106 HIGHWAY CONSTRUCTION 



to wear in small ridges. The most recent practice for laying blocks 

 on 3 per cent grades, has been to remove from the top of one side of 

 each block a strip J inch thick and 1^ inches deep, extending the length 

 of the block. When the blocks are laid and driven closely together, 

 there is a quarter-inch opening or joint extending clear across the street 

 in each course. These joints are filled with Portland cement grout. 

 Fig. 63 shows a section of pavement having this form of joint. 



Filling for Joints. The best materials for filling the joints are 

 bitumen for the lower two or three inches, and hydraulic cement grout 

 for the remainder of the depth. The cement grout protects the pitch 

 from the action of the sun, and does not wear down very much below 

 the surface of the wood. 



ASPHALT PAVEMENTS. 



Asphaltic Paving Materials. All asphaltic or bituminous pave- 

 ments are composed of two essential parts namely, the cementing 

 material (Matrix) and the resisting material (aggregate). Each has a 

 distinct function to perform; the first furnishes and preserves the co- 

 herency of the mass ; the second resists the wear of traffic. 



Two classes of asphaltic paving compounds are in use, namely, 

 natural and artificial. The "natural" variety is composed of either 

 limestone or sandstone naturally cemented with bitumen. To this 

 class belong the bituminous limestones of Europe, Texas, Utah, etc., 

 and the bituminous sandstones of California, Kentucky, Texas, Indian 

 Territory, etc. The "artificial" consists of mixtures of asphaltic 

 cement with sand and stone dust. To this class belong the pavements 

 made from Trinidad, Bermudez, Cuban, and similar asphaltums. 

 For the artificial variety, most hard bitumens are, when properly 

 prepared, equally suitable. For the aggregate, the most suitable mate- 

 rials are stone-dust from the harder rocks, such as granite, trap, etc., 

 and sharp angular sand. These materials should be entirely free from 

 loam and vegetable impurities. The strength and enduring qualities 

 of the mixture will depend upon the quality, strength, and proportion 

 of each ingredient, as well as upon the cohesion of the matrix and its 

 adhesion to the aggregate. 



Bituminous limestone consists of carbonate of lime naturally 

 cemented with bitumen in proportions varying from 80 to 93 per cent 

 of carbonate of lime and from 7 to 20 per cent of bitumen. Its color, 

 when freshly broken, is a dark (almost black) chocolate brown, the 



