In this second installment of our Pave the Way series, we bring you the story of Associated Asphalt’s StellarFlex FR®. I recently sat down with Ron Corun and John Mims from Associated to chat about their fuel-resistant binder. Over the course of our conversation, they told me the story of Associated Asphalt’s unique solution to an incredibly high-stakes problem: degradation of airport pavements.
Margin of Error: Slim to None
We take planes for granted these days, but if you consider what happens every time a plane lands, it’s really quite remarkable.
Via a carefully engineered balance of aerodynamic forces, a 400-ton behemoth of metal and cargo descends from the sky, touching down on an asphalt runway at 180 miles an hour. This happens tens of thousands of times a day, every day, for passenger and freight lines alike. After landing, these planes are unloaded, refueled, repaired, reloaded, and then they’re back up in the air for their next trip. Our economy, our defense, and Amazon two-day delivery depend on this process going smoothly.
In short: the stakes are high and the room for error is slim to none.
On Shaky Ground
Unfortunately for airports, there are two aggressive forces that cause their pavements to quickly deteriorate: the enormous weight of slow-moving aircraft and exposure to the corrosive chemicals found in jet fuels. These forces cause raveling, rutting, and the creation of foreign object debris (FOD).
FOD is defined as any foreign thing that can harm an airplane- including birds, trash, and loose bits of gravel. It’s critical to keep FOD away from planes, especially from points of vulnerability like the engines. FOD is responsible for irreparable damage to engines, disastrous crashes, and even forced landings like the Miracle on the Hudson.
Unfortunately, petroleum products like jet fuel and hydraulic oil are like acid to asphalt- they eat right through it, turning the once sturdy pavement into a crumbling mess that subsequently becomes FOD. Areas where planes are refueled or parked are particularly vulnerable to damage because this is where spills are most likely to happen. Because these petrol derivatives are chemically compatible with asphalt, spills can weaken the bonds in the binder, causing it to detach from the aggregate. This aggregate becomes FOD.
Coal Tar Band-Aid
Before StellarFlex FR®, the go-to solution for preserving airport pavements was coal tar sealant. It technically did the job- after application, FOD was reduced, cracking and raveling were slowed, and the life of the pavement was extended for a bit. But coal tar actually causes cracking over time- because it expands and contracts at a different rate than asphalt, coal tar ends up damaging the very pavement that it’s initially applied to protect. It doesn’t solve the problem so much as it delays it by a little.
To make matters worse, coal tar is carcinogenic. Around the time StellarFlex FR® was being developed, several states had already outlawed its use and it looked like it would be illegal nationwide soon enough. Because of these issues, the FAA was looking into coal tar alternatives by the early 2000s.
A Star is Born
Luckily, a solution was on the horizon, thanks to the development of their fuel-resistant binder. After a series of problems caused by damaged pavements- including an incident where a plane got stuck in a rut so deep that it had to be towed out by a tug– the NY/NJ Port Authority was looking for a better way of constructing and maintaining their airfield.
In order to make a pavement that wouldn’t soften, rut, ravel, or crack, Associated knew they had to account for two destructive forces: the massive weight of the planes and the corrosive chemicals to which airport pavements are frequently exposed. Their solution was a highly polymer-modified asphalt binder that would resist the deleterious effects of jet fuel while also bringing increased crack resistance to the airfield pavement. This new product became known as StellarFlex FR®– the Fuel Resistant (FR) version of their StellarFlex® line of products.
In 2002, they paved their first job at LaGuardia, reconstructing a taxiway that was in need of repair. They milled out two inches of pavement, and then paved two inches of a P-401 mix with their product. One year later, the team went back to LaGuardia to see how the project was performing
It was in perfect condition.
Sixteen years later, a pavement survey of the entire LaGuardia airport was conducted to do an assessment of all airfield pavements. Out of all the pavements the team looked at, only the taxiway that had StellarFlex FR® was not rutted.
The next job came in 2004, at Logan Airport in Boston, MA. Engineers at Logan were looking for a solution to a particularly bad problem with their pavement. Planes taking off from Runway 4L-22R had to make a sharp 180° turn to get from the taxiway to the runway. The wheels of these massive jets grinding into the pavement were causing major deformations- pushing and shoving so badly that they were having to repave it every year.
The Massachusetts Port Authority (MassPort) adopted a spec that would include StellarFlex FR®. The problem area at Logan was paved, and everyone waited to see if this new product would withstand the elements. A year later the turn at Runway 4L-22R was in perfect condition!
Logan chose to use StellarFlex FR® again to deal with a different problem: airport pavements getting destroyed by the de-icing chemicals used on their planes. Logan doesn’t have a dedicated de-icing area; planes are de-iced in the alleyways near the gates. The de-icing chemical they used is glycol-based, like antifreeze, and it corrodes the asphalt binder. That, combined with fuel spills that occasionally happen in the alleys, made them extremely susceptible to damage.
Logan Airport chose to pave the alleyways with StellarFlex FR® in 2005. Just like the other pavements using this binder, the alleyways held up wonderfully. Logan ended up paving three alleyways in three consecutive years because they were so impressed with its performance.
