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|Federal Highway Administration > Publications > Public Roads > Vol. 73 · No. 6 > Leaving a Smaller Footprint|
Publication Number: FHWA-HRT-10-004
Leaving a Smaller Footprint
by Suneel N. Vanikar, Jim Grove, and Leif Wathne
Seizing opportunities in the design, construction, and maintenance phases of concrete pavements can provide a more sustainable highway infrastructure.
The Federal-aid highway system consists of about 160,000 miles (256,000 kilometers) of concrete, asphalt, and other materials that stitch together a roadway network critical to the Nation's economy, defense, and mobility. Though the system continues to provide efficient and cost-effective movement of people, goods, and services, due to the ever-increasing traffic levels and age of the system, thousands of miles now require maintenance, rehabilitation, or reconstruction every year.
How transportation agencies address these needs for system preservation can play a critical role in ensuring the sustainability of infrastructure, especially in light of growing debate among scientists, policymakers, and the public regarding climate change and greenhouse gas emissions, such as carbon dioxide (CO2).
Concrete, one of the most widely used materials on Earth, is a key building material in all sorts of modern public works projects -- from water treatment facilities, aqueducts, underground utilities, hydroelectric dams, containment structures, and buildings to slabs, foundations, levees, tunnels, safety barriers, bridges, and of course pavements. According to the Carbon Dioxide Information Analysis Center, the manufacture of cement for use in concrete accounts for roughly 4 percent of global CO2 emissions.
Responsible, sustainable practices in the selection, design, and construction of concrete pavements can limit the material's environmental footprint, putting highway agencies in the driver's seat to become part of the solution to the Nation's sustainability challenges.
"Concrete pavement preservation projects offer a long-term repair solution engineered to last many years, even decades," says John H. Roberts, executive director of the International Grooving & Grinding Association® (IGGA). "They can be designed and packaged for bid in a matter of days. Stiff competition within the industry and advancements in technology ensure that pricing is typically far less than alternative repair treatments."
Indeed, transportation agencies can seize opportunities in all phases of a concrete pavement's life cycle -- before, during, and after construction -- to bring about more sustainable infrastructure.
Opportunities Before Construction
The design phase of a concrete paving project affords two main opportunities to focus on sustainability. Transportation agencies can optimize pavement design to maximize longevity and minimize use of virgin materials and energy.
When developing concrete mixtures to meet project-specific needs, pavement designers can incorporate industrial byproducts, such as fly ash from coal-burning plants and blast furnace slag, and other constituents that might have lower carbon footprints than conventional materials. By incorporating industrial byproducts as replacements for virgin aggregates and cementitious materials, designers can produce a more sustainable concrete and improve performance at the same time. For example, in 2007 researchers with the Iowa Department of Transportation found that a test section construction on I-29 in 1994 incorporating a blended pozzolan (calcined clay) cement was in "excellent condition," while other parts of the project experienced early pavement deterioration.
In the context of sustainability, longevity is paramount. A long-lasting concrete pavement does not require rehabilitation or reconstruction as often as a typical pavement and therefore consumes fewer materials in the long run. Because a long-lasting pavement requires less frequent reconstruction, it entails fewer work zones and thus has less impact on traffic congestion, with accompanying energy savings and a reduction in vehicle pollutants. The latter is particularly important: By most estimates, the sustainability footprint associated with the usage phase of a roadway facility dwarfs the footprint associated with highway materials production and construction.
The longevity of concrete pavements is well documented. Countless concrete highways in North America have lasted 50 years or more, supporting traffic volumes much greater than originally anticipated. A notable example is I-10 in California's San Bernardino Valley, originally constructed in 1946 as part of Route 66. Portions of this concrete highway still carry more than 200,000 vehicles per day. The California Department of Transportation (Caltrans) has renewed the highway three times by surface grinding over its 60-plus-year life, testament to concrete pavement's sustainability.
Other key opportunities to improve the sustainability of concrete pavements before construction lie in use of the latest pavement design guidelines, two-lift pavement design, modular concrete pavement, materials selection and mixture design, and choice of cementitious materials.
