10 



BULLETIN 555, U. S. DEPARTMENT OF AGRICULTURE, 



Physical prop- 

 erties. 



Grading. 



Method of sam- 

 pling. 



Methods of 

 testing. 



The stone shall meet the following requirement : 



Per cent of wear not more than 



(or French coefficient not less than ) 



The broken stone, when tested by means of laboratory screens, 

 shall meet the following requirements : 



Per cent. 



Passing inch screen 100 



Passing * inch screen and retained 



on inch screen to 



Passing inch screen not more than 



Stone shall be sampled for quality and grading in accordance 



with the method described on page , paragraphs , 



of these specifications. 



Tests of the physical properties and grading of the stone shall 

 be made in accordance with the methods described or referred to 

 on pages of these specifications, tests Nos. 



SPECIFICATION FOR STONE OR GRAVEL SCREENINGS FOR FINE AGGREGATE 

 FOR PORTLAND-CEMENT CONCRETE BASE. 



General. 



Physical prop- 

 erties. 



Grading. 



M o r t a 

 strength tests. 



Method of sam- 

 pling. 



The screenings shall consist of material obtained by crushing 

 hard, durable rock or gravel, all of which shall pass a revolving 

 screen having circular openings three-eighths of an inch in diam- 

 eter. They shall be free from lumps of clay and at the time of 

 use from lumps or crusts of hardened material. 



If the screenings are produced from rock, the rock shall meet 

 the following requirement : 



Per cent of wear not more than 



(or French coefficient not less than ) 



The screenings 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 J-inch screen not less than 



Passing 20-mesh and retained on 50-mesh 



sieve to 



Passing 100-mesh sieve not more than 



When that portion of the screenings which passes a one-quarter 

 inch laboratory screen is mixed with Portland cement in the 

 proportions of 1 part of cement to 3 parts of screenings, 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 f per cent of the strength de- 

 veloped in the same time by mortar of the same proportions 

 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. 



Stone shall be sampled for quality in accordance with the 

 method described on page , paragraphs , of these 



* It is recommended that a %-inch screen be specified here. 

 t It is recommended that the minimum strength ratio be 90 per cent 

 for this purpose. 



