In business, companies are always looking for an edge, a way to be the first, angles to capture a new market. When successful, these companies become very profitable. In government, saving the taxpayers money is a major driving force—not profitability.
Agencies look for more efficient methods to deliver services to the public; ways to stretch the dollar with tightening budgets. For departments of transportation, road-user cost and environmental impact are key elements in deciding and performing road maintenance and construction. These elements have been the focus of numerous forums and workshops sponsored by various organizations and government agencies.
The type of pavement constructed and materials used play a big role in road-user cost and environmental impact. Pavements must last longer and require less maintenance; they must be smoother for the traveler and quieter to everyone.
Over the last 15 years, the Federal Highway Administration (FHWA) has sponsored several trips to learn what their non-North American counterparts were doing with pavement materials and technologies. One such trip in 1990 focused on the long-life asphalt materials used in Germany. As a result of this trip, stone-matrix asphalt (SMA) was discovered and the technology was brought back to the U.S. Soon afterward, several departments of transportation began experimenting with SMA. Likewise, the Virginia Department of Transportation (VDOT) became interested in SMA’s potential. With little information and no proven material specifications for Virginia’s materials and paving practices, VDOT attempted a trial section in 1992. While the project was a failure, VDOT was not deterred. Three years later, a joint industry and DOT delegation made the first of two German trips to see the SMA technology firsthand.
Now they understand
Like most state departments of transportation, VDOT is maintaining a highway infrastructure that has performed longer than the original design life. The majority of the interstate system was constructed from the mid-1960s through the late 1970s. Most of these pavements (flexible, jointed-reinforced concrete [JRC] and continuously reinforced concrete [CRC]) have required various levels of maintenance or rehabilitation to support the increasing traffic and loading levels.
In eastern Virginia, I-95 runs from the North Carolina to Maryland state line. A majority of this highway was constructed with JRC pavement. Between Richmond and Washington, this JRC pavement was typically constructed of a 9-in. PCC slab over a 6-in. sub-base material; the joint spacing was 61.5 ft. Based on a study conducted by the Virginia Highway and Transportation Research Council in 1975, these pavements predominate failure was joint spalling. Over the next 25 years, the original JRC pavements were overlaid with asphalt concrete to restore ride quality.
At the same time I-95 was being overlaid to improve the functional condition, a section of I-81 in western Virginia was in need of rehabilitation. A 17-mile section of JRC pavement was exhibiting severe joint failure, mid-slab cracking and faulting. The pavement condition required constant maintenance. Unfortunately, the funding was not available to reconstruct this section of highway, so an asphalt overlay was programmed for the 1995 and 1996 construction seasons.
Since the initial SMA failure, VDOT had learned from the earlier mistakes and had been able to place a few successful trial sections. In late 1994 and early 1995, VDOT developed contracts to overlay a section of I-95 and I-81 with SMA. Prior to starting work on the I-81 section, the contractors awarded the work decided to see the German SMA firsthand. Therefore, industry personnel along with Maryland State Highway Administration, Delaware Department of Transportation and VDOT employees traveled to Germany in early 1995. This trip allowed them to observe European SMA production, to discuss materials specifications and to retrieve German SMA aggregates.
While on the trip, the SMA surface mix was placed on I-95. This placement was over a deteriorated composite pavement surface. Despite several problems encountered during production and placement, the project was deemed a success. For the I-81 overlays, the contractors were able to apply the knowledge gained in Germany. Unlike I-95 where only a 11?2-in. overlay was used, I-81 required a SMA intermediate mix as well as a SMA surface mix, resulting in a total of 31?2 in. of SMA. The SMA intermediate mix gradations were developed by VDOT; the surface mix was based on the FHWA Technical Working Group guide specification for SMA. Once completed, the new smooth surface was welcomed by the traveling public.
After the initial projects, SMA was used on a limited basis around Virginia. All projects were located on the interstate system. Typically, the existing pavement was JRC or CRC in poor condition. The specifications were revised in 1999 to reflect VDOT’s move from the Marshall hammer to the Superpave gyratory compactor. Although far from routine, VDOT managed to place over 600,000 tons of SMA from 1995 to 2002.