Many States have begun to implement the procedures and designs described in the new Mechanistic-Empirical Pavement Design Guide published by the American Association of State Highway and Transportation Officials (AASHTO). The latest guide replaces earlier ones that served highway agencies well for half a century but were based on limited foundational data. The new guide can help designers optimize pavement thickness to meet the design-life criteria with greater reliability, but without overbuilding. Further, it enables a pavement designer to optimize use of materials, thereby minimizing waste and reducing a highway's environmental footprint.
Two-Lift Design and Pervious Concrete
Two-lift concrete pavement design is making a comeback in the United States. Two-lift construction involves placement of two wet-on-wet layers of concrete instead of the homogenous single layer commonly placed in concrete paving. The bottom layer is thick and consists of lower quality (lower durability and strength) aggregate. The top layer is thin and consists of high-quality aggregate that provides better resistance to freeze-thaw damage, reduced noise, and improved friction. In the past, cost, mixture design, and construction concerns inhibited use of two-lift paving.
Lately, however, changes in the availability of a good quality aggregate, advances in materials knowledge and construction equipment, and increasing demands for pavement surfaces that meet specific noise, durability, and safety objectives are prompting the need to reconsider two-lift paving. Currently, the greatest resistance to this technique is economic: Two-lift paving would likely result in the use of two concrete plants, two slipform paving machines (although single machines do exist to perform this operation), and a special haul road, all of which add to a project's cost.
A promising advancement for two-lift pavements is use of recycled concrete, or even recycled asphalt, material in the lower lift, which enables transportation agencies to avoid using virgin materials that require mining or processing. If the existing concrete pavement is to be recycled for use in the lower lift, agencies also can take advantage of a process for recycling pavement in place. Developed more than 15 years ago, the technique significantly reduces material hauling and saves energy.
In another design approach, agencies can use roller-compacted concrete to meet design criteria. This alternative requires less cementitious material than conventional concrete, which helps to minimize the CO2 emissions required to produce the final concrete mixture.
Pervious concrete can provide environmental advantages, such as reduced stormwater runoff. To date, this technology has been used successfully for parking lots, city streets, and alleys. (See "Greener Alleys" on page 26 in this issue of Public Roads.) Research is underway in Australia and the United States to develop a procedure that would allow placement of pervious pavement as the top lift of a two-lift pavement and thereby reduce noise and increase safety through reduced splash and spray.
Modular Concrete Pavement
Modular concrete pavement technologies facilitate accelerated pavement construction and provide long-lasting concrete pavements. They are particularly effective in urban areas and in situations with high traffic volumes. Precast, prestressed and nonprestressed concrete systems are available, and several such projects have been constructed in the United States over the last 10 years.
Materials Selection And Mixture Design
In some areas of the United States, engineers have successfully incorporated blast furnace slag generated from steel production as coarse aggregate in new concrete. The slag aggregate is highly absorptive, and experience shows it can help support a durable, long-lasting concrete. Using blast furnace slag as aggregate not only puts a former waste material to productive use, avoiding landfilling and reducing the demand for virgin aggregate, but also the slag releases absorbed water during hydration that increases strength and lowers permeability of the concrete.
The concrete component that has the greatest carbon footprint is cement. Cement manufacture produces CO2 from both the calcination of limestone and the combustion of fuels to generate the high kiln temperatures needed to produce clinker. Therefore, if engineers can reduce the amount of cement in a concrete mixture, or reduce the amount of clinker needed to produce cement, they can thereby reduce concrete's carbon footprint.
In 2004, ASTM International changed the specification for cement manufacture to allow up to 5 percent uncalcined limestone. This allowance directly reduced the amount of clinker needed, plus the CO2 footprint per unit mass of cement by the same amount. A report by the Portland Cement Association, The Use of Limestone in Portland Cement: A State-of-the-Art Report, concludes that there is no discernable difference in the properties of concrete resulting from this change.
