2 BULLETIN 670, U. S. DKl'AHTAIKNT OF AGRICULTURE. 



The percentage variation in strength of 153 granites and gneisses 

 and 156 limestones and dolomites, the total number of these types 

 of rock tested up to January 1, 1918, is shown graphically in Fig- 

 ure 1. In this chart the percentage of total samples tested having 

 various values for crushing strength are plotted as indicated. As 

 an example, the chart shows that 7 per cent of all granites and 

 gneisses tested have a crushing strength of 21,000 pounds per square 

 inch. The sum of all per cents to the left of the 21,000-pound line 

 totals 50; that is, 50 per cent of all granites and gneisses tested have 

 a crushing strength of less than 21,000 pounds. On the other hand, 

 7 per cent of all the granites and gneisses show a crushing strength of 

 20,000 pounds per square inch, and by summing up all the per cents 

 to the right of the 20,000-pound line, it will be seen that 50 per cent 

 of the samples tested have a crushing strength greater than 20,000 

 pounds per square inch. In other words, the average crushing 

 strength of tins type of material lies between 20,000 and 21,000 

 pounds per square inch. By a similar interpretation, the average 

 crushing strength of limestones and dolomites lies betw.een 18,000 

 and 19,000 pounds per square inch. Further study of the chart 

 shows that the crushing strength of limestones and dolomites closely 

 approaches that of granites and gneisses. 



INTERPRETATION OF RESULTS OF PHYSICAL TESTS. 



To interpret the results of the physical tests made of road-build- 

 ing rock, the office has adopted a table of general limiting test values 

 for broken stone for the various types of road construction which 

 is printed on the back of the form for reporting tests. For general 

 reference, these limiting values, together with comments upon limits 

 shown, are given in Table IV. By comparing the results of tests 

 on a sample of rock with the limits shown in the table, a general 

 idea of the types of road construction for which it is best suited 

 may be obtained. Table III contains the total number of rock 

 samples received from the various States which have been tested 

 up to January 1, 1918. 







GPvA lv ' TF 



A 



niMFIS*? 















153 1 



eats 

























































ill 



1 



1 







III 



| 



, 1 , 







LIMESTONE $ DOLOMITE - 



136 t"tJ 















































. 1 



1 ll 



ill II 



1 1 1 



1 ll 



1 









I 



l ! 



1 1 



> 



1 



c 



i 

 i 



< 



c 



3 C 

 3 C 



> C 



I c 



c 



o 

 o 



CRUSHING STRENGTH —LBS. PER 50. IN. 



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



