Since CMUs and mortar have high compression strength but low tension strength, manufacturers have determined many steel products to help achieve stronger and more serviceable walls. Rebar can be added in masonry walls, with cores grouted solid, to substantially increase the load carrying capacity of a wall. Generally the Structural Engineer designates the size of the rebar and the spacing. Since CMU cores are usually on 8" centers, the common spacing is 8", 16", 24", 32" or 48" on center.
There are also specialty CMU products that allow rebar to be run horizontally. Bond beam are designed for this purpose, as are open ended CMUs. When using these products, grout bonds the rebar to the CMUs making an integral wall. An important item for the Construction Supervisor to watch concerns the grout used in this reinforced masonry. Many times the Structural Engineer specifies the grout to be a concrete grout, say 3,000 psi, with pea size aggregate. It's easiest for the Masonry Contractor just to use the same mortar he's using to lay the wall. If no one pays attention, a 1,200 psi mortar may be installed instead of the specified 3,000 psi concrete grout. Obviously, this becomes a difficult problem to go back and fix.
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