by Ron Corun and John Davis
Central Park, located in the heart of Manhattan in New York City, has been offering recreational activities to millions of visitors since 1857. The Park consists of 843 acres and offers numerous attractions including hiking, biking, rollerblading, a zoo, boating, athletic fields, concerts and horse-drawn carriage rides.
The horse-drawn carriages have caused pavement maintenance problems for the New York City Department of Transportation (NYC DOT) for many years. The carriage wheels are very narrow and are covered with solid rubber, which creates high-stress point loadings and severe rutting in the asphalt roads inside the park.
In addition, puddles of horse urine cause pavement deterioration, which result in potholes. Central Park is the starting point for the New York City marathon and the NYC DOT wants the pavement to look good when the cameras covering the race are focused on it. To ensure attractive pavement photos, the DOT has repaved portions of Central Park roads every year for ten years.
In 2007, NYC DOT was in search of a solution to this problem. During a discussion of the issue with one of the hot mix asphalt (HMA) suppliers for the city, Willets Point Asphalt, they were told about a recent paving project at New York’s La Guardia Airport. The airport had been experiencing damage to its asphalt pavements from jet fuel spills and Willets Point had supplied a fuel-resistant asphalt mix using a new polymer-modified (PMA), fuel-resistant asphalt binder.
The project had gone well and Ken Tully, President of Willets Point Asphalt, suggested to the NYC DOT that the new binder might also resist horse urine. The city contacted the producer of the new binder, NuStar Asphalt, and testing was performed at Rutgers University to determine if this product would resist horse urine. The asphalt binder had already been tested by soaking a compacted mix sample in jet fuel for 24 hours and measuring weight loss after removal. The weight loss had to be less than 1 percent to demonstrate fuel resistance. The same test was repeated using horse urine. The sample showed no weight loss or degradation and the NYC DOT decided to try the new product in Central Park.
The project included milling 2 inches of the existing asphalt pavement and repaving it with 2 inches of a New York State DOT 6F mix containing StellarFlex FR asphalt binder. The 6F mix is a ¾-inch maximum aggregate size Marshall Mix Design mix. It is a heavily polymer-modified asphalt (PMA) that is classified PG 94-22. The mix was designed as a 50-blow Marshall mix with target air voids of 2.5 percent. The Marshall Mix design was modified to produce an impermeable asphalt pavement that would not allow the intrusion of horse urine and would maximize the asphalt content, which increases the durability of the mix. The revised Marshall criteria resulted in a mix with a design asphalt content of 7.2 percent.
NYC DOT paves approximately one million tons of HMA per year with city crews and equipment. A city-owned HMA plant produces the majority of the mix, but the DOT also relies on contractor-owned plants to supply the remaining mix. Willets Point Asphalt was chosen to supply the 5,000 tons of mix for the Central Park project because of their experience with the fuel-resistant material at La Guardia airport.
All work was performed at night to minimize the impact on the park?s visitors. Work hours were from 9 p.m. to 6 a.m. Monday through Thursday. NYC crews performed all the milling, cleaning and paving operations. The city paving crew had no prior experience with polymer-modified asphalt so NuStar gave them a short training course on working with PMA before they started paving. The crew adapted quickly and, despite no experience with the PMA binder, said it was one of the best asphalt mixes they ever placed. The mix was produced at the Willets Point plant in Queens, New York, at approximately 335ºF and hauled to the Central Park paving site by NYC DOT trucks. The mix was placed at 325ºF by paver and by hand without any problems. The city crews used two double-drum vibratory rollers to compact the mix. Pavement cores showed an average in-place density of 96.8 percent of theoretical maximum specific gravity. Overall pavement appearance and smoothness were excellent.
The project began at 72nd Street and Central Park West and meandered south to 59th Street to the park entrance at Central Park South. Curious residents and visitors followed the paving crew every evening during the project. Bicyclists and rollerbladers tested the pavement the day after paving and were pleased with the new surface. “Because I’m commuting by bicycle to work every day, you can be sure that I’ll be loving the smooth ride and the new surface,” said one bicyclist.
NYC DOT officials have also been pleased with the new pavement. After four years, the fuel -and urine-resistant mix is in excellent condition. There is no evidence of rutting or deterioration from horse urine. “The need to repave Central Park every year has been eliminated, and despite the additional cost of polymer-modified asphalt, it has proven to be a cost-effective solution,” said one NYC DOT official.
The paths must stand up to “a very aggressive chemical environment,” says Galileo Orlando, Assistant Commissioner in New York City’s Transportation Department. “So far, so good,” says Orlando. “The PMA is more expensive upfront, but when you have better pavement life, the higher cost is justified.”
The 2007 Central Park paving project has convinced the NYC DOT that a PMA-engineered asphalt mix can provide a high performance pavement, and the DOT has indicated that they will pave another section in 2011 with the same mix. Asphalt pavements and horse-drawn carriages can indeed work successfully together to provide a memorable ride through Central Park for millions of visitors.
Ron Corun is Technical Support Manager for NuStar Asphalt Refining
John Davis is a contributor to Asphaltmagazine.com