Page 129 - Construction Waterproofing Handbook

3.18 CHAPTER THREE
a chemical bond with the substrate. The advantage of siloxanes over silanes is that their
chemical structure does not promote a high evaporation rate.
The percentage of siloxane solids used is substantially less (usually less than 10 percent
for vertical applications), thereby reducing costs. Chemical reaction time is achieved faster
with siloxanes, which eliminates a need for wetting after installation. Repellency is usually
achieved within 5 hours with a siloxane.
Siloxane formulations are now available that form silicone resins without the catalyst—
alkalinity—required. Chemical reactions with siloxanes take place even with a neutral sub-
strate as long as moisture, in the form of humidity, is present.
These materials are suitable for application to damp masonry surfaces without the
masonry turning white, which might occur with other materials. Testing of all substrates
should be completed before full application, to ensure compatibility and effectiveness of
the sealer.
Siloxanes do not change the porosity or permeability characteristics of a substrate. This
allows moisture to escape without damaging building materials or the repellent. Since
siloxanes are not subject to high evaporation rates, they can be applied successfully by
high-pressure sprays for increased labor productivity.
Siloxanes, as other silicone-based products, may not be used with certain natural stones
such as limestone. They also are not applicable to gypsum products or plaster. Siloxanes
should not be applied over painted surfaces, and if surfaces are to be painted after treat-
ment they should first be tested for compatibility. (See Table 3.9.)

Silicone rubber
These systems are a hybrid of the basic silicone film-forming and the silicone derivatives
penetrating sealers. The product is basically a silicone solid dissolved in a solvent carrier
that penetrates into the substrate, carrying the solids to form a solid film that is integral
with the substrate. Unlike the penetrating derivatives, silicone rubbers do not react with the
substrate to form the repellency capability.
The percentage solids, as high as 100 percent, carried into the substrate supposedly cre-
ate a thickness of product millage internally in the substrate to a film thick enough to
bridge minute hairline cracking in the substrate. This elongation factor, expressed as high
as 400 percent by some manufacturers, does not produce substantial capacity to bridge
cracks, since the millage of the film that creates movement capability is minimal with clear
repellents. Only existing cracks less than 32 in are within the capability of these materials
1
to seal, and new cracks that develop will not be bridged since the material is integral with
the substrate and cannot move as film-forming membranes are allowed to do.
Through chemical formulations and the fact that they penetrate into the substrate, the
silicone rubber products have been UV-retardant, unlike basic silicone film-forming seal-
ers. At the same time they retain sufficient permeability ratings to permit applications to
typical clear repellent substrates. These systems are also applicable to wood, canvas, and
terra cotta substrates that other penetrating sealers are not applicable, since the rubber sys-
tems do not have to react with the substrate to form their repellency.
Silicone rubber systems are applicable in both horizontal and vertical installations and
make excellent sealers for civil project sealing including bridges, overpasses, and parking
garages. Like the generic silicone compounds, silicone rubber does not permit any other