Building Design and Construction

Concrete Masonry Units

A wide variety of manufactured products are produced from concrete and used in building construction. These include such items as concrete brick, concrete block or tile, concrete floor and roof slabs, precast wall panels, precast beams, and cast stone. These items are made both from normal dense concrete mixes and from mixes with lightweight aggregates. Concrete blocks are made with holes through them to reduce their weight and to enable masons to grip them.
Nominal size (actual dimensions plus width of mortar joint) of hollow concrete block usually is 8 x 8 x 16 in. Solid blocks often are available with nominal size  of 4 x 8 x 16 in or 4 x 21⁄2 c 8 in. For a list of modular sizes, see ‘‘Standard Sizes of Clay and Concrete Modular Units,’’ ANSI A62.3.

Properties of the units vary tremendously—from strong, dense, load-bearing units used under exposed conditions to light, relatively weak, insulating units used for roof and fire-resistant construction.
Many types of concrete units have not been covered by adequate standard specifications.
For these units, reliance must be placed upon the manufacturer’s specifications.
Requirements for strength and absorption of concrete brick and block established by ASTM for Type I, Grades N-I and S-I (moisture-controlled), and Type II, Grades N-II and S-II (non-moisture-controlled), units are summarized in Table 4.7.
Manufactured concrete units have the advantage (or sometimes disadvantage) that curing is under the control of the manufacturer. Many methods of curing are used, from simply stacking the units in a more or less exposed location to curing under high-pressure steam. The latter method appears to have considerable merit in reducing ultimate shrinkage of the block. Shrinkage may be as small as 1⁄4 to 3⁄8 in per 100 ft for concrete units cured with high-pressure steam. These values are about one-half as great as those obtained with normal atmospheric curing. Tests for moisture movement in blocks cured with high-pressure and high-temperature steam indicate expansions of from 1⁄4 to 1⁄2 in per 100 ft after saturation of previously dried specimens.

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