What causes expansion joint failure?

Expansion joints fail due to the degradation of the sealant material (from UV or age), 'compression set' (where the sealant loses its ability to bounce back), or 'three-sided adhesion' (where sealant bonds to the bottom of the joint, tearing it apart when the joint moves). Failure allows water and debris to enter, causing damage to the joint edges (arris) and leaking into the structure below. Repair involves removing the old sealant and installing a new system with a bond breaker.