The Strategic Highway Research Program (SHRP) ended in March, 1993. The activity of this highway program included $50 million devoted to the study of asphalt cement binder tests and specifications, as well as tests and specifications on hot-mix asphalt (HMA) materials. The end result of the SHRP work was an overall asphalt/aggregate mixture design and analysis system called Superpave. The Superpave system proposes test procedures and criteria for asphalt binders, aggregates, mixes, designs, and performance prediction methods for HMA materials. The test procedures proposed with the SHRP system are a positive step forward to improve quality and performance of HMA.
A large part of the testing procedures introduced by the SHRP system is the use of the gyratory compactor.
It is important to remember that the laboratory compaction process is intended to duplicate field conditions, not the other way around. We must not let laboratory equipment limitations dictate substandard construction practices or eliminate the use of good performing mixes.
The gyratory compactor approximates the force to high vibrations per minute (VPM), as opposed to amplitude (the height of a vibrating drum off the mat in one rotation of the drum) or a pounding impact moment that we have utilized with the Marshall design.
In the past two years, as the utilization of the Superpave system has been initiated, we have seen varying results in the attempt to compact this system's mix designs.
First let me dispel a myth-the Superpave system mixes can be compacted productively; however, this is a learning process, and it takes a good field management process. If the construction process is not being controlled with the existing system, adopting a new system will not improve pavement performance.
The contractor must be responsible for process control. In order to accomplish this, mix design and analysis should be the responsibility of the contractor. There are four primary components in productively achieving specified density with Superpave mixes:
- Communications-process control;
- Selection of compaction equipment;
- Testing and establishing rolling patterns; and
- Operator training.
The use of Superpave does not eliminate the need for some engineering judgement in pavement mixture design and analysis. There is no fool-proof or text book process for mix design. Bituminous Quality Control Engineers must fully participate in the decision-making process to include the other three components in achieving productive density.
Selection of compaction equipment
The primary roller that has been utilized as the breakdown, or front roller, is a tandem vibratory with a drum width of 66 to 78 in. or 84 in. The 78-in. and 84-in. rollers have the advantage of covering a standard paving width of 12 ft in two converages.
The placement of this roller behind the paver in relationship to the mat temperature is critical. We want to keep this roller in the 290¡F to 300¡F temperature zone, with both drums vibrating.
The selection of VPM and amplitude varies according to the controls on the roller. On a two amplitude machine with variable frequency, we are making two passes in high amplitude at 3,000 - 3,200 VPM; and two passes in low amplitude at 3,000 - 3,400 VPM. On a 12-ft-wide mat, this is a total of eight passes with the front roller. This is the maximum number of passes we have made on a 19.5-mm, (3-in.) lift thickness of binder.
The higher VPM can run the breakdown roller and move at a faster travel speed in order to stay in the 290¡F to 300¡F temperature zone.
Example: We want to maintain a minimum of 10 impacts/foot with a vibratory roller. At varying VPMs we establish the production rate with the breakdown roller: 2,500 VPM (divided by) 10 Impacts = 250 FPM.
A vibratory roller compacts by particle rearrangement. With the larger volume of aggregate in a Superpave design, the higher VPM we can run, the faster we set the aggregate in motion and get the initial lockup of aggregate particles at high mat temperature.
Behind the breakdown roller, we have seen several of the Superpave mixes go through a tender zone at a surface temperature of between 200¡F to 240¡F. You do not want to put the intermediate or second roller right behind the breakdown roller at a temperature above 240¡F.
Trying to roll with the second roller above 240¡F, we have seen a decrease in density from the initial density obtained during breakdown rolling. You have to allow this mix to reach a stable temp-viscosity zone before applying the intermediate roller. Between rolling and intermediate rolling, this mix has to rest.
We have utilized both a pneumatic roller and a tandem vibratory roller as the intermediate roller. The pneumatic has been either a 12 1Ú2-ton nine wheel with 7.50 x 15, 14 ply tires, exerting 80-85 lb per sq in. (PSI) ground contact pressure, or a 22-ton, 8 wheel with 11 x 20, 16 ply tires, exerting 75-80 PSI ground contact pressure.
On the job we applied the larger pneumatic at a temperature between 200¡F to 240¡F and made a total of four passes to cover a 12-ft-wide mat. Because we were in a milled area with supported sides, we rolled right to the edge of the mat. With non-supported edges, generally we will hold off the edge 8 in. with the pneumatic roller. Because the pneumatic roller gives us the compaction forces of both pressure and manipulation or kneading, we are taking the force straight down into the lift and compacting from the bottom up. This helps control the lateral movement of the mat in the tender zone.
We also applied, in place of the pneumatic not in addition to, a tandem vibratory roller. The temperature zone was 200¡F to 240¡F, and the pass pattern was, two passes low amplitude-3,400 VPM, and one static pass for a total of six passes on a 12-ft wide mat.
The third roller was a tandem vibratory being used in a static mode, at a mat surface temperature of 150¡F to 160¡F. We were achieving our maximum density with the finish roller making three static passes, or a total of six passes on a 12-ft-wide mat.
The density requirement of 94% MTD was achieved with this rolling pattern. There was an incentive on density above 95% MTD, and most of the time we were making the incentive density.
On 9.5-mm surface mix being placed in lifts from 1 1/2 - 2 in., 12 ft wide, we have reduced the total number of breakdown passes with a tandem vibratory roller from eight to six passes. We made two vibratory passes (low amplitude at 3,200 VPM) and one static pass. The intermediate and finish rolling was done with one tandem vibratory roller making three to four passes, or a total of six or eight passes on a 12-ft-wide mat.
When placing a 9.5-mm surface mix, 1 1/2 in. depth and 14 ft wide (12-ft mainline 2-ft shoulder), confine the mix during the breakdown rolling and the intermediate (tender zone). The addition of a tandem vibratory roller, with a 48-in. drum width, could be utilized to compact this. Compaction should be done with a 2-ft shoulder ahead of the breakdown and intermediate rolling. This roller should be set in low amplitude at high VPM; 3,500 - 3,900 VPM.
Testing and establishing a rolling pattern
The establishment of any rolling pattern begins with the breakdown roller and a test strip. It is essential to utilize a test strip or control zone to develop a pass density pattern. Also take into account the intermediate roller and establish a pass density pattern for the second roller based on the temperature of the mat and the tender zone between 200¡F - 240¡F.
Because we have achieved final density during the finish rolling, we have to run pass density patterns with the finish roller to minimize the number of passes in the proper temperature zone. The test strip should be established with enough tonnage to test all three phases of rolling.
The communication and management on the job should take the entire compaction process and roller operators into account. During the control strip, the roller operators should understand that there will be changes made, so they should be thoroughly trained on the controls and operation of the rollers to maximize their contribution during the control strip.
Upon establishment of the rolling patterns that maximize productive density, the roller operator should be communicated with, and trained to stay within the rolling patterns established at given vibratory settings, travel speeds and temperature zones.
The Superpave mixes can be produced, laid and compacted productively by establishing good field management, communication on the job, continual evaluation of the results, utilization of talents, ingenuity and drive for quality hot-mix asphalt paving in the U.S.