By Dwight Walker
Performance Graded (PG) asphalt binders were introduced in the mid-1990s and the PG system has since become the basis for specifying and accepting asphalt binders in the U.S. To date, a comparable system of performance grade asphalt emulsions has not been developed. However, considerable effort has been made to develop and implement such technology.
A workshop held at the January 2013 Transportation Research Board (TRB) meeting covered the advancements that have occurred. Efforts to develop a new emulsion testing system began in 1998 when the FHWA (Federal Highway Administration) Office of Pavement Preservation and Construction hosted meetings to initiate discussion. From this initial work, research needs were identified but adequate funding was not provided to conduct the effort.
FHWA created the Emulsion Task Force (ETF) comprised of a group of technical experts. The ETF functions as the materials portion of FHWA’s Pavement Preservation Task Group. Any test methods or specifications reviewed and accepted by the ETF are, in turn, forwarded to the American Association of State Highway and Transportation Officials (AASHTO) for approval and adoption. ASTM International has also been working to improve emulsion test methods and specifications.
A circular titled “Progress Toward Performance-Graded Emulsified Asphalt Specifications” was published following the workshop, and it provides an excellent progress report. The preface from that document describes the efforts as intending to “…define the beginnings of new test methods that eventually will redefine how the industry determines specifications and performance of emulsified asphalts.” The report consists of an overview of the workshop and six papers on specific advancements and research efforts.
Note: The following information is summarized from the author’s review of the workshop report. All of the papers are not covered here, and information provided here is much abbreviated. Anyone desiring more detailed information should access the report.
Asphalt Emulsion Recovery Update – Arlis Kadrmas
Arlis Kadrmas is the chairman of the ETF subcommittee working on the recovery and testing of emulsion residues. Currently oven evaporation or distillation procedures use temperatures (260˚C or 500˚F) that damage the polymers included in many emulsions. Recovery temperatures were lowered (177 or 204˚C, 400˚F), and a vacuum procedure was also introduced to reduce recovery temperature. However, these temperatures are still substantially higher than actual field application temperatures. More recently, evaporative techniques using temperatures of 60˚C (140˚F) have been used to yield a residue similar to field conditions.
Two low-temperature evaporative techniques have been approved, AASHTO PP72, Methods A and B. These procedures use a thin film comparable to a chip seal (Method A) or a tack coat (Method B) and 60˚C temperatures. Method A takes 48 hours (24 hours at 25˚C, 77˚F or ambient room temperature and 24 hours at 60˚C). Method B requires 6 hours at 60˚C. Method B has become the ETF subcommittee’s preferred procedure. Various vacuum methods are being evaluated to further reduce 6-hour recovery time. The recovered material is tested using the DSR.
Techniques for Accelerating Recovery of Asphalt Emulsion Residues at 60 C by Using Thin Film Procedures and Test Methods Suited to the Limited Amount of Residue Obtained to Characterize the Recovered Binder – Gerald Reinke, et al.
Reinke and his associates used a 2-hour, 60˚C, 5- to 10-mm Hg vacuum recovery procedure to recover the emulsion residue, which compared closely to the base asphalt binder. Additional work was planned to see if 3 hours is needed for high-float emulsions.
The group also used “…a novel emulsion residue recovery procedure using a DSR to break, cure and test the emulsion residue…” in a single, continuous operation. There are few tests or procedures to assist in selecting the type of emulsion to be used. Most of the existing tests are empirical with little established relationships between emulsion residue properties and performance. Most emulsions are selected and specified based on experience without measuring the effects of temperature, traffic and aging. With the increased emphasis on preserving pavements, there is a need for a better means of choosing emulsions.
Development of a More Rational System for Selecting Emulsions for Surface Treatment – Hanz, Bahia, et. al
This paper proposes using the DSR in emulsion characterization and discusses the Bitumen Bond Strength (BBS) test. The DSR evaluates emulsion residue-related distresses, and the BBS test evaluates the emulsion to aggregate bond. The paper discusses using PG-related procedures to evaluate resistance to bleeding, raveling, cracking (both fatigue and thermal), and for measuring elastic recovery properties.
A survey of state agencies found aggregate (chip) loss and bleeding as the two most common chip seal distresses. The Multiple Stress Creep Recovery (MSCR) test was selected to evaluate bleeding susceptibility. The report describes the BBS as “… a pneumatic adhesion test adapted from the paint and coatings industry (ASTM D4541) …” This procedure has been accepted as a provisional AASHTO test (TP-91).
The report concludes that “…the performance evaluation framework proposed in this study shows promising potential to differentiate between emulsion types and to quantify the effects of modification …”
Performance Evaluation of Chip Seals – Richard Kim, et al.
This paper provides a summary of the test methods used to perform analyses of emulsions and surface treatment (chip seal) mixtures. Some conclusions and developments are as follows:
Uniform chip gradation is one of the most important factors in good chip seal performance. The Performance Uniformity Coefficient (PUC) is used as an indicator of the degree of uniformity of chip gradation.
Polymer modified emulsions extend chip seal performance life, particularly on higher traffic roadways, and improve chip retention in cold weather.
A new performance-based chip seal mix design method has been developed as part of this work. The procedure considers traffic and environmental conditions and uses a 3-D laser profiler and volumetric relationships to determine the optimal design. A procedure was developed to determine the maximum allowable traffic volume for a given chip seal. The method was based on the relationship between the reduction in Mean Profile Depth (MPD) and the traffic level and the relationship between chip loss and MPD reduction. MPD is a measure of the exposed texture depth of a chip seal and is inversely related to the embedment depth of the chips.
With the increased focus on preserving asphalt pavements, emulsions play a major role. The introduction of PG technology to selecting, specifying and evaluating emulsion performance offers significant promise to improving emulsion tests to a “performance graded” (PG) system.