PORTLAND CEMENT CONCRETE PAVEMENTS. 21 



were worked out by a contractor of considerable experience. Labor- 

 ers for preparing the subgrade, setting the forms, and for covering 

 the concrete with earth should be provided in addition to those 

 called for in the diagrams. 



HANDLING MATERIALS. 



One of the most difficult problems which has to be solved in con- 

 nection with concrete road construction is that of determining the 

 proper methods to employ in handling and delivering the materials 

 for the concrete. The different kinds of material required must be 

 delivered to the mixer in definite proportions at the same time, and 

 it is evident that the location of the several sources from which the 

 materials are obtained, with respect to each other and to the road, 

 will have a very great influence in determining the most economical 

 transportation methods. 



Consider, for example, a project on which is used a concrete 

 mixer of the street-paving type which mixes a batch containing 

 three sacks of cement. If the work is to progress normally, the 

 quantities of the different materials required each day will be 

 approximately as follows : 



Cement barrels 320 



Sand cubic yards 70 



Coarse aggregate do 140 



Water ; gallons— S, S00 



In addition to the above, if the mixer runs continuously, about 

 10,000 gallons of water will be required each day for keeping wet 

 that part of the pavement which will have been laid during the two 

 preceding weeks, and for sprinkling the subgrade before the con- 

 crete is placed. This makes the total weight of water which may 

 be required each day about 75 tons, and the total weight of all the 

 materials combined about 420 tons per day. 



The importance of the water supply is not always appreciated by 

 contractors and engineers, and the provision made for delivering 

 water on the work has sometimes been entirely inadequate. Another 

 frequent error is that of overestimating the amount of water which a 

 chosen stream is capable of supplying. In general, the most practi- 

 cable method of delivering the water is to pump it through a pipe line 

 laid along the road. The pipe should be at least 2 inches in diameter, 

 and for the mixer under consideration the pump should be capable 

 of furnishing about 25,000 gallons of water in 10 hours to any point 

 on the pipe line. Ordinarily at least 10,000 feet of pipe will be 

 required if the concrete is to be sprinkled for two weeks after it is 

 laid. 



