208 ENGINEERING ON THE FARM 



speed of motor vehicles requires some type of construction 

 similar to city pavements. Pavement construction for 

 country highways has been developed in the country since 

 the advent of the motor car and is of two general types: (i) 

 some form of a block laid on a solid foundation; (2) a tough, 

 hard material that will stand the wear united in a homoge- 

 neous mass by a cementing material. 



The type of construction most favored for this purpose 

 is a good foundation covered with a wearing surface made 

 of a mixture of tough, hard sand and pebbles or broken 

 stone bound together by a cementing material. The cement- 

 ing materials are of two kinds: (1) bitumen, a material found 

 in natural asphalt, asphalt, petroleum oils, and tar, in which 

 the cementing qualities are physical, being due to sticky 

 properties which develop when the material is heated; 

 (2) Portland cement, in which the cementing properties 

 are due to chemical action set up when water is added. 



Block paving. Block paving materials that have proved 

 satisfactory for pavements under certain conditions are 

 vitrified brick, stone block, wood block treated with a pre- 

 servative, and bituminous block. The blocks are usually 

 laid on a foundation made of Portland cement concrete 

 4 to 6 inches thick. The block pavements for country high- 

 ways are used only to a limited extent. 



Bituminous surfaces. Bituminous road surfaces are con- 

 structed in a variety of ways and under numerous trade 

 names. Bitumen is the substance that gives certain oils 

 previously mentioned their value as road material. A thin 

 oil applied to a road surface is known as a dust layer. A 

 coat applied to the surface and then covered with rock 

 screening or sand is called a bituminous carpet. A bitu- 

 minous macadam is a macadam road properly built and 

 shaped and then treated with a hot bituminous binder which 

 penetrates to a depth of approximately one inch in sufficient 

 quantities to bind the surface in a solid sheet. A bitumi- 

 nous concrete road has a wearing surface 2 inches thick of 



