Building Materials, Third Edition

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The mould used for the test specimen is shown in Fig. 10.31.
The internal dimensions of the mould are bottom diameter =
200 mm, top diameter = 100 mm, and height = 300 mm.
The mould is filled in with fresh concrete in four layers,
each approximately one-quarter of the height and tamped
with twenty-five strokes of the rounded end of the tamping
rod. The strokes are distributed in a uniform manner over the
cross-section and for the second and subsequent layers should
penetrate into the underlying layer. The bottom layer is tamped
throughout its depth. After the top layer has been rodded, the
concrete is struck off level with a trowel or the tamping rod, so
that the mould is exactly filled. The mould is removed
immediately by raising it slowly and carefully in a vertical
direction. This allows the concrete to subside and the slump is
measured immediately by determining the difference
between the height of the mould and that of the highest
point of the specimen being tested (Fig. 10.32). The slump
measured is recorded in terms of millimetres of subsidence
of the specimen.

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X This test is more precise and sensitive than the slump test. It is

particularly useful for concrete mixes of medium and low workabilities as are normally used

when concrete is to be compacted by vibration; such dry concretes are insensitive to slump test.

For concrete of very low workability of the order of 0.7 or below, the test is not suitable, because

this concrete cannot be fully compacted for comparison in the manner described in the test.

A diagram of the apparatus used in compacting factor test is shown in Fig. 10.33.

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