45 



"percentage of voids," that is, the number of parts in 

 100 parts of crushed stone, which are occupied, not by 

 stone, but by air (when it is in the stock pile). We 

 wish to determine this volume in order to tell how 

 much mortar to use. There are two ways of finding 

 this out. One cubic foot of solid limestone weighs 

 165 Ib. Make a wooden box exactly I ft. each way, 

 inside measure. Fill it with the broken stone, and 

 weigh it. This weight will vary, determined by the 

 degree of crushing and screening. Assume that you 

 find that I cu. ft. of crushed stone weighs 105 Ib. 



The ratio of 105 to 165, or 63^ per cent, equals the 

 percentage of solid matter. 



100% 63y2%=36 l / 2 %, or the "percentage of voids." 

 In other words, i cu. ft. of crushed stone requires 

 .361/2 cu. ft. of sand and cement to fill all the voids. 



This same method can be applied to the sand to 

 determine the percentage of voids. A solid cu. ft. of 

 quartz weighs about 165 Ib. and assume the voids in 

 the sand to be 40 per cent. Therefore, i cu. ft. of 

 sand requires .40 cu. ft. of a "filler." in this case ce- 

 ment, to make it solid. 



If i cu. ft. of sand requires .40 cu. ft. of filler, then 

 the .36. r /2 cu. ft. with the i.o cu. ft. of stone will require 

 .141/2 cu. ft. of "filler" or cement. 



We then see that in this cnse to produce an ideal 

 concrete we could take 



i .00 cu. ft. of crushed stone. 

 .361/2 cu. ft. of sand 



. .1^/2 cu. ft. of cement. 



These proportions reversed and referred to I cu. ft. 

 of cement as a base, give us a proportion of i cu. ft. 

 of cement, 27 cu. ft. of sand and 6.9 cu. ft. of stone. 

 For better work, increase the proportion of the 

 cement to provide for the cement used to coat the sur- 

 face of the stone. a factor of safety, as it 

 were, increase the proportion of sand. 



