Highway construction is vastly different than it was even a few years ago. The recession continues to plague agencies and the construction industry. As Congress weighs decisions about the future of the federal-aid highway program, road builders and agencies are challenged to do more with less and to live within the context of continuing resolutions at the federal level, as well as tight budgets at the state and local level. As a result, most of the work performed by contractors and agencies these days is either for preservation, maintenance or repair.
Austerity has created greater awareness about every aspect of building and maintaining our nation’s highways. More people are now viewing waste, not as a problem, but as an opportunity to reuse, and so it is accurate to say there is a greater awareness and appreciation for sustainability.
Economic cycles may come and go, but sustainable construction practices are likely here to stay. Although the sharper focus on sustainability may have been influenced somewhat by the current economic conditions, there is a greater imperative than ever to decrease the carbon footprint, conserve natural resources and adopt other sustainable construction practices. In pavement design and construction, there are many opportunities to improve sustainable practice. Much of the focus historically has been on doing so during the phases of design, construction and even maintenance. This is what we will refer to as the conventional approach to sustainability. However, there is increasing awareness of the significantly larger opportunities available by considering effects during the use phase or operational phase of a pavement’s life span. Consideration of these factors is part of an emerging view of sustainability in the highway arena.
At any time
For the highway engineer, sustainability initiatives are usually focused on structural design, the pavement materials or the construction operation itself. This is natural, as these are items that are largely within the engineer’s control. Items such as recycling, use of industrial byproducts (fly ash, slag cement, etc.), resource conservation, CO2 footprint and embodied energy tend to get a fair amount of attention in this context. Broadly, these sustainability opportunities can be captured before, during or after concrete-pavement construction.
The before
In the design phase, there are opportunities to improve sustainability by designing for maximum longevity, as well as by minimizing both energy consumption and the use of virgin materials.
Long-lasting pavements do not require frequent rehabilitation or reconstruction and, therefore, use less construction materials and less fossil fuel over time. Because long-lasting pavements also allow longer intervals between rehabilitation or reconstruction, there also are fewer work zones, which means fewer road-user delays, less congestion and lower consumption of fossil fuels by motorists stuck in traffic.
Other ways designers can influence sustainability include:
- Optimizing pavement design: Tools such as the “Mechanical-Empirical Pavement Design Guide” (M-E PDG), the latest highway analytical and design tool adopted by the American Association of State Highway & Transportation Officials (AASHTO), enables designers to optimize thickness, reduce overdesign and improve reliability. The guide also allows pavement designers to optimize use of materials, thereby minimizing waste and reducing a highway’s environmental footprint. Designers also can improve sustainability through the use of analytical and design tools such as the American Concrete Pavement Association’s (ACPA) StreetPave (for street- and road-pavement design) and ACPA’s PerviousPave (for designing pervious pavements);
- Using supplementary cementitious materials (SCMs) and blended cements: Many SCMs are made from industrial byproducts. Examples include fly ash (from coal-burning power plants) and blast furnace slag (a byproduct of iron production). Incorporating these materials into a concrete mixture diverts them from landfills, while at the same time lowering the carbon footprint of the mixture and improving its performance and longevity. While fly ash and slag are among the most common SCMs, there is a wide range of other materials, including silica fume (a byproduct of silicon manufacture), metakaolin, rice hull ash and other natural pozzolans that could be used in a concrete-paving mixture. Blended cements, those containing SCMs, also have become more popular in recent years. Our hope is that states and municipalities will further embrace the incorporation of SCMs in their concrete-pavement mixtures;
- Designing with two-lift concrete: Two-lift construction involves placement of two wet-on-wet layers of concrete instead of a single layer used in traditional concrete paving. This method allows for a more efficient use of aggregate resources in the pavement section. A designer can use highly durable and abrasion-resistant aggregates at the surface, where they are needed most, and recycled aggregates or lower-quality aggregates in the bottom bulk-layer, where they are perfectly suited. The top layer also can use highly specialized aggregates that provide optimal noise and friction requirements, where conditions dictate. A few recent examples of two-lift pavements include I-70 in Abilene, Kan., (placed in 2008) and U.S. 141 in St. Louis (placed in 2010). The St. Louis project features photocatalytic cement—sometimes called “smog-eating” cement—in the top lift. In addition to the favorable sustainability aspects, the costs of these two-lift concrete-pavement projects were reported to be competitive with conventional jointed plain concrete pavements;
- Designing with pervious concrete: Pervious concrete is typically used in low-volume applications such as parking lots, but is seeing increased use in streets and roads. Pervious concrete pavements are considered to be a valuable storm-water management tool under the requirements of the EPA Storm Water Phase II Final Rule. Phase II regulations provide programs and practices to help control the amount of contaminants in our waterways; and
- Precast panels: Precast concrete pavement panels can help accelerate pavement construction while also providing enhanced durability and longevity. They are particularly effective in highly trafficked urban areas, where rapid replacement and extreme longevity are paramount. In these cases the increased cost of precast solutions may be justified.
