Concrete Blocks – Key Masonry Units


Concrete is an amalgamation of Portland cement, aggregate and water. A Concrete Block, sometimes called a concrete masonry unit (CMU), is one of several precast concrete products, primarily used as a building material in the construction of walls. The term precast refers to the fact that the blocks are formed and hardened before they are brought to the job site. Often the aggregate used in the manufacture of concrete block is fly ash, or bottom ash. Both fly ash and bottom ash is the residue, or cinders, resulting from burning coal. Hence, some concrete blocks are known as cinder blocks. Most concrete blocks have one or more hollow cavities, and their sides may be cast smooth or with a design. In use, concrete blocks are stacked one at a time and held together with fresh concrete mortar to form the desired length and height of the wall.

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Characteristics in a brief :

  • Made from a mixture of Portland cement, blended cement, various types of aggregates, and water. Lightweight aggregates were either natural materials, by-products or manufactured. Natural aggregate materials included pumice. By-products aggregate materials included cinders and slag. Manufactured aggregate materials included expanded shale, clay and slate.
  • If granulated coal or volcanic cinders are used instead of sand and gravel, the resulting block is commonly called a cinder block. This produces a dark grey block with a medium-to-coarse surface texture, good strength, good sound-deadening properties, and a higher thermal insulating value than a concrete block. A typical cinder block weighs 26-33 lb (11.8-15.0 kg).
  • Lightweight concrete blocks are made by replacing the sand and gravel with expanded clay, shale, or slate. Expanded clay, shale, and slate are produced by crushing the raw materials and heating them to about 2000°F (1093°C) at which the material bloats, or puffs up, because of the rapid generation of gases caused by the combustion of small quantities of organic material trapped inside. A typical light-weight block weighs 22-28 lb (10.0-12.7 kg). Expanded blast furnace slag, as well as natural volcanic materials such as pumice and scoria, are also used to make lightweight blocks.
  • To ensure uniform building construction the most common standardized block sizes are 6 X 8 X 16 inches (15.24 X 20.32 X 40.64 cm) and 8 X 8 X 16 inches (20.32 X 20.32 X 40.64 cm). These blocks weigh about 40 to 45 pounds (18.14 kg to 20.41 kg).
  • The earliest known example of a house built in this country entirely of concrete block was in 1837 on Staten Island, New York. The first hollow concrete block was designed in 1890 by Harmon S. Palmer in the United States.
  • Advantages – Inexpensive, lightweight, durable, easy to install, fireproof, low maintenance, and could be ornamented. Advantages of using cinder blocks included its strength, ability to receive nails and ease of installation.
  • Face plates were used to create a variety of surface finishes, including cobble stone, brick, ashlar and rock-face (the most common type); more decorative finishes included designs of scrolls, wreaths and roping.
  • They may be solid or hollow with two or three cores for such stretcher blocks; various other types of standard shapes are also often available and one should consult the local market to determine availability.
  • Block ends may be flat or flanged.
  • Compressive strength and fire resistance of the each block is dependent upon the block’s configuration.
  • Typical historical uses for concrete block include – Foundation walls, Basement walls, Partition walls, Exterior walls, Back-up material for cavity wall construction.

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