—Load-transfer mechanisms

360R-24 ACI COMMITTEE REPORT
shear. Dowels force concrete on both sides of a joint to deflect
approximately equally when subjected to a load and help
prevent damage to an exposed edge when the joint is subjected
to wheeled traffic. Table 5.1 provides recommended dowel
sizes and spacing for round, square, and rectangular dowels.
For dowels to be effective, they should be smooth, aligned,
and supported so they will remain parallel in both the hori-
zontal and the vertical planes during the placing and
finishing operation. All dowels should have sawn and
deburred end edges. Properly aligned, smooth dowels allow
the joint to open as concrete shrinks.
Dowel baskets (
Fig. 5.9
 and
 5.10
) should be used to maintain
alignment of dowels in sawcut contraction joints and alignment
devices, similar to what is shown in 
Fig. 5.5
, should be
incorporated into the bulkhead of construction joints. In exterior
slabs, wet conditions, or corrosive environments, the designer
should consider corrosion protection for the dowels. Round
dowels should be placed no closer than 12 in. (300 mm) from
the intersection of any joints because the maximum movement
caused by curling and dry shrinkage occurs at this point, and
the corner of the slab may consequently crack.
Diamond-shaped load plates (a square plate turned so that
two corners align with the joint) can be used to replace
dowels in construction joints. The diamond shape allows the
slab to move horizontally without restraint when the slab
shrinkage opens the joint (
Fig. 5.11
). Table 5.2 provides the
recommended size and spacing of diamond-shaped load
plates. Square and rectangular dowels cushioned on the
vertical sides by a compressible material also permit horizontal
movement parallel and perpendicular to the joint (
Fig. 5.12
).
These types of load-transfer devices are useful in other slab
types where the joint should have load-transfer capability
while allowing some differential movement in the direction
of the joint, such as might be necessary in post-tensioned and
shrinkage-compensating concrete slabs,
 
or in slabs with two-
directional doweling (Schrader 1987, 1991; PTI 2000; Ringo
and Anderson 1992; Metzger 1996; Walker and Holland 1998;
American Concrete Paving Association 1992). These types of
load-transfer devices may be placed within 6 in. (150 mm) of a
joint intersection (
Fig. 5.12
 and 
5.13
).
Less effective as a load-transfer mechanism than those just
discussed is aggregate interlock. Aggregate interlock
depends on the irregular face of the cracked concrete at joints
for load transfer. The designers that choose to use aggregate
interlock as the load-transfer mechanism at joints are
cautioned that, for unreinforced concrete slabs, the joint
spacings recommended in Fig. 5.6 are intended to minimize
the potential for midpanel out-of-joint random cracking, and
Fig. 5.6—Recommended joint spacing for unreinforced slabs.

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