Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 83

suppressants. This old dictum has recently been taken to a new level by
Kaplan, de Larrard, and Sedran (2005) in a research project involving a
specially assembled 148 m closed circuit of piping and over 60 truckloads
of concrete. A technique using a standard pressure air metre with tetra-
chloroethylene instead of water and so measure water squeezed from the
concrete was employed to measure bleeding under pressure. It was found
that the rate rather than the quantity of bleeding was significant. This was
fortunate because it allowed a rapid result to be obtained. The pumping
procedure was also found to be very important. Avoiding delays between
trucks, defective joints in the line, and pumping slowly during priming as
well as when difficulties are experienced is well-known advice. An interest-
ing new observation was the importance of the first concrete intermingling
with the priming slurry if it was too fluid. This confirms our collective
experience that a cohesive slurry, preferably similar to the concrete without
aggregate is required.
VMAs are particularly important when SCC of relatively low strength
(and therefore low cementitious content) is involved. SCC has been reported
to demonstrate excellent pumpability on the Eureka building in Melbourne,
Australia, currently the tallest in the Southern hemisphere.
4.3.9 Self-consolidating concrete (SCC)
Self-consolidating concrete (SCC) was discussed as a current hot topic in
the third edition. It was highlighted that its ranking in terms of production
volume was nothing like its ranking in volume of technical literature.
However, there has been a considerable increase in the volume of SCC used,
particularly in the precast industry where the advantages of ease of place-
ment and high-quality finish have made SCC the preferred concrete used
in many markets. The greater quality control required is easier to achieve
under the factory conditions of a precast plant than in a standard premix
plant. There has also been greater resistance to the use of SCC for in situ
construction because of the increased materials cost as a result of the higher
binder content and admixture dosage. One region where this has not been a
significant issue has been in foundation elements in the Middle East where
the durability requirements of the specification would normally require a
low water to cementitious ratio and a high binder content, often with sig-
nificant replacement with supplementary cementitious materials. Therefore
modification of aggregate grading and possibly the use of suitable viscosity
modifying agent may be the only changes to achieve self-consolidating
characteristics at minimal additional cost. The fact that SCC could greatly
reduce the placement time and the size of the concrete placing team means
that SCC could provide significant cost savings in these applications.
The improved capability of superplasticisers has played an important
role in expanding the capability of SCC to achieve the required fresh and