Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 81

and capillarity and water immersion), they did not consistently reduce water
penetration in the pressure tests (namely, DIN 1048 and vacuum soaking)
(Aldred, 1989). These would be considered nonhydrostatic water resisting
admixtures. Roy et al. (1995) also found significant variability in short-
term penetrability of the different water resisting admixtures tested. Again
the same hydrophobic pore-blocking ingredient was the only product that
consistently reduced penetrability.
Another important aspect of water resisting admixtures is long-term
performance. Neville (1981) mentions that “water-proofing admixtures …
are supposed to repel water by an electrostatic charge which they form after
reacting with calcium ions on the walls of the capillaries in the hydrated
cement paste. It is doubtful whether this effect persists over long periods”.
This is a valid concern and there are examples of hydrophobic admixtures
that have not provided acceptable long-term performance. Only water
resisting admixtures where the chemistry has been proven to achieve long-
term performance should be considered as a suitable “waterproofing” or
durability enhancing systems.
In addition to reducing penetrability, effective admixtures appear to
assist in reducing cracking by reducing interfacial surface tension (similar to
shrinkage reducing admixtures). Another benefit in real structures appears
to be limiting lateral water movement from cracks and voids, which makes
localised repair easier.
The use of fly ash, ground-granulated blast-furnace slag (GGBS), silica fume,
and other supplementary cementitious material (SCMs) reduces permeability.
These materials have been dealt with in Chapter 2. It is interesting that in
spite of the widespread use of SCMs that significantly reduce permeabil-
ity, there is a growing trend toward using water resisting admixtures. This
appears to be due to ease with which faults in the concrete can be repaired
and the preparedness of better suppliers to warranty performance. It is also
probably due to continued problems with membrane waterproofing.
4.3.7 Shrinkage reducing admixtures
Capillary tension theory is the leading theory to explain autogenous and
drying shrinkage of concrete. Shrinkage reducing admixtures (SRAs)
are typically polyoxyalkylene alkyl ether or similar (ACI 212.3R) which
reduce surface tension thereby reducing the tension that develops in the
capillaries during drying (either to the environment or due to hydration).
Bentz et al. (2001) measured the effect of the addition of a 6% solution of
a dipropylene glycol ether blend in water. The surface tension was reduced
by 57% compared to distilled water.
The reduction in autogenous and drying shrinkage is reportedly up
to 50% or more using these admixtures. Therefore this type of admix-
ture can be an important tool for designers and contractors to be able to