Ready to expand
In the mid-1990s, an asphalt “revolution” was sweeping America: Superpave. For seven years, VDOT focused resources on the Superpave effort—switching to performance-graded asphalt binders, modifying materials specifications and reviewing material performance.
By 2002, over 2 million tons annually of Superpave mixes were being specified for all maintenance and construction projects. While the initial mixes (placed between 1998 and 2000) experienced problems due to low AC content and coarse aggregate grading, those problems were largely addressed through modifications to the specifications and ensuring quality paving practices.
With this focus and effort on Superpave, the SMA program was not expanding. Notably though, the SMA sections placed on I-81 and I-95 in 1995 were performing much better than expected and were outperforming VDOT’s conventionally designed dense-graded mixes. After observing this proven performance, VDOT and the industry recommitted to SMA and the 2003 implementation effort was initiated.
The first phase of the implementation effort involved reviewing and revising the 1999 SMA specifications. The 1999 specification included two mixes—an SMA surface mix and an SMA intermediate mix. In keeping with the precedent set by Superpave, the original SMA surface was redesignated SMA 12.5, and the SMA intermediate was re-designated SMA 19.0. Additionally, a new finer surface mix was added—SMA 9.5. This new mix was based on new information gathered from Germany. Allowing for two performance-graded (PG) binders per gradation (PG 70-22 and PG 76-22), a total of six mix designations were possible.
The second phase of the implementation effort was identifying SMA sites around Virginia. Each of VDOT’s nine districts were asked to find one or more locations to place SMA. Sites had to have high traffic levels and require at least 5,000 tons of material. At the end of 2002, 22 sites accounting for 255,000 tons were either under construction or identified for construction in 2003.
The final phase of the effort was providing education/training for VDOT and industry employees. In October 2002, VDOT and the Virginia Asphalt Association organized an SMA workshop. Attendees learned about aggregate requirements, mix designs, production issues and placement considerations. After the workshop, follow-up assistance was given during mix design, production and placement.
Riding it out
For the traveling public, the functional attributes of a pavement defines its quality. No matter how strong a material is or how long it lasts, it must be smooth and safe. To address smoothness, an International Roughness Index (IRI)-based ride quality specification was employed on each site. For safety, skid-resistance testing was performed on each site using an ASTM E-274 trailer with a blank tire. Since 2003, all sites have been routinely tested for ride and skid to monitor performance.
Ride quality
SMA sites were tested for ride quality in accordance with the VDOT Special Provision for Rideability, which sets targets for smoothness in terms of the IRI (ASTM E1926). Initial testing was conducted within 30 days of completion of the final surface course. Follow-up testing was conducted over the next two years on each of the sites to determine whether the roughness had changed since initial testing and significant traffic had been introduced.
A total of 22 sites consisting of 185 lane-miles were tested for roughness after completion of paving. The majority of the sites were located on interstate routes (18 sites), while the remainder were located on four-lane divided primary routes (four sites). All of the interstate sites and one primary site consisted of the 12.5-mm surface mix (SMA 12.5). The remaining three primary sites consisted of the new 9.5-mm surface mix (SMA 9.5).
Ride-quality testing conducted on the sites showed a wide range in achieved smoothness. Table 1 summarizes the measured IRI for each mix classification. The weighted statewide average was 66 in./mile. The highest (87 in./mile) and lowest (46 in./mile) average project IRI values were found among the three projects (and 26 miles) that represented the SMA 9.5 (70-22) mixes. The projects that used SMA 12.5 (70-22) did not exhibit the broad range of IRI values (87 to 61 in./mile), but did represent the roughest general category of surface. Interestingly, the VDOT statewide maintenance award for asphalt production and placement was given to the SMA site with an average ride quality of 46 in./mile.
For the contractor, this was their first experience with SMA. Their attention to detail during production and placement proved SMA could be produced as well or better than Superpave mixes.
In January 2004, follow-up testing was conducted on the SMA sites to determine if changes had taken place since initial testing due to traffic and environmental loading. The results show a slight increase in roughness since final paving (see Figure 1). This increase in IRI was expected and partially due to the difference between monitor and ride-spec testing. Only one testing pass was made for monitoring; two passes are made for ride-spec testing and the lowest value for each 0.01-mile section is used for averaging purposes. The IRI based on monitoring increased from 66 to 68, or about 4%. The difference between the original ride spec and monitoring tests was more pronounced for the 9.5-mm mixes, increasing 6% from 61 to 65, while the 12.5-mm mixes increased in IRI by 2% from 67 to 68. Overall, the ride quality had not changed significantly since initial placement.
