This month, we’re back to my series of columns on factors affecting pavement performance. This time I’m talking about pavement structure and thickness. I’m not covering the “how-to” of pavement design, just some things to consider. A lot of good resources for actually determining pavement thicknesses are available. We will get into more regarding those resources later. First, some background information.
Pavement thickness design is all about determining the correct thickness for the traffic/loading, underlying soils/subgrade, and existing pavement (if applicable) conditions. Under-designing the thickness leads to poor performance and pavement failure. And significantly over-designing can be a waste of money. But some over-design is not a bad thing; a small amount of added thickness can provide a meaningful increase in the performance life. Also, traffic always seems to grow faster than we anticipate, so the extra thickness will probably be needed. Many of the pavement problems we see can be attributed to the growth of the trucking industry and the pavement being designed for considerably lower stress conditions than actually experienced.
Historically, thickness design procedures have been empirically based. Pavements were built and evaluated and design charts were developed from the information gathered. The current trend is to move to a mechanistic-empirical (ME) approach. As mentioned earlier, there are numerous manuals and software programs to assist designers in determining pavement thicknesses. For high-type facilities, these include “MEPDG,” perpetual pavement and Asphalt Institute (AI) manuals and software. Simplified procedures are available from AI, state asphalt pavement associations and other sources.
MEPDG is the AASHTO Mechanistic-Empirical Pavement Design Guide. This procedure transitions from an empirically-based design method to one combining both mechanistic and empirical procedures. MEPDG uses analytical modeling and field performance data from the SHRP database. This approach models the pavement as a multi-layered elastic system and the stresses and strains due to loading of the pavement are calculated. The strains are compared to known pavement performance measurements to determine the required thickness.
Perpetual pavement is another approach to asphalt pavement thickness design. Perpetual pavements are designed and built to last more than 50 years without major structural work. These pavements incorporate a fatigue-resistant base layer, a rut-resistant intermediate layer and a rut-resistant, renewable surface course. The base layer is specifically designed to be strong and flexible in order to stop fatigue cracks from forming at the base of the pavement. The intermediate layer serves as the primary load-carrying part of the structure. The surface is designed to serve many years before scheduled restoration. It is designed to be rut-resistant and to resist top-down cracking.
Software to design perpetual pavements is available as a free download from the Asphalt Pavement Alliance. The software uses a ME approach and is available in two versions. PerRoad 3.5 is the current procedure for designing high-type facilities. PerRoadXpress 1.0 is an easy-to-use program for designing low to medium volume roads and parking lots.
For light traffic facilities (for example, driveways), there are practical minimum thicknesses that should be placed, not for load-carrying purposes but for constructability. There must be sufficient thickness of mixture to accommodate particle re-orientation during compaction. For base mixes (with a top-size aggregate of 3/4 or 1 inch), I recommend a minimum of 3 inches. For surface mixes (with a 3/8-inch top size), place at least a 1-inch layer. And remember I am referring to the finished, compacted thickness—not the loose, uncompacted depth.