Page:America's Highways 1776–1976.djvu/460

 Stage construction was also used for other reasons. Some road projects were let to contract for the grading work alone, or perhaps the drainage work and grading work, leaving the surfacing work to be done by later contracts. This permitted the newly graded embankments to settle for a couple of years before placing surfacing materials. It was advantageous to let nature compact these embankments over a period of time because many States still did not use effective compaction procedures in their grading work. This also permitted the natural stabilizing of any slips and slides that might occur in the generally steep back slopes which were employed at that time in roadway excavation.

Between 1926 and 1944, BPR and the American Association of State Highway Officials (AASHO) worked with various associations in the highway industry toward the common goal of building more and better highways. For example, the Portland Cement Association (PCA) contributed financially to the establishment in 1929 of the Cement Reference Laboratory by the National Bureau of Standards in cooperation with the American Society for Testing and Materials (ASTM). The PCA was also instrumental in the development of soil cement for low-cost surfacing. AASHO published the first edition of their materials books in 1931, and then in 1940 and 1942 they published their Standard Specifications for Concrete Pavement and Bituminous Surface Treatment, respectively. In the 1920’s and early 1930’s, the Asphalt Institute, in cooperation with the BPR and various other public agencies, standardized asphalt cements and liquid asphalt products, thus, greatly reducing the number of grades and thereby facilitating the writing of asphalt specifications. Also, the Asphalt Institute in cooperation with the American Farm Bureau Federation was instrumental in initiating a program of paving farm-to-market roads. Like the capillaries of one’s blood system, these feeder roads played an important role in the transportation system of today. In cooperation with public highway authorities, all of the industry associations, in one way or another, were major contributors to the tremendous development of highway construction during this period.

Constructing a stabilized soil road. A scarifier prepares road for the mixing of soil and cement.

Probably the two most important advances in modernizing construction equipment during the 1920’s and 1930’s were: (1) The introduction of the diesel engine on tractors and graders in 1931, and (2) the development of large pneumatic tires which were used on scrapers as early as 1932, and on dump trucks in 1934, making it easier to operate on soft ground off the highway.

In the 1940’s self-powered equipment for onsite material stabilization was developed. This machine mixed soil and cement and added water to create a stabilized base. 454