Page 249 - Construction Waterproofing Handbook

5.8 CHAPTER FIVE
TABLE 5.1 Coefficients of Thermal Expansion for
Common Envelope Materials
Material Coefficient of thermal expansion, in/in/°F
Aluminum 0.000013
Concrete 0.000008 to 0.000005
EIFS 0.0000075
Granite 0.000005
Limestone 0.000005
Marble 0.000007
Masonry 0.000004 to 0.000003
Plate glass 0.000005
Structural steel 0.000007
Wood 0.000002 to 0.000003

Backing systems provide three critical
functions for sealant joints:
1. Control the depth of sealant application
2. Prevent three-sided adhesion
3. Form the hourglass shape necessary for
proper joint design

It is imperative that sealant materials be
allowed to expand to their maximum capa-
bility without exerting unnecessary stress at
the adhered substrate area. Thick beads of
sealant are more difficult to elongate (a
FIGURE 5.7 Backing detailing in sealant joints. thick rubber band is harder to stretch than a
thin band), which places more stress on the
sealant. If this stress exceeds a sealant’s
bonding capability, adhesive failure will
occur. Stress that exceeds a sealant’s cohe-
sive capability results in sealant tear failure.
To prevent failure, a backing material is
inserted into joints to provide a large
adhered contact area, at only two sides of
the joint, with a thin bead of sealant. This is
shown in Fig. 5.7. This backing material, or
backer rod (Fig. 5.8) as it is commonly
referred to, is of major importance in joint
design and installation. Besides ensuring
proper joint design, the backer rod allows
applicators to monitor proper depth of
material installation. Figure 5.9 shows
FIGURE 5.8 Backer Rod material is supplied in
rolls of various sizes. (Courtesy of Nomaco) failed material due to excessive thickness of