Three challenges users must consider
By Amma Wakefield, P.Eng.
Specifications for paving-grade asphalt binders such as the Performance Graded (PG) Asphalt Binder Specification, are traditionally applied to the asphalt binder before it has been used in an application. These types of specifications are governed by the American Association of State Highway and Transportation Officials (AASHTO), ASTM International or the agency.
However, with the increased use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixes, many user agencies are looking for ways to evaluate the properties of the blended asphalt binder (i.e. new binder and old binder from RAP or RAS) since this also has an impact on the asphalt pavement performance. One way to evaluate the blended binder is to extract the binder from the mixture with solvent and then recover the binder from the binder/solvent solution. This produces an extracted and recovered binder sample that can be tested.
There has always been an interest in determining the properties of asphalt binder in-situ asphalt mixtures, for research or forensic investigation purposes, or as a way of confirming that the correct asphalt binder grade was utilized during production. Other reasons agencies see value in testing the recovered binder are to: 1) protect against excessive aging during storage and mix production, 2) protect against the excessive use of RAP or RAS that will produce a mix that is excessively aged and 3) protect against an excessively aged blended binder. However, using recovered asphalt binder properties – particularly in a specification – is not without some potential concerns.
The process of mixing an asphalt binder with aggregates and possibly RAP and/or RAS, coupled with the aging that occurs during production (whether in a lab or mixing facility) and post production while in-service (field only), can significantly impact the properties of the blended asphalt binder (virgin and recycled). In addition, the extraction-recovery procedures and solvents used can also potentially have an impact on the recovered asphalt binder properties.
Regardless of the reason for conducting the procedure, asphalt technologists and user agencies should recognize the following three main concerns associated with evaluating physical properties of asphalt binder recovered from asphalt mixtures. (1)
1: The choice of procedure and solvent can have an impact on the resulting physical properties of the recovered asphalt binder.
The process of determining the material properties of a recovered asphalt binder starts with extracting that binder from asphalt mixtures or pavement samples, followed by recovery of the binder for testing. ASTM and AASHTO procedures are available for performing extraction and recovery. The centrifuge solvent extraction method is the only method currently referenced in subsequent asphalt binder recovery procedures for determining recovered asphalt properties. The most common solvents used in the extraction are either trichloroethylene (TCE) or Toulene. The two recovery methods used are either by Abson or by Rotavapor.
Research performed by the National Cooperative Highway Research Program (NCHRP) evaluated the effect of different extraction-recovery procedures and solvents on the high-temperature properties of recovered asphalt binders (i.e. G*/sin δ). Findings from that work have shown that (I) the extraction-recovery procedures used, (II) the type of solvent used, and (III) and type of material being evaluated, can all impact the results of physical property testing.
In particular, the Abson recovery method was more likely to leave the residual solvent in the recovered asphalt binder, thereby lowering the measured stiffness of the recovered asphalt binder. The effect was more noticeable if the binder was harder. (2)
The Asphalt Institute has conducted research using toluene as a primary solvent for extraction in combination with the Rotary Evaporator. The temperature of the oil bath in the procedure was increased by approximately 10°C since toluene has a higher boiling point than TCE, however a benefit to the change is that toluene does not have 1,2-epoxybutane as a stabilizer. This stabilizer can cause problems with asphalts containing acids during the extraction process, which will affect the properties of the recovered binder. Asphalt Institute advanced the procedure to ASTM and it was approved as ASTM D7906 in 2014.(3)
2: Physical property test results of recovered binders have much higher variability compared to unrecovered binders.
AASHTO re:source (formerly AMRL or the AASHTO Materials Reference Laboratory) conducts proficiency sample programs (PSPs) on an annual or semi-annual basis for many tests used in the asphalt industry. The summary report data is publicly available for analysis and use on their website although individual data is only available to the participating lab. One of the PSPs that has been conducted for some time has been on solvent extraction and recovery of hot-mix asphalt.
To analyze the variability in testing, summary report data was collected from 2003-2017 for each pair of proficiency samples. For their analysis, AASHTO re:source had segregated the results into physical properties of asphalt binders recovered using the Abson method and physical properties of asphalt binders recovered using the Rotavapor method. To address the variability that occurs when two separate labs obtain a split sample, one must consider multi-laboratory variability or reproducibility.
In Figure 1, the multi-lab variability of the Rotavapor-recovered G*/sin δ value is an average of 86 percent (range from 38-121 percent). The multi-lab variability of the Abson-recovered G*/sin δ value is an average of 71 percent (range from 57-96 percent). By contrast, the multi-lab variability for the same property (G*/sin δ) measured on RTFO-aged, unrecovered asphalt binder is 17 percent. Although the Abson and Rotavapor procedures on average appear to produce similar multi-lab variability, the variability of recovered G*/sin δ appears to increase by approximately four-five times compared to the unrecovered values.
The data showed similar variability when other properties (i.e. absolute viscosity) were measured for the recovered binders. This suggests that the increase in variability seen in recovered asphalt binder properties is not a function of the test or the recovery procedure, but rather the process of recovering an asphalt binder from the mix.
3: The effect of solvent extraction on polymer modified binders and impact on physical properties is not well understood and still being investigated.
The asphalt industry has been increasingly using polymers to modify asphalt binders – polymer modified asphalt or PMA – to enhance their performance. However, the extraction-recovery research so far has primarily used neat (non-polymer modified) binders.
How do the polymers behave when a modified asphalt is extracted and recovered for subsequent testing? What does this mean for the elastic properties of the recovered binder? The research is limited; however, some key findings are (I) Polymers may adhere strongly to aggregate and there is no guarantee that extracting 98 percent of asphalt will also extract 98 percent of the polymer modifier, (II) Dissolving (extracting), then re-precipitating (recovering) the PMA will most definitely cause morphological rearrangement of the polymers. This means the rheology (or flow properties) of extracted PMA, may be different from those in the in-situ PMA in asphalt mixtures. Why does this matter? Because the properties of asphalt used to specifications and used to predict its performance are dependent on its rheological (or flow) properties.
Take home
The purpose of testing asphalt binder and other materials is to measure their characteristics and behavior, to determine their suitability for various applications. When it comes to analyzing the test results and relating those results to performance, we always consider three sources of variability: materials, sampling and testing. Our quality control and quality assurance testing programs are meant to focus on the variability in the material, by limiting the variability from sampling and testing.
In the case of physical property testing on recovered asphalt binder, the research shows that the process of recovering an asphalt binder from solution results in physical properties with much higher variability than experienced if performing the same physical property tests on unrecovered (i.e. virgin) asphalt binder. As such, it is recommended that users should exercise caution when comparing values of recovered asphalt binder to test criteria and variability derived for unrecovered (virgin) asphalt binder.
Wakefield is an Asphalt Institute Regional Engineer based in Ontario, Canada.
1. Wakefield A, Anderson RM, McKay Z, Tighe SL. “A Review of Solvent Extraction-Recovery Procedures and their Effect on Recovered Asphalt Binder Properties”, Proceedings for 63rd Annual Canadian Technical Asphalt Association Conference, Vol. LXIII, 475-496, Regina, SK (Nov 2018).
2. McDaniel RS, Soleymani HR, Anderson RM, Turner PA, Peterson RL. “Recommended Use of Reclaimed Asphalt Pavement in the Superpave Mix Design Method”, National Cooperative Highway Research Program (NCHRP) Web Document 30, Transportation Research Board, National Research Council, National Academies, Washington, D.C. (2000).
3. ASTM D7906: Standard Practice for Recovery of Asphalt from Solution Using Toluene and the Rotary Evaporator (2014)