Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/149296
RoadScience by Tom Kuennen, Contributing Editor The Durability Factor The goal is performance over time D esign durable roads and you are designing them "not to fail." A variety of tools now exists that civil engineers can use to design pavements that will stand up to traffic and the elements, optimizing long-term expenses while minimizing disruption to the motoring public. • Life-cycle cost analyses (LCCA) permit engineers and agency owners to evaluate alternative infrastructure design options for optimal long-term investment. • The Mechanistic-Empirical Pavement Design Guide being promoted by the American Association of State Highway and Transportation Officials (AASHTO) provides more reliable pavement designs for all pavement types, based on inservice conditions. • The asphalt perpetual pavement philosophy provides a path to deep asphalt pavements that resist fatigue cracking while ultimately permitting easy removal and replacement of stressed driving courses after they have served their missions. Also, grooved next-generation concrete surface (NGCS) pavements now exist for quieter travel, increased friction and improved ride, even as high-performance concrete (HPC) made from ternary blends of cement-enhanced durability. Value-added materials such as polymer modifiers for asphalt pavements, and chemical admixtures for concrete, make those paving media construct faster and last longer. Recycled materials have become a major component of sustainable pavement design, providing a real benefit to society while giving a defense against attacks by the Green movement. And in an age of reduced funding, the philosophy of pavement preservation provides a pathway to extended pavement life, so long as the pavements are well-constructed to begin with, and the preservation treatment is applied at the right point in a pavement's life cycle. Even as new materials and new philosophies of pavement engineering for longer life structures proliferate, it's instructive to note that it's a clear reversal of how the Interstate system came to be. Beginning in 1956, when Interstate highways began to be designed and built, deep-section pavements were discouraged, and the program goal was to put down pavement as fast as possible, toward a goal of total system completion by 1975. Today's themes of spending more up front for longerterm performance, contractor design and warranties, and contractor-certified quality acceptance were either strongly discouraged, or actually illegal. The program operated under very tight, top-down supervision of pavement design and specs, with the Federal Highway Administration reviewing and approving every pavement design. Because the intent of Congress was to complete 42,000-mile Interstate system by 1975 – in only 19 years – the federal emphasis was on laying down as much highway as possible. Back then maintenance was to be the complete responsibility of the states. FHWA engineers reviewed all pavement designs to make sure states were not placing more robust pavement designs than needed to complete the system in time. Because deeper pavements reduced future state maintenance costs, it was perceived that federal funds were being used to defer maintenance spending at the expense of other states and timely completion of the system. Life-Cycle Costing Evolves For more than two decades, the concepts of pavement life cycle cost analysis – and now, life-cycle assessment – have influenced design of both flexible (asphalt) and rigid (concrete) pavements. Proponents of each paving medium have developed rationales for LCCA favoring use of one over the 6 August 2013 Better Roads RoadScience_BR0813_old.indd 6 8/1/13 10:25 AM