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b. Contraction (Weakened Plane) Joints. Contraction joints should be used to control cracking in
the pavement due to volume changes resulting from a temperature decrease or a moisture decrease
and to limit curling and warping stresses from temperature and moisture gradients in the pavement.
Contraction joints are formed in concrete by partial depth sawing or by installing sawable inserts. The
saw cut joint or formed groove provides a weakened plane which will crack through the full slab depth
during shrinkage and contraction of the concrete as it cures. Contraction joints are required in the
transverse direction and also in the longitudinal direction depending upon slab thickness and spacing of
the construction joints. See Figure 12-30 for contraction joint details.
c. Construction Joints. Construction joints are used between paving lanes or when abutting slabs
are placed at different times. Longitudinal and transverse construction joints may be required.
Transverse construction joints will be required when it is necessary to stop concrete placement for a
length of time sufficient to allow the concrete to begin to set. Longitudinal construction joints are
generally spaced 6 meters (20 feet) apart but may be multiple lane width, depending on the construction
(1) Transverse construction joints. When possible, locate all transverse construction joints at
the same location as regularly spaced transverse joints. Provide for load transfer or a thickened edge.
(2) Longitudinal construction joints. Construct longitudinal construction joints as shown in
Figure 12-32 and indicated below.
(a) Keyed joint. Keyways have been used extensively to provide load transfer along
longitudinal joints. However, there has been a substantial amount of keyway failure under heavy aircraft
loading on thinner slabs. Keyed joints may only be used on slabs 225 mm (9 in.) thick or greater.
(b) Butt joint. A butt joint may be used for longitudinal construction joints on pavements less
than 229 mm (9 in.) thick constructed with a stabilized base.
(c) Thickened-edge joint. A thickened-edge joint may be used for longitudinal construction
joints. The thickened-edge joint may be used for any pavement thickness and base type.
d. Joint Spacing. The standard slab size for pavements is 3.8 by 4.6 meters (12.5 by 15 feet).
Transverse joint spacing is 4.6 meters (15.0 feet) and longitudinal joint spacing is 3.8 meters (12.5 feet).
For slabs having a thickness greater than 300 millimeters (12 inches), joint spacing can be increased to
a maximum of 6.1 meters (20 feet). The transverse joint spacing shall not vary from the longitudinal joint
spacing by more than 25 percent. Figure 12-29 shows standard joint spacings.
e. Load Transfer Design. A properly designed joint must provide adequate load-transfer across the
joint. Load transfer efficiency is normally defined as the ratio of deflection of the unloaded side to the
deflection of the loaded side of the joint. Good load transfer will aid in preventing deterioration such as
corner breaks, transverse and longitudinal cracking, faulting, pumping, and spalling. Different amounts
of load transfer can be obtained through the use of aggregate interlock, dowel bars, keyways, a
stabilized base, or a combination of these.
(1) Aggregate interlock. Aggregate interlock can provide adequate load transfer across joints
when the pavement is originally constructed or during hot weather. However, as joint movements due to
temperature variation and load applications increase and the joint begins to open, aggregate interlock is
lost and load transfer is greatly reduced. The effectiveness of aggregate interlock may be improved by
increasing base strength and the angularity of coarse aggregate and shorter spacing of joints.