Expansion joint waterproofing of concrete Expansion joints are installed to manage the coefficient of thermal expansion over large spans of concrete. The waterproofing material must not only cope with the movement of the building, but also be designed to resist aging that leads to the weakening of the bond on the concrete and causes failure of the system. On vertical applications: apply approved sealant (typically polyurethane for concrete-to-concrete joints) in the opening with an approved backer rod to control the depth of the sealant. Apply the waterproofing manufacturers approved reinforced flashing material over the opening and up the vertical substrate. Expansion joints are put in place before the concrete is poured. Expansion joints are used to allow the slab to move and not put stress on whatever it abuts. These joint are placed where a slab meets a building, where a slab meets another slab, and where a pool deck meets the coping.