4 REINFORCED CONCRETE CONSTRUCTION 



openings less than 1/10 in. in diameter; while for larger openings 

 sheet brass is used, having circular openings drilled to the 

 required dimensions. The woven brass wire sieves are given 

 commercial numbers which approximately coincide with the 

 number of meshes to the linear inch. The actual size of hole, 

 however, varies with the gauge of wire used by different manu- 

 facturers and every set of sieves must be calibrated separately. 

 A common defect in sieves is the displacing of the wires so that 

 they are not perpendicular to each other; such sieves should be 

 discarded. Sieves are made to fit together in nests, so that when 

 a sample of sand is placed in the upper (or coarsest) sieve and 

 the nest of sieves is thoroughly shaken, the quantity caught on 

 each sieve can be determined at once. For analyzing sand the 

 following sizes 1 are desirable: 



Commercial No...' 10 20 30 40 50 80 100 200 



Approximate size of hole in 



inches 0.073 .034 .022 .015 .011 .007 .0055 .0026 



A screen with 1/4-in. openings is generally employed for separat- 

 ing out large material from sand. 



Specifications should limit the maximum amount of loam or 

 clay to be allowed in any given work. Loam should never be 

 permitted, but clay to the amount of 5 to 10 per cent, if evenly 

 divided, is often beneficial in lean mortars. In rich mortars the 

 strength and density is decreased by even slight additions of 

 clay; but in lean mortars the clay helps to fill the voids of the 

 sand, and causes the cementing material to coat the grains 

 better and to bind them together more strongly. 



Broken stone screenings have a small percentage of voids and, 

 when free from clay, usually make excellent sand. These 

 screenings ordinarily give a stronger mortar than natural sand 

 but are likely to contain an undue amount of dust, especially 

 when obtained from soft stone; in such a case the mass should 

 be screened before being used in mixing mortar. Gravel screen- 

 ings also constitute a good material in place of sand. All 

 material passing a 1/4-in. screen is generally considered as sand, 

 or fine aggregate; while all material larger than this size is 

 classed as coarse aggregate. 



4. Stone. For the coarse aggregate, either crushed stone or 

 gravel is generally used. Any stone is suitable which is clean 

 and durable and which has sufficient strength to prevent the 

 strength of the concrete from being limited by the strength of 



1 From American Civil Engineers' Pocket Book, 1st edition, page 415. 



