Expansion JointAn expansion joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials. They are commonly found between sections of slabs, bridges, and other structures.
The "assembly" can be as simple as a caulked separation between two sections of the same materials. More recently, expansion joints have been included in the design of, or added to existing, brick exterior walls for similar purposes. In concrete and concrete block construction, the term applied is "control joint," but serves similar purposes.
Throughout the year, building faces and concrete slabs will expand and contract due to the warming and cooling of our planet through the seasons. The structures would crack under the stress of thermal expansion and contraction if expansion joint gaps were not built into the structures. Even today the expansion joint gaps are often neglected during the design process, and simple caulking is used to fill these gaps to complete a project. This simple caulking cannot handle the thermal expansion due to the changing seasons, ultimately leaving a leak point in the structure. This expansion joint becomes the main source of leakages in the structure which can ruin the interiors of the building if not sealed or treated confidently.
Anybody can make an expansion joint appear watertight in cross-section. However, joints leak at changes in plane, direction and where dissimilar joint materials meet.