aggregate; concrete placed in the tops of walls, piers, parapets and curbs; and concrete placed in tunnel inverts and horizontal, or nearly horizontal, slabs.

(2) Four inches maximum slump: concrete pumped or placed in side walls and arch of tunnel linings.

(3) Three inches maximum slump: all other concrete.

169. CONCRETE BATCHING AND MIXING PLANT. Before the large batching and mixing plant for full concrete production was available, the contractor erected a temporary plant to supply his plant construction needs and for supplying concrete for lining the diversion tunnels. This temporary plant was erected on the right side of the damsite and adjacent to the aggregate stockpiles. This plant was equipped with two 4-cubic-yard mixers, automatic weigh batchers, and automatic recording devices for batch weights and consistency. Final screening of aggregate was performed adjacent to the plant and cement was stored in three 2,000-barrel-capacity silos. Bulk cement from the Permanente Cement Co., Lucerne, Calif., was transported by truck and furnished by the contractor. Discharged into 4-cubic-yard cableway buckets, concrete was transported on flat-bed semitrailers to a cableway erected for the diversion tunnel construction. The plant was capable of producing about 80 cubic yards of concrete per hour. No significant difficulties were experienced with the operation of this plant.

Late in 1957, the Noble Co., Oakland, Calif., was given the task of designing a plant which could produce concrete on an average hourly rate of 420 cubic yards, and a maximum rate of 480 yards for short periods of time. The plant was to be as automated as possible and still maintain quality control to produce concrete in compliance with the specifications.

Usually called the mixing plant, the concrete batching and mixing plant was located on a bench excavated from the west canyon wall at elevation 3540 just upstream from the right abutment of the dam. The bench also served as a roadbed for the tracks of the electric-powered transfer trains which moved the concrete-loaded ladle buckets from the mixing plant and across a structural steel trestle for transfer to the cableway buckets.

Concrete footings for the plant were placed during March 1959, and erection of the supporting steel frame began in April. Steady progress was made erecting steel

until July when labor difficulties stopped all work on the project until January 1960. Erection of the plant was then resumed and was essentially completed in April 1960. Tests and checking out of all electrical and mechanical components were made during May and the mixing plant began operation in June 1960.

The octagon-shaped mixing plant (fig. 258) was 217 feet in height and contained six main floors. A large 3,000-ton-capacity aggregate storage bin was located near the middle of the plant. This bin contained eight compartments for the two types of sand and the six different sizes of coarse aggregates required for concrete production. The portion of the plant above the storage bin was called the screening tower; the batching and concrete mixing floors were beneath the storage bin.

Sand and coarse aggregates were transported to and entered the plant by belt conveyors. At the top of the plant, coarse aggregates passed over sloping, vibrating screens (final screening) and dropped into proper bin compartments. Sand was delivered directly from the sand stockpiles by a separate conveyor system to the mixing plant storage bins. Sand and aggregates were weighed through batching scales, discharged into one of six 4-cubic-yard mixers, and combined with cement, water, and required admixtures into a batch of concrete. The concrete was discharged from the mixers into one of three holding hoppers for loading into the 12-cubic-yard ladle buckets on the transfer trains. Two of the holding hoppers had a capacity of 26 cubic yards and the other 12 cubic yards.

Location of the plant on the rock bench below the canyon rim allowed almost horizontal delivery of materials to the plant and retained the advantage of gravity flow during production. Another prime factor in this location of the plant was that about five-sixths of the almost 5 million yards of concrete required would be located below the bench at elevation 3540, resulting in a substantial savings in the cost of operating the cableways.

(a) Arrangement.-At the top of the mixing plant, on floor 6, sand and coarse aggregates entered by conveyor from the cooling chamber and dewatering screens. A 42-inch reversible transfer conveyor, 10 feet in length, transferred sand to the proper storage bin compartment for "natural" sand or for heavy-media treated sand. Coarse aggregates passed through a transfer chute with the flow divided between two 6- by 14-foot sloping vibrating screens, located below on floor 5, where the aggregate received the first step in final screening.

