Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 78

4.3.6 Water-resisting admixtures
One important limitation of conventional concrete, even of good qual-
ity, is the presence of microcracks, capillaries, and microcapillaries into
which water is able to penetrate, sucked in by surface tension or driven by
an external hydrostatic pressure. Where concrete is in contact with damp
soil or is below the water table, water is drawn through the concrete into
the structure. This can lead to unacceptable dampness as well as damage
to carpets, furnishing, and equipment. Water transmission under such
conditions can also result in the dangerous accumulation of aggressive
salts in the concrete leading to corrosion of the reinforcing steel and dete-
rioration of the concrete itself. Similar problems occur in concrete exposed
to periodic wetting with water containing salt or other aggressive agents
such as in the splash zone or bridge decks. Therefore, control of water
(salt) movement is often vital to achieve the required performance and
durability.
In the past designers attempted to isolate the concrete from water by the
use of membranes or surface coatings. However, it is extremely difficult,
if not impossible, to ensure there are no weak points or faults through
which water along with any dissolved salts or acids can penetrate leading to
leakage, dampness, and possibly corrosion. As a result of these problems,
attention is now focused on water-resisting admixtures to control water
and moisture movement as well as improving concrete durability.
Unfortunately, the information available on this class of admixture
has been full of generalisations based on little, if any, controlled data.
A Building Research Advisory Board report (1958) said that in the opin-
ion of the majority of 61 observers, “dampproofing admixtures are not
… effective or acceptable in controlling moisture migration through
slabs-on-ground”. On the other hand, Robery (1987), based on work on
a hydrophobic pore-blocking ingredient (HPI), takes a more positive view:
“Hydrophobic additives will produce the most marked improvement in
average quality concrete subjected to low hydrostatic pressures (<10 m
head). The waterproofed concrete thus competes directly with conventional
tanking and roof membrane systems”. Rixom and Mailvaganam (1986) in
their summary of concrete hydrophobic admixtures also support this more
positive view. Comparative testing has shown significant variation in short-
and long-term performance of hydrophobic admixtures, which has led to
the conflicting views on this class of admixtures in the literature.
ACI 212.3R-10 (Table 4.1) refers to permeability reducing admixtures
(PRAs) and subdivides them into nonhydrostatic (PRAN) and hydrostatic
(PRAH). As permeability is defined as water flow due to a hydraulic gra-
dient, water permeability necessarily involves the hydraulic pressure.
Accordingly, the attempt to create a subcategory of permeability reducing
admixtures for nonhydrostatic conditions is technically invalid.