Page 275 - Construction Waterproofing Handbook

5.34 CHAPTER FIVE
Precast concrete panels
Precast panels, including tilt-up and prestressed ones, are now produced in a variety of
sizes, textures, and finishes. These have become a common building facing material for all
types of structures. Problems arise not with the panels themselves but with sealers, finishes,
or coatings applied to them.
Form-release agents are used in all precast panel fabrications. Since panel edges typi-
cally become sides of joints after erection, problems with adhesion occur. Oil- and petroleum-
based products used for curing the panels will cause deterioration of silicone and polysulfide
sealants. Film-forming curing and release agents can act as bond breakers between sealant
and concrete.
Substrate adhesion testing often tests a sealant’s ability to adhere to the form-release or cur-
ing agents rather than to a panel itself. Therefore, all precast joints should first be abraded or
chemically cleaned to remove all residue of these compounds before sealant application.
Often precast panels are designed with exposed aggregate finishes. Although aestheti-
cally pleasing, exposed aggregate often prevents proper joint sealing.
When panels are manufactured with aggregate turned or exposed onto panel sides (that
later become joint sides), proper sealing is impossible (Fig. 5.31). Sealants will not adhere
properly to exposed aggregate, and the aggregate will prohibit proper movement charac-
teristics of the sealant.
Figure 5.32 shows a typical improperly manufactured panel. If someone attempts to
chip the stone out at a project site, pockets are created that must be patched with a cemen-
titious grout. If such a repair is at tempted, grout repairs can actually be pulled away from
precast substrates during joint movement and cycling. The only acceptable repair method
is to replace panels with precast panels, which are manufactured with no aggregate
exposed within joints.
Project specifications often require coating application to panels after installation.
These coatings include water repellents, antigraffiti coatings, color stains, and elastomeric
coatings. Coatings applied before sealants can create problems. Sealant application should
be completed first and protected during coating application; otherwise, sealants would be
bonded to coatings rather than to the panel surface.
If a panel finish is porous and water absorption rates are high, water may enter the panel
substrate and bypass sealant joints. Water causes bonding problems if faces of joints
remain wet or if open-cell backing materials become saturated after sealant installation.
This wet backing causes sealants to reemulsify.
Wet substrates also cause the release of primers used for sealant installations.
Absorptive panels require sealing with a water repellent after sealant installation, to pre-
vent these problems.
Panels should be erected and securely attached to prevent slippage, bowing, or creeping,
which causes shearing and ripping of sealants. Panels must be installed so joints are uniform
from top to bottom, to prevent joints that are too narrow or too wide for proper sealing. In
these situations an applicator may not bother to change the backing material size when the
joint width changes, causing performance problems with sealants after installation.
Joints should also be kept uniform from one to the next. For instance, panels meant to
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have 2-in joints should not be installed with one joint 1 in wide and an adjacent joint
virtually closed. Such variances will considerably shorten the life-cycling of sealants.