By: Brian D. Prowell, P.E., and Graham C. Hurley Contributing Authors
The Federal Highway Administration (FHWA), the National Asphalt Pavement Association (NAPA) and the National Center for Asphalt Technology (NCAT) are working with material suppliers to evaluate a group of technologies commonly called warm-mix asphalt. The processes are designed to facilitate compaction at lower temperatures. In some cases they have been used as compaction aids for stiff mixtures and also may allow the paving season to be extended. Lower production temperatures result in reduced emissions and fuel savings.
Background information on warm asphalt mixtures was provided in the February issue of Roads & Bridges (See Expect cooler temperatures p 34). Initially, there were three processes developed in Europe:
- Aspha-min zeolite developed by Eurovia;
- Sasobit developed by Sasol International; and
- WAM Foam by Shell and Kolo Veidekke.
A new process, called Evotherm from MeadWestvaco Asphalt Innovations, was developed in the U.S. and is currently under evaluation.
Aspha-min zeolite is a man-made material that is sold in a granular form. It contains approximately 2% water by weight. The water is released from the Aspha-min when it is heated to temperatures of 212 to 392°F. When the water is released in a hot-mix asphalt (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 compaction. The recommended addition rate for Aspha-min zeolite is 0.3% by weight of total mix or 6 lb per ton. Sasobit is a Fischer-Tropsch wax. Fischer-Tropsch wax is a fine crystalline, long-chain aliphatic polymethylene hydrocarbon produced from coal gasification using the Fischer-Tropsch (FT) process. Sasobit acts to reduce viscosity at mixing and compaction temperatures while having the same or increased viscosity at in-service pavement temperatures. An example of Sasobit’s effect is shown in Fig. 1. Fig. 1 presents rotational viscosity and dynamic shear rheometer test results for two PG 64-22 binders, one with and one without Sasobit. As can be seen in the figure, Sasobit reduces the mixing and compaction temperatures by approximately 30°F, but has the same stiffness at in-service pavement temperatures. Sasobit can be co-modified with SBS polymer and a special cross-linking agent. This combination is referred to as Sasoflex.
Evotherm is a non-proprietary technology based on a chemistry package that includes additives to improve coating and workability, adhesion promoters and emulsification agents. The chemistry is delivered in an emulsion with a relatively high asphalt residue (approximately 70%). Unlike traditional asphalt binders, Evotherm is stored at 176°F. The water in the emulsion is liberated from the Evotherm in the form of steam when it is mixed with the hot aggregate. The resulting warm mix appears like hot mix in terms of coating and color.
Two of these processes, the Aspha-min zeolite and Sasobit, were evaluated in laboratory studies conducted by NCAT. Evotherm is currently under evaluation. The studies evaluated the performance of these processes under typical paving practices and conditions found in the U.S. These studies’ findings and recommendations are summarized below:
- Warm asphalt processes appear to reduce the optimum asphalt content determined with the gyratory compactor. However, the studies recommend that the optimum asphalt content for warm mixes be determined without using the warm-mix processes. Basically, one would determine the optimum asphalt content as for a normal hot-mix design. Additional samples should then be produced with the warm-mix additives so the targets for the field volumetric properties can be adjusted.
- The studies compared the compactability, modulus, rutting potential and moisture sensitivity of the warm mixtures produced at temperatures of 190, 230, 265 and 300°F with HMA. Based on these comparisons, a minimum mixing temperature of 275°F and a minimum compaction temperature of 250°F is recommended. If the mixing temperature is below 275°F, then the high-temperature grade should be bumped by one grade to account for reduced short-term aging of the binder during construction. Performance testing can be conducted to predict field performance. Field compaction will dictate the true minimum compaction temperature depending on a number of factors.
- Moisture sensitivity can increase with decreasing mixing temperatures. Moisture sensitivity testing should be conducted at the anticipated field production temperatures. If test results determined are not favorable, an anti-stripping agent should be added to the mix to increase the resistance to moisture.
- More research is needed to further evaluate field performance, the selection of the optimum asphalt content and the selection of binder grades for lower production temperatures.
The complete reports can be downloaded at www.NCAT.us.
Parallel with NCAT’s laboratory study, several field trials were constructed using all three processes. To date, three field demonstration projects have been conducted in the U.S. with Aspha-min zeolite. The first was constructed in February 2004 at Hubbard Construction’s equipment yard in Orlando, Fla. The control mix met the requirements of a Florida Department of Transportation Traffic Level C, fine-graded, 12.5-mm NMAS Superpave mix. The mixture was produced with crushed granite aggregate and 20% reclaimed asphalt pavement (RAP).
Traffic level C is designed for 3 to 10 million ESALs with an Ndesign = 75 gyrations. The warm mix was produced by adding Aspha-min zeolite to the control mixture at the rate of 0.3% by weight of total mix.
