.9 bbl. cement at $2.46 (including $0.59 per bbl. storage)................. $2.214

.28 cu. yd. shell, at $0.45 per cu. yd.......128

.47 " sand, at 0.12 " " ......056

.80 " stone, at 2.95 " " ..... 2.360

Total materials.......... $4.758

mixing and placing............. $0.623

forms....................370

Total.............. .993

Total Cost per cubic yard ...... $5.751

349. Defenses San Francisco, Cat3

Defenses San Francisco, Cat3 — Cubical mixer; materials drawn from bins into measuring cars, hoisted by elevator and dumped into hopper of mixer. Mixer given twelve to fourteen turns and concrete dumped into cars, pushed by hand out on trestles, and dumped in place. Average capacity plant, 280 cubic yards per day of eight hours. Itemized Cost of 8,328 cubic yards of concrete in place was as follows:

1 Report Lieut. Robert P. Johnson, Corps of Engineers, U. S. A., Report Chief of Engineers, 1899, p. 906.

2 Report Lieut. Frank C. Boggs, Corps of Engineers, U. S. A., Report Chief of Engineers, 1900, p. 931.

3 Maj. Charles E. L. B. Davis, Corps of Engineers, in charge. Report Chief of Engineers, 1900, p. 980.

1 Capt. George A. Zinn, Corps of Engineers, U. S. A., in charge. Report Chief of Engineers, 1900, p. 3467.

.758 bbl. Portland cement, at $3.03 per bbl . . . $2.298

.887 cu. yd. rock, at $1.80 per cu. yd...... 1.597

.41 cu. yd. sand, at 0.73 per cu. yd....... .299

water................... .016

Cost of materials......... $4.210

Concrete plant, erection per cu. yd. concrete . . $0.269

Concrete plant, running expenses per cu. yd. . . .022

Concrete plant, taking down......... .020

Cost for plant, exclusive of purchase price............. .311

forms — materials............. $0.272

forms — labor in erecting.......... .346

forms — labor taking down......... .079

Cost forms............ .697

Labor mixing, placing and ramming...... .626

Total Cost per cubic yard...... $5.844

350. In the building of a concrete dam for the enlargement of the head of the Louisville and Portland Canal,1 comparison of Cost of hand and machine mixing is given by Asst. Engr. j. H. Casey.

1.63 bbls. natural cement, at $0.635 per bbl. . . $1.034 2 volumes sand, .47 cu. yd., at .87 per cu. yd. . . .408 5 volumes broken stone, .89, cu. yd. at $.84 . . .756

Cost of testing cement............ .081

forms, material for............ .107

forms, labor making and setting up...... .168

Cost materials and forms...... $2.554

Hand Mixed Concrete

Cost of mixing............... $1.917

Cost of placing and tamping......... .791

Cost of mixing and placing.......... $2.708

Total Cost hand mixed concrete per cu. yd. in place.......... $5.262

Machine Mixed Concrete

Charging and running mixer . ........ $0.864

Placing and tamping............. .585

Cost mixing and placing........... $1.449

Total Cost machine mixed concrete per cu. yd. in place........ $4.003

Difference in favor machine mixing...... $1.26

Since the above concrete was placed in large masses, the costs of labor are considered high, and it is probable the work was done with exceptional care.

351. In the construction of the lock at the Cascades Canal1 the concrete plant was so arranged that the materials did not have to be elevated, but much of the work of transportation was done by gravity. The mixing of about eighteen hundred yards by hand permits a comparison to be.made with machine mixing by which method about seven thousand eight hundred yards were made. The costs were as follows:

352. In the construction of the retaining walls for the Chicago Drainage Canal,2 a special plant was designed for the work on account of the large quantities of concrete required, and this, combined with the low Cost of materials and the character of the work, resulted in a very low Cost concrete. On Section 14 the stone was selected from the spoil banks along the canal and could usually be obtained within one hundred feet. This stone, which was delivered, to the crusher by wheelbarrows, required some sledging to reduce it to crusher size. An Austin jaw crusher was mounted on a flat car with the Sooysmith mixer. "The cement, sand and stone were raised from their respective bins by means of belt conveyors running at the same rate of speed, but carrying buckets spaced proportional to the required ingredients." "The Cost of a second hand plant used on this section was estimated at $9,600, including two crushers and two mixers at $1,500 for each machine. common labor Cost $1.50 per day; firemen, enginemen, and carpenters from $2.00 to $3.00 per day. The itemized cost is as follows:

1 Maj. Thomas H. Handbury, Corps of Engineers, U. S .A., in charge. Report Lieut. Edward Burr, Report Chief of Engineers, 1891, Vol. v. Abstracted, Engineering News, June 2, 1892.

2 " Construction of Retaining Walls for the Sanitary District of Chicago," by Mr. James W. Beardsley, and discussion by Mr. Charles L. Harrison. Jour. W. Soc. Engrs., Dec, 1898.

The amount of concrete used on this section was 23,568 cu. yds.

353. On Section 15 of the same work the conditions were somewhat different. The stone had to be quarried within about a thousand feet of the crusher. The stone, after being broken to crusher size, was delivered on the tipping platform of the No. 7 Gates crusher in cars drawn by a cable hoist. "The average output of the crusher for a day of ten hours was about 210 cubic yards." The materials were transported to the mixer in four and one-half yard dump cars drawn by a light locomotive. The mixer was of the spiral screw type and deposited the materials on a rubber belt conveyor. The mixer and operating machinery were mounted on a car which propelled itself by means of rope and winch. The plant for this section was new and estimated to Cost $25,420, including $12,000 for one crusher.