Though its engineers and contractors didn’t realize it at the time, Eugene, Ore., was making some groundbreaking advances in road construction during the 1960s and 1970s. Ironically, it took more than 20 years for the discovery to surface.
“We became aware that our full-depth pavements were acting as long-lasting pavements in the early 1990s, when we hired a consultant to evaluate a main arterial,” said Paul Klope, principal engineer of Eugene.
“We expected an extensive repair, given all the pop-outs and raveling on the surface,” he said, “but it turns out the pavement was 10 to 12 inches thick, requiring only 2 inches of milling and overlay. It’s the first time I witnessed pavement failing from the top-down, rather than the bottom-up.”
More examples of long-lasting pavements were discovered in Eugene when a pavement preservation program was initiated in 2002. Though hard numbers are not available, Klope said there are several full-depth pavements within the city’s jurisdiction, proving that early discoveries of pavements that cracked from top down were not isolated incidents. One of the city’s streets originally constructed in 1952 is still in service, and has had only one structural overlay (in 1969). While the base is 55 years old, it is still functioning like new with no signs of deterioration.
Long-lasting or Perpetual Pavements are defined as those “built for long life without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses confined to the top of the pavement.” As defined, the “Perpetual” label could easily be applied to these aging full-depth pavements in Eugene, most of which received no overlay treatments within their first 25 years of use.
According to Klope, the city of Eugene had several reasons for building full-depth pavements in the ’60s and ’70s. First, it was faster. Constructing full-depth pavement required only one operation rather than two, since multiple paving layers were not involved. This minimized traffic disruption and other impacts.
Full-depth pavements often require less excavation as well. This reduces the potential for disruption of utility services, and lowers construction costs.
Finally, full-depth pavements were found to be less expensive to construct — not only over the life of the pavement when lower maintenance costs are factored, but also at original installation (referred to as “first cost”).
Klope said no hard figures have been calculated at the city of Eugene to quantify the cost benefits of constructing full-depth pavements, but noted that “milling and filling” the surface typically averages about one-fourth the cost of the complete reconstruction that would be required in pavements demonstrating full-depth failures.
Advantages of Full-Depth Pavements
Beyond the fact that full-depth installations outlast traditional structures and cost less to rehabilitate, several other points came to light upon further examination. One was that traditional gauges may not be the best method for assessment of pavement condition.
The pavement condition index (PCI), for example, is based on surface deficiencies like cracking, rutting, raveling and shoving. Klope said that PCI is a fine method of assessment for applications involving traditional pavement structures that fail from the bottom up, but can falsely signal poor conditions beneath the surface when pavement fails from the top down.
Some pavements in Eugene that showed typical surface distress, like raveling, pop-outs, and alligator or age-related cracking, required only surface rehabilitation, though the PCI figures based on these distress marks pointed to deficiencies beneath the surface that were not there. Upon further inspection, the bases were found to be in “like-new” condition with no signs of distress, even after more than 30 years of service.
This lesson showed the value of testing to obtain accurate assessments of a pavement’s true structural condition. Core samples and other methods are used to accomplish this, rather than relying too heavily on the conditions that surface appearances might imply.
As for future construction, will the city of Eugene intentionally make full-depth pavements the structure of choice?
“That’s where I’m starting the discussion for debate,” Klope said. “We should consider full-depth pavements because of how they’ve performed in the past, especially if first cost is competitive with that of other paving options.”
The city also is discussing changes to its design standards based on what’s been learned.
“We’ve been making practical changes along the way,” Klope said, “but the concept of changing design standards has not been on the table until now.”
As an aside, Klope mentioned the poor quality of soils in Eugene. Most “r values” — measuring relative strength of the soil — fall in the poor to very poor range.
“Because the soils are so poor in Eugene, our inclination might be to avoid full-depth pavement,” Klope said. “But with addition of some sub-base material the method actually worked quite well. If it worked here, it should work even better in locations with more conducive soils.”
Jim Huddleston is the executive director of the Asphalt Pavement Association of Oregon. This article was reprinted from HMAT magazine, November/December 2007, Volume 12, Number 6.