230 PLAIN CONCRETE 



PLAIN CONCRETE 



MATERIALS USED IN CONCRETE 



DEFINITIONS AND TERMS 



Concrete is usually made of cement, sand, and broken 

 stone. The cement in a plastic state, either by itself or with 

 the sand that is generally mixed with it, is called the matrix, 

 and the broken stone, gravel, or other material used as a 

 filler is called the aggregate. The sand is correctly classed as 

 a part of the aggregate, although some engineers include it 

 with the matrix. The aggregate is used to cheapen con- 

 crete. Pure, or neat, cement, when wet with water, would 

 in a way fulfil all the physical requirements of concrete, but 

 it would be too expensive. 



In the concrete of today, hydraulic cement is used almost 

 exclusively. For this reason, the term concrete, as com- 

 monly used, refers only to that variety. In specifying any 

 other kind of concrete, the usual custom is to mention it 

 by its full name, as bituminous concrete, lime concrete, etc. 

 Such varieties, however, are of comparatively little impor- 

 tance. 



The term concrete, besides being restricted to hydraulic- 

 cement concrete, has another restriction: the aggregate must 

 not be sand alone, although it may be partly sand. A mix- 

 ture of hydraulic cement, sand, and water is called by the 

 special name of mortar. 



Concrete is usually named from the kind of aggregate used . 

 For example, stone concrete embodies the use of broken 

 stone or coarse pebbles, while in cinder concrete, the aggre- 

 gate consists of cinders or broken slag. 



The proportion of cement and sand to the broken stone 

 depends on the spaces between the stones, which are known 

 as voids. In all instances, there must be sufficient mortar 

 to fill the voids entirely and to cover all surfaces of the 

 separate stones. 



