Concrete masonry units, or CMUs, form the building blocks of many constructions. They are a less expensive alternative than poured concrete, are available in a variety of fashions, and a few manufacturers offer a selection of colors other than gray. The face, or even exposed front facet of the concrete cube, may be smooth or it might have a layout element, like texture. Two common uses for CMUs are foundations and retaining walls.
The American Society for Testing and Materials, or ATSM, designates CMU types. ATSM C 90 blocks are acceptable for load-bearing wall installation, while ATSM C 129 is for non-load-bearing walls. The standard size CMU is an 8-by-8-by-16-inch cube. Half blocks can be found in all fashions so that you do not have to cut them. Some blocks are solid, while others have open places within their center and ATSM sets minimum thicknesses for almost any CMU with an opening in its middle. ATSM also establishes that the water resistance of almost any lock and designates a few blocks as waterproof, which means that they are acceptable for outdoor installations. Blocks also receive passion retardation ratings from ATSM.
Split-faced CMUs give a wall the overall look of natural stone. The production process moulds two blocks together, then splits them apart. The splitting procedure leaves a rough surface and also exposes the rock aggregate used in the concrete mix. Blocks are available with the rough surface on either side or on one side only. Some manufacturers let you personalize the colour of the aggregate used in the blocks. The rough surfaces are perfect to use on walls in which you do not want graffiti because the surface is difficult to paint.
Scored CMUs give the illusion of using more blocks than you really use. You can align the shallow vertical marks cut into the face of the blocks in a number of patterns. Standard vertically scored blocks have two, three, five or seven vertical lines etched into their faces.
Ribbed CMUs are created by molding the concrete in fluted molds. The moulds create vertical ribs along the face of the cube. Blocks have four, six or eight ribs. The ribs might have rounded edges or they may be squared. You must align the ribs during installation. It is difficult to get rid of excess mortar that’s caught from the ribs, so you must take particular care to maintain the faces clean when setting ribbed CMUs.