APAI Article Archive

(APAI Newsletter, May 2003)

Pavements that last and last...

Back in the early 1960s most asphalt pavements were built with very simple designs. Not much data was available to predict performance over time, so it was an educated guess how long these roadways would hold up. As the years pass, some pavements are in need of replacement, while others have served the motoring public well for more than 40 years.

Two of these 1960s pavements on I-80, one in Cedar County and another in Iowa and Johnson counties, were recently awarded the Asphalt Paving Alliance’s 2002 Perpetual Pavement Award. This national recognition was given to six different sections of the Interstate Highway System nationwide. The fact that two of those six pavements are in Iowa is a testament to the quality of roadways produced in our state for many years. As part of the award, these pavements are now entered into a database kept by the asphalt paving industry to be studied with other exceptional performing pavements around the country.

The first section named in the award was constructed in 1962 and is located in Cedar County. It consists of 16 inches of asphalt-treated hot-mix base (ATB) placed directly on the fine-grained subgrade, plus four-and-one-half inches of hot mix asphalt surface course. Since then two surface restoration overlays were placed in 1976 and 1990.

The second award-winning interstate pavement is located in Iowa and Johnson counties. Following construction of the Cedar County project, and due to unusually wet general subgrade soil conditions, design changes made in 1964 to the Iowa/Johnson project included a six-inch soil aggregate subbase as a working platform with a corresponding two-inch reduction in the ATB. The surface thickness was reduced to three inches with a planned stage construction final surface course of two inches to be placed two years later. The actual final surface was not put in place until 1968, or four years after initial construction. Since 1968 there has been only one surface reconstruction on this section.

So, how long should a pavement last? “As these pavements have shown, with the correct pavement design, quality initial construction and proper maintenance, there is no reason a pavement can’t last forever. That’s what’s meant by perpetual pavement,” said Mike Heitzman, DOT bituminous engineer. “If the pavement is designed and constructed well, surface maintenance should be the only work necessary to sustain the roadway indefinitely, even with changes in traffic and load levels.”

The traffic data we collect shows conditions have changed significantly in the past 40 years. Traffic load levels are more than five times greater on I-80 than when the pavements were built in the early 1960s. “We couldn’t place the 1960s pavement design today and expect a similar 40-year life,” said Heitzman. “The pavements would look similar, but the mixes and thickness would be different to account for the high traffic load. In the 1960s we used a generalized design for pavements. Today we have technology to allow a more detailed, engineered approach to each project using materials that have properties shown to perform in a given situation. The goal of good design is to minimize the strain on the bottom of the pavement to eliminate stress cracks. This will make the base last. We design the pavement thick enough to bend, but never break.”

These material properties are kept in a database and computer models using that data are now used for pavement design. “We take data from monitored projects and develop the models using real-world information,” said Heitzman. “This way hot mix asphalt pavement design can be very precise. If we build the structure well enough and with the proper materials, we only need to renew the surface on a 10- to 15-year cycle. This will have slightly higher up-front cost, but the long-term performance of the roadway will be very cost effective.”