Although the mix design of the Superpave system still presents some challenges, this system is still the best tool we have to improve the performance of hot-mix asphalt (HMA) pavements.
One of these challenges has been productively compacting coarse-graded Superpave mixes to meet specified density, smoothness and production.
The Superpave system has developed test procedures and criteria for asphalt binders, aggregates, mixes, design and performance prediction methods for HMA materials.
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.
In trying to compare the type of lab equipment utilized to the competitive forces used by various types of rollers, the gyratory compactor approximates the compactive forces used by a pneumatic-tired roller. The gyrations of the lab specimens are more like the confined manipulation and pounds per square inch (PSI) compactive effort of the pneumatic-tired roller.
Utilization of the tandem vibratory roller to compact Superpave mix designs has been compared to the gyratory lab compactor.
The vibrations per minute (VPM) does not directly relate to the gyrations of the gyratory compactor. There are vibratory lab compactors that utilize the same VPM as vibratory rollers to compact lab specimens.
Amplitude (the height a vibratory drum moves off the HMA mat surface in one rotation of the vibratory drum) or a pounding impact moment is more comparable to the Marshall Design and Marshall Hammer.
We do get more information during the lab compaction with the gyratory compactor than we have with previous lab methods. There are a couple of areas that need to be improved utilizing the gyratory compactor. Even though AASHTO reduced the number of gyrations and limited the gyratory levels versus traffic volume, I believe the gyrations are still too high for the high volume roads. There are still coarse-graded mix designs where we are crushing more aggregate than we are compacting to achieve a specified density or bonus for achieving density. Also, we need to further develop the proper number of gyrations to achieve N design density for low volume roadways and commercial applications utilizing Superpave mix designs.
As I stated, there have been some challenges in productively compacting some Superpave mixes. These mixes are: those on the coarse-graded side of the maximum density line; stay below the restricted zone; and have been placed at high ambient temperatures 85-100û F.
The Superpave system mixes can be compacted productively, however, this is a learning process. If the construction process is not being controlled with the existing system, adopting a new system will not improve pavement performance.
There are four primary components in productively achieving specified density with Superpave mixes:
• Communication-process control;
• Selection of compaction equipment;
• Testing and establishing rolling zones and rolling patterns; and
• Training for foremen, quality control engineers and roller operators.
The use of the Superpave system does not eliminate the need for some engineering judgment on pavement mixture design, lift thickness and communication between foremen, quality control engineers and roller operators. Bituminous Quality Control Engineers must fully participate in the decision-making process to include the other three components in achieving productive density.
Coarse-Graded Superpave Design: The coarse-graded Superpave designed mixes with high coarse aggregate content typically act differently than the fine-graded mixtures during compaction.
Time Available for Compaction (TAC): Coarse-graded mixtures have a tendency to cool more quickly, resulting in less time to obtain density productively.
Lift Thickness: With the higher coarse aggregate content, we should be aware of a recommended change in lift thickness versus nominal maximum aggregate size. The past rule of thumb has been that the lift thickness should be 2 1/2 times the maximum size aggregate in the mix. With Superpave, the nominal maximum aggregate size is specified, and usually 100% of the material does not pass this sieve size. So a mixture that used to be 19 mm maximum size is likely now to be referred to a 12.5 mm nominal size. The recommendation is that the lift thickness should be three times the nominal maximum size aggregate; and some states have specified four times. The thicker lift thickness for coarse-graded Superpave mixes is a major factor in productively achieving specified density due to a faster cooling rate with coarse-graded mix designs and the idea that we need enough weight of material (lift thickness) to move this larger volume of coarse aggregate and lock up the aggregate particles during initial rolling with a vibratory roller.
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 in., 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 coverages.
The placement of this roller behind the paver in relationship to mat temperatures, base temperatures, ambient temperatures and lift thickness is critical. We want to keep this roller in the 285-300û F mat surface temperature zone with a rolling zone of no more than 200 ft off the screed of the paver.
We want to place the vibratory roller in this 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 up to 3400 VPM, we are using high amplitude on 19-mm, 3-in. lift thickness material at 3000-3400 VPM; and low amplitude on 9.5-mm, 1-in. lift thickness at 3000-3400 VPM.
