CMU

A concrete masonry unit, or CMU, is a prefabricated hollow block made from portland cement, water, and aggregates like sand and gravel. The blocks are cast in molds, cured, and sold ready to lay. Standard CMU dimensions in North America are 8 inches wide by 8 inches tall by 16 inches long, though 4-inch, 6-inch, 10-inch, and 12-inch widths are also common. The hollow cavities reduce weight and material cost while creating space for steel reinforcement, grout, and insulation.

CMUs are the workhorse of single-wythe wall construction in commercial and industrial buildings. A mason lays them in running bond or other patterns, using mortar joints typically 3/8 inch thick. The blocks interlock mechanically; vertical steel rebar inserted into the cavities and grouted solid provides tensile strength. Partial grouting, called open-end grouting, fills only the cores where rebar sits, reducing material and weight. Solid-grouted walls, where every cavity is filled, offer greater shear strength and better fire rating.

Material grade and strength

CMUs are graded by compressive strength, typically ranging from 1,000 to 3,500 pounds per square inch. Low-density blocks using lightweight aggregates such as expanded slate or shale weigh less and offer better insulation but have lower strength. Standard-weight blocks made with normal stone aggregate run denser and stronger. The type chosen depends on structural loads, climate, and building code requirements. Moisture resistance matters in wet climates; some blocks are produced with water repellents or lower water absorption rates.

Common failure modes include moisture infiltration if mortar joints are poor, corrosion of rebar in damp cavities if grout is skipped or incomplete, and cracking under uneven settlement or thermal stress. Wide CMU walls, especially 12-inch blocks, require careful detailing of horizontal and vertical reinforcement to control shrinkage and expansion. Building codes specify maximum unsupported heights and lateral bracing depending on CMU grade and grouting pattern.

CMUs compete with brick, adobe, stone, and cast-in-place concrete. They are faster to install than masonry units laid individually, cheaper than precast panels, and more flexible than concrete frames. In seismic zones, CMU walls must be properly reinforced and anchored or they can fail catastrophically. The National Concrete Masonry Association publishes design guides and standards for layout, reinforcement, and grouting practice.