Page 72 - Design of Reinforced Masonry Structures

MASONRY UNITS: APPLICATIONS, TYPES, SIZES, AND CLASSIFICATION 2.25

All units have to meet a minimum compressive strength calculated on average net area as
follows:
Individual unit minimum 1700 lb/in 2
Average of 3 units minimum 1900 lb/in 2

1
Permissible variations in the dimensions are set at ± ⁄ 8 in. For special units, different stan-
dards may be applied.
2.6.2 ASTM Standards for Concrete Masonry Units
Concrete masonry units are available in various sizes of rectangular shapes, which are
manufactured according to the following ASTM Standards:
1. Load-bearing concrete masonry units (hollow load-bearing units conforming to ASTM
C90-06: Specification for Load-Bearing Concrete Masonry Units, commonly referred
to as concrete block [2.42].
2. Nonload-bearing concrete masonry units conforming to ASTM C129-05: Specification
for Concrete Brick [2.43].
3. Concrete brick conforming to ASTM C55-03: Specification for Concrete Brick [2.44].
4. Prefaced concrete and calcium silicate masonry units conforming to ASTM C744-05:
Specification for Prefaced Concrete and Calcium Silicate Masonry Units [2.45].
The focus of discussion in this book is load-bearing hollow concrete masonry units;
therefore, these are discussed in considerable detail in this section. ASTM C90 [2.42] is
the most frequently referenced standard for load-bearing concrete masonry units. A com-
pilation of various ASTM Standards applicable to concrete masonry units can be found
in Ref. [2.6].

2.6.3 Solid and Hollow Load-bearing Concrete Masonry Units

Solid masonry units are defined as those units whose net cross-sectional area is not less than
75 percent of the gross cross-sectional area; units not conforming to this requirement are
classified as hollow units. These units are made from portland cement, water and mineral
aggregates, with or without other aggregates, in two grades:

1. Grade N: For general use in walls above and below grade which may or may not be
exposed to moisture or weather.
2. Grade S: For use in above-grade exterior walls with weather-resistant protective coat-

ings, or walls not exposed to weather.
2.6.4 Relationship Between Unit Strength of Concrete Masonry Units and
Specified Compressive Strength of Concrete Masonry, ′ f m
The specified compressive strength, ′ f , of a masonry is greatly influenced by the variability
m
in mortar strength, grout strength, and their interaction in generating compressive strength.
As such, the relationship between the masonry unit strength and the strength of the masonry
assemblage ( ′ f ) varies considerably, depending on the properties of mortar and grout used
m
in the preparation of the masonry assemblage. The relationship between the compressive
strength of concrete masonry units and the type of mortar is given in Table A5 (App. A).