Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 72

4.2 POSSIBLE REASONS FOR USING AN ADMIXTURE
Reasons to use an admixture are as follows:
1. To save money and reduce peak temperature by reducing cementitious
content for a given strength and workability
2. To improve concrete properties, including:
a. Reduction of bleeding or segregation
b. Compensation for aggregate grading deficiencies
c. Reduced permeability
d. Improved pumpability
e. Reduced shrinkage
f. Improve durability
3. To compensate for weather conditions or haulage distance, for example,
retarders and accelerators
4. To reduce labor costs—Superplasticisers/HRWRs
5. To produce self-compacting concrete to facilitate placement in difficult
locations, provide good off-form finish and reduce labor
6. To facilitate the use of marginal cementitious materials or aggregates.
4.3 TYPES OF ADMIXTURES AVAILABLE
4.3.1 Water reducers
The most common water reducers are lignosulphonates, which are natural
retarders but may be modified by the addition of accelerators such as
triethanolamine (hopefully no longer calcium chloride as in the past).
A water reduction of the order of 5% to 10% is obtained and the admix-
ture is used basically to enable cement reduction. Some of the water reduction
is due to the entrainment of 1.5% to 2% of air by this type of admixture.
When an accelerator is used to reduce retardation, it can cause an increase
in shrinkage but this is offset to some extent by the water reduction. There is
some evidence that early shrinkage is less compensated than later shrinkage
and this may lead to slightly increased susceptibility to early cracking.
The time of addition of these admixtures may be important, a delayed
addition giving substantially more effect. In some cases readiness for
trowelling of slabs may be delayed even when the 24-hour compressive
strength is not reduced.
Water-reducing strength increasers containing polymers such as
hydroxycarboxylic acids and polysaccharides can be very similar to
lignosulphonates. The cement saving is of a similar order but the action is a
little different since water reduction is slightly less and there is a small direct
strength increase at a given water/cement ratio. These admixtures may be
a little more effective in cement saving than lignosulphonates (especially