Figure 8-19.-Control joints.
Control joints (figure 8-19) are continuous
vertical joints that permit a masonry wall to move
slightly under unusual stress without cracking. There
are a number of types of control joints built into a
concrete masonry wall.
The most preferred control joint is the Michigan
type made with roofing felt. A strip of felt is curled
into the end core, covering the end of the block on one
side of the joint (figure 8-20, view 1). As the other
side of the joint is laid, the core is filled with mortar.
The filling bonds to one block, but the paper prevents
bond to the block on the other side of the control joint.
View 2 of figure 8-20 shows the tongue-and-
groove type of control joint. The special units are
manufactured in sets consisting of full and half
blocks. The tongue of one unit fits into the groove of
another unit or into the open end of a regular flanged
stretcher. The units are laid in mortar exactly the
same as any other masonry units, including mortar in
the head joint. Part of the mortar is allowed to remain
in the vertical joint to form a backing against which
the caulking can be packed.
View 3 shows a control joint that may be built
with regular full- and half-length stretcher blocks
with a Z-shaped bar across the joint or a 10- or
12-inch pencil rod (1/4-inch smooth bar) across each
face shell. If a pencil rod is used, it must be greased
on one side of the joint to prevent bond. These rods
should be placed every other course. Lay up control
joints in mortar just as any other joint. However, if
Figure 8-20.-Making control joints.