Smooth as a baby's...

Dec. 28, 2000
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).

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.

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