38 Concrete Silos 



if such a supply is available, it is a slow process, and 

 one requiring the employment of expensive skilled 

 labor, to work this stone into shape and lay it up in the 

 wall. 



Then, too, ordinary building stone at the best will 

 usually show a greater degree of porosity than good 

 concrete. It is easy to see why this is so. Concrete is, 

 to be sure, composed of stone possibly even a poorer 

 grade of stone than would be considered fit for ma- 

 sonry work; but in concrete each particle of stone is 

 covered with a film of cement, while in stone masonry 

 there is no such protective coating. But with a scarc- 

 ity of stone, there is always a probability that one 

 who wants to build with this material will be satisfied 

 with Less than the best, thus working into the structure 

 a material of even greater porosity, and possibly one 

 also which will disintegrate rapidly under the action 

 of the elements. Neither will stone give a sufficiently 

 smooth interior surface, making it necessary to give it 

 a plaster coat if the best results are to be secured. 



Brick is open to the same objection as stone, so far 

 as porosity is concerned. Tests made at various times 

 and places have established beyond question the fact 

 that a well made concrete has a greater density, and 

 will consequently exclude air and moisture more per- 

 fectly, than any grade of brick which would be likely 

 to be used in silo building. Brick is also inexpedient 

 for this purpose because of the fact that it usually has 

 to be hauled a considerable distance, >and on top of 

 this haulage cost is often a freight bill for shipment 

 by rail from a distant brick yard. In the case of 

 brick, too, practically all the material has to be hauled 

 in, the only local material used being a small supply 



