Paving the Way

HISTORICAL NOTE Paving the Way On the road of progress, where did the road surfaces were slabs of stone laid upon passing vehicle to the underlying soil with asp_halt come from? a strong foundation of rubble or broken out damaging the road surface. Paving ma Throughout history, the creation and stone. Other Roman roads used a concrete terial must resist the degradation caused maintenance of roads has been a barome made from lime and pozzolana (volcanic by traffic and must also provide a smooth ter for the rise and fall of civilizations. ash). surface for a comfortable ride. Ideally, pav While other new science and technology For centuries after the downfall of the Ro ing material should resist damage caused developed over the centuries, surprisingly man Empire (about the fifth century A.D.) by changes in temperature, moisture, and little progress was made in the construc long-term environmental chemical action. tion of roads or the materials and tech In the 1770s two self-taught engineers niques used to surface them. changed roadbuilding methods and mate The word "highway" is derived from the rials used throughout Europe-John Roman roads built on a mound of dirt piled The United States and Loudon McAdam and Pierre Tresauguet. up from ditches on either side, raising the Europe have nearly Tresauguet became inspector general of route above the surrounding ground and bridges and roads for France in 1775. He making it a "high way:' The term "road" 5,000,000 total miles of proposed a new theory for a light road sur comes from the Anglo-Saxon rad ("to paved roads-most of face, suggesting that the subsoil should ride"), for the path on which travelers support the load of traffic, not the surface rode. which are paved with the itself. The earliest roads developed from paths same substance used by McAdam devised an inexpensive, self worn into the ground by travelers. Actual sealing pavement of crushed stone, called road building did not develop until local the Babylonian king a "macadam surface;' applied directly to governments were powerful enough to Nabopolassar in the subsoil of a roadbed. McAdam began take on such projects. The first civilizations his work at Bristol, then the second most to build simple roads probably appeared in 3800 B.C. important city in Britain. In 1816, when southwestern Asia. The Mesopotamians McAdam was chosen the general surveyor undertook an extensive project to develop of the area, the roads were in poor condi a travel route from the Babylonian Empire tion. He decided that roadbuilding should to Egypt. The Egyptians later adapted these roads degenerated with no central be conducted in keeping with fundamen these road building skills to provide a solid authority to maintain them. Dirt paths be tal scientific principles, and he experi track for hauling gigantic limestone blocks came rough and uneven, dusty or muddy mented with local roads to see how they for the pyramids. depending on the weather. Most traffic withstood traffic and time. Like Tre In the seventh and sixth centuries B.C., consisted of people traveling on foot or on sauguet, he believed that the roads should processional roads connected the cities of horseback and farmers moving livestock. be well drained, with a compacted subsoil Assur, Babylon, and Tall as-Asmar. These Merchandise was carried on pack trains. In to carry most of the load. His philosophy roads were paved with burnt brick and about 1600, though, the wheeled coach be was that the pavement should act only to stone laid into bituminous mortar; paved gan to see popular use for intercity travel, shed water and to cover the subsoil. city streets were often elaborately deco and the heavy iron tires caused great dam The macadam roc.d consisted of two or rated with colored tile, bricks, or stones. age to road surfaces. This forced the devel more layers of crushed stone compacted In India, archaeological excavations have opment of improved paving materials. with a heavy roller and then bonded to shown that the Indians used similar mate At first these improved roads were sur gether with a fine stone powder pressed rials and techniques (burnt bricks ce faced with new stone. Stone-paved toll into the surface. The stone powder and the mented with bitumen) much earlier, as far roads appeared in Britain, across Europe, crushed stone made for a sturdy surfa back as 3250-2750 B.C. By A.D. 75, the In and even in the American Colonies. In ce ... adding water to the compaction proc dians also used brick pavement, stone slab many cities cobblestones were used until ess made the surface even stronger. The pavement, and primitive concrete (both as the middle of the nineteenth century. method was so effective that it was widely a foundation or as the road surface). The Maintaining a good road surface de