Page images

Figure 33. --View from top of mixing plant showing aggregate unloading hopper, aggregate stockpiles, and conveyor system. A pile of riprap material is shown at the extreme left. P328-701-3010, August 16, 1951.


Percent of Maximum size

concrete placed of aggregate, inches
95 3
4 1-1/2
1 3/4

A large percentage of the 3/4-inch aggregate was used in concrete to cover freshly excavated shale in the foundation of the spillway.

Nonshrink concrete for machinery bases and for filling blockouts was produced by adding 0.005 percent aluminum powder by weight of cement to the mix.

During cold weather the concrete was mixed with heated water, the heat being

supplied by a steam boiler which was connected with pipes to a steam coil in the mixing water storage tank. Aggregates were heated by electrical strip heaters attached to the outside of the aggregate bins. These heating methods were not completely satisfactory because the temperature of the water and aggregates varied in accordance with the rate at which the materials were being used in the mixer. During freezing weather, to supplement the heating of ingredients, all concrete except mass concrete was mixed with calcium chloride dissolved in the mixing water (sec. 55(a)).

54. Control. - Prior to mixing concrete, scales for batching the cement, aggregate, and water were tested for compliance with specifications tolerances. Later, periodic tests and adjustments were made when necessary to keep the equipment in an accurate working condition.

Soon after a mixer was put in operation, samples of concrete were taken at several locations in the mixer for determining mixing efficiency. To maintain uniform concrete, tests were taken each day during concrete production to determine gradation, moisture content, and specific gravity of all aggregates. Slump tests were made frequently on the freshly mixed concrete during each placement, or lift. In general, concrete samples were obtained for each 100 cubic yards of concrete mixed for determining air content, unit weight, and compressive strengths. Six- by 12-inch test cylinders were cast for use in determining compressive strengths at 7 and 28 days of age. In addition, one test cylinder was cast each week for testing at 90 days and one each month for testing at 360 days. Typical mixes and other concrete data are shown on figure 34.

Equipment for conducting slump, air content, and unit weight tests, and for casting test cylinders was maintained at the mixing plant. The laboratory, housed in the caretaker's garage, was equipped with a fog room, a 200,000-pound-capacity testing machine, aggregate screening equipment, drying ovens, scales, and other items for conducting various tests.

Very little trouble was experienced with deleterious or foreign matter in the finished concrete. Corrective measures, consisting of slowing down the raw feed or changing the screen sizes, were immediately taken when gradings exceeded specification percentages. During the dumping, hauling and stockpiling operations for Clear Creek aggregate, much of the weathered and unsound material was broken down into sand sizes and dust. These fines were rejected from the rescreening plant.

55. Placement. - Equipment used for concrete placement and handling included several dumpcrete trucks (special concrete dump trucks), a 3-1/2-cubic-yard crane, 2-cubic-yard pneumatically operated concrete buckets, two 1-cubic-yard manually operated buckets, several electric vibrators, one pneumatic vibrator, and one 36-footlong vibrating screed board. The dumpcrete trucks were originally equipped with 20inch-wide discharge chutes and gates, but it was found that concrete with 3-inch-maximum aggregate and less than 3-inch slump could not be discharged from the truck without vibrating. The contractor successfully overcame this difficulty by lengthening and widening the chutes, and eliminating the gates on the trucks. The concrete placing equipment had a capacity of 60 cubic yards per hour, but the average placing rate was only about 40 cubic yards per hour.

[ocr errors][graphic][merged small][merged small][merged small]

Figure 35. --Workmen using a vibrating screed in finishing a spillway floor slab. Wood floats were used for the final finishing work. P328-701-3634, May 8, 1952.

Figure 36. --Placing concrete in a spillway pier with a crane and bucket. P328-701-4648, March 18, 1953.


The initial concrete placing operations were restricted to the canal outlet works, the spillway floor and walls between stations 30+80 and 35+55, the foundation for the gate structure, and the left nonoverflow abutment. Spillway floor panels 18 inches high or over were placed in two lifts to assure fresh concrete on the entire surface for finishing. About 2 hours time was needed to place and vibrate one panel, requiring 50 cubic yards of concrete, and one additional hour for finishing operations. Some difficulty was experienced in finishing floor slabs during very warm or windy days. To improve the quality of finishing, these slabs were placed only during night shifts or cool weather. A floor slab finishing operation is shown in figure 35.

Initially, for placement of concrete in spillway wall panels, hoppers and flexible chutes were used. This proved inefficient, and after several wall panels had been placed, the hoppers were discarded and concrete was discharged directly from the bucket into forms. The latter method also proved to be economical for placing concrete in the nonoverflow abutment and piers of the spillway (fig. 36). Because the wall panels were placed monolithically and in a short period of time, the top 6 or 8 feet were revibrated after 1 or 2 hours to settle the concrete as much as possible prior to setting. Concrete in the higher walls of the stilling basin was placed in lifts of 19 feet or less. In general, the height of lifts in the gate structures and nonoverflow abutments was governed by the most convenient elevation for installing metalwork and reinforcement steel or by location of reinforcement steel splices.

Because of anticipated high shrinkage, concrete in the 50-foot-long bridge spans was placed with special care. The concrete had a water-cennent ratio of 0.47 and a maximum slump of 2 inches, and was placed in lifts about 12 inches thick. Before a final set had taken place on the bridge span, a warehouse broom was drawn laterally across the surface to produce numerous small parallel grooves.

(a) Cold Weather Operations.--In order to maintain specified minimum temperatures of concrete placed during freezing weather, several methods were used in addition to heating of ingredients (sec. 53). On small jobs, the concrete was enclosed in prefabricated canvas shelters and heat supplied by airplane heaters. For casting walls about 17 inches thick at the base and 12 inches thick on the top, insulated wall forms and concrete with admixture of 1 percent of calcium chloride were used. The top of the wall was protected with a layer of sand or rock-wool blankets. Insulated forms were also used for bridge piers. Measurement of the concrete temperature after placement indicated a rise of about 30° F during the first 48 hours and a gradual decrease of from 30° to 40°F during the next 5 days.

Floor slabs were placed only in the afternoon when frost had left the gravel beneath the slab. Prior to placing of the slabs, construction joints at the base of wall panels were washed with hot water. After placement and finishing operations were completed, the slab was coated with a membrane curing compound and enclosed in a canvas shelter which was heated with airplane heaters. Heat was applied overnight or until the curing compound had set, when the heating facilities were removed and the slab covered with 6 inches of sand.

56. Curing. - Several methods were used for curing concrete. Construction joints were water-cured with burlap or sand and frequent sprinklings of water. Finished and formed concrete was usually cured by application of a membrane curing compound. The interiors of control houses, gate chambers, stair wells, and spillway gallery were water-cured or cured by leaving the specially coated forms in position.

Dampproofing, consisting of two coats of an asphalt emulsion, was used for curing the outside of the horseshoe conduit, the outside surface of the terminal well, and a portion of the nonoverflow abutment. To keep concrete and embankment work progressing, the contractor was permitted at times to apply only one coat of the dampproofing as a sealing compound until about a day before backfilling operations, when the second coat was applied. Because of temperature and shade requirements for the dampproofing, large volumes of concrete placed at one time presented a critical curing problem with this method of curing.

57. Contraction Joint Grouting. - Two keyed contraction joints, each provided with grout outlets and header and return pipes, were constructed in the spillway crest

« PreviousContinue »