2 BULLETIN 370, U. S. DEPARTMENT OF AGRICULTURE. 
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. 
Traffic. — 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 cause 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 result 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 economy is concerned, but need not 
be considered in this bulletin. In addition, the type of road to be 
