NYC Uses a High Performance Thin Overlay to Solve a Hard Problem

New York City’s First Avenue in Manhattan is now safer for vehicles, bicycles and pedestrians, and quieter for residents thanks, in part, to a new surface of High Performance Thin Overlay (HPTO) mix with a highly polymer-modified asphalt binder. The $7 million resurfacing project is part of a $6 billion NYC DOT investment in nearly 1,000 projects over the past six years to keep road infrastructure in a state of good repair.

First Avenue is a northbound one-way street and handles much of the northbound truck traffic through Manhattan. It was re-paved in 1983 with an 1894 thick Reinforced Portland Cement Concrete pavement (RPCCP). The pavement was the first new concrete pavement in NYC in 30 years and had been touted to have a design life of 40 years. After 30 years, the pavement was in very poor condition, and the NYC DOT began searching for a repair solution. The cost of removing and replacing the 1894thick RPCCP pavement was beyond the NYC DOT budget. In addition, aged water, gas, sewer and steam lines beneath First Avenue would likely break during the RPCCP removal process. Repairing and replacing utilities would have only added to the prohibitively expensive project.

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First Avenue-February 2012

Curb clearances prevented the use of a thick asphalt mix overlay and traditional thin asphalt overlays would not last long on the heavily cracked concrete surface with the continual stop-and-go bus and truck traffic. The NYC DOT contacted Axeon Specialty Products (formerly NuStar Asphalt) in 2012 and asked for recommendations for repair options. Axeon SP and Rutgers University had developed a High Performance Thin Overlay (HPTO) mix in 2005. The mix design differs from a Superpave 4.75mm mix. It is slightly gap-graded and has a minimum asphalt content of 7% to ensure good fatigue resistance. NJDOT adopted the HPTO design and placed it on I-295 in2007. It has performed very well to date.

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NJ I-295 with HPTO Overlay

Axeon SP proposed placing a one-block test section of a 1 BD94 thick HPTO overlay over the existing First Avenue pavement, after crack-sealing and patching as required. The asphalt binder chosen was a conventional Polymer-Modified Asphalt (PMA), grading as PG 76-28. In addition, they proposed a second test section using the HPTO mix with a high performance asphalt binder developed by Kraton Polymers for even greater rutting and cracking resistance. This Highly Modified Asphalt (HiMA) asphalt binder had a continuous grading of PG 95-31.

Laboratory testing at Rutgers University showed that both HPTO mixes performed much better in rut and fatigue resistance than conventional Superpave mixes. However, the HPTO mix with HiMA binder performed much better than the HPTO mix with conventional PMA (labeled as FlexGard in the research report).

chart 1

Figure 1 shows the rut resistance of the two mixes, as compared by the AMPT Flow Number. The HiMA mixes, both aged and unaged, had flow numbers 67% greater than the conventional FlexGard asphalt. Current recommendations require a minimum Flow Number ? 740 for traffic levels > 30 million ESALS.

chart 2

The Texas Overlay Tester is a severe test that simulates the horizontal movement at PCC pavement joints and TXDOT specifications typically require a minimum of 300 cycles. The standard NYC DOT 9.5mm mix completed only 39 cycles. Both HPTO mixes far exceed the TXDOT requirements, but the HiMA mix more than doubled the number of cycles of the conventional PMA mix.

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LTOA=Long Term Oven AgedA0
Figure 3: Flexural Beam Fatigue Test Results

The Flexural Beam Fatigue Test is a measure of crack resistance to movement in the vertical direction. A conventional 12.5mm mix with PG 76-22 polymer-modified asphalt fails at only 1,200 cycles at a strain level of 900 micro-strain. In comparison, the HPTO mix with FlexGard fails at 100,000 cycles and the HPTO mix with HiMA fails at 300,000 cycles.

As a trial, NYC DOT paved two blocks of First Avenue from 100th to 102nd Streets in September 2012. The two blocks included one with a conventional PG 76-28 PMA binder and one with the HiMA binder. In the ensuing year, both sections performed well (no observed cracking or rutting). The DOT decided to use the HPTO design for the full length of the project, from 72nd Street to 125th Street and, based on the laboratory data, chose to use the HiMA binder.

The NYC DOT chose to first micro-mill the entire project to improve the ride and restore the proper profile. This was followed by crack sealing and patching with highly-modified Crafco PolyPatch material.

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Micro-Milled Surface with Crack Sealing and Patching

Trying to add another level of performance, the NYC DOT also decided to install a reinforced paving fabric prior to paving. They selected Mirafi PGMG4, which is reinforced longitudinally, transversely and diagonally. PG 76-22 polymer modified asphalt was applied as the tack coat, followed by the fabric installation.

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Paving HPTO Mix on Paving Fabric

Finally, the 1 BD94 layer of HPTO mix with HiMA asphalt was placed and compacted. EvothermAE warm mix additive was added to the HiMA binder to lower mix temperatures, improve workability and increase density. Despite the 7.5% polymer content in the HiMA asphalt, the mix was produced at the Willets Point Asphalt plant at 290-310B0F. The NYC DOT paving crew placed the mix and performed necessary handwork around innumerable manholes and utility boxes with ease. One vibratory pass, followed by 4-5 static passes were sufficient to achieve 95% density. The finished project has an excellent appearance and ride. It took the city only three weeks of night paving to apply 10,000 tons of HPTO and HiMA technology to 53 blocks of concrete, which allowed streets to remain open during the day versus shutting them down for several months and interrupting vital transportation systems.

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Completed First Avenue

HPTO and HiMA technology are expected to enhance the life of New York City’s resurfaced streets at tremendous savings compared to removal and replacement of the existing PCC pavement. The repaved streets will also help protect the city’s vital underground utility infrastructure, which is a key safety benefit. In addition to an attractive appearance, the new, smooth surface will give motorists and cyclists a much better ride, facilitate pedestrian and handicap crossings, and reduce noise for residents.

Ronald Corun,A0Director Asphalt Technical Services andA0Robert Q. Kluttz,A0Staff Research Chemist atA0Kraton Polymers