Warm Mix Asphalt - Gaining momentum
By Dwight Walker, P.E.
Warm mix asphalt (WMA) is the latest high impact innovation in the asphalt industry. Surprisingly, WMA is not a new mix or materials system. At the most basic level, it is a means of producing an asphalt mix without heating the aggregate to the normal hot mix asphalt (HMA) levels.
WMA technology allows mixing and placement at temperatures significantly lower than those used for HMA. Using one of the WMA processes, asphalt mixes can be produced as much as 100o F (37o C) lower than by conventional HMA means.
The current interest in WMA is incredible. And unlike Superpave or SMA or other recent asphalt innovations, the momentum is not coming from a government push. It is coming from hot mix producers who are looking for a better way to produce asphalt mixes. WMA was debuted in Europe in 1995 and the first public U.S. demonstration of WMA was in 2004. Since then the U.S. total is approaching 500,000 tons of WMA.
The National Asphalt Pavement Association (NAPA), along with the Federal Highway Administration (FHWA), and the American Association of State Highway and Transportation Officials (AASHTO) sponsored a conference in Nashville, Tennessee, in November 2008. More than 650 people attended the sold-out event, and it was web-cast to 70-plus sites around the world.
One of the advantages of using WMA is immediately obvious—fuel savings. Other benefits include fewer emissions and improved workplace conditions. There are also paving benefits. These include: acting as a compaction aid, facilitating cold weather paving, allowing longer haul distances/times, and allowing higher percentages of RAP.
Fuel savings on the WMA projects monitored thus far indicate that burner fuel savings of 20 to 35 percent are possible. Fuel savings are dependent on the temperature reduction, aggregate moisture content, and plant settings.
Using WMA should result in reduced volatile organic compounds (VOCs), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and particulates. Since most plant emissions result from burning fuel and WMA uses less fuel, there should be lower emissions. The amount of emissions reduction will depend on the temperature reduction, the type of fuel used, plant settings, moisture content of the aggregate and RAP use.
Workers should find that WMA provides a more comfortable working environment. The cooler material should be more pleasant to work around, and there should be fewer burns. Worker exposure to asphalt fumes will be reduced.
WMA can be easier to compact than HMA. Some WMA methodologies have been added to mixes containing highly modified binders (PG 82s, etc.) to facilitate aggregate orientation, which results in better field densities. Lab specimens of WMA routinely have less air voids and VMA than corresponding HMA specimens.
The reduced viscosity of WMA mixes’ binders and the slower cooling rate of WMA mixes allow paving to be completed under cooler conditions than normally associated with HMA. This condition allows for extended paving seasons or longer haul distances and times.
Some additional benefits have been found from the early WMA projects. One advantage is the ability to avoid the common problem of bumps forming when paving over crack sealants. The reduced mat temperature does not trigger the formation of the bumps. Another benefit observed in Texas has been reduced cracking, both in the lab from their Overlay Tester and on the road.
There are multiple WMA methodologies; at least 14 suppliers are currently marketing to the U.S. The WMA methodologies can be roughly grouped as those which use some type of organic additive or wax, those which use a chemical additive or surfactant, and those which use water for foaming.
The WMA processes which use organic additives or waxes have a decrease in viscosity when heated above the melting point of the wax, allowing mixing and coating. The warm mix processes using surfactants work via a variety of different chemical mechanisms. The processes with water use the volume expansion from the conversion of liquid to gas/steam to cause an expansion of the asphalt binder and a resulting decrease in the mix viscosity. The water can be introduced through a foaming operation or by a material containing internal water, such as a zeolite, or from a moist aggregate.
The choice of WMA process depends on several factors. One consideration is how many tons will be produced. Some of the methodologies have higher initial equipment costs. Others using additives have higher costs per ton produced. Another consideration is how much temperature reduction is desired. Some technologies offer more temperature reduction than others. Some WMA additives can affect the final PG binder grades. Typically both the high-temperature and low-temperature grades are raised slightly.
Some of the WMA processes used to date in the US include:
- Akzo Nobel’s Rediset WMX. Rediset uses surfactants and organic additives in pellet form. The surfactants improve the wetting ability of the asphalt for better coating, and the organic additives provide a reduction of the binder viscosity and a lubricating effect for easier coating and compaction. Rediset can be blended directly into the asphalt or directly into the mixing drum near where the asphalt is introduced.
