132 ENGINEERING ON THE FARM 



fine and coarse material. All of this material which will pass 

 i hrough a screen with quarter-inch openings is known as fine 

 aggregate, and that which remains on the quarter-inch 

 screen is known as coarse aggregate. 



To construct good concrete, the selection of good aggre- 

 gate is as important as the selection of good cement. A 

 satisfactory aggregate must be strong, and the individual 

 grains or pieces must be so free from dirt that the wet 

 cement can adhere to each. The size of the aggregate 

 should be such that, when thoroughly mixed, the smaller 

 particles fill in between the larger and reduce the voids to 

 a minimum. 



Coarse aggregate. Coarse aggregate should be hard, 

 strong stone, such that the strength of the concrete will 

 not be limited by the strength of the stone. Gravel, lime- 

 stone, granite, trap-rock, and the harder sandstones are 

 used. Soft, rotten rock which easily splits, scales, or breaks 

 should be avoided, as it is weaker than the cement matrix. 

 In fireproof construction where strength is not necessary 

 cinders are used. Crushed rock or gravel used as coarse 

 aggregate should be clean and graded in size from the largest 

 to the smallest, so that the percentage of voids will be at a 

 minimum, the same as in fine aggregate. The limit of 

 maximum size of coarse aggregate depends upon where the 

 concrete is to be used. In heavy walls the maximum size 

 may be as much as 3 inches in the largest diameter and 1 

 inch for thin sections and reinforced work. Where large 

 mass is required, rubble concrete is used, in which the 

 maximum diameter of stones may be 5 or 6 inches. 



Fine aggregate. Fine aggregate, which is sand or screenings 

 from crushed rock, should be hard grains, free from loam, 

 clay, vegetable matter, or dirt of any character. The word 

 "sharp" in the description of fine aggregate means that the 

 material, when rubbed in the hands, gives a sharp feeling 

 on account of containing the proper proportion of various- 

 sized, clean grains. The term should not be understood to 



