(e) Delays.-During operation of the plant, three difficulties were encountered which slowed down concrete production at times. As previously mentioned, the final rescreening of coarse aggregate was performed at the top elevations of the plant, and plant storage was about 3,000 tons, or each aggregate bin had a capacity of 275 tons when full. However, it was not possible to maintain all the bins full all of the time and the operational level for each coarse aggregate was closer to 200 tons. In addition, the 36-inch conveyor delivering material from the stockpiles could not deliver more than three materials at one time. It was not possible to load 3- and 6-inch rock on the 36-inch conveyor belt at one time because all feeder gates were manually operated and required different settings for these fractions. Consequently, as these materials were alternated from one gate to the other, the storage bins had a tendency to become layered with the results that the aggregate grading in each bin was not uniform.

The rescreening plant had sufficient capacity if it had been possible to feed all the different materials uniformly at one time. This condition would have been improved by utilizing a wider conveyor belt between the reclaim tunnel and the chilling chamber and installing a belt feed in the reclaim tunnel for the 3and 6-inch aggregate. When rescreening the two sizes of heavy-media-processed aggregate, only these two sizes of aggregate could be screened. Consequently, with sustained operation above 400 cubic yards of concrete per hour, the storage of aggregate was gradually depleted and separation increased within the bins. If a repair delay of more than 1 hour occurred, the supply of some size of aggregate was exhausted, usually the 3-inch size.

Another difficulty in the plant involved obtaining a suitable timing sequence for charging the various materials to each of the six mixers. A timing sequence that would be good for one or two mixers would not be satisfactory for the other mixers. This difficulty was related to the 35-foot distance of travel of materials between the weighing hoppers and the mixers, and the different angles of approach the swivel chute would have in relation to each weighing hopper and each mixer. This difference in travel time of materials for charging the mixer was improved by installation of two more timing devices. Best results were obtained, after considerable experimentation, by having three mixers on one sequence, two mixers on another, and one mixer on separate sequence.

An unusual problem was presented as the result of the lead time needed to allow a pause in production because of delays at the placing site. These delays frequently resulted when the cableway was used for yarding, leaving up to 136 cubic yards of concrete in storage between the placement and the mixers. This yardage was distributed as follows: 24 cubic yards in the mixers, 64 cubic yards in the three "gob" hoppers, and 48 cubic yards in the transfer cars. This naturally created a tendency for slump loss, especially during warmer weather, which further slowed down placing operations. This situation would have been somewhat improved by closer coordination of the placing and batching operations in providing lead times and deceleration of placements.

(5) Cooling placed concrete with water or brine through embedded pipes.

As used in sustained production, the first four items, involving reduction of the mix to below 50° F., required about 40 percent of the plant output. Twenty percent was used in initial cooling of the coarse aggregate prior to delivery to the batching plant.

Several features in the installation were of special interest. For example, refrigeration coils were installed beside each aggregate bin at the batching plant to chill the air that cooled the rock. At another location, one central bank of coils was installed with air ducts to the different bins. Temperature of the aggregates in the bins was controlled by thermostats to prevent freezing. In addition, the refrigeration coils were cleaned of frost and dirt accumulations by automatic water flushing systems operating on a timeclock.

Another interesting feature was the automatic operation of the ice batching system. Most ice systems have been semiautomatic, requiring a man in the ice storage bin to keep the ice friable and moving into screw conveyors. In this plant, however, the flake ice fell from the ice-making machine onto a fast-moving rubber belt which carried it some 50 feet to an 8-inch-diameter chute where it dropped 125 feet. It fell into a small storage bin immediately adjacent to the batching plant weigh hopper floor. This bin contained about a 30-minute supply of ice. To insure an adequate supply of ice at all times, 22 ice-making machines were installed. An automatic signal from the batching plant ice bin activated more or fewer of these ice-making machines as necessary to keep up with mixing operations.

The plant included a battery of 18 ammonia compressors most of which were arranged for multiple-purpose use. Four 125-horsepower compressors were used solely to provide refrigeration for the 22 ice-making machines which produced 20 tons of flake ice per hour. The other units were connected with appropriate controls so that they could be used effectively for varied load demands. These other units consisted of eight 600-, two 100-, one 450-, two 150- and one 25-horsepower compressors. The compressors were serviced by eight 48-inch condensers. Excess heat, peaking at more than 1,000,000 British thermal units per minute, was wasted to the air by a cooling tower installation. The tower recirculated water at 15,000 gallons per minute to the condensers.

