Oregon Answers Perpetual Pavement
Analysis with a Field Test

Oregon gets a head start on incorporating permanent pavement
as part of the design process.

by Tiffany Estes

The concept of perpetual pavement — one designed to last longer than 50 years without requiring major structural rehabilitation or reconstruction — is not a new one. It has been established by the successful performance of well-constructed, thick asphalt pavements built as early as the 1960s.

“What is new,” said Jim Huddleston, executive director of the Asphalt Pavement Association of Oregon, “is the development of perpetual pavement as a design concept.”

A 10-mile segment of Interstate 5 in Oregon has just been reconstructed to incorporate a design that was not necessarily intended to be a perpetual pavement, but upon analysis at Oregon State University, was found to possess perpetual pavement qualities. As part of its construction, instrumentation was installed that may help validate the assumptions and conclusions of that analysis. By relating field measurements to those assumptions and conclusions, valuable information may be gained to assist in the formulation of guidelines for future perpetual pavement designs.

The Interstate experience

Like most other Departments of Transportation, Oregon has traditionally designed their pavements to weather 20 years of traffic, following AASHTO Design Guide procedures. For interstate systems, this approach typically calls for deep-strength asphalt thickness between 10 and 14 inches.

Conventional design theory would suggest that after 20 years of traffic, these pavement structures are likely to incur distress from the bottom up, with cracks reaching the surface after having traveled the entire depth of the pavement. Oregon’s interstate experience, like that of many other states, proves the opposite. Cracks at the surface originate at the top and work their way down, with original base layers retaining all of their original structural integrity. This pattern is confirmed by detailed investigations, including deflection testing and coring.

The most logical explanation for this behavior is strain limitation — a theory suggesting that limitation of bending strain at the bottom of the asphalt layer to an established threshold (currently believed to be 70 microstrain) will prevent a bottom-up crack from developing. This theory is being validated in laboratory and field studies nationwide.

The challenge in Oregon and across the country is to develop traffic and material inputs, and calibrate the design models to suit local conditions so reliable predictions of bending strain can be made. The instrumentation and subsequent evaluation of the Interstate 5 project will go a long way toward helping the Oregon DOT achieve that goal. Comparison of results with those from other instrumentation sites nationwide should provide valuable information for other jurisdictions as well.

The rubber meets the road

The section of I-5 that has just been reconstructed to incorporate principles of perpetual pavement design lies in Oregon between Salem and Albany. It encompasses a 10-mile stretch of two southbound lanes. A 4.5-mile subsection contains the instrumentation that will help engineers compare field results to performance expectations established through analysis conducted at OSU.

Originally constructed in 1958, the I-5 section consisted of 8 inches of reinforced, jointed concrete placed over an aggregate base that was 9- to 12-inches thick. Since original construction, the pavement has been overlaid with 3 to 6 inches of asphalt concrete.

In places where the old pavement had been thinned for grade purposes, there was evidence of extensive reflective joint cracking. There were also instances of raveling and potholing, along with joint faulting in the underlying concrete.

Though ODOT’s design for reconstruction of this failing section was not necessarily intended to be a perpetual pavement, conversations between Liz Hunt, pavement services engineer for ODOT, Jim Huddleston of APAO, and Dr. Jim Lundy, professor of civil engineering at OSU, led to the question of whether it could be classified as such. The group agreed to have additional analysis conducted at OSU to test that hypothesis.

Lundy said the design’s thickness and material properties would enable it to be considered a perpetual pavement. The analysis estimated fatigue resistance only, and determined that little or no fatigue life would be consumed in this pavement through the course of over 50 years of traffic. Lundy notes, however, that numerous other variables in the construction process can impact the longevity of the pavement structure. Those variables and other assumptions made during analysis are among the items ODOT seeks to validate through data collected over the next year at the site.

A layered approach

The 4.5-mile subsection of the project contains the instrumentation that will be used for data collection. It is built upon a base of rubblized concrete — material that was once part of the original roadway that has been broken up for reuse as a pavement base. The failed concrete was rubblized to provide more uniform support, and to reduce or eliminate reflective cracking that can originate in concrete joints.

Atop the rubblized concrete are 12 inches of asphalt divided into three layers. The bottom two layers consist of 0.75-inch dense-graded mix, and the top layer consists of 0.75-inch open-graded mix.

The bottom layer of asphalt is an 8-inch base designed for fatigue resistance and durability. It was designed at 100 gyrations and the binder content was selected at 3% air voids instead of the usual 4% that is targeted. The resulting asphalt content was 5.9%, about one-half percent more than the layer immediately above it. The higher binder content, coupled with a higher field density requirement, ensures in-place air voids averaging nearly 5%. ODOT’s conventional base design averages in-place air voids of about 7%.

Implementation of the rich bottom base was a deviation from the original reconstruction design, and was proposed by ODOT and APAO’s joint Quality Improvement Committee.     The higher binder content in the base will contribute to improved durability and fatigue resistance, which are critical to the long-term survival of a perpetual pavement.

The top 4 inches of the pavement structure consist of two layers that are each 2-inches thick. The first is a dense-graded Superpave mix with 5.4% asphalt content, and the second is a wearing course. The wearing course is comprised of a 0.75-inch open-graded mix. ODOT selected the open-graded wearing course for its ability to reduce splash, spray, and road noise.

Comparing notes

A variety of measuring devices that will potentially validate the analysis conducted at OSU were installed in the 4.5-mile segment of this project.

Gauges were inserted on top of the rubblized concrete base to measure the strain at the bottom of the asphalt. One of the industry standards for classification as a perpetual pavement is a strain of 70 microstrain or less at the bottom of the asphalt layer. OSU’s analysis determined that the vast majority of the strains on this pavement would be below 70 microstrain. These gauges will allow direct comparison of the measured strain to that predicted by the OSU analysis.

Temperature gauges were also installed in the segment. The stiffness of asphalt is highly influenced by temperature, and it is critical to understand how the temperature varies seasonally within the pavement in order to make reliable predictions on bending and strain. Analysis based on data collected from these instruments will include review of performance from season to season and comparison of field results to performance expectations established in the lab.

The project also includes a portable weigh-in-motion site that will monitor truckloads as they cross. Data collection will include truck weight and number of axles on the truck, as well as axle configuration. Results will be used to relate axle load and configuration to the measured strains.

What it means

Data from the I-5 project will be collected over the course of the next year. In the short  term, it will provide measurement of the strain present in this particular project, and will potentially validate performance expectations suggested by OSU’s analysis.

“This project will enable us to refine our material property inputs. We’ll have a better understanding of how they vary with changes in temperature and moisture, and how they are affected by different loading conditions. And of course, we’ll be able to refine our models for predicting strain,” said Bruce Patterson, ODOT pavement materials engineer.

“The whole country is moving toward mechanistic-empirical design,” said Liz Hunt. “This data will help us establish guidelines in Oregon for future perpetual pavement projects.”

Tiffany Estes represents the Asphalt Pavement Association of Oregon.

Reprinted from Better Roads Magazine
November 2005

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