Since about 1900, the Black Canyon and nearby Boulder Canyon had been investigated for their potential to support a dam that would control floods, provide irrigation water and produce hydroelectric power. In 1928, Congress authorized the project. The winning bid to build the dam was submitted by a consortium called Six Companies, Inc., which began construction on the dam in early 1931. Such a large concrete structure had never been built before, and some of the techniques were unproven. The torrid summer weather and the lack of facilities near the site also presented difficulties. Nevertheless, Six Companies turned over the dam to the federal government on March 1, 1936, more than two years ahead of schedule.
Hoover Dam holds back Lake Mead, the largest reservoir in the United States by volume. The dam is located near Boulder City, Nevada, a municipality originally constructed for workers on the construction project, about 30 miles southeast of Las Vegas, Nevada. The dam's generators provide power for public and private utilities in Nevada, Arizona, and California. Hoover Dam is a major tourist attraction; nearly a million people tour the dam each year. Heavily travelled U.S. 93 ran along the dam's crest until October 2010, when the Hoover Dam Bypass opened.
The first concrete was poured into the dam on June 6, 1933, 18 months ahead of schedule. Since concrete heats and contracts as it cures, the potential for uneven cooling and contraction of the concrete posed a serious problem. Bureau of Reclamation engineers calculated that if the dam was built in a single continuous pour, the concrete would take 125 years to cool and the resulting stresses would cause the dam to crack and crumble. Instead, the ground where the dam was to rise was marked with rectangles, and concrete blocks in columns were poured, some as large as 50 feet square and 5 feet high. Each five-foot form contained a series of 1 inch steel pipes through which first cool river water, then ice-cold water from a refrigeration plant was run. Once an individual block had cured and had stopped contracting, the pipes were filled with grout. Grout was also used to fill the hairline spaces between columns, which were grooved to increase the strength of the joints.
The concrete was delivered in huge steel buckets 7 feet high and almost 7 feet in. These buckets, which weighed 20 tons when full, were filled at two massive concrete plants on the Nevada side, and were delivered to the site in special railcars. The buckets were then suspended from aerial cableways, which were used to deliver the bucket to a specific column. As the required grade of aggregate in the concrete differed depending on placement in the, it was vital that the bucket be maneuvered to the proper column. Once the bottom of the bucket opened up, disgorging 8 cubic yards of concrete, a team of men worked it throughout the form. Although there are myths that men were caught in the pour and are entombed in the dam to this day, each bucket only deepened the concrete in a form by an inch, and Six Companies engineers would not have permitted a flaw caused by the presence of a human body.
A total of 3,250,000 cubic yards of concrete was used in the dam before concrete pouring ceased on May 29, 1935. In addition, 1,110,000 cubic yards were used in the power plant and other works. More than 582 miles of cooling pipes were placed within the concrete. Overall, there is enough concrete in the dam to pave a two-lane highway from San Francisco to New York. Concrete cores were removed from the dam for testing in 1995; they showed that "Hoover Dam's concrete has continued to slowly gain strength" and the dam is composed of a "durable concrete having a compressive strength exceeding the range typically found in normal mass concrete" Hoover Dam concrete is not subject to alkali–silica reaction as the Hoover Dam builders happened to use nonreactive aggregate, unlike that at downstream Parker Dam, where the alkali–silica reaction has caused measurable deterioration.
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