14 



BULLETIN 704, U. S. DEPARTMENT OF AGEICULTURE. 



construction specifications will be based \ipon a fixed proportion of 

 cement to combined coarse and fine aggregate and that the rela- 

 tive amounts of broken stone and sand will be varied as directed 

 by the engineer so as to approximate definite proportions of coarse 

 and fine aggregate. Thus the concrete should be composed of 1 

 part of Portland cement and 4J parts of fine- and coarse aggregate, 

 each measured separtely and accurately by volume. The concrete 

 should approximate a 1 : 1* : 3 mix, but the engineer should vary 

 the relative proportions of fine and coarse aggregate, depending 

 upon their exact grading so as to obtain a concrete of maximum 

 density. 



The concrete should be mixed in a batch mixer for not less than 

 1^ minutes and the water should be reasonably clear, free from 

 oil, acid, alkali, or vegetable substance, and neither brackish nor 

 salt. Sufficient water should be used to produce a concrete which 

 ^^^ll flatten out and quake when deposited in place, but not enough 

 to cause it to flow or to separate mortar and coarse aggregate. 



SPECIFICATION RC-2, 



Broken Stone or Gravel, and Sand for Portland Cement Concrete Base. 

 BROKEN STONE OR GRAVEL FOR COARSE AGGREGATE. 



General. 



Grading. 



The broken stone or gravel for coarse aggregate shall consist of clean, 

 hard, toiigh, durable stone. It shall be free from soft, thin, elongated, 

 or laminated pieces, disintegrated stone, vegetable, or other deleterious 

 matter. 



When tested by means of laboratory screens the coarse aggregate 

 shall meet the following requirements : 



Per cent. 



Passing 3-inch screen, not less than 95 



Total passing IJ-inch screen 40 to 75 



Retained on J-inch screen, not less than 85 



SAND FOR FINE AGGREGATE. 



General. '^^^ sand shall consist of clean, hard, durable, uncoated stone particles, 



preferably siliceous, free from lumps of clay, soft or flakey particles, loam, 

 and all organic matter. 



Gradlns ^* shall be well graded from coarse to fine, and when tested by means 



of laboratory screens and sieves shall meet the following requirements : 



Per cent. 



Passing 5-inch screen 100 



Total passing 20-mesh sieve, not less than 20 



Total passing 50 mesh sieve, not more than 50 



Passing 100 mesh sieve, not more than 10 



Not more than 5 per cent by weight shall be removed by the elutriation 

 test. 

 Mortar Stren<rth ^^en the sand is mixed with Portland cement in the proportion of 1 

 Tegt. part of cement to 3 parts of sand, by weight, according to standard 



methods of making 1 : 3 mortar briquets, the resulting mortar at the 

 age of 7 and 28 days shall have a tensile strength of at least 75 per cent 

 of that developed in the same time by mortar of the same proportions 

 and consistency, made of the same cement and Ottawa sand. 



Preliminary acceptance samples shall be subjected to both 7 and 28 

 day tests, and acceptance based thereupon. Samples tested during the 

 progress of the work will be accepted on the basis of the 7-day test. 



