The transportation infrastructure in general, and roadways in particular, has a significant impact on the environment. 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.
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. 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.
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. Imad Al-Qadi, ICT director, notes: “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.”
Matt Mueller, chair of the Technical Review Panel overseeing the project says, “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 (SCB; see Figure 1) 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.
The study showed that using only fracture energy could be sometimes misleading. Different mixes can have similar fracture energies represented by area under the load-displacement curve. The result is that some poorly performing brittle mixes can be grouped with good performing mixes. However, on the basis of the shape of the load-displacement curve (Figure 2), it is evident that one mix is more flexible than the other. This issue prompted a review of several methods to separate these mixes appropriately. The effort focused on the speed of crack growth rate as reflected in the slope of load-displacement curve at inflection between the peak and termination of the test. The result was the development of the Flexibility Index (FI).
After evaluation of several approaches, the Flexibility Index (FI) took the following form:
FI = A x (Fracture Energy/slope at inflection)
where A is the calibration coefficient for unit conversions and age shifting for lab versus plant versus field materials.
As results of this research effort are presented to the HMA community, the approach is gaining favor from researchers, agencies, and practitioners. The simple test geometry, specimen preparation, and temperature conditions—along with increased discrimination of the FI—are all expected to result in the test being widely adopted.
Other testing devices can cost as much as $50,000 and will not differentiate between good and poorly performing HMA as well as the proposed Illinois modified SCB (SCB-IL) with use of the Flexibility Index. It is estimated that the SCB-IL can be adopted for approximately $10,000. Approximately 40 labs could be outfitted in Illinois. While the cost savings are substantial, the development of a repeatable, easy-to-conduct test that properly ranks HMA is the real benefit.
The study is currently evaluating a huge array of field cores obtained from all the districts in Illinois. This important step will link laboratory-compacted mixes and the test result thresholds to field performance. This final step will result in establishment of specification values used to accept or reject proposed HMA.
The selection of proper values is a very important step in order to properly flag poorly performing mixes for redesign. As the specification is refined and equipment is procured, it is anticipated that the Flexibility Index will be used starting in 2016 and that the State of Illinois could lead the way with this new suite of mix performance prediction tests.
Imad Al-Qadi, David Lippert, Hasan Ozer, and Marshall Thompson contributed to this article.