Page:America's Highways 1776–1976.djvu/339

 lightweight concrete, on the performance of concrete highway bridges.

With the beginning of the Interstate System came the need for fitting bridges to much more restrictive and sophisticated geometries to provide for the needs of high-speed traffic in congested areas while maintaining structural esthetics and economy. Straight span members were no longer always adequate for the needs, and the understandings of the structural design of curved members was limited. Research undertaken, both in the form of multi-State pooled-fund laboratory and analytical studies and the extensive use of cooperative Federal-State field studies, helped fill this urgent and critical need for improved curved girder design technology. As a result of this research, bridges today are designed and built with curved alinement and warped decks in a manner unheard of two decades ago.

Following World War I, a prime objective of the State and Federal highway program was to reconstruct the surfaces and pavements of main highways which had received widespread damage from the greatly increased traffic of heavy trucks with solid rubber-tired wheels. The light surfaces, designed for horsedrawn or other light vehicles, failed under the impact of the heavier vehicles. Road test data were needed to determine the supporting capability of the various subgrade soils, the stresses induced in rigid and flexible pavements by the impact of motor vehicles, the effects of expansion and contraction of road surfaces caused by variations in temperature, the wear of traffic upon pavement surfaces, the distribution of loads upon bridges, and many other factors. To solve some of these problems, the Bureau of Public Roads and the States initiated a series of road tests.

The Bates test road and test vehicles.

The Arlington Road Test. The first road test was initiated in 1918 at BPR’s Experimental Farm in Arlington, Virginia, to measure qualitatively the impact forces of various wheel loads. The tests, conducted with Army trucks equipped with solid rubber tires, were on concrete, brick and bituminous slabs on circular tracks. The early tests indicated the major effect of wheel impact forces and led to further studies with more refined measuring instruments and the inclusion of pneumatic tires.

The Bates Road Test. For the Bates Road Test, the Illinois Division of Highways constructed 68 test sections, including six types of pavement on 2 miles of road and, with the assistance of BPR engineers, conducted tests on them from 1920 to 1923. Trucks were operated with controlled wheel loads that were progressively increased from 2,500 to 13,000 pounds. The test data gave the pavement type and thickness required for a specified loading and showed the need for control of wheel loads. A direct result of this study was the use of a thickened edge concrete pavement.

The Pittsburg Road Test. Between 1921 and 1922, another road test was conducted in Pittsburg, California, to determine the efficiency of both reinforced and plain concrete pavements of variable thickness and designs on certain types of subgrade soil. The test was conducted by the Columbia Steel Company on an elliptical track 560 feet in length containing 13 concrete pavement test sections. Much new information was learned, particularly concerning the effectiveness of longitudinal joints in preventing longitudinal cracking.

The combined findings from these three contemporary road test studies led to major advances in pavement design practices. The relations were determined between concentrated wheel loads and the thickness of several common types of pavement, which were directly usable on new highway plans. From the range of wheel loads studied, highway officials were able for the first time to reach agreement on the use of a 9,000-pound maximum value as the economic standard for highway pavement designs. More directly evident to the public was the abrupt phasing out of solid-rubber tires on trucks because of their high impact destruction on all types of pavements. Within a few years after 1926 the use of pneumatic tires became universal. Also, these studies made evident the need for amplified research on soil support values to attain better pavements.

The Hybla Valley Road Test. From 1944 to 1954 a series of studies on flexible pavements was conducted on the Hybla Valley test track in Alexandria, Virginia, by BPR in cooperation with the Asphalt Institute and HRB. A 2,000-foot oval track for full size vehicles was built with selected soil foundation and paved with asphaltic concrete. Using newly developed measuring equipment, data were obtained on the vertical displacements of the pavements from both static and moving wheel loads of varying intensities.

A significant finding from these tests was that there is considerable elastic movement within a nonrigid pavement structure under load. A great deal was also learned about instrumentation and field measurement procedures which greatly aided later test road studies. 333