Superpave Use Spreads in Iowa

Dec. 28, 2000
The city of Des Moines has implemented new asphalt technology on two of its city streets over the past two years. The 1995 construction of east 29th Street was the first project in Iowa to utilize both Superpave binder and mix design. The 1996 construction of Douglas Avenue was an overlay project, completed in one day, which utilized both Superpave binder and SHRP gradation. SHRP projects in Iowa have been the result of state, county, city and/or contractor vision. It is this vision that has allowed those involved to take the next step, hoping to further improve hot-mix asphalt.
The city of Des Moines has implemented new asphalt technology on two of its city streets over the past two years. The 1995 construction of east 29th Street was the first project in Iowa to utilize both Superpave binder and mix design. The 1996 construction of Douglas Avenue was an overlay project, completed in one day, which utilized both Superpave binder and SHRP gradation. SHRP projects in Iowa have been the result of state, county, city and/or contractor vision. It is this vision that has allowed those involved to take the next step, hoping to further improve hot-mix asphalt. Although the mixes produced in Iowa have improved greatly under Quality Management Asphalt (QMA), the research results of Superpave are promising enough to experiment with.

29th Street project

The 1995 project was made possible by the joint efforts of Public Works director John Bellizzi and Des Moines Asphalt and Paving Co.

The project, which began Aug. 21, 1995, and was completed Sept. 13, 1995, is located on east 29th Street. This roadway was completely rehabilitated. The existing curbs and gutter were removed and the intakes rebuilt. The power poles were relocated and water stop boxes installed. The existing asphalt base was cold milled, driveways replaced, handicap ramps built and SHRP binder and a 2-in. SHRP wearing surface placed.

The Des Moines Public Works Department was responsible for approximately 70% of the work leaving 30% of the work for the Des Moines Asphalt and Paving Co--this involved cold milling and placement of the wearing surface. The total contract amount equaled $225,000, with $150,000 going to the public works department.

The key element of the SHRP design system is that the mixture is tailored to unique performance requirements dictated by traffic, the environment and structural section. The binder for this project was rated PG58-34, which makes the mixture adequate for pavement temperatures up to 58 deg C and down to -34 deg C.

To achieve this specification the asphalt had to be modified. The key to the strength of the mix used is the stone-on-stone contact obtained, as well as the high film thickness.

After the milling process and preparatory work was completed, a new 18-in. curb and gutter was installed. The SHRP binder was then placed by the Des Moines Public Works Department. This lift, which was approximately 600 tons, was placed to correct the crown and shape of the existing roadway. At that point the contractor placed the wearing surface with double pavers operating in tandem to achieve a full-width pavement surface with a hot centerline joint. The project is therefore designated as seamless.

The contractor utilized some new equipment in the paving process. A Dynapac CC501 50 series roller, was used along with a Caterpillar PS-300B pneumatic roller. At the time of the project the Cat PS-300B was a prototype model on loan from Caterpillar through its field-follow program. Since January 1996 the roller has been available commercially. The pavers used were Cedarapids CR551, with the asphalt being loaded from a Floy-Boy and standard dump trucks.

The project involved the paving of 3,340 lin ft and used 600 tons of asphalt binder and leveling course, and 1,400 tons of wearing course.

Douglas Avenue project

The 1996 project consisted of patching and overlaying one mile of Douglas Avenue. The overlay work was to be completed on consecutive Sundays with the binder course on the first Sunday and the surface course the next Sunday. AC-10 (similar to a PG58-28 binder) was specified, and the state assisted by conducting 109 gyrations on the gyratory compactor over the course of the project. Superpave gradation was used for both the surface and binder course.

Prior to Des Moines Asphalt and Paving moving in, Midwest Contractors did the concrete patching, and The Underground Co. did the intake repairs. The project was now ready for milling, which was completed by the paving contractor. The asphalt base course (3,000 tons at a depth of 2 in.) and asphalt surface course (also 3,000 tons at a depth of 2 in.) were ready to be placed. The traffic count on this stretch of Douglas Avenue was over 18,000 vehicles per day. In order to reduce the inconvenience to the traveling public, the state decided to close the street completely, allowing the contractor higher production over a shorter time. It was also decided to use a high performance SHRP mix in the surface course without additional cost, to accommodate the high amount of traffic and public visibility.

Des Moines Asphalt and Paving assembled two pavers, two asphalt plants, seven rollers and 20 trucks to tackle the job. The two pavers, a Cedarapids CR551, ran off of a ski, and the Roadtec 230, which used sonic sensors, were operated in tandem with over 250 tph of production. Paving began at 6:00 a.m. Sunday morning, was finished at 4:00 p.m., and the street was opened for traffic later Sunday evening after the striping had been completed by Dennis Parking Lot Maintenance.

Although a tight schedule was kept, the contractor received 100% of the smoothness incentive. This is another example of efficient, high-production paving without sacrificing quality. Since the first Superpave project in Des Moines in 1995, others have followed throughout the state. Some of these include: nine miles of resurfacing with PG-graded binders of U.S. 71 in Sac and Buena Vista counties, a seven-mile project with SHRP gradation on Iowa 175 in Hardin County, a 9.7-mile, full-depth project on Iowa 3 in Plymouth County, and work on Highway 63 in Howard County.