8 THE TESTING OF ROAD MATERIALS. 



All of these tests were designed for substances of a homogeneous 

 nature, and are consequently not at all suited to any of the road mate- 

 rials. Further than this, it can be seen that in their conception of 

 hardness some of the investigators differ greath T . All of these meth- 

 ods, as well as modifications of them, are based either on abrasion or 

 penetration. 



The valuing demands of technology give rise to different definitions 

 and methods of testing, and the method used in any particular case 

 must give a measure of the value of a new material for the purpose 

 for which it is intended. To permanently deform a substance by com- 

 pression with a pointed or spherical instrument tests altogether differ- 

 ent properties from those opposed to abrasion. 



Only one test has yet been devised for determining the hardness of 

 road materials, and that, the writer believes, gives the value of this 

 property as understood by road builders in a satisfactory manner. 

 This is the Dorry test of the French School of Roads and Bridges which 

 consists in grinding specimens with sand of a standard size and qualit} 7 . 

 This method of grinding with a powder has the advantage of having 

 been used as a test for hardness by a number of the most able students 

 of the subject, and at the same time valuable results have been obtained 

 from it on the very class of materials in which \ve are most interested. 

 Hardness, therefore, will be defined as the resistance which a material 

 offers to the displacement of its particles by friction. The measure 

 of hardness will be, inversely, as the loss of weight arising from the 

 scoring by an abrasive agent. 



TOUGHNESS. 



In the consideration of road materials toughness is understood to 

 mean the power possessed by a material to resist fracture under 

 impact. As the surface of a road is continually subjected to the 

 pounding of trallic, it can be seen that toughness is an important 

 property from the standpoint of the road builder. From the labora- 

 tory standpoint the problem is not altogether a simple one. and con- 

 siderable difficulty has been found in designing a suitable tet I'm 

 measuring the degree to which a road material possesses this property. 



With homogeneous, structureless, brittle materials, resistance to 

 impart may be due to a relatively low modulus of elasticity combined 

 with high elastic limit. Provided a blow is delivered by a Mat strik- 

 ing head with small local damage, on such a material toughness will 

 be almost wholly due to elasticity. In this case there will be a critical 

 energy of blow below which the specimen under test will not be 

 broken by an indefinite number of blows, and in excess of which it 

 will be broken by a single blow. The toughness of a road material 

 in this instance will vary directly as the square of the elastic limit, 

 which equals the ultimate strength, and inversely as the modulus of 



