Determine at what point the least amount of repair investment produces the greatest improvement in pavement condition. Photo: Sergey Kravtsov | Dreamstime.com
Although “perpetual pavement” wasn't formally defined until a decade ago, public works departments built such pavements as far back as the 1960s. These roads were called “deep-strength,” in which asphalt surface and asphalt base layers are supported by a minimal aggregate base layer; or “full-depth”: thick asphalt courses placed directly on subgrade roads.
While they probably didn't associate the word “sustainability” with their work, designers were concerned with delivering economical pavements of lasting value; and highways based on these principles have served the nation well for decades.
Routes 82 and 9 in Connecticut, where the bedrock subgrade is gneisses that consist primarily of quartz and calc-silicate rock, are two examples.
A 2¾-mile section of 28-mile Route 82 in East Haddam — the 2007 Asphalt Pavement Alliance Perpetual Pavement Award winner — has received just one 2-inch overlay, in 1996, since opening to traffic in 1971. The pavement structure consisted of a 4-inch asphalt surface layer on a 6-inch asphalt base on top of a 10-inch granular subbase.
An adjacent 1-mile section of Route 9 had a similar history and overall structure. Also a rural arterial, this freeway opened to traffic in 1970. It had a 4-inch asphalt surface layer, a two-part base — 3 inches of asphalt on top of 4 inches of chemically stabilized base — and a 10-inch granular subbase. It received a crack seal in 1987, a 2-inch overlay in 1999, and a thin asphalt overlay of ¾ inch in 2008.
Rutting, which usually occurs in the top 2 inches of pavement, is easily remediated by milling and overlaying the affected area without increasing pavement thickness.
The goal of perpetual pavement is to make those top 2 inches wear- and rut-resistant. This is accomplished by placing a rut-resistant intermediate layer of asphalt pavement on top of a fatigue-resistant base layer. This method eliminates the potential for fatigue cracking by reducing the strain at the bottom layers below a level at which damage would occur.
Traffic and environment-related top-down longitudinal, block, and transverse cracking can be sealed or filled to keep moisture from intruding through existing openings, but those techniques are practical only if cracking is limited and there's no significant structural cracking.
Overlays of less than 2 inches can extend pavement life if applied to pavement in good to fair condition with no significant fatigue cracking and no high-severity transverse cracking. They address friction loss, raveling/weathering, and bleeding in additional cracking when applied to a section.
Decide whether to use the U.S. Army Corps of Engineers' Pavement Condition Index (PCI) or World Bank's International Roughness Index (IRI) to gauge pavement performance. Additionally, some agencies use individual distresses and optimization processes to determine optimal timing.
Typically, the maximum benefit/cost ratios are obtained for several treatment alternatives applied at different ages or conditions and then compared to determine the optimal timing and optimal treatment. The benefit/cost ratio is expressed in percent benefit per unit of Equivalent Uniform Annual Cost (EUAC).
After performing the calculations for Route 9, research by the authors of this article concluded that a 2-inch overlay should have been conducted when the pavement was still in satisfactory rather than poor condition, which would have resulted in lower costs over time.
Also, if the crack sealing had been applied to the road just two years earlier, there would have been a drastic decrease in EUAC — from $2,287 in a “fair” scenario to $306 in a “good” scenario — a significantly higher benefit/cost ratio, and longer treatment life (nine years in a “good” scenario versus slightly less than two and a half years in a “fair” one).
— Zofka (firstname.lastname@example.org) is an assistant professor at the University of Connecticut; Yut (email@example.com) and Nener-Plante (firstname.lastname@example.org) are graduate research assistants.
How pavement fatigue life is determined
In 2000, a review of Interstate 90 in Washington State revealed that pavements with asphalt thicknesses greater than 8 inches had never been rebuilt for structural reasons after in service, since the road was built over several years. The age at resurfacing ranged between 17 and 22 years.
That same year the Kansas DOT built four 500-foot-long sections with 11- to 15-inch-thick asphalt layers placed on a clay subgrade on U.S. Route 75.
Seven sessions of pavement response measurements under known vehicle loads were performed between July 2005 and October 2007. During that time, the longitudinal and transverse strains at the bottom of the asphalt layers did not exceed an endurance limit of 70 microstrains.
This suggested that no cumulative fatigue damage would be present at the bottom of the asphalt layer, and that pavement would have a virtually infinite fatigue life.