40 BULLETIN 1077, U. S. DEPARTMENT OF AGRICULTURE. 
by providing expansion devices on the pipe line or by "snaking" the 
line. For cold- weather construction drain valves should be placed 
at all low points in order that the pipe may be drained to avoid 
damage by freezing. 
Either steam or gasoline pumps may be used for supplying water. 
The horsepower required to deliver a stated quantity of water at any 
given point will depend upon the length and size of the pipe line 
and the height the water has to be raised from the source to the 
work. A method of computing the horsepower required for the 
delivery of different quantities of water is given in the appendix, 
page 63. To avoid overloading the pump, a relief valve should be 
placed in the pipe line near the pump. This valve should be set 
to open when the pump pressure exceeds that needed, and provision 
should be made to discharge the water back into the source of supply 
so that waste of water will be avoided. 
The amount of water required for concrete-pavement construc- 
tion is approximately 30 gallons per square yard of pavement. A 
4-sack mixer laying an average of 800 square yards of pavement per 
10-hour day will require 24,000 gallons of water, or 40 gallons per 
minute, for mixing and curing. The failure of the water supply is 
responsible for many of the delays in concrete construction. These 
delays may be overcome to a marked extent by using double-unit 
pumps. The added expense of this type of pumping plant is usuallv 
justified on work of any considerable magnitude. 
MIXING AND PLACING. 
The quantities of all materials entering into the concrete should be 
accurately measured before they are placed in the mixer. If wheel- 
barrows are used, their capacity should be checked by means of a 
1-cubic-foot measuring box. No size of batch should be permitted 
which would require fractional sacks of cement. Concrete for pave- 
ments should invariably be mixed by means of mechanical mixers. 
If it is mixed at a central plant and hauled to the road, any satis- 
factory type of building mixer may be used. If it is mixed on the 
road, a paving mixer provided with traction and equipped with a 
device for distributing the concrete will be the most economical to 
use. Figure 1, Plate V, shows one type of mixer and a finishing 
machine. 
The device to convey the concrete from the drum of the mixer to 
its place in the road may consist of a bucket and boom attachment 
or a chute. The bucket and boom device is believed to be preferable 
for pavement work, especially if a relatively dry mix is required. In 
chute distribution the tendency is to mix the concrete rather wet so 
that it will readily flow down the chute, and this is objectionable 
