Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 73

at higher cement contents) but are more sensitive to variations in cement
characteristics. Newer types of admixture (described as “synergised” by
some manufacturers) often combine polymers and lignosulphonates in an
attempt to get the best of both characteristics.
4.3.2 Superplasticisers or high range
water reducers (HRWRs)
HRWRs have become distinctly more important in the years since the first
edition of this topic. It is hard to imagine a high-performance concrete (HPC)
without an HRWR. Their wider use and greater importance have been accom-
panied by a better understanding of their strengths and weaknesses. It is becom-
ing apparent that denser packing of the paste fraction of concrete is the key to
higher strength, reduced permeability, and so on. This can be achieved by the
use of finer materials such as silica fume, finer cement, and superfine fly ash.
Such finer materials have a higher water requirement, which can offset their
benefit. The answer to this is to use the fine material together with an HRWR
to counter the higher water requirement. It has also become apparent that
not all HRWRs are compatible with all cements and cementitious materials.
The best way to check on this is to use the admixture at the intended dose in
an otherwise normal Vicat setting test. Better still the test can be repeated
at different dosage rates to establish the saturation dosage (i.e., that dosage
above which no further water reduction is obtained) as well as checking on
the possible rapid workability loss which is the nature of the incompatibility
of some admixtures and cements. Alternatively it may be found that excessive
retardation of set is experienced in some cases. It is also desirable to include in
this test any pozzolanic materials intended for use in the concrete.
The original superplasticisers were melamine formaldehyde and sulpho-
nated naphthalene. The former originated in Germany and the latter in Japan.
These are effective water reducers with a limited period of effectiveness and
apparently no significant detrimental effects on retardation or air entrain-
ment. They are relatively expensive and use in higher volume compared
to normal water reducers and cannot be justified on cement reduction
grounds for ordinary concrete but usually can be for concrete with higher
performance requirements.
They can be used in four ways:
1. To produce “flowing” concrete—Such concrete can be virtually
self-compacting if appropriate modifications to the mix are made and
may be justified on labor-saving grounds. It may also be worthwhile
where excellent surface finish (on vertical formed surfaces) is required
or for very congested sections (see also Section 4.3.9).
2. To produce very high strength or durability—At normal workability
the water reduction can give high strength increases. This may only