472 ANNUAL REPORTS OF DEPARTMENT OP AGRICULTURE. 



accelerated wear test was continued. By the end of the fiscal year 

 the specially designed machine equipped with four wheels, carrying 

 solid rubber tires, had made approximately 50,000 trips. The ma- 

 chine has been operated at a speed of 22 miles an hour, each wheel 

 being loaded Avith 600 pounds per inch of tire. The results of the 

 experiment to date are not such as to warrant drawing definite con- 

 clusions other than the statement that actual surface wear under 

 rubber-tired traffic without impact is exceedingly slow. 



NONBITUMINOUS ROAD MATERIALS RESEARCH. 



In addition to the major researches previously described the fol- 

 lowing investigations of the properties and uses of nonbituniinous 

 highway materials have been under way during the year. Studies 

 regarding the effect of alkali on the strength of concrete, mentioned 

 in last year's report, have been continued with special reference to 

 methods of retarding or neutralizing the destructive action of the 

 alkali salts. Interesting and encouraging results have been obtained 

 in connection with the use of water-gas tar and paraffin as protective 

 agents. The study of the effect of organic impurities on the strength 

 of concrete to which attention was called in last year's report was 

 also continued, special attention being given to practical methods 

 of neutralizing such harmful ingredients. Several methods which 

 have been proposed from time to time for curing concrete pavements 

 without the use of water were studied. 



The investigations relative to the suitability of blast-furnace slags 

 as concrete aggregate have been finished. Results of the investiga- 

 tion, which are reported in a paper presented before the American 

 Society for Testing Materials, indicate that slag is suitable for such 

 use and that the strength of slag concrete is equal to the strength of 

 concrete in which other acceptable aggi'egates are used. 



A bulletin giving cost data on the production of broken stone as 

 obtained in the field from 23 quarries was completed. Various types 

 of quarries and crushing plants were included in the study, such as 

 pit, open-face, high and low face, as well as large and small plants. 

 Quarries were selected so that data might be obtained according 

 to various kinds of rock. A unit sj^stem of recording costs Avas 

 adopted and it is believed that the results will be of service to all 

 engineers engaged in rock excavation. 



Laboratory studies in cooperation with the committee on tests of 

 the American Association of State Highway Officials and various 

 technical committees of the American Society for Testing Materials 

 were conducted with special reference to such problems as the 

 standardization of strength tests for fine aggregate used in concrete, 

 compression tests of rock, and abrasion tests of crushed rock. Studies 

 involving the scientific proportioning of concrete by the sieve methods 

 which have been proposed were likewise carried on during the year. 



BITUMINOUS ROAD MATERIAL RESEARCH. 



In addition to the bituminous stability investigations, the follow- 

 ing bituminous material researches were conducted during the year. 

 The study of samples of asphaltic pavements taken from a number of 

 large cities, which was begun last year in cooperation with the As- 

 phalt Association and the street departments of the cities involved, 

 was continued during the fiscal year. The testing of the samples 



