ICT engineering for performance and sustainability of future paving asphalt materials

7/28/2015

The transportation infrastructure in general, and roadways in particular, has a significant impact on the environment. Getting the best performance and sustainability of pavements is just one of the many projects at the Illinois Center for Transportation (ICT), part of the Department of Civil and Environmental Engineering at Illinois. The quantity of emissions depends on total fuel combusted in different types of vehicles, whose quantities are in turn affected by the quality of roadway materials, construction, and performance, as well as road conditions.

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The transportation infrastructure in general, and roadways in particular, has a significant impact on the environment. Getting the best performance and sustainability of pavements is just one of the many projects at the Illinois Center for Transportation (ICT), part of the Department of Civil and Environmental Engineering at Illinois,  

According to a 2013 EPA report, greenhouse gas (GHG) emissions associated with the transportation economic sector totaled 1,829 million metric tons of CO2, out of a total of 6,702 million metric tons of CO2 in 2011. Within the transportation sector, a large quantity of GHG emissions—and consequently energy—is associated with vehicles traveling along the highway system. The quantity of these emissions depends on total fuel combusted in different types of vehicles, whose quantities are in turn affected by the quality of roadway materials, construction, and performance, as well as road conditions.

"The most common and visible performance measure of hot-mix asphalt (HMA) sustainability is measuring how many tons—or how high a percentage—of recycled materials can be placed in the mix. Upfront cost savings are also pointed to as a rationale for increased recycling. Unfortunately, this approach concentrates only on the material production phase of the pavement life cycle and ignores the fate of the pavement during its service life," explained Imad Al-Qadi, ICT director and a Founder Professor of Engineering at Illinois.

"To be truly sustainable, a pavement must be built with the least environmental impact and life-cycle costs from 'cradle to grave,' while providing desired performance for an extended time with little interruption by maintenance activities," Al-Qadi added. "Roads deteriorating at faster rates burden both the users and the owner agencies. Users are burdened with a reduction in ride quality, the cost of additional fuel, and vehicle damage. Worsening overall roadway infrastructure health and additional maintenance costs are borne by owner agencies. In addition, shrinking and fluctuating funds for infrastructure projects increase the importance of maintaining or improving the overall health of pavement infrastructure."

With an increasing emphasis on recycling, reducing energy demand, and reducing greenhouse gases to improve the sustainability of pavements, new analysis methods are needed to measure the environmental impacts and benefits of engineering decisions in addition to traditional cost assessment methods such as life-cycle cost analysis (LCCA). While not new, life-cycle assessment (LCA) in pavement engineering is an emerging analysis tool to accomplish this goal. The use of LCA allows looking into energy and emissions over the life of the pavement, including material acquisition and production, construction, use, maintenance, and recycling/landfilling phases. In doing so, each phase of the pavement life disposition provides additional insight into the characteristics of good pavement engineering. With this detailed approach, in conjunction with economic assessment using LCCA, the process of assessing the sustainability of a pavement project can be improved.

One such analysis was recently undertaken at ICT. Typical and high-recycle HMA overlays were evaluated for economy, energy use, and differences in GHG emissions and compared under slightly different performance scenarios. Using a roughness progression model developed at ICT, performance scenarios simulated cases in which overlay performance exhibited subtle differences of reaching the point of “rough riding” (measured by an International Roughness Index reaching 130 inches per mile) in 15 years for a good pavement.

The economic and environmental assessment focused on evaluating the trade-offs between using a greater amount of recycled materials to reduce upfront economic costs and environmental burdens and life-cycle impacts emanating from the various phases, including service, of the pavement. It was found that for high-volume roads with an average daily traffic (ADT) count of 60,000, the increased energy used by a vehicle to traverse a rougher road eradicated any upfront energy savings and GHG emissions that might have resulted from increased recycling if the pavement performance is reduced by only one year or less.

On the basis of the LCA results, it is fairly apparent that HMA that allows high-recycle contents—yet provides good long-term performance—are needed. Performance-based or performance-related specifications are critical in achieving a balance between material selection and its long-term performance. The key is to adopt a performance test that offers the best way to achieve the desired performance outcome before the HMA is allowed in the roadway. For the greatest benefit, the test must be able to provide screening at the project level for the actual mix being proposed.

The concern with high-recycle mixes is that typical materials added to the mix, such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), introduce hard and brittle asphalt binders (believed to contribute to thermal and fatigue cracking) into pavements that need an optimum level of flexibility over their life. Hence, blending and homogenization of aged binder and new binder is usually the goal. Although some information is available on the blending of RAP binder with new binder, limited information is available for RAS.

Illinois semi-circular bending (SCB-IL) test apparatus and geometry.
Illinois semi-circular bending (SCB-IL) test apparatus and geometry.
Recent ICT research has studied various fracture and fatigue test methods for HMA, testing conditions and ultimately the reliability of the testing/screening process in evaluating brittleness of mixes. Typically, fracture tests are run at cold temperatures—just above that of the low-temperature asphalt binder grade being used. For example, a mix with an asphalt binder graded as PG 64-22 would be tested at –12°C and a loading rate of 0.7 mm per minute. However, measuring fracture energy at low temperatures could not accurately and consistently discriminate between mixes that contained varying amounts of brittle binder.

For that reason, part of the ICT research was to review and understand the pros and cons of each test method and come up with a reliable, practical test method that can be used at the mix design stage. Fracture behavior was investigated at varying temperatures and loading rates to evaluate differences in the resistance of the mixes to cracking and damage. By controlling the testing conditions, researchers found that fracture energy calculated at or around intermediate temperatures with relatively fast loading rates provides the largest discriminatory potential between mixes with different volumetrics and other engineering properties related to field performance.

Therefore, an Illinois modified semi-circular bending (SCB-IL) test was developed and proposed to IDOT. 

“Four criteria were considered when selecting the test method—reasonable and consistent spread of fracture energy, correlation to independent tests and engineering intuition, applicability and seamless implementation by pavement practitioners, and simplicity, repeatability, efficiency, practicality, and cost effectiveness,” Al-Qadi noted.

Typical outcome of the SCB-IL test illustrating different load-displacement behavior and slopes for similar fracture energy results.
Typical outcome of the SCB-IL test illustrating different load-displacement behavior and slopes for similar fracture energy results.
Matt Mueller, chair of the Technical Review Panel overseeing the project said, “The Illinois Department of Transportation sees great opportunity in moving away from method specifications toward performance specifications. A few years ago, there was an opportunity to borrow the work of several states to implement a mix rutting test. Unfortunately, existing cracking tests and protocols did not adequately screen mixes for Illinois weather and loading conditions. This work, under ICT project R27-128, 'Testing Protocols to Ensure Performance of High Asphalt Binder Ratio Mixes Using RAP and RAS,' over the last several years has led to a very promising protocol using relatively inexpensive equipment that requires minimal training to operate and uses standard gyratory pucks or field cores for specimens. Best of all, the test can be run in a single day, from specimen preparation to final calculation of the Flexibility Index. The department looks forward to providing this new performance test to our industry partners as they seek out new combinations and new materials to optimize mix performance for Illinois roads.”

The Illinois modified SCB test is conducted at an intermediate test temperature of 25oC and a testing rate of 50 mm per minute. This testing scheme resulted in increased data range and the ability to better separate good and poorly performing mixes. This test (and resulting index) has the added benefit of eliminating the need for costly environmental chambers.
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This story was published July 28, 2015.