HARDNKSS OK MATK.UIAI.S. 7 



Laboratory lia> carefully studied (he methods of burning clay for bal- 

 last in use by a number of western railroad-. I'.y some\\ hat modify- 

 ing these methods very satisfactory roads can undoubtedly he made at 

 a moderate cost in districts where the day is of a suitable nature. 

 Kxperiments are, also being made with mixtures of crude petroleum 

 and asphaltum which give promise of good results. 



Before taking up any of the above subjects in detail it will be well 

 to consider first some of the important physical and mechanical prop- 

 erties of road materials, and also some of the principal agents in the 

 destruction of roads. 



IMPORTANT PHYSICAL AND MECHANICAL PROPERTIES OF ROAD 



MATERIALS. 



There are three chief properties essential to good road materials, 

 and only these will be considered here. They are hardness, tough- 

 ness, and cementing or binding power. Although these properties, 

 at least hardness and toughness, have long been recognized by those 

 familiar with the subject, yet they have never been properly defined 

 and the terms have been very much confused. This is not at all sur- 

 prising, for hardness and toughness are closely related. It would be 

 well, therefore, to define these terms from the road maker's stand- 

 point before going further. 



HARDNESS. 



There is no widely accepted measure of the property of hardness. 

 Even in the case of metals, the hardness of which has received much 

 study, there are many tests based on different conceptions of the term. 

 The many methods devised for measuring hardness may be summar- 

 ized under the following heads: Drawing substance under a point; a 

 drawing a point over substance and determining resistance;* grooving 

 with a standard edge and determining the depth of groove; c boring 

 substance with a standard point and ascertaining depth of hole and 

 number of revolutions;' 7 grinding with a standard powder and taking 

 the loss inversely; 6 compressing lenses on plates of substance and 

 taking as the hardness the limit of pressure per unit of surface multi- 

 plied by the cube root of the radius of curvature;^ compressing a 

 point into substance and taking the volume of indentation inversely. '' 



"Pekarek, Sitz. v. k. k. Akad. Wirn. is.VJ, vol. 1:5. 'Contains complete bi< 

 phy of earlier papers.) 



^Turner, Proceedings Phil. Society, I'.irniinirliain, Is 



''Pfaff, Sitz. k. k. liayer. Akad., i 



^Pfaff, ibid., 1884. 



Rosiwal, Yerhandl. k. k. Geol. Reichwmstalt, IxOfi, 17: 475. 



/Auberbach, Wied. Ann., 1891-96. 



Report of Experiments on Metal for Cannon, United States Ordnance Depart- 

 ment, 18o6. 



