PORTLAND CEMBNT 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. Rubber 
hose of 1J 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 
