass disc on top of the concrete cone. The electrical vibrator is
switched on and the concrete is allowed to spread out in the pot. The vibration is continued
until the whole concrete surface uniformly adheres to the glass disc and the time taken for this
to be attained is noted with a stop watch. The consistency of the concrete is expressed in VB
degree which is equal to the recorded time in seconds. The required slump is obtained on the
basis of the consistency scale given in Table 10.6. The curve in Fig. 10.35 indicates the relationship
between slump in mm and Vee-Bee degrees covered by the consistency scale in seconds and are
given in Table 10.6.
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