Buildings move. Sometimes movements come from temperature related expansion and contraction. Or perhaps structural loads from wind, snow, or vibration get movement started. Of course, foundation settlement also moves buildings. Due to the tendency for masonry to crack easily (low tension strength), movement control must be considered for proper construction of masonry walls.
One common method of dealing with masonry wall cracking is to improve the tensile strength of the wall. Bond beams utilize special cmu shapes that allow rebar to be placed horizontally and grouted full. Steel horizontal joint reinforcement also can be used at 8" to 24" on center and laid directly in the horizontal masonry joint. Different styles of horizontal joint reinforcement can be found at: http://www.dur-o-wal.com/prod/index.html.
Control joints are used to relieve masonry tensile stresses and allow movement to occur. Typically a pre-formed rubber gasket is used at vertical control joints to transfer the shear load (normally wind load) across the joint but still allowing the joint to move horizontally. Masonry control joints should be shown in Contract Documents. The following table gives guidelines for control joint spacing:
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