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NCAT Plans to Expand Preservation Research in 2015 Research Cycle

By Buzz Powell, P.E.

September 2014 —  The NCAT Pavement Test Track is an accelerated performance test facility managed by the National Center for Asphalt Technology at Auburn University. Every three years, experimental pavements are built in 200-ft. test sections around the 1.7-mile NCAT test oval. After construction, five heavy triple trailer trains are used to compress a design lifetime of damage (10 million equivalent single axle loadings, or ESALs) into two years of fleet operations.

Findings from the first four research cycles at the NCAT Pavement Test Track have resulted in rapid implementation by state DOTs of improvements to asphalt mix design, as well as flexible pavement design and construction. A pavement preservation technology experiment on a local county road (Lee Road 159) was added to the research program for the fifth (2012) research cycle in order to quantify the life extending benefit of pavement preservation.

Truck traffic in/out of a quarry and an asphalt plant on the dead end county road induces pavement damage in real time at a fraction of the accelerated rate on the NCAT Pavement Test Track. Performance data from Lee Road 159 is now being used to construct objective and unique life extending benefit curves for select preservation treatments and treatment combinations.

The planning process now is underway for the sixth (2015) research cycle, in which the continuation of the successful work on Lee Road 159 will be complemented with longer pavement preservation sections on a higher ADT, lower truck traffic roadway. Longer sections will facilitate the inclusion of in-place recycling technologies.

Completion of the 2012 Research Cycle

Pavement preservation test sections on Lee Road 159 consist of individual treatments as well as treatment combinations. Examples of final surfaces (placed either as a standalone treatment or in combination with other underlying treatments) include chip seals (7), crack sealing only (1), micro surfacing (6), rejuvenating fog seal (1), and thin asphalt overlays (8). All treatments were placed with a very high level of quality control in order to prevent poor placement quality from confounding results.

Weekly distress measurements make it possible to quantify the life extending benefit for each treatment or combination of treatments by continuously comparing current and pretreatment conditions. All 23 treated sections, each 100 ft. long by two lanes wide, are subdivided into 45-ft. by 10-ft. subsections in which pavement condition is discretely monitored. The time/traffic needed to return each subsection to pretreatment condition is being quantified over the course of the experiment, meaning that each independent life extending benefit curve will ultimately consist of 40 data points.

The changing condition of the two untreated control sections also is measured on a weekly basis. The life cycle curve generated from these sections will be used to estimate the “untreated” condition of the 40 subsections within all 23 treated sections. The difference between the treated and estimated “untreated” data points are being used to construct condition improving benefit curves. These curves will quantify the benefit of pavement preservation compared to the “no action” option.

The benefits of pavement preservation are also being quantified in ways that may not be directly visible on the surface of the roadway. For example, probes are used to measure seasonal fluctuations in subgrade moisture content. Lower moisture contents are measured in wetter months in treated sections compared to the two untreated control sections. Additionally, new methodologies that only require small amounts of dry auger-recovered binder are being utilized to measure the effect of preservation on the age hardening of underlying asphalt binders.

Traffic to and from the quarry and the asphalt plant provides a load spectra that is both low ADT and high truck percentage. Although the pavement damage that results from this traffic/load distribution is in excess of 100,000 ESALs a year (in the more heavily loaded outbound lane), it may not be enough to produce a return to pretreatment condition of the more robust preservation treatments within a relatively short timeframe. For this reason, and to directly connect results between the two locations, select treatments on Lee Road 159 also were installed on the NCAT Pavement Test Track. In this manner, higher ESAL data points on the far right of the life-extending benefit curves will be achievable.

At the request of sponsoring state DOTs, a secondary product of the preservation research on Lee Road 159 will be recommended specifications and practices that equip specifying agencies to achieve the same high level of construction quality seen in this experiment. Existing resources will be used as the starting point for this process (e.g., ISSA documents) whenever possible, in order to avoid duplication of effort. Emphasis will be placed on practical, implementable methods. Additionally, a training program will be proposed that will be designed to promote successful, rapid implementation.

Planning for 2015 Cycle

Based on feedback from existing track sponsors and other state DOTs which have expressed an interest in joining the pooled fund, NCAT researchers are optimistic that the 2012 Preservation Group study will become the “PG15” study in the coming 2015 research cycle.

It will be necessary to continue to monitor sections that do not return to pretreatment condition (i.e., cracking, roughness, macrotexture, rutting, friction, etc.) in order to fully quantify life extending benefit curves.

In the 2015 research cycle, sponsors may choose to rebuild select sections in the more heavily trafficked outbound lane on Lee Road 159 that do return to pretreatment condition, leaving the more lightly trafficked inbound lane in place to capture as many points as possible on the life extending benefit curve.

Select sections could be rebuilt with a structure that is identical to one(s) previously tested on the track in order to directly connect the two test sites, then original preservation treatments and treatment combinations from the summer of 2012 could be reapplied in order to quantify the life-extending benefit of true pavement preservation.

Any and all options will be considered for the next research cycle that take full advantage of the “free” trucking and convenience of extended pavement preservation research on Lee Road 159.

Although the changing condition of the two control sections will allow for the estimation of the “untreated” condition of all the treated sections/subsections, it will only be an estimate. In order to precisely quantify the condition improving benefit, it would be necessary to directly compare treated sections to untreated controls that have a similar condition at the time the treatments are applied.

In order to address this research need to accurately quantify the condition improving benefit of pavement preservation, and to relate the work begun in the 2012 research cycle to higher ADT roadways with more diverse load spectra, another open roadway will be identified on which the pavement preservation treatments from Lee Road 159 will be installed in 1/10-mile test sections. This work is tentatively being referred to as the “Phase II” component of the “PG15” study.

The selected road will be a long, straight, relatively flat four-lane highway on which treatments can be applied in the outside (“truck”) lane behind traffic control maintained during construction by the Alabama DOT. This road will ideally be at least 10 years old so differences in pavement condition between treated and untreated sections can be expected within a few years. A staging area will be needed near the test area to facilitate material storage, equipment setup, rate calibrations, etc.

The placement of these sections will provide an opportunity to fill knowledge gaps identified in the findings from Lee Road 159. For example, extensive video will be shot of placement activities to facilitate training inspection personnel, and newly developed quality control tools will be proven. A formal process for micro surfacing proportion verification will be utilized. Additionally, material sampling and analysis methodologies will be test driven.

Noting that the NCAT Pavement Test Track has been approved for one hundred percent State Planning and Research (SP&R) funding, agencies interested in having a voice in the design of the experiment by joining the Transportation Pooled Fund to provide financial support for the PG15 study can send an email to buzz@auburn.edu.

Implementation of Research Findings

At the end of each three-year research cycle, NCAT hosts a track conference with a focus on implementation of practical research findings. This multi day event consists of test section inspections, targeted technical sessions, and the publication of a comprehensive synthesis report. With the addition of pavement preservation in the 2012 research cycle and plans for “Phase II” in 2015, the scope of the next track conference has been expanded to include implementable findings from Lee Road 159.

Designed to complement the Pavement Preservation & Recycling Summit in Paris the week before, the next track conference is scheduled to run from 8:00 a.m. March 3 through 12 noon March 5, 2015. Practitioners from government, industry and academia with an interest in asphalt mix design, flexible pavement design, pavement preservation, and/or APT are encouraged to attend this important event.

Registration information for the 2015 Track Conference can be found at http://www.ncat.us/newsroom/2015conference.html.

Powell is assistant director and Test Track manager, National Center for Asphalt Technology