Testing - Concrete Mix Design, Quality Control and Specification - page 160

6

5

4

3

2

1

0

0 30 60

Plastic Viscosity (Pa.s)

90 120

Figure 7.8 Relationship between plastic viscosity and friction factor.

rheology (http://www.sensocrete.com). Grace has also developed automated
process control equipment for managing batches of ready-mixed concrete
and reducing process variations (http://www.na.graceconstruction.com).
The equipment reportedly accurately calculated the slump for a range of
materials and mixture proportions and correctly adjusted the water con-
tent of individual batches to reach a target slump. At the ACI Convention
in Dallas, Dr Denis Beaupre presented data on another truck measuring
system (the IBB probe), which has been used in some production concrete
and the results look encouraging. A comparison of the slump tests in accor-
dance with the European and U.S. procedures showed the probe generated
significantly lower coefficients of variation (2.8% versus 7.5% and 8.2%,
respectively (Figure 7.9). This would be due to the number of readings pos-
sible for the probe. However, using such systems for compliance with slump
requirements would appear to be missing the opportunity to get full benefit
from rheological data. The fact that a number of suppliers are providing
truck-mounted rheology monitoring systems is encouraging for future pro-
duction quality control.
The problem with the slump test is that it is a very widely and firmly
established test but is a poor measure of the relative workability of different
mixes. It survives because of its simplicity and robustness and also because
it is (when properly conducted) quite a good measure of the relative consis-
tency (i.e., wetness) of successive deliveries of the same mix. With today's
much more accurate batching and using the Day's mix suitability factor
(MSF) we can have defined and controlled the other aspects of workability
so that it may now be adequate to accept the slump test as defining consis-
tency for the particular mix (especially if an “equivalent slump”, adjusted
for time delay and temperature, is used). What we must not do is to use