Innovations and advances in research are changing the way highways are built in America.
The Egyptians were pouring concrete in 2500 BC, and the Romans used it to construct the Pantheon and the Colosseum. By the mid-1800s, Europeans were building bridges with concrete, and the first “modern” concrete highway pavements appeared in the latter part of the 19th century. Naturally occurring asphalts, which have been used for waterproofing for thousands of years, came into common use in road construction in the 1800s. The first iron bridge was constructed in 1774, but by the end of the 19th century steel had largely replaced iron in bridge construction. These materials—concrete, asphalt, and steel—are now the mainstays of highway and bridge construction throughout the world, as well as of most types of public works infrastructure. Concrete and steel, the most versatile of these materials, are used for bridges and other highway structures; concrete and asphalt are used for roadway pavements.
Everyone is familiar with concrete, asphalt, and steel, and some of us have worked with them, perhaps on home improvement projects. This familiarity, coupled with the long history of their many uses, has led many otherwise technically savvy people to believe that these materials are well understood, that their performance can be easily and reliably predicted, and that the technical challenges in using them for highways were overcome long ago. However, such notions are largely incorrect and misleading.
For example, consider concrete, which is a mixture of portland cement, sand, aggregate (gravel or crushed stone), and water. Its performance characteristics are determined by the proportions and characteristics of the components, as well as by how it is mixed and formed. The underlying chemical reactions of concrete are surprisingly complex, not completely understood, and vary with the type of stone. Steel may be added for tensile strength (reinforced concrete), and a variety of additives have been identified to improve the workability and performance of concrete in particular applications and conditions. Damage and deterioration to concrete can result from excessive loadings and environmental conditions, such as freeze-thaw cycles and chemical reactions with salts used for deicing.
Many factors contribute to the
urgent need for innovation in
Concrete is the most heavily used substance in the world after water (Sedgwick, 1991). Worldwide, concrete construction annually consumes about 1.6 billion tons of cement, 10 billion tons of sand and crushed stone, and 1 billion tons of water (M.S. Kahn, 2007). Given transportation costs, there is a huge financial incentive to using local sources of stone, even if the properties of that stone are less than ideal. Thus concrete is not a homogenous material. In truth, an unlimited number of combinations and permutations are possible.
Much the same can be said of asphalt—technically, asphaltic concrete—which is also a mixture of aggregate (gravel or crushed stone), sand, and cement (asphalt binder); economics promote the use of locally available materials; and the underlying chemistry is not well understood. The characteristics of asphalt binder, for instance, vary depending on the source of crude oil from which it is derived.
The metallurgy of steel is probably better understood than the chemistry of either asphalt or concrete, but it too is a mixture with virtually limitless combinations. Strength, toughness, corrosion resistance, and weldability are some of the performance characteristics that vary with the type of steel alloy used and the intended applications.
As uses evolve and economic conditions change, we have a continuing need for a more sophisticated understanding of these common materials. Even though they are “mature” products, there is still room for significant incremental improvements in their performance. Because fundamental knowledge is still wanting, there is also considerable potential for breakthroughs in their performance.
Factors That Affect Highway Construction
All other things being equal, stronger, longer lasting, less costly highway materials are desirable and, given the quantities involved, there are plenty of incentives for innovation. In highway transportation, however, all other things are not equal. A number of other factors contribute to the urgent and continuing need for innovation.
First, traffic volume and loadings continue to increase. Every day the U.S. highway network carries more traffic, including heavy trucks that were unimagined when the system was originally conceived and constructed. The 47,000-mile interstate highway system today carries more traffic than the entire U.S. highway system carried in 1956 when the interstates were laid out. The U.S. Department of Transportation (DOT) estimates that in metropolitan areas the annual cost of traffic congestion for businesses and citizens is nearly $170 billion (PB Consult, Inc., 2007). On rural interstates, overall traffic more than doubled between 1970 and 2005; at the same time, the loadings on those highways increased six-fold, mainly due to the increase in the number of trucks and the number of miles they travel. (Truck traffic increased from about 5.7 percent of all vehicle-miles traveled on U.S. highways in 1965 to 7.5 percent in 2000 [FHWA, 2005]).
