Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 70

but all the concrete covered by the reports contained a superplasticiser.
There may be a temptation to think that the use of silica fume, or high
strength, is the outstanding characteristic of high-performance concrete but
probably its most basic and essential feature is the use of a superplasticiser
or high range water reducer (HRWR).
Admixture technology is both extensive and virtually a foreign language
to many in the concrete industry and related professions. It is easy to pro-
vide more detail than can reasonably be absorbed and retained by non-
specialists. This chapter is therefore aimed at providing general guidance
rather than at providing detailed knowledge. What is new is that the situa-
tion has now become so complex that even the technical representatives of
major admixture suppliers do not have all the answers.
It is important to realise both the complexity of the situation and the
inaccuracies inherent in any attempt to compare the relative value of
different admixtures. Different admixtures can have significantly different
relative benefit when used with different cementitious materials or other
different conditions. A particular brand name of admixture may be differ-
ently formulated in different parts of the world. A difference in the time of
addition (relative to that of the cement first coming into contact with the
water) can substantially affect the performance of an admixture. Different
results may be obtained from the same mix and admixtures when mixed in
a laboratory mixer or in a truck.
The basic cost of most admixture raw materials is relatively low compared
to the selling price of the admixture. This is at least partly due to the very
considerable costs of research and development, quality control, technical
service, and marketing. However, with the possible exception of very large
concrete producers with good facilities and very knowledgeable staff, the
availability of technical assistance from an admixture supplier may be good
value for the money.
If one admixture enables the saving of 5 kg of cement per cubic metre of
concrete more than another, this may save several hundred tons of cement
per annum. However, the strength difference at the same cement content
would only be of the order of 1 MPa and this may be within the margin of
error of the trial mixes used. There has been a worrying trend to use much
higher cementitious contents than necessary and not to use admixtures to
reduce cementitious contents. Specifications that require minimum cementi-
tious contents are part of the reason but ignorance or laziness by premix
suppliers is also a component. Working toward the optimum minimum
cementitious content should be the aim of everyone in the industry in this
era where sustainability is becoming more important.
On the whole it is probably of greater importance to select the correct
type of admixture and to use it in the most advantageous way than to
obtain the most cost-effective admixture. It is therefore again emphasised