By John I. Duval, P.E and Robert Humer, P.E
Warm mix asphalt (WMA) is the “darling” of the asphalt industry. After all, what’s not to like? WMA promises lower emissions, energy savings, fewer fumes, improved worker safety, better density, improved durability and increased capability to incorporate reclaimed asphalt pavement (RAP) to name a few of its benefits.
Interest in WMA Builds
More than a promise, WMA is proving its merits on countless highways, roads, streets and airports across the country. As the number of successful WMA projects climbs, mix producers are increasingly calling on state departments of transportation (DOT) and other public agencies for “permissive specifications” allowing the use of WMA on any DOT project at the “contractor’s option.”
WMA performance questions
While the benefits of WMA mixtures are many, there are some lingering performance questions that are focused primarily on moisture damage susceptibility and rut resistance. In general, WMA mixtures are more susceptible to moisture damage (stripping) due to the possibility that entrapped water in the aggregates may not be driven off at lower WMA temperatures. WMA rut resistance comes into question because of the lack of beneficial short-term aging of the asphalt binder that occurs at HMA temperatures.
An additional question, specific to the asphalt binder, is the effect of various WMA technologies on the performance grade (PG) of the binder. It is important to be aware that introducing additives or altering the process used to produce asphalt mixtures can have an effect on the final PG grade of the binder.
This has led some asphalt binder suppliers to be concerned about their “downstream liability” with regard to certificates of compliance and modifications at the plant with WMA additives or foaming processes. The American Association of Highway and Transportation Officials (AASHTO) provides specific guidance in their “R26, Standard Practice for Certifying Suppliers of Performance Graded Asphalt Binder”, which states… “if any modification…is made at the HMA plant, the HMA producer shall be the supplier and must provide the certification.”
The fact is that the speed of implementation of WMA in the U.S. has outpaced our ability to test WMA in the laboratory. Hundreds of thousands of tons of WMA are being produced each year and as the table shows, the list of WMA technologies has grown to twenty-one technologies. The National Cooperative for Highway Research Program Study 9-43 “Mix Design Practices for Warm Mix Asphalt” is underway but it is not clear when or to what extent this project will provide specific guidance.
Laboratory evaluation of WMA additives and foaming processes
In the meantime, state DOTs must determine the extent to which they will accommodate warm mix asphalt, knowing that some WMA technologies are difficult to mimic in the lab and thus examine their performance in the field. In practical terms, most WMA additives are relatively easy to introduce into an asphalt mixture in the laboratory. Wax, chemical and mineral additives have been successfully blended into asphalt mixtures in the laboratory at warm mix temperatures.
Asphalt foaming technologies, on the other hand, pose a greater challenge as questions remain on how to best develop foamed asphalt mixtures in the laboratory that mirror conditions in the field. Some DOT laboratories are already equipped with an asphalt foaming device that allows them to create foamed asphalt. Such devices are available from Wirtgen GmbH and Pavement Technology, Inc.
Another consideration is that not all asphalt binders foam well. Depending on the crude source, refining methods employed and modifiers used to produce the grade of asphalt, some binders foam readily while others do not foam much at all. In addition, there is an ever-increasing number of different foamed asphalt systems on the market, currently numbered at fourteen.
Various DOT approaches to WMA
In light of both the positive benefits of WMA and the concerns about WMA mix evaluation, state DOTs find themselves in a predicament. On one hand, they want to encourage the use of WMA and would prefer not to put up roadblocks to its use. On the other hand, most highway agencies are not keen to “open the floodgates” to WMA without evaluating WMA mixtures in the laboratory. In response to this situation, public agencies are moving in different directions.
A growing number of states have adopted permissive specifications that allow contractors to substitute WMA for HMA on state projects. These DOTs do not necessarily evaluate the WMA mixtures in the laboratory. Rather, they evaluate the contractor’s submitted HMA mixture at hot temperatures. This is done without introducing WMA additives or simulating any WMA process modifications such as foaming. The asphalt content is selected and rut resistance and moisture damage susceptibility evaluated at standard elevated temperatures without the WMA. The contractor is then allowed to switch on the water-injection system, introduce additives and lower plant temperatures to produce WMA for the job.
