Masonry Magazine February 1996 Page. 47
Fig. 9. Examples of Control Joints:
(a) Michigan type of control joint, (b) use of tongue-and-groove units, and (c) use of premolded joint insert with a sash block.
Control joints are used in concrete masonry construction to control possible cracking resulting from shrinkage. Control joints consist of a continuous, vertical weakened section built into the wall. In addition to accommodating longitudinal movement resulting from shrinkage, a control joint must usually provide lateral stability to the wall. Special units, accessories, or fabrication procedures may be used to accomplish this. Perhaps the most common examples are the Michigan type of control joint, the use of tongue-and-groove units, and the use of a premolded rubber insert Fig. 9). Regardless the type of control joint, it must be installed such that it permits ready longitudinal movement away from the joint. For example, horizontal joint reinforcement must be terminated at the control joint for the control joint to function properly.
Expansion joints are used in clay masonry construction provide a continuous void space in the wall plane that will accommodate movement resulting from expansion of the masonry. It is important that this expansion joint be free of any non-compressible material such as mortar, grout, wood, or other construction materials. A variety of compressible expansion joint fillers are used to help assure that he expansion joint is not filled with mortar or other non-compressible materials (Fig. 10). The mason must properly install these materials and assure that the expansion joint can function as intended.
Tooling mortar joints. Tooling compacts the mortar to a dense, neat surface with good durability (Fig. 11). Tooling o a concave or a V shape provides optimum resistance to water penetration. These shapes are recommended because they do not allow water to rest on the joint, and they esult in the mortar being pressed toward both the lower and the upper masonry unit. This reduces weathering and elps ensure maximum bond between the mortar and the masonry units.
Fig. 10. Compressible expansion joint fillers are used to assure that the expansion joint is not filled with mortar.
Timing is important to achieving a uniform appearance in the finished mortar joint surface. The mortar should be allowed to stiffen until it is hard enough that a thumbprint just barely shows. Tooling too soon will bring more paste to the surface, resulting in a lighter appearance. Mortar smears are more likely to occur when tooling joints early. In addition, the high water content and fines contained in this surface layer of paste may result in visible shrinkage cracks. Tooling mortar joints after they have hardened too much results in an uneven dark appearance, sometimes termed "tool burning." Tooling at the proper time optimizes appearance and performance. The initial rate of absorption of units, ambient temperature, humidity, wind, and whether or not the area is exposed to direct sunlight all affect the rate at which mortar stiffens in the wall. Therefore, a mason must evaluate whether or not a joint is ready to tool by observation and touch rather then by setting a specific elapsed time from placement until tooling.