Concrete Mixing.

The plant used in mixing the concrete for the land tunnels was pulled down and re-erected before the concrete work in the river tunnels was begun. At the New York shaft two new bins for sand and stone were built, bringing the total capacity up to 950 cu. yd. Two No. 6 Ransome mixers, driven electrically by 30-h.p. General Electric motors, using current from the contractor's generators, were set up on a special platform in the intercepting arch.

At Manhattan the sand and stone were received from the bins in chutes at a small hopper built on the permanent upper platform of the intercepting arch. Bottom-dumping cars, divided by a partition into two portions, arranged to hold the proper quantities of sand and stone for a 4-bag batch of concrete, were run on a track on this upper platform, filled with the proper quantities of sand and stone, and then run back and dumped into the hoppers of the mixer. After mixing, the batch was run down chutes into the tunnel cars standing on the track below. The water was brought in pipes from the public supply. It was measured in barrels by a graduated scale within the barrels. The water was not put into the mixer until the sand and stone had all run out of the mixer hopper. The mixture was revolved for about 1½ min., or about 20 complete revolutions.

At Weehawken Shaft the mixing plant was entirely rebuilt. Four large bins, two for sand and two for stone, were built in the shaft. Together, they held 430 cu. yd. of stone and 400 cu. yd. of sand. The sand and stone were dumped directly into the bins from the cars on the trestle which ran from the wharf to the shaft. The materials were run through chutes directly from the bins to the hoppers of the mixers, where they were measured. Two No. 6 Ransome mixers, electrically driven, were used here, as at New York, and, as there, the water was led into measuring tanks before being let into the mixer.

The quantity of water used in the various parts of the concrete cross-section, for a 4-bag batch consisting of 1 bbl. (380 lb.) of cement, 8.75 cu. ft. of sand, and 17.5 cu. ft. of stone, is given in [Table 31.]

TABLE 31.— Quantity of Water per 4-Bag Batch of Concrete, in U.S. Gallons.

Portion of cross-section.	Maximum.	Minimum.	Average.
Invert	40	20	26
Duct bench	36	21	27
Arch (excluding key)	37	19	25
Key of arch	27	15	20
Face of bench	31	22	27

The maximum quantities were used when the stone was dry and contained more than the usual proportion of fine material, the minimum quantity when the sand was wet after rain.

The resulting volumes of one batch, for various kinds of stone, are given in [Table 32].

TABLE 32.— Volume of Concrete per Batch, with Various Kinds of Stone.

Mixture.	Description of Stone.		Resulting volume per barrel of cement, in cubic yards.	Remarks.
Mixture.	Passed screen.	Retained on screen.	Resulting volume per barrel of cement, in cubic yards.	Remarks.
1 : 2½ : 5	1½-in.	⅜-in.	0.815	Measured in air
1 : 2½ : 5	2½-in.	Run of crusher.	0.827	Measured in air.
1 : 2½ : 5		General average.	0.808 [D]	Measured from plan.
1 : 2½ : 5	2-in.	1½-in.	0.768 [E]	Measured from plan.

[D] Average for whole of River Tunnel section.

[E] Average from 7,400 cu. yd. in Land Tunnel section.

The sand used was practically the same for the whole of the river tunnel section, and was supposed to be equal to "Cow Bay" sand. The result of the mechanical analysis of the sand is shown on[ Plate XLVI]. The stone was all trap rock. For the early part of the work it consisted of stone which would pass a 2-in. ring and be retained on a 1½-in. ring, in fact, the same as used for the land tunnels. This was found to be too coarse, and for a time it was mixed with an equal quantity of fine gravel or fine crushed stone. As soon as it could be arranged, run-of-crusher stone was used, everything larger than 2½ in. being excluded. About three-quarters of the river tunnel concrete was put in with run-of-crusher stone. The force was:

At Manhattan.

1	Foreman	@	$3.00	per	shift
4	Men on sand and stone cars	"	1.75	"	"
4	Men handling cement	"	1.75	"	"
2	Men dumping mixers	"	1.75	"	"

At Weehawken.

1	Foreman	@	$3.00	per	shift
2	Men hauling cement	"	1.75	"	"
2	Men dumping mixers	"	1.75	"	"

The average quantity of concrete mixed per 10-hour shift was about 117 batches, or about 90 cu. yd. The maximum output of one of the mixers was about 168 batches, or 129 cu. yd. per 10-hour shift.