2 BULLETIN 704^ U. S. DEPARTMENT OF AGRICULTURE. 



number of standard sizes necessary for the various tyjDes of roads in 

 which broken stone is used. It was found that, within a ver^^ re- 

 stricted area, revolving screens, with as many as 28 different size 

 openings, were being used to produce broken stone for a compara- 

 tively few types of roads. As a rule, the exact size of product for 

 any one type of road is selected by the individual engineer without 

 reference to other products which the crushing plant may be called 

 upon to supply, also without reference to what size or sizes the en- 

 gineer himself will select for other types of roads. Moreover, great 

 lack of uniformity in the method of specifying sizes of broken stone 

 was found to exist so that the entire subject of broken stone specifi- 

 cations is at present in an almost chaotic condition. 



Following the very logical recommendations of the First Confer- 

 ence of State Highway Testing Engineers and Chemists,^ the speci- 

 fications for sizes of broken stone, as given in this bulletin, are based 

 upon laboratory screen tests, rather than upon the size of openings 

 in the commercial screen which produces the material. Based upon 

 data secured in the survey previously mentioned, a limited number of 

 sizes has been selected. It is believed that for the commercial produc- 

 tion of each size sufficient tolerance has been allowed in the specifi- 

 cations to make practicable the use of revolving screens with the 

 same openings as the maximum and minimum laboratory screens 

 specified for that size. In most cases, the amount passing an inter- 

 mediate laboratory screen has also been specified with limits (from 

 25 to 75 per cent) sufficiently wide to include all acceptable commer- 

 cial products between the maximum and minimum size but reject 

 most products which would be unacceptable. 



The different size broken stone products which may be used satis- 

 factorily in constructing various types of road are given in Table I. 

 It will be seen that not more than five different size openings in re- 

 volving screens are required for any one type. In order to meet 

 reasonable working conditions, a general tolerance of not less than 5 

 per cent retained on the maximum screen, and 15 per cent passing 

 the minimum screen, has been allowed. In the case of screenings, 

 when only the maximum size revolving screen would be required 

 ordinarily, as the product would include dust and all other smaller 

 material, from 40 to 80 per cent of material passing the |-inch screen 

 is specified. The only other exception is the broken stone for Topeka 

 type bituminous concrete, where at least 20 per cent of material is 

 required to be retained on the |-inch screen. 



It is realized that just as satisfactory roads may be constructed 

 with sizes of broken stone varying considerably from these specifi- 

 cations. It is believed, however, that the sizes here covered repre- 

 sent average practice and they are recommended for general use as 



