PLAIN CONCRETE 231 



AGGREGATES OTHER THAN SAND 



The aggregates or broken stone used in concrete work 

 should possess three qualities: (1) They should be hard 

 and strong, so as to resist crushing and shearing or transverse 

 stresses; (2) they should have surface texture that will per- 

 mit the cement mortar to adhere to their surfaces; and 

 (3) where the concrete is to be used for building construc- 

 tion, such as in reinforced-concrete work, and for fireproofing, 

 they should possess refractory, or fire-resisting, qualities. 

 Usually, aggregates that break in such a way as to allow the 

 smallest spaces, or interstices, between the particles, will 

 make the strongest concrete for construction purposes 

 because the voids can be most economically filled with 

 cement mortar. 



Size of Aggregates. In measuring broken stone, the size 

 of the stone is determined by the size of the ring through 

 which it will pass. For instance, a 2-in. stone is one that 

 will pass through a ring, or hole, that is 2 in. in diameter. 



The broken stone used in concrete work varies in size with 

 the nature of the work. For foundation and mass con- 

 struction, it is the custom to use broken stone of a size that 

 will pass through the 2- or 2$-m. ring. For filling the 

 spandrels of bridges or the spaces between walls, where 

 mere mass is desired, broken stone of a much larger size is 

 used. 



In reinforced-concrete work, the broken stone must be 

 small, owing to the narrow spaces in the forms. For columns 

 and wall work, stone that will pass through a 1- or i-in. ring 

 is suitable, while for filling the beam and girder forms, where 

 numerous reinforcing rods occur, the broken stone is some- 

 times so small as to pass through a i-in. ring. 



The latest practice in making concrete is to use stone as 

 it comes from the crusher, without screening it. While 

 such stone, termed the run of crusher, contains broken stone 

 of a size specified, it also has smaller particles of stone and 

 such stone dust as is carried along with the broken stone 

 from the crusher. Where the run of crusher is used, the 

 proportion of the cement and sand must be changed, because 

 the stone dust takes the place of some of the sand. In 



