BUREAU OF PUBLIC ROADS. 471 



only 12 of the slabs have been tested, but the work is proceeding as 

 rapidly as possible. 



In this connection it is pertinent to observe that in many of its 

 physical researches the bureau is dealing with forces and phenomena 

 which have not heretofore been measured or defined. It is not unus- 

 ual to find that there is no measuring instrument in existence capable 

 of use to secure the necessary scientific data. Under such circum- 

 stances it is necessary to halt the investigation until suitable instru- 

 ments can be devised. The development of measuring instruments 

 occupies much of the time of the investigators, and, although the 

 progress of the research is retarded, the scientific apparatus contrib- 

 uted by the bureau in this way is in itself exceedingly valuable. 



In connection with the impact tests several important pieces of 

 apparatus have been designed, among them a strain gauge for meas- 

 uring and recording graphically the instaneous deformation which 

 takes place in a road slab when it is subjected to impact. This ap- 

 paratus, which is so designed that it can be placed anywhere in a 

 pavement slab, makes it possible to determine the amount and distri- 

 bution of the stress in the slab when it is subjected to the impact 

 of a loaded truck or other moving vehicle. A special accelerometer, 

 the purpose of which is to determine the force of impact by measure- 

 ment of the deceleration of the truck wheel on contact with the road 

 surface, has also been constructed. 



The tests of each of the concrete slabs subjected to impact include 

 the application of impact forces the values of which are determined 

 by means of the accelerometer, and the determination of the effect 

 of the impact on the slab by means of the specially designed graphic 

 strain gauge, which records the instantaneous deformation, and 

 another instrument which records the instantaneous deflection. 



The results of the tests so far are not sufficient in number to per- 

 mit any final conclusions to be drawn as to the relative strength of 

 the slabs, but the instruments have been proved to be sufficiently ac- 

 curate and sensitive to develop the data from which such conclu- 

 sions can be drawn. 



The investigation of the relative stability of bituminous pave- 

 ments was practically completed during the fiscal year, so far as the 

 actual tests are concerned. Considerable study and additional work 

 will be required, however, to locate definitely the factors which 

 cause or prevent the characteristic shoving of the surface. 



For the purpose of the experiment a number of sections of bitumi- 

 nous concrete pavement were constructed in the form of a circular 

 track approximately 600 feet in circumference and 13 feet wide. 



The object of the test was to study the effect of variations in the 

 size of aggregates, percentage of asphalt, and hardness of asphalt 

 on the resistance of the bituminous concrete to shoving under motoi-- 

 truck traffic. By the end of the fiscal year approximately 40,000 

 trips of a 3-ton loaded motor truck, operating at a speed of 10 to 12 

 miles per hour, had been made. The experimental results were very 

 pronounced. Shoving and displacement of the surface in the direc- 

 tion of the traffic was marked in a number of sections. As stated 

 above, the actual testing has been practically completed. 



The study of the relative wear of the 62 sections of experimental 

 concrete pavement constructed li'st year and subjected to an 



78007— AGR 1923 31 



