2 BULLETIN 370^ U. S. DEPARTMENT OF AGEICULTUEE. 



AGENCIES CAUSING ROAD DETERIORATION. 



Roads may deteriorate from both external and internal causes. 

 The destructive agencies may be classified as mechanical, chemical, 

 and physical, but in some respects it is more convenient to consider 

 deterioration as being due to the effect of (1) traffic, (2) climatic con- 

 ditions, and (3) faulty construction. The first two are external 

 agencies and the latter is internal. 



Tra-fjic. — Traffic divides itself into two classes, (a) horse-drawn 

 vehicles and (b) self-propelled or motor-driven vehicles. In the 

 former the impact of horses' feet tends to disturb the position of indi- 

 vidual fragments of rock in the wearing course and also to fracture 

 the rock. At the same time wheels, especially steel-tired wheels, 

 not only exert an abrasive action which grinds away the rock sur- 

 faces, but tend to crush the fragments of rock in proportion to the 

 load per unit width of tire. 



Automobile traffic exerts a severe shearing action upon the road 

 surface which tends to loosen the individual fragments and, ulti- 

 mately, to remove them from the road. Where chains or armored 

 tires are used, considerable abrasion may also result, especially under 

 those conditions which favor slipping or skidding. 



Climatic agencies. — So far as the rock itself is concerned, climatic 

 or weather conditions are not important destructive agencies. While 

 it is true that rain and surface waters gradually dissolve or react 

 with certain rock-forming minerals, the action is so slow as to be 

 practically negligible as a source of deterioration during the life of a 

 road. Frost may cavise some deterioration in the more porous types 

 of rock, but both rain and frost are more destructive to the road 

 structure than to the rock of which it is built. Wind also is a negli- 

 gible factor so far as the rock is concerned. 



Faulty construction. — Faulty construction may resuH in rapid 

 deterioration of the road proper, due to a number of causes, such as 

 poor drainage, lack of proper consolidation, the use of the wrong 

 size or wrong grading of broken stone, etc. Destruction or disinte- 

 gration of the fragments of rock may also be hastened by these errors 

 in construction. 



FACTORS INFLUENCING THE SELECTION OF ROCK FOR ROAD 



BUILDING. 



In accordance with the preceding discussion it is evident that 

 from the standpoint of destructive agencies traffic conditions are the 

 most important factors to be considered in the selection of rock for 

 road building. Availability as well as relative cost are also impor- 

 tant factors in so far as ultimate econom}^ is concerned, but need not 

 be considered in this bulletin. In addition, the type of road to be 



