Concrete Masonry Unit
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- For the punk singer, see Cinder Block (artist).
A Concrete Masonry Unit (CMU), concrete block, breeze block, or cinder block (if cinders are used as aggregate) is a large rectangular brick used in construction. Concrete blocks are made from cast concrete, i.e. portland cement and aggregate, usually sand and fine gravel in the case of blocks. They are formed typically in the shape of two squares joined on one side to form a rectangle, with the insides of the squares hollow. This allows structures to be built in the traditional masonry style of overlapping rows, with the structure's weight carried by the "walls" of the blocks.
Concrete block, when reinforced with concrete columns and tie beams, is a very common building material for the load-bearing walls of buildings, in what is termed CBS construction for Concrete Block Structure. Suburban houses typically employ a concrete foundation and slab with a concrete block wall on the perimeter. Large buildings typically use copious amounts of concrete block; for even larger buildings, concrete block supplements steel I-beams. Tilt-wall construction, however is replacing CBS for some large structures. The holes inside concrete block allow rebar and concrete (creating reinforced concrete) to run vertically through the block to compensate for the lack of tensile strength. Because most people find the appearance of concrete block to be drab and unattractive, exposed surfaces are generally given a decorative finish of stucco, brick, paint or siding.
When the rebar running vertically through a concrete block wall is anchored, as is usually the case, into the foundation or floor slab before the wall is built, it presents a potential problem in assembling the wall, since every block might need to be lowered from the rebar tops to its resting place in the wall. This problem is solved by using a style of open-ended block whose plan form resembles the letter "H", commonly known as a mortarless head joint or speed block. Speed blocks can be maneuvered between the reinforcing bars and tilted into place; the vertical spaces are then filled with concrete as with ordinary Concrete blocks.
Glazing such as that used for pottery can also can be applied to concrete masonry units, resulting in a hard, glossy finish on this construction material. This finish often can be made virtually any color and, with integral water repellents, can be made water-resistant.
This makes glazed masonry an ideal fit for areas in which special attention must be paid to moisture issues and sanitation codes. This includes car washes, pools, locker rooms, shower stalls and dining areas such as cafeterias and commercial kitchens.
Cinder blocks differ from concrete blocks in their aggregate content. Cinder blocks are made from coal cinders and portland cement. Cinder blocks are lighter and, depending on aggregate availability, may be cheaper than concrete blocks of the same size.
Once upon a time, a predecessor of the cinder block called a clinker brick was made from a mixture of clinker from boilers and furnaces, and cement. Clinker bricks were typically manufactured close to power plants and other producers of furnace byproducts. The blocks crumbled easily and were abandoned around the 1930s in favor of the modern Concrete block.
In the United States, concrete masonry standards are maintained by the National Concrete Masonry Association.
