412 



Missouri Agriculhiial lU pori. 



sand aud stone, and proportion the mixture to perfectly fill them. 

 An easy method of determining the amount of voids is to fill a vessel 

 of known A^olnme with the material under examination, then pour in 

 water till the vessel is full, taking note of the amount of water. The 

 volume of Avater, divided by the volume of the material, will give the 

 percentage of voids. 



Materials for 1 cubic yard of concrete, in place; sand voids, 35 per cent; rock voids, 45 per 



cent. 



(Weiglit of concrete is about 150 pounds per cubic foot.) 



Table of safe worldng strengtli of Portland cement concrete in direct compression (factor J). 



(For a vibrating or pounding load take one-half; for tensile strength, use one-tenth; for 

 shearing value assume 60 pounds per square inch). 



Sand and Mortar — The sand should be reasonably clean, sharp 

 (angular, not round and smooth) and coarse. Any foreign substance 

 of an organic or decaying nature is detrimental beyond a doubt. Any 

 loam or matter adhering to the particles of sand will prevent the 

 close contact and adhesion of the cement as well as the ditferent par- 

 ticles of sand. Also more cement is necessary to produce a mortar of 

 a given strength with fine than with coarse sand. 



The weight of good mortar sand varies with the amount of water 

 it carries, but is usually considered as about 2,500 pounds per cubic 

 yard. The voids are 30 to 40 per cent, depending upon the character 

 of the sand. 



Aggregate — Rock and gravel are the most commonly used materials 

 for the aggregate. There is little choice between a good hard rock 

 and a good clean gravel. Chats (tailings from the lead and zinc 

 mines) is an excellent concrete material. For floors of buildings 

 and where little weight is desired, cinder concrete is sometimes used. 

 The strength of cinder concrete is about one-half that of rock or 

 gravel concrete. A general order for the relative strength of con- 



