Masonry Magazine February 1976 Page. 11
NCMA-TEK
An Information series from National Concrete Masonry Association
Sizes and Shapes of
Concrete Masonry Units
Sizes
After modular standards were introduced by ANSI in 1946, the concrete masonry industry adapted production to these standards. Today, all plants can furnish units normally used in modern construction of a nominal size that will fit the 4 inch module. The nominal dimensions of these standard units provide that with the 3/8 inch mortar joint, wall lengths and heights of modular standards can be obtained. The illustration of Fig. 1 shows the difference between actual and nominal dimensions using an 8" x 8" x 16" unit as an example. These are, of course, special units such as screen block, ornamental block, etc., to which these references do not apply.
Table 1 summarizes the types, grades, and intended usages for several load bearing units as specified in the ASTM Standards. The ASTM Specifications permit a maximum variation in overall dimensions (length, width and height) of plus or minus 1/8" from the actual dimensions listed by the manufacturer. It is the usual practice, however, to manufacture the units to a tolerance of plus or minus 1/16". For hollow units, ASTM Specification C-90 contains minimum thickness requirements for face shells and webs. Table 2 lists these minimum thickness requirements as related to the width, or thickness of the unit. Although these thicknesses may vary slightly from plant to plant the minimum values can be used to estimate properties for preliminary design. Table 3 shows calculated values for properties of hollow units having minimum permitted face shell and web thickness. Additionally, Table 4 presents information on percent solid volume and equivalent thickness for typical 2 and 3 core hollow units in the most popular block widths.
| GRADE | USE |
| ----------- | ----------- |
| N-I, N-II | For general use such as in exterior walls below and above grade that may or may not be exposed to moisture penetration or the weather and for interior walls and back-up. |
| 5-1, 5-11 | For use above grade in exterior walls with weather-protective coatings and in walls not exposed to the weather. |
Table 1 Types and Grades of Concrete Masonry Units for Various Uses