THE FLOW OF WATER IN" CONCRETE PIPE. 99 



It is suggested that concrete pipe, especially where it is to be laid 

 on or near the surface of the ground, should where possible be built 

 in cool weather rather than hot weather, both because of the tem- 

 perature of the air and also on account of the temperature of the 

 water. If the pipe is built in warm weather it is apt to shrink in 

 cold weather and when cold water is run through it in such a way 

 as to develop objectionable circular cracks. 



In 1907 or 1908, at the time the Los Angeles Aqueduct was being 

 designed, investigation was made to determine the value of the 

 coefficient n in the Kutter formula in a number of the conduits of 

 southern California. The result of these investigations was pub- 

 lished about that time in the Engineering News over my signature. 

 An interesting feature that developed as a result of these investiga- 

 tions was that the carrying capacity of the ditch and the value of 

 the coefficient varied immensely with the question of whether the 

 particular ditch or conduit was exposed to the rays of the sun or 

 whether it was covered. If it were covered, the lining kept clean 

 and smooth, but when it was exposed to the action of the sun a 

 vegetable growth immediately set in. The growth resembled both 

 the lichens on stone and long streaming grass. The result was that 

 we found values of n of about 0.012 for ordinary covered conduits, 

 and anywhere from 0.014 to 0.018 for these uncovered conduits. 1 



It was also observed that in ditches where the sand-box arrange- 

 ments were poor that small bars of sand would follow down the 

 conduit or even through the pipe lines and very materially, in fact 

 totally, change the carrying capacities of the ditch. In other words, 

 the capacity of the ditch is affected by the provisions that are made 

 to remove sand and gravel that might otherwise enter. 



My experience is that plastered conduits of any kind should if 

 possible be covered. It has been found by experiments made, I 

 think at the University of Michigan, that the expansion and con- 

 traction of concrete varies greatly with the richness of the mix, the 

 richer mixes expanding more than the leaner mixes. It is customary 

 to make the plaster richer than the main bulk of the concrete in 

 order to get a smoother surface. The different rates of expansion 

 and contraction of the body of the concrete and the plaster is largely 

 responsible for the scaling of the plaster. If a plaster is desired it 

 should have about the same ratio of sand to cement as is used in the 

 main mix between sand and cement, and much more labor should 

 be put on than ordinarily in producing the smooth surface. By 

 careful forming in conduits and by troweling it is found to be possible 

 in many cases to obtain a perfectly smooth surface without the use 

 of plaster. 



I believe that it is entirely possible to build a jointed concrete 

 pipe that roots will not enter. A number of years ago I made an 

 examination of the irrigation system at Rialto. This was all built 

 of cement pipe which had been in service for a term of years, perhaps 

 15 or 20. The pipe ran under trees of all classes and kinds — cypress, 

 eucalyptus, orange, and lemon trees and deciduous fruits. We cut 

 into the pipe in many places and by means of mirrors threw the 

 light back through the pipe so that it could be carefully examined. 

 The pipe was clean, in every instance showing evidence of course of 



1 The above experiments are described as Nos. 58, 59, and 60. 



