By: James A. Scherocman, P.E.
Editor's note: This is the second installment of a two-part series on asphalt compaction. In the August issue, Scherocman, a consulting engineer in pavement design and construction located in Cincinnati, discussed the importance of compaction and the relationship between density and the air-void content of hot-mix asphalt (HMA) pavement. For an HMA to perform well under traffic, it must be compacted to an air-void content in the range of 3% to 8%. Proper use of rollers will help achieve this result.
Five primary variables are under the control of the roller operator during the compaction process: roller speed, number of roller passes, rolling zone, rolling pattern, and vibration frequency and amplitude for vibratory rollers.
The faster a roller moves over a particular point on a new HMA pavement surface, the less time the weight of the roller dwells on that point. This means that less compactive effort is imparted to the mixture at higher roller speeds than at lower speeds. As asphalt roller speed increases, the density achieved with each roller pass decreases.
For static steel-wheel rollers and vibratory rollers that have a maximum frequency of 2,400 vibrations per minute (vpm), 21¦2 mph typically is accepted as the maximum speed a roller should travel. For a vibratory roller capable of applying compactive effort at a rate of 3,600 vpm, the roller can be operated up to 4 mph. For a pneumatic-tire roller, the maximum speed should also be 4 mph. Rollers can move faster or slower than the recommended speed, but compaction varies directly with roller speed.
Paving roller speed also is governed by the lateral displacement or tenderness of the HMA. If the mixture moves excessively under the rollers, the speed of the compaction equipment should be reduced. Roller speed should be kept constant. If the paver speeds up and the rollers also speed up, less density will be obtained in the mix for the same number of passes of each roller over each point of the pavement surface. It is very important that both the paver and the roller maintain a consistent speed to obtain consistent density.
The actual number of passes needed over a point in the pavement surface by each of the rollers is a function of many variables. The type of compaction equipment is one primary variable. Three-wheel, static steel-wheel rollers apply different compactive effort than tandem static steel-wheel rollers; pneumatic-tire rollers and single- or double-drum vibratory rollers all apply different compactive effort, as well.
However, these capabilities vary with layer thickness, mix temperature, mix design (asphalt content and aggregate gradation) and environmental conditions. In addition, the number of asphalt roller passes required of a particular roller depends on its position in the roller train-breakdown, intermediate or finish rolling. It may be possible, for example, to obtain a significant increase in the density of the mat for the same number of roller passes when a pneumatic-tire roller is moved from the intermediate position behind a vibratory roller to the breakdown position in front of that same vibratory roller.
To determine the minimum number of roller passes needed to achieve the required density level, a test strip should be constructed at the beginning of the project. Typically, only one combination of rollers is tested with one combination of passes of each roller. To pick the most efficient and economical number of roller passes, it is suggested that more than one test strip be constructed, with each test area using different rollers in different positions behind the paver.
Roller passes must be distributed uniformly over the width and the length of the HMA layer. Most often, the center of the paver lane receives more passes from the rollers than do the outside edges of the lane. The number of roller passes applied by each roller must be the same over each and every point in the pavement surface to obtain consistent density.
Compaction process
Compaction must be achieved while the mix is still hot enough for the applied compactive effort to reorient the mix's aggregate particles to reach maximum density. If the HMA is stable under the rollers, the rollers should operate as closely behind the paver as reasonably possible. Both the breakdown and intermediate rollers should be within 500 ft of the laydown machine.
With a stable mix, fewer roller passes are needed to obtain a given level of density when rolling is accomplished directly behind the paver-where the mix is the hottest. More density is usually obtained with one pass of the roller when the mix temperature is 250 F than with a similar pass when the mat is at 220 F.
If the mix is tender and moves either longitudinally and/or transversely under the compaction equipment, the breakdown rolling is often delayed to avoid excessive shoving or checking of the mix by the rollers. This, however, is the wrong solution to the problem.
The properties of a mix that cannot be compacted immediately behind the paver need to be modified. It is very difficult to obtain the required degree of density with a tender mix-the rollers can't compensate for a poor mix design. When a tender mix is encountered, the mix design-not the compaction process-should be changed.
Until the mix design can be changed, often the best way to compact a tender mix is to use a pneumatic-tire roller in the breakdown position, directly behind the paver. A vibratory roller can be operated in the intermediate position, some distance away from the laydown machine. While most of the required density is obtained with the pneumatic-tire roller, care should be taken that the vibratory roller does not operate too close to the paver and cause the mix to move-shove or check-and thereby cause a density reduction instead of the desired increase in density.