copied, and macadam roads spread Indians filled crevices in the road with pends primarily on the type of pavement, throughout the western world. lime, gypsum, or a bituminous mortar. the compaction of the underlying material, McAdam had also recommended bind Many roads in ancient China were well and the drainage of the surface. Drainage ing the stone powder and crushed rock to built and surfaced with stone; however, af depends on the engineering of the road it gether with slag or a bituminous mixture. ter the original construction, these roads self. Techniques for compacting the under Bitumen, or natural asphalt, paving was were not maintained. In fact, a popular lying material improved as roadbuilders first laid in Paris in 1854, using rock asphalt saying remarked that a Chinese road was began to bring in many shallow layers of from Switzerland. good for seven years and then bad for 4000 material. Historically, sheep were driven Asphalt is a material that had been used years! over fresh layers to compact the soil. Later, in building and paving since ancient times The Romans, master roadbuilders, workers used a "sheepsfoot roller" with (in fact, archaeologists have established made systematic highways that were re small steel protrusions like sheeps' feet that it is one of the oldest adhesives known markably straight, with solid foundations over the new road surface. to man). Early Buddhist writings call as and cambered surfaces. Some of the best A pavement must transmit the load of a phalt "earth butter:' The Mesopotamians 66 MRS BULLETIN/FEBRUARY 1989 HISTORICAL NOTE used it as a mortar for building stones and tury, the primary purpose was to control ing methods used high-temperature atmo paving blocks as early as 3800 B.C. dust. The mixture could be sprayed on spheric distillation. Today, modern Asphalt is a black or dark brown cemen roads as a diluted emulsion. Asphalts fractional-distillation methods have greatly titious material that gradually liquefies could also be used as a binder to fill voids in increased the efficiency of asphalt produc when it is heated. Asphalts are complex a macadam base. tion. mixtures of high-molecular-weight hydro Asphalt paving is smooth and easily By 1915 heavy automobile traffic showed carbons, which can be separated into res cleaned, which made it ideal for city that asphalt paving could withstand the ins, oils, and asphaltenes (brown to black streets. The first U.S. stretch of asphalt scuffing and abuse caused by the high semicrystalline solids that are hard and road was laid in 1870 in front of the city hall speed and heavy weight of automobiles brittle). in Newark, New Jersey. Now, about 90% of and cargo trucks. The heavy truck traffic of Natural asphalt, such as that used by the U.S. city streets are paved with asphalt. World War I demanded extremely durable ancients, was occasionally found in seeps While most secondary roads in the pavement, such as asphalt or concrete and pools, a natural residue of petroleum United States are now paved with asphalt, (which had been used in Britain and Eu deposits from geologic processes. Typical country roads for a long time remained vir rope since the nineteenth century). pools are the La Brea tar pits in Los tually impassible in wet weather... until Highway surfacing materials have been Angeles, California, or Pitch Lake on the roadbuilding interest was stimulated again developed and tested in the decades since island of Trinidad, where Columbus with the advent of bicycle use. Bicycle rid World War I, with little major improve stopped to caulk his ships on the way ing required a somewhat better road sur ment. The United States and Europe have home to Spain. Pitch Lake and Bermudez face than horseback riding. Later, the nearly 5,000,000 total miles of paved Lake in Venezuela furnished most of the automobile demanded even better roads. roads-most of which are paved with the asphalt used for paving in the United At the beginning of the motor age, petro same substance used by the Babylonian States up until the 20th Century. leum refineries began making a host of by king Nabopolassar in 3800 B.C. He The exploitation of these two major products, one of which was asphalt. After claimed that he had found his realm in sources of natural asphalt stimulated inter 1900, large-scale production of asphalt was mud and had left it "laced with roads glis est in using the material for street paving. accomplished through refining crude petro tening with asphalt:' When asphalt (and oil or tar) was first used leum ... most crude petroleum contains for paving roads in the late nineteenth cen- some asphalt, up to 50%. Early simple refin- KEVIN J. ANDERSON Asphalt paving is smooth and easily cleaned, which makes it ideal for city streets.

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