- Astec Double Barrel Green. This system is a drum-plant modification that uses a series of nozzles to introduce one pound of water per ton of mix produced.
- The Hubbard Group’s Aspha-Min. This process uses a synthetic zeolite containing about 20 percent water of crystallization. Upon heating above the boiling point of water, a small amount of water is released, creating foaming. For batch plants, the zeolite material can be added to the pug-mill. For drum-plants, a feeder is needed to introduce the material into the binder stream.
- McConnaughay Technologies Low Energy Asphalt (LEA). In the LEA process, the coarse aggregate is mixed with the full amount of asphalt binder. Both the coarse aggregate and asphalt are heated to conventional temperatures. The asphalt is treated with 0.5 percent by weight of binder of the additive which promotes coating and adhesion just prior to the mixing operation. After the coarse aggregate is coated, it is mixed with the cold, wet fine aggregate and RAP, if applicable. The resulting steam foams the asphalt on the coarse aggregate and coats the fine aggregate.
- MeadWestvaco’s Asphalt Innovations Evotherm DAT. The Evotherm DAT chemical additive is introduced into the asphalt line just before mixing for drum plants or directly into the pug-mills of batch plants. The Evotherm chemicals enhance coating, adhesion and workability at reduced temperatures.
- PQ Corporations’s Advera. This technology uses a synthetic zeolite to create micro-foaming. It is added to the pug-mill of a batch plant, and it is added close to the binder introduction point in a drum plant.
- Revix from Mathy Technology and Engineering Services and Paragon Technical Services. Revix uses a combination of materials to reduce the friction between aggregate particles and the asphalt film rather than using foaming or viscosity reduction. Revix-treated binder can be terminal- or plant-blended, or Revix dry additives may be added at the plant to the drum or pug-mill, or directly to the binder or aggregate.
- Sasobit. Sasobit uses a synthetic paraffin wax to reduce the asphalt viscosity at mixing temperatures. Sasobit can be added directly to the binder, added through a RAP collar, or through a feeder similar to a fiber feeder.
- WAM Foam. The WAM Foam process initially coats the coarse aggregate with a soft binder then water is added to foam the hard binder. For batch plants, a second asphalt feed line, as well as a foaming nozzle and expansion chamber are needed. For drum plants, a second asphalt line and a water line are needed.
As with any new technology, further study of warm mix is needed. Some of the concerns identified thus far include the need for a comprehensive specification, finding a mix design strategy, determining if moisture susceptibility is a consideration, etc.
A significant need for the long-term adoption of WMA is the development of a specification or product approval process. Proprietary specifications are suitable for trial applications but a more long-term approach is needed. To start this process, the Warm Mix Asphalt Technical Working Group has developed a guide specification that is available at www.warmmixasphalt.com/Publications-Guidelines.aspx.
When WMA was introduced, engineers recommended that the hot mix design be followed for producing the warm mix. It is now recommended that a mix design be performed using the applicable WMA process at the anticipated production temperature. However, for processes using a foaming process, this may be a problem. Laboratory foaming devices are available but are not common. Lab compaction temperatures should correspond to the actual field compaction temperatures. Some of the WMA processes may yield lower air voids and VMA than the companion HMA design. Current practice is to use the HMA design binder content with the reduced voids to safeguard against durability concerns. NCHRP 9-43 is responsible for developing a mix design procedure for WMA. The report is expected in 2010.
The two most common performance concerns about WMA are potential for increased rutting and moisture damage. Several WMA installations have been subjected to heavy traffic, and no rutting problems have been observed. Regarding moisture damage, the tensile strength ratio (TSR) of some tests performed on plant-produced WMA has been lower than comparable HMA samples. This lower TSR has been attributed to the reduction in binder aging at the lower production temperatures. Cores taken from WMA pavements after some service time have shown an increase in TSR and have not shown any moisture damage/stripping. European practice is to use anti-stripping agents in WMA.
The greatest need for follow-up is an evaluation of long-term performance. Even with all the enthusiasm for WMA, the proof lies with performance. In order for warm mix to be a success, it must perform as well as hot mix.
|Dwight Walker is a Contributing Editor for Asphalt Magazine.