Page:America's Highways 1776–1976.djvu/332

 soil stabilization. Beginning in 1954, and continuing until about 1970, the Bureau in cooperation with the chemical industry evaluated about 50 chemicals for soil stabilization. A few were found to be marginally useful, but none were economically competitive with Portland cement or lime for subgrades.

Throughout modern highway history, there has been a continuing search for a rapid, effective way to explore conditions on or beneath the ground without actual excavation or ground surveys. Beginning about 1932, Public Roads promoted the development and use of remote sensing methods for obtaining information on soil and ground conditions in place. The “cumulative-curve” method of presenting electrical resistivity data was developed and adapted to determine depth to bedrock, delineate sand-gravel deposits and thickness of portland cement concrete pavement. Also, aerial photographic interpretation principles were adapted to engineering-soil and materials mapping. Cooperative work between BPR field offices and other agencies (U.S. Geological Survey, National Aeronautics and Space Administration (NASA), the University of Michigan and Purdue University) in 1960-1967 demonstrated that color aerial photography was a good way to evaluate soil and terrain. Widespread use of these methods has followed. The development and evaluation of ground and aerial remote sensing methods for obtaining information on terrain and environmental features continued into the 1970’s.

Over the years, Public Roads has played a predominant role in the development of asphalt technology and test methods for bituminous materials. In 1903 it cooperated with the American Society for Testing Materials in organizing a committee on road and paving materials to develop essential standard test methods and material specifications.

The BPR tried various tests on bituminous materials to determine their physical and chemical properties and their suitability for use in road construction. Conclusions on which properties were significant in relation to performance and on the best methods for testing materials were disseminated in a series of Department of Agriculture bulletins, beginning in 1911, which included the methods in use by the Office of Public Roads at that time, and provided a progression of guidelines for procedures in the use of bituminous materials. Many of the early ASTM and AASHO standards for material and test method specifications, which are used in highway construction, were derived from this series. The latest edition, now called Standard Specifications and Methods of Tests for Highway Materials, was published by AASHO in 1974.

Beginning in the 1920’s and continuing through the mid-1960’s, most of the research effort in the bituminous area was concerned with studies of properties of asphalts and tars produced in the United States, and the relation of such properties to the performance of pavements. Often such studies resulted in the adoption of new specification requirements.

One of the most significant developments was the simplification of specifications for liquid asphalts and asphalt cements. A survey of penetration grade asphalts in 1923 showed that 88 different specifications for these materials were being used in the United States. A joint conference of representatives of Public Roads and users and producers of asphalt recommended that only nine grades were sufficient to provide the necessary materials. Those nine grades were adopted by national groups and most of the States. Subsequently, the number of grades has been reduced to five.

In 1930 a similar survey showed that there were 119 different tests, including all variations, being used in specifications for acceptable grades of liquid asphaltic materials in the United States. After a cooperative study of the materials being produced and a series of conferences throughout the United States sponsored by Public Roads, the Asphalt Institute, and the petroleum industry, 13 grades were adopted by most States and national groups. In most cases, the reduced number of grades for both penetration grade and liquid asphalts were adequate to fill the needs for pavement construction and resulted in considerable economic benefit to the user and producer of the materials.

The control of the hardening of asphalt in hot-plant mixing and in mixtures in service has been of major concern to the engineer and chemist since asphalt was first used. In 1940 the Public Roads Administration developed a laboratory test that would predict the amount of hardening of asphalt in hot-plant mix construction. This became known as the “Thin-Film Oven Test” and has been adopted as a standard specification test by ASTM, AASHO, and essentially all State highway departments. This test was also adopted by several foreign countries to measure and control asphalt hardening.

The effect of moisture on asphalt paving mixtures, macadams and surface treatments has been one of the primary causes of pavement distress or early failure. Tests were available to measure the effect of moisture on different aggregates coated with asphalt, but a test was needed to measure the effect of water on complete compacted mixtures representative of those used in construction. To meet this need, the immersion-compression test was developed, based on a laboratory study made by the Bureau in 1942. The test produced valuable information on the susceptibility of asphalt mixtures to loss in strength by water action. The immersion-compression test has been adopted as a standard by ASTM, AASHO, and many State highway departments and has been a valuable tool for investigating the cause of distressed pavements. 326