Concrete masonry unit

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A pallet of "8-inch" CMUs
An interior wall of painted CMUs
Concrete masonry blocks
A building constructed with concrete masonry blocks

A concrete masonry unit (CMU, also known as a cinder block, breeze block or by various other terms) is a standard-size rectangular block used in building construction. The use of blockwork allows structures to be built in the traditional masonry style with layers (or courses) of staggered blocks.

Concrete blocks may be produced with hollow centers (cores) to reduce weight, improve insulation and provide an interconnected void into which concrete can be poured to solidify the entire wall after it is built.

CMUs are some of the most versatile building products available because of the wide variety of appearances that can be achieved using them.[1]


Those that use cinders (fly ash or bottom ash) as an aggregate material are called cinder blocks in the United States, breeze blocks (breeze is a synonym of ash)[2] in the United Kingdom, and hollow blocks in the Philippines. In Australia, they are known as Besser blocks or Besser bricks, because the Besser Company was a major supplier of machines that made concrete blocks.[citation needed] Clinker blocks use clinker as aggregate.


Concrete blocks are made from cast concrete (e.g. Portland cement and aggregate, usually sand and fine gravel, for high-density blocks). In the western United States, where they are easily obtainable, porous lava rock gravels are used for weight reduction. They also have an added decorative effect to certain types of block, such as split-face, due to their distinct red and black colors. Lower density blocks may use industrial wastes, such as fly ash or bottom ash,[3][4] as an aggregate.[5]

Recycled materials, such as post-consumer glass, slag cement, or recycled aggregate, are often used in the composition of the blocks.[6] 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; these are widely used for construction in Finland and other Scandinavian countries, as well as in central Europe, for the material's inherent thermal isolation characteristics, as are expanded clay aggregate blocks.

Sizes and structure[edit]

Blocks come in modular sizes, with the most popular typically referred to (by their thickness) as "4-inch", "6-inch", "8-inch", and "12-inch". In the US, CMU blocks are nominally 16 in (410 mm) long and 8 in (200 mm) wide. Their actual dimensions are 38 inch (9.5 mm) less than the nominal dimensions (to allow for 38-inch mortar joints between blocks in any orientation).[7] 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 Australia, 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 their top surface (as laid) has a greater surface on which to spread a mortar bed and for easier handling. Most CMUs 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.[8] Cores may also be filled with expanded-polystyrene (EPS) block foam insulation, substantially increasing the R-value of the resulting wall to be in compliance with the US national energy code.[9]

A variety of specialized shapes exist to allow special construction features. U-shaped blocks, or knockout blocks, have 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.[10]

CMUs 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.[11] 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.[12]


Concrete block, when built with integral steel reinforcement, is a very common building material for the load-bearing walls of buildings, in what is termed concrete block structure (CBS) construction. One of the common foundation types for American suburban houses is the "crawl space foundation" which consists of a concrete block wall around the perimeter on which dimensional lumber floor joists are supported. Retaining walls, which also can be constructed of concrete blocks, either using blocks designed to be set back each course and used with a sand base and without mortar or reinforcing (gravity wall), or using blocks (typically an architectural style of block or clad with a veneer such as brick) with a concrete base, steel reinforcing and mortar (piling wall). Other very common, non-structural uses for concrete block walls (especially in American schools) are as interior fire-rated and extremely durable partition walls, and as exterior backup curtain walls for attachment of building envelope systems (rigid foam insulation and an air/vapor barrier) and veneers (stucco, steel, brick, or split-face concrete block).


Concrete masonry walls may be ungrouted, partially grouted, or fully grouted, the latter two enhancing their structural strength. Additionally, steel reinforcement bars (rebar) 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 allow only fully grouted walls in their building codes. The American design 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 (TMS 402/ACI 530/ASCE 5).[citation needed]

The compressive strength of concrete masonry units and masonry walls varies from approximately 3.4 to 34.5 MPa (500–5,000 psi) based on the type of concrete used to manufacture the unit, stacking orientation, the type of mortar used to build the wall, it depends on whether it is a load-bearing partition or not and other factors. [13][14][15][16]

See also[edit]


  1. ^ "Concrete Masonry". Retrieved 31 January 2023.
  2. ^ "breeze, n.3". breeze. The Oxford English Dictionary (2nd ed.). Oxford University Press. 1989. Archived from the original on May 24, 2011. Retrieved 2007-11-30.
  3. ^ "A new invention by two female engineers could be the solution to Gaza's rebuilding problems - The Independent". 28 February 2018. Archived from the original on 28 February 2018. Retrieved 20 October 2018.
  4. ^ "Rebuilding Gaza: 'Green Cake' could be the phoenix rising from Gaza's ashes - Middle East Eye". 17 April 2017. Archived from the original on 17 April 2017. Retrieved 20 October 2018.
  5. ^ "Block Making Machine aggregate example". Web Editor.
  6. ^ "Masonry & LEED v4: Continuing Education from Echelon Masonry". Retrieved 2019-02-12.
  7. ^ "Concrete Block (CMU) Sizes, Shapes, and Finishes".
  8. ^ Beall 1987, pp. 61–63.
  9. ^ "EPS Insulation | Geofoam, Styrofoam, EPS & Polystyrene | Universal Foam Products". Retrieved 2020-10-26.
  10. ^ Beall 1987, pp. 66–68.
  11. ^ Beall 1987, pp. 68–70.
  12. ^ Ching, Francis D.K. (2012). A Visual Dictionary of Architecture (2nd ed.). John Wiley & Sons, Inc. p. 168. ISBN 978-0-470-64885-8.
  13. ^ "ASTM Specifications for concrete masonry units".
  14. ^ "Compressive Strength of Masonry" (PDF). Portland Cement Organization. Retrieved June 5, 2016.
  15. ^ "Comprehensive Strength of Hollow Concrete Blockwork" (PDF). Government of Canada. 4 June 1986. Archived from the original (PDF) on 2012-09-16.
  16. ^ "Properties of Concrete Blocks — Strength".


  • Beall, Christine (1987). Masonry Design and Detailing for Architects, Engineers and Builders (2nd ed.). New York: McGraw-Hill. ISBN 0-07-004223-3.

External links[edit]