Page 151 - Handbook of Adhesives and Sealants

120 Chapter Three

A parabolic stress deformation of a sealant also occurs when the
sealant is in compression. Figure 3.25 shows that a compressed seal-
ant will extrude outside the joint area under load. This may become
subject to abrasion and provide a safety hazard as well as an unsightly
joint. It is advisable in horizontal joints that are subject to traffic to
1
keep the level of the sealant a distance ⁄8 in. below the substrate level.
An improvement in the rectangular cross section can be achieved by
using a curved back-up material in the joint and tooling the top sur-
face as shown in Fig. 3.26 (top). The tooling must be done with care,
because if the sealant is too thin at its center it will buckle under
compression as shown in Fig. 3.26 (bottom).
Elastic sealants should be bonded to only two sides of the joint in
order to perform properly. The bottom surface of the sealant must be

Figure 3.25 Sealant under compression. 14

Figure 3.26 Tooled sealant showing (top) uniform substrate stress
and (bottom) under compression if the sealant is too thin. 14