Page:America's Highways 1776–1976.djvu/340

 Road Test One—Md. In 1949 the Interregional Council on Highway Transportation originated a test road study on a 1.1-mile section of highway south of La Plata, Maryland. The study and report was a cooperative effort of 11 eastern States, the District of Columbia, Public Roads, truck manufacturers, the petroleum industry, the Department of Defense and HRB. The objective was to obtain data, for use both in vehicle weight regulations and in pavement design, on the relative effect of four different axle loads on the existing concrete pavement (four lanes), then in excellent condition.

Test trucks with 8,000- and 22,400-pound single axle loads were operated on adjacent lanes and trucks with 32,000- and 44,800-pound tandem axle loads operated on the other pair of lanes. Nearly 240,000 single axle truck passes were made and about half as many of the tandem axle.

The data on the progressive pavement deterioration, evidenced by cracking, internal structure failure, and joint depression, showed that damage was proportionate to axle loads. Significant data also were reported concerning soil support features and stresses at various points in the pavement slab.

WASHO Road Test. In 1951 the Western Association of State Highway Officials (WASHO) set up a test road to obtain data for use in establishing load limits and in designing flexible pavements. Cooperating in this test were 11 western States, truck and truck trailer manufacturers and three petroleum companies. The Highway Research Board supervised the construction and testing to determine the effects of four-axle loads on selected designs of flexible pavement. The Bureau of Public Roads assisted with funds, personnel, instrumentation, photography, supplies and equipment. This test was run from 1952 to 1954 at a site near Malad, Idaho.

Two four-lane test loops, each with 1,900-foot tangents, were constructed with five pavement structural sections ranging in depth from 6 to 22 inches on each tangent. On one loop the pavements were subjected to 18,000- and 22,000-pound single axle loads and on the other 32,000- and 40,000-pound tandem axle loads. The lighter loads were operated on the inner lanes. About 240,000 truck passes were recorded on each of the loops.

The test data yielded significant relations of stresses within and failures of the several flexible pavement design sections as the truck loadings progressed. Some of the general findings were:


 * The 4-inch asphaltic concrete top course was markedly superior to the 2-inch.
 * The heavier axle loads resulted in considerably more distress.
 * A paved shoulder contributed to the pavement support.
 * Greatest distress occurred in the spring and least in the fall.
 * Distress from the tandem axle was equivalent to that of a single axle of about two-thirds of the tandem weight.

The AASHO Road Test. After the WASHO Road Test, the American Association of State Highway Officials (AASHO) in agreement with the Bureau of Public Roads decided to conduct a comprehensive national test to obtain data on all significant variables. In 1955, AASHO sponsored and requested the Highway Research Board to direct this test. The purpose was to study the performance of pavement and bridge structures of known characteristics under moving loads of known magnitude and frequency. Portland cement concrete and asphaltic pavements, as well as certain types of bridges, were included in the specially constructed test facility.

This $27 million project, located near Ottawa, Illinois, was financed by the highway departments of the 48 States, Hawaii, the District of Columbia, and Puerto Rico. Financial support was also provided by the Bureau of Public Roads, the Automobile Manufacturers Association, the petroleum industry, and the American Institute of Steel Construction. The Department of Defense furnished the heavy vehicles and drivers for their test run operations. In addition, there were many services and contributions by the automotive, petroleum, tire, cement, and steel industries.

Traffic on loops during the AASHO road test. 334