Use of supplementary cementitious materials (SCMs) as a replacement for cement can improve the performance of concrete, reduce environmental impacts, and lower the cost. Concrete made with SCMs produces less heat during the hydration process (a benefit for constructability in hot, summer weather), is more workable, and has greater strength and lower permeability.
Most SCMs are byproducts from other industries. A common SCM in concrete pavements is fly ash, which is a byproduct of coal burning at electricity generating stations. Others include ground granulated blast furnace slag (a byproduct of extracting iron from iron ore), silica fume (a byproduct of silicon manufacture), metakaolin, rice hull ash, and other natural pozzolans, which are used to increase concrete strength, density, and resistance to chemicals. (California recently included in its specifications the option to incorporate rice hull ash into concrete mixtures.)
The volume of SCMs designed into a mixture depends on the intended use of the concrete. Engineers at CanmetENERGY (Canada's center for clean energy research and technology development) are working on projects in India that utilize high volumes of fly ash. The approach of using high volumes of fly ash is not new, but its sustainability benefits have rekindled interest in determining how such a mixture can be used most successfully.
Blended cements have become more popular in recent years. They are the product of blending or intergrinding cement with one or more SCMs, which can take the place of as much as 50 percent of the cement in concrete, thereby reducing the clinker that needs to be produced, also by up to 50 percent. Engineers also are evaluating alternatives to portland cements. Geopolymer cements, particularly those made from fly ash, have vastly reduced environmental impacts yet have similar properties to portland cements.
Photocatalytic cement, which uses energy from the sun's ultraviolet rays to oxidize most organic and some inorganic compounds on the surface of the concrete structure, offers significant air pollution benefits. The active ingredients (including titanium dioxide) are not consumed during these reactions; they act solely as catalysts. Water from rain and melted snow then wash the solid pollutants away from the surface, and the process continues. Pavements made with this type of concrete can reduce nitrogen oxides, major contributors to ozone and smog, by as much as 80 percent.
Opportunities During Construction
Transportation agencies have many opportunities to enhance the sustainability of concrete pavements during construction. These opportunities include using locally available materials, recycling, accelerated construction, contracting flexibility, and equipment innovations.
By using locally available materials, such as from nearby aggregate quarries, contractors can reduce hauling distances, fuel use, and carbon footprints. Use of mobile batch plants onsite (instead of hauling concrete from fixed concrete plants distant from the construction site) offers similar sustainability benefits. Contractors often select local materials for economic reasons, but certain project characteristics could make it even more attractive to use such materials.
For example, with two-lift pavements, locally available yet marginal aggregates that ordinarily would not be suitable for the surface layer of a pavement could be used for the bottom (bulk) layer. In this case, only the upper, much thinner layer would require aggregates brought in from outside the local area. Using well-graded and durable aggregates that further extend the longevity of the bulk layer could further reduce the pavement's carbon footprint.
In-place recycling is another means to enhance pavement sustainability during construction. By using recycled concrete aggregates in new pavements, agencies can virtually eliminate the need for mining and transporting virgin aggregates. Furthermore, recycling the existing concrete pavement onsite eliminates the need to transport the old concrete to an offsite crusher and processing facility and back to the concrete plant.
The strategy of using recycled aggregate in new concrete pavements has proven successful in many applications across the United States, resulting in excellent long-term concrete performance. One notable example is I-35 near Guthrie, OK, where the Oklahoma Department of Transportation (ODOT) reconstructed a 6-mile (10-kilometer) section in 1988.
By way of a two-sentence notation on the construction plan, ODOT allowed the contractor the option of salvaging the old concrete to produce coarse aggregate for the new concrete mixture. According to the contractor, the yield of coarse aggregate (#57 stone) that could be reused in the project was about 40 percent of the total crushed. The remaining 60 percent of the old concrete became about 25 percent chips and 35 percent screenings. Although ODOT did not incorporate the latter materials into the pavement in that project, today the chips would be a valuable intermediate aggregate for an optimized combined aggregate.
