Concrete expansion joint failing — Zaandam, North Holland

Expansion joints are the flexible seams cut into driveways, sidewalks, and patios that let concrete slabs expand and contract with temperature changes without cracking. When the filler material dries out, crumbles, or pulls away, water seeps into the gap, erodes the sub-base, and accelerates slab movement and cracking. A concrete mason cleans out the old filler, applies a backer rod for proper depth control, fills the joint with a self-leveling polyurethane sealant, and tools the surface flush to restore long-term flexibility.