From beer to bratwurst, the German influence in the city of Milwaukee is quite unmistakable. Although some European influences on the city are undeniable, a less-known influence is the concept of warm-mix asphalt. Although the idea of lowering the production and placement temperatures of hot-mix asphalt to conserve energy and reduce emissions is a new concept to Milwaukee, the practice has been used for over a decade in Germany and other European nations.
One of the major influences on European countries developing warm-mix technologies has been the Kyoto Protocol, which is an international treaty to reduce greenhouse gases by more than 5% over the next four years. Germany has pledged to reduce their emissions by 25%.
The U.S. has not formally ratified the Kyoto Protocol, but Milwaukee has taken the leadership role like Germany and has gone beyond what is required nationally and internationally. The city has formed an office of environmental sustainability. The office’s primary function is to reduce energy consumption and promote economic development in the green-technology sector.
“We believe strongly about the direction and the commitment the city has made to environmental sustainability and believe warm-mix asphalt is a crucial element of that commitment,” said Ghassan Korban, coordination manager of the Department of Public Works.
Brown has green feel
As part of the city’s initiative, Payne and Dolan Inc. of Waukesha, Wis., just recently completed a warm-mix paving project in a brownfield development area. The project required using green construction practices from road design to building construction. To fully fit the bill of environmental sustainability for the city, Payne and Dolan used warm mix for the project as well as 25% recycled asphalt in the mix.
“We met with the city of Milwaukee public works department to ensure we all understood the specifications and what was expected prior to construction of the project, and everyone was on the same page prior to paving. Meeting with the city as well as the technical support from PQ Corp. on their Advera product were all critical steps to ensure things ran smoothly and the city got exactly what they wanted,” said Todd Hughes, project manager for Payne and Dolan. The project was scheduled for construction in early November 2007, so everything is taken into account during the planning, from the expected ambient air temperatures to the mix design in the laboratory.
The mix designs chosen for the project were a 19-mm E-1 Superpave base layer and a 12.5-mm E-1 Superpave surface layer. The asphalt binder used in both mixes was a PG 64-22. Both the surface course and base course designs included 25% fractionated reclaimed asphalt pavement (RAP).
The use of fractionated RAP has become more commonplace in the industry because the material is split similar to that of the virgin aggregates in the mix. This type of splitting allows the use of higher RAP percentages and thus maintains correct mix properties.
The design laboratory batched and mixed the optimum aggregate structure together with optimum binder content adding 0.25% of Advera by weight of mix and compacted it at a reduced temperature of 225°F, which is 50° lower than normal. The mixture volumetrics in the design lab produced 4% air voids for both mixes. Maintained in the field, this reduction would result in a minimum fuel reduction of 10% in the drying process of the aggregate alone.
Achieving the required volumetrics in the design lab was met with normal protocols. When field production began, achieving the specified volumetrics required the total added binder to be slightly reduced to raise Va in both mixes. The aggregate blend gradation for each mix was as follows:
The asphalt produced for the job was at Payne and Dolan’s plant located in Waukesha, which was only a few miles from the project. The plant included a counterflow Dillman drum with a 500-ton-per-hour rating, a Hauck ES 150 burner and a Standard Haven baghouse with a CFM rating of 70,000. The Advera product was added through the fiber port of the drum. The material is blown in very similar to the method cellulose fibers are added for stone-matrix asphalt mixes. The fiber port is located just after the entry point of the asphalt binder. The Advera warm-mix product contains approximately 18% moisture, which is released as steam into the mix. The workability of the mix is improved by this process, which produces what could be considered a foaming process by releasing only 0.03% water into the mix.
Based on experience from previous warm-mix projects, Payne and Dolan made some minor plant adjustments to ensure proper drying of aggregates at the reduced temperatures. The drying zone in the Dillman drum allows flighting to be shut or turned down. The third row of flighting in this particular drum was closed to force more heat into the baghouse and maintain temperatures above 225°.
To ensure proper temperature and to aid in monitoring mix temperatures, Payne and Dolan installed Lay-tec infrared temperature probes at the discharge chute. Direct wiring to the control house allowed accurate instantaneous reading of mix temperatures during production. The most noticeable difference with the plant was the minor amount of smoke and emissions noticed during the load-out procedure. This is even more remarkable considering the ambient air temperatures were in the 40s.
The paving operation consisted of paving approximately 4,500 tons of mix through an industrial corridor including several crossovers. On the first day of paving, a 19-mm E1 base course was placed. Payne and Dolan used a Blaw Knox PF 3200 paver with a Carlson EZ IV screed for letdown. The ambient temperature that morning was roughly 40°F and rose into the mid-40s as the day progressed.
The first couple of loads of mix were shipped to the project at normal HMA temperatures to warm up the equipment. As the project progressed, however, the mix temperatures were continually lowered until reaching the approximate target of 255°F delivered. Although the mixture did not exhibit any of the typical blue smoke normally associated with HMA when dumped into the hopper or extruded from the screed, the material still seemed to flow as though it was 290°F. Even at 245°F the material did not exhibit any dragging or tearing under the screed, and the paving crew was able to shovel and rake the material with ease.
Behind the breakdown roller, the material compacted extremely well even though it was at a temperature 50°F cooler than normally compacted. The densities behind the breakdown roller were consistent with those obtained with conventional HMA mixtures, which is roughly 89 to 90% of the theoretical maximum density (TMD). Amazingly, the material continued to compact all the way down to temperatures of roughly 120°F. A Hypac 778 roller was used in the breakdown position, and a second Hypac 778 was used in the cold roller position. Additionally, the rate of heat loss of the material seemed to be reduced when compared with HMA.
“What really surprised me was the material took over an hour and a half to cool from 245°F down to 120°F even though the ambient temperature was near 40°F,” said Brett Stanton, corporate engineer for Payne and Dolan. Using just two vibratory steel-drum rollers, final densities for the base course layer surpassed the specifications of 92% of TMD, despite the lower ambient temperature and lower mixture temperatures.
After the two layers of base course were placed, an E1 12.5-mm surface layer was laid beginning on Nov. 12. Again, the mixture arrived at the project nearly 50°F cooler than conventional HMA. Once more, although the mixture did not produce any blue smoke it continued to flow as though it was 50°F warmer. Densities behind each roller were similar to the previous day’s results and consistent with other HMA mixtures. Once more, the mixture continued to compact easily down to temperatures unheard of for typical HMA applications.
In short, the Advera additive demonstration near Miller Park was a success. The mixture containing the additive exhibited flexibility down to temperatures unheard of for most conventional HMA mixtures, even when those mixtures are placed in the middle of the summer in 80°F-plus weather. The overall average density for all layers exceeded those specified, and the mixture’s appearance was exceptional. Finally, Advera performed exceptionally well and seems to be an extremely viable product for producing true warm-mix asphalt.