What is the purpose of bond beams in concrete masonry walls?
Bond beams are courses of block constructed with special units designed to receive horizontal reinforcement and grout. These units are used to integrate the horizontal reinforcement with vertical reinforcement bars in a reinforced masonry wall. Bond beams often are placed at regular intervals in the wall to permit placement of more reinforcement than would be possible using bed joint reinforcement. Bond beams can be used in masonry bearing walls--to serve as horizontal members along the top of the walls, tying the walls together. They can be used below a line of bar joists, so that joist anchors can be embedded into the grout of the bond beam. Bond beams often are used as lintels over doors and windows. They sometimes are located at the bottom of walls that span over an opening to make a deep beam. Bond beam reinforcement also can be used for crack control. In this application, there must be a break between bond beams at the control joints in a wall. The area of steel required for bond beams used for crack control should be greater than that required for joint reinforcement. This is because the yield strength of the bars often is slightly less than that of the joint reinforcement and because the walls will undergo greater shrinkage due to the wetting effect of the grout during construction. Moisture from the grout causes the concrete masonry units to expand during construction. The CMUs will shrink as this moisture dissipates over time.