and dolomitic limestone. About 0.5 percent of the aggregate consisted of chert which reacted deleteriously with high-alkali cement. The aggregate contained an excess of deeply weathered and otherwise unsound particles, but because of the good service of this aggregate in numerous types of concrete structures in the region, these deficiencies were discounted. About 86, 088 tons of aggregate were obtained from this deposit. The aggregate was received in three sizes--3/16 to 3/4 inch, 3/4 to 1-1/2 inches, and 1-1/2 to 3 inches--and each size was stockpiled separately. When gravel was drawn from the stockpiles, all three sizes were intermingled to prevent them from rolling off the conveyor belt during transportation to the rescreening plant. Also, the rescreening plant could handle more tons of material per hour when supplied with several sizes of intermingled aggregate.

During the latter part of 1953, when the Brannon Sand and Gravel Co. was unablto obtain sufficient railroad cars for aggregate shipments, the contractor purchased 1,244 tons of crushed gravel from the Guernsey Rock Co. at Guernsey, Wyo. This gravel, composed of limestone, was intermingled with Clear Creek aggregate for use in concrete mixes. Batching weights were altered slightly to compensate for the variance in the specific gravities of the two materials.

About 3, 425 tons of aggregate crushed to 3-inch size were obtained from Table Mountain quarry at Golden, Colo. This material was composed mainly of granite gneisses. No physically unsound particles or deleteriously reactive materials were found in the samples tested.

(c) Cement.-- Because of the alkali-reactive material in the concrete sand and in the foundation materials, type II low-alkali cement was used for Trenton Dam. This cement, a mixture of pozzolan and portland cement, manufactured at the Louisville, Nebr., plant of the Ash Grove Lime and Portland Cement Co., was low-alkali type and conformed to Federal specifications No. SS-C-192.

As soon as the portland-pozzolan cement was available, the field laboratory conducted tests to determine the most economical mix consistent with workability, durability, and strength requirements. The maximum water-cement ratios indicated in specifications No. 3047 for parts of structures subjected to certain conditions are indicated below:

However, in order to meet a minimum compressive strength requirement of 3, 000 pounds per square inch at 28 days, the water-cement ratios for structures b and c, listed above, were reduced to 0.51 and 0.53, respectively. For concrete mixes using 3-inch maximum aggregate the cement content was increased for structures under conditions b and c from 1.00 to 1.23 barrels and 1.00 to 1. 12 barrels, respectively. Concrete mixes using other sizes of aggregates were also altered.

51. Reinforcement Steel. In accordance with specifications No. 3047, all reinforcement steel was to be furnished by the Government. Approximately 45 percent of the total amount required was obtained from a surplus supply at Enders Dam, Nebr. Attempts were made to purchase the remainder through competetive bids on the open market, but because of national defense priorities in effect at the time and the general steel shortage, no bids were received for a sufficient quantity to supply the job requirements. A large quantity was procured from a surplus supply at the Army Engineer's Office at Atlanta, Ga., and a surplus at Shasta Dam at Redding, Calif.; the remaining

Figure 31.--Panels for forming spillway wall sections. Form was held

rigidly in position by five steamboat jacks bolted to the concrete footing. P328-701-3134, September 19, 1951.

quantity was obtained from several sources. Because numerous sizes of steel were received which did not entirely conform to construction drawings, some substitution of steel sizes to provide equivalent areas was made in parts of the spillway.

Some of the steel was rusted, partly painted or bent. Paint was removed by an acetylene torch or by wire brushing. Cutting and bending or straightening of reinforcement bars was done at the steel storage yard located about one-half mile downstream from the spillway crest.

52. Forms. Forms for concrete structures were constructed of a variety of materials. These materials included metal, plyboard of various thicknesses, tongueand-groove flooring material, masonite, and heavy timbers. Cables, bolts, and jacks were used for keeping the forms in position during concrete placement. Forms for walls and other exposed surfaces were generally faced with 1- by 4-inch flooring material or plyboard. For reasons of economy, some forms were designed for many reuses.

Spillway wall forms (fig. 31) consisted of panels 32 feet by 19 feet 4-1/2 inches, and were faced with flooring material. The flooring material was placed vertically and was backed by diagonally placed 2- by 6-inch tongue-and-groove material which was solidly attached to 4- by 6-inch timbers. The facing material was held in place by tapered bolts which extended through the forms. Jacks with their bases anchored to a concrete base were used to hold the back form rigidly in position. After these panels were used about 12 times, the floor facing began to curl and required smoothing with a sander. When the curling became excessive, the flooring material was replaced with new lumber. Two replacements were necessary on each form during the entire construction program.

Forms for the upstream end of the outside bridge piers of the spillway were faced with 1/4-inch plyboard and backed with two layers of 3/16-inch masonite and a layer of 1/4-inch plyboard. Frames for these forms consisted of a double layer of 2by 12-inch planks spaced at 15-inch intervals and cut to fit the pier curvature. Support for the forms was provided by a 12- by 12-inch strong back with a number of small studs placed at the leading edge of the pier.

Three 5-foot I-beams were placed in blockouts left in the piers and were used to support the forms and reinforcement steel for the center bridge spans over the spillway structure. Heavy timbers were placed across these beams at 5-foot centers for supporting the bridge forms. Correct elevation for the forms was obtained by wedging.

Forms for the 5.5-foot circular conduit were assembled in a jig to one side (fig. 32). After the inside and outside forms with reinforcement steel were assembled, the jig was placed in position with a dragline. The interior form was faced with masonite and arranged for easy removal. The outside form was constructed of shiplap. Metal forms were used for forming the 8-foot 2-inch diameter conduit.

53. Batching and Mixing. Concrete for Trenton Dam structures was batched and mixed at a central plant located about 500 feet east of the spillway outlet channel at station 53+00. This plant consisted of a partitioned 225-ton gravel bin, a 100-barrel cement bin, a 90-cubic-foot batching hopper equipped with a full reading dial scale for cumulative weighing of aggregates, a separate automatic hopper for weighing cement, a semiautomatic water batcher, a 2-cubic-yard tilting mixer equipped with a timing and locking device, a 160-cubic-foot concrete collection hopper, and conveyor systems for the cement and aggregates. Cement was stored in a 1, 750-barrel storage silo located adjacent to the mixing plant. The aggregate bins were provided with vibrating screens. Air-entraining agent was added to the mix by means of a dispenser. Mixing water was obtained from a shallow well located under the plant. A view of the layout of the mixing plant is shown in figure 33.

Three sizes of aggregates were used in the concrete. The percentages of concrete placed with the maximum size of aggregate used are indicated below:

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.

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.