By Robert Horan, P.E.
There was a time when it was fairly common to go to a conference and hear a presentation about how to build a high-performance intersection that would stand up to the combination of heavy loading and slow or stopped trucks. That was a popular presentation topic at that time because many agencies were struggling with the challenge of coming up with the rut resistant asphalt mixtures required at those locations.
It is less common to hear a presentation like that now, and the reason is simple. With the adoption of PG binder grading system and Superpave mixture design procedures (as well as mixture performance testing) in the mid-1990s, agencies now have the tools to design and construct asphalt mixtures that are both stable enough to dramatically reduce rutting even in extreme loading scenarios at intersections and durable enough to have long service lives.
The West Virginia Department of Highways (WVDOH) tackled the challenge of solving a big problem of early and severe rutting at heavily loaded intersections on coal hauling routes. The results are impressive.
Recently, representatives of Asphalt Institute and WVDOH made a site visit to evaluate the performance of a high-performance asphalt pavement intersection that was built in July 2002.
The location is at the intersection of US 52 and WV 37 in Wayne County where two coal hauling routes come together – known as the Fort Gay intersection.
A 2002 “Asphalt” magazine article previously told the story of the intersection with its extreme loading conditions. WVDOH engineers said that the intersection probably had the heaviest vehicle loading in the state and that there was a history of early failures of the previous efforts to address severe rutting problem using mill and overlay with standard mixes containing unmodified binders. This approach had repeatedly resulted in severe rutting at the intersection within 6-9 months after construction. WVDOH decided that a different approach to solving this rutting problem was needed.
The 2002 project included full-depth rehabilitation of some areas with a 37.5 mm base mix and a 12.5 mm surface mix where the channelized rutting was the worst and a two-inch mill and fill overlay using 12.5 mm surface mix for the rest of the intersection.
While preparing the project in 2002, WVDOH worked with contractor Mountain Enterprises to design, produce and construct asphalt mixtures that could stand up to the extreme loading conditions on the project. A major change was WVDOH specifying the use of polymer modified asphalt (PMA) meeting the requirements of PG 76-22. The goal was to develop a rut resistant asphalt mixture that could be used to build a high-performance intersection at this location and could also be used at heavily loaded locations throughout the state.
For both the base and surface mixes, a combination of limestone aggregate and angular blast furnace slag was used in the mix design. The surface mix contained 95 percent slag. As part of the mix design process, WVDOH conducted laboratory rut testing using the Asphalt Pavement Analyzer (APA) on the Superpave Gyratory Compactor specimens to measure the rut resistance properties of both the base and surface mixes. The APA is lab testing equipment that simulates the application of a large number of heavy loadings on the lab compacted specimen. Both mixes easily met the 6 mm maximum rut depth criteria.
Fast forward 17 years
At the time of the visit, the intersection had been in place for almost 17 years and only minimal rutting had occurred despite being subjected to hundreds of very heavily loaded coal haul (slow-moving or stopped) trucks on a daily basis.
During the recent visit, rut measurements throughout the project found that rutting had not significantly worsened compared to measurements taken over ten years ago. No rut depths over 5/8” were measured even at the locations where the heavily loaded coal trucks would be stopped for extended periods of time. The average rut depth for all measurements was around 3/8”. However, moderate to severe cracking in the project pavement was noted throughout the project.
John Crane of the Materials Division of WVDOH is the lead technical person in the state when it comes to asphalt materials and pavements had this to say about the performance of the Fort Gay intersection:
“Untouched for seventeen years, the Fort Gay intersection is an amazing success story as WVDOH’s first project utilizing a highly modified asphalt binder. This success has confirmed the investment that the DOH made back in 2002 and solidified the use of highly modified asphalt mixes as a permanent tool in our toolbox. With all great stories, there are ups and down, and while the Fort Gay intersection has performed extremely well at resisting rutting, the seventeen years of coal truck after coal truck have not been kind to it. Over the last few years, the two-inch overlay has begun exhibiting environmental distresses and cracking throughout the entire section. It appears as though the time to preserve our investment is upon us so that we may keep the success of the Fort Gay intersection alive.”
Crane continued: “Throughout West Virginia, we see PG 64E-22 (new name for PG 76-22) mixtures being used to combat similar situations at other intersections and on heavily traveled roadways in hopes to extend the overlay’s life expectancy. Throughout the last five years, the WVDOH has constructed over 100 projects, roughly 30 of which included intersection rehabilitations, totaling over 525,000 tons of highly modified asphalt concrete.”
Stormy Brewster of Marathon Petroleum Company LP, the supplier of the polymer modified binder PG 76-22 binder for this project, is very pleased with the excellent performance.
“The Fort Gay intersection is a perfect example of why we use polymer modified construction materials on high volume roads. The road needed rut resistance and fatigue cracking resistance which is why it was necessary to specify a polymer modified PG76-22. Now a 17-year-old road, the WVDOH may have spent more money upfront but we have saved a lot in longevity. As has been demonstrated here and on many roads and labs across the world, polymer modified asphalt will increase durability and cracking resistance,” said Brewster.
The key takeaway from the excellent performance of the asphalt pavement at the Fort Gay intersection project? WVDOH was able to use a combination of polymer modified binder, a savvy mix design process and good construction practices to solve their rutting problems at heavily loaded intersections. To their credit, they have built on what they learned from that experience to solve similar problems throughout the Mountain State.
Horan is Asphalt Institute Senior Regional Engineer based in Virginia.