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 usually 

 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 



