BUREAU OF PUBLIC ROADS. 491 



the warping and movement of road surfaces due to natural causes, 

 accelerated tests of various surfacing matc:ials to determine their 

 resistance to the wear of traffic, and bridge investigations, including 

 the distribution of stress in skew arches and the impact stresses set 

 up in bridges by moving loads. 



To the State Highway Department of Illinois and the Highway 

 Commission of California, cooperating with the Columbia Steel Co. of 

 Pittsburg, Calif., the country is indebted for two researches, con- 

 ducted at Bates, 111., and Pittsburg, Calif., on a scale never before 

 attempted. Both of these investigations had for their purpose the 

 testing of actual road surfaces of various designs under actual traffic. 

 By the boldness with which these tests were conceived and the 

 thoroughness with which they were conducted the two State depart- 

 ments and the steel company have made the outstanding contribu- 

 tions of the year to the advancement of the science of highway 

 engineering. 



The researches completed last year at .^lington and those at the 

 Bates road in Illinois and at Pittsburg, Calif., in which the bureau 

 cooperated, have thi'own considerable light on the traffic-resisting 

 qualities of road surfaces of different designs laid on various subgrade 

 materials, and have led to certain tentative conclusions as to the 

 proper thickness of slab for different weights of vehicle. Immediate 

 results are apparent in the altered standards of design in some States, 

 and as the information is gradually disseminated other States will no 

 doubt follow. Results of the motor-truck impact tests are being used 

 as the basis for the amendment of laws governing the weight of motor 

 vehicles. 



There is still need, however, for much more investigation. Infor- 

 mation is badly needed as to the physical characteristics of subgrade 

 materials which are causing certain roads to give short service, the 

 proper manner of draining soils of different types or their possible 

 treatment with admixtures of stabilizing materials, and the relative 

 strength of bituminous surfaces on concrete bases as compared with 

 concrete slab surfaces. 



RESEARCHES CONDUCTED DURING THE YEAR. 



In addition to the motor-truck impact and subgrade investigations 

 continued from the previous fiscal year, the following specilic re- 

 searches have been carried forward during the year: 



Preparations have been completed for the investigation of relative 

 wear of different concrete surfaces. These surfaces are constructed 

 on a circular track 625 feet in circumference, in which there are 62 

 sections of different qualities of concrete. These will be sul)jected 

 to the wear of a rubber-tired truck and the results will be compared 

 with physical laboratory tests. 



Another circular track has been surfaced with bituminous mixtures 

 of different qualities. These sections will be subjected to actual 

 motor-truck traffic for the purpose of stud.ying their stabihty and 

 determining the laws governing the stability of bituminous mixes. 

 This investigation has been instituted with the idea of rendering 

 bituminous roads less likely to wave under traffic. 



Special studies have been conducted in the field for the purpose 

 of analyzing the causes qf rhythmic corrugations in gravel roads with 



