VAASPHALT.ORG 23 Looking to the Future of Pavement Design Today, much of the pavement engineering community focuses on complex design approaches developed through research projects. This is valid and crucial for specific U.S. and world applications: we are seeing new aircraft wheel gear configurations emerging, super heavy loads at port facilities, and special loadings on highways becoming more and more common. Will closely-spaced truck platooning with super single tires and no lateral wander change the overall thickness? Today’s pavement design procedures rely heavily on modeling and assumptions to predict performance and required thickness. The PaveME procedure developed by AASHTO uses layered-elastic theory combined with past climate data and distress modeling to predict future performance. The pavement thickness and materials properties can be modified until performance criteria are satisfied. Yet, the local, regional and climate data of the past cannot predict the future climate. Current rainfall, drought, heat and cold events are more extreme than what has been recorded in hundreds or thousands of years. The modeling of pavement distresses such as cracking and ride quality have a wide coefficient of variation and low R-squared values. While the trends of the data plots may make sense, their accuracy is low. I’ll also add that, as a person who moved from a blackbox desk engineer doing pavement designs to one who got out and observed the construction and paving processes, accuracy to the nearest tenth of an inch is meaningless when tolerances to the half-inch or full inch are allowed for roadway construction specifications. Realistically, the vast majority of new and rehabilitated pavements will not be designed by the pavement engineering community. This responsibility will fall to local public works officials or design consultants. From experience and observation, they may implement what works in the local area even though the accepted pavement design method does not confirm its long-term performance. We go back to our original question: build based on design, or based on what works? The answer to future pavement performance is simple: a design catalog generated through mechanistic principles and validated through local experience. The materials selected will differ across a state or even region, but the approach would remain the same. Generating the designs would be as simple as performing analysis with a range of subgrade and sub-base layer strengths and performing an analysis, since we now know that the key to long-term performance does not lie with the overall thickness as much as with the paving materials selection. From Virginia’s research project in the early 2000s to subsequent field investigations, we have found the “sweet spot” for flexible pavement is between eight and fourteen inches of asphalt concrete on a solid sub-base foundation and adequate drainage depending on truck traffic levels. At thicknesses below eight inches, the risk of bottom-up failure increases. At thicknesses over fourteen inches, tax-payer dollars are being spent without a big return on investment. In that range, adding an inch of asphalt doubles the structural life of a pavement, according to the work done by Jim Huddleston and other engineers. The research documented by NCAT in their report 15-05R indicates a worst-case scenario of 15.5 inches of total asphalt on a weak base and subgrade layer. Most high-volume pavements in Virginia would not be constructed on a subgrade of 5,000 psi. Time, effort and money to calculate a thickness to the nearest tenth or quarter of an inch miss the bigger picture. We often miss the little things that turn out to be the big things. With pavement design and performance, we can’t take materials or construction techniques for granted. Over thousands of years, we have learned what works, and that should remain our basis of pavement design. Interstate 66 cracking due to mid-layer striping PAVEMENT DESIGN: A PHILOSOPHY CONSIDERED
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