Backer rod use in joint sealing?

• A. Provides a compressible support at the bottom of the joint to control sealant depth and allow movement.
• B. Bonds the sealant to the concrete surface.
• C. Serves as a permanent filler to seal the joint.
• D. Acts as an adhesive to stick forms.

Answer: (A)

Backer rod provides a compressible support at the bottom of the joint so the sealant depth is properly controlled and the sealant can accommodate movement without tearing or excessive adhesion to the bottom. By acting as a backstop, it ensures the sealant bonds mainly to the sides of the joint and forms the correct bead profile, which is essential for a durable, flexible seal. It also reduces the amount of sealant needed and prevents sag during application. It is not used to bond the sealant to concrete, nor is it a permanent filler, nor an adhesive for forms, since its purpose is the spacer and movement allowance, not adhesion or permanent filling.