BULLETIN 537, U. S. DEPARTMENT OF AGRICULTURE. 



and properly faced, are bedded in plaster of Paris. The cylinder 

 then is crushed in a 200, 000-pound universal testing machine. A 

 small 2-inch spherical bearing block is placed between the moving 

 head of the machine and the upper surface of the specimen. The 

 average of at least two determinations is reported as the crushing 

 strength, calculated in pounds per square inch. Crushing strength 

 tests are made upon samples of road-building rock only when especi- 

 ally requested. Of a total of 282 compression tests made up to 

 January 1, 1917, Table II shows that 97 were made on granites, 

 13 on gneisses, 78 on limestones, 42 on dolomites, 28 on sandstones, 

 and 24 on various other types of material. 



The percentage variation in the strength of the 110 granites and 

 gneisses and the 120 limestones and dolomites is shown graphically 



in figure 1. In this 

 chart the per cent of 

 total samples tested 

 having various values 

 for crushing strength 

 are plotted as indi- 

 cated. For instance, 

 the chart shows that 

 10 per cent of . all 

 granites and gneisses 

 tested show a crush- 

 ing strength of 20,000 

 pounds per square 

 inch. Likewise, by 

 summing up all the 

 per cents to the left 

 of the 20,000-pound 

 line we find that 48 per cent of all samples of granite and gneiss tested 

 have a crushing strength of less than 20,000 pounds per square inch, 

 which shows that the average crushing strength of this type of 

 material lies between 19,000 and 20,000 pounds per square inch. 

 Likewise, the average crushing strength of the limestones and dolo- 

 mites lies between 18,000 and 19,000 pounds per square inch. 



INTERPRETATION OF RESULTS OF PHYSICAL TESTS. 



A discussion of the interpretation of the results of physical tests 

 was given in Bulletin 370, to which reference has been made. Since 

 the publication of that bulletin, however, a table of general limiting 

 test values for broken stone for various types of road construction 

 has been adopted by the office and printed on the back of its form 

 for reporting tests. For general reference these limiting values, 

 together with comments upon limits shown, are given in Table IV. 









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TOT! 5 



s 





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ill 



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 10 











(120 



Test* 



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. II 



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 c 

 c 



1 I 



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 o 



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CRUSHING STRENGTH - LBS PER 5Q. 'N. 



Fig. 1.— Variations in the crushing strength ol rock. 



