HIGHWAY CONSTRUCTION 



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mechanical mixers, and the ingredients will he more thoroughly mixed, 

 than by hand. Thorough incorporation of the ingredients is an essen- 

 tial element in the quality of a concrete. When mixed by hand, how- 

 ever, the incorporation is rarely complete, because it depends upon the 

 proper manipulation of the hoe and shovel. The manipulation, 

 although extremely simple, is rarely performed by the ordinary laborer 

 unless he is constantly watched by the overseer. 



Several varieties of concrete-mixing machines are in the market. 

 A convenient portable type is illustrated in Fig. 60. The capacity of 





Fig. 60. Concrete Mixing Machine. 



the mixers ranges from five to twenty cubic yards per hour, depending 

 upon size, regularity with which the materials are supplied, speed, etc. 



Gravel Heaters. Fig. 61 illustrates a device commonly employed 

 for heating the gravel used for joint filling in stone-block pavements. 

 These heaters are made in various sizes, a common size being 9 feet 

 long, 5 feet wide, and 3 feet 9 inches high. 



Melting Furnaces, for heating the pitch or tar for joint filling, are 

 illustrated by Fig. 62. Various sizes are on the market. 

 WOOD PAVEMENTS 



Wood pavements are formed of either rectangular or cylindrical 

 blocks of wood. The rectangular blocks are generally 3 inches wide, 