There were 10 shell and tube chillers. Two of these, 32 inches in diameter, were used to chill the mixing

water. Three, 52 inches in diameter, chilled water for cooling the coarse aggregate on the conveyor belts. The other five, 38 inches in diameter, cooled water for circulating through the cooling pipes embedded in the dam. These units were initially located at the base of the dam for cooling of the first 2 million cubic yards of concrete, but were moved to the right canyon rim for cooling of the remaining 2.9 million cubic yards.

The refrigeration plant was designed to meet a concrete placing rate of 480 cubic yards per hour. With a normal temperature of aggregates at 87.5° F., cement at 150° F., pozzolan at 120° F., and water at 80° F., the temperature of the resulting mix would be about 95° F. This is 45° F. higher for placing than the allowable 50° F.

To bring the concrete mix temperature to 50° F., the following four steps were used:

(1) Spray the aggregates. This step involved spraying the coarse aggregate (3/16 to 6 inches) on the conveyor belt as it moved from the stockpile area to the batching plant. This required 830 tons of refrigeration and involved the use of 2,200 gallons of water per minute at 35° F. The water was recovered for recirculation. Accumulated particles were settled out of the water before recirculation. The aggregate was passed over dewatering screens before proceeding to the batching plant. This process cooled the rock to 50° F. and would reduce the mix temperature to 72° F.

(2) Air cool the aggregate. The aggregate was stored at the batching plant in eight bins with a total capacity of 3,000 tons. Four of these bins were equipped for air cooling. These were the 3-to 6-inch, the 1-1/2- to 3-inch, and the two bins holding 3/4- to 1-1/2-inch aggregates. One of these latter bins held aggregate from which lightweight rock had been removed by the heavy-media process. No attempt was made to air cool aggregates smaller than the three-fourths inch.

Air cooling of the larger aggregates required 270 tons of refrigeration and reduced the rock temperature to 30° F. This action would further reduce the mix temperature to 64° F. As mentioned, each of the bins had its own individual air-cooling coils which were automatically defrosted and cleaned. Air circulation was on a closed circuit basis with air moved by blowers.

(3) Chill the mix water. In this step, the mix water was chilled from 80° to 35° F. This required

180 tons of refrigeration; the chilled water, together with the aggregate cooling, would reduce the temperature of the resulting mix to 59° F.

The chilled water, including the water in the surge tank at the batching plant, was continuously circulated. The chilled water was also used by the ice plant.

(4) Add flake ice to the mix. Flaked ice was fed direct to the surge tank at the batching plant without intermediate storage. This tank had capacity for 30 batches. Automatic operation started and stopped ice machines as necessary to keep the surge tank filled. The ice was constantly agitated in the tank to provide ready delivery to the weigh hopper. The 22 ice machines could provide about 20 tons of ice per hour which required 400 tons of refrigeration effort. By adding 300 pounds of ice per 4-cubic-yard batch, the mix was finally cooled to about 47° F. This is about 30 below the specifications requirement, but this margin was needed to insure that the final placing temperature at the forms was 50° F. or less.

Cooling of placed concrete was accomplished in two stages and required 1,600 tons of refrigeration effort. The first phase was for removing the heat of hydration and used water at 38° to 47° F. circulating through 1-inch aluminum tubing embedded at the base of each 7.5-foot lift. Specifications required that water be cooler than the concrete being placed. Circulation of this water was required for 12 days. The purpose of the first phase of the cooling was primarily to control potential cracking in the concrete. Secondary cooling was used to reduce the temperature of all concrete in the dam below elevation 3450 to 40° F. The required temperatures from 3450 to the crest elevation of 3715 varied from 40° to 50° F. Time for secondary cooling was about 52 days for the 7.5-foot lifts used. To get the dam concrete to 40° F., chilled water at 38° F. was used initially and was switched to brine at 35° to 28° F. in the final stages. Secondary cooling was used to open up the contraction joints which permitted effective grouting to make a monolithic structure of the dam. Instruments embedded in the dam provided temperature controls during construction and will provide structural behavior data under reservoir loading conditions.

171. CONCRETE CONTROL OPERATIONS. To perform the assigned duties of the branch, a complement of 40 engineers, technicians, and inspectors of various grades and experience were employed under the direction of the branch chief. A

supervisory engineer was assigned to correlate and direct inspection and testing on the site for the three shifts of concrete production.

