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FARM BUILDINGS IN SOUTH AFRICA 



3. Proportions of 1 : 3 : 6 give a lean mixture which should only be 

 employed for large mass work, or as a backing to stone 

 masonry. 



If the proportions be fixed by the experiments already referred to, 

 leaner mixtures than the above may be used ; this is called " scientific 

 proportioning." 



Quantities of Material. — We may look upon the ideal proportions of 

 concrete as being such that there is sufficient sand present just to fill 

 the voids between the stones, while the amount of cement used slightly 

 exceeds that which is necessary just to fill the voids in the sand. The 

 excess of cement is required to coat each grain of sand and each stone, 

 so that the mass may be properly bound together. 



It may here be stated that, if the stones are unscreened or just as 

 they have come from the crusher, being of all sizes, say from \ inch 

 up to 2 inches, less sand will be required than if the stones were 

 all of uniform size. This is because the smaller stones partially fill 

 the voids between the larger ones. As a consequence, less cement also 

 is required. 



In the same way, if the sand contains a large variety of sizes of 

 grains, less cement is required than if the grains of sand are all of 

 one size. 



If the sand and cement exactly filled the voids between the stones, 

 we might expect to get the same volume of concrete as we had of stone 

 to commence with, notwithstanding that we have added sand and 

 cement. As, however, the cement and sand are in excess of the voids 

 between the stones, we always get a volume of concrete larger than 

 that of the stone with which we started. Thus to produce 1 cubic yard 

 of concrete we will require less than 1 cubic yard of broken stone. 



The following table gives the materials required to produce 1 cubic 

 yard of concrete in various proportions : — 



In the above table, a barrel which contains 376 lbs. of Portland 



