Sustainability, in its simplest form, is the capacity to endure. Evolving from the Latin word sustinére (from sus meaning up and tenére meaning to hold), sustainability has different meanings, including to give support, prolong, and withstand. The term is applied broadly to almost every facet of life, although it is increasingly being used in the context of human sustainability on earth.
Consideration of sustainability or sustainable practices often leads to a focus on the structural design, pavement materials, or construction operation. These items are largely within a roadway engineer’s control. Practices—such as recycling, use of industrial byproducts such as fly ash and slag cement, resource conservation, CO2 footprint, and even embodied energy—tend to get a fair amount of attention. However, there are significant sustainability opportunities missed by ignoring the operational or use-phase of a pavement’s life.
Recent research suggests it is the long-term cumulative benefits that really matter—the things that contribute in a positive way to sustainability every hour of every day of a pavement’s lifetime. Mostly these relate to fuel consumption of traffic, energy, and CO2 savings associated with light-reflective “cool” pavements.
The central question is: In the context of sustainable practices, are the long-term operational sustainability benefits accounted for when pavement selection and design decisions are made. Is the focus on the right things?
What is meant by sustainability?
Fundamentally, how do we balance the natural environment, societal needs, and the economic vitality when talking about building pavements? Even though much has been written about sustainable practices in the roadway sector over the last decade, significant confusion remains about how to tackle this challenge. There have been major advancements in the use of recycled materials and industrial byproducts in pavements, as well as efforts to improve upon machine efficiency. There also has been much focus on ways to reduce the energy footprint of the building materials themselves, by optimizing mixtures and using less energy intensive cementitious systems.
So, how does one make sense of all these opportunities? The concept of cradle-to-grave, or end-to-end analysis 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 standardized by the International Organization for Standardization (ISO) in the late 1990s. The purpose of the LCA approach is to ensure all the effects, factors, and loads are accounted for in the analysis, from the moment any component is extracted or processed, all the way to its end of life. It involves a cumulative analysis of a product’s environmental or sustainability impact throughout all stages of the product’s life, including impacts not usually considered in more traditional analyses. Figure 1 illustrates the concept of a LCA, including inputs, outputs, and the system boundary.
Only through this kind of a comprehensive cradle-to-grave LCA will transportation officials and other decision makers be able to properly account for and consider the cumulative impacts of their decisions. In other words, LCA will enable us to ascertain what sustainability really means in the context of roadway pavements.
Focusing on the long term
It is important to clarify that every one of the commonly adopted sustainability strategies are important and significant benefits that can be derived by embracing each of these sustainable practices. Infrastructure professionals always should strive to find ways to improve the sustainability profile in all arenas, and support moving these technologies forward.
However, it is useful to know what aids the highway administrator in making the most informed decisions about the sustainability impacts of the various pavement infrastructure choices available. It does not make sense to make pavement design and selection decisions without considering sustainability from a comprehensive perspective.
A recent LCA study examined the impact of 12 different environmental factors on six different pavement structures. Environmental factors included greenhouse gases, energy, ecotoxicity, smog, odor, solid waste, and more. The analysis indicates the overall impact from the use-phase dwarfs impacts from all other phases of the pavement’s life cycle. In fact, with the sole exception of solid waste, the impact of the use-phase, or traffic in this case, was at least 10 times greater than all other phases.
Figure 2 illustrates how the different life-cycle phases of a typical road contribute to the total footprint across the 12 different environmental factors. In this case, truck and car traffic dominate. Just a 2% to 3% improvement in truck and car traffic would offset the entire construction and maintenance ecoprofile. Because pavements remain in service for decades, lying exposed every hour of every day 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.
What are these use-phase or operational-phase impacts? The most prominent opportunities are vehicle fuel consumption rates, which relate to pavement rigidity and smoothness, and pavement albedo, as it relates to urban heat island, lighting, and global cooling.
Fuel and pavement
Because fuel consumption of traffic has such an influence on the roadway’s overall sustainability footprint (or ecoprofile), any strategies that positively impact fuel consumption will be of great importance. Even small changes in traffic’s fuel consumption likely would result in hundreds of thousands of gallons of marginal fuel consumption over a pavement section’s service life.
Can such considerable fuel savings via pavement type be realized alone? Can the fuel economy of vehicles be increased by 1%, 2%, or more just by selecting a rigid pavement structure? According to a growing body of evidence, the answer is yes.
