BUILDING PERPETUAL PERMANENT PAVEMENTS
What defines a Perpetual Pavement? A perpetual pavement is a mechanistic/empirical designed asphalt pavement built to diffuse the vertical strain of traffic loading along a wide horizontal strain. This is accomplished through constructing a highly-flexible, asphalt-rich base course, a high-aggregate, ultra-strong intermediate course with a rut-resistant, high friction surface course. This revolutionary Perpetual design eliminates pavement failure in the base course and allows for periodic reconditioning of the surface course with no need to ever completely reconstruct the pavement.
The National Asphalt Pavement Association defines their Perpetual Pavement Award Winners as full-depth asphalt pavements that are a minimum of 35 years old and have not been overlaid more often than once every 15 years. Iowa was recognized in 2005 for three sections of Perpetual Pavement on Interstate 80 in Jasper, Iowa/Johnson and Cedar Counties. These sections were originally constructed in 1960-61 and have never been reconstructed (although the rest of the interstate has been). Thinking that there might be a few more long-lived full-depth sections in Iowa, the IDOT was asked to check their database for any other pavements that would meet these criteria. 160 sections of US Highways in Iowa meet those criteria. Ages for these pavements range from 82 years old for Hwy 64 in Jackson Co. to a 64 year old Hwy 146 in Marshall Co. to a 44 year old Hwy 6 in Pottawattamie Co. along with every age in between.
These 160 highway sections in Iowa were not constructed as perpetual pavements, yet many have achieved the pavement longevity of what we expect from today’s perpetual pavements. For the sake of discussion, let’s call these long-lasting, never-reconstructed roads Permanent Pavements. The premise of these roadways was to first establish the road. Many of these sections of highway, and untold miles of county roads were originally gravel or sealcoat roads where traffic volumes increased. These roads eventually warranted a 2”-3” asphalt layer, or maybe they utilized 4” of asphalt treated base (ATB), a coarse graded aggregate mixture with approximately 4% asphalt cement. The engineer would add a two-inch overlay every 15-25 years as traffic levels increased. This “staged” construction worked well in keeping the pavement equal to the traffic load as well as keeping the cost of construction amortized over many years. This method is still viable today.
The underlying premise of both the permanent and perpetual pavements is that neither pavement will need to be fully reconstructed. The surface layers of the asphalt can be strengthened with today’s newer stronger asphalts, or can be recycled with a “mill and fill” operation, or revitalized with emulsions or foamed asphalt with cold-in-place recycling.
A recent review of the IDOT Pavement Management System (PMS) found that the previous belief that PCC lasts 40 years with no maintenance and asphalt lasts 40 years with an overlay at year twenty is false.
Service Live
1992-2010
Chart 1
Information from IDOT Pavement Management System (PMS)
As you can see from Chart 1, low-volume and medium volume truck traffic roadways are lasting 29 and 30 years or longer before needing rehabilitation. High truck volume roadways are lasting 25 years on average before rehabilitation. PCC pavements are nearly identical in their performance. The service lives of flexible and rigid pavements are statistically the same for all traffic levels.
Conventional wisdom has theorized that maintenance is higher on asphalt roadways – IDOT data proves a different theory.
Maintenance Cost*
Chart 2
* Reflects all contract maintenance costs within the right of way
Information from IDOT Pavement Management System (PMS)
Perpetual and Permanent pavements differ in their design methods. Permanent pavements utilize empirical designs derived from the AASHO 1958 Road Tests where more traffic equaled more thickness. Perpetual pavements utilize mechanistic/empirical design and are better utilized for high-volume designs over 3M ESALs to prevent over-designing pavement thicknesses.
The APAI, in cooperation with ISU, has developed I-Pave, a program that utilizes empirical design (AASHTO 93) to provide comparable sections of asphalt and concrete (Click here to experience I-PAVE) on low volume roadways. The Asphalt Paving Alliance (APA) offers a perpetual pavement design guide for higher volume roadways called PerRoadXpress (click here to open PerRoadXpress ) developed by Dr. David Timm of Auburn University. These two design guides will allow you to design low-volume and high-volume full-depth permanent perpetual asphalt pavements.
Although the design parameters for the Permanent and Perpetual designs are different, the overriding theme is that both work. Iowa asphalt contractors have been building long-lasting, full-depth asphalt pavements for at least 82 years. The long-held belief that PCC pavements last longer, with less maintenance, has been proven false with IDOT records. It’s time to let go of the fallacies and adopt the realities. Asphalt is just as durable as PCC but it’s faster to construct, smoother to drive on, and, if built perpetually, will never need to be reconstructed.