PHYSICAL TESTS OF EOAD-BUILDING ROCK. 5 



per cubic foot is calculated from the specific gravity of the material 

 obtained on a 10-gram sample by the usual displacement method. 

 The gain in weight of this fragment after four days' continuous 

 immersion in water is used to calculate the water absorption in 

 pounds per cubic foot of the solid rock. 



VARIATIONS IN RESULTS OF TESTS. 



Because of the fact that the various rock families, when subjected 

 to the tests outlined above, give results which are more or less dis- 

 tinctive of a group or type, these results can best be discussed in many 

 cases collectively. There are 14 families of rock which are more 

 or less commonly used in macadam-road construction. The varia- 

 tions which have been found to exist in the three principal tests for 

 each of these are shown in graphic form in the accompanying chart. 

 The values of the tests are arranged as abscissae, with the zero points 

 to the left and the values numerically increasing toward the right. 

 The ordinates or A^ertical lines represent the percentages of the total 

 number of samples having values corresponding to the abscissae on 

 which they are plotted. The figures in parentheses in the upper 

 right-hand corner of each block represent the total number of de- 

 terminations from which these percentages were calculated. 



TRAP-ROCK GROUP. 



The first six rock families, Andesite, Basalt, Diabase, Diorite, 

 Gahhro, and RhyoUte, comprise the well-known group of road-build- 

 ing rocks commonly known as "trap." They are all of igneous origin, 

 but are denser and finer grained than the granites, possessing as a 

 rule a peculiar interlocking crystalline structure which imparts to 

 them their distinguishing characteristic — high toughness. Thus, by 

 referring to the chart, it will be noted that the average toughness of 

 all the traps, with the exception of gabbro, which runs somewhat 

 lower, is about 18. This is a considerably higher average than that 

 shown by any of the other types or groups. The same relationship 

 holds true in the abrasion test, the average French coefficient of wear 

 running from about 13 to 15. Comparatively slight variations in 

 hardness are noted for any family or for the group as a whole, the 

 average hardness for which is about 18. The binding power of the 

 traps, as determined by test, varies through wide limits, depending 

 largely on the degree of weathering they have undergone, as shown 

 by Lord.^ The specific gravity of this group averages about 2.9, 

 giving an average weight per cubic foot of 180 pounds. Individual 

 samples are seldom less than 2.7 nor more than 3.2 specific gravity. 

 Water absorption may vary from a few hundredths of 1 per cent to 

 over 7 per cent. 



1 United States Department of Agriculture Bulletin No. 348. 



