Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 82

increase the size of concrete panels and to reduce or eliminate cracking in
heavily restrained concrete. Reducing autogenous shrinkage using SRA is
very helpful in higher strength concrete with low w/cm. The problem with
autogenous shrinkage is that it is a through section strain and usually com-
pounded by thermal shrinkage.
By changing surface tension, SRAs change other properties of the concrete.
Bentz et al. (2001) found that the desorptivity was decreased and resultant
drying profile changed from uniform drying to a sharp front. Aldred (2008)
found that SRA reduced sorptivity, desorptivity, and wick action.
Shrinkage can also be reduced by shrinkage compensators such as finely
divided iron or calcium sulphoaluminate. These materials work but require
careful use to avoid the expansion tendency being disruptive. Also it must
be remembered that they do not actually work by reducing shrinkage. In
both cases an expansion is produced while the concrete is kept damp (i.e.,
before any shrinkage occurs) and the concrete then shrinks normally. The
initial expansive tendency is restrained by reinforcement or by abutting
concrete and develops a compression that dies away under the later influ-
ence of shrinkage. In addition to the risk of excessive expansion causing
disruption, there can also be a threshold effect in which the expansive ten-
dency is inadequate and the precompression is all lost in creep of the con-
crete, leaving no effect on subsequent shrinkage.
In the United States shrinkage compensating cements are available and
even expanding cements designed to automatically apply prestress to cast-
in steel tendons. This is done by the incorporation of calcium sulphoalumi-
nate in the cement during manufacture.
4.3.8 Viscosity modifying agents (VMAs)
Viscosity modifying agents (VMAs) include
• Wax emulsions
• Thickening agents (methyl cellulose, polyethylene oxide)
• Fly ash
• Silica fume
Wax emulsions and thickening agents do improve pumpability, but the
improvement is not dramatic and the expense and difficulty may be appre-
ciable. Fly ash is very useful if available. Silica fume at low replacement
levels of 2% to 3% is very effective.
It has been said that the only satisfactory test for pumpability is to pump
the concrete. However a most effective cheap and simple test is bleeding.
It is probably true that concrete that bleeds excessively will not pump, but
the reverse is not necessarily the case. Too high a workability can be as
harmful as too low a workability if the mix has inadequate cohesion. It
can be seen that the aforementioned admixtures are all in effect bleeding