356 Annual Report Agricultural Experiment Station 



it was found that certain specimens failed at very low pressures. 

 The pipe walls when broken showed that the water had penetrated 

 from one-fourth to one-third of the thickness. Apparently the ex- 

 pansion of the inner portion of the wall produced tension in the un- 

 wetted portion and caused the pipe to burst. This action, which 

 may be called differential expansion, doubtless is the main cause 

 of the failures of pipe lines by longitudinal cracks. 



The intensity of differential expansion must vary with many 

 factors, including the thickness of the pipe walls, the richness of 

 mixtures, the mortar consistency, and the climate. It is believed also 

 that the magnesia content in the cement affects the amount of 

 expansion. 



Several methods of overcoming the danger have been proposed, 

 but it is believed that the only thoroly safe method is to prevent 

 the drying of the pipe between the curing and laying. The curing 

 should be continued up to the time of laying. 



METHODS OF TESTING CEMEjNT PIPE 



The technique of cement pipe testing has not been well stand- 

 ardized and different methods are in use by those engaged in pipe 

 testing. It is quite impossible, therefore, to make comparisons be- 

 tween pipes made and tested in different places. Factors of safety 

 in design must depend on the methods employed in testing. A 

 paper on this subject, discussing certain alternative methods and 

 pointing out the importance of standardizing the condition of the 

 test specimens as well as the methods of testing, has been con- 

 tributed to the technical press.* 



REINFORCEMENT FOR CEMENT PIPE 



Several trials of reinforcing cement pipe were made during 

 the year, but in all cases the reinforced pipe was found to be weaker 

 than plain pipe. Electro-welded wire rings, hog wire, and Triangle 

 Mesh were tried. Reports from other sources show similar disap- 

 pointing results. This matter needs further extensive investiga- 

 tion. If a means of making the reinforcement effective can be 

 found, the field of usefulness of cement pipe will be widened greatly. 



The publication of the bulletin on cement pipe noted in the 

 last annual report has been delayed, but the bulletin is now in 

 press and will be issued shortly as No. 86 of the Station series. 



TRACTOR POWER ON FARMS 



This department has watched the development of traction en- 



♦Concrete, VoL 13, No. 5, p. 156. 



