Page 274 - Construction Waterproofing Handbook

SEALANTS 5.33
Following are descriptions of substrates commonly found in construction and their
requirements for proper sealant installation.

Aluminum substrates
A common building component, aluminum substrates present difficulties when choosing
sealants. This is due to the architectural coatings now being applied to aluminum to pre-
vent aluminum from oxidizing and to provide color.
By themselves, aluminum surfaces must be cleaned chemically or mechanically to
remove any trace of oxidation that will prevent sealant adhesion. With coated aluminum,
it is more difficult to choose a sealant.
Baked-on finishes and other coatings contain oils, carbon, and graphite residues, which
act as release agents for sealants. Some coatings themselves may have poor adhesion to
aluminum, thereby making it impossible to achieve proper adhesion with any sealant
application. Other coatings may soften or deteriorate when solvents in sealants or primers
come into contact with the finish.
The only positive method to test adhesion with a coated aluminum is actual testing
before application. Silicones and butyls have acceptable adhesion to aluminum and are
often used in aluminum curtain wall construction. But even these materials should be tested
for proper adhesion. Elastomeric sealants such as urethanes are often used around aluminum
frame perimeters, and as such should be checked for adhesion especially when coated
aluminum products are used.

Cement asbestos panels
Cement asbestos panels are produced with a variety of finishes, including exposed aggre-
gate and tile. The panels are attached to a wood or metal stud frame for support and attach-
ment to a structure.
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Often panels without finishes are less than 2 in thick. This composition prevents
proper width-to-depth ratios for backing and sealant installation. Cement asbestos panels
present difficult problems for sealant applications because of their high moisture absorption
properties and thinness of the panel itself. After sealant installation, any water absorbed
into a panel can bypass sealant joints and cause damage to interior areas and panel support
systems.
Cement asbestos panels have thermal coefficients similar to concrete panels, with
movement at joints often exceeding movement capability of sealants. Compounded with
absorption rates of panels, long-term performance of any sealant is questionable. In addi-
tion form-release agents or sealers used on panels often contaminate joints, prohibiting
proper adhesion of sealants. Some panels are manufactured with aggregate exposed in
joint sides, which also prevents proper sealing.
These factors all contribute to problems in sealing and keeping panels watertight. To
prevent such problems, adequate connections must be incorporated into panel design.
Accelerated weathering testing of a panel design with wind and structural loading should
be completed to verify the effectiveness of proposed sealant systems. Panel design should
include details that make joints acceptable for sealant installation. These include joint sides
at least 1 in thick, panel edges clean of any form-release agents or sealers, and aggregate
not exposed on joint sides.