APAI Article Archive

The City of Des Moines - always a leader in Iowa paving innovations - has done it again by implementing a new HMA technology strategy on a municipal street rehabilitation project. The city cashed in on past decisions to design and build quality HMA overlays that may now enable them to last over 50 years or be considered perpetual.

Since the early 1980’s, The City of Des Moines - always a leader in Iowa paving innovations - began paving with high quality HMA on its city streets in an effort to eliminate major defects in its asphalt surfaces and to project extended life. The city also embarked on improved placement methods to minimize future maintenance, traffic delays, and business interruptions. Des Moines has been credited with:

-Quality HMA designs utilizing high percentages of crushed durable aggregates and polymerized binders

-The first Iowa municipal seamless construction

-The first full-depth major arterial street, a fast-track project paving record exceeding 600 tons per hours

-The state of Iowa’s first pure Superpave project in 1995.

The city is now in a position to capitalize on the fruits of these improved HMA design and construction methods.

Well, just ask The City of Des Moines - always a leader in Iowa paving innovations. They are at it again by implementing a new HMA technology strategy on a municipal street rehabilitation project.

Fleur Drive is the heaviest traveled municipal arterial street in the state of Iowa. It is predicted to carry over 50,000 vehicles per day and approximately 10 million Equivalent Single Axle Loads (ESALs).

In a 1985 effort to minimize future rehabilitation costs and obtain extended service life, Fleur Drive received a rehabilitation 3 inch HMA overlay on the existing PC base pavement. The 1985 HMA design utilized, for the first time in the city, 75% crushed ½ inch Type 4 aggregates with Type 2 (quartzite) required above the No. 4 sieve and a polymerized (SBS) asphalt cement binder. Seamless construction was also implemented at that time with pavers in tandem eliminating centerline or cold joints. The Department of Transportation was asked, and assisted the city, with mix design review and quality control testing.

Time has proven that the structure’s surface was designed and built for durability, rut resistance, wear resistance, and long life. But by 2001, time, weather and traffic caused noticeable distress to the Fleur Drive HMA surface. However, the oxidation, pitting and raveling, and rutting were confined to the top ½ inch of the surface. To evaluate surface reconstruction, 3-inch cores were taken through the HMA to the top of the PCC base. The cores clearly indicated that the existing HMA overlay was sound, intact, and alive just below the thin distressed surface. A 10-foot straight edge revealed less than ½ inch rutting and wear at the wheel tracks, in fact, measurements were predominately ¼ inch.

In consideration for the possible use of Fleur Drive as a new state highway connecting the Des Moines freeway with Iowa 5, the city’s south loop by-pass, 80% federal dollars were budgeted by the state of Iowa for a 3.8 mile rehabilitation project. In 2002, 17 years after placement of the existing HMA 3-inch overlay, the first 9,100 foot rehabilitation section was designed and constructed. In 2003, the second 4,900 feet was built. The last segment is scheduled for 2005.

A city design team was formed to study the Fleur Drive 2002 rehabilitation project. The design team concluded that the conventional method of removing the entire existing asphalt surface, prior to new overlay, be rejected in lieu of a new perpetual overlay approach. Armed with a confirmed knowledge that the minimum depth of new overlay be three times the maximum aggregate size, it was determined that only 1½ inches of the existing 3-inch overlay be removed by cold milling and replaced with 1¾ inches of high quality HMA (7,200 tons). Due to the condition of the existing overlay the bottom 1½ inches would be considered perpetual and not removed. The extra ¼ inch in thickness on the new overlay would allow passage of the paving machine screeds without adjustment over the appurtenances and produce a smooth surface.

In June 2003, the second segment of the Fleur Drive rehabilitation project was constructed. Like the first, not only was the cost saving perpetual overlay concept utilized, but the city’s standard fast track seamless construction was also implemented. The construction in June also consisted of cold milling the top 1½ inch layer of the existing surface, minor patching, crack sealing, cleaning, and placement of a new 1¾ inch HMA surface. All this work was performed under traffic with absolute minimum interruption to the abutting businesses and the international airport which fronted the west side of the project. The 3,300 tons of HMA consisted of 85% crushed Type 4 aggregate with a PG 70-28 binder which was modified up two grades with SBS. It was accomplished in 8 hours with 2 laydown machines in tandem and one ahead placing turning lanes. The overlay paving project began at 5:30 a.m. and by 1:30 p.m. the street was totally completed with lane striping in place. The contractor for both the 2002 and 2003 projects was the experienced Des Moines Asphalt and Paving Company.

Due to a reduction of 7,500 tons, the perpetual overlay concept of partial depth HMA removal and replacement was estimated to save in construction costs over $450,000, versus conventional complete removal and replacement methods, for the first 14,000 feet (2.6 miles) of the Fleur Drive rehabilitation project.

In the early 1980s, the City of Des Moines made a conscious decision to obtain long-lasting, durable HMA pavements. The goal was to minimize to future reconstruction costs utilizing the best design and construction alternatives. That decision now appears to enable the city to cash in on past expenditures to obtain virtually perpetual overlay structures on its arterial street network. Those structures, in the 20-year periodic time frame, may require only rehabilitation at the very top surface with abandonment of conventional overlay rehabilitation methods. This will enable the city to rehabilitate 30-40% more HMA surfaces annually with its present budget.

The City of Des Moines’ past investment in quality HMA pavements enables design of rehabilitation projects with advantageous life cycle cost analysis that includes not only user delay costs, but also a perpetual overlay theory. Future roadway funding uncertainties requires this type of innovative thinking. One look at the Fleur Drive rehabilitation project, together with the knowledge of the history of construction, maintenance, and traffic loads, is extremely convincing that this new perpetual overlay concept has great potential.

John P. Bellizzi, P.E. & P.L.S.

APAI Consultant