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A common mistake is not to specify the correct joint
spacing, Waalkes says. “We recommend 24 times the pavement thickness as the
maximum spacing when the pavement is on an unstabilized base,” he notes.
“And the spacing should be 21 times the thickness when the pavement is on an
asphalt or cement stabilized base. For bonded whitetopping the range of
spacing is 12 to 18 times the pavement thickness.”
Another way to look at it is the ratio of length to
width of slabs. In general, transverse joints should be spaced so that the
ratio of slab length to width is between 1.25 and 1.50. “Typically,
transverse joint spacings work out to be 15 feet on an 11- or 12-foot lane,”
says Waalkes. “That 15 feet is a pretty good rule of thumb on most highways
and streets. Also, keep slabs square or rectangular. Avoid odd-shaped slabs.
“Moreover, joints need to intersect in-pavement
objects, such as manhole covers, drainage inlets, and water-main valve
covers,” Waalkes says. For example, manhole covers should have a square or
circular isolation joint placed around them, at a minimum distance of 1 foot
from the manhole. Typically, then the joints will intersect the isolation
joints at the corners of the box-out.
“You can build square isolation joints,
diamond-shaped ones, or circular joints, but the transverse and longitudinal
joints need to intersect there,” says Waalkes. “If you put a manhole in the
middle of a slab, the slab will crack into four pieces. Joints must radiate
out in four directions from that manhole.”
Design the plan
A jointing plan needs to be drawn up and put into
effect before concrete is ever delivered to the site, Waalkes says. A
complete illustration of a joint system plan is beyond the scope of this
article, but ACPA publishes a document called Intersection Joint Layout,
which explains the process. A computer-aided design system can be used for
this. Briefly, the steps for an intersection are:
1. Draw all pavement edge and back-of-curb lines
in plan view.
2. Lightly draw the circumference return, the
taper-return, and the cross-road return lines. The circumference return
line is a line 1.5 to 3 feet from the face of the gutter along the curve
between the edges of the intersecting roads. The taper-return line is
lightly drawn from the face of the gutter at the start of a turn-lane
taper. A cross-road return line is drawn from the edge of the mainline
roadway at a skewed intersection. Any cross-road longitudinal joint will
meet a transverse joint for the mainline roadway at the cross-road
return line.
3. Draw all lines that define lanes on the
mainline and cross road.
4. Define the mainline lanes for paving. Find
all the points where the mainline lanes intersect the circumference
return or taper-return lines.
5. Add transverse joints at all locations where
the pavement changes width, and extend those joints through the curb and
gutter.
6. Add transverse joints between and beyond the
joints defined in Step 5, but do not add joints to the center of the
intersection yet. Attempt to keep the distance between joints less than
the maximum desirable length.
7. By extending the edge of pavement lines for
the cross road and any turning lanes, define the intersection box. This
is the box formed by the edge of the mainline and intersecting paving
lanes.
8. Check the distances between the intersection
box and the surrounding joints.
9. If the distance is more than the desirable
joint spacing, then add transverse joints at an equal spacing. Do not
extend these joints past the circumference return or cross-road return
lines.
10. Lightly extend radius lines from the center
of the curve(s) to the points defined by the intersection box and points
along any islands. Add joints along these radius lines. Finally, make
slight adjustments to eliminate doglegs in mainline edges.
A common mistake that contractors make is to delay
sawing joints until after cracks have developed in uncontrolled locations.
“The natural cracking occurs elsewhere,” says Waalkes.
“The contractor needs to get on the pavement and saw
the joints, usually within four to 24 hours. The time depends on the mix,
how fast it cures, the ambient temperature, the wind speed, and such
factors. The faster it’s curing, the sooner you need to saw the joints.”
Contractors usually use a simple scratch test, he says. Scratch the pavement
with a nail and test the hardness. If you can put the saw and operator on
the pavement without making an imprint, it’s time to saw joints.
Load transfer
The performance of concrete pavement depends on its
ability to transfer loads from one side of a joint to the other. This is
called load transfer. Good load transfer results in lower deflections, which
reduces faulting, spalling, and corner breaks, thereby increasing pavement
life. Load transfer across joints for street pavements is accomplished
either by aggregate interlock or dowel bars.
Aggregate interlock is the interlocking action
between aggregate particles at the face of the joint. It relies on the shear
interaction between the aggregate particles at the irregular crack faces
that form below the saw cut. Aggregate interlock has been found most
effective on roadways with short joint spacings and low truck volumes. A
1985 study by the Minnesota Department of Transportation found that
aggregate interlock load transfer provides acceptable pavement performance
when truck semi-trailer volumes are fewer than 80 to 120 trucks per day per
lane.
Dowel bars are round, smooth, steel bars placed
across transverse joints to transfer loads without restricting horizontal
joint movements due to thermal and moisture contractions and expansions.
These bars also keep slabs in horizontal and vertical alignment. Dowels
reduce deflections and stresses due to traffic loads. In turn, that prevents
faulting, pumping, and corner breaking on roadways that carry a large number
of trucks and/or have longer joint spacings.
The use of dowel bars for minimizing faulting and
pumping should be considered when the slabs are longer than 20 feet, or when
truck semi-trailer traffic exceeds 80 to 120 per day per lane, or when the
accumulated design traffic exceeds four or five million ESALs per lane as
defined by AASHTO. Typically, this truck traffic level requires an
8-inch-thick slab or greater.
Because most city streets do not experience truck
traffic levels that high — and recommended joint spacings are not greater
than 15 feet — dowels are generally not necessary on those pavements. But
for 8-inch slabs or greater, dowels and/or the following methods are
recommended for most highway applications:
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Thicker slabs.
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Stiffer subbase and subgrade.
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Less erodable subbase, for example cement
treated base or asphalt.
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Edge support such as tied concrete shoulders or
tied integral curb-and-gutter.
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A drainable subgrade.
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Longitudinal edge drains.
“We’re seeing some inexperience in the design and
construction of concrete pavements,” says Waalkes. “We have a whole new
generation of people involved — and it’s important that they know how to
properly design and build a concrete pavement.” |
