Category Construction Glossary
A concrete masonry unit (CMU) , concrete block, cement block or foundation block 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 for high-density blocks. Lower density blocks may use industrial wastes as an aggregate. Those that use cinders
(fly ash or bottom ash) are called cinder blocks in the US. Clinker blocks use clinker as aggregate. Concrete blocks that
do not contain cinders are often mistakenly called cinder or breeze blocks in everyday speech. Lightweight blocks can
also be produced using aerated concrete.
Concrete blocks may be produced with hollow centres to reduce weight or improve insulation. The use of block work
allows structures to be built in the traditional masonry style with layers (or courses) of overlapping blocks. Blocks come in
many sizes. In the US, the most common size is 8 in × 8 in × 16 in.. The actual size is usually about 3/8 in smaller to allow
for mortar joints.
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. US 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 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
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
In the United States, concrete masonry standards are maintained by the National Concrete Masonry Association