A concretemasonry unit is a standard size rectangular block used in building construction. CMUs are some of the most versatile building products available because of the wide variety of appearances that can be achieved using them. Those that use cinders as an aggregate material are called cinder blocks in the United States, breeze blocks in the United Kingdom, and hollow blocks in the Philippines. In New Zealand and Canada they are known as concrete blocks. In New Zealand, they are also called construction blocks. In Australia they are also called Besser blocks and Besser bricks, because the Besser Company was a major supplier of machines that made concrete blocks. 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.
Composition
Concrete blocks are made from cast concrete. Lower density blocks may use industrial wastes, such as fly ash or bottom ash, as an aggregate. Recycled materials, such as post-consumer glass, slag cement, or recycled aggregate, are often used in the composition of the blocks. Use of recycled materials within blocks can create different appearances in the block, such as a terrazzo finish, and may help the finished structure earn LEED certification. Lightweight blocks can also be produced using autoclaved aerated concrete.
Sizes and structure
The use of blockwork allows structures to be built in the traditional masonry style with layers of staggered blocks. Concrete blocks may be produced with hollow centers to reduce weight or improve insulation. Blocks come in modular sizes, with the most popular typically referred to as "4-inch", "6-inch", "8-inch", and "12-inch". In the US, CMU blocks are nominally long and wide. Their actual dimensions are less than the nominal dimensions. In Ireland and the UK, blocks are usually excluding mortar joints. In Australia, New Zealand and Canada, blocks are usually excluding mortar joints. Block cores are typically tapered so that their top surface 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 to span courses in order to increase tensile strength. This is accomplished by grouting the voids of blocks containing rebar with concrete. Thus reinforced, CMU walls are better able 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 block may be scored in the middle to simulate masonry, with the grooves filled with mortar and struck to match the true joints.
Concrete masonry walls may be ungrouted, partially grouted, or fully grouted, the latter two enhancing their structural strength. Additionally, steel reinforcement bars can be used both vertically and horizontally inside a CMU wall to maximize its structural performance. The cells in which the rebar is placed must be grouted for the bars to bond to the wall. For this reason, high seismic zones typically only allow fully grouted walls in their building codes. The Americandesign code that guides design engineers in using CMU as a structural system is the Masonry Standards Joint Committee's Building Code Requirements & Specification for Masonry Structures. The compressive strength of concrete masonry units and masonry walls varies from approximately to based on the type of concrete used to manufacture the unit, stacking orientation, the type of mortar used to build the wall, and other factors.
