Page 253 - Construction Waterproofing Handbook

5.12 CHAPTER FIVE

FIGURE 5.13 Outgassing of closed cell rod can cause failure of sealant material. (Courtesy of Nomaco)

DUAL-CELL BACKER ROD

Dual-cell rod was developed to eliminate the problems of both open- (moisture entrap-
ment) and closed-cell rod materials (gassing). It is manufactured from extruded polyolefin
foam that contains a combination of both open- and closed-cell structures (bi-cellular).
The dual-cell materials prohibit the absorption of too much moisture that can damage the
sealant, but in sufficient quantities to facilitate the proper air or moisture curing of the
sealant. At the same time, the dual-cell structure does not permit out-gassing when ruptured.
The dual cell is ideal for both vertical and horizontal joints and all types of sealant mate-
rials. It assists in making the sealant application on a project with various types of materi-
als and joint design “idiot-proof” by being applicable in all situations. This eliminates the
possibility of a mechanic choosing a closed-cell rod when an open-cell is required.

BACKING TAPE

Many construction joints are constructed that do not permit sufficient size or shape for the
use of round backer rod materials (Fig. 5.14). In these instances, a backing tape is used.
The tape must act as a bond breaker, not permitting the adherence of the sealant material
to the tape itself as well as the backside of the joints.
Backing tape is a specially produced tape that does not facilitate adherence to sealants. It
is not similar to masking tape or duct tape, which are not acceptable products to use as back-
ing tape. Backing tape is typically of polyethylene composition, 12–20 mil thick and pro-
vided in several widths, although cutting of the tape to the exact width of the joint is required.
It is extremely important that the tape not be so narrow that the sealant is allowed to
adhere on the backside of the joint (Fig. 5.15). Tape should also not be so wide that it turns
up joint sides, preventing proper adhesion (Fig. 5.16). A proper fit can be accomplished by
cutting installed tape that is slightly larger than the joint’s width along the joint edges with
a razor knife and removing excess tape.

JOINT DETAILING

The relatively insignificant cost of sealants compared to the overall cost of a specific build-
ing project should facilitate the use of superior joint detailing to eliminate the common