The during
Just as there are opportunities to improve sustainability in the design phase, so too are there ways to make improvements during construction. A few examples of how sustainability can be improved during the construction phase include:
- Using locally available materials: For economic reasons alone, contractors traditionally use materials from local quarries. By using locally available materials, contractors reduce hauling distances, fuel use and carbon footprints. Using on-site mobile mix plants, contractors avoid hauling concrete from fixed ready-mix plants distant from the construction site, saving fuel, money and emissions in the process;
- Recycling: By using recycled concrete aggregates (RCA) in new pavements, agencies and contractors can virtually eliminate the need for mining and transporting virgin aggregates. Recycling concrete pavement on-site eliminates the need to haul old concrete to an off-site crushing and processing facility and then back to the concrete plant. RCA has been used successfully in new concrete pavements for several decades, although it is typically used for sub-base layers. Concrete pavements are 100% recyclable, and most concrete-pavement-recycling projects today put all the old concrete to use. In fact, according to the Construction Materials Recycling Association, concrete is the most recycled material in America—an estimated 140 million tons is recycled annually;
- Accelerated construction: Accelerated construction methods, such as fast-track paving, generally minimize work-zone impacts, and as a result, can lower the carbon footprint by minimizing traffic congestion and reducing air pollution from cars and trucks idling;
- Contracting flexibility and equipment innovations: Contract incentives for completing projects on time or early, or using lane-rental charges, a commonly used technique to assess a fee to the contractor for every hour a lane is taken out of service, also can enhance sustainability by encouraging project acceleration and efficiency. Contractors often meet these requirements by lengthening the workday or increasing the size of construction crews; and
- Innovative materials and equipment: Equipment and materials innovations are allowing contractors to achieve greater efficiency and thereby are accelerating project delivery. Just a few examples of equipment innovations that allow this are variable-width pavers, dowel-bar inserters and stringless paving. Nondestructive testing also has helped expedite project delivery, and examples of equipment and instruments that allow this include maturity meters for in-place strength testing and real-time smoothness equipment. Likewise, concrete mixtures with different set times, mixtures containing SCMs and specialty cements/admixtures also can accelerate project delivery while also improving sustainability by imparting properties that contribute to the pavement’s longevity and other performance features.
The after
After a pavement has been constructed and is in service, a highway agency has essentially only one opportunity to ensure and enhance the sustainability profile: maintenance, preservation and restoration activities. The objective is to extend the life of the pavement as long as possible, while minimizing disruptions and maximizing resource efficiency. Such practices include concrete-pavement-restoration (CPR) activities like dowel-bar retrofits, cross-stitching, partial-depth repairs, joint and crack resealing, slab stabilization and, most important, diamond grinding.
Diamond grinding involves removing a thin layer of a concrete pavement’s surface using closely spaced diamond saw blades, exposing a renewed and smooth concrete-pavement surface. According to data published by Caltrans, diamond grinding extends the service life of concrete pavement nationwide by an average of 14 years. Depending on its initial design, concrete pavement can be diamond ground up to three times before major reconstruction is required. Consequently, diamond grinding can, when combined with other needed CPR techniques, extend the service life of the pavement to twice its normal design life. In addition, this enhanced smoothness and longevity is accomplished without extracting or processing additional raw materials such as aggregates or binders. Two other major advantages to diamond grinding also are achieved: improved texturing/skid resistance and reduced tire-pavement interface noise levels. Both of these benefits have positive sustainability implications from a societal perspective.
Finally, another pavement-preservation technique that can extend pavement life significantly is concrete overlays. Concrete overlays are increasingly being adopted by state DOTs and municipalities as a way to preserve the structural value of a concrete pavement near the end of its service life as they look for proactive, sustainable pavement-maintenance and rehabilitation strategies that provide long life at a reasonable cost. Concrete overlays offer cost-effective, versatile, long-term solutions for the full range of concrete-pavement needs—and they can extend the pavement life by 25 to 30 years.
Proactive programs
Given the increased interest and focus on sustainability and green approaches, what are roadway agencies and pavement engineers currently doing to meet the challenge? A variety of programs have emerged in recent years where focus has been on improving the sustainability of our highways, streets and pavements. These programs include environmental life-cycle assessment (LCA) programs, sustainability rating systems as well as direct implementation efforts of what is regarded as sustainable practices. Likely because of the complexity and unfamiliarity associated with performing a complete LCA, most software applications and ratings systems take a conventional approach and focus primarily on the pavement design, construction or maintenance periods.
The most prominent and commonly referred to programs in the U.S. today include the Green Highways Partnership, the Greenroads system (www.greenroads.us) and, most recently, the Federal Highway Administration’s (FHWA) Sustainable Highway Self Evaluation Tool (www.sustainablehighways?.org). There are others as well, including: the New York State DOT’s Green LITES program; the Illinois DOT’s Livable and Sustainable Transportation Rating System and the Wisconsin DOT’s BEST program.
