How did engineers cool the concrete of Hoover Dam?

Concrete gives off heat as it cures, so engineers did not pour Hoover Dam as one mass. They built it as a series of separate blocks and columns, and they embedded thin steel pipes in each block. River water ran through the pipes first, followed by chilled water from a refrigeration plant. Afterward the pipes were sealed with grout. [1] [2]

Cement reacts with water in a process called hydration, which releases heat. According to the Bureau of Reclamation, engineers calculated that a single continuous pour would have taken 125 years to cool to ambient temperatures, and the resulting stresses would have cracked the dam. A 1938 tourist guide quoted by the National Park Service gives a figure of 150 years instead, so the exact estimate varies by source, though all agree it would have been a century or more. [1] [3] [4]

The very dry concrete mix that gave the dam its strength also left little time to move each bucket from the mixing plant to the form before the concrete began to set. [1]

The Bureau of Reclamation describes trapezoidal columns rising in five-foot lifts. The National Park Service says the design called for an estimated 215 vertical columns, built from square forms ranging from 25 to 60 feet on a side. Letting each small block shed its heat was the first line of defense against cracking. The blocks had interlocking vertical and horizontal grooves, called keyways, so that they could later be bonded together. [1] [5]

Each form held cooling coils of one-inch thin-walled steel pipe. Once concrete was poured, river water circulated through the coils for an initial cooling. Chilled water from a refrigeration plant on the lower cofferdam then finished the job. One trade article says the plant could make up to 1,000 tons of ice a day and was the largest refrigerator in the world at the time. [1] [3]

After a block had cooled, the pipes were cut off and pressure grouted at 300 pounds per square inch. Grout was also forced into the joints between blocks, bonding the structure into one monolithic whole. The pipes remain buried in the concrete. [1] [4]

The total length of pipe is given differently: 590 miles by the National Park Service, over 650 miles by the American Society of Civil Engineers, about 600 miles by a trade article, and 581 miles in the 1938 guide. The sources do not explain the differences. A trade article also says crews paused 72 hours after finishing a layer, a detail the other sources here do not mention. [5] [6] [3] [4]