With the river diverted and the foundations for the base and walls cleared, construction of the dam itself could begin. The first concrete was placed into the dam on June 6, 1933. The ultimate size of Hoover Dam created many challenges for the dam’s engineers. Many novel and previously untried construction techniques were used in building Hoover Dam. The most significant issue facing the engineers dealt with the amount of concrete needed to build the dam, and then length of time it takes to cure under normal circumstances. Concrete heats up and contracts as it cures and the uneven cooling and contraction of the concrete posed a serious problem. The Bureau of Reclamation engineers calculated that if the dam were built in a single continuous pour, the concrete would have taken 125 years to cool to the site’s ambient temperature.
To solve this problem, the dam was built in a series of interlocking trapezoidal columns. Each column was built in layers with each pour no more than six inches deep. To further cool the concrete, each form contained cooling coils of 1 inch thin-walled steel pipe. River water was circulated through these pipes to help dissipate the heat from the curing concrete. After this, chilled water from a refrigeration plant on the lower cofferdam was circulated through the coils to further cool the concrete. After each layer had sufficiently cooled, the cooling coils were cut off and pressure grouted by pneumatic grout guns. The concrete is still curing and gaining in strength as time goes on.
Despite popular folklore, the six inch depth of each section makes it extremely unlikely that construction workers were accidentally buried alive in the concrete
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