Second, traffic disruptions must be kept to a minimum during construction. Our overstressed highway system is not very resilient. Thus disruptions of any sort, such as lane and roadway closings, especially in major metropolitan areas and on key Interstate routes, can cause massive traffic snarls. This means that repair and reconstruction operations must often be done at night, which introduces a variety of additional complexities and safety issues. Occasionally, heroic measures must be taken to keep traffic moving during construction. For example, during construction of the “Big Dig” in Boston, the elevated Central Artery was in continuous service while cut-cover tunnels were constructed directly below it.
Third, environmental, community, and safety requirements have become more stringent. For many good reasons, expectations of what a highway should be, how it should operate, and how it should interact with the environment and adjacent communities are constantly evolving. Designs to promote safety, measures to mitigate a growing list of environmental impacts, and attention to aesthetics have fundamentally changed the scope of major highway projects in the United States. For example, on Maryland’s $2.4 billion Intercounty Connector project in suburban Washington, D.C., which is now under construction, environmental mitigation accounts for 15 percent of project costs, or about $15 million per mile (AASHTO, 2008).
Fourth, costs continue to rise. Building and maintaining highways cost effectively is an ever-present goal of good engineering. But cost increases in highway construction have been extraordinary due in part to the expanded scope of highway projects and construction in demanding settings. In addition, the costs of the mainstay materials—portland cement, asphalt binder, and steel—have risen dramatically as the world, particularly China, has gone on a construction binge. The Federal Highway Administration’s cost indices for portland cement concrete pavement, asphalt pavement, and structural steel increased by 51 percent, 58 percent, and 70 percent respectively between 1995 and 2005 (FHWA, 2006).
Fortunately, research and innovation in construction have never stopped, although they are not always sufficiently funded and they seem to fly beneath the radar of many scientists and engineers. Nevertheless, there have been great successes, which are cumulatively changing how highways are built in America.
The Superpave Design System
In response to widespread concerns about premature failures of hot-mix asphalt pavements in the early 1980s, a well funded, congressionally mandated, crash research program was conducted to improve our understanding of asphalt pavements and their performance. The seven-year Strategic Highway Research Program (SHRP), which was managed by the National Research Council, developed a new system of standard specifications, test methods, and engineering practices for the selection of materials and the mix proportions for hot-mix asphalt pavement.
The new system has improved matches between combinations of asphalt binder and crushed stone and the climatic and traffic conditions on specific highways. State departments of transportation (DOTs) spend more than $10 billion annually on these pavements, so even modest improvements in pavement durability and useful life can lead to substantial cost savings for agencies and time savings for motorists (TRB, 2001).
SHRP rolled out the Superpave system in 1993, but it took years for individual states and their paving contractors to switch to the new system, which represents a significant departure, not only in design, but also in the procedures and equipment used for testing. Each state DOT had to be convinced that the benefits would outweigh the modest additional costs of Superpave mixes, as well as the time and effort to train its staff and acquire necessary equipment.
FIGURE 1 - Superpave application on Michigan State Route 95, near Iron Mountain. The Superpave system, which matches the combinations of asphalt binder and crushed stone to the climatic and traffic conditions on specific highways, was developed under the National Research Council–managed Strategic Highway Research Program and is in use in every state. (Photo: Larry L. Michael.)
A survey in 2005 showed that 50 state DOTs (including the District of Columbia and Puerto Rico) were using Superpave (Figure 1). The remaining two states indicated that they would be doing so by the end of 2006. Throughout the implementation period, researchers continued to refine the system (e.g., using recycled asphalt pavements in the mix design [TRB, 2005]).
It may be years before the cost benefits of Superpave can be quantified. A 1997 study by the Texas Transportation Institute projected that, when fully implemented, Superpave’s annualized net savings over 20 years would approach $1.8 billion annually—approximately $500 million in direct savings to the public and $1.3 billion to highway users (Little et al., 1997). Moreover, analyses by individual states and cities have found that Superpave has dramatically improved performance with little or no increase in cost. Superpave is not only an example of a successful research program. It also demonstrates that a vigorous, sustained technology-transfer effort is often required for innovation in a decentralized sector, such as highway transportation.
