Civil Engineering Project Management (4th Edition)

244 Civil Engineering Project Management
Fig. 19.3. Suitable range of aggregate gradings for concrete (based on modified versions
of graphs by H.M. Walsh: How to make good concrete)
since slump varies with size and sharpness of aggregates used, as well as the
amount of fines, cement and water in a mix. A truncated metal cone, 300 mm
high by 100 mm diameter at the top and 200 mm at the bottom, is filled in
three equal layers with concrete, each being rodded with 25 strokes of a
16 mm rod, rounded at both ends. On removal of the cone the ‘slump’ or drop
in level of the top of the concrete below the 300 mm height is measured.
Another site test uses the compacting factor apparatus, which works on the
principle of finding the weight of concrete which falls via a sequence of two
hoppers into a cylinder. The ‘compacting factor’ is the ratio of the weight of
concrete falling into the cylinder as compared with the weight of concrete
compacted to fill it. The higher this ratio is, the more workable is the concrete.
These and other laboratory tests are described in BS 1881:1983. In practice,
workability can be judged by eye as described in Section 19.6.
There is a substantial reduction of the workability of a concrete mix during
the first 10 min after mixing, as anyone who has hand mixed concrete will
know. This is primarily due to absorption of water by the aggregate so that
the reduction in workability is less if the aggregate is wet before use. On a