Full-Scale Performance of Warm-Mix Asphalt with High Recycled Content
This project was selected as a potential collaborative project between the European Union (EU) and the U.S. Department of Transportation's (DOT) Federal Highway Administration (FHWA) Turner-Fairbank Highway Research Center (TFHRC). [Read more about this partnership.]
For EU Candidates
Selected projects will assess performance of warm-mix asphalt (WMA) with high recycled materials, and should capitalize on the existing experience available in the United States. It is expected that the European Commission (EC) will fund an EC project on this topic and the FHWA will cooperate closely with the contractor selected for the relevant EC funded project.
The United States work on this topic will be conducted onsite at the Turner -Fairbank Highway Research Center.
Full-Scale Performance of Warm-Mix Asphalt with High Recycled Content
FHWA’s Every Day Counts (EDC) initiative is rapidly implementing WMA technologies to State and local highway agencies. The biggest potential impact in terms of cost and energy savings is the use of WMA technologies combined with high recycled materials (e.g., reclaimed asphalt pavement and asphalt shingles) content. However, there is no long-term pavement performance information for WMA and high RAP (recycled asphalt pavement) mixtures and specifically the combination of the two technologies. At the same time, major National Cooperative Highway Research Program (NCHRP) projects on WMA (NCHRP Project 9-43) and high RAP mix design (NCHRP 9-46) will conclude, providing guidance for mix design practices with WMA and improved mix design and evaluation of hot-mix asphalt (HMA) with high RAP content. The findings and recommendations of these studies need to be validated in a full-scale research environment in order to provide highway agencies with the utmost comfort level for using WMA combined with recycled materials. Further, six more NCHRP projects are under way to address WMA use and these studies findings and recommendations will need to be validated, as well.
Research is needed to achieve the following objectives:
- Comparatively quantify the differences in short-term and long-term load-associated cracking performance (fatigue, top down, bottom up) of combined use of WMA and High RAP for different WMA processes
- Establish realistic boundaries for high-RAP mixtures employing WMA technologies based on percent binder replacement and binder grade changes when using high RAP with WMA
- Verify guidance from ongoing RAP and WMA NCHRP projects including mix design and associated performance test recommendations
Planned Research Activities
A top-down approach has been utilized to identify the broad characteristics of a streamlined experimental design in order to complete the loading, analyze the data, and make recommendations within 3 years. The experimental plan will consist of both laboratory and field analysis and testing. The FHWA’s Pavement Materials Laboratories (Aggregate and Petrographic Laboratory, Binder Laboratory, Bituminous Mixtures Laboratory and Chemistry Laboratory will be used to conduct the testing and analysis. In addition, the FHWA may partner with other laboratories to complete laboratory testing. The field testing will be accomplished using FHWA’s Pavement Testing Facility (PTF).
A draft experimental design follows:
1. The Interdependency of Accelerated Loading Frames (ALFs) Operations and Test Pavement Structure
- TFHRC PTF has a unique capacity of two ALFs to increase the productivity with simultaneous loading of paired-test sections.
- Both TFHRC ALFs have received new upgrades to modernize the electronic control systems, which will significantly increase the reliability and decrease down-time for troubleshooting and repair.
- A specific focus on the fatigue cracking resistance under a controlled intermediate temperature such as 20 degrees Celsius. Specific test sections and wheel loading configurations to capture high temperature rutting are tentatively not being scoped. The implications of this is that only 8 to 10 test sites will be needed of the available 48 test sites. Comparative rutting performance can still be quantified in the fatigue loading sections, but only at an intermediate temperature.
- Research has suggested the fatigue performance of RAP mixtures with higher recycled contents potentially exhibits a greater sensitivity to strain. If the ALF conditions were to employ a relatively high wheel load (e.g., 70 kN used in the past experiments) this could jeopardize the experiment by unfairly evaluating the recycled materials under conditions that don’t reflect inservice pavements. Thus, to minimize nonlinear pavement structural responses such as excessively high asphalt tensile strains and unbound layer deformations, the experiment is proposed to configure the ALFs with a 44 kN single wheel load equivalent to a 89 kN axle that is more near legal limits.
- A total thickness for the asphalt concrete layers is being scoped at 100 mm balance (a) the planned completion schedule of the experiment in 36 months with (b) the 44 kN wheel load. A review of the TFHRC ALF’s historical pavement structures and performance suggest a 6th order exponent for load level effects in this asphalt thickness range. If the asphalt mixtures avoid highly polymer modified asphalt, then mechanistic-empirical engineering computations indicate each test site will require between 300,000 to 600,000 passes to achieve fatigue cracking failure.
- A calculated estimate for scoping the experiment’s schedule is each lane achieving approximately 515,000 passes in 5 months. This is based on each lane achieving 65,000 passes the first month because data collection is more intense in the early stage followed by 90,000 passes for the next four months. Both of these rates are easily achievable and based on a factor of safety whereas the highest achievable rate monthly ALF performance is 160,000 passes.
- The remaining structure under the high-RAP/WMA sites for the primary component of the experiment will be comprised of the existing unbound aggregate base layer 22 to 26 inches thick and the existing A-4 subgrade.Two different types of WMA technologies will be specified. However, FWHA will not prescribe the particular product. It will be the contractors’ discretion, but it must utilize water foaming and a WMA chemical additive so as not change the effective asphalt binder grade. It is planned to require only one level of temperature reduction for the WMA mixtures to be nominally 28 degrees Celsius (50 degrees Fahrenheit) below the control HMA counterpart mixtures.
- 15 percent RAP content reflecting the state-of-the-practice will be compared against 40 percent RAP content state of the art.
Specific recommendations will be made regarding implementation of high recycled content and WMA mixtures and pavement design based on the findings of this study.