Fuel-Resistant Asphalt Binder Resists Asphalt Breakdown in St. Augustine
BY Asphalt Testing Solutions & Engineering LLC
FDOT uses StellarFlex FR asphalt binder and Evotherm M-1 warm mix asphalt technology to combat pavement issues presented by horse-drawn carriage traffic.
One of the main attractions in St. Augustine, Florida, is the horse and carriage rides. Every day the horse-drawn carriages line up on State Road 5A (SR 5A) along the bayfront waiting for visitors to book a historic, romantic or haunted tour around the city. The tours follow the same routes day after day in a concentrated area, so the pavement is highly abused. The chemistry of the horses’ urine softens and breaks down the asphalt. The combination of the soft asphalt, the concentrated loading of the carriage wheels and horse-shoed hooves, creates an abusive environment for the pavement. It’s ideal for rutting.
The Florida Department of Transportation (FDOT) sought a solution to extend the pavement’s lifespan. During its research, FDOT learned about a job completed in Central Park (New York City) in 2007. The carriage rides through Central Park caused the same issues to the pavement that SR 5A in St. Augustine was experiencing. The New York City Department of Transportation used a fuel-resistant asphalt binder, which it found also resists the asphalt breakdown from the horse urine.
FDOT decided to use the same asphalt binder in St. Augustine on SR 5A on two projects; a maintenance job in 2015 and a reconstruction job in 2019. The fuel-resistant asphalt mix contained StellarFlex FR asphalt binder (PG88-22FR) from NuStar Asphalt, now Associated Asphalt, as well as Evotherm M-1 Warm Mix Asphalt technology and anti-strip from Ingevity.
The components already have history. For example, StellarFlex FR was first used in the United States in 2002 at La Guardia airport on Taxiway GG. Asphalt binder can dissolve when exposed to the lighter-end fuels such as diesels and jet-fuels. Since then FAA Standard Specifications for the Construction of Airports Item P-601 (2014)—which was updated to P-404 in 2018—outlines the requirements for a fuel-resistant asphalt mix pavement where pavements are likely subjected to fuel spills. The first section of pavement constructed in 2002 at LaGuardia is still in service today. After a pavement survey of the entire airport was conducted fall 2018, Taxiway GG was the only portion of the airport not rutted.
Ingevity’s Evotherm M-1 Warm Mix Asphalt technology and anti-strip was used in the mix as a dual-purpose additive. The warm-mix technology is designed to promote a smooth, uniform texture of the pavement surface and acts as a compaction aid, providing additional construction time with thin lifts and in cool weather.
For the SR 5A projects, the fine friction course mix was designed to target 2.5 percent air voids. The modified Superpave design is intended to create an impermeable asphalt pavement that resists the horse urine from penetrating the asphalt pavement and maximize the asphalt content (6.3 percent); both of which increase the durability of the mix.
Steve McReynolds, director of operations for Asphalt Testing Solutions & Engineering LLC (ATS), formerly with FDOT, contributed to the development of the fuel-resistant specifications and determined the limits for the SR 5A maintenance project in 2015. “Fuel-resistant mixes are typically used at airports and weigh stations; anywhere fuel spills are common,” he said. “It’s unusual to use a fuel-resistant mix on a main roadway, so adjustments were made to the specification for use on an artillery road as opposed to an airport.”
Duval Asphalt was awarded the SR 5A maintenance job in 2015. The purpose of the maintenance job was to test the performance of the fuel-resistant mix. The project included milling 1.5 inches of friction course and replacing it with the new fuel-resistant mix. Since this mix is considerably more expensive to create and anticipating a reconstruction project a few years later, McReynolds took care in determining the project limits. The horses follow a very specific route through the St. Augustine streets, and McReynolds mapped the routes carefully to ensure the fuel-resistant mix was only applied where needed.
The asphalt placed in 2015, as part of the maintenance project, held up so well under the horse and carriage conditions, FDOT approved a full reconstruction this year using the fuel resistant mix. Duval Asphalt was again awarded the project. ATS was hired for quality control with McReynolds overseeing mix production and placement on the job.
“I’m very familiar with the specs and the desired performance, since I helped develop these parameters the last time this mix was used on this road.”
Duval Asphalt milled a total of 3 inches and replaced 1.5 inches of structural course and 1.5 inches of friction course along SR 5A.
The fuel-resistant mix was placed over the horses’ paths and a typical FDOT 12.5-mm friction course mix was placed in the areas untouched by the horses. A total of 350 tons of fuel-resistant mix was placed. They used a material transfer vehicle (MTV) to keep the paving process continuously moving, keep the polymer-modified mix homogenous and to prevent segregation.
One 12-ton oscillating roller and an 8-ton finish roller were used to compact the mix. ATS worked closely with the roller operators to ensure smoothness and proper density was achieved. Lab testing of the cores confirmed an average 93.5 percent in-place density.
Performance testing was completed on the fuel-resistant mix to identify performance standards and predict the future of the pavement. ATS Pavement Materials Engineer Tanya Nash confirmed the lab performance results of the mix are positive.
“Since the performance distresses seen in the past were primarily rutting as a result of softening of the asphalt from the consistent exposure to the horses’ urine, rut testing and fuel-resistance testing was performed,” Nash said. “Both tests met the industry standard and the FDOT specification requirement, indicating that the mixture will resolve the performance issues of the past and ultimately perform well.”
Although the mix was developed to combat the rutting ultimately caused by the horses, residents and tourists alike will enjoy the new, smooth ride. With an average of 500,000 visitors to St. Johns County each month, shutting down lanes to pave is not an easy nor convenient feat. Although this mix has a high cost up front, in this particular application, it extends the pavement’s lifespan. Rather than requiring frequent maintenance and resurfacing, construction will take place approximately every 15 to 17 years. This helps alleviate the higher cost and inconvenience.
Asphalt pavement provides a smooth ride. Now, a smooth ride can be enjoyed via horse and carriage through the beautiful streets of the nation’s oldest city for many years to come.