In 2014, the pavements at Logan airport were examined closely for damage. They found that the asphalt was almost perfect. The problematic 180° turn on Runway 4L-22R wasn’t damaged by the turning aircraft- a pavement that had previously been replaced annually was now ten years old and pristine. The alleyways looked fantastic as well. Other than sealing joints in the pavement, no repair or preventative work had been done since the initial paving a decade earlier.
Based on the success of this job, Associated Asphalt decided it was time to scale up and start moving StellarFlex FR® to more markets.
A Spec-tacular Change
Because the Federal Aviation Administration (FAA) was already looking for a coal tar alternative in the early 2000s, StellarFlex FR®️ could not have been developed at a better time.
Associated Asphalt tweaked the existing FAA P-401 specification in order to get more liquid asphalt into the mix. This does two things: 1) it makes the mix impermeable to fuel spills and 2) it makes the mix more durable and crack resistant. This specification was proposed to MassPort, who adopted its usage.
The FAA is known to be very tough when it comes to changing their specifications. Because of the extremely high stakes of airfield paving, the FAA is rightfully cautious to adopt new technologies. But after the successful jobs at Logan and LaGuardia, the FAA updated their specifications and adopted a version of the MassPort spec into their own guidelines. The fact that the FAA adopted a spec around this product is an incredible testament to the efficacy of StellarFlex FR®.
Proof in the Pavement
The FAA was persuaded to change the spec in part because of the well-documented success of the product, and its endurance through rigorous testing.
One study, conducted at Rutgers University, ran multiple tests comparing the standard pavement at Logan Airport with StellarFlex FR® (referred to in the report as P601). The results revealed a lot in its favor. First, they found that it was able to achieve better stiffness properties than the standard Logan Airport asphalt. Next, when simulating approximately ten years of service life, it showed lower age hardening than the Logan asphalt. Finally, using the AMPT Repeated Load Flow Number test, StellarFlex FR® was found to have better rutting resistance.
In their own labs, Associated Asphalt ran a test specifically designed to measure jet fuel resistance. In this test, Marshall samples of asphalt are weighed, then submerged in jet fuel for 24 hours. The samples are weighed again after the 24 hours is up. The difference in the pre- and post-test weight indicates how much binder was dissolved by the jet fuel. Typical loss for normal asphalt is about 10%. StellarFlex FR® had a maximum of 1.5% loss, with averages usually closer to 0.5 to 1%. This test gives proof that it is, as the name says, fuel resistant.
StellarFlex FR® also shows promising results when predictive models are used. The FAA has their own pavement design software called FAARFIELD which they use to build and test hypothetical pavements. When comparing SFFR to a normal P401 mix, FAARFIELD predicted a ten-year increase in pavement life, from 20 to 30 years.
StellarFlex FR® has also been utilized in New York City’s Central Park. The NYC Marathon ends in Central Park, in an area that is also used by the horse-drawn carriages that give rides through the park. The carriages, while charming, do a fair amount of damage to the pavements they use. The narrow carriage wheels caused severe rutting, and the urine left behind by the horses caused potholes. These road hazards posed serious risks to the marathon runners.
To prevent injuries, New York was repaving this particular section of Central Park every single year- and had been doing so for a decade. NYC representatives voiced their frustration about this annual repaving project at a meeting with their asphalt mix supplier, who happened to be the same supplier that had worked on the project at LaGuardia. This supplier suggested that if StellarFlex FR® could withstand the destructive power of jet fuel and heavy airplanes, maybe it was tough enough to withstand carriages and horse urine as well.
NYC paved the problem area in Central Park with a mix containing StellarFlex FR® in 2007. In the 12 years since this project was completed, the pavement has not been repaved and is still in excellent condition.
After seeing the success NYC found using StellarFlex FR® to protect their pavements from horse-drawn carriages, the city of St. Augustine, FL was inspired to try it as well. St. Augustine is a tourist town, and there are horse-and-carriage rides that visitors can take around town. Unfortunately, the urine deposited by the horses softens and ultimately degrades the asphalt. St. Augustine repaved the streets commonly used by the horses with StellarFlex FR® and saw a marked improvement in pavement life. In fact, after experiencing the long-term benefits to this historic city, they have utilized this binder on three separate occasions, in three different locations. Now the people of St. Augustine can enjoy a smooth ride- whether it be by car or carriage.
The proven longevity of StellarFlex FR®, along with the FAA’s vote of confidence, has paved the way for its growth.
Since their initial jobs in the early 2000s, Associated Asphalt’s binder has been utilized on a runway in Charlotte, a freight airfield in Baltimore, aprons in Portland, Maine, an airport in Hollywood-Burbank, California, an airfield in Philadelphia, and numerous other general aviation projects. Their team is currently working on increasing the distribution and availability of StellarFlex FR®️ beyond their current east coast market. To make this product available to airports all over the country, they are working on growing the number of business partners that can manufacture and sell this product.
StellarFlex FR® continues to be popular in general aviation airports, and it might be coming to Department of Defense airfields soon. Associated Asphalt is currently in talks with the DOD to get a spec written that would include their product. A project was completed at Hulbert airfield in Florida and is currently being monitored by the Army Corps of Engineers. Pending the success of this job, the DOD will consider moving forward with a StellarFlex FR® spec of their own.
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