Expect Cooler Temperatures

Feb. 10, 2005

A decade ago, a typical compaction temperature for hot-mix asphalt (HMA) was 275°F. Though we all knew we should determine the equiviscous compaction temperature, a single compaction temperature was typically used for commonly available binder grades such as AC-20 or AC-30. In 1994, the Superpave mix design system was introduced. The implementation of the Superpave system brought renewed emphasis for the use of equiviscous mixing and compaction temperatures in the laboratory. In the field, agencies placed an increased emphasis on in-place density.

A decade ago, a typical compaction temperature for hot-mix asphalt (HMA) was 275°F. Though we all knew we should determine the equiviscous compaction temperature, a single compaction temperature was typically used for commonly available binder grades such as AC-20 or AC-30. In 1994, the Superpave mix design system was introduced. The implementation of the Superpave system brought renewed emphasis for the use of equiviscous mixing and compaction temperatures in the laboratory. In the field, agencies placed an increased emphasis on in-place density. There also was a significant increase in the use of polymer-modified binders. It is now commonplace to see inspectors and contractor personnel frequently checking mat surface temperatures with infrared thermometers. They have even been mounted on rollers. All of these factors have tended to increase production temperatures.

In many cases, contractors could probably lower production temperatures by at least 10°F with no ill effects. Some of our current situation results from the thought that if hot is good, hotter is better. Many modifiers actually begin to degrade at temperatures somewhere around 350°F. (This is highly dependent on the modifier. The binder producer should be consulted for specific recommendations.) However, for larger reductions in temperature, a different approach is needed.

Lowering the temperature at which HMA is produced and placed is desirable both for cutting emissions and for reducing fuel consumption. Dr. Mary Stroup-Gardiner and Dr. Cliff Lange, both of Auburn University, have clearly shown that increasing production temperatures results in increased emissions. In some non-attainment areas, heavily modified binder grades cannot be used due to concerns over emissions. Throughout the HMA industry, there are growing concerns about emissions and the higher production temperatures seen in recent years.

The leaders of the National Asphalt Pavement Association (NAPA) learned of cutting-edge technologies in Europe for reducing production temperatures and, during the summer of 2002, initiated a European study tour to research and evaluate three of these processes. They visited asphalt plants, paving sites and completed roads in Germany and Norway.

Finding that the technologies showed promise, NAPA began efforts to introduce the concept to the U.S. The program included a panel of European experts at NAPA’s 2003 annual convention, outreach to the highway press and a live demonstration of paving with warm mix at the World of Asphalt 2004 Show and Conference.

In order to answer questions about the technologies’ applicability to U.S. conditions and paving practices, NAPA also initiated a research program at the National Center for Asphalt Technology (NCAT) in cooperation with the companies that developed the European technologies. The research effort is jointly funded by the product suppliers, the Federal Highway Administration, NAPA and the State Asphalt Pavement Associations.

Ways to look at it

The three processes that the NAPA group evaluated in Europe have been in use for up to six years. Each process acts to reduce the necessary compaction temperature required to achieve the required in-place density, thus reducing the required production temperature. In theory, there should be no reduction in the quality of the resulting HMA. The three processes are:

  • Aspha-min zeolite developed by Eurovia;
  • Sasobit developed by Sasol International; and
  • WAM Foam developed by Shell International and Kolo Veidekke.

Zeolites are crystalline hydrated aluminum silicates. They can be naturally occurring or man-made. Aspha-min is a man-made zeolite material that is sold in a granular form with approximately a No. 50 mesh size. Aspha-min zeolite, hereinafter referred to as zeolite, contains approximately 21% water by weight. The water is released from the zeolite when it is heated to temperatures of 212 to 392°F. When the water is released in an HMA mixture, it microscopically foams the asphalt. The foamed asphalt has greater workability and allows coating of the aggregate particles at lower temperatures as well as improved compactability.

