Smooth roads are one of life's unsung pleasures. When a car glides across
a glass-smooth, like a skater on ice, there is no need to reduce speed to
negotiate bumps and dips. Belly rolls do not jiggle nor do teeth rattle
as one zips down such a highway. A smooth road can make a drive enjoyable,
safer and cause less wear and tear on a vehicle. In addition to comfort,
smooth roads last longer and hold up better under traffic loads.
One reason smooth roads can do all this is its ability to reduce dynamic
loads (also called impact loads). Dynamic loads occur when a vehicle drives
over a bumpy road. As it bounces vertically on its suspension, the vehicle's
mass accelerates downward. Because an accelerating mass has a higher force
than a static mass (force = mass ¥ acceleration) increased stress is
placed on the pavement.
The heavier, the vehicle the more force placed on the road. Naturally, dynamic
loading will cause further deterioration and more bumps, which will accelerate
the problem like a snowball rolling downhill. This leads to a shorter life
span for the road.
So a smooth surface lessens dynamic loading, allowing a road to enjoy a
longer life. Smoothness can be ensured through careful planning during the
design and construction phases, and by a dedicated implementation of the
CPR maintenance system. (see Georgia Boasts Smoothest Roads, 4/96).
Base type, vertical and horizontal alignment, grades, pavement type and
concrete mix all affect a road's smoothness and should be addressed during
the design phase. A smooth, stable paver trackline should be planned in
advance, rather than waiting to resolve any problems caused by a bad trackline
with the stringline.
The American Pavement Association's technical bulletin, Constructing Smooth
Concrete Pavements, states that a stable trackline can be achieved by extending
the base outside the pavement edge by 3 ft on each side. The plan and bid
schedule should take into account this extension of the base.
Horizontal and vertical curves must be addressed. By making adjustments
to the paving machine-and paying close attention to the machine's operation
as the degree of horizontal curvature increases-roughness can be decreased.
Establishing a proper staking interval can help correct any problems caused
by vertical curves. Staking intervals for the stringline should be based
on the rate of change of gradient in the vertical curve.
Careful attention to embedded items-manholes, inlets, dowel baskets and
reinforcement-is necessary to prevent reinforcement ripple, spring-back
or damming, all of which cause uneven surfaces. Adjustments and attention
during the paving process can help. Use of dowel insertion equipment-rather
than baskets-is also recommended and should be written into design specifications.
Accuracy during surveying also is important. With the availability of Global
Positioning System technology, surveying accuracy to 1 cm can be realized.
Care should be taken while surveying, and methods that improve survey accuracy
should be considered and used whenever possible.
During the construction phase, accurate setting of the stringline is important.
When paving on a stable platform, results can be improved by using one stringline
to control the surface elevation, while the other side of the paver operates
freely or from an averaging ski.
On rough surfaces, a dual stringline can help. Once the stringline is placed,
it is important to never touch it, because any movement of the stringline
will translate into a bumpy, rough road.
Quality control of the concrete mix and batching operations must be implemented
throughout all phases-hauling, placing and finishing-in order to produce
a smooth road. An aggregate gradation must not be too harsh and unworkable.
A paving machine works harder to spread a harsh mix than a smooth mix. Harsh
mixes also create extra work for the finishing crew.
The paver forms the surface of the pavement. If it is forced to stop during
a job, this stoppage can cause dips and bumps. Therefore, it should maintain
a constant speed at all times. To ensure constant speed, the plant should
provide a steady supply of concrete.
By having a sufficient number of trucks to haul the concrete and with constant
communication between the plant and the job site, an operator can keep the
paver's speed constant. However, on occasion, stops are unavoidable. The
major cause of unpredictable stops is mechanical breakdown. One way to cut
down on this is a preventative-maintenance program. Whenever a paver is
forced to stop, always record the area in order to pinpoint problems that
may occur so they can be corrected later.
Monitoring the smoothness also will contribute to the rideability of the
road. By measuring the smoothness of the previous day's paving, corrections
can be made and results improved. A profilograph-a rolling straight edge
that measures vertical deviations from a moving reference plane-is used
to check a pavement's smoothness. The recorded measurements are used to
compile a profile trace, which is used to identify bumps on the newly paved
road. These bumps are then ground down to acceptable smoothness levels with
diamond grinding equipment.
A road that is built with smoothness in mind also will last longer and provide
better service to the driving public. In addition, the longevity of such
a road can be further extended by the application of CPR methods.