Testing - Concrete Mix Design, Quality Control and Specification - page 163

such as a cold joint or pump blockage as a result of the delay to replace the
rejected truck. Unfortunately inexperienced supervisors are likely to reject
nominal noncompliance without realising the damage that could be caused,
particularly in high-temperature environments.
Although continuous perfection is impractical, a slump test will only
be asymmetrical if it has been produced by an asymmetrical process. It is
often possible to know where the slump operator has stood, how he has
used his scoop, and how he has held his rod, all by looking at the resulting
slumped concrete after the test. A failure to rotate the scoop will usually
cause a higher coarse aggregate content opposite the point of discharge
from the scoop. This will often cause the cone to lean toward the point of
discharge on stripping. It is not easy to rod the foot of the cone opposite
the operator if the rod is held in a dagger grip. To accomplish this the
operator must project his elbow over the slump cone in order to rod each
layer of the concrete parallel to the side of the cone around the entire cir-
cumference. An alternative is to use a rope grip, that is, to hold the rod as
though pulling a rope.
The slump test is based on a standardised degree of semicompaction, unlike
compression test specimens, which should be fully compacted whatever it
takes. Therefore it is important that the correct number of strokes be used in
the slump test while being only a required minimum in compacting compres-
sion specimens. It is also important that the rod have the correct end shape. A
flat-ended rod (e.g., a piece of reinforcing bar) pushes coarse aggregate to the
bottom and tends to leave a hole rather than compact. The British rod has a
hemispherical end, which is a distinct improvement over a flat end. However.
the Australian and American rods, which taper to half the original diameter
before having a hemispherical end give greater compaction. It should also be
realised that slump measurement is different in the United Kingdom, United
States, and Australia. In the United Kingdom, measurement is to the high-
est point, in the United States to the point on the centerline of the original
cone, and in Australia to the average of the original top surface. One may
have personal preferences, but the important thing is to be consistent on a
particular project and to be on the lookout for new operators who may have
been trained by site engineers of different nationality.
A concept proposed by Day (1986) is that of an equivalent slump. As
Bryant Mather (1987) has so firmly pointed out, slump loss is proportional
to temperature and leads to the (strictly incorrect but workable) view that
water requirement increases with temperature. Everyone realises that slump
reduces with time. Putting the two effects together, it is clear that slump
only has a real meaning if accompanied by a time and temperature reading.
Day's proposal in the third edition was to combine the time and temperature
into an equivalent age according to Arrhenius (see Section 7.4 on early-age
strength for more detail). Thus an equivalent slump could be evaluated, being
the slump that would be obtained had the concrete been kept for 30 minutes