In Florida, for example, the DOT has placed 300,000+ tons of WMA on 26 projects as of July 2010. Florida’s permissive specification allows WMA to be used at the contractor’s option after normal HMA mixture design and evaluation. Florida DOT engineers are monitoring properties of the WMA mixes produced and placed on their projects and expect to use a “data driven” approach to making future decisions about the use of WMA in the state.
Other states are holding firm to the mixture evaluation systems that have served them well for many decades by insisting on laboratory mixture and evaluation. For example, CalTrans is planning for a major increase in WMA tonnage, likely three-quarters of a million tons to be specified and produced over the next two years. CalTrans insists that its existing mixture evaluation system will remain intact.
“In my opinion, WMA mix design procedures must replicate the mix being produced,” says CalTrans District Materials Engineer Joe Peterson, P.E.
TxDOT has been recognized as a leader in the implementation of WMA and for good reason—more than a million tons of WMA has been placed in the state as of 2010. The Texas DOT was uniquely prepared to lead WMA implementation in the U.S. because it routinely evaluates rutting and moisture susceptibility of all HMA mixtures using the Hamburg Wheel Tracking Device (HWTD) a minimum of three times over the course of a project. During the mixture design phase, TxDOT conducts a traditional HMA mixture evaluation in the laboratory. At the start of the project, the contractor is required to produce a 50-ton trial batch which is sampled and tested. This is where TxDOT is able to evaluate the exact WMA mixture that will be produced during the project. After an evaluation of mixture properties and HWTD performance, TxDOT approves the project to move on to full production. At least once during full production, WMA is sampled again and tested using the HWTD. This procedure, which is identical for HMA mixtures, allows TxDOT to fully evaluate WMA mixtures prior to and during production.
The New York State DOT is embarking on a large-scale WMA evaluation program with 24 experimental projects over the next two years. Each project will include an HMA test section (minimum 1,000 tons) to allow comparison to the majority of the mix which will be WMA. Contractors will be allowed to select from a list of NYSDOT-approved WMA technologies. Prior to production, both HMA and WMA mixtures must be tested for rut resistance and moisture sensitivity and results submitted to NYSDOT. According to Zoeb Zavery, P.E., NYSDOT Civil Engineer II, WMA test specimens must be made from plant-produced mix if the laboratory preparation process does not simulate the production process.
Moving forward with WMA
As the utilization of WMA increases in this country, DOT materials engineers, asphalt suppliers and WMA producers should not simply sidestep this issue of WMA evaluation. We need to move forward with WMA, but without giving up the right to fully examine the asphalt mixture that is intended to be produced and placed.
We recommend that DOT materials engineers continue the practice that has been utilized for the better part of the past century—conduct a mix design and evaluation using the actual materials at anticipated mixing and compaction temperatures. In some cases, a laboratory foaming device may be an acceptable method to simulate foamed asphalt mixes. For those WMA technologies that are not compatible with laboratory mixing, we advocate the evaluation of plant produced mixtures for volumetric properties, rut resistance, and moisture sensitivity, at a minimum.
WMA appears to represent a big part of the future of the asphalt industry, but in order to be confident of the long term promise of warm mix asphalt, we must not shy away from the opportunity to test and evaluate it before it is used on major DOT projects.
Warm Mix Asphalt
Twenty-two technologies are available in the United States as of August 2010.
- Rediset WMX
- Aquablack foam
- Double Barrel Green/Green Pac
- Low Emission Asphalt (LEA)
- Meeker Warm Mix foam
- Terex foam
- Tri-Mix foam
- Ultrafoam GX
Source: Federal Highway Administration
John Duval is a consulting engineer in Portland, Oregon. He is a former Regional Engineer for the Asphalt Institute and is a former Technical Marketing Manager for MeadWestvaco, the manufacturer of Evotherm® WMA additive.
Bob Humer is a Senior Regional Engineer for the Asphalt Institute in Los Angeles. He has 30+ years experience in asphalt technology and mixture design and evaluation.