The contractor's operations at the Wahweap aggregate washing and screening plant were inspected on all shifts of operation. On day shift, two inspectors were scheduled and one each on the other shifts. Special tests, as well as routine tests were performed on the day shift, such as aggregate production tests. Although the specification requirements limited the acceptance of materials at the batching plant as batched, it was found more practical to perform routine tests, such as sand gradation, quantity of light-weight materials in heavy-media processed aggregate, and soluble sulphate content in the fine aggregate as produced at the production site. These same tests were performed, but not as frequently, on all the material as it was batched to assure that these materials had not been contaminated or mishandled during transit to the batching bins. This procedure proved satisfactory, and when materials did not meet specifications during production, it was more practical and economical to reject material at the source of production rather than waste large quantities in the various stockpiles. Gradation tests on coarse aggregate were not performed at the aggregate screening and washing plant because coarse aggregate was later screened at the concrete batching plant.

Owing to the nature of the concrete production facilities and the intended use, adequate quality control was necessary for each step in the manufacture of the component materials, the production of the concrete mixture, and the placement. Adequate inspection and production testing were therefore provided to insure acceptable concrete in accordance with the specifications. Being a key feature in the whole operation, a substantial part of the concrete control effort was expended at the batching and mixing plant.

The inspection force for each shift of operation consisted of an engineer and shift chief, who was assisted by four inspectors. On the day shift tour of duty, a supervisory engineer was assigned to the plant to coordinate the work and test procedures on all shifts, calibrate and maintain all testing equipment used at the plant, make mix adjustments for gradation, supervise the adjustments necessary for mixer charging sequences to obtain mixing efficiency, and supervise the checking of all batch weighing scales.

Duties of the plant inspectors for an average shift consisted of selecting and setting up the proper

concrete mixes for all authorized concrete placements, performing tests and insuring conformance with the specifications. This involved observing and recording the scale weighing cycle for accuracy; observing the mixing action of each mixer visually and also checking the recorded record of concrete consistency for each batch; obtaining test batches of concrete on each major mix used on the shift; obtaining a breakdown from the test batches to show aggregate moisture and gradation of aggregate, slump, temperatures, entrained air content, unit weight; and casting at least four 6- by 12-inch test cylinders for later testing of compressive strength. During the shift, other routine tests were made to verify slump, temperatures, entrained air content, and unit weight. Two mixer efficiency tests were made on each shift per week, which constituted an efficiency test on each mixer for approximately every 6,500 cubic yards of concrete mixed.

The tests performed at the central laboratory required the use of more precise equipment and temperature and humidity controls. Among these tests were the physical properties, acceptance tests on pozzolan, the storage and testing of all test cylinders for compressive strengths (see fig. 259 for typical test results), the soluble sulphate tests on sands, and all other special tests.

The government furnished all cement and pozzolan to the contractor at his unloading facilities. This required that all cement and pozzolan be acceptance tested and weighed before delivery to the contractor. No unusual difficulties were experienced with the delivery of cement. During the winter months, heavy snowfalls near Flagstaff, Ariz., caused hazardous road conditions and shipments were delayed on several occasions, but the project supply was never depleted.

For the performance of all tests at the mixing plant, two field laboratories were located within the plant, one on the batching floor and one on the mixer deck. All operation records and test data were compiled on proper report forms and turned in at the end of the shift to the concrete control branch office.

The central laboratory and control office was located in a separate building in Page, Ariz., and was the center of coordination between all concrete activities in the field and the main project office. Headed by a supervisory engineering technician and assisted by approximately nine engineers and technicians, their work alternated between the office and the laboratory special tests. At times they served as supervisory inspectors on construction contracts, such as on the transmission lines, the completion contract and other smaller contracts, and served as routine relief inspectors in the field.

Although even greater problems in supply were experienced with pozzolan, the project supply of this material was also never completely depleted.

The Government scale house was located on the west side of the canyon between the highway and the storage silos and operated with one technician on each shift. Aside from weighing cement and pozzolan, the technician obtained samples and performed site acceptance tests on both products. Acceptance tests on cement included one for temperature, which was not to be above 180° F. in the cement as delivered; and a false set test, which was not to exceed 17 millimeters between the initial and final readings. All other acceptance tests on cement were performed by the National Bureau of Standards at the cement plant. The project tests were performed on about every tenth truckload of cement (or 1,400 barrels) delivered under normal conditions. No difficulty was experienced with

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temperature. During the summer of 1962, some difficulty was experienced with false set in cement. For a short time thereafter, all truckloads of cement were tested before unloading and several truckloads were rejected. The cause of the condition was apparently corrected promptly, as no further difficulties were experienced with false set.

