National Asphalt Pavement Association PERPETUAL PAVEMENTS

T he concept of perpetual pavements, orlong-lasting asphalt pavements, is notnew. Actually, full-depth and deep-strength asphalt pavement structures have been constructed since the 1960s. Full-depth pavements are constructed directly on subgrade soils, and deep-strength sections are placed on relatively thin granular base courses. One of the chief advantages of these pavements is that the overall section of the pavement is thinner than those employing thick granular base courses. As a result, the potential for traditional fatigue cracking may be reduced, and pavement distress may be confined to the upper layer of the structure. Both are advantages to perpetual pavements. Thus, when surficial distress reaches a critical level, an economical solution is to remove the very top layer and replace it to the same level. Recent efforts in materials selection, mixture design, performance testing, and pavement design offer a methodology to obtaining long-lasting performance from asphalt pavement structures (greater than 50 years) while periodically replacing the pavement surface. The thrust is to combine a rut resistant, impermeable, and wear resistant top structural layer with a rut resistant and durable intermediate layer and a fatigue resistant and durable base layer as shown in Figure 1. Though the use of a perpetual pavement is focused at high-volume traffic, the justification may be made for medium- and low-volume roads as well. The criteria used in Maryland and California for high-volume traffic is discussed below. Maryland incorporates a thick asphalt structure surfaced with a stone matrix asphalt (SMA) on roads where the posted speed limit is 55 mph or greater, and the traffic is at 2000 equivalent single axle load (ESAL)/day or greater. California uses such designs when the truck traffic exceeds 15,000 vehicles/day or the average daily traffic is greater than 150,000 vehicles/day. Thus, it is conceivable, that with the right design approach, the use of perpetual pave-ments could become common on medium- and low-volume roads as well. Currently, most pavement design procedures do not consider each pavement layer to explain its characteristics relating t