TM 5-809-1/AFM 88-3, Chap. 15
(1) Doweled butt joint. The doweled butt
(3) Thickened-edge joint. Thickened-edge-
joint is considered to be the best joint for providing
type joints maybe used instead of other types of
load transfer and maintaining slab alignment. It is a
joints employing load-transfer devices. When the
desirable joint for the most adverse conditions such
thickened-edge joint is constructed, the thickness of
as heavy loading, high traffic intensity, and lower
the concrete at the edge is increased to 125 percent
strength foundations. However, because the
of the design thickness. The thickness is then
alignment and placement of the dowel bars are criti-
reduced by tapering from the free-edge thickness to
cal to satisfactory performance, this type of joint is
the design thickness at a distance of 5 feet from the
difficult to construct, especially for slipformed con-
longitudinal edge. The thickened-edge butt joint is
crete. However, the doweled butt joint is required
considered adequate for the load-induced concrete
for all transverse construction joints in
stresses. However, the inclusion of a key in the
nonreinforced pavements. Doweled construction
thickened-edge joint provides some degree of load
joints are shown in figure 5-9.
transfer in the joint and helps maintain slab
(2) Keyed joint. The keyed joint is the most
alignment; although not required, it is recommended
economical method, from a construction standpoint,
for pavement constructed on low- to medium-
of providing load transfer in the joint. It has been
strength foundations. The thickened-edge joint may
demonstrated that the key or keyway can be
be used at free edges of paved areas to
satisfactorily constructed using either formed or
accommodate future expansion of the facility or
slipformed methods. Experience has proved that the
where wheel loadings may track the edge of the
required dimensions of the joint can best be
pavement. All floor slabs accommodating vehicular
maintained by forming or slipforming the keyway
traffic will be thicked at doorways to have an edge
rather than the key. The dimensions and location of
thickness of 1.25 times the design thickness as
the key (fig 5-10) are critical to its performance.
shown in figure 5-11. The use of this type joint is
The structural adequacy of keyed ed construction
contingent upon adequate base-course drainage.
c. Isolation joints. Isolation joints are provided to
joints in rigid floor slabs, however, can be impaired
seriously by such factors as small changes in the
prevent load transfer and allow for differential
dimensions of the key and positioning the key other
settlement between the floor slab and other building
than at the middepth of the slab. Exceeding the
components. Isolation joints also allow for some
design values for the key dimensions produces an
horizontal movement. Isolation joints should be
oversize key, which can result in failure of either the
placed at locations where slabs abut walls or their
top or bottom edge of the female side or the joint.
foundations and around columns, column
Similarly, construction of an undersizes key can
foundations, and other foundations that carry
result in shearing off the key. Keyed joints should
permanent dead load other than stored material.
riot be used in floor slabs 8 inches or less in
Isolation joints are provided by placing 30-pounds
thickness except where tie bars are used. Tie bars in
asphalt, coal-tar saturated felt, or equivalent
material between the floor slab and the building*s
the keyed joint, will limit opening of the joint and
provide some shear transfer that will improve the
structural components before the floor is placed.
performance of the keyed joints. However, tied
Such sheets should be placed or fastened to the
joints in floor widths of more than 75 feet can result
buildings components to prevent any bonding or
in excessive stresses and cracking in the concrete
direct contact between the floor slab and the
during contraction. When a longitudinal
building component. This requires that the sheets
construction joint is used at the center of a floor two
have a height equal to the floor slab thickness and
paving lanes wide, a keyed joint with tie bars should
be placed at the same elevation as the floor slab, as
be used. When a keyed longitudinal structure joint
shown in figure 5-12.
is used at the center of a floor four or more paving
lanes in width, dowels should be used.
5-19
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