A Control/Expansion Joint is a groove which is formed, sawed, or tooled in concrete or masonry to regulate the location and amount of cracking and separation resulting from the dimensional change of different parts of the structure with expansion and contraction.
A break in a plaster coat will also prevent cracks from spreading. Similarly, expansion joints should be provided to prevent large expanses of floor tiling or timber flooring from buckling or cracking. The joints should be filled with flexible sealant. Control joints should not be filled with non flexible material such as grout or mortar.
BSA TECHNICAL INFORMATION FLOOR TILES.