A structural expansion joint exists precisely because that part of the building moves, and treating it with a rigid material guarantees the waterproofing will crack in a short time.
Why the joint moves
Expansion joints separate parts of the structure that expand and contract differently because of temperature, differential ground settlement or vibration. That movement, even if only a few millimetres, is constant and repeated over the years.
Elastic sealant, not rigid mortar
On an expansion joint you can't apply a conventional mortar or rigid waterproofing: a specific elastic sealant is needed (polyurethane, structural silicone or prefabricated elastic bands) able to stretch and recover its shape with each movement cycle without losing watertightness.
How to spot a badly treated joint
A straight, regular crack that always reappears at the same point, however much it is repaired with conventional mortar, usually indicates an expansion joint treated with an unsuitable material. Identifying the origin properly before quoting avoids repeating a repair doomed to fail again.
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