FIGURE 2 - Aerial view of the east span of the San Francisco-Oakland Bay Bridge, Labor Day 2007, with the replacement deck piece newly installed. Demolishing the old viaduct sections and moving the prefabricated section into place was accomplished in 70 hours—11 hours ahead of schedule. (Photo: Barrie Rokeach, Aerial/Terrestrial Photography.)
The offsite manufacturing of steel and other components of reinforced concrete for bridges and tunnels is nothing new. But the need for reconstructing or replacing heavily used highway facilities has increased the use of prefabricated components in startling ways. In some cases components are manufactured thousands of miles from the job site; in others, they are manufactured immediately adjacent to the site. Either way, we are rethinking how design and construction can be integrated.
When the Texas Department of Transportation needed to replace 113 bridge spans on an elevated interstate highway in Houston, it found that the existing columns were reusable, but the bent caps (the horizontal connections between columns) had to be replaced. As an alternative to the conventional, time-consuming, cast-in-place approach, researchers at the University of Texas devised new methods of installing precast concrete bents. In this project, the precast bents cut construction time from 18 months to slightly more than 3 months (TRB, 2001).
As part of a massive project to replace the San Francisco-Oakland Bay Bridge, the California Department of Transportation and the Bay Area Toll Authority had to replace a 350-foot, 10-lane section of a viaduct on Yerba Buena Island. In this case, the contractor, C.C. Myers, prefabricated the section immediately adjacent to the existing viaduct. The entire bridge was then shut down for the 2007 Labor Day weekend, while the existing viaduct was demolished and the new 6,500-ton segment was “rolled” into place (Figure 2). The entire operation was accomplished 11 hours ahead of schedule (B. Kahn, 2007).
FIGURE 3 - Casting basin constructed adjacent to Four Point Channel during Boston’s Central Artery-Tunnel project, for manufacturing 30- to 50-ton concrete tunnel sections; when the sections were complete, the basin was flooded, and the sections were floated into position and submerged. The casting basin was 1,000 feet long, 300 feet wide, and 60 feet deep—big enough to hold an aircraft carrier or three Titanics side by side. (Photo: Massachusetts Turnpike Authority.)
Probably the most extensive and stunning collection of prefabricated applications on a single project was on the Central Artery/Tunnel Project (“Big Dig”) in Boston. For the Ted Williams Tunnel, a dozen 325-foot-long steel tunnel sections were constructed in Baltimore, shipped to Boston, floated into place, and then submerged. However, for the section of the tunnel that runs beneath the Four Points Channel, which is part of the I-90 extension, bridge restrictions made this approach infeasible. Instead, a huge casting basin was constructed adjacent to the channel where 30- to 50-ton concrete tunnel sections were manufactured (Figure 3). The basin was flooded and the sections winched into position with cables and then submerged.
An even more complicated process was used to build the extension tunnel under existing railroad tracks, which had poor underlying soil conditions. Concrete and steel boxes were built at one end of the tunnel, then gradually pushed into place through soil that had been frozen using a network of brine-filled pipes (Vanderwarker, 2001).
Specialty Portland Cement Concretes
New generations of specialty concretes have improved one or more aspects of performance and allow for greater flexibility in highway design and construction. High-performance concrete typically has compressive strengths of at least 10,000 psi. Today, ultra-high-performance concretes with formulations that include silica fume, quartz flour, water reducers, and steel or organic fibers have even greater durability and compressive strengths up to 30,000 psi. These new concretes can enable construction with thinner sections and longer spans (M.S. Kahn, 2007).
Latex-modified concrete overlays have been used for many years to extend the life of existing, deteriorating concrete bridge decks by the Virginia DOT, which pioneered the use of very early strength latex-modified concretes for this application. In high-traffic situations, the added costs of the concrete have been more than offset by savings in traffic-control costs and fewer delays for drivers (Sprinkel, 2006).
When the air temperature dips below 40?F, costly insulation techniques must be used when pouring concrete for highway projects. By using commercially available admixtures that depress the freezing point of water, the U.S. Cold-Weather Research and Engineering Laboratory has developed new concrete formulations that retain their strength and durability at temperatures as low as 23?F. Compared to insulation techniques, this innovation has significantly decreased construction costs and extended the construction season in cold weather regions (Korhonen, 2004).
