By Dwight Walker, P.E.
This is the third article in our series on asphalt emulsions. The first article provided basic, overview information and the second one looked at some high performance applications. These articles are available online in the archives of our magazine.
In this article, we asked a group of asphalt industry professionals to share their thoughts on what is ahead for emulsions. The panel includes:
• Barry Baughman, Technical Director for Ultrapave Corporation. Baughman is a former president of AEMA and serves as a member of the Pavement Preservation Expert Task Group’s (PPETG) Emulsion Task Force (ETF).
• Mark Ishee, Vice President, Pavement Preservation, Ergon Asphalt & Emulsions. Ishee is on AEMA’s Board of Directors, a PPETG member, and on TRB’s Committee on Pavement Preservation.
• Gayle King, Consultant at GHK, Inc. King serves as a member of the ETF and as chairman of the Emulsion Residues Test subcommittee.
The next few pages contain what our panelists said.
ASPHALT: WHAT ARE SOME OF THE ISSUES AND CHALLENGES FACING THE USE OF EMULSION?
BAUGHMAN: Historically, emulsion availability has generally driven the agencies’ decisions on which emulsions to specify. Today’s technology has brought more manufacturing choices and capability to agencies such that a “performance based” specification is desired.
Most emulsion specifications continue to use viscosity-graded asphalt binders or penetration-based specifications. A specification that incorporates a standard grading system will serve to more uniformly describe asphalts and base asphalt for asphalt emulsions.
The new specification needs to address testing at temperatures normally seen by surface treatment applications, chip retention, fatigue at room temperature, ability to penetrate cracks of varied widths, and adhesion at high and low temperatures, and include climate and traffic in the binder selection process. Asphalt emulsion requirements must include a mechanism for acquiring the emulsion residue such that the residue is representative of the binder on the road.
ISHEE: Regional consistency of emulsion specifications, nomenclature and agency certification would help mitigate some of the challenges the industry faces.
We are seeing an increased rate of turnover in agency personnel nationwide. This puts an enormous amount of importance on education. We, as an industry, have to step up the efforts to keep everyone involved familiar with the latest technological advancements as well as the fundamentals of emulsion usage.
KING: It is hard to separate emulsion issues from the broader need to expand the use of pavement preservation technologies. FHWA’s Pavement Preservation Roadmap identifies two critical research needs. The first is performance-graded emulsion specifications. FHWA created the Emulsion Task Force to oversee a comprehensive effort to bring emulsion specifications into the Superpave era.
A second key need is to identify material properties near the pavement surface that predict imminent damage. Engineers could then optimize the selection and timing of preservation applications to best counter the damage.
We should think in terms of “staged preservation,” much like we talk about “staged construction” to add new pavement layers as traffic increases. The first treatment when damage is concentrated in the top ½-inch might be a rejuvenator fog seal. When oxidative damage goes a bit deeper, the second treatment might be a 1-inch hot-in-place recycling application with an emulsified asphalt/rejuvenator blend. The other option is to reduce the rate of oxidation, perhaps by applying a chip seal to reduce the rate at which oxygen can reach the underlying asphalt surface layer. If this works, chip seals might even be applied to a newly constructed pavement.
ASPHALT: IS SUPERPAVE APPLICABLE TO EMULSIONS? WHAT IS BEING DONE?
BAUGHMAN: Soon after the PG binder specification was finalized, the Texas Department of Transportation (TXDOT) funded research on the applicability of the PG binder specification to chip seal binders. The report indicated that there was no technical reason that the PG specification could not be used to specify chip seal binders.
There are complications, however, in that the Superpave PG binder specification was designed to address HMA distresses, and more sophisticated chip seal binders containing polymer modifiers did not fit into the PG system well. Additionally, traffic and climate were unaccounted for in selection of chip seal binders.
TXDOT funded additional research to develop a new specification, based on PG testing equipment, to address chip seal distresses. Proposed specifications based on this work included initial stiffness requirements to address loss of aggregate early in chip seal life and aged low temperature properties to address low temperature aggregate loss later in chip seal life. Climate and traffic levels were not investigated.
ISHEE: Almost as quickly as the Superpave binder specifications were implemented, people started to ask the same question. Can we apply this protocol to asphalt emulsions? The answer is yes, but industry owes the issue thoughtful research to assure equal or improved performance of products as well as to assess the effect on product selection balanced with the ability to supply.
This work is being accomplished through many individual fronts, and the Asphalt Emulsion Task Force represents our industry from a national perspective.
KING: Two elements of Superpave are critical to PG emulsion specifications. First is the recognition that material performance depends upon the local pavement temperature. AASHTO has always specified a single penetration range for chip seal emulsions (e.g. CRS) for the same U.S. highway network that identifies approximately fifteen different climates for its PG-grading system with temperature increments of 6°C.
Climate-based specifications would clearly increase the number of emulsion grades for a given application, forcing emulsion producers to adapt production systems accordingly. However, given technology advancements such as inline blenders and in-line viscometers, the tools are available to make this happen without severe heartburn.
