In pavement design and construction, there are many opportunities to improve sustainable practice. For the highway engineer, sustainability initiatives are usually focused on structural design, the pavement materials or the construction operation itself. As agencies increase their understanding of the sustainability impacts associated with the construction and operation of roadways and pavements, and as research into sustainable practices continues to evolve and advance, the awareness of life cycle analyses and use-phase impacts will continue to grow.
It is important to look at the concept of “cradle-to-grave” analysis, which has emerged in the pavement arena. Carrying an analysis from cradle-to-grave is the central idea of a life cycle assessment (LCA). LCA allows for a sophisticated and complete means of examining resource use and availability and was in the late 1990s standardized by the International Organization for Standardization (ISO). The purpose of the LCA approach is to ensure that all the effects, factors, loads, etc. are accounted for in the analysis, from the moment any component is extracted or processed, all the way to its end of life. It essentially involves a cumulative analysis of a product’s environmental or sustainability impact throughout all stages of the product’s life cycle, including impacts not usually considered in more traditional analyses. Figure 1 illustrates the concept of a LCA, including inputs, outputs and the system boundary.
A comprehensive LCA study undertaken by Centre d’Energétique de l’Ecole des Mines de Paris (Mines Paris Tech) examined the impact of six different pavement structures in reference to 12 different environmental factors (including greenhouse gases, energy, ecotoxicity, smog, odor, solid waste, etc). (Figure 2) From the analysis it can be seen that the overall impact from the use-phase dwarfs impacts from all other phases of the pavements life cycle. In fact, with the sole exception of the solid waste factor, the impact of the use-phase (traffic in this case) is at least 10 times greater than all other phases. Just a 2 or 3 percent improvement in the truck traffic and car traffic portions of the ecoprofile would essentially offset the entire construction and maintenance ecoprofile. Recent and ongoing research conducted by the Massachusetts Institute of Technology (MIT), Technische Universitt Mnchen (TUM), National Concrete Pavement Technology Center (CPTech Center) and others are drawing similar conclusions.
Essentially because pavements remain in service for decades, lying exposed every hour of every day, and typically supporting millions of vehicles during that time, use-phase impacts are likely to be the dominant factor when assessing sustainability and should therefore be the chief focus of our efforts. The most prominent of these impacts likely come from either vehicle fuel consumption rates (related to pavement rigidity and smoothness) or pavement albedo (as it relates to urban heat island, lighting and global cooling).
Vehicle fuel consumption and pavement
Since 1989, several important studies have examined the link between vehicle fuel consumption rate and pavements. Most of the studies suggest that because vehicles (particularly trucks) cause greater deflections on flexible pavements than on rigid pavements, more of the energy intended for propelling the vehicle is absorbed, causing those deflections.
Arguably the most statistically rigorous of these efforts, a comprehensive, multi-phase study on the effects of pavement structure on vehicle fuel consumption, was published in 2006 by the National Research Council Canada. The study concluded that tractor-trailers traveling on rigid pavements consume significantly less (on average about 3.8 percent) fuel than those traveling on flexible pavements. Research is ongoing at MIT’s Concrete Sustainability Hub to verify and expand on this work.