VOIDS. 



Voids vary greatly with the humidity and method of 

 filling. For detailed discussions, see Marsh, and Taylor 

 & Thompson. It is advisable to test the various sands 

 to find which one has the greatest compactness. The 

 amount of cement and sand to make a unit volume of 

 mortar should also be found. Mortar dropped from a 

 trowel should leave it perfectly clean; it should be easily 

 molded into a ball and if dropped 20 in. should retain its 

 rounded shape without cracking. 



M. Feret determined that: 



(1) The compactness of a concrete increases with the 

 size of stone. 



(2) It varies inversely as the proportion of the fine 

 grains in the mortar. 



(3) It decreases generally as the proportion of the 

 mortar to the same volume of stone increases. Table 

 page 5. 



The following table was made by Mr. Baker at Water- 

 town Arsenal on 12 x 12 x 12 in. cubes: 



i:< Uitive to Strengths of Concretes Tested 600 Days 

 After Moulding. 



