Monitoring of a warm-mix asphalt placement job in Michigan.

Warm-mix asphalt is steadily gaining acceptance throughout the asphalt paving industry. However, there is still much to prove when it comes to performance.

The National Asphalt Pavement Association introduced warm-mix asphalt technology to the United States from Europe in 2002, spurring immediate interest among hot-mix asphalt engineers and producers because of warm-mix asphalt’s potential environmental and engineering benefits. Today, at least 45 states either actively use warm-mix asphalt technology or have constructed trial projects. A number of states, including Alabama, California, Florida, Illinois, New York, North Carolina, Ohio, Pennsylvania, Texas, Virginia, Washington, and Wisconsin, have adopted permissive specifications allowing the use of warm-mix asphalt on many highway projects.

The basics and benefits
How exactly is warm-mix asphalt different from conventional hot-mix asphalt, and what specific benefits make it so appealing to pavement engineers? Warm-mix asphalt is a comprehensive term for a variety of technologies that allow asphalt paving mixtures to be produced and compacted at lower temperatures without any loss in workability. Temperature reductions of 50 to 100°F have been documented, which results in environmental benefits of reducing fuel consumption and greenhouse gas emissions. In addition, other potential benefits include the ability to pave in cooler temperatures, use longer haul distances while still obtaining density targets, and incorporate higher percentages of reclaimed asphalt paving at reduced temperatures. Some highway agencies have also found that warm-mix asphalt can be used when constructing an overlay to eliminate or reduce the bumps caused by the expansion of underlying crack sealant. Pavement construction workers also benefit; tests have shown a significant reduction in asphalt aerosols/fumes and polycyclic aromatic hydrocarbons with the use of warm-mix asphalt compared with hot-mix asphalt.

Warm-mix asphalt has been used for all asphalt types, including dense-graded, stone-matrix, porous, and mastic asphalt. It has also been used in a range of layer thicknesses and in a wide variety of applications, from parking lots and low-traffic roadways to interstate highways.

Warm-mix asphalt technologies
Warm-mix asphalt technologies can be classified in a few different ways. The first way is by degree of temperature reduction, as there is a wide range of production temperatures in warm-mix asphalt that separate warm-mix from half-warm-mix asphalt. Warm-mix asphalt technologies can also be classified by distinguishing those that foam the asphalt binder and those that use some form of chemical additive to improve mix workability at lower temperatures.

Asphalt foaming technologies include adding small amounts of water to the hot asphalt just prior to mixing with aggregate, using damp aggregate, or zeolite, which contains adsorbed water. When that water turns to steam at atmospheric pressure, it is dispersed into the hot asphalt and results in binder expansion and a reduction in mix viscosity. The warm-mix asphalt technologies that use organic additives or specially formulated waxes reduce the viscosity of the asphalt binder above the melting point of the wax. The wax must be carefully selected to ensure that its melting point is higher than temperatures expected to occur when the asphalt is in service and to minimize asphalt brittleness at low temperatures.

When using warm-mix asphalt additives, there are additional material costs to consider, but no plant modifications are typically required. Alternatively, producing warm-mix asphalt using the water injection/foaming process requires an initial equipment investment but essentially no additional material costs.

Some state agencies specify that contractors use state-approved warm-mix asphalt processes or products. These agencies require verification of satisfactory performance in order to approve new warm-mix asphalt technologies for use on state-maintained roads. Since trial projects can be challenging and costly for warm-mix asphalt manufacturers to set up in many different states, the National Center for Asphalt Technology (NCAT) has begun a warm-mix asphalt certification program that will involve detailed evaluation of well-controlled test sections on the NCAT Pavement Test Track and a very comprehensive laboratory testing program.

Design, production, and placement
Research is currently under way through the National Cooperative Highway Research Program (NCHRP), project 9-43, to develop a standard mix design procedure for warm-mix asphalt. Currently, hot-mix asphalt mix design procedures are used, followed by a trial plant run using the warm-mix additive or process for verification. Quality control and quality assurance (QC/QA) testing is performed at the warm-mix temperature and must meet standard hot-mix asphalt requirements.

Production of some warm-mix asphalt technologies require minor plant modifications and fuel burners may need to be adjusted to operate efficiently at lower temperatures. Care should also be taken for mixes containing absorptive coarse aggregate since aggregates with higher water absorption can make it difficult to dry the aggregate completely at lower production temperatures.

It should also be noted that several warm-mix asphalt processes modify the asphalt binder, which may affect its specification grade. Lower mixing temperatures also reduce aging of the binder during production. Therefore, as recommended in several studies, performance testing should be used to assess warm-mix asphalt mix designs before the field trial stage.

Placement practices usually do not differ between warm-mix asphalt and hot-mix asphalt, with the important exception of lower compaction temperatures. Improved compactability is often cited as a potential benefit for warm-mix asphalt technologies.

Early warm-mix asphalt performance is favorable
Overall, early performance of warm-mix asphalt projects has been very good — comparable to that of conventional hot-mix asphalt. However, laboratory tests have indicated an increased potential for rutting and moisture susceptibility for some warm-mix asphalt mixes. Whether or not these test results are good predictors of performance is being carefully examined. Lower temperatures could result in incomplete drying of the aggregate, compromising the bond between asphalt and aggregate. With reduced mixing temperatures there is also less binder aging, which can result in lower tensile strengths and/or increases in rutting in lab tests.

However, field evaluations have not shown any practical difference in rutting between warm-mix asphalt and control hot-mix asphalt sections. One case study is a warm-mix asphalt project in Missouri, which has no appreciable rutting in any of the warm-mix asphalt or hot-mix asphalt sections after two years of heavy traffic. The warm-mix technologies used in the Missouri trial were Aspha-min (a zeolite), Sasobit (a wax), and Evotherm ET (a chemical additive). In addition, no moisture damage was observed in the warm-mix asphalt sections evaluated, and tensile strength of the warm-mix asphalt sections have increased over time to similar values as the hot-mix asphalt after two years. A field trial in Ohio also revealed that warm-mix asphalt sections with Aspha-min, Sasobit, and Evotherm performed as well as or better than hot-mix asphalt control mixes in laboratory testing for rutting and moisture susceptibility.

NCHRP 9-47A
NCAT is leading a team of specialists working on NCHRP 9-47A, a project that will document field performance of warm-mix asphalt technologies across the United States, compare engineering properties of warm-mix asphalt to hot-mix asphalt, and evaluate energy savings and emissions reductions for warm-mix asphalt production. The research team will monitor short-term warm-mix asphalt performance during a two-year period and will use the Mechanistic-Empirical Pavement Design Guide to predict long-term performance.

Compaction temperatures of warm-mix asphalt (top) compared with hot-mix asphalt (bottom).

NCAT and Advanced Materials Services are currently documenting 14 regionally diverse warm-mix asphalt projects and testing materials from the eight projects that are new construction (see Table 1). A variety of warm-mix asphalt technologies are included. Documentation at several of the projects will include an asphalt plant energy audit and stack emissions testing, as well as workspace respiration monitoring to measure asphalt fume exposure levels for paving crew members. Six existing sites, as shown in Table 2, are being monitored for performance. Both new and existing sites have control hot-mix asphalt sections for comparison.

Karen Hunley has been working with the National Center for Asphalt Technology (NCAT) at Auburn University as a communications specialist since June 2010. Courtney Jones has been writing for NCAT since January 2009. She has a bachelor’s degree in civil engineering from Auburn University and has been involved with the asphalt industry since 1995. They can be reached at karen.hunley@auburn.edu and courtneyj@centurytel.net, respectively.