BUREAU OF PUBLIC ROADS. 473 



has been practically completed. Considerable work, however, re- 

 mains in connection with the final analysis and correlation of results. 



Tests of the toughness and adhesiveness of bituminous materials 

 and mixtures were carried on in cooperation with the committee on 

 tests of the American Association of State Highway Officials, and a 

 progress report was made at the last meeting of the committee. 



Work in connection with the standardization of the tests of ma- 

 terials in cooperation with the American Association of State High- 

 way Officials and the American Society for Testing Materials was 

 conducted with special reference to the float test for the consistency 

 of tar products and the penetration test for asphalt cements. 



AUTOGRAPHIC TRAFFIC COUNTER DEVELOPED. 



One of the instruments devised during the year is an autographic 

 traffic counter which, when installed in a road, will record the num- 

 ber and weight of all vehicles passing the point of installation. It 

 is a fundamental principle of economic highway improvement that 

 the roads built should be adequate for the traffic they are expected to 

 carry, and a corollary of this principle is that no road should be 

 improved to a degree in excess of the requirements of the traffic. 

 A precise knowledge of the volume of traffic and the weight of the 

 vehicular units is essential to the practical application of these 

 principles, and the new instrument developed by the bureau gives 

 promise of usefulness in securing this information with a minimum 

 expenditure of effort and money. 



COOPERATIVE RESEARCHES. 



In addition to the researches conducted solely by the bureau, a 

 number of investigations have been carried on in cooperation with 

 several of the State highway departments, universities, and experi- 

 ment stations. These investigations, which involve studies of high- 

 way design, materials, and economics, include the following : 



With the Engineering Experiment Station of Purdue University. 



Experiments have been conducted to develop a ball test for mortar 

 and concrete. The load required to press a steel ball into the surface 

 of a material has been found to be a good index of the general 

 strength and qualities of some materials. In the form of the Brinell 

 test it is used for steel, and it also applies to wood. The purpose of 

 this cooperative research was to determine the applicability of a 

 similar test to concrete, and the results indicate not only that such a 

 test can be applied to determine the surface strength of concrete and 

 mortar, but that it may throw valuable light on the question of 

 fatigue of such materials under repeated applications of load. 



Other experiments in connection with the fatigue of mortar and 

 concrete and stresses in road slabs, as affected by moisture content, 

 have also been conducted in connection with this university. The 

 significance of these experiments in connection with the design of 

 concrete road surfaces which are subjected to frequent repetitions 

 of concentrated loads, and which are exposed to moisture from all 

 sides, will be readily apparent. Observation of concrete pavements 

 under actual service conditions has suggested the possibility that 

 there may be a gradual depreciation in their strength as the re- 

 sult of repeated deformation under load and the tests thus far made 



