concrete masonry unit

A concrete masonry unit, or CMU, is a hollow or solid rectangular block made from portland cement, aggregates, and water, cast to a uniform dimension and used as the primary building element in load-bearing and non-load-bearing walls. The most common CMU in North America measures 7.625 inches wide by 7.625 inches high by 15.625 inches long, designed to work with standard 3/8-inch mortar joints to produce an 8 by 8 by 16-inch nominal dimension. This modular sizing coordinates with other building materials and simplifies layout and calculation on job sites.

CMUs are manufactured in dozens of variants tailored to different structural and aesthetic demands. Hollow-core units, the standard type, contain two or three vertical voids that reduce weight and material cost while maintaining strength when reinforced and grouted. Fully solid units appear less frequently and cost more but provide higher compressive strength for heavily loaded applications. Split-face, slump, fluted, and colored units serve architectural purposes, allowing masons to create visual texture and pattern without applied finishes. Density varies considerably: standard-weight units run 40 to 50 pounds and use gravel aggregate; lightweight units, often made with expanded shale or coal slag, weigh 25 to 35 pounds and offer better thermal performance.

The compressive strength of a CMU typically ranges from 1,500 to 3,500 pounds per square inch, depending on mix design and curing. Load-bearing applications require units meeting ASTM C90 criteria; non-load-bearing units, made to a looser standard, are cheaper and still useful for interior partitions or infill in steel-frame buildings. Moisture absorption is critical: a CMU that absorbs more than 15 percent of its dry weight in water becomes prone to efflorescence, salt deposits that bloom on the finished wall surface, and freeze-thaw damage in cold climates.

Installation and Common Problems

Masons lay CMUs in running bond, stacked bond, or other patterns, bedding each unit in a 3/8-inch to 1/2-inch layer of mortar. Vertical cells in load-bearing walls must be filled with concrete grout as construction proceeds. Inadequate grouting, insufficient mortar coverage on face shells, and poor workmanship create weak joints and water infiltration; water trapped inside the wall cavity causes efflorescence, mold, and structural deterioration. Grade beams, lintels, and pilasters are cast or built into CMU walls to handle concentrated loads.

CMUs dominate commercial and industrial construction because they are faster to install than poured concrete, cheaper than brick, and can be built directly by semi-skilled labor under supervision. The cavity created by hollow cores allows utilities, electrical conduit, and plumbing to run vertically within the wall thickness. In seismic zones, CMU walls must be reinforced with rebar and grout to meet code; unreinforced CMU construction is restricted or forbidden in areas of high seismic risk. The term masonry unit encompasses brick, stone, and tile as well, but in trade speech CMU almost always means concrete block.