10 



BULLETIlsr 370^ U. S. DEPAETMEISTT OP AGEICULTUEE. 



group low results are frequently shown by laboratory tests for rocks 

 which bind quite satisfactorily upon the road, provided traffic is suf- 

 ficiently heavy to supply the requisite amount of fine material. Cer- 

 tain granites, gneisses, and schists which are not suitable for use as 

 binding material give good results in this test. In such cases it is 

 usually found that the highly altered nature of the material reduces 

 its toughness and resistance to wear to such an extent as to condemn 

 it, for use. 



Experience has shown that in general the following table of limit- 

 ing values for the French coefficient of wear, toughness, and hardness 

 may be used in determining the suitability of a rock for the con- 

 struction of the wearing course of a macadam road : 



Table II. 



-Limiting values of plujsical tests of rock for toater-Jjound macadam 

 road construction. 



Character of traffic. 



Limits of tests. 



French coefficient of wear. 



Toughness. 



Hardness. 



Light 



5-8— (5-8 per cent wear) 



5-9 



10-18 



Over 18 



10-17 







Over 14 



Heavy 





Over 17 









With relation to the limitations for hardness it may be noted that 

 as a result of comparing hardness and toughness tests of some 3,000 

 samples, the authors^ have show^n that when any given value for 

 toughness falls within certain limits which define the suitability of 

 the material for macadam road construction under given traffic 

 conditions, the corresponding Value for hardness will fall within 

 similar limits for hardness. In this connection it will be seen, in 

 Table II, that a maximum limit for hardness is only given in the 

 case of light traffic. It has been found that the great majority of 

 samples having a French coefficient of wear of from 5 to 8 and a 

 hardness of over 17 are granites, quartzites, and hard sandstones, 

 which are unsuited for use in the wearing course of water-bound 

 macadam roads due to their lack of binding power. 



BITUMINOUS ROADS. 



For broken-stone roads which are maintained with dust palliatives, 

 the same limits for French coefficient of wear and toughness should 

 hold as for ordinary macadam roads. 



In bituminous work observations indicate that in some cases it is 

 advantageous to use a rock of relatively high absorption rather than 

 one with low absorptive qualities, owing to a better adhesion of the 

 bituminous material by a partial surface impregnation of the rock. 



1 Relation Between the Properties of Hardness and Toughness of Road-Building Rock, 

 Journal of Agricultural Research, Vol. V, No. 19, D-3. 



