Civil Engineering Reference
In-Depth Information
trol joints) should be constructed to permit transfer of
loads perpendicular to the plane of a slab or wall. If no
contraction joints are used, or if they are too widely
spaced in slabs on ground or in lightly reinforced walls,
random cracks may occur; cracks are most likely when
drying and thermal shrinkage produce tensile stresses in
excess of the concrete's tensile strength.
Contraction joints in slabs on ground can be made in
several ways. One of the most common methods is to saw
a continuous straight slot in the top of the slab (Fig. 11-28).
This creates a plane of weakness in which a crack will
form. Vertical loads are transmitted across a contraction
joint by aggregate interlock between the opposite faces of
the crack providing the crack is not too wide and the
spacing between joints is not too great. Crack widths at
saw-cut contraction joints that exceed 0.9 mm (0.035 in.) do
not reliably transfer loads. The effectiveness of load
transfer by aggregate interlock depends on more than
crack width. Other factors include: slab thickness, sub-
grade support, load magnitude, repetitions of load, and
aggregate angularity. Steel dowels (Figs. 11-6 and 11-29b)
may be used to increase load transfer at contraction joints
when heavy wheel loads are anticipated. Sizes and spacing
of dowels, which are placed at the center of the slab depth,
are shown in
Farny (2001)
. See
ACI Committee 302
and
PCA (1982)
for further discussions on doweled joints.
Sawing must be coordinated with the setting time of
the concrete. It should be started as soon as the concrete
has hardened sufficiently to prevent aggregates from
being dislodged by the saw (usually within 4 to 12 hours
after the concrete hardens); sawing should be completed
before drying shrinkage stresses become large enough to
produce cracking. The timing depends on factors such as
mix proportions, ambient conditions, and type and hard-
ness of aggregates. New dry-cut sawing techniques allow
saw cutting to take place shortly after final finishing is
completed. Generally, the slab should be cut before the
concrete cools, when the concrete sets enough to prevent
raveling or tearing while saw cutting, and before drying-
shrinkage cracks start to develop.
Contraction joints also can be formed in the fresh con-
crete with hand groovers or by placing strips of wood,
metal, or preformed joint material at the joint locations.
The top of the strips should be flush with the concrete sur-
face. Contraction joints, whether sawed, grooved, or pre-
formed, should extend into the slab to a depth of at least
one-fourth
the slab thickness or a minimum of 25 mm (1
in.) deep. It is recommended that the joint depth not
exceed one-third the slab thickness if load transfer from
aggregate interlock is important.
Contraction joints in walls are also planes of weakness
that permit differential movements in the plane of the
wall. The thickness of the wall at a contraction joint should
be reduced by 25%, preferably 30%. Under the guidance of
the design engineer, in lightly reinforced walls, half of the
horizontal steel rebars should be cut at the joint. Care must
be taken to cut alternate bars precisely at the joint. At the
corners of openings in walls where contraction joints are
located, extra diagonal or vertical and horizontal rein-
forcement should be provided to control cracking.
Contraction joints in walls should be spaced not more
than about 6 meters (20 ft) apart. In addition, contraction
joints should be placed where abrupt changes in wall
thickness or height occur, and near corners—if possible,
within 3 to 4 meters (10 to 15 ft). Depending on the struc-
ture, these joints may need to be caulked to prevent the
passage of water through the wall. Instead of caulking, a
waterstop (or both) can be used to prevent water from
leaking through the crack that occurs in the joint.
The spacing of contraction joints in floors on ground
depends on (1) slab thickness, (2) shrinkage potential of
the concrete, (3) subgrade friction, (4) environment, and
(5) the absence or presence of steel reinforcement. Unless
reliable data indicate that more widely spaced joints are
feasible, the suggested intervals given in Table 11-2 should
be used for well-proportioned concrete with aggregates
having normal shrinkage characteristics. Joint spacing
should be decreased for concrete suspected of having high
shrinkage characteristics. The panels created by contrac-
tion joints should be approximately square. Panels with
excessive length-to-width ratio (more than 1
1
⁄
2
to 1) are
likely to crack at an intermediate location. In joint layout
design it is also important to remember that contraction
(control) joints should only terminate at a free edge or at
an isolation joint. Contraction joints should never termi-
nate at another contraction joint as cracking will be
induced from the end of the terminated joint into the adja-
cent panel. This is sometimes referred to as sympathetic
cracking. Refer to Fig. 11-31, which illustrates one possible
joint layout solution to eliminate the potential for induced
sympathetic cracking.
Fig. 11-28. Sawing a continuous cut in the top of a slab is
one of the most economical methods for making a
contraction joint. (69947)
