Page:America's Highways 1776–1976.djvu/417

 claims as to their superiority over macadam, and lumber being available, thousands of miles were built in many States during the following decade. In a few years, the public discerned that the life of any road is limited by the lasting qualities of the material of which it is built. It took about 10 years for the wooden planks to rot away, and the plank road era ended rather abruptly.

Dust palliatives were applied on gravel and macadam road surfaces as early as 1898. Tar and asphalt were used as protective surface coatings and later as binders.

Bituminous pavements were constructed in Washington, D.C., and New York City as early as about 1870. Bituminous macadam experiments began in Boston in 1906, and a 1-mile section was constructed there in 1907.

Recovering from the ravages of the Civil War, the South needed a road surface that could be built and maintained at a small cost from local materials in general abundance. Sand-clay surfacing, because it was lower in cost, adequate for light traffic, less dusty and noisy, and more resilient than macadam, was the logical answer to the road problems of the South Atlantic and Gulf States in 1885. It was used widely in these areas.

The first brick pavement on a rural road was placed near Cleveland, Ohio, in 1893. The roadway was 32 feet wide, but the brick pavement, 8 feet wide, was placed near one side of the roadway, leaving the remaining width unsurfaced.

Mention has been made of early macadam, brick, and bituminous surfaces and pavements on rural roads. By the 1890’s the heavy-load hauling demands in the large cities had led to construction of heavy street pavement sections of the types which had shown good stability. The horsedrawn drays hauling heavy loads on steel-rimmed wheels had pulverized all but the hardest pavement surfaces. Consequently, the main streets of the large cities were built very heavily and surfaced with granite blocks or hard paving bricks. Concrete bases were used on some. The minor business streets and residential streets were commonly of macadam or gravel. Asphalt paving, begun in the early 1870’s, was immediately popular because of its smoothness, silence, lack of dust, and ease of cleaning. By the 1890’s, many of the city streets were asphalt surfaced to gain these advantages.

Credit for first surfacing a rural public road with portland cement concrete is conceded to Wayne County, Michigan, where a 1-mile section was built in 1909. The pavement was laid in two courses, 18 feet wide and having a total depth of 6½ inches. The first course was made of 1–2½–5 mix of portland cement, sand and limestone 4 inches deep and the second course of a 1–2–3 mix of portland cement, sand and crushed cobblestone 2½ inches deep. It was laid in 25-foot sections.

The heavy truck traffic during World War I inflicted widespread damage upon surfaces built during the preceding generations to carry horsedrawn vehicles. After the war, there was a public clamor for improved roads, at once and everywhere. Responsible engineers agreed that they lacked the essential information required for the design and construction of a nationwide system of paved highways as envisioned by the framers of the 1916 Federal Aid Road Act. This led to the formulation and carrying out of the continuing and extensive series of engineering research studies by the State highway departments, Bureau of Public Roads and related industry. The Highway Research Board, created in 1920, served to nationally correlate and disseminate the ensuing results of the studies, delving into the proper relationship between highway loads, road surfaces, and subgrades. The system of soil classification and analysis was developed, together with data on the many detailed characteristics of aggregates, materials and mixes under the varied field conditions and traffic loads. Traffic counting and weighing facilities were set up, and the procedures for predicting traffic volumes and loadings were developed and put into use in the late 1930’s.

As the numbers, sizes, weights and speeds of vehicles increased, highway officials struggled to provide pavements with adequate structural strength to meet the demands of traffic. Practices of highway design, construction and maintenance were progressively revised upward. Some of these revisions were the result of practical experience and some were based on results of engineering research. In the 1920’s, many highway engineers recognized that the supporting ability required in the pavement structure of roads was determined principally by the axle loads of the vehicles. There was also general recognition of the need for more factual information on all of the pavement and soil elements.

Beginning in 1920, several research projects using specially constructed test tracks produced significant advances in the science of pavement design and construction. In 1920, the Bureau of Public Roads expedited its field tests on the large circular track containing different pavement sections at Arlington, Virginia. Experimental roads were constructed in 1921 by the Columbia Steel Company cooperating with the California Highway Commission at Pittsburg east of San Francisco. The Bureau of Public Roads intensified its laboratory tests and initiated a countrywide field study of subgrade soils. Universities became beehives of research activity. One major effort to obtain such information was the Bates Experimental Road, a test conducted by the Illinois Division of Highways in 1922 and 1923 near Springfield, Illinois, on a 2½-mile roadway divided into 63 test sections of varied materials and design. The test vehicles were trucks with solid rubber tires on which wheel loads were increased from 2,500 to 13,000 pounds as the testing progressed.

Within a few years, there were four principal types of findings: (1) Subgrade soil tests were developed which, together with traffic studies and other information, became valuable aids in pavement design. (2) The destructive impact of the solid rubber tire was isolated and overcome by the introduction of the softer pneumatic tire. (3) Agreement was reached on the 9,000-pound wheel load as a logical basis upon which to plan a long-term paving program. (4) The thickened edge design of rigid pavements was adopted. These findings soon brought about the adoption of laws regulating vehicle weights. 411