406 ROADS AND PAVEMENTS 



with a 2- to 5-T. roller. When the bricks have been settled to a 

 firm and solid bearing, the joints are filled full either with a 

 grout composed of equal parts of hydraulic cement and fine, 

 clean, sharp sand, or with a tar filler composed of No. 6 coal-tar 

 distillate. After the joints have been filled, the entire surface 

 is covered with a layer of sand J in. deep, which after a few days 

 is swept up and removed. 



Asphalt Pavement. Asphalt is the solid form of bitumen, 

 either in a state of purity or combined with other matter. 

 Bitumen is a complex hydrocarbon considered to be the ultimate 

 product of the decomposition of certain vegetable and animal 

 matter. The best known sources of asphalt are those on the 

 island of Trinidad, in the West Indies, and in the state of Ber- 

 mudez, Venezuela, where it is usually found in the form of large 

 deposits, or lakes. It is rarely found in a pure state and it is 

 usually refined by a heating process, the product obtained 

 being called refined asphalt. Many of the refined asphalts are 

 too brittle for use. To remedy this defect, the asphalt is 

 mixed with a softening agent called the flux. The result- 

 ing mixture is called asphalt cement or asphaltic cement. The 

 agents most extensively employed for a flux are maltha and 

 residuum oil , the latter of which is obtained by the distillation 

 of petroleum. A concrete in which the matrix consists of 

 asphalt cement or coal tar is called bituminous concrete. 



It is very essential that all asphalting pavements be sus- 

 tained by a solid unyielding foundation, as the asphalt is suit- 

 able for a wearing surface only. The foundation is made 

 either of hydraulic-cement concrete or of bituminous concrete. 

 The former is more durable and is, therefore, generally preferred. 

 On the other hand, with hydraulic cement the bond between 

 ihe foundation and the wearing surface is not very perfect. 

 When bituminous concrete is used a layer of clean, well- 

 screened, broken stone is spread on the prepared roadbed to the 

 proper depth, and thoroughly consolidated by rolling, as in the 

 construction of broken-stone roads, after which a coating of 

 coal tar or bituminous cement is spread on it. The proportions 

 used should be about 1 gal. of cement to each square yard of 

 foundation. Bituminous concrete is less expensive than 

 hydraulic-cement concrete. 





