Page 90 - Design of Reinforced Masonry Structures

2.42 CHAPTER TWO

2.9 MORTAR JOINTS

Mortar joints play an important role in the bond patterns and texture of a wall. Concrete
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masonry units are placed with a gap of ⁄ 8 in. between the adjacent units, both horizontally
and vertically, in order to provide for the mortar joints between them. Thus, the mortar
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joints appear as ⁄ 8 in. wide stripes on the face of a masonry wall. However, these joints
are not made solid through the wall thickness, that is, they are not continuous through the
thickness (width) of a hollow concrete masonry wall. Rather, only the vertical head joints
and horizontal bed joints are required to receive mortar; mortar is not placed on interior
cross-webs or across the full width of head joints. Mortar joints are only as thick (measured
perpendicular to the wall thickness) as the thickness of the face shell of a hollow concrete
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masonry unit. The face shell of a typical 8 in. (nominal) concrete masonry unit is 1 ⁄ 4 in.
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thick; therefore, mortar joints are 1 ⁄ 4 in. thick (measured perpendicular to wall) by ⁄ 8 in.
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wide (measured on the face of the wall) on only the head and bed joints of the concrete
masonry units. Thus, when the heads of two closed-end units are mortared together at a
vertical joint, it results in a ⁄ 8 in. wide air space between the adjacent masonry units that
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remains even if the wall is solid grouted. Of course, if necessary or preferred, a wall can also
be built without any built-in air voids. This can be accomplished by using open-end units.
When the masonry surface is not to be plastered, all mortar joints of the masonry are
finished as specified. In some cases, the joints are accentuated by deep tooling. Basically,
three kinds of joint finishes are used in masonry construction, with some variations in each:
flush joints, tooled joints, and raked joints. Joint finish is given with the help of appropriate
finishing tools.
Various kinds of masonry joints are shown in Fig. 2.41. Flush joints are prepared by
removing or cutting off the mortar flush with the face of the wall with a trowel. Tooled
joints can be concave or V-shaped, and are prepared by striking the joints with a metal joint-
ing tool. Racked joints are recessed joints prepared by racking out the mortar at a distance
of ⁄ 8 in. from the face and then tooled with a square jointer or the blade of a tuck pointer
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(Fig. 2.42). Another form of joint is the squeezed joint. This type of joint is obtained simply
by using an excess of mortar while laying the units. During this process, some of the mortar
is squeezed out as the units are set and pressed into place; this squeezed out mortar is not
trimmed off but left in place.

Concave joint Raked & tooled joint V joint

Weather joint Flush joint Squeezed joint
FIGURE 2.41 Various kinds of joint finishes for masonry walls. (Courtesy: NCMA.)