CONCRETE 5 



the stone. Traps, granites, limestones, and the more compact 

 sandstones are generally employed. Aggregates containing 

 soft, flat, or elongated particles should never be used. 



All that has been said concerning voids in sand applies with 

 equal force to the coarse aggregate. Fig. 2 illustrates the analy- 

 sis of a bank gravel and of a crushed stone. Screens varying by 

 a quarter of an inch from 1/4 in. up are desirable, but a very 

 useful analysis may be made with fewer screens. A uniform 

 size of stone filled with mortar does not make as dense or as 



Q. 



100 



025 0.50 0.75 100 I.E5 1.50 



Diameter of- Particle in Inches. 

 FIG. 2. Typical mechanical analyses of bank gravel and crushed stone. 



strong a concrete as one in which the coarse aggregate is well 

 graded that is, where the small stones partly fill the larger 

 interstices. A straight line on a mechanical-analysis diagram 

 indicates a uniform grading of size. 



Other things being equal, the larger the stone, the stronger 

 and denser the concrete. Experience has shown that for rein- 

 forced concrete the maximum size should not be more than about 

 1 in. to 1 1/2 in., in order that the concrete may fit itself closely 

 around the reinforcing metal. The smaller the stone, the greater 

 the surface to be coated, and the greater the amount of cement 

 required. 



Most gravels are sufficiently durable for use in concrete. They 

 should be at least reasonably clean, although a quantity of finely 

 divided clay equal to 5 to 10 per cent of the gravel may add to 



