One of the most employed fillers for expansion and control joints is sealant caulking. Used for their highly elastomeric properties, silicone and polyurethane sealant products are specified most frequently. Silicone is an inorganic material with high resistance to ultraviolet (UV) light exposure. Polyurethane is an organic product well suited for use with wood substrates, and, unlike silicone, it can be painted. Latex and acrylic caulking are also paintable, but are prone to shrinking. Sealant caulking joints are often used in conjunction with compressible foam backer rod or polymer-based bond-breaker tape behind the sealant. The foam or tape does not adhere to the back of the sealant, preventing it from having a three-point connection, for when sealant adheres to more than two points, it can fail. Backer rod can be closed-cell rigid foam (allows a tighter seal but can off-gas if punctured and cause joint failure), open-cell (allows air movement and faster curing of the sealant), or hybrid (also allows a tighter seal but reduces the risk of off-gassing). As an alternative to field-applied caulking, preformed expanding high-density foam bellows are sometimes used for joints in commercial applications for their ability to bridge larger gaps and endure heavy traffic. In some historic masonry walls, copper expansion joints were employed, as with metal roofing. Typically, the joint incorporates a V-shaped piece of sheet metal with two flanges, secured behind the face masonry, which widens when the wall expands and narrows when it contracts.
Joint responsibilities: Structural integrity
Many joints play a critical role in bearing gravity loads or physically resisting lateral loads. This is especially true of most masonry assemblies, which contain mortar joints. Mortar allows individual masonry units to transfer their weight uniformly to adjacent units and facilitates chemical bonding between them, so the assembly acts as a unified whole to resist wind and seismic forces. However, in most cases, mortar alone is not sufficient to join masonry.
Historically, masonry wythes (vertical layers) were also joined through interlocking units by alternating stretchers (the long side of a brick or block set parallel to a wall) and headers (the long side set perpendicular to the wall), creating different bonding patterns (e.g. common bond, Flemish bond, English bond, etc.). In walls with large blocks, ferrous metal anchors might be installed between mortar joints to further secure the assembly. Contemporary brick masonry wall assemblies typically rely upon steel reinforcement set in the joints to bond to the mortar, such as wall-ties secured to the back-up, ladder, or truss ties set every several courses.
Contemporary mortar is a mixture of Portland cement (chemical binder), aggregate (sand), lime (inorganic calcium oxide and hydroxides), and water. Grout, sometimes confused with mortar, differs in that it typically does not contain lime, may include epoxy, and, as a more viscous material, is primarily used to fill joints between tiles, bond metal reinforcement to masonry, or fill cracks in masonry and concrete. Depending on the precise ratio of its components, mortar can vary in strength and hardness.
As a workable material, mortar allows for leveling and plumbing of load-bearing masonry units that commonly have subtle variations in size and surface texture. Enclosures with thin brick cladding can be the exception, as they are sometimes prefabricated as panels that are mechanically attached to their substrate; if the joints are filled with mortar, they generally do not serve a structural bearing, adhesive, or leveling purpose. Although mortar is relatively hard and rigid compared to some other joint materials, it is important that it is still softer than the surrounding masonry to provide flexibility to absorb movement stress. Otherwise, it can generate cracking or displacement in the masonry. Sand-swept joints may be used in lieu of mortar for masonry pavers, often set in a bituminous setting bed over grade. This is because such pavement can be more subject to movement from frost heave (the upward swelling of wet soil from freezing), and sand and bitumen will not crack like mortar, but rather help hold the masonry in place.
