By David L. Lippert, Senior Sustainability Implementation Engineer, Illinois Center for Transportation
The following ICT/IDOT projects demonstrate just a few of the sustainability synergies of pavement research being conducted at the Illinois Center for Transportation …
Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS (R27-128)
The impetus for this project was the increased complexity of modern hot-mix asphalt (HMA) and the need for mixes to resist cracking over the expected pavement service life. The adoption of increasing amounts of asphalt binder replacement (ABR) from reclaimed asphalt pavement (RAP) and, more recently, from recycled asphalt shingles (RAS) permits very hard and aged asphalts into an HMA.
To combat the negative impact of hard asphalts from recycled material, most states—including Illinois—adopted ABR limits and asphalt grade-bumping specifications to reduce the risk of pavement cracking. The Illinois Department of Transportation (IDOT) wanted to increase ABR limits to realize cost savings and be more sustainable up front—but with little consideration for the long-term performance of the pavement. True sustainability is a balancing act between recycling as much as possible while maintaining pavement performance and service life. The need to abandon arbitrary methods specifications in favor of engineering performance tests was becoming increasingly clear and was the genesis of this research.
This study began with the goal of developing test methods to identify mixes prone to low-temperature and block cracking (cracking associated with aged and hard asphalt). Project deliverables included developing testing protocols, procedures, and specifications to evaluate potential cracking in the actual mix being proposed for a roadway. The ultimate goal was to create a win–win situation in which IDOT would receive mixes known to perform well from a cracking standpoint and the contracting community would have freedom to develop the most economical mix.
ICT efforts have led to an important milestone with the development of a practical and reliable test method to determine cracking resistance of HMA. The test method is preliminary and currently being evaluated by IDOT. The method centers around the use of the semi-circular beam (SCB) fracture test. Although SCB is not a new test, the conditions of the test are unique and provide ease of adoption at a minimal cost. The test is run at room temperature (77°F/25°C) using a test frame common to tensile strength ratio (TSR) testing under AASHTO T283. Existing equipment can be used with modifications. An important advantage of the newly developed method is the elimination of special environmental chambers, which are costly to purchase and maintain.
The test method has several advantages: (1) it correlates with field performance; (2) it provides a significant and meaningful spread in test outcomes that represent a mix’s cracking resistance, (c) it offers repeatability, practicality, low cost, and easy implementation by districts and contractors; and (d) it correlates with other independent cracking test methods.
In addition to the test method, the development and introduction of a flexibility index (FI) derived from SCB results provides a simple quality indicator for cracking performance. It is expected that the FI can be used in place of existing specifications that limit the amount of recycled material allowed in a mix. Preliminary results indicate that high amounts of both RAP and RAS with proper asphalt grade adjustments can provide good FI values.
Construction and Performance Monitoring of Various Asphalt Mixes (R27-161)
A study was initiated to evaluate mixes with a variety of asphalt binder replacement (ABR) levels in hot-mix asphalt (HMA) through field performance monitoring and extensive laboratory investigation over a three-year period. Also to be studied are ABR types—reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS)—and various asphalt binder grades.
The use of RAP in Illinois’ pavement is not new, dating back to the early 1980s. Use of RAP was relatively unchanged until the late 2000s when allowable percentages of RAP could be increased if materials were sized or fractionated similar to virgin aggregates (based on the results of an ICT study). Because of the cost reductions for construction, the percentage of allowable ABR was increased each year from 2010 to 2013. To show that it was possible to produce and pave HMA with very high levels of ABR, in 2013 IDOT demonstrated the use of total recycle asphalt (TRA), which allowed up to 60% ABR with only recycled materials for aggregate.
Upfront cost are important, but so are annual maintenance cost and pavement longevity. Another way to look at this is that every year of pavement life is worth 5% to 7% of the upfront construction cost. High-recycle mixes can reduce costs by 10% to 20%, but if pavement performance results in early repairs and its overall life suffers, the savings are not truly realized. These figures are based on a recent report, Illinois Highway Materials Sustainability Efforts of 2013.
This ICT/IDOT study was designed to follow the laydown and early-life performance of five construction projects using eight different surface mix designs, including TRA. Two of the projects were completed in 2014, and the remaining three will be completed in early 2015. Material sampling and testing will establish baseline values of the mixes at construction. Annual coring and pavement distress surveys will be conducted to document changes the pavement experiences with time.
The goal is to evaluate pavement performance and determine the mix test methods and criteria that will most accurately predict performance of HMA. The mixes are also being tested under the method developed recently at ICT for potential cracking to determine the mix flexibility index (FI). The study is expected to provide the much-needed link between mix properties at production and longer-term performance. These test methods and criteria can then be made part of mix design criteria for accepting a proposed mix design.
For further information about these studies, please contact Imad Al-Qadi, David Lippert, or Hasan Ozer at ICT, or IDOT engineers Matt Mueller or Jim Trepanier.