STONE AND CEMENT CONSTRUCTION. 



487 



1. Sand constitutes about one-third to one-half of the material 

 in making concrete. 



8. Sand is the small grains that will pass through a one- fourth 

 inch mesh. 



9. Gravel and pebbles remain upon a quarter inch mesh. 



10. A 40-mesh screen is a screen with 40 holes to the lineal inch. 



11. Sand that will pass through a 40-mesh screen is generally unfit 

 for concrete work. 



12. In using a screen it should be placed upright at an angle of 

 45 degrees. 



Proportions for Mixing. 



Four proportions have been arbitrarily made by some au- 

 thorities and are given here as a guide to the selection of mate- 

 rials for different classes of work. The following are given as 

 such proportions: 



1. Rich 1:1:3 for columns, and structural parts for carrying 

 heavy weights. 



2. Standard 1 :2 :4 for floors, beams, tanks, sewers and for columns 

 requiring reinforcing. 



3. Medium 1:2^:5 for walls, piers, sidewalks. 



4. Lean 1:3:6 for heavy masses. 



One part of cement, two parts of sand and four parts of 

 gravel are known as a i : 2 : 4 mixture. Concrete materials are 

 mixed in this stated proportion when used in^small amounts, but 

 in large structures it becomes necessary to determine approx- 

 imately the amount of each different kind of material used in a 

 cubic foot of concrete. 



TABLE I. 



QUANTITIES OF MATERIALS AND AMOUNT OF CONCRETE FORMED BY 

 MIXING WITH A TWO-BAG BATCH OF CEMENT. 



