F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

Assessment of workability 47
f
Slump
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Fig. 2.6-1 Slump test
and a cylindrical container (Fig. 2.6-2). The upper hopper is first filled
with a sample of the freshly made concrete, which is then dropped through
the trap door into the second hopper. Again, by opening the trap door of
this second hopper, the concrete is dropped into the cylindrical container
below. Any surplus concrete above the top of the cylinder is then struck
off flush, and the weight of the concrete in the cylinder is determined.
The ratio of this weight to that of an equal volume of fully compacted
concrete from the same batch is called the compacting factor. The latter
weight is usually determined by filling the cylinder in several layers, each
layer being vibrated to achieve full compaction. The compacting factor test
provides a good indication of workability, and is particularly useful for the
drier mixes of medium to low workabilities.
For measuring the workability of concretes which are so stiff that
compaction by vibration is always necessary, the VB consistometer test
(BS 1881: Part 104), developed by V. Bahrner of Sweden, is more
satisfactory than either the slump test or the compacting factor test. The
VB apparatus (Fig. 2.6-3) consists essentially of a standard slump cone
placed concentrically inside a cylinder of 240 mm internal diameter and
200 mm high, the cylinder being mounted on to a vibrating table. To carry
out a VB test, the slump cone is filled with concrete as in a standard slump
test. The slump cone is then removed, which allows the cone of concrete to
subside. The swivel arm enables the transparent disc (230 mm diameter)
to be swung into position over the subsided cone of concrete, as shown
in Fig. 2.6-3. The screw on the swivel-arm holder is then tightened and
vibration is started. The remoulding of the concrete in the cylinder is
observed through the transparent disc. The VB time is the time in seconds
between the start of virbration and the instant when the whole surface
of the transparent disc is covered with cement paste, i.e. it is the time of
vibration required to change the shape of the cone of concrete, left
standing after lifting of the slump cone, into that of the cylinder with a level
top surface. Thus, the stiffer and the less workable the concrete mix, the
higher will be the VB time.