By: Gerald Voigt, P.E.
To ensure the
concrete pavements we are building now will continue to serve the needs of the
traveling public well into the future, it is essential to take into account all
design and construction aspects that may affect the performance and durability
of the pavement.
Joint design is extremely important, but sometimes not given
its due importance in an overall pavement design. A proper jointing system for
concrete streets ensures that the structural capacity and ride quality of the
pavement is maintained at the highest level of performance at the lowest
possible cost.
A proper jointing system will control cracking; divide the
pavement into practical geometric increments for construction; accommodate slab
movements; and provide sufficient load transfer. Concrete pavement joint design
has evolved from a tremendous body of theoretical studies, laboratory tests,
experimental pavements and performance evaluations of in-service pavements.
Understanding project-specific considerations such as traffic and environmental
considerations is of paramount importance in design of slab dimensions and
jointing details.
The primary purpose of joints in a concrete pavement is to
control the location of transverse and longitudinal cracking, which results
from stresses caused by normal drying shrinkage, temperature and moisture
differentials and applied traffic loadings. If the stresses are not relieved,
uncontrolled cracking will occur.
Designers also must consider climatic and environmental
conditions; slab thickness; load transfer; shoulder, curb and gutter
construction; and traffic. It's important to note that late or improper joint
formation may cause cracks to occur at locations other than those
intended.
Crack control
Observing slab behavior of unjointed and plain pavements in
service has provided excellent perspectives on how joints are used to control
cracking. To obtain adequate workability for placing and finishing concrete,
more mixing water is used than needed to hydrate the cement.
As the concrete consolidates and hardens, some of the excess water bleeds to the
surface and evaporates. Mix water also is consumed in hydration, the chemical
reaction of cement hardening. With the loss of mix water, the concrete shrinks,
occupying slightly less volume.
A second major source of early volume reduction is caused by
the pavement's temperature change. The heat of hydration and temperature of the
concrete normally peak a short time after the final set. After peaking, the
temperature of the concrete declines, and as the temperature drops, the
concrete pavement contracts.
The initial cracks will occur from about 40 to 150 ft apart,
depending on the concrete properties, variations in subgrade friction and
climatic conditions. Evaluating the combined effect of restrained temperature
curling and moisture warping is complicated because of their opposing nature.
Repetitive traffic loads compound the problem, but in any case the curling and
warping, combined with traffic loads, will cause additional transverse cracks
between the initial contraction cracks. Also, a longitudinal crack will form
along the approximate centerline of pavements with two lanes of traffic.
Load handling
For jointed pavements to perform adequately, traffic
loadings must be transferred effectively from one side of the joint to the
other. Adequate load transfer, as this is called, results in lower deflections.
The net effect is less faulting and spalling. Load transfer is generated by
aggregate interlock or the use of dowel bars.
Aggregate interlock refers to the interlocking action
between aggregate particles along the irregular crack faces that form below a
saw cut. This form of load transfer has been found to be adequate on roads with
short joint spacings and low truck volumes.
Dowel bars, which are cylindrical, smooth bars (usually
epoxy-coated steel), when placed across transverse joints will provide
mechanical interaction without restricting horizontal joint movements. They
also keep slabs in horizontal and vertical alignment and reduce deflections and
stresses occurring under various traffic loads. This, in turn, prevents or
reduces faulting and pumping. This form of load transfer is well suited for
roadways with longer joint spacings and for those that carry a large number of
trucks. Dowel bars should be considered necessary when slabs are longer than 20
ft, when truck traffic exceeds 80 to 120 per day per lane or when the
cumulative design traffic exceeds 4 million equivalent single axle loads per
lane.
What's your type?
There are four general types of joints for concrete streets:
* Transverse
contraction joints: Constructed transversely to the street's centerline and
spaced to control transverse slab cracking. These are placed primarily to
control natural cracking. Their spacing, saw cut depth and timing of joint
formation are all critical to performance;
* Transverse
construction joints: Installed at the end of a day-long paving operation or
other placement interruption. Planned joints are butt-type joints and to
perform properly require dowel bars extending through the joint;
* Longitudinal
joints: Placed parallel to the pavement centerline to control cracking and
delineate traffic lanes. Longitudinal joints on arterial streets also should be
spaced to provide traffic and parking lane delineation. On most streets the
pavement is laterally restrained by the backfill behind the curbs, so there is
no need to tie longitudinal joints with deformed tiebars. But when lateral
movement is not restrained, tiebars must be placed at mid-depth of the slab to
prevent the joint from opening as a result of slab contraction; and
* Isolation joints: Placed to allow movement of the pavement
without damaging adjacent pavements, intersecting streets, drainage structures
or other fixed objects.
Isolation joints isolate the pavement from a structure,
another paved area, or an in-pavement fixture. Proper use of these joints
reduces compressive stresses that develop between the pavement and a structure
or between two pavement sections.
Expansion joints are generally full-depth, full-width
transverse joints placed at regular intervals of 50 to 500 ft, with contraction
joints in between. This is an old practice that was used to relieve compressive
forces, but unfortunately one that caused transverse joints to open wide near
the expansion joint. These open joints were prone to spalling, pumping and
faulting. Proper design, construction and maintenance of contraction joints has
virtually eliminated the need for expansion joints, except under special
circumstances.
Proper tips
In any application, it is essential to understand all the
factors that can influence or can be influenced by pavement joints. Here are
some general tips and guidelines to keep in mind when designing a joint layout:
1. Do not exceed maximum transverse joint spacing for
streets, usually the lesser amount of either 24 times the slab thickness or 15
ft;
2. Longitudinal
joint spacing should not exceed 12.5 ft;
3. Slabs should
be kept as square as possible, with a maximum length-to-width ratio of
1.25;
4. Transverse
contraction joints must be continuous through the curb and gutter and must have
a depth equal to 1/4 to 1/3 the pavement thickness, depending on sub-base type;
5. In isolation
joints, the filler must be full-depth and extend through the curb;
6. If there is
no curb, longitudinal joints should be tied with deformed tiebars;
7. Offsets at
radius points should be at least 1.5 ft wide, and within this region all joints
should intersect the edge of pavement at 90°. Joint intersection angles
less than 60° also should be avoided;
8. Minor adjustments in joint location to meet inlets and
manholes will improve pavement performance; and
9. When the pavement area has drainage structures, joints
should be placed to meet the structures, if possible.
Although these are general guidelines and tips that apply to
proper joint design and construction, there are many other factors and
variables that must be considered. ACPA has a number of technical publications
that address joint design in various applications. They include:
"Design and Construction of Joints for Concrete
Highways" (TB010P);
"Design and Construction of Joints for Concrete
Streets" (IS061.01P);
"Intersection Joint Layout" (IS006P);
"Joint and Crack Sealing and Repair for Concrete
Pavements" (TB012P);
"Proper Use of Isolation and Expansion Joints in
Concrete Pavements" (IS400P); and
"Concrete Pavements with Undoweled Joints for Light
Traffic Facilities" (IS405P).
Pricing, ordering information and additional details about
these publications may be found on ACPA's website at www.pavement.com, or by
calling 800/868-6733. Fax requests to 847/966-9666.
About The Author: Voigt is COO and vice president of technical operations for ACPA, Skokie, Ill.
