Transcript for Hoover Dam Construction, segment 08 of 17
Sand and gravel for the four and one-half million cubic yards of concrete required for the construction of the dam and its appurtenant works were obtained from a detrital deposit located on the Arizona side of the river some twelve miles upstream from the dam site. Here the raw material was excavated by dragline and hauled by train to the gravel screening and washing plant which was the largest of its kind ever built, being capable of producing twenty thousand tons of crushed, screened, grated, and washed materials every twenty-four hours. Arriving at the plant, the raw material was dumped into hoppers from which it was conveyed into the plant over endless belts. Here it passed through the various stages of screening by which it became suitable aggregate material for the manufacture of concrete of a quality meeting the most rigidly uniform requirements.
Oversized cobbles, that is, those measuring more than nine inches in one dimension, were first screened out, then crushed and returned to the plant for regrading along with raw material. The screening plant itself consisted of four towers of similar design, each equipped with screening apparatus and each separating out gravel of a different given size from the mass of raw material reaching the unit over the conveyer belts. An endless stream of raw pit-run gravel passed over the screens and the selected material carried to the stockpiles over the conveyer belts, each of the four sizes being stocked separately, ready at hand when needed at the mixing plants. A clarifier tank provided six million gallons of clear water daily for washing gravel and sand. Raw material for concrete manufacture which fell into the size classified as sand was grated a second time into three sizes after the very fine and undesirable sand had been discarded. The three selected sands were then recombined into a uniform mixture to meet specifications for the concrete to be used in the dam and the appurtenant structures of the project. The sand was then stored, awaiting the requirements of the construction program. As the grated materials were called for, conveyer belts loaded the sand and gravel into railroad cars for transportation to the concrete mixing plants located at the dam site several miles distant.
Concrete was mixed in two plants, one located in the bottom of the canyon upstream from the dam and the other on the canyon rim on the Nevada side immediately over the dam site, both equipped with the most advanced machinery for concrete manufacture. Upon arrival at the concrete mixing plant, sand and gravel of the various sizes were stored in separate bins. A railroad system of sizable proportions was required to maintain a steady flow of material from the gravel plant to the screening plant and from the screening plant to the concrete mixing plants. Bulk cement was unloaded from railroad cars by means of a pneumatic pump and conveyed into the bins of the blending plant where cements from several production sources and of varying physical and chemical characteristics were combined into a uniform product. This was necessary because of the requirement for standardization in concrete workability, strength, texture, color, and other properties.