Even if a control joint contains mortar, the initial moisture shrinkage opens the joint sufficiently to accommodate subsequent thermal and moisture expansion.
Concrete masonry experiences a permanent shrinkage as the latent moisture from the manufacturing process evaporates. Control joints are required for concrete masonry. Even if a control joint contains mortar, the initial moisture shrinkage opens the joint sufficiently to accommodate subsequent thermal and moisture expansion. Clay masonry, in contrast, experiences permanent moisture expansion as the units reabsorb atmospheric moisture after firing. In order to accommodate these volume increases, an expansion joint for clay masonry must be void of mortar and other non-compressible materials. Mortar that protrudes into an expansion joint keeps the joint from closing to accommodate the enlargement of adjacent wall panels. To avoid this problem, expansion joints are sometimes installed with a filler material. Any filler material that is used to keep mortar out of an open joint during construction must be of a soft sponge rubber that is at least as compressible as the sealant to be used. If a more rigid filler material is used, such as asphalt-impregnated fiber board, it must be removed or it will obstruct joint closure as the masonry expands. Asphalt-impregnated fillers also are chemically incompatible with most sealants. If a filler material is adequately soft, it can stay in place, but it must be at a depth within the joint that does not interfere with proper placement of the sealant. Sealants in a rigid control joint with mortar are installed over a bond breaker tape, while sealants in an open expansion joint are installed over a sponge backer rod. The mortar in a rigid control joint must be raked out to the proper depth while it is still plastic. Bond breaker tape prevents adhesion to the back of shallow three-sided joints, which can cause splitting failure in the sealant. Polyethylene tape is the most commonly used material, but it can be difficult to handle in cold weather. Butyl tape is a good substitute for easier handling at lower temperatures. Weather conditions at the jobsite affect both the substrate materials and the sealants themselves. Cold, damp surfaces inhibit adhesion, and moisture that is trapped in a joint can cause sealant failure. For most sealants, it is sufficient for the substrate to be dry to the touch. Urethanes, though, are more sensitive to moist surfaces than other sealants; so it is best to allow at least a full day's drying time after rain before applying multicomponent urethanes and two to three days drying time before applying single-component urethanes.