Each of these tools is focused primarily on either the materials production or the construction phase of the pavement’s life span. They all involve capturing/measuring/rating the more conventional sustainability aspect of our highways and pavements. The state of practice within the U.S. highway sector is focused primarily on strategies to improve the sustainability profile of materials production operations and construction/maintenance operations. Even in the area of measurement and rating of sustainability, little is being done to capture the long-term cumulative sustainability benefits that present themselves during the use or operational phase of a pavement’s life cycle.
The most impact
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 emphasize that every one of the conventional sustainability strategies mentioned in the previous sections are important, and significant benefits can be derived by embracing each of these sustainable practices. The concrete-paving industry should always strive to find ways to improve the sustainability profile in all arenas.
However, it is useful to know where the industry can make the biggest difference. This will aid highway administrators and engineers 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. With increased understanding of the truly important characteristics and the most significant sustainability factors, engineers and administrators will be in a better position to make meaningful advances in sustainable highway practice.
So, what are the most significant factors? How do we know what the most effective sustainability strategies, features and factors are? To address this question, the concept of cradle-to-grave analysis has emerged in the pavement arena. Carrying an analysis from cradle to grave is the central idea of an 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. 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. 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 1)
From the analysis it can be seen that 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 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% 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 Universität München, the National Concrete Pavement Technology Center (CP Tech 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 efforts in the concrete-pavement industry.
What are these use-phase or operational-phase impacts? 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
Can we realize such considerable fuel savings via pavement type alone? Can we increase the fuel economy of a vehicle by 1%, 2% or more just by selecting a rigid pavement structure? According to a growing body of evidence, the answer to that question is yes.
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, multiphase 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%) fuel than those traveling on flexible pavements. Research is ongoing at MIT’s Concrete Sustainability Hub to verify and expand on this work.
Pavement smoothness also is a factor. The smoother a pavement is, the less fuel will be required to propel vehicles along the roadway. Any roughness along the way will translate 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 there are significant fuel-efficiency gains associated with pavement-smoothness gains. An FHWA report published in 2000 suggests that a reduction in International Roughness Index from 150 in./mile to 75 in./mile (i.e., improvement in smoothness) on asphalt pavement results in an accompanying 4.5% improvement in truck fuel economy.
These benefits are relevant not only for new pavements (i.e., specifying smooth pavements), but it also is important when deciding on maintenance strategies and schedules. As highlighted in the previous section, diamond grinding is a particularly useful technique to restore pavements and improve smoothness. This can extend the service life of a smooth concrete pavement to twice its design life.
Pavement and light reflectance
The other critical operational-phase impact that should be considered in a sustainability assessment 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. This has obvious visibility and safety implications, but the higher albedo that concrete pavement can provide is advantageous for other important sustainability-related reasons as well. High-albedo pavements can significantly reduce the amount of energy needed for artificial roadway illumination during nighttime.
High-albedo, or cool, pavements also can reduce the amount of energy needed to cool urban environments associated with the urban heat island effect. Much has been written about this in the literature. Cool pavements also can 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 can offset CO2. By reflecting more of the sun’s energy away from the surface and back out into space, they reduce the amount of solar radiation absorbed by the earth’s surface; in effect, this is a change in the global energy balance. The study indicates that from a global-warming perspective, an increase in albedo for pavement of 0.15 (say from 0.10 to 0.25) 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 CO2 offset for increasing the albedo for all paved surfaces by 0.15 is over 20 gigatons. This clearly suggests that use of high-albedo pavements has a significant sustainability impact and may even prove to be a useful tool in helping mitigate climate change. Moreover, because pavements remain in service for decades, lying exposed every hour of every day, the cumulative impact of these use factors over the pavement’s service life are enormous.
More needs to happen
Highway officials and the industry have made great strides in understanding and embracing sustainability concepts and practices. To date, most of the highway and pavement community’s efforts have been focused on conventional sustainability approaches—those that focus on the material acquisition, production and construction phase of the pavement life cycle (things like optimized design, recycling, reuse, use of industrial byproducts, etc.). Although these approaches are all important, and significant benefits can be derived by embracing sustainable practices with each of these, there are significant sustainability opportunities that are missed by ignoring the benefits presented by considering the operational or 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.
Embracing this emerging approach to sustainability is going to be essential if engineers and officials are going to be able to make meaningful advances in sustainable highway practice. To successfully move this emerging approach forward, several things must happen.
LCA research, such as that conducted by the FHWA, CP Tech Center and the MIT Concrete Sustainability Hub, must continue to be supported to provide objective and meaningful results that agencies and industry can use. Education and outreach by all stakeholders also is important. ACPA is working closely with the public sector, industry and academia to advance both the understanding and use of sustainable construction practices as well as LCA. For more than two decades, ACPA has focused on environmental features and benefits of concrete pavements and continues to work with agencies and the industry to refine and embrace best practices in this important area.
ACPA also believes sustainability initiatives must be objective, well-balanced, achievable and measurable. More research, education and outreach are needed, but it is clear that the emphasis on sustainability must go beyond the design and construction phases to realize the significant benefits that can be derived during the use phase of pavements. From what we know today, rigid, smooth and light-reflective pavement surfaces will be a major focus of sustainable roadway practices moving forward.
