Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 76

than calcium chloride. Sodium silicate and aluminate as well as sodium or
potassium carbonates are powerful set accelerators but reduce strength at
later ages. Triethanolamine and salicylic acid are only mild accelerators and
are not used alone.
Hot mixing water or steam curing can also be used to accelerate set
and strength gain. Hot water is in fact often a quite suitable choice as an
accelerator, especially in cold climates. A major project involving thousands
of very large precast segments for an elevated roadway again demonstrated
this. Faced with a requirement to attain 18 MPa in 7 hours, only 2 weeks
were available to solve the problem. It took only a theoretical analysis and
two sets of four trial mixes each to convince the client that hot mixing water
was a more economical solution than steam curing, chemical accelerators,
or extra cement. The point is, given the very short curing period, that hot
mixing water takes immediate effect, whereas steam curing has to be grad-
ually applied. Of course, a superplasticiser was also used and Day's early
age system (see Chapter 7) was an integral part of the solution. Insulation
also plays an important role when using hot water for acceleration.
Superplasticisers are very useful for high early strengths, because they
enable low water/cement ratios, which not only increase eventual strength but
also increase the proportion of that strength developed at earlier ages. Also
they give a strong dispersing effect, which makes more effective use of high
cement contents. Some producers, particularly in tropical climates, find that
using a superplasticiser is an economical substitute for steam curing precast
units. Of course, such a substitution provides a very large strength margin at
later ages. The continued development of superplasticisers coupled with hot
water and insulation will probably completely replace traditional accelerators.
An important recent development is a patented product called x-seed. This
contains particles of calcium silicate hydrate (CSH), which act as nucleation sites
for further CSH formation. The result is reduced dormant phase and strength
increase similar to steam curing. The resultant increase in strength appears to
be without the detrimental effect of traditional accelerators on shrinkage.
4.3.5 Air entrainers
It is of interest that most concrete of up to 30 MPa (4,500 psi) in Australia
contains entrained air but the practice appears unusual in Southeast Asia
and Europe. Worldwide, one of the principal benefits of air entrainment
is greatly enhanced resistance to damage by freezing and thawing, but in
Australia, as in Southeast Asia, this is not a problem.
The other reasons for using air entrainment are
1. Reduced bleeding
2. Improved cohesion
3. Grading rectification