The discharge temperatures from the drum were 336°F for the control mix and 300°F for the Aspha-min warm mix. The mix was dumped directly into the paver at the site. Temperatures behind the screed ranged from 293 to 315°F for the control mix and 256 to 260°F for the warm mix. Breakdown rolling was conducted with two 10-ton rollers (Bomag BW9AS and Hypac C340C) rolling in echelon. The rolling pattern was established with the control mix and consisted of four static breakdown passes followed by two static finish roller passes. The paving crew observed that the warm mix was more workable than the control mix. Densities were monitored by nuclear gauge tests and cores. Although the warm mix was compacted approximately 40°F lower than the conventional hot mix, the resulting in-place densities were almost identical.
After one year, the site was visually inspected and cores were taken to assess the current condition of the warm mix. Indirect tensile tests were performed and results indicated that in the field the Aspha-min warm mix was equally resistant to moisture damage as the control mix. No distresses were noted at the site.
Additional Aspha-min demonstrations were constructed in Nashville, Tenn., in March 2004 as part of the World of Asphalt and in Charlotte, N.C., in September 2004. All three projects used drum plants to produce the warm mix. A specially built feeder was used to introduce the Aspha-min zeolite into the mixing chambers. Numerous projects also have been constructed in Europe using Aspha-min.
Sizzling success
Since 1997, more than 142 projects were paved using Sasobit totaling more than 2,716,254 sq yd of pavement. The projects were constructed in 19 countries around the world. Addition rates ranged from 0.8 to 4.0% Sasobit. In June 2005, Maryland State Highway Administration (SHA) completed a series of deep patches using Sasobit. The Sasobit was added not to produce warm mix, but to act as a compaction aid in a stiff 19-mm NMAS mix containing RAP. Three combinations were evaluated: the mix without any additive, the mix with 1.5% Sasobit by total weight of binder at normal production temperatures and the mix with Sasobit at reduced temperatures (approximately 20°F). Tom Harmon from FHWA stated that Maryland SHA wanted to “leverage warm-mix technology to improve compaction of difficult mixes.”
F.O. Day produced the mix using an Astec Double Barrel Drum Plant. The Sasobit was pneumatically fed into the mixing chamber using an existing fiber feed line. Larry Michael from Maryland SHA, who arranged the trial, said that the original mix was almost “unworkable.” With the Sasobit the mix flowed well, was workable and more readily compacted at both temperatures. Maryland SHA has additional plans to use more Sasobit in the near future.
The first Evotherm construction project was conducted in South Africa in November 2003. The first public U.S. trial of Evotherm was constructed in Boone County, Ind., an Indianapolis suburb, in July 2005. The Heritage Group oversaw the project. The Heritage Group is a vertically integrated company with quarries, binder and emulsion production capabilities, hot-mix plants, laydown crews and engineering and environmental services. When Herb Wissel, Heritage Research Group’s assistant director for research, was asked why the Heritage Group was interested in warm asphalt he responded, “The Heritage Group is interested in both the environmental aspects of reduced emissions as well as the potential for fuel savings and improved performance.”
The Evotherm emulsion was produced by Asphalt Materials Inc. using MeadWestvaco’s chemistry package. The warm-mix asphalt was produced by Milestone Contracting using a hybrid batch/drum plant in drum plant mode. Triangle Asphalt laid 660 tons of warm mix on county road 900E.
The Evotherm was added to a 12.5-mm NMAS coarse-graded Superpave mix produced with crushed dolomite and 15% RAP. The substitution of Evotherm for conventional binder requires that the mix batch weights be recalculated to account for water in the Evotherm emulsion (approximately 30%) to produce the same residual binder content. The MeadWestvaco and Heritage Group team wanted to minimize production temperatures so that emissions would be minimized and fuel savings maximized. Discharge temperatures from the mixing drum stabilized at approximately 200°F. Even at such low temperatures, the aggregate was completely coated and appeared like conventional hot mix with none of the brown or grey coloration often associated with emulsions. Steam release was evident at times from both ends of the mixing drum and the slat conveyor but varied in quantity throughout the production run. The baghouse was examined after all of the warm mix was produced. The bags and fines appeared dry.
The haul distance to the paving site was approximately 10 miles. There were no problems with mix sticking in the trucks. The mix was laid at an average depth of approximately 2 in. using a Blaw-Knox paver. Temperatures behind the screed ranged from approximately 160 to 200°F for the warm mix. There were no visible emissions around the paver. Breakdown rolling was accomplished with an Ingersoll-Rand DD-90 applying two vibratory and a single static pass. The mix did not show any signs of tenderness. A Hyster 350C applied three static-finish passes. After placing approximately 400 tons of warm mix, the paver backed up to pave the other lane. The dump trucks then hauled over the freshly laid mat. No damage to the fresh mat was noted. However, one of the screed operators on the paver noted, “I miss the sizzle.”
Once the 660 tons of Evotherm warm mix were laid, hot mix was shipped to the site for a control section. Hot-mix temperatures at the discharge of the drum were in the neighborhood of 320°F, and mat temperatures were 300-317°F behind the screed. Walking beside the paver, the heat was immediately evident. The “sizzle” was back.
More warm-mix trials using the three technologies described in this article are scheduled later in 2005. Though there is still a lot to learn about warm mix, the technologies appear promising. The lower temperatures are certainly attractive if the performance is equal to hot mix and the economics work out.
