Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

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Forms for Architectural Concrete 401
FIGURE 14-2 Coil tie with loose (spreader) cones for architectural concrete.
(Source: Dayton Superior Corporation)
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Construction Joints
All concrete structures, except possibly small and simple types, require
construction joints, which serve several purposes. Concrete undergoes
shrinkage as it sets and cures, which usually induces tension stresses.
If these stresses are excessive, and if no provision is made for relieving
them, it is probable that cracks will occur. Such cracks are unsightly,
and they cannot be patched satisfactorily. Cracks are especially objectionable
if they appear in the surfaces of architectural concrete.
Because forms for architectural concrete frequently contain molds
and other ornament-producing features, excessive stresses or movements
in the concrete resulting from shrinkage may damage or
destroy some of the ornaments. These stresses and movements can be
minimized by locating construction joints reasonably close together.
For example, if concrete heads above windows are placed in one
operation, it is probable that the shrinkage will cause cracks to appear
at their tops. For this reason, it is good practice to provide horizontal
construction joints directly under the window heads.
Another reason for using construction joints is that concrete for
most structures is placed in stages, rather than continuously. Therefore,
it is desirable to provide construction joints to control the appearance
at each stage of concrete placement. When an architect or an
engineer designs a concrete structure, he or she usually designates
the locations of construction joints, giving consideration to the effects
on the appearance of the structure and also to the quantity of concrete
required between construction joints.
Because the surface appearance of architectural concrete is of significant
importance, the location of construction joints should be
selected carefully. Frequently, joints may be located so that they are
inconspicuous. Horizontal joints may be located along the sills or
heads of windows, or they may coincide with recesses in the concrete
surface extending around the structure. Vertical construction joints
may be located along the edges of pilasters or other intersections of
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