Tests 



127 



TABLE XII. MODULUS OF RUPTURE OF MACHINE-MADE CEMENT PIPE, DRY 



Group 



Diam. 



Min. 

 thickness 



Breaking 



load per 



lineal foot 



Radius 

 of center 



line 



Maxi- 

 mum 

 bendingr 

 moment 



Modulus 



of 



rupture 



Inches 

 12.0 

 14.0 

 16.1 



20.2 



Inches 



1.28 

 1.35 

 1.62 



1.84 



Pounds 



2633 

 1721 

 2828 



2780 



Inches 



6.64 



7.68 



8.86 



11.02 



In.— Lb. 



29\ 

 220 

 417 

 510 



Pounds 



1067 

 725 

 953 

 904 



The strength of the machine-made pipe in each group is com- 

 paratively uniform. This should be characteristic of pipe made by- 

 machinery, all pipe being compressed exactly the same way and 

 the same amount. Such uniformity is very improbable in the case 

 of hand-made pipe. 



The 16-inch hand-made pipe broke at loads three-fifths as great 

 as the machine-made pipe. This may be accepted as the approxi- 

 mate ratio of strength of the two classes of pipe, inasmuch as the 

 wall thicknesses and ages were approximately the same. 



The data obtained on pipe made of washed sand are somewhat 

 equivocal. The tests on 12-inch pipe do not show any advantage 

 from the washed sand, and the tests on 20-inch pipe are not con- 

 clusive. , 



Owing to the difficulties previously had with the internal press- 

 ure tests, it was thought that the strength might be reduced ser- 

 iously by thoroughly wetting the pipe just before testing them. To 

 investigate this possibility, pipes Nos. 8, 9, and 10 were immersed in 

 a tank of water, then weighed and tested immediately. No. 8 was 

 immersed 36 minutes, it gained 1.5 pounds, and broke at 4560 

 pounds, a reduction of 18 per cent from the strength when tested 

 dry. Nos. 9 and 10 were soaked 46 minutes, and lost 36 per cent in 

 strength. 



These tests, while few in number, are sufficient to establish the 

 point that cement pipe, cured and dry, is weakened by immersing in 

 water. The weakening may be due, in part, to differential expan- 

 sion, or it may be due wholly to the condition of saturation. Van 

 Ornum found that the loss of strength of concrete cylinders is tem- 

 porary, and the original strength is regained and perhaps exceeded 

 in the course of a few weeks* ; but the maximum load on a pipe line, 

 laid dry, may occur when it is at the lowest point of its strength. 

 Usually it is possible to apply the load by degrees ; deep trenches 

 can be backfilled in part only, until sometime after water has been 



•Trans. Amer. Soc. C. E.. Vol. LXXVII. p. 438. 



