Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 106

been included in a previous specification and so mindlessly continued
in the current document. The objectives may be to avoid high shrink-
age, segregation, and bleeding or to avoid an excessive w/c ratio leading
to inadequate strength or durability. However, any of these faults can
be encountered at almost any slump, however low, and avoided at any
slump, however high. It is also easy to detect from a theoretical mix
submission, which mixes will be subject to one or other of these prob-
lems. The contractor should therefore be permitted to submit his mix
for approval at whatever slump he chooses, provided it is designed to
accommodate his own slump limit without detriment. It is quite possible
to produce fully flowing (250 mm slump or more) concrete having none
of the potential faults noted and to produce almost all these faults in a
50 mm slump mix.
5.4.2.2 Self-compacting concrete (SCC)
A whole new ball game in workability has been opened up with the con-
cept of self-compacting concrete, also called self-consolidating concrete or
superworkable concrete. This is a relatively new concept, having originated
in Japan in the 1980s and originally met with a degree of skepticism in most
of the rest of the world. Now it seems quite possible that it will become one
of the most widely used kinds of concrete in the not too distant future. This
is already true in much of the precast industry.
5.4.3 Pumpability
Bleeding rate (segregation resistance) plays an important role in preventing
blockage during pumping. Pumpability should not be considered an intrin-
sic property of concrete but involves concrete composition, configuration
of the plant, and the pumping procedure. Blockages generally occur during
priming and restarting after a prolonged delay but not during steady-
state pumping. Predicting the expected pumping pressure is a vital part
of pumping concrete up super-tall buildings. The pressure required will
depend on frictional losses within a concrete pipeline and the pressure
head. Kaplan et al. (2005) demonstrated that bends and so on did not
influence pressure losses. Pressure calculation is based on Bernoulli's
equation. The pressure head component is unavoidable as it is simply a
function of the density and vertical height. In the case of the uppermost
pour for the Burj Khalifa, the pressure head component alone was over
140 bar. Output (i.e., velocity) and pipe diameter are constructability
issues. The critical parameter that will determine the feasibility of pump-
ing to a particular height is the friction factor. This is typically estimated
with site trials.