15 March 2001
BACKER AND SEPARATING MATERIALS
7.1. Description. Backer material is placed in the sawed crack to minimize excess stress on the
sealant material from improper shape factors and to prevent three-sided adhesion that would
inhibit the ability of the sealant to expand and compress under thermal stress. Typically, backer
materials are rod-shaped and are often referred to as "backer rod" (Figure 7.1.). The backer
material must be chemically inert to prevent reaction with the sealant, flexible to conform to the
shape of the crack path, nonabsorptive to prevent water retention, nonshrinkable, and
compressible to allow for easy installation. Typical backer materials are polychloroprene,
polystyrene, polyurethane, and polyethylene closed-cell forms. Paper, rope, or cord should not
be used. The melting temperature of the backer material must be at least 25 degrees Fahrenheit
(14 degrees Celsius) higher than the sealant application temperature to prevent damage during
sealant placement. The uncompressed backer rod must have a diameter at least 25 percent
larger than the sealant reservoir to ensure that it remains in position during the sealing operation.
Backer rod is the recommended material for repairing cracks.
7.2. Separating Materials. Separating tape may be employed when the sealant reservoir
dimensions correspond to that for the proper shape factor and the use of backer material would
lead to an incorrect shape factor for that sealant material. Separating materials are usually a thin
adhesive tape or a flexible plastic strip employed to prevent three-sided adhesion of the sealant.
These materials must be flexible enough to deform with the sealant as the concrete expands and
contracts. However, this repair method should only be used when the crack has been sawed to
provide a reservoir of the proper depth.