DRAINAGE METHODS FOE 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 knowledge 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 
from those shown. 
Approximate average iveiglits 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 \ and 2 inches 2, 700 
• Crushed trap screened between \ 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 suppty 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 vard and the number of laborers 
