DEAIISrAGE METHODS FOR COUNTY EOADS, 75 



In purchasing sand and coarse aggregate for concrete the unit of 

 measure is quite frequently the ton instead of the cubic yard. After 

 the quantities have been estimated from the preceding table, there- 

 fore, it may be necessary to convert cubic yards into tons before the 

 cost can be estimated. This can not be done accurately without 

 some definite laiowledge as to the relative heaviness of the materials 

 it is purposed to use, but the following list of average weights will 

 afford a means of making an approximate conversion. In using this 

 list it should be remembered that the extreme- variations in weights 

 of crushed stone may be as much as 10 per cent in either direction 

 f rorii those shown. 



Approximate average toeights per cubic yard of various concrete materials. 



Pounds. 



Standard Ottawa sand 2, 500 



Average clean bank 2, 600 



Well-graded clean bank sand 2,900 



Average washed gravel 2, 700 



Well-graded washed gravel 3, 000 



Crushed limestone screened between i and 2 inches 2, 500 



Crushed sandstone screened between | and 2 inches 2, 400 



Crushed granite screened between 4 and 2 inches 2, 700 



Crushed trap screened between i and 2 inches 2, 800 



The cost of quarrying and crushing stone has been discussed in 

 connection with crushed-stone foundations, and need not be discussed 

 further here except to state that stone for use as concrete aggregate 

 must be screened more carefully than when intended merely for use 

 in a crushed-stone foundation. The additional cost occasioned by 

 this extra care should not exceed about 5 or 6 cents per ton where 

 adequate machinery is employed. 



In discussing gravel foundations some information as to the cost 

 of screening has been included. In a well-arranged plant the cost of 

 washing and screening should scarcely exceed the average cost for 

 screening alone. Large producers of washed gravel ordinarily mar- 

 ket their product at from 30 to 40 cents per ton f. o. b. at the plant. 

 Where the supply of gravel is limited, or the demand is small, the 

 price may be increased greatly ; sometimes to more than twice what is 

 given above. The cost of sand at the pit averages about the same 

 as that of gravel. 



The cost of mixing and placing concrete may be estimated from 

 the labor required to operate an ordinary street-paving concrete 

 mixer, considered together with the mixer's capacity. There are two 

 sizes of such mixers in common use. The larger size has a batch 

 capacity of about one-half cubic yard and requires about 35 laborers 

 of all classes for its efficient operation, while the batch capacity of the 

 smaller size is about one-third cubic yard and the number of laborers 



