Another system of delivering concrete deserves mention, not because it has been applied to any large mass work such as a dam, but because it seems to possess possibilities for development in that direction. Briefly it is the mixing of concrete and forcing it through pipes by means of compressed air at one operation. Fig. 34 shows the hopper with plunger actuated top door and an 8-in. diameter delivery pipe leading from the bottom. A 3/4-in. air inlet is near the top of the hopper and a 2-in. air inlet enters the elbow in the delivery pipe, pointed in the direction of the flow. The materials are dumped in any order, into the hopper from measuring hoppers, the door is closed and the air is applied. With 1/4-cu. yd. and 1/2-cu. yd. batches (the present development of the method) the forcing of the materials from the hopper into the pipe and through several hundred feet of pipe results in the discharge of a satisfactory mixture. To date, this method has been used in tunnel work up to something over 1000 ft. in length of delivery pipe, and in ordinary building work it has pushed concrete up 80 ft. vertically. Four 1/2-cu. yd. batches per minute have been delivered for limited periods of time, and 300 cu. yd. per ten hours is a common and easily maintained rate of progress. Two or more hoppers may be connected to one delivery pipe, resulting in practically a continuous issuing stream. The compressor capacity advised for a 1/2-cu. yd. machine operating at 30 cu. yd. per hour, with moderate distances, and few elbows in delivery pipe, is 500 cu. ft. of free air per minute raised to 80 lb. pressure. For maintaining the same rate at distances over 500 ft. add about 1cu. ft. of air for each foot of distance; also add about 2 1/2 ft. of air for each foot of elevation to be overcome. Moderate receiver capacity should be provided, and the pressure will fall during the delivery of a batch from So lb. to 50 lb. or 60 lb. The 8-in. delivery pipe may be of thin steel weighing about 250 lb. per 20-ft. length, and the joints may include special gaskets permitting considerable flexibility. The machines are installed upon a royalty basis which naturally varies with the situation and the saving which may be made over any other system of delivery. In other words the charge is "what the traffic will bear".
Fig. 34. Hopper for pneumatic mixing and delivery of concrete.
Naturally the greatest advantage of the system lies in its application to a situation difficult of access by any other system, such as in a tunnel. As it accomplishes mixing, elevating (within limits) and horizontal transportation in one simple operation, and as the delivery pipe can easily cover a wide range of work that would require many elevators and chute equipments, this system should find a wide field for application. However, its advisability in connection with a large masonry dam would be a matter for some study. Thus, compared with the system of chutes and conveyors at Lake Spaulding the delivery pipe would be very much cheaper to construct and easier to shift and raise. It would require, however, say three times the power and much greater compressor capacity. The same rate of progress as at Lake Spaulding would require for pneumatic delivery some 2500 cu. ft. of free air per minute, which would mean 350 h.p. or more according to elevation above sea level.
Various devices are used for transporting some of the materials which enter the masonry. Attention will be briefly called to them and their possibilities.
At the Roosevelt dam, cement and sand were brought to the dam by a Leschen tramway. (See page 139.) The buckets averaged 8 cu. ft. of sand and about 5.2 cu. ft. of cement. The labor cost per cu. yd. transported was about $0.175 when supplying cement and sand for the maximum masonry progress, and about $0.243 when at two-thirds maximum masonry progress. Obviously the distance has a very slight effect upon the cost of transportation by such a method. It could have been ten times as far at practically the same cost.
At the Big Creek hydro-electric development (see page 169) pit sand and gravel traveled 154 ft. on a belt conveyor up a 35-deg. slope. At the Arrowrock dam, cement is being transported 1500 ft. by blowing it with compressed air through a 4-in. diameter pipe. The cement drops into the pipe through the riser of a 4-in. Tee into one end of which the air is introduced through a 3/4-in. pipe. Several 3/4-in. air nozzles are inserted along the line as boosters. The system is said to be entirely satisfactory.
Although in the present work the subject of excavation for the foundation of a dam is only incidentally mentioned, it should not be lost sight of in figuring upon the plant required for the masonry.
The amount of excavation may be enough so that a saving of a few cents per cu. yd. may materially reduce the plant account chargeable to masonry if the same plant is adapted to both operations. In general, it may be said that in a situation where a system of tracks is advisable for delivery of the materials for the masonry it will generally be found that the excavation can be disposed of immediately upstream or downstream from the trench.
Where cableways are indicated for handling the masonry it usually means a valley so narrow that the excavation must be disposed of in a dump paralleling the stream. In such case the cableways are probably the best means of conveying the material longitudinally to that station where it must be transferred to cars for lateral travel to or along such dump. It should be remembered that at the transfer station storage should be provided, as by bins or hoppers, into which the cableways may dump, and under which the cars may run. This is in order that one will not have to wait on the other, that coordination of functions may not require the wasteful coordination in point of time which was pointed out in discussing the delivery of masonry materials via train to masonry derricks.
For an account of the economical handling of a large quantity of excavation at the Arrowrock Dam the reader is referred to Eng. News, July 17, 1913.