BLOCK SIZES AND SHAPES
Concrete masonry units are available in many
sizes and shapes to tit different construction needs.
Both full- and half-length sizes are shown in figure
8-4. Because concrete block sizes usually refer to
nominal dimensions, a unit actually measuring
7 5/8-by-7 5/8-by-15 5/8-inches is called an
8-by-8-by-16-inch block. When laid with 3/8-inch
mortar joints, the unit should occupy a space exactly
ASTM (American Society for Testing and
Materials) specifications define a solid concrete block
as having a core area not more than 25 percent of the
gross cross-sectional area. Most concrete bricks are
solid and sometimes have a recessed surface like the
frogged brick shown in figure 8-4. In contrast, a
hollow concrete block has a core area greater than 25
percent of its gross cross-sectional area-generally 40
percent to 50 percent.
Blocks are considered heavyweight or
lightweight, depending on the aggregate used in their
production. A hollow load-bearing concrete block
8-by-8-by-16-inches nominal size weighs from 40 to
50 pounds when made with heavyweight aggregate,
such as sand, gravel, crushed stone, or air-cooled slag.
The same size block weighs only 25 to 35 pounds
when made with coal cinders, expanded shale, clay,
slag, volcanic cinders, or pumice.
The choice of
blocks depends on both the availability and
requirements of the intended structure.
Blocks may be cut with a chisel. However, it is
more convenient and accurate to use a power-driven
masonry saw (figure 8-5). Be sure to follow the
manufacturers manual for operation and
As with all electrically powered
equipment, follow all safety guidelines.
BLOCK MORTAR JOINTS
The sides and the recessed ends of a concrete
block are called the shell. The material that forms the
partitions between the cores is called the web. Each
of the long sides of a block is called a face shell. Each
of the recessed ends is called an end shell. The
vertical ends of the face shells, on either side of the
end shells, are called the edges.
Bed joints on first courses and bed joints in column
construction are mortared by spreading a 1-inch layer of
mortar. This procedure is referred to as full mortar
For most other bed joints, only the upper
edges of the face shells need to be mortared. This is
referred to as face shell mortar bedding.
Head joints may be mortared by buttering both
edges of the block being laid or by buttering one edge
on the block being laid and the opposite edge on the
block already in place.
Properly mixed and applied mortar is necessary
for good workmanship and good masonry service
because it must bond the masonry units into a strong,
well-knit structure. The mortar that bonds concrete
block, brick, or clay tile will be the weakest part of the
masonry unless you mix and apply it properly. When
masonry leaks, it is usually through the joints. Both
the strength of masonry and its resistance to rain
penetration depend largely on the strength of the bond
between the masonry unit and the mortar. Various
factors affect bond strength, including the type and
quantity of the mortar, its plasticity and workability,
its water retentivity, the surface texture of the mortar
bed, and the quality of workmanship in laying the
units. You can correct irregular brick dimensions and
shape with a good mortar joint.
Workability of Mortar
Mortar must be plastic enough to work with a
trowel. You obtain good plasticity and workability by
Figure 8-5.-Masonry saw.