107 



A person interested in this method of proportioning will 

 flnd on trial that other sands and stones available in the 

 vicinity will give other depths in the pipe, and it is prob- 

 able that by looking around and obtaining the best avail- 

 able materials the strength of the concrete obtainable will 

 be very materially increased. 



As a guide to obtaining the best concrete, the propor- 

 tion of cement remaining the same, the following are the 

 results of extensive tests: 



The stone should all be of one size or should be evenly 

 graded from fine to coarse, as an excessive amount of 

 the fine or middle sizes is very harmful to strength. 



All of the fine material smaller in diameter than one- 

 tenth of the diameter of the largest stone should be 

 screened out from the stone. 



The diameter of the largest grains of sand should not 

 exceed one-tenth of the diameter of the largest stone. 



The coarser the stone used the coarser the sand must 

 be, and the stronger, more dense and watertight the prop- 

 erly proportioned work becomes. 



When small stones only are used the sand must be fine 

 and a larger proportion of cement must be used to obtain 

 equal strength. 



A set of test beams has shown the following decrease 

 in strength, due to decrease in density: 



Modulus of Rupture. 

 Proportions. Lb. sq. in. 



1:2:6 319 



1:3:5 285 



1:4:4 209 



1:5:3 151 



1:6:2 102 



1:8:0 41 



By inspecting the above table it is seen that although 

 the amount of cement in each of the above beams was 

 the same (namely, 1-9 of the total material), some of 

 the beams were over 700% stronger than others. 



Proportioning of mixtures can be done by means of the 

 following formula: 



S I 



(I) ~~ = 



to 



