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Rolling with the punches

Asphalt Article December 28, 2000
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All other things being equal in the asphalt compaction process, an increase in the ground speed of a vibratory roller can trans


All other things being equal in the asphalt compaction process, an increase in the ground speed of a vibratory roller can translate directly into an increase in profit or decrease in expense, depending on which side of the equation you are on. But there’s one catch–anyone understanding the theory of vibratory compaction knows that the above statement can only be true with a corresponding increase in vibration frequency.


Unlike static compactors, vibratory compactors offer a combination of both static and dynamic forces to achieve the specified density in the shortest possible time frame. The dynamic component of the vibratory roller is the component which generates the extra high forces needed to reach density fast. This dynamic force is the result of a centrifugal force created by an eccentric (offset) weight driven into rotation by a hydraulic motor.


The speed at which this eccentric weight is driven is the frequency of impact measured in vibrations per minute (VPM). A vibratory roller moving at a speed of, say, 3 mph (264 ft per minute) with a 2000 VPM vibration frequency impacts the material every 1.58 in., or 7.6 impacts per foot. Normally, one would think that a faster rolling speed would be better, but in the case of a vibratory roller, an increase in rolling speed translates into a decrease in density. Maintaining the above 2000 VPM vibration frequency and increasing the roller speed to 4 mph increases the impact spacing to 2.11 in. Spreading out the spacing of these impacts and reducing the dynamic component applied to the material results in a decrease in the density. The thicker the lift, the more apparent this disparity becomes.


Since density is the primary reason for rolling asphalt in the first place, users certainly don’t want to downgrade the result by speeding up the roller. In the example given above, increasing the ground speed of the roller increases the impact spacing, having a negative effect on density.


As long as a roller rolling at 3 mph can keep up with the paver and meet coverage, productivity and density requirements, there’s no reason to change anything. If it is determined that density can still be reached while increasing ground speed, then the speed should be increased to help meet productivity requirements. Proper ground speed will be determined by running a test strip.


Finding a soft spot


There are a number of parameters which will determine the ground speed of the vibratory roller. These include the width of the paver, laydown speed, lift thickness, mix properties, laydown temperature, speed of trucks, distance to the plant, ambient temperature and maximum frequency of the roller.


Over the years, vibratory roller patterns have been adjusted and refined to handle a wide range of mix characteristics with optimum results. Most recent and most challenging of these are the new Superpave mix designs. During compaction, Superpave mixes have shown a tendency to act tender at mat temperatures around 240û. This means that applying a vibratory roller, or any roller, in an attempt to meet density requirements at this temperature will cause displacement and marking of the mat, which in turn necessitates additional rolling after the mat temperature cools.


Roller speed is important. In order to get more compaction work done before the Superpave mix cools, it is necessary to speed up the compaction process. To handle Superpave mixes, Sakai has developed an entirely new vibration process which increases the vibration frequency to 4000 VPM, thus allowing faster rolling speeds, tighter impact spacing and shorter rolling times. This new process has been tested and approved for use on Superpave.


Using the above example, we can maintain the same impact spacing and, therefore, density while we double the roller ground speed and thus productivity.


Exceeding the limit


Until now, the typical Superpave roller train would include two double drum vibratory rollers acting as breakdown rollers prior to the tender zone, a seven-wheel, pneumatic-tired roller as an intermediate roller following the tender zone and a balanced three-wheel steel drum roller for finish.


In most cases, the lower VPM of the conventional vibratory rollers prevents those rollers from speeding up and reaching density before reaching the tender zone. On these jobs, a Sakai TS600 pneumatic-tired roller and a Sakai R2H balanced three wheeler, both approved for Superpave work, are used for final density and finish following the tender zone.


Some manufacturers, understanding this Superpave phenomenon, have pushed the limit of their vibration systems. Manufacturers with systems designed for 2400-3000 VPM operation, which is certainly suitable for most asphalt mixes, have cranked up the RPM of their hydraulic motors, stretching the limits of the eccentric shaft bearings which were designed to handle lesser stresses. This stretching of the existing design also has created some limitations in output force, which could affect the end result.


Addressing those problems, Sakai brought in Superpave experts and Sakai engineers completely redesigned these systems for their new SW800 and SW850 Superpave machines, which were recently introduced at ConExpo/ConAgg ’99. These 4000 vpm machines let the operator speed up his roller to increase productivity while maintaining the force and impact spacing characteristics necessary to achieve density.


It is relatively easy to calculate how a 4000 VPM vibration frequency permits increases in rolling speeds, and thus decreases in operating costs with any asphalt mix. This same increase in frequency and speed will on many Superpave projects allow two 4000 VPM rollers to reach density and finish requirements and get off the mat before reaching the tender zone.


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