Page:America's Highways 1776–1976.djvu/336

 The Bureau of Public Roads roughometer measures the roughness of the pavement in units of vertical spring motion per unit of distance the wheel travels.

Research was also performed on reinforcing materials for concrete pavement. Bond studies in 1926 or steel reinforcing bars of various shapes and types of deformation included pullout tests to evaluate the contribution of the different types of bar deformation to the load-carrying capability of concrete slabs. The superior performance of several specific types of bars was established. In addition, results in 1958 of an extensive laboratory study of the performance of load transfer dowels in concrete pavement joints had an immediate effect in pavement design. Recommended dowel bar lengths and diameters for effective load transfer were widely adopted and are still in use today.

The Bureau of Public Roads was an early leader in efforts to provide a means of obtaining quantitative measures of pavement roughness. The result was a compact single-wheel trailer that measured and recorded the roughness of the wheel path in units of vertical spring motion per unit of distance traveled. With this device, commonly known as the BPR roughometer, it became possible to easily rate the roughness of both new and old pavements. The term “present serviceability index” as a function of roughness was used to express the results in numerical terms. This index was used to specify the degree of roughness for design standards and in rating construction.

Among other items of special equipment designed by Public Roads engineers was a device to measure the vertical displacement of pavement, both elastic and permanent, caused by static and moving wheel loads of various intensities and at different distances from the point of load application. With these displacement data, it was possible to make design analyses for better subgrades and pavements. This ingenious device, named the Benkelman Beam for its inventor, received nationwide attention and was duplicated by many State highway departments.

In addition to tests on roads in their normal condition, studies were made in the 1920’s to determine the impact forces of wheel loads generated by an “artificial” bump. One study was on a ramp 30 inches long with a drop-off varying from 1½ to 3 inches. These tests demonstrated the superiority of pneumatic tires versus solid rubber tires. Partly as a result, solid rubber tires were rapidly phased out in the late 1920’s.

Highways do wear out. Because they wear out, highway programs must include continuous resurfacing and reconstruction operations to maintain highways in a usable and safe condition. Accordingly, a knowledge of service lives of highway pavements is essential.

Taking deflection measurements with the Benkelman Beam to determine pavement displacement when a load is applied.

Testing impact of vehicle wheels on pavement. 330