76 BULLETIN 124, U. S. DEPARTJVtEXT OF AGRICULTURE^ 
usually employed in operating it is about 30.- The hourly output of 
the larger size should average about 18 cubic yards of concrete and 
of the smaller size about 12.5 cubic yards. Assuming the cost of 
labor at 20 cents per hour and making a reasonable allowance for 
depreciation of equipment and supervision, the cost of mixing and 
placing the concrete may usually be estimated at from 60 to 80 cents 
per cubic yard* 
NOTES ON SPECIFICATIONS. 
The following notes on specifications are given as typifying what 
is believed to be the best' practice so far developed in connection with 
the various features of highway improvement that have been dis- 
cussed in the preceding pages. These notes are given with a view to 
making the information contained in the text readily applicable to 
contract work. In order to. avoid repetition the notes on materials 
are separated from those on the methods of construction. 
MATERIALS. 
Cement. — The cement for use in this work shall meet the requirements of 
the standard specifications for Portland cement of the American Society for 
Testing Materials, adopted in 1904 and revised in 1916, serial designation 
C9-17. 
Provisions are made in these specifications for testing and storing the cement. 
Under certain conditions, if the cement meets the 7-day tests satisfactorily, it 
need not be held until the results of the 28-day tests are available. 
Fine aggregates: Sand. (See Bulletin 555, p. 19.) 
Stone or gravel screenings. (See Bulletin 555, p. 10.) 
Coarse aggregates : Broken stone or gravel. ( See Bulletin 555, p. 9. ) 
Broken slag. (See Bulletin 555, p. 15.) 
Cushion or bedding: Sand. (See Bulletin 555, p. 21.) 
Granulated slag. (See Bulletin 555, p. 15.) 
Grouting sand. (See Bulletin 555, p. 20.) 
Water. — The water used in the mixing of concrete or grout shall be free 
from oil, acid, alkali, or vegetable matter, and fairly free from clay or silt. 
Cobblestones. — Cobble paving shall be constructed of bank or river cobbles, 
sound in all respects and fairly symmetrical in shape. No stone shall be 
used which is less than 6 inches or more than 10 inches in its longest dimension 
or which does not have one lesser dimension at right angles to the length of 
at least 4 inches. If preferred by the contractor, quarried stones which, in the 
judgment of the engineer, are equal to the cobbles above specified, may be sub- 
stituted for the cobbles, provided that the limiting dimensions of the individual 
blocks of quarried stone shall be the same as those specified for cobbles. 
CLAY FARM DRAIN TILE. 
NOT EXCEEDING 12-INCH DIAMETER. 
[Copied largely from A. S. T. M. Standard Specifications C4-16.] 
The tile shall be made from shale, fire clay, or surface clay by such method 
of manufacture as will insure excellence of product and uniformity in quality. 
The presence in the tile of visible grains or masses of caustic lime, iron 
