ROADS 155 



undergone the test placed upon it in its formation and trans- 

 portation by water and ice. 



Size of Gravel. It is desirable that the pebbles in the 

 gravel for road surfacing be not too large. It is customary 

 to screen out all pebbles or stones larger than % to 1 inch in 

 diameter. In some cases where larger pebbles exist they 

 are screened out and used for the first courses in the con- 

 struction of the road bed. If large pebbles or stones are 

 left in the gravel they are quite apt to come to the surface 

 through the action of the traffic and of frost. Gravel 

 should also vary in size, so that there will be small pebbles 

 to fill the open spaces between the larger ones, and in 

 turn the space between the smaller pebbles should be filled 

 with sand grains. When the gravel varies in size in this 

 manner a very dense mixture is obtained, which is ideal for 

 road material. In some cases where the different sized 

 pebbles do not exist naturally in the proper proportion to 

 make a dense mixture, it may be profitable to screen the 

 gravel and remix it more nearly as it should be. 



The Binder. In order that the gravel shall form a 

 satisfactory surfacing material for a road, it must contain or 

 be mixed with some material which will hold the pebbles 

 together. In most instances this binding material is clay. 

 Clay exists to some extent in all gravels. When the gravel 

 will stand vertically in the bank, and when it resists the 

 action of frost and must be loosened with a pick, it is quite 

 likely to contain the proper amount of binding material. If 

 a sufficient amount of clay is not present to fill the open 

 spaces between the pebbles and cause them to be packed 

 into a dense structure, additional clay should be added. 

 Clay has several characteristics which recommend it as a 

 binding material. It is cheap, can be easily reduced to a 

 finely divided state, and is usually found to a more or less 