A temperature acceptance test on pozzolan was performed at the scale house. The upper allowable temperature limit for pozzolan was 170° F. and a 15-pound sample of pozzolan was obtained from each 150 tons, or every sixth truckload delivered. At least two truckloads were sampled each day for determining the moisture content of pozzolan. Truckloads were not accepted before the Blaine test had been completed, which generally consumed approximately 45 minutes of time. When the test failed on a truckload, tests were taken on each subsequent truckload until consecutive truckloads met the specifications. On different occasions, several consecutive truckloads of pozzolan were rejected.

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The delivery of pozzolan of acceptable quality and in sufficient quantities at the peak of construction caused some concern. At one time it was necessary to ship in several carloads of fly ash from the Chicago area, and on two other occasions pozzolan was trucked 250 miles from Panaca, Nev., in order to keep the prime contractor in supply. Although the project supply was never depleted, it was down to less than a 1-day supply on several occasions, and part of the pozzolan in the concrete was replaced by cement.

The concrete control office compiled all concrete data from the field and prepared its part of the daily and weekly progress reports and the monthly L-29 report. The daily, weekly, and monthly compilations of these data were valuable in analyzing the progress of the work, and aided the branch chief in making any necessary changes in mix design, test procedures, or the requirement for additional tests. Data were also used for the purpose of ordering cement and pozzolan for project needs in a regular and orderly manner on a weekly and monthly basis.

172. CEMENTING MATERIALS. The Government-furnished type II, low-alkali cement for the construction of Glen Canyon Dam and Powerplant was supplied under invitation No. DS-5023. This contract was awarded to the American Cement Corp. of Los Angeles, Calif., on April 3, 1958, on their low bid of $9,741,900 for the estimated quantity of 3 million barrels of cement. The Phoenix Cement Co., a subsidiary of the American Cement Corp., proceeded

to construct a mill at Clarkdale, Ariz., which was completed and in operation late in 1959. The Clarkdale area was chosen because there were large limestone deposits in the immediate area, other necessary materials for making cement were readily available, and there were no other mills in northern Arizona.

Two types of limestone, Redwall limestone and Lakebed limestone, are found near Clarkdale, each being created in a different geologic age. The Redwall limestone is a high-quality white limestone laid down in Pennsylvania times about 100 million years ago. This limestone is found in a fractured state due to the movement of the earth since it was deposited and the pieces are coated with Supai sandstone, making them red in color, which was also deposited in the same era. The Lakebed limestone, also a high-quality white limestone, was laid down by a fresh water lake 750,000 years ago or in the Pleistocene age. The argillaceous materials needed for cement came from slag from the old Clemenceau smelters, located only 4 miles away from the mill, and from other nearby quarries. Gypsum was obtained from the Verde Gypsum Co., mined near Camp Verde about 20 miles from the Clarkdale mill.

Utilizing the dry-process manufacturing method, the Clarkdale plant was reportedly capable of producing 5,500 barrels of cement daily. It was estimated that the maximum monthly quantity of cement required at Glen Canyon would not exceed 120,000 barrels. The contractor was able to meet all cement requirements during the life of the contract. Inspection of the manufacturing process and plant testing of the cement was made at the plant by a resident inspector from the National Bureau of Standards. Maximum temperature and false set tests were performed by the Bureau's concrete control section at the construction site.

Phoenix Cement Co. issued a subcontract to Belyea Trucking Co. of Los Angeles for transporting the cement from the mill to Glen Canyon, a one-way distance of 188 miles. Twenty truck-tractors pulling two hopper-type trailers, specially designed to haul the maximum load allowed under Arizona State laws, were used to make the haul. Each tractor-trailer combination had an overall length of 59 feet and 11-1/2 inches and was capable of hauling 142 barrels or 27-3/4 tons of bulk cement.

Had the Clarkdale mill not been constructed, all of the cement would have had to have been hauled from southern Arizona or southern California which would have substantially increased the cost of the cement to the Government. A total of 3,046,441 barrels of cement were purchased for use by the prime contractor