The I-35 project has a rare educational value because ODOT paved only the southbound lanes with the recycled aggregate; the agency paved the northbound lanes with virgin limestone aggregate. Offering further testament that sustainable practices can yield long-term success, David M. Howard, president/CEO of Koss Construction Co., says, "Today the stretch of roadway is 20 years old and there is no evidence of any difference in quality between the lanes."
Congestion is a significant contributor to energy waste, CO2 emissions, and pollution. Any construction activity, including resurfacing, rehabilitation, preservation, and reconstruction, can result in some degree of congestion. Accelerated construction strategies, however, can reduce congestion and minimize the overall environmental footprint.
One of those strategies, fast-track paving, generally entails using the least time-consuming construction techniques that project-specific conditions will allow. This technique requires well-planned construction sequencing and sound traffic-handling plans. Contractors and specifying agencies should be aware that operation adjustments will be necessary while paving crews become accustomed to the characteristics of fast-track concrete mixtures.
Contracting Flexibility and Equipment Innovations
For concrete pavements, many other approaches can accelerate construction. Traditional acceleration methods include contract incentives and disincentives for completing projects on time, 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. Contractors often meet these requirements by lengthening the workday or increasing the size of construction crews.
Transportation agencies have other opportunities for shortening construction times, such as giving contractors flexibility to use different materials, technologies, and equipment. Some agencies have allowed contractors to use concrete mixtures tailored to a particular situation, rather than applying just one mixture approved for all placements. This approach not only encourages innovation but makes good engineering sense.
For example, use of several different mixtures often is appropriate for a single project, as the first concrete placed during a construction window will have more time to gain strength than the last concrete placed. A variety of cementitious materials and specialty admixtures are available to contractors that can help optimize the strength gain for any given situation.
Allowing use of early-age joint sawing -- sawing to induce cracks at desired locations on a pavement -- helps minimize uncontrolled cracking in fast-track paving. The typical time sequence for joint sawing and sealing is not necessarily compatible with accelerated construction and early opening to traffic. A contractor must take into consideration that sawing is necessary much sooner after paving with accelerated construction than with normal concrete. Light saws that handle easily and are more versatile generally will be more effective for accelerated construction projects.
In-place and nondestructive testing to determine concrete strength is particularly attractive for accelerated construction projects, as there is little time to evaluate strength using test specimens cast onsite during construction. Nondestructive tests evaluate strength, or load-carrying capacity, by monitoring internal concrete temperature in the field and can help establish when the concrete pavement is ready for sawing or to accept traffic.
The basis for determining when a concrete pavement is ready to accept traffic should be its strength and not an arbitrary time after placement, such as 1 day or 12 hours. The required concrete strength is a function of the type and volume of traffic as well as pavement thickness and geometry. Restricting use to automobile traffic during early ages can accelerate the time to opening.
Recent equipment innovations also can facilitate rapid construction. Slipform paving technology continues to improve and now can accommodate variable-width paving, which reduces equipment setup and adjustment times, and speeds overall operations. Automatic dowel bar inserters and tie bar inserters can reduce paving time as well by eliminating the time typically needed to place and affix baskets. Also, stringless paving technologies (involving a computer-aided electronic guidance system controlling the slip form paver instead of the conventional string line guidance) not only eliminate the time required to set up and maintain a paving string line, but promise to improve pavement smoothness.
"The recent push for stringless paving technology in the mainline concrete paving market has accelerated equipment innovation," says Ron Meskis, national sales manager at Guntert & Zimmerman Const. Div., Inc. "With the savings from string line, stringless paving technology puts concrete at its highest advantage."
Opportunities After Construction
Once a concrete pavement is placed and opened to traffic, what can be done to help enhance its sustainability? The only opportunities at this point involve preservation and restoration strategies. In general, preservation and restoration include a series of engineering techniques to manage the rate of deterioration and renew a pavement. The techniques restore the pavement to a condition close to original and reduce the need for major and more costly repairs later.
Such practices include 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 -- analogous to refinishing a hardwood floor with a drum sander. Diamond grinding at once smoothes the pavement to improve the ride and improves its frictional characteristics, thereby enhancing safety.
