Page:America's Highways 1776–1976.djvu/459



Equipment for highway construction was undergoing some tremendous changes during the period 1916–26. The steam shovel was being replaced with gasoline engine-powered excavators. The lines of horsedrawn wagons were disappearing as dump trucks were taking their places. Crawler tractors pulling rotary scrapers were replacing horse or muledrawn fresnos and drag scrapers. Monstrous early models of the self-propelled scrapers were being developed in the early 1920’s. A manual hoisting bulldozer blade was put on a crawler tractor in 1923, and by 1925 hydraulic controls were in use on the bulldozer blades.

The self-propelled motor grader was introduced in the early 1920’s, but it did not get widespread use until much later. By 1922, an all-welded tractor drawn scraper with a 6-cubic yard capacity was in use and by 1924 electric handsaws were put to use in construction. The steam crusher was being replaced with gasoline engine models with revolving screens and belt conveyors, and in 1923 a completely portable crushing and screening plant was placed on the market.

Paving equipment was also coming into more use. In 1919, a bituminous distributor was introduced to help improve the quality and uniformity of penetration asphalt pavements; 1920 brought batching plants (volumetric) for concrete aggregates, eliminating a large amount of hand shovel and wheelbarrow work. In 1923, a traveling concrete mixer was introduced on pavement projects. An old issue of Engineering News-Record contains a detailed description of the construction of a concrete pavement in 1918:

The rough grading was done entirely by machine: meaning a pull-type grader equipped with a scarifier. Fine grading was done by hand. A concrete mixer rode between wooden forms, with volumetric proportioning of the mix. The concrete was struck off and tamped with an iron-shod wooden templet, finished with a wood float and a coarse wire stable broom. The concrete gang (23 men) averaged 500 feet of 9-foot wide, 8-inch thick pavement per 9-hour day.

The Federal-aid system of building roads was well established by this time. Construction followed the same basic procedure with the State highway departments selecting, designing and constructing their own projects by contract, all with the approval of the Bureau of Public Roads and with partial funding from Federal appropriations.

The 1930’s witnessed an important advance—the introduction of the diesel engine on tractors. This diesel-powered tractor teamed with a 5-cubic yard capacity scraper moved approximately 16,000 yards of earth in 14 days.

Pressure from the general public was increasing to “get the farmer out of the mud”; and the consolidated school system required the improvement of many miles of roads in order to facilitate the busing of more children to the wider dispersed schools. To get more miles improved with the funds available, many of the highways were built as “stage” construction so that the road could be brought up to a higher type later with additional improvement as traffic volumes grew and the weight and speed of vehicles increased. 453