As useful as these and other specialty concretes are, nanotechnology and nanoengineering techniques, which are still in their infancy, have the potential to make even more dramatic improvements in the performance and cost of concrete.
Waste and Recycled Materials
Highway construction has a long history of using industrial waste and by-product materials. The motivations of the construction industry were simple—to help dispose of materials that are otherwise difficult to manage and to reduce the initial costs of highway construction. The challenge has been to use these materials in ways that do not compromise critical performance properties and that do not introduce substances that are potenti-ally harmful to people or the environment. At the same time, as concerns about sustainability have become more prominent in public thinking, the incentives to use by-product materials have increased. In addition, because the reconstruction and resurfacing of highways create their own waste, recycling these construction materials makes economic and environmental sense.
Research and demonstration projects have generated many successful uses of by-product and recycled materials in ways that simultaneously meet performance, environmental, and economic objectives. For example, “crumb rubber” from old tires is increasingly being used as an additive in certain hot-mix asphalt pavement designs, and a number of patents have been issued related to the production and design of crumb rubber or asphalt rubber pavements (CDOT, 2003; Epps, 1994).
Several states, notably California and Arizona, use asphalt rubber hot mix as an overlay for distressed flexible and rigid pavements and as a means of reducing highway noise. Materials derived from discarded tires have also been successfully used as lightweight fill for highway embankments and backfill for retaining walls, as well as for asphalt-based sealers and membranes (Epps, 1994; TRB, 2001).
Fly ash, a residue from coal-burning power plants, and silica fume, a residue from metal-producing furnaces, are increasingly being used as additives to portland cement concrete. Fly-ash concretes can reduce alkali-silica reactions that lead to the premature deterioration of concrete (Lane, 2001), and silica fume is a component of the ultra-high-performance concrete described above.
After many years of experimentation and trials, reclaimed asphalt pavement (RAP) is now routinely used in virtually all 50 states as a substitute for aggregate and a portion of the asphalt binder in hot-mix asphalt, including Superpave mixes. The reclaimed material typically constitutes 25 to 50 percent of the “new” mix (TFHRC, 1998). The National Asphalt Pavement Association estimates that 90 percent of the asphalt pavement removed each year is recycled and that approximately 125 millions tons of RAP are produced, with an annual savings of $300 million (North Central Superpave Center, 2004).
FIGURE 4 - Engineers can use visualization to resolve construction sequencing issues and develop cost-efficient approaches. (Images: Charles L. Hixon III, Bergman Associates.)
Visualization, Global Positioning Systems, and Other New Tools
For more than 20 years, highway engineers have used two-dimensional, computer-aided drafting and design (CADD) systems to accelerate the design process and reduce costs. The benefits of CADD systems have derived essentially from automating the conventional design process, with engineers doing more or less what they had done before, although much faster and with greater flexibility.
New generations of three- and four-dimensional systems are introducing new ways of designing roads, as well as building them (Figure 4). For example, three-dimensional visualization techniques are clearly useful for engineers. But, perhaps more importantly, they have improved the communication of potential designs to affected communities and public officials; in fact, they represent an entirely new design paradigm. Four-dimensional systems help engineers and contractors analyze the constructability of proposed designs well in advance of actual construction (Figure 5).
FIGURE 5 - Visualization image prepared for a design-build reconstruction of U.S. Highway 52 near Rochester, Minnesota. The image shows the versatility of visualization, incorporating plan views, 3-D rendering, and final projection. (Image: URS Corp.)
Global positioning systems are being used in surveying/layout, in automated guidance systems for earth-moving equipment, and for monitoring quantities. Other innovations include in situ temperature sensors coupled with data storage, transmission, and processing devices that provide onsite information about the maturity and strength of concrete as it cures (Hannon, 2007; Hixson, 2006).
The examples described above suggest the wide range of exciting innovations in the design and construction of highways. These innovations address materials, roadway and bridge designs, design and construction methods, road safety, and a variety of environmental, community, and aesthetic concerns. Looking to the future, however, challenges to the U.S. highway system will be even more daunting—accommodating more traffic and higher loadings; reducing traffic disruptions during construction; meeting more stringent environmental, community, and safety requirements; and continuing pressure to reduce costs. Addressing these challenges will require a commitment to innovation and the research that supports innovation.
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