FARM STRUCTURES 



411 



ground and should set or harden neither too quickly nor too 

 slowly. It should show a high tensile strength when hard- 

 ened and sufficiently aged. It should not check, crack, or 

 crumble upon hardening. Where cement is to be used in 

 considerable quantities it should be carefully tested by 

 standard tests. 



Sands. Sand should be clean, durable, coarse, and free 

 from vegetable and other foreign matter. Coarse sand is 

 preferable to fine sand because the percentage of voids or 

 open space between the sand grains is less. 



Concrete. In a general way concrete consists of mortar 

 in which there is imbedded more or less coarse material, like 



C£M£NT 



*SjA//> 



JTO/Vf 



COA/C/ffTf 



Fig. 261. Material required to make concrete to the proportion of 

 1 part of cement, 2 parts of sand, and 4 parts of broken stone. 



gravel or broken stone, called the aggregate. Thus it is seen 

 that if the aggregate be good, durable material and the mortar 

 be sufficient in quantity to surround all of the aggregate, the 

 whole will be as strong as the mortar. In preparing concrete, 

 therefore, it is desirable to obtain as dense a mixture as is 

 practical. 



The mixtures indicated in the following table are in com- 

 mon use, and the amount of material required to- make a 

 cubic yard of concrete in each case is also given. 



A rich mixture is used for beams, columns, and water-tight 

 constructions. 