Since 1989, several important studies have examined the link between vehicle fuel consumption rates and pavements. Most of these studies suggest that because vehicles, particularly trucks, cause greater deflections on flexible pavements than on rigid ones, more of the energy intended for propelling the vehicle is “absorbed” causing those deflections.
A comprehensive, multiphase study on the effects of pavement structure on vehicle fuel consumption, published in 2006 by the National Research Council Canada, concluded that tractor-trailers traveling on rigid pavements consume on average about 3.8% less fuel than those traveling on flexible pavements.
Pavement smoothness is also a factor; the smoother a pavement, the less fuel required to propel vehicles along the road. Any roughness translates into vertical motion and consequently heat in vehicle suspension systems, leaving less energy available for forward motion. This concept is very similar to the hypotheses associated with rigid versus flexible pavements. Any energy that is “bled off” to do such things as deflect the pavement, or excite the suspension system, will not be available to propel the vehicle forward. Hence, more energy is required to propel the vehicle, and fuel economy suffers.
A number of studies published since 1990 suggest that there are significant fuel consumption efficiency gains associated with pavement smoothness. A 2000 U.S. Federal Highway Administration report suggests a reduction in International Roughness Index from 150 to 75 inches per mile on asphalt pavement results in a 4.5% improvement in truck fuel economy.
These benefits are relevant not only for new pavements, but also for maintenance strategies and schedules. Concrete pavement restoration activities, such as diamond grinding, is particularly useful to restore pavements and improve ride quality, noise, and surface texture. This is important because it is possible to diamond grind concrete pavements up to three times before major reconstruction is needed, which could extend the service life of a concrete pavement to twice its normal design. In addition, this enhanced smoothness and longevity is accomplished without extracting or processing additional raw materials, such as aggregates or binders.
In overall transportation sustainability, the potential fuel savings and greenhouse gas reductions associated with selecting and maintaining rigid and smooth pavements dwarfs the sustainability benefits from all other phases of the pavement life cycle.
Pavement and light reflectance
Another critical operational-phase impact relates to concrete pavement’s capacity to reflect light. This characteristic of pavement, generally referred to as albedo or solar reflectance, is a function of both type and age of the material. The higher albedo concrete pavement provides is advantageous for multiple reasons. High-albedo pavements significantly reduce the amount of energy needed for artificial roadway illumination during nighttime.
High-albedo or cool pavements reduce the amount of energy needed to cool urban environments associated with the urban heat island effect. Cool pavements also mitigate the greenhouse effect and contribute to global cooling by reducing the amount of solar radiation absorbed by the earth’s surface. A recent study details how changing the albedo of pavement surfaces offsets CO2. By reflecting more of the sun’s energy away from the surface, pavements reduce the amount of solar radiation absorbed by the earth’s surface. The study indicates from a global warming perspective, an increase in albedo for pavement of 0.15 is equivalent to eliminating 38 kg of CO2 per square meter of pavement surface. When considered in the 100 largest cities on earth, the equivalent for this CO2 offset is more than 20 gigatons. This clearly suggests the use of high-albedo pavements has a significant sustainability impact and can mitigate climate change.
Bringing things into focus
Sustainability considerations must be included in the decision making process for highway administrators and engineers. To date, the focus remains on material acquisition, production, and construction phases of the pavement life cycle. These factors are all important, and significant sustainable benefits can be derived, but sustainability opportunities are missed by ignoring the benefits of the use-phase of a pavement’s life. These are things that contribute in a positive way to sustainability every hour of every day of the pavement’s lifetime. As agencies recognize the limitation of the current approach, they explore comprehensive LCA approaches and need tools to account for all the impacts, including those in the long term.
The development of these tools and guidelines will provide agencies with direction in the area of sustainability. The transportation construction community will have the opportunity to positively impact the sustainability by selecting, designing, and optimizing pavements. From what is known today, rigid, smooth, and light-reflective pavement surfaces will be a major focus of sustainable roadway practices.
Leif Wathne, PE, is the vice president of highways and federal affairs at the American Concrete Pavement Association, Washington, D.C.
Editor’s Note: This article is based on “Sustainability Opportunities with Pavements: Focusing on the Right Things” (SR306). This and other resources are available in printed and PDF form in ACPA’s bookstore at www.acpa.org/bookstore.