Both of the two sloping vibrating screens on floor 5 were double decked. The top deck on each screen had a 3-1/2-inch mesh for final screening for the 3- to 6-inch coarse aggregate, which passed directly down a rock ladder to a bin compartment. The two lower decks had 1-3/4-inch mesh screens and gave final screening to the 1-1/2- to 3-inch coarse aggregate, which also passed by rock ladder to a bin compartment. Aggregate passing the 1-3/4-inch deck screens dropped below to sloping vibrating screens on floor 4.

There were two double-deck, 6- by 14-foot vibrating screens on floor 4, each with 7/8- and 3/16-inch mesh screens. The top deck on each screen removed the 3/4- to 1-1/2-inch coarse aggregate which passed by chute to a storage compartment. The lower screen decks gave final screening to the 3/16- to 3/4-inch coarse aggregate which also passed by chute to a storage compartment. Material passing each of the 3/16-inch mesh screens was wasted. When screening the 3/4- or 1-1/2-inch maximum size aggregates, there were flow arrangements available to divert either size of aggregate to one of two storage compartments depending on whether the material had received heavy-media treatment.

Floor 3 consisted of decking over the main storage bins and provided access to inspect the various bin compartments and repair worn chutes and rock ladders. Floor 2 was located below the 3,000-ton storage bin and was known as the batcher floor. Here the individual weigh hoppers were suspended below the storage bin compartments, along with others to weigh cement, pozzolan, water, ice, air-entraining agent, water-reducing agent, and calcium chloride solution. Gates to fill and empty each weigh hopper were operated electronically through an automatic sequence, or individually by manual control. The contractor's plant office, a tool and workshop room, and a materials laboratory were also located on this floor. The laboratory was maintained by the Bureau in order to check coarse aggregate and sand samples for grading and moisture content, and for control of liquid admixtures by specific gravity tests.

Adjacent to floor 2 and between the plant and right abutment wall two surge silos were located, one 750-barrel silo for bulk cement and one for 75 tons of pozzolan, and a 5-ton storage bin for crushed ice. Cement, pozzolan, and ice were delivered to their weigh hoppers, located inside the plant on floor 2, by remote-controlled screw conveyors. All weigh hoppers loaded to the predetermined amounts required for the mix of concrete being produced.

The six tilting mixers were installed on floor 1 in

a circular pattern, being supported by a separate concrete foundation and structural members to reduce vibration in the rest of the plant. Each mixer had a capacity of 4 cubic yards and was charged from the front. The mixers were charged by a collecting hopper and a swivel chute, which is tapered to an offset angle with an opening of about 32 inches at the lower end. When the swivel chute was centered in front of a mixer, the lower end contacted a shorter length of chute, called the retractable chute, and the two sections provided a clear passage for charging that mixer with materials from the weigh hoppers on floor 2.

The control room at the top of the screening tower located above the 3,000-ton storage bin was equipped with pushbutton controls to enable the tower operator to draw sand and coarse aggregate from the storage stockpiles for delivery to the mixing plant on belt conveyors. A 36-inch-wide conveyor was used from the reclaim tunnel to the chilling chamber, a distance of 2,000 feet, having two shuffle points. A belt conveyor 42 inches in width and 250 feet in length was used through the chilling chamber. After the coarse aggregates passed through the chilling chamber, they passed through a vibrating double-deck dewatering screen by gravity, and then were conveyed to the top of the mixing plant, a distance of 150 feet. The plant storage bins were originally equipped with high and low bin indicators to show when the bins were nearly empty and when filled. This system was replaced with a closed-circuit television system which enabled the operator to see the actual bin level and to maintain better control on filling the compartments.

Coarse aggregates drawn from the storage stockpiles traveled through a covered section enclosing the conveyor called the chilling chamber. During hot weather, the aggregates were sprayed here with water chilled to 34° to 40° F. to lower their temperatures. Sand was delivered to the mixing plant on a separate conveyor system, but did not pass through the chilling chamber. The storage compartments for natural 1-1/2, 3-, and 6-inch maximum aggregates and the 1-1/2-inch maximum size heavy-media treated aggregate were insulated with exterior fiberglas blankets. Further chilling of these aggregates was provided by means of cold, refrigerated air, injected and circulated through the compartments.

