Perpetual Pavements

"Perpetual Pavement" is a term used to describe a long-lasting structural design, construction and maintenance concept for HMA pavements. If properly maintained and rehabilitated, a perpetual pavement can be designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses confined to the top of the pavement.

The concept of perpetual pavements, or long-lasting HMA pavements, is not new. Full-depth and deep-strength HMA pavement structures have been constructed since the 1960s and before, and those that were well-designed and well-built have been very successful in providing long service lives under heavy traffic (APA, 2001¹). Advancements in milling, recycling and HMA technology over the last few decades have created HMA pavements that perform better, longer and with lower life-cycle costs than was previously possible.

As Michael Nunn pointed out in 1998², flexible pavements over a minimum strength are not likely to exhibit structural damage even when subjected to very high traffic flows over long periods of time. He noted that existing pavements over about 370 mm (14.5 inches) should be able to withstand an almost infinite number of axle loads without structural deterioration due to either fatigue cracking or rutting of the subgrade. Deterioration in these thick, strong pavements was observed to initiate in the pavement surface as either top-down cracking or rutting. Further, Uhlmeyer et al. (2000³) found that most HMA pavements thicker than about 160 mm (6.3 inches) exhibit only surface-initiated top-down cracking. Therefore, if surface-initiated cracking and rutting can be accounted for before they impact the structural integrity of the pavement, the pavement life could be greatly increased.

Conventional structural design sizes each layer for the traffic expected over a design life of typically around 20 years, with trucks and buses rather than passenger cars controlling the thickness. Perpetual pavement design instead uses the threshold at which damage below the surface from even the heaviest vehicles becomes negligible. Distress is then confined to rutting and cracking in the top layer, which can be corrected by milling and overlaying. Because conventional design does not identify this threshold, many existing pavements built to conservative designs already act as perpetual pavements, and in some cases a perpetual design calls for a thinner section than an over-conservative conventional design.

Researchers have used this idea as well as pavement materials research to develop a basic perpetual pavement structural concept. This concept uses a thick asphalt over a strong foundation design with three HMA layers, each one tailored to resist specific stresses (TRB, 2001⁴):

1. HMA base layer. This is the bottom layer designed specifically to resist fatigue cracking. Two approaches can be used to resist fatigue cracking in the base layer. First, the total pavement thickness can be made great enough such that the tensile strain at the bottom of the base layer is insignificant. Alternatively, the HMA base layer could be made using an extra-flexible HMA. This can be most easily accomplished by increasing the asphalt content. Combinations of the previous two approaches also work.
2. Intermediate layer. This is the middle layer designed specifically to carry most of the traffic load. Therefore it must be stable (able to resist rutting) as well as durable. Stability can best be provided by using stone-on-stone contact in the coarse aggregate and using a binder with the appropriate high-temperature grading.
3. Wearing surface. This is the top layer designed specifically to resist surface-initiated distresses such as top-down cracking and rutting. Other specific distresses of concern would depend upon local experience.

Typical thicknesses are about 38 to 75 mm (1.5 to 3 inches) for the wearing surface, which may be dense-graded HMA, warm mix asphalt, stone matrix asphalt or an open-graded course; about 100 to 175 mm (4 to 7 inches) for the high-modulus, rut-resistant intermediate layer; and about 75 to 100 mm (3 to 4 inches) for the flexible, fatigue-resistant base layer.

In order to work, the above pavement structure must be built on a solid foundation. Nunn (1998²) notes that rutting on roads built on subgrade with a CBR greater than 5 percent originates almost solely in the HMA layers, which suggests that a subgrade with a CBR greater than 5 percent (resilient modulus greater than about 7,000 psi (50 MPa)) should be considered adequate. As always, proper construction techniques are essential to a perpetual pavement's performance. Figure 1 shows an example cross-section of a perpetual pavement design to be used in California on I-710 (the Long Beach Freeway) in Los Angeles County.

A perpetual pavement also depends on timely renewal of its wearing surface. Distresses expected in the top layer, particularly top-down cracking, must not be allowed to propagate into the layers below, so the timing of milling and overlay directly affects the life of the whole structure. Because the work is confined to the surface, it can often be done overnight with little disruption to traffic, which keeps user delay costs low.

Finally, the most important point in this brief perpetual pavement discussion is that it is possible to design and build HMA pavements with extremely long design lives. In fact, some HMA pavements in service today are living examples of perpetual pavements. For instance, two sections of Interstate 40 in downtown Oklahoma City are now more than 33 years old (built in 1967) and are still in excellent condition. These sections, which support 3 to 3.5 million ESALs per year, have been overlaid but the base and intermediate courses have lasted since construction without any additional work (APA, no date given⁵).

@misc{pavint-perpetual-pavements,
  author = {{Pavement Interactive}},
  title = {Perpetual Pavements},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/design/structural-design/perpetual-pavements/}},
  note = {Accessed [date]}
}