After January 2004, the SMA sites were tested three more times—June and December 2004 and July 2005. A general trend (see Figure 1) showed a slight increase in roughness over the two-year period. The SMA 12.5 (76-22) remained the smoothest mix and the SMA 12.5 (70-22) stayed the roughest. Except for the SMA 9.5 (70-22), the 24-month change in IRI was less than 3 in. per mile. This minimal change was exceptional given the high levels of traffic these sites experience.
Skid
With the high AC content and high film thickness of SMA mixes, early-age skid resistance was a concern. Friction testing was conducted on the SMA sites beginning in September and October 2003 and continued through July 2005.
Figure 2 depicts the average friction measurements (skid number, or SN) for the first 20 to 24 months of service life of the 2003 SMA surface mixes. Initial tests indicated lower numbers for the 9.5-mm mixes (average SN=34.0) as compared to the 12.5-mm mixes (average SN=44.1). One major difference between the SMA 9.5 and SMA 12.5 mixes was the minimum required AC content. The minimum design AC content for the SMA 9.5 was specified at 6.8% compared to 6.5% for the SMA 12.5. For most of the aggregate quarried in Virginia, the higher AC content was not a concern. However, with the higher aggregate specific gravities in the northern part of Virginia where the SMA 9.5 was used, contractors had difficulty meeting the minimum AC requirement and voids in total mix (2-4%) during production.
Over the next 18 months, the average friction had increased for each mix. The largest increase over the shortest time frame occurred with the SMA 9.5 mixes between October and December 2003. Based on visual observations, the traffic wore off the excessive surface asphalt exposing the coarse aggregate. After reaching a peak friction value in May 2004, the average value was lower in Fall 2004 (September/October) and Summer 2005 (July). This drop off in friction corresponded with more mastic noted on the surface in the wheel paths. Therefore, additional field testing was performed.
Coring was conducted in each wheel path and the center of the lane for comparison purposes. For the two separate locations investigated, the in-place field density ranged from 96.7% to 97.8% in the wheel paths. This density was 1% to 3% higher than the center of the lane not exposed to traffic. Slight rutting (1?16 in.) was measured in each wheel path; this was attributed to the additional densification from the traffic (average density at time of construction ranged from 94% to 96% of maximum theoretical). Visual inspection of the cores showed good stone-on-stone contact. Based on these experiences, the minimum AC content has since been lowered to 6.3% in an effort to recognize the impact of aggregate specific gravity.
As proven with the ride quality, SMA can provide friction equivalent to Superpave mixes. Once the asphalt film on the aggregate exposed to traffic was removed, the SMA friction after paving increased. To investigate the longer-term friction characteristics of SMA and Superpave mixes, VDOT has tracked the skid resistance of 90 sites statewide since construction. Both types of mixes follow a seasonal trend, with SMA having an equivalent skid resistance. Figure 3 displays data from three VDOT districts (31 sites) where SMA has been used prior to 2003. While SMA has a higher AC content, skid should not be a concern when produced and placed correctly.
Over 400,000 served
SMA is a premier asphalt material. Many transportation agencies around the U.S. and the world have increased the use of SMA. VDOT has successfully used SMA on high-volume interstate highways since 1993. With a few exceptions, all of these SMA sites are still in service today. From the recorded and observed performance of these early SMA sites, VDOT expanded the SMA program in 2003. This expansion exposed many contractors and VDOT personnel to SMA for the first time. Despite a normal assortment of “growing pains,” the results of the ride and skid testing confirm good early-age performance of SMA.
Since 2002, Virginia has expanded the use of SMA on construction and maintenance overlay projects. From just over 150,000 tons awarded in 2003, VDOT has increased the use of SMA to almost 400,000 tons in 2005. Now a recognized national challenge is to see how SMA fits into the new mechanistic-empirical pavement design guide. VDOT will continue to monitor the performance of SMA over time.