In selecting VPM and amplitude, we should set up the controls with the highest VPM and lowest amplitude setting we can compact a mix, in the fewest number of passes. There are several reasons for running higher VPM’s and lower amplitudes:
• We want to maintain a minimum of 10 impacts per 12 in. of asphalt mat or 1.2-in. impact spacing. The higher VPM allows us to travel faster with the roller and stay in the 285-300û F temperature zone; gain density in the fewest number of passes to maintain production and smoothness; and help set the higher production rate 10 impacts at 2500 VPM = 250 FPM travel speed (10 impacts at 3400 VPM = 340 FPM travel speed).
• A vibratory roller compacts by particle rearrangement. With the larger volume of coarse aggregate in a Superpave design, the higher VPM we can run, the faster we set the aggregate in motion and obtain the initial lock up of aggregate particles at high mat temperature.
• We do not want to pound on this mix in the breakdown rolling phase by using high amplitude ranges. Trying to obtain 100% of specified density during breakdown rolling can cause smoothness problems, fractured aggregate particles with high permeability and high voids to density relationship. We will literally pound the life out of Superpave pavements trying to achieve 100% of specified density during the breakdown rolling.
In order to discuss the intermediate and finish rolling phases, we need to discuss the three temperature zones we have with Superpave mixes on the coarse-graded side below the restricted zone.
On a number of jobs with this mix design we have the following rolling and mat surface temperature zones:
• Breakdown rolling with tandem vibratory—285-300û F.
• Intermediate rolling—In a temperature zone from 200-250û F we have identified a tender zone. The mix can be compacted above this range or below this range, but the mix is tender within this temperature range and cannot be compacted.
• Finish rolling—From 120-170û F we are obtaining final density with a tandem vibratory roller rolling in the static mode or utilizing a static tandem roller.
On actual jobs with a density spec of 92%, maximum theoretical density for 100% pays off. We are obtaining 92% - 93% in the breakdown rolling phase with one or two tandem vibratory rollers. We have established some patterns with two tandem vibratory rollers rolling in echelon, or side-by-side, in order to cover the mat and make the initial passes in 285-300û F mat surface temperature zone.
In the tender zone we are either not rolling in this temperature zone or utilizing a pneumatic-tired roller or rollers. The size of pneumatic-tired rollers ranges from 12-ton to 25-ton ballasted weight with a compactive effort of 80 to 90 PSI (pounds per square inch). The pneumatic-tired roller, like the gyratory lab compactor, compacts with confined manipulation and PSI. By utilizing the pneumatic-tired roller in this tender zone, we are confining the mix and not allowing the horizontal and lateral displacement we have seen under steel vibratory or steel static rollers. The pneumatic-tired roller might have a problem with modified asphalt if proper procedures are not utilized to keep the mixture from adhering to the tires. There are non-adhering pneumatic tires available to the industry that have been tested on neat asphalt mixes without having "pick-up" problems. With modified asphalt, some release agent should be used to eliminate "pick-up."
We have achieved some additional density in the tender zone with the rubber-tired roller from 92% behind the breakdown roller. There has been an increase by .5 to 1 lb of density.
The finish rolling zone
From 120-170û F mat surface temperature, we are gaining 3-4 lb of density from 92-93% to 95-97%. We are utilizing tandem vibratory rollers in the static mode or static tandem rollers and are not vibrating in this temperature zone. If we do not apply a pneumatic roller in the tender zone, our choice is to wait for the mix to cool to the temperature range it can be confined and compacted in this finish rolling zone.
If the tenderness problem yields a pavement with poor in-place density, or if the paving train is excessively long due to the time required for the mixture to cool, adjustments to the mix design must be made to eliminate or reduce the temperature tender zone.
Testing/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 200-240û 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 settings for VPM, amplitude, tire inflation pressures and travel speed of the rollers utilized in the test strip should be the same as we expect to run in production.
Communication between foremen, quality control engineers and roller operators is critical in establishing rolling patterns, changing procedures on the job and maintaining roller patterns and rolling zones to maximize production and gain density.
Before placing the test or control strip, the foreman, operators and quality control engineers should understand the controls and settings on all the rollers to be utilized. There should be an initial plan laid out for each phase of rolling, but the roller operators should understand there will be changes made during the test strip.
Upon establishment of the rolling patterns that maximize productive density, the foreman, roller operators and quality control engineers should meet and discuss the rolling patterns, density control and paving and rolling speeds that balance plant production. You cannot place any more material than you can get compacted at passing density.
Superpave mixes have offered some challenges, and we are continuing to learn and make modifications to this system and mix design.
We will see changes in this mix design, but Superpave mixes today and in the future can be produced, laid and compacted productively with good field management, communication on the job, continued evaluation of results, utilization of individual talents and a commitment to quality HMA pavements.