The other critical tool from Superpave that must translate to emulsion residue specifications is the Dynamic Shear Rheometer (DSR). Perhaps rutting and fatigue are not relevant to many emulsion applications. However, this instrument is amazingly adaptable, both in terms of the physical properties it can identify, and the temperature range over which it can now operate. This tool can do much more than determine binder consistency at high and intermediate temperatures as used for Superpave.
Research at labs including the University of Wisconsin, Texas A&M, North Carolina State, the Western Research Institute, the Asphalt Institute, PRI, MTE and others is ongoing. If this research bears fruit, the DSR could be the sole rheological tool needed to specify emulsion residues.
ASPHALT: ARE NEW TEST PROCEDURES IN THE WORKS? NEW SPECS?
BAUGHMAN: The goal is a performance-based specification using a testing protocol that is efficient, reliable and accurately characterizes field behavior. The ultimate specification, with a target testing cost of $1,000 per individual certification, will only use the test conditions needed for a specific application.
For the Federal Lands study, field project samples were tested over a broad range of temperature and loading conditions to give a better understanding of material properties, the feasibility of the draft specification and potential specification limits. All of the data collected has been posted on the National Center for Pavement Preservation’s website. It is expected that the data will begin a database used by other researchers for the development of emulsion performance-based specifications.
Four documents have been submitted to AASHTO TSP2 Section for consideration:
• “Standard Practice for Certifying Suppliers of Emulsified Asphalt”
• “Recovering Residue from Emulsified Asphalt using Low Temperature Evaporative Techniques”
• “Determining Asphalt Binder Bond Strength by Means of the Bitumen Bond Strength Test (BBS)”
• “Performance-Graded Asphalt Binder for Surface Treatments (Surface PG Spec).”
ISHEE: Residue recovery and subsequent testing of the residue is a hot topic. There are several efforts underway to improve the repeatability and reliability of that testing while providing more meaningful data than from current methods.
KING: Regarding emulsion recovery methods, the high temperature distillation methods will disappear in favor of newer techniques that never heat the emulsion residue above 60°C. The same recovery method must be applicable to all asphalt emulsions. Researchers are working through the ETF to evaluate the various proposals and identify the preferred option for recovery.
PAV aging is relevant for emulsion residues, but we have no data to know what aging times and temperatures are appropriate. We’re working with much thinner films to speed recovery time, so the rate of aging in the PAV is not equivalent for the same aging time and temperature using PG-defined films. The same issues apply to the other emulsion surface treatments, such as slurry/ micro, but the results will probably be unique for each application.
Some test methods originally developed for other needs are getting attention as emulsion residue tests. WRI has applied newly available DSR compliance corrections to develop a new 4mm-plate method for low temperature consistency measurements. Given the small amount of material available from thin film recovery methods and the preferred limitation not to reheat recovered residue, this test may replace the BBR when measuring low temperature consistency of any emulsion residues.
ASPHALT: HOW ABOUT NEW EMULSION MATERIALS OR TREATMENTS/APPLICATIONS? WHAT IS THE FUTURE OF EMULSIONS?
BAUGHMAN: We continue to evaluate new products and to improve existing products. As we move to performance-based specifications, we will develop a better understanding of our products and their applications. We will continue to see innovations to improve our products and applications.
As president of AEMA in 2008, it was a dream come true to sit with Jim Sorenson, along with other industry and academia members to discuss and establish the Emulsion Task Force under the PPETG. The emulsion industry needed to take the next step to move to truly performance specifications. I was happy to be part of this movement and continue to support it. Emulsions are coming into their time to meet the needs of the pavement preservation movement.
ISHEE: Improvements to asphalt emulsions are ongoing and inclusive. The emulsion producers continually work with agencies at the federal, state and local levels to improve and evolve techniques to address the growing maintenance and preservation needs.
As importantly, industry partners, including equipment manufacturers, emulsifier producers, and latex and polymer suppliers, are expending significant capital on research and improvements. Our industry is working together to provide a more consistent, reliable and efficient product.
The emulsion industry’s future is bright indeed. Pavement Preservation has taken flight as more people tasked with the responsibility of expending taxpayer funds to maintain the transportation infrastructure embrace the common sense approach to “keeping good roads good.”
The spotlight on the environment and the effect of our actions on it also bodes well for the future of emulsions. It is well established that emulsion production and its usage in the maintenance and construction of our roadways result in a dramatic reduction in the carbon footprint and provides an eco-efficient solution.
KING: Agencies clearly show preferences for certain preventive maintenance applications, and tend to use the same ones repeatedly. That perspective should be broadened to better satisfy the preservation mantra – Right Application – Right Place – Right Time. To do this, pavement engineers need a variety of products that meet the needs over the life of the pavement.
Now that the need has been recognized, new treatments are coming, and new emulsions are being developed to improve performance. For example, we already see emulsions with crumb rubber, or emulsions with fiber woven into them in-situ. The challenge, or course, is to be sure that new specifications anticipate and can be adapted to reflect the performance of new materials.
I believe the most important challenge to emulsion technology will come in restoring and reusing the RAP already on the road. Society has made enormous investments in those materials, and the old asphalt is irreplaceable.