1、原文 Highway Design and Construction: The Innovation ChallengeAuthor: Robert E. Skinner Jr. 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 th
2、e 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
3、 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 materialsconcrete, asphalt, and steelare now the mainstays of highway and bridge construction throughout the world, as well as of most types of publ
4、ic 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 im
5、provement 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 fo
6、r 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 characteri
7、stics 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 i
8、dentified 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 fact
9、ors contribute to the urgent need for innovation in highway construction. _ 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 bil
10、lion 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
11、are possible. Much the same can be said of asphalttechnically, asphaltic concretewhich 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 chara
12、cteristics 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,
13、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
14、 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 thing
15、s 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 continuin
16、g 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
17、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
18、 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 tra
19、veled 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
20、 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 constr
21、uction. 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, expectati
22、ons 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 th
23、e scope of major highway projects in the United States. For example, on Marylands $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).
24、 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 additio
25、n, the costs of the mainstay materialsportland cement, asphalt binder, and steelhave risen dramatically as the world, particularly China, has gone on a construction binge. The Federal Highway Administrations cost indices for portland cement concrete pavement, asphalt pavement, and structural steel i
26、ncreased 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. Never
27、theless, 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
