CMUs

CMUs, or concrete masonry units, are rectangular blocks used in building walls and other structures. They are manufactured from cementitious materials, typically Portland cement, aggregate, and water, and may be hollow or solid. Hollow CMUs have cores that reduce weight, provide space for vertical reinforcement, and improve insulation; solid blocks offer higher strength and sound reduction. Mortar joints hold CMUs together, and openings for doors and windows are supported with lintels and reinforced elements as required by design.

Sizes and formats vary by region. In the United States, the common nominal size is 8 by 8 by 16 inches, with actual dimensions usually around 7 5/8 by 7 5/8 by 15 5/8 inches. Other shapes include solid and hollow forms in different thicknesses, including lighter-weight varieties designed for improved thermal performance.

Use and construction: CMUs can be employed in load-bearing or non-load-bearing walls. Walls may be built as single wythe units or as multiwythe assemblies with interior reinforcement and grout-filled cores. Mortar joints are typically about 3/8 inch thick, and reinforcement bars (rebar) may be placed through the cores according to structural design.

Standards and performance: Concrete masonry units are manufactured to regional standards, with ASTM C90 serving as a primary specification in the United States for load-bearing units, along with related standards covering testing, mortar, and grout requirements. Regional variations exist, and high-strength blocks are available for specialized applications.

Other uses: CMU is also the acronym for Carnegie Mellon University, a private research university in Pittsburgh, Pennsylvania. While widely recognized in higher education, in construction contexts CMU almost always refers to concrete masonry units.