FARM DEVELOPMENT 



The selection of materials for macadam roads.* No one rock can 

 be said to be a universally excellent road material. The climatic 

 conditions vary so much in different localities, and the volume 

 and character of traffic vary so much on different roads, that the 

 properties necessary to meet all the requirements can be found 

 in no one rock. If the best macadam road be desired, that material 

 should be selected which best meets the conditions of the particular 

 road for which it is intended. 



In most cases the selection of a material for road making is deter- 

 mined more by its cheapness and convenience of location than 

 by any physical properties it may possess. But when we consider 

 the number of roads all over our country which are bad from 

 neglect and from obsolete methods of maintenance that would be 

 much improved by the use of any rock, this regard for economy 

 is not to be entirely deprecated. At the same time, as a careless 

 selection leads to costly and inferior results, too much care can- 

 not be used in selecting the proper material when good roads are 

 desired at the lowest cost. 



In selecting a road material it is well to consider the agencies 

 of destruction to roads that have to be met. Among the most 

 important are the wearing action of wheels and horses' feet, frost, 

 rain, and wind. To find materials which can best withstand these 

 agencies under given conditions is the great problem that con- 

 fronts the road builder. 



Before going further, it will be well to consider some of the 

 physical properties of rock which are important in road building, 

 for the value of a road material is dependent in a large measure 

 on the degree to which it possesses these properties. There are 

 many such properties that affect road building, but only three 

 need be mentioned here. They are hardness, toughness, and 

 cementing or binding power. 



By hardness is meant the power possessed by a rock to resist 

 the wearing action caused by the abrasion of wheels and horses' 

 feet. Toughness, as understood by road builders, is the adhesion 

 between the crystal and fine particles of a rock, which gives it 

 power to resist fracture when subjected to the blows of traffic. 

 This important property, while distinct from hardness, is yet 

 intimately associated with it, and can in a measure make up for 

 a deficiency in hardness. Hardness, for instance, would be the 

 resistance offered by a rock to the grinding of an emery wheel; 

 toughness, the resistance to fracture when struck with a hammer. 

 Cementing or binding power, is the property possessed by the dust 

 of a rock to act after wetting as a cement to the coarser fragments 

 composing the road, binding them together and forming a smooth, 

 impervious shell over the surface. Such a shell, formed by a rock 

 of high cementing value, protects the underlying material from 

 wear and acts as a cushion to the blows from horses' feet, and a^ 

 the same time resists the waste of material caused by wind and 

 rain, and preserves the foundation by shedding the surface water. 



*Extracts from a paper on this subject, from the Yearbook, Department of Agri- 

 culture, for 1900, by Logan Waller Page. 