The recommended addition rate for Aspha-min zeolite is 0.3% by weight of total mix or 6 lb per ton. The zeolite can be introduced into the asphalt plant by a number of methods. On a batch plant, it can be manually added directly into the pug mill or automatically using a weigh bucket. On a drum plant, the zeolite could be added through the RAP collar, however the preferred method requires a specially built feeder. A calibrated vane feeder controls the quantity and then the material is pneumatically blown into the drum. At a trial section produced by Orlando Paving Co., an existing fiber line was used to introduce the zeolite at the same point as the asphalt.

Sasobit is a high-molecular-weight Fischer-Tropsch wax. Fischer-Tropsch waxes are produced through the Fischer-Tropsch synthesis of coal or natural-gas feedstocks. Unlike paraffin waxes, which can cause problems with permanent deformation or rutting, Fischer-Tropsch waxes have actually been shown to reduce rutting susceptibility. Sasobit acts to reduce viscosity at HMA mixing and compaction temperatures while having an increased viscosity at in-service pavement temperatures. The addition of Sasobit also increases the binder’s softening point and reduces the penetration value. The reduced viscosity at mixing and compaction temperatures acts to improve compactability. The addition of Sasobit may worsen the low-temperature properties of the binder. For example, a PG 58-28 with the addition of 2.5% Sasobit might grade as a PG 64-22. To combat this effect, Sasol International developed a special cross-linking agent to allow the incorporation of SBS polymer with Sasobit. This one-pack co-modifier system is called Sasoflex. In ongoing trials at NCAT, 4% Sasoflex was added to a PG 64-22 to produce a PG 76-22, where the low-temperature grade was held the same.

The addition of Sasobit or Sasoflex does not require high-shear blending. It can be terminal blended or blended in the contractor’s tank by circulation. In Europe, Sasobit has been added directly into the pug mill of a batch plant by liquid dosing from a molten tank.

Warm Asphalt Mixtures (WAM) Foam is a process that utilizes a two-stage addition of the asphalt binder. In the first stage, the aggregate is coated with a very soft binder, which might be referred to as a flux. The properties of this soft binder control the minimum mixing and compaction temperatures for the mixture. Then a harder binder is added with the addition of a very small amount of water. The water foams the hard asphalt, allowing the expanded binder to coat the aggregate and improving workability during compaction. The combined properties of the soft asphalt and foamed hard asphalt should meet the specified performance grade for the project.

WAM Foam requires at least two tanks to store the two different binder grades as well as two heated asphalt lines into the mixing chamber. The asphalt line for the hard asphalt requires a special nozzle to allow the introduction of a small amount of water in order to foam the asphalt.

Made for the USA?

Though all three of these processes have been successfully used in Europe, there were some concerns that differences between typical European and American paving practices could possibly cause some performance problems. Batch plants are much more prevalent than drum plants in Europe. European production rates tend to be lower than those in the U.S. In some cases, pavements in Europe (particularly Germany) are allowed to cure before opening to traffic.

One of the primary concerns related to quick turnover to traffic. If the warm asphalt additives improved workability, when did that workability end? If traffic was returned to the pavement while it was still warm (above 140°F), would the pavement be susceptible to rutting? To address this and other concerns, an experimental plan was developed to evaluate these products. The experimental plan includes evaluations of:

  • Compactability;
  • Strength gain with time (curing);
  • Resilient modulus;
  • Rutting susceptibility; and
  • Moisture susceptibility.

The first phase consists of a laboratory evaluation followed by field trials. To date, the laboratory phase has been completed for the Aspha-min zeolite and is under way for the Sasobit product.

The laboratory study used two aggregate sources, a granite and a limestone, both in a coarse-graded 12.5-mm nominal maximum aggregate size Superpave mixture. Two binder grades from two different crude sources were used, a PG 58-28 and a PG 64-22. The warm asphalt mixtures produced with the zeolite or Sasobit were compared with control mixtures without any additive. A PG 76-22 also was produced with Sasoflex.

Field compaction is difficult to simulate in the lab. Research has shown that the density of samples compacted in the Superpave gyratory compactor (SGC) are not very sensitive to compaction temperature. To evaluate compactability, samples were compacted in both the SGC and in Pavement Technology’s vibratory compactor. Samples were compacted at 300, 265, 230 and 190

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