Published: Jan 29, 2016 12:00:00 AM
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Dr. Buzz Powell overlooks the micro-milling process on U.S. Route 280.
After the successful construction of new test sections this summer, NCAT has begun its sixth cycle of accelerated pavement testing, which includes structural experiments, surface mix experiments, and pavement preservation studies. The scope of the pavement preservation research has been expanded in this cycle to include continued evaluation of Lee County Road 159, a low volume route, and new treatment sections on U.S. Route 280, a much higher traffic volume roadway.
NCAT’s testing sites for the 2015 research cycle include the Pavement Test Track, Lee County Road 159, and a portion of U.S. Route 280.
Sequence of a research cycle
Initially constructed 15 years ago, NCAT’s Test Track is a 1.7-mile oval comprised of 46 200-ft test sections sponsored on three-year cycles. The first phase of the 2015 research cycle consists of constructing test sections on and off the Test Track. Representatives from research sponsors make on-site visits during this phase in order to ensure that the sections satisfy their research needs. This cycle, NCAT used Pavia’s HeadLight software to document and share construction data in real time among NCAT staff and track sponsors. To facilitate lab to field performance correlations, a large amount of mixture samples from construction are tested and analyzed in the laboratory. Various parameters are also measured during the construction process to capture in-place properties of the sections.
In the second phase, each section on the track is subjected to 10 million equivalent single axle loads (ESALs) of heavy truck traffic applied over a period of two years. The performance of each track section is closely monitored on a weekly basis. A new automated pavement distress data collection vehicle will be used to quantify roughness, macrotexture, rutting, and cracking in the same manner used by most state highway departments for their pavement management systems. Other tests such as surface friction, falling weight deflectometer, sound, and permeability are also conducted. Similar performance data is conducted on the off-track sections, but due to the open traffic on these roadways, data collection is conducted on a less frequent basis than the closed-loop track.
The final part of the three-year cycles at the test track involves forensic analyses of damaged sections in order to determine the contributing factors to pavement distresses. Many forensic investigations conducted during this stage include destructive testing such as trenching and coring.
For structural sections on the Test Track, NCAT closely monitors how the pavement sections respond to loading and environmental changes. A wireless network covers the entire track to safely and effectively transmit high-speed data from stress, strain, and temperature sensors built into the structural sections. Measurements from the sensors can then be compared between test sections, monitored over time, and linked to performance in order to develop pavement response models and improve mechanistic-empirical design procedures.
A major focus of the sixth research cycle is to quantify the life-extending benefits of different pavement preservation treatments by determining the field performance of treatments applied at various stages of pavement life and decay. In this highly controlled experiment, quantifying the benefits of various pavement preservation alternatives as a function of pretreatment condition will provide transportation agencies an objective basis for pavement management decisions.
In 2014, pavement preservation treatments were applied to several Test Track sections at predetermined distress levels. Traffic and examination of these sections, as well as those on Lee County Road 159, will continue throughout the sixth research cycle. Sponsors will also select consensus treatments for placement on comparable roadways in Minnesota through a partnership with the Minnesota Department of Transportation.
Experimental test sections were installed in 2012 on Lee County Road 159. Traffic and examination will continue throughout NCAT’s sixth research cycle.
New pavement preservation treatments have also been installed on a nearby section of U.S. Route 280, expanding the scope of our research to include a higher volume route and cold recycling techniques. These sections serve as a complement to existing sections on the Test Track and Lee County Road 159 and are essential to accurately quantify the condition improving benefit curve of each treatment/treatment combination under different levels of traffic. The core objective of this research is to quantify the benefits of pavement preservation on a more typical highway, somewhere between the accelerated damage environment of the track and the low ADT environment of Lee County Road 159. Sections on U.S. 280 are 0.1 miles in length and include 34 treatment and treatment combinations such as microsurfacing, crack sealing, chip seals, fog seals, scrub seals, cape seals, thin asphalt overlays, and cold recycled sections using foamed and emulsified asphalts in both central plant and in-place production.
Sealant is precisely applied to cracks in a U.S. Route 280 pavement preservation test section.
Cold central plant recycling is one of the innovative pavement preservation methods used on U.S. Route 280.
Cold in-place recycling reduces the life cycle cost of the pavement structure by reusing the existing asphalt pavement.
Microsurfacing is slurry of polymer-modified emulsion, fine aggregate, cement, and water. Specialized equipment mixes the components and spreads the slurry in a continuous operation.
FiberMat® is designed to act as a crack-absorbing membrane and incorporates two applications of polymer-modified asphalt emulsion with a layer of fiberglass strands between them. A layer of crushed aggregate is then spread and rolled into the fiberglass-reinforced emulsion.
Results from the expanded 2015 preservation experiment will provide a rational starting point for the implementation of a life cycle cost based preservation treatment selection process that can be refined over time with location-specific pavement performance feedback. A second objective is to develop guide specifications and recommend guidelines for quality assurance testing and inspection of pavement preservation treatments.
Asphalt cracking performance testing
While many tests have been developed to evaluate cracking potential of asphalt mixtures, it is not clear which ones correlate the best with field performance and what criteria could be used in specifications. This is the second focus area of the 2015 Pavement Test Track cycle. The objective is to validate laboratory cracking tests by establishing correlations between test results and measured cracking in pavement test sections. New track sections have been constructed with asphalt surface layer mixtures that have a range of expected cracking susceptibilities. Other factors that can impact top-down cracking, such as underlying pavement structure and differences in aging, have been controlled to the highest degree possible. A battery of laboratory cracking tests will be performed, which will include semi-circular bend tests using the Illinois and Louisiana methods, the energy ratio method, and the Texas overlay test. The analysis of the laboratory cracking tests will identify which method provides results that best correlate with field cracking while also considering practicality for use in mix design control testing, cost-effectiveness, and the ability to accommodate recycled materials, new and future additives, and mix combinations.
Test Track findings have been and will continue to be used to improve materials, tests, specifications, and design policies in many ways. Research experiments based on single test sections or groups of test sections provide sponsors the confidence to move concepts into practice. This cycle is expected to yield information to advance the use of laboratory performance tests, novel materials and additives, and structural design.
Sponsors funding research
Sponsors of the sixth cycle include highway agencies and private sector partners. The number of sixth cycle sponsors funding on and off-track research is currently at 18, and additional sponsors are anticipated.
The sixth cycle offers a new chapter in real-world pavement research through a partnership with MnROAD, which brings about expansion of experiments to include a cold-weather climate. Many of the same pavement preservation treatments installed by NCAT in Alabama will also be installed on low and high volume routes in Minnesota. In 2016, MnROAD will also build complimentary cracking group sections to validate low-temperature cracking tests. These parallel studies are aimed at producing findings that can be directly implemented by a larger geographic base of state departments of transportation. With the addition of this new partnership, researchers are confident that this full-scale pavement testing cycle will provide far-reaching impacts in advancing pavement technologies.