Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 74

be financially worthwhile when the strength required cannot be
obtained by increased cement contentitious. On the other hand a
superplasticiser is very desirable with high cementitious content, as
the cement may not otherwise be adequately dispersed.
3. To limit shrinkage—In thin walls with congested reinforcement a small
aggregate, high slump mix may be necessary to achieve full compaction.
Such concrete would have excessive shrinkage if the high workability
were attained by increased water and cement content, but not if obtained
by using a superplasticiser at normal water and cement contents.
4. To limit cementitious content—In massive elements where excessive
temperature rise could occur or where sustainability concerns war-
rant a reduction in total cementitious content.
These remarks apply to what are now described as first-generation super-
plasticisers. Nowadays, melamine formaldehyde has virtually disappeared
from the market. The situation has now become much more complicated in
that there are second- and third-generation HRWRs that retain their action
over a considerable period of time (in some cases more than 2 hours).
The original materials derived their effectiveness not so much from a new
property as from an absence of two old properties. They can be used at much
higher dose rates than normal water reducers because they do not either
retard set or entrain air. As an example of this, it was required to produce a
highly fluid mortar with a very low water to cement ration (w/c) to surround
and protect a steel tension pile (or ground anchor). High strength was really
only essential at the rock anchorage over 30 m below ground level. A super-
plasticiser was considered, but it was realised that a normal water reducer at
the same dosage would produce a similar water reduction at lower cost. It
was an advantage that a very long retardation resulted (because the mortar
was placed first and the pile was lowered into it). The high air percentage
was reduced to a very modest amount by the fluid pressure at the full depth.
There is now an enormous variety of HRWRs available, from a dozen or
more different countries. The original materials have been supplemented or
replaced by others, including lignosulphonates formulated to entrain reduced
amounts or air and produce less retardation. Their cost, relative to the cost
of labor, is reducing. The value of very high-strength concrete is becoming
more widely realised. Perhaps more important still, it is being realised that
these materials are not only labor-content reducers but also skill-requirement
reducers. For all these reasons, the use of superplasticisers is on the increase.
The new kid on the block is polycarboxylate ether (PCE). These admix-
tures are particularly favored for use in self-compacting concrete (SCC),
having longer workability retention with less set retardation and apparently
giving some bleeding resistance. A problem with this type of admixture
is that it tends to entrain more air, which is countered by the inclusion
of a defoaming agent (i.e., air entrainment suppressor). However, some