Page 31 - Masonry and Concrete

Understanding Concrete
30 CHAPTER TWO

2.5.1 Control Joints
Control joints are used to prevent random shrinkage cracking and
instead make the concrete crack in straight lines at predetermined
locations. Control joints can be hand-tooled into fresh concrete with a
special jointing tool, sawed into partially cured concrete with a circu-
lar saw, or formed with fixed divider strips of wood or of specially
molded fiber, cork, or sponge rubber (Figure 2-14). The depths of
tooled and saw-cut control joints are typically one-fourth the thick-
ness of the concrete. This weakened section causes cracks to occur at
the bottom of the joints where they will be inconspicuous. Divider
strips that will remain in place should be
the full thickness of a concrete slab so that
HAND they create separate panels that can
1 /2" MAX.
TOOLED
RADIUS expand and contract independently of one
JOINT
another.
Figure 2-15 shows recommended maxi-
mum control joint spacing for concrete
slabs based on concrete slump, maximum
aggregate size, and slab thickness. Using
SAWED
JOINT the maximum spacing recommendations
from the table as a guideline, it is best to
subdivide concrete into panels that are
square in shape rather than elongated.
Rectangular areas that are more than one-
and-a-half times as long as they are wide are
16d GALVANIZED
NAILS AT prone to cracking. For a 10-ft.-wide drive-
16" 0. C.
ALT. SIDES way that is 4-in. thick, has a 5-in. slump
and 1-in. maximum aggregate size, the
table recommends control joints every 10
ft., which would result in square panels.
For a 3-ft.-wide sidewalk with the same
thickness, slump, and aggregate size, how-
ever, 10-ft. spacing would create elongated
rectangular panels, so the spacing should
be much closer than the maximum table
FIXED DIVIDER STRIP recommendation. The sidewalk is less
FIGURE 2-14 likely to crack if control joints are spaced 3
Concrete control joints. ft. apart to form square panels.

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