NCAT Anticipates Preservation in Sixth Test Cycle in 2015
By Buzz Powell, P.E.
June 2014 – Preservation Group research in pavement preservation technology at the National Center for Asphalt Technology on Lee Road will continue through March 2015, with the sixth cycle of testing at the track itself to begin later that year. NCAT researchers are optimistic that the 2012 Preservation Group study will continue as the “PG 15” study in the coming research cycle beginning in the summer of 2015.
Pavement preservation research at NCAT began in the summer of 2012, and was initiated and sponsored by seven state DOTs, and also underwritten by FP2 Inc. and its supporters.
The 2012 NCAT Pavement Test Track represents the fifth research cycle, with plans to rebuild the facility again in summer 2015. The focus of research at the track has logically grown in conjunction with NCAT’s expanding mission from just mix performance in the original 2000 research cycle, to both structural performance and pavement preservation in the current (2012) research cycle.
The inaugural NCAT Pavement Test Track (www.pavetrack.com) was completed in 2000 as a partnership between Auburn University and the Alabama DOT. The 309-acre site was purchased by the university, with subsequent improvements (including earthwork, buildings and robust track foundation) made by Alabama DOT.
This investment makes it possible for other state DOTs to fund the operation of the 1.7-mile track in a prorated manner through a pooled fund mechanism based upon the number of 200-ft. pavement test sections they choose to build and test in three-year project cycles.
A design lifetime of truck traffic (10 million equivalent single axle loadings, or ESALs) is then applied to the surface of experimental pavements in an accelerated manner using a fleet of heavy triple trailers. All non-steer axles in the triple trailer trains are loaded at the federal legal bridge limit in order to induce accelerated damage on a scale that can be directly related to open roadways.
The focus of research on experimental track pavements is either surface performance on perpetual foundations or structural performance on thinner, fully instrumented buildups. By comparing the performance of two or more sections, NCAT provides recommendations to member states on how to optimize performance (for example, via fighting rutting or cracking), and reduce the life cycle cost of their transportation infrastructures (for example, via optimized use of reclaimed/recycled materials or new binders/modifiers).
In recent years, state DOTs have been pressured by diminishing resources and expanding missions to broaden their thinking beyond traditional mill-and-fill pavement management philosophies. This has resulted in the implementation of decision trees that seek to match existing distresses with both traditional and nontraditional treatments that are objectively selected to minimize life cycle cost.
Although these decision trees are typically well thought out and logically constructed, insufficient life cycle performance data on treatment alternatives currently exist to reliably select the treatment that represents the best investment over the life of the pavement. That’s because many treatments on good roads may perform well, while the same treatments on bad roads may perform poorly. As a result, performance expectation ranges found in literature are too broad to be realistically inserted into decision trees and used for objective – that is, nonpolitical – treatment selections.
NCAT researchers noted that this limitation could be overcome by linking performance expectation to the pretreatment condition of the pavement. As a result, pavement preservation was added to the research plan for the 2012 NCAT Pavement Test Track.
Lee Road 159 Tests
Although a number of track test sections at the end of the 2009 research cycle had representative distresses that made them good candidates for pavement preservation research, it was decided that the scope of a properly designed experiment needed to encompass a thinner structural buildup with traffic that was not applied in an accelerated manner.
A search of local open roadways led researchers to Lee Road 159. This road was considered to be an ideal test candidate because it provides dead-end access to both a quarry and an asphalt plant. Traffic volumes are low with extremely high truck traffic.
After obtaining permission from the Lee County Commission to place pavement preservation test sections on Lee Road 159, data sharing agreements were secured with both the quarry and the asphalt plant to supply date, time, axle configuration, tare weight, and loaded weight for every truck that entered (usually empty) and exited (usually loaded) Lee Road 159. The existing 5½-inch flexible pavement was fully characterized for roughness, rutting, macrotexture, cracking, and deflection.
Pavement condition on Lee Road 159 was generally the best in the left (inside) wheelpath of the inbound lane, slightly worse in the left (inside) wheelpath of the outbound lane, slightly worse still in the right (outside) wheelpath of the inbound lane, and worst overall in the right (outside) wheelpath of the outbound lane.
A working partnership with professionals from FP2 – which ultimately became a funding partner in the 2012 Preservation Group study – produced a consensus experiment that was presented to representatives from sponsoring state DOTs, and select treatments were placed in the summer of 2012 with FP2 professionals onsite to provide technical support.
Now, traffic has been accumulating on all 25 test sections on Lee Road 159 since the summer of 2012. The annual ESAL rate resulting from trucks traveling unloaded into the quarry and the asphalt plant is a fraction of the accelerated damage rate (five million ESALs a year) on the NCAT Pavement Test Track itself.
In comparison, the annual ESAL rate from loaded trucks is substantially higher in the outbound lane than the inbound. While still heavy for a county road, the much lower rates on Lee Road 159 are expected to provide results that are more applicable to open roadways. Comparison sections on the track will then bridge the gap between accelerated and nonaccelerated traffic rates.
As of spring 2014, weekly performance testing has revealed the recurrence of at least some pretreatment distresses in only a small number of sections. Roughness, rutting, macrotexture, subgrade moisture, and deflections will continue to be measured on a regular basis until the end of the 2012 research cycle in March 2015.
The 2015 Research Cycle
Based on feedback from existing track sponsors and other state DOTs who have expressed an interest in joining the pooled fund, NCAT researchers are optimistic that the 2012 Preservation Group study will become the “PG 15” study in the coming 2015 research cycle.
It will be necessary to continue to monitor sections in which distresses do not fully recur (e.g., cracking, roughness, macrotexture, rutting, friction, etc.) in order to fully quantify life extending benefit curves. Points farther down the x-axis (meaning more traffic and time are required to induce a return to pretreatment condition) may represent the highest return on investment for preservation treatments that are the best performers.
In the 2015 research cycle, sponsors may choose to rebuild select sections in the more heavily trafficked outbound lane that have reached the point of full distress recurrence, 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 NCAT Pavement Test 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.
Representatives from state DOTs interested in having a voice in the PG15 study by joining the pooled fund for the 2015 research cycle are encouraged to contact Dr. Buzz Powell at firstname.lastname@example.org.
Powell is assistant director and Test Track manager, National Center for Asphalt Technology