Early NCAT Results Favor Preservation
Wednesday, September 27, 2017
Posted by: Kristi Olson
Reprinted with permission from the Fall 2017 issue of Pavement Preservation Journal of FP2 Inc.
By Dr. Buzz Powell, P.E.
| Two-year-old test sections on U.S. 280 in Alabama
Full-scale accelerated pavement testing (APT) has been conducted for predominantly southern states at the National Center for Asphalt Technology (NCAT) Pavement Test Track since 2000, while the Minnesota DOT’s Road Research Facility (MnROAD) has been conducting similar research for northern states since 1994.
These proven and practical APT facilities are for the first time officially engaged in a research partnership that is cooperatively funded by numerous state DOTs from all over the country for the purpose of executing flexible pavement studies targeting national research priorities.
The objective of the preservation group (PG15) experiment, in which FP2 Inc. and its allies are an equal funding partner with DOTs from 15 different states, is to quantify the benefits of pavement preservation on both low volume and high volume roadways with results that are practical and implementable in both northern and southern US climates. Funding is provided through the national Transportation Pooled Fund that supports the NCAT Pavement Test Track (http://www.pooledfund.org/Details/Study/496).
LOW, HIGH TRAFFIC TESTS
In order to objectively quantify the life-extending and condition-improving benefits of pavement preservation, test sections were built on local roadways near both facilities.
In the southern half of the experiment, low traffic test sections were built on Lee (County) Road 159 (near the main NCAT location in Auburn, Ala.) in summer 2012, and high traffic sections were built on U.S. 280 (near the NCAT Pavement Test Track in Opelika) in summer 2015.
In the northern half of the experiment, low traffic sections were built on County Road 8 and high traffic sections were built on U.S. 169 near Pease, Minn. (both about 45 minutes north of the main MnROAD facility) in summer 20016.
Field performance is monitored with similar automated technologies at both locations, and data is stored in a common database. As of Oct. 1, 2017, the number of ESALs supported since treatment placements will have totaled approximately 70,000, 790,000, and 1.3 million for Lee Road 159 northbound, Lee Road 159 southbound, and U.S. 280, respectively. Traffic volumes on the northern sections are not yet published, but are reported to be similar to those in the southern sections.
PRELIMINARY FINDINGS, OBSERVATIONS
Both low and high traffic sections in the NCAT southern preservation experiment continue to show excellent performance after five and two years of service, respectively.
The benefit of crack sealing as a standalone treatment -- and in combination with other treatments -- is evident in low, high, and accelerated traffic. Scrub seals have exhibited the combination benefit of crack sealing and chip sealing that is often promoted, an observation that is supported by monthly subgrade moisture content measurements.
Although multilayered chip seals have performed well in low traffic, some flushing and bleeding has been observed under high traffic. Micro surfacing has worked better in combination treatments, either as a double layer placement or as the riding surface on a cape seal.
Thinlays have performed well with both virgin and aged binder replacement designs. Aged-binder design thinlays on 100 percent RAP cold recycle binder layers are some of the best performing sections on U.S. 280.
After just one winter, every northern test section, both low and high traffic, has exhibited some amount of thermal crack reflection. An unexpected result from the northern half of the experiment is the good performance of permeable thinlays, which still look very good on the MnROAD low volume loop as a thin overlay on both flexible and rigid pavement. That's significant as northern states will not use permeable thinlays out of concern for freezing conditions and snow plow durability.
QUANTIFYING PRESERVATION BENEFITS
Ultimately, the life-extending and condition-improving benefit of the various treatments and treatment combinations will be quantified in terms of pretreatment MAP-21 performance measures.
||Life-extending benefit using MAP-21 criteria to band pretreatment condition; banding refers to grouping pretreatment cracking into good, fair, and poor ranges as defined by MAP-21. All the subsections with less than 5 percent of the area cracked prior to treatment are averaged to produce a single life extending benefit curve; the same is done for fair and poor ranges
Reduction in cracking is shown as an example; however, the methodology will also be applied to both roughness and rutting. The main implementable product will be life extending and condition improving benefit curves for low, high, and accelerated traffic in both hot and cold climates.
State agencies can then interpolate between these plots for their particular climate and traffic level. It is expected that pavement condition data, which is a relatively low cost now that sections have been installed, will be collected from both the southern and northern sites for many years.
Other deliverables will include guide construction specifications (e.g., quality testing for micro surface), best practices (e.g., quick reference materials for new inspectors), and support for implementation (e.g., training).
Powell is assistant director and Test Track manager, National Center for Asphalt Technology, Auburn, Ala. Images provided by NCAT