

BINM>IN<; I'oWKi; OF MA'l KKI.M.S. V) 



elasticity. In testing such materials under impart it would he neces- 

 sjiry to apply a number of Mows of successively increasing energy, 

 and note that Mow which causes failure. A test involving this prin- 

 cipal will l>e described further on in detail. 



With heterogeneous materials like most of the rock> used in roacj 

 building, toughness depends on a number of factors. Among these 

 may be mentioned interlocking of the crystals, the nature of the crys- 

 tals themselves, and in some cases the nature of the cementing or 

 binding agent. This matter has received but little study. 



There is another class of road materials whose toughness is due to a 

 combination of properties not considered under the above definition. 

 it includes those materials capable of considerable deformation with- 

 out rupture when stressed beyond the elastic limit. Asphaltum and 

 tar practically constitute this class. They distinctly come under the 

 head of tough substances; but it is obvious that toughness in this 

 sense is viscosity and malleability. 



CEMENTING OB BINDING POWER. 



The binding power or, as it has now come to be called, the cement- 

 ing value of a road material is the property possessed by rock dust 

 or other finely divided material found in nature to act as a cement on 

 the coarser fragments composing crushed stone or gravel roads. 

 This property varies enormously, not only with different kinds of 

 rocks, but also with those which are practically identical in classifica- 

 tion and chemical composition. The absence of cementing power i> 

 so pronounced in some varieties of rock that they can never be made 

 to compact with the road roller or under traffic. As the binder sur- 

 face of a macadam or gravel road is most exposed to the action of 

 wind and rain, as well as the wear and tear of traffic, it can be seen 

 that the presence of this property is most essential to good results. 

 Further than this, the hardness and toughness of the binder surface. 

 more than of the rock itself, constitutes the hardness and toughness 

 of the road, for if a load be sufficient to destroy the bond of cementa- 

 tion of the upper surface of a road, the stones below are soon loosened 

 and forced out of place. The impervious shell obtained by the use of 

 a rock of high cementing value gives the greatest protection to the 

 foundation of a road. Moreover, it is a matter of common observa- 

 tion that a good surface which binds well is less dusty and !<-- 

 muddy, while the advantage from the standpoint of economy i- very 

 great, as it is only the loose, unbound material which is ordinarily 

 carried away by wind and water. 



In view, therefore, of the importance of this property, it has been 

 made the subject of especial study in this laboratory. It was most 

 important to know the cause of the cementing value in order to deter- 

 mine what could be done to improve the conditions of service. The 



