A working sealant has to stretch and compress along its whole width. If it also sticks to the joint bottom, the back edge cannot move and stress concentrates there, so the sealant tears cohesively or pulls off the sides within a few movement cycles. It happens when backer rod or bond breaker is left out, or when gaps at the rod ends let sealant reach the joint bottom. Inspectors find it by cutting a sample out of the joint.
Cut a section out and it's stuck to the concrete at the back too, three-sided adhesion, no wonder it split.
A sealant bead with a flat back rather than a curved one is a giveaway that it was gunned onto the joint bottom without rod.
Also heard as
- 3-sided adhesion
- three-point adhesion
Related terms
Backer rod
A round foam cord pushed into a joint to set sealant depth and keep the sealant from bonding to the back.
Bond breaker
A tape or material at the back of a joint that sealant will not stick to, preventing three-sided adhesion.
Cohesive failure
A sealant tearing apart within itself while staying stuck to both joint faces.
Hourglass profile
The ideal cross-section of a sealant joint: thick at both bond faces and thinner in the middle.