Concrete Masonry Unit - Sizes and Structure

Sizes and Structure

Concrete blocks may be produced with hollow centres 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, with an R-Value of 1.11 the most common nominal size is 16×8×8 in (410×200×200 mm); the actual size is usually about 3⁄₈ in (9.5 mm) smaller to allow for mortar joints. In Ireland and the UK, blocks are usually 440×215×100 mm (17×8.5×3.9 in) excluding mortar joints. In New Zealand, blocks are usually 390×190×190 mm (15×7.5×7.5 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. There may be two, three or four cores, although two cores are the most common configuration. The presence of a core allows steel reinforcing to be inserted into the assembly, greatly increasing its strength. Reinforced cores are filled with grout to secure the reinforcing in proper relationship to the structure, and to bond the block and reinforcing. The reinforcing 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 notches 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 (e.g., an 8" x 16" block may be scored in the middle to simulate 8" x 8" masonry), with the grooves filled with mortar and struck to match the true joints.