The main plant control room was located above the mixers on a small mezzanine floor, adjacent to the downstream side of the plant. The control room was glass enclosed, giving a clear view of the mixers and

swivel chute action, and was maintained under slight air pressure to reduce maintenance on electronic devices and meters due to dust. An open-mesh catwalk circled in front of and above the six mixers, enabling the operator and technicians to visually spot check the actual concrete mixing action in any one or all of the six mixers.

(b) Operation.-The control room operator could operate the mixing plant, either automatically or manually, by switches and pushbuttons conveniently arranged on a single console board. The operator had a choice of 12 different concrete mixes, which had been previously computed by a Bureau representative. A large panel of monitoring dial instruments were electrically connected to each of the weigh hoppers on floor 2 to show the progress of weighing materials used in a batch of concrete. The operator could readily see if the weigh hoppers were filled correctly or if any material batches were under or over the required weight desired. Underloads which if not immediately corrected disrupted the mixing cycle, and overloads greater than permissible specification limits would be corrected by addition or deletion of materials in proper amounts.

When the plant was set to operate on automatic production, a complete mixing cycle for each mixer took 3 minutes; one-half minute was allowed for filling and emptying the weigh hoppers and 2-1/2 minutes were used for the mixing time for each mixer. After any mixer was charged with the batch ingredients, the swivel chute automatically rotated clockwise to the next mixer. With the completion of the first charging cycle to all mixers, the plant continued to produce 24 cubic yards of concrete each 3 minutes until the charging and mixing cycle was stopped or the operator placed the plant on manual control.

During 2 years of maximum concrete production, an average of 335 cubic yards per hour on a 24-hour day, 5 or 6 days a week, was produced. During one 8-hour shift the plant produced the maximum plant capacity of 3,840 cubic yards, and the maximum production for a single 24-hour period was 10,127 cubic yards.

An automatic recording machine printed a graphic record of the weight of each material used in a batch of concrete. This record, or roll, was used to check the number of batches of each kind of concrete produced by shift, and to check on the amounts of materials weighed. Another recording machine, called a consistency meter, recorded the electrical current required to turn any mixer. Concrete having a "dry" consistency or low slump took more current to turn

the mixer than a mix of a "wet" consistency or higher slump concrete. Both of these records became part of the permanent Government data and were retained for a period of time after completion of the work.

Communications throughout the various floors of the mixing plant and to other features of the jobsite, were facilitated by an elaborate system of telephones, intercoms, and two-way radios. Access throughout the plant was by stairways, ladders, and an outside elevator to all main floors.

(c) Cement.-Four silos were provided to store bulk cement, each having a capacity of 10,000 barrels. Three silos, each holding 1,000 tons, were provided to store pozzolan. Cement was delivered by truck from the Phoenix Portland Cement Co., Clarkdale, Ariz., a distance of 188 miles, under invitation No. DS-5023. Pozzolan was supplied under invitation No. DS-5053 by J. G. Shotwell from a plant located on pumice deposits about 25 miles north of Flagstaff, Ariz., and trucked 110 miles to the damsite.

Trucks containing cement or pozzolan were received at the Government scale house for weighing and then proceeded to the storage silos for unloading. The trucks bottom-dumped into a 14-inch screw conveyor which passed the cement or pozzolan to a bucket elevator where the material was lifted to air slides at the top of the silos and transported to the respective silos. Cement was transported to the mixing plant by screw conveyors from the bottom of the silos, then transported by air slides and gravity to the balancing storage silos, located adjacent to the plant at the batching floor elevation.

On the canyon rim adjacent to the plant, bulk storage tanks contained calcium chloride solution, air-entraining agent, and liquid water-reducing setretarding agent, which were piped for gravity flow to the batching floor for dispensing.

(d) Crew.-The contractor operated the mixing plant with a crew of 12 men on each shift, with a general superintendent in charge of all work except the refrigeration plant. Each shift consisted of one foreman, one operator at the cement and pozzolan silos, one operator at the dewatering tower, one operator for rescreening operations at the top of the plant, one operator for batching and mixing, one dispatcher at the bottom of the plant for dispatching concrete, one signalman for routing trains, one electrician and one mechanic for maintenance and general repair, three or four general laborers for cleanup and general help.