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A concrete masonry unit (CMU) is a standard size rectangular block used in building construction.
Those that use cinders (fly ash or bottom ash) are called cinder blocks in the United States, breeze blocks (breeze is a synonym of ash) in the United Kingdom, and hollow blocks in the Philippines. In New Zealand and Canada they are known as concrete blocks or more commonly as just blocks. In Australia they are also known as Besser blocks or Besser bricks, because the American-based Besser Company was a major supplier of concrete-block-making machinery. Clinker blocks use clinker as aggregate. In non-technical usage, the terms cinder block and breeze block are often generalized to cover all of these varieties.
Concrete blocks may be produced with hollow centers (cores) to reduce weight or improve insulation. The use of blockwork allows structures to be built in the traditional masonry style with layers (or courses) of staggered blocks. Blocks come in many sizes. In the US, the most common nominal size is 16 in × 8 in × 8 in (410 mm × 200 mm × 200 mm); the block measures a ⅜ in shorter, allowing for mortar joints. In Ireland and the UK, blocks are usually 440 mm × 215 mm × 100 mm (17.3 in × 8.5 in × 3.9 in) excluding mortar joints. In New Zealand and Canada, blocks are usually 390 mm × 190 mm × 190 mm (15.4 in × 7.5 in × 7.5 in) excluding mortar joints.
Block cores are typically tapered so that the top surface of the block (as laid) has a greater surface on which to spread a mortar bed and for easier handling. Most CMU's have two cores, but three- and four-core units are also produced. A core also allows for the insertion of steel reinforcement, tying individual blocks together in the assembly, with the goal of greatly increased strength. To hold the reinforcement in proper position and to bond the block to the reinforcement, the cores must be filled with grout (concrete). Reinforcement is primarily used to impart greater tensile strength to the assembly, improving its ability to resist lateral forces such as wind load and seismic forces.
A variety of specialized shapes exist to allow special construction features. U-shaped blocks or knockout blocks with notches to allow the construction of bond beams or lintel assemblies, using horizontal reinforcing grouted into place in the cavity. Blocks with a channel on the end, known as "jamb blocks", allow doors to be secured to wall assemblies. Blocks with grooved ends permit the construction of control joints, allowing a filler material to be anchored between the un-mortared block ends. Other features, such as radiused corners known as "bullnoses" may be incorporated. A wide variety of decorative profiles also exist.
Concrete masonry units may be formulated with special aggregates to produce specific colors or textures for finish use. Special textures may be produced by splitting a ribbed or solid two-block unit; such factory-produced units are called "split-rib" or "split-face" blocks. Blocks may be scored by grooves the width of a mortar joint to simulate different block modules. For example, an 8-by-16-inch (200 mm × 410 mm) block may be scored in the middle to simulate 8-by-8-inch (200 mm × 200 mm) masonry, with the grooves filled with mortar and struck to match the true joints.