Transportation agencies typically combine diamond grinding with at least one other restoration procedure when significant structural distresses are present. Diamond grinding restores ride or smoothness, while the other procedures address structural problems.
"Concrete preservation projects use environmentally friendly products manufactured in the United States that don't contain a drop of foreign oil," says IGGA's Roberts. "And motorists benefit from the resulting smooth, safe, and quiet ride after pavements are completed."
Based on a Caltrans study of 76 test sections nationwide (including pavements in freeze-thaw zones), the average longevity of a diamond-ground project is about 14 years. Crews can diamond-grind concrete pavements up to three times before major reconstruction is needed. Combined with routine maintenance, diamond grinding can extend the service life of a pavement to twice its normal design life. Plus, diamond grinding is done during off-peak hours, thereby minimizing disruption to traffic flow.
The benefit for the environmental footprint is that the rehabilitation activities use no virgin aggregates or binder materials. The rehabilitation has two other major advantages: (1) improved texturing, resulting in enhanced skid resistance, and (2) reduced tire/pavement interface noise levels.
In addition, highway agencies increasingly are adopting concrete overlays as a way to preserve the structural value of concrete and asphalt pavements near the end of their service lives. Concrete overlays, which are applications of thin layers of new concrete over old and worn concrete or asphalt, offer cost-effective, versatile solutions for the full range of pavement needs. And they can last 15 to 30 years.
The future looks promising for improving the sustainability of highway infrastructure through innovations in concrete pavement design, construction, and maintenance. "Sustainability in concrete pavements is simply good engineering, which always involves working with limited resources to achieve the best product possible," says Thomas Van Dam, program director at Applied Pavement Technology, Inc.
Researchers in the private sector continue to develop innovative manufactured materials, such as ceramics, epoxies, and polymers. Synthetic aggregates, such as lightweight and slag aggregates, already are available. More synthetics and carbon-sequestering aggregates might be available in the near future.
Photocatalytic cements have caught the interest of commercial and public specifiers alike and could see wider use in highway infrastructure -- not just to remove smog from the air in urban areas but also to reduce the urban heat island effect.
The latest design procedures for concrete pavements, available in AASHTO's design guide, will likely see widespread adoption over the coming years. Improved quality assurance and quality control tools will further facilitate construction of cost-effective and sustainable concrete pavements. Researchers at universities and in the private sector in the United States and overseas continue to pursue nanotechnology applications for cement and concrete, zero-carbon cements, composite materials, and equipment innovations that will further enhance the sustainability of concrete pavements.
"Implementation of new technologies for sustainable concrete pavements is as important as the research and development that create them," says Peter Stephanos, director of the FHWA Office of Pavement Technology. "Collaboration among researchers, developers, and practicing engineers representing customers and stakeholders is the key to quick adoption of sustainable approaches to concrete pavements."
Suneel Vanikar, P.E., leads the concrete group in the FHWA Office of Pavement Technology. An FHWA employee for more than 30 years, he is responsible for activities related to concrete pavements and materials, including policy, guidance, and technology transfer. He earned his M.S. degree in civil engineering from Colorado State University.
Jim Grove, P.E., is the senior project engineer in FHWA's Office of Pavement Technology, where he is involved in special projects concerning concrete materials, mixture design, durability, and pavement construction; quality testing and assurance; nanotechnology; and sustainability. He worked for 16 years with the Iowa Department of Transportation and 5 years with the National Concrete Pavement Technology Center at Iowa State University. He has a master's degree in transportation engineering and a bachelor's in civil engineering, both from Iowa State University.
Leif Wathne, P.E., currently serves as the American Concrete Pavement Association's (ACPA) vice president of highways and Federal affairs in Washington, DC, where he is responsible for various technical, policy, and Federal/State agency issues related to concrete highway pavements. He has a bachelor's degree in civil engineering from the University of Connecticut and a master's degree in civil engineering from Pennsylvania State University.
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