The rolling zone for the static steel-wheel finish roller is the position where marks from other rollers can be removed from the surface of the layer without adding new marks by the finish roller itself. Finish rolling normally takes place within a temperature range of 185 F down to 160 F. Finish rolling for a stable mix is accomplished at higher temperatures than finish rolling for a tender mix.
Rollers operate whenever the paver operates. Interestingly enough, when the paver stops, often the breakdown and intermediate rollers also stop. When the paver restarts, the rollers follow suit. While the paver and rollers are stopped, the mix that has not been completely compacted is cooling. Depending on the length of the shutdown, it may be difficult to obtain the desired level of density if the mix has cooled too much. It is very important that rollers continue their pattern, regardless of what the paver does, until the required number of roller passes are applied to the pavement surface and the compaction process is finished.
For each roller used on the project, the width of the paved lane should be divided by the width of the compaction rolls on each roller to determine the number of passes needed to cover each transverse point of the surface. A tandem static steel-wheel roller, 41¦2 ft wide, for example, would need to make at least four passes across the width of a 12-ft-wide lane. This allows for a minimum overlap of 6 in. over each longitudinal edge of the lane and a minimum 6-in. overlap between each roller pass.
However, a 7-ft.-wide double-drum vibratory roller could cover the full 12-ft-wide lane in only two passes across the width, still allowing for a minimum overlap of 6 in. over each longitudinal edge and between each roller pass. Thus, in terms of a roller pattern, the 7-ft-wide roller is twice as efficient as the 41¦2-ft-wide roller. A roller that is 51¦2-ft-wide would need to make three passes up and down the 12-ft-wide lane to obtain complete transverse coverage of the surface, allowing for the minimum overlap between passes.
In the longitudinal direction, the rollers should not stop at the same transverse end point with each pass of the roller. The reversal points should be staggered to prevent shoving of the mix. When paving is suspended for a period due to a lack of haul trucks, for example, the roller should not sit on the hot layer. The rollers should be parked either on an adjacent lane, on the shoulder or at the back of the cold, fully compacted layer.
Vibration frequency
Vibratory rollers have two additional variables that the operator must control during the compaction process. The first is the frequency of the vibration. Most vibratory rollers have a range of vibratory frequencies available.
With very few exceptions, the maximum possible frequency setting available should be chosen. This permits the roller to maximize the amount of compactive effort applied to the mix by minimizing the spacing between impacts.
Frequency is measured in terms of vibrations per minute (vpm). At the same roller speed, a vibratory roller operated at a frequency of 2,400 vpm will provide more impacts per foot than will the same roller operated at a frequency of 2,000 vpm.
More impacts per foot provides more compactive effort for each pass of the roller. Frequency of vibration, in conjunction with roller speed, plays a very significant role in the ability of the vibratory roller to efficiently obtain density in the HMA material.
The amplitude setting (impact height) on a vibratory roller depends on the thickness of the layer being compacted. For the vast majority of mixes placed, the roller should be operated at the lowest amplitude setting. Only when the lift thickness is greater than about 3 in. should the use of a higher amplitude setting be considered.
The amplitude setting also is dependent, in part, on the characteristics of the mix. If the mix is tender, only the lowest amplitude setting should be used. If the HMA is stiff and stable, and the lift thickness is at least 21¦2 in., use of a higher amplitude may be possible. A high amplitude setting on a thin lift (less than 2 in.) will typically cause the vibratory roller to bounce, making it very difficult to obtain the desired density level.
For very thin lifts, 1 in. or less in thickness, the vibratory roller should not be used in the vibratory mode. Instead, operate the unit in the static mode.
The compaction of an asphalt-concrete mix is really common sense. Because density, or its inverse air-void content, is the single most important variable affecting the long-term durability of an HMA material, it is very important that proper attention is applied to those primary factors that affect the time available to compact the mix-air temperature, base temperature, mix laydown temperature, layer thickness and wind velocity.
In addition, the five main variables that can be controlled by the roller operator during the compaction process should also be carefully monitored-roller speed, number of roller passes, rolling zone, rolling pattern and vibration frequency and amplitude.
The proper air-void content in the mix must be obtained at the time of construction in order to prevent the mix from failing prematurely under the applied traffic loads.
