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High-Volume Preservation Tests
Placed in New NCAT Phase

By Mary Robbins, Ph.D.

A stretch of U.S. 280 in Alabama – a route that sees an average of 17,000 vehicles per day – has been converted to a high-volume project site for the MnROAD-NCAT pavement preservation study, with the application of more than 30 treatments completed in September.

Started in 2012, the study was expanded to address national-level needs in the 2015 research cycle through a partnership with MnROAD within the NCAT Transportation Pooled Fund. This pavement preservation study, aimed at determining the life-extending and condition-improving benefits of pavement preservation treatments, grew under the MnROAD-NCAT partnership to encompass both cold and warm climates and low and high-volume routes. Sections on U.S. 280 serve as the higher-volume project site for the warm climate-side of the partnership, with cold climate test sections to be placed at MnROAD.

In the meantime, the low-volume route – Lee Road 159 in Auburn, Ala. – received preservation treatments in summer 2012, and continues to be monitored as part of the study.

Dual-Lane Highway

Located just a short drive from the NCAT Pavement Test Track between Salem and Opelika, Ala., the 4.6-mile long, dual-lane, high-volume project includes 46 tenth-mile test sections lining the westbound driving lane.

As part of NCAT Pavement Preservation Study, chips are placed by contractor Vance Brothers on U.S. 280 in summer 2015

While construction on 34 treated sections began in August, work on the high-volume site began well before any treatments were placed. Pretreatment condition data obtained earlier in the year enabled researchers to establish the layout of the sections along the route, carefully selecting the location of each treatment based on measured distresses.

Although thirty-four sections received some type of treatment or combination of treatments, six sections were left as untreated control sections. These control sections play a critical role in understanding condition improving benefits of the treatments. These control sections encompass the range of rutting, cracking, texture and roughness along the entire length of the project, which will allow researchers to more accurately assess the relative improvement observed in treated sections over those left untreated, and gain valuable insight into the optimal timing for placing preservation treatments.

Cold, Warm Climate Tests

An important aspect of the study is that the core treatments placed as part of the original experiment constructed in 2012 are replicated in both warm and cold climates, and on the low- and high-volume routes. Therefore many of the 23 treated sections included on Lee Road 159 were repeated on U.S. 280, and also will be placed in Minnesota on routes similar to Lee Road 159 and U.S. 280.

Treatments such as fog seal, crack sealing, FiberMat chip seal, chip seals applied as single-, double-, and triple-layer, scrub seals, single- and double-layers of micro surfacings, thin overlays, cape seals, and an ultra-thin bonded wearing course thin overlay also were repeated on U.S. 280.

Unique to the study on Lee Road 159 were the many different combinations of treatments, such as the use of crack sealing beneath a chip seal, and micro surfacing. Other combinations include cape seals that used micro surface atop a scrub seal, chip seal and FiberMat chip seal, hot-mix asphalt cape seal which placed a thin asphalt overlay on top of a FiberMat chip seal, and a thin overlay on top of a 100 percent recycled foamed inlay using a cold-central plant recycling process. In addition to the single treatment types, these unique combinations were also placed on U.S. 280.

IMAGE CREDIT: NCAT

Cracks are sealed on U.S. 280 to judge preservation treatment performance under high-volume traffic

While many of the 23 treatments or combinations of treatments placed on Lee Road 159, were replicated on U.S. 280 with some adjustments made for the change in the traffic-level, a number of additional treatments were also installed. The list of treatments was expanded based on a consensus agreement from the sponsors to include additional micro surfacing sections, cold recycling techniques, HMA cape seals, and to also include open-graded friction course thin overlays, and other new and unique combinations of treatments.

Additional micro surfacing sections included a micro surface with fibers, a highly-modified asphalt micro surface, and micro surfacing applied on top of a thin asphalt overlay. Although an HMA cape seal was placed on Lee Road 159, the concept was extended to include two additional surface treatments; one with a chip seal as the first layer, and the other with a scrub seal, and just as the HMA FiberMat cape seal was constructed at both sites, a thin overlay was placed on top ¾-in. thick.

Several OGFC thin overlays were placed at ¾-in., with varying bond (tack) coat types. Although the variants of a 4.75mm NMAS HMA used for the thin overlays were reduced from five placed on Lee Road 159 to two mixtures on U.S. 280 – virgin control and an aged binder ratio (ABR) mix – the virgin control mixture links the two roadways. With the exception of the 1-in. thick thin overlays placed on top of the cold recycled inlays, all of the thin overlays were placed on U.S. 280 at just ¾-in. thick. Also, new to the experiment, were sections placed using cold-central plant and cold-in-place recycling techniques. For each technology used, cold recycled mix was produced with both foamed and emulsified asphalt.

Back at the Track

Pavement preservation treatments were applied to 100-ft.-long sections at the NCAT Pavement Test Track, an accelerated pavement testing facility, in spring 2014 as part of this study, placing preservation treatments in a heavily-loaded traffic environment.

In addition, construction of two more sections on the NCAT Pavement Test Track was completed in September 2015, with the placement of two HMA cape seals along the north tangent of the oval track. Both sections feature a scrub seal as the first layer, and differ only by the 4.75 NMAS mixture placed on top. Two 100-ft.-long untreated control sections on the test track also were added to the study.

With construction on U.S. 280 and at the NCAT Pavement Test Track complete, efforts shift to monitoring the performance of each test section, and planning for the upcoming construction in Minnesota.

Data to develop life-extending benefit curves and to evaluate condition-improving benefits continue to be collected on the 100-ft.-long sections on Lee Road 159. Consistent with those measurements made on Lee Road 159, data also will be collected in the same manner on the tenth-mile sections at the complementary high-volume site.

Meanwhile, researchers at NCAT and MnROAD will be working together to support the development of life-extending benefit curves and evaluation of condition improving benefits in a cold climate as they prepare for construction of tenth-mile-long sections on both high and low volume sites in Minnesota in summer 2016. Researchers in Minnesota will be collecting pre-treatment condition data on each of the low and high-volume routes to develop a layout for each site that pairs treatments with the appropriate distresses and to strategically select untreated control sections to represent the ranges of pretreatment condition on each route.

State highway agencies interested in participating in this pavement preservation and recycling research should contact either Ben Worel (Ben.Worel@dot.state.mn.us) at Minnesota DOT, or Buzz Powell (buzz@auburn.edu) at NCAT for more information.

Robbins is assistant research professor, National Center for Asphalt Technology