POETLAND CEMHNT CONCRETE ROADS. 39 



short sections of industrial track. (See Fig. 2, PI. III.) The princi- 

 pal advantage of this system is that the materials can be placed on 

 the shoulders of the road before the grading is begun, thereby al- 

 lowing the teams or trucks to use the road before it is disturbed. 

 Where materials are delivered to the mixer in batch boxes a derrick 

 is necessary to hoist the boxes from the cars and swing them over 

 the mixer skip. For this purpose the derrick may either be attached 

 to the mixer or independent of it. A derrick attached to the mixer 

 may be operated either by utilizing the power developed by lowering 

 the skip or by independent power obtained from the mixer. That 

 which utilizes the power developed by lowering the skip requires 

 fewer working parts and less power expenditure than any other 

 method. It is not as flexible, however, as a derrick operated by 

 independent power and has the disadvantage that the same relative 

 elevation must be maintained between the track and the subgrade 

 in order that a constant height of lift may be secured tO' swing the 

 boxes free of the cars. There is no particular advantage in using 

 a derrick independent of the mixer when batch boxes are discharged 

 into the mixer skip. The added expense of operation does not appear 

 to be justified. However, for very large mixers, with overhead 

 charge, a crane independent of the mixer must be used. These mixers 

 are usually not equipped with traction and therefore depend upon 

 an autocrane for movement. 



Water. — The usual sources of water supply are city mains, run- 

 ning streams, lakes, ponds, or wells. A city main is the most 

 satisfactory source of supply that can be obtained, as a uniform 

 pressure is secured and no pump is required. It is seldom, how- 

 ever, that the work is located so that city water can be used. A 

 frequent error on the part of engineers and contractors is that 

 of overestimating the amount of water which can be obtained from 

 any given stream or pond. Information should be obtained locally 

 as to dry-season flow before placing dependence on small streams 

 for water supply. 



The most practicable method of delivering water is to pump 

 it through a pipe line laid along the road. The diameter of the 

 pipe line should be not less than 2 inches. If very large mixers are 

 used, a pipe of larger diameter will be necessary in order to ob- 

 tain sufficient water for curing. Tees for supplying water to the 

 mixer and for sprinkling should be placed in the pipe line at 

 intervals of from 200 to 300 feet. Gate valves should be spaced 

 about 1,000 feet apart and unions about 500 feet apart. Eubber 

 hose of 1|- inch diameter should be used for connecting the pipe 

 line with the mixer, while 1-inch hose is usually used for sprinkling. 

 Provision should be made for the expansion of the pipe either 



