Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 84

hardened properties. High fine aggregate percentage and often reduced
maximum aggregate size coupled with low w/cm and high workability
would only be possible with considerable dosage of an effective superplas-
ticiser. The advances in viscosity modifying or viscosity enhancing admix-
tures (VMAs or VEAs) are potentially more important as they help to
ensure adequate segregation resistance, even with variation in aggregate
grading. The active marketing of VMAs to provide robust SCC with lower
binder contents is expected to further expand the application of SCC for
in situ applications. One of the main points for advocating SCC has been
to reduce the cost of repairing honeycombing and other defects in concrete
elements. As the admixture suppliers often sell the repair products, it is
commendable that they are trying to eliminate these defects (but then again
the repair products are probably in another division!). Olafur Wallevik has
done a great deal of research on low binder content SCC and has been able
to achieve 40 MPa SCC with only 280 kg/m³ binder.
Improved quality control procedures, greater acceptance of simple practi-
cal test procedures to assess segregation resistance (see Chapter 7), and the
development of standardised SCC mixes by premix suppliers will also help
expand the application of SCC for in situ construction. Educating specifiers
in the advantages and limitations of SCC is also an important factor.
As most of the test procedures on SCC focus on passing or filling ability,
there can be a tendency to add too much superplasticiser to reduce the
viscosity. This can lead to reduced segregation resistance, particularly if
the retardation level is high. We would suggest that segregation resistance
is at least as important as adequate flow and is rarely assessed on site.
For example, on one project a concrete mix was modified to reduce the
VMA dosage and increase the superplasticiser content, as the L-box value
was less than the guideline given by EFNARC. A site trial was conducted,
which showed no segregation. However segregation did occur during initial
casting. Further trial mixes demonstrated a tendency for the slump flow of
the mix to increase over time and the 5 minute V funnel increased markedly
after 90 minutes indicating reduced segregation resistance. This suggested
a limited effective duration for the VMA, which was not compensated by
hydration effects due to the increased retardation. This was an example of
the dangers of using arbitrary performance criteria to define the concrete
mix design without a full appreciation of the interaction of the different
parameters. Testing procedures for SCC together with their advantages and
limitations are discussed in Chapter 7.
An interesting historical note is that Thomas Edison was very interested
in concrete and set up a concrete company. He was convinced that self-
consolidating concrete was the way forward (Figure 4.3). He envisioned
whole structures being pumped from one point. He did not have the tools
of superplasticisers, and VMAs but he had the vision and would be pleased
to see the progress being made.