Radiant cooling is a great way to deal with sensible cooling loads in high-performance building design. However, one of the biggest radiant cooling projects in the history of the United States had an entirely different purpose.
The world’s largest hydroelectric dam at the time of construction, Boulder Dam was initially planned for an area of the Nevada and Arizona border called Boulder Canyon. It required a network of pipes filled with cold water to facilitate construction. This smart usage of radiant cooling ensured the successful building of a structure that continues to serve as a modern marvel of engineering.
The narrow span of the Colorado River at Boulder Canyon was the first choice for the dam construction; however, the project was eventually moved down the river to Black Canyon. The name one used to talk about the hydroelectric construction project may have tipped your hat to your political leanings, notes History.org.
In 1930, the dam was renamed after President Herbert Hoover. Three years later, when Franklin Roosevelt took office, the dam was changed back to the Boulder Dam. The use of Boulder Dam might signal that one was a Democrat, while Hoover Dam could signal Republican. Much like modern politics, there was much grandstanding on both sides to lobby for one name or the other. It wasn’t until 1947 that the name was officially confirmed/restored by Congress as the Hoover Dam.
It was going to take a lot of concrete to build this dam. The volume of concrete in the Hoover Dam could have been used to pave a 4-foot-wide sidewalk long enough to wrap around the world at the equator, the U.S. Bureau of Reclamation (USBR) says. Unlike a simple sidewalk, pouring that much concrete stacked in a canyon was an enormous engineering challenge.
The Hoover Dam wouldn’t still be standing if all the concrete was poured at the same time. USBR engineers broke the construction up into more than 200 different blocks, so the site looked like a few stacks of Lego blocks. It was smoothed into an arch later in construction. The dam is about 660 feet wide at the base and is more than 700 feet tall. The inside of the dam would have taken more than a hundred years to cure naturally if the construction crews had not integrated a radiant cooling system.
At the simplest mixture, concrete is composed of cement, aggregate and water. The size of the aggregate and a myriad of different additives make different types of concrete; the amount of water is crucial. To make the Hoover Dam as strong as possible, engineers aimed for a very dry concrete mixture. They also sought to have the exposed edges of the slabs cure at a similar rate as the inside volume. This is no easy task in a dam.
How did radiant cooling help with the curing of the Hoover Dam? Popular Mechanics provides more background on the chemical consistency of concrete: “Portland cement is produced by mixing ground limestone, clay or shale, sand and iron ore. This mixture is heated in a rotary kiln to temperatures as high as 1,600 degrees Celsius. The heating process causes the materials to break down and recombine into new compounds that can react with water in a crystallization process called hydration.”
Hydration essentially activates like glue in between all the aggregate.
USBR engineers realized that the heat in the center of the dam would have taken too long to cure, compared to the outside. The stress could have caused the dam to stress crack and break apart. To create the most durable structure possible, builders had to cool it from the inside, with water. Six hundred miles of steel pipe was installed through the different layers of concrete pours.
Once the 1-inch diameter, thin-walled steel piping was installed and concrete was poured, engineers pumped river water through it. After the first pass with cool river water, the second stage of cooling began. They built a refrigeration plant on-site to produce a maximum of 1,000 tons of ice per day. It was the largest refrigerator in the world at the time.
Next, icy water was circulated to bring the cores of the slabs down in temperature. After the concrete had cured to engineers’ satisfaction, the steel pipe was filled with grout, pumped in at 300 psi.
Breaking up the pours into different layers allowed the crews to move very quickly when they were mixing cement in a plant on the canyon wall. According to a “Stuff You Should Know” podcast, a crane would deliver an enormous bucket of concrete every 78 seconds to a different area in the dam. A team of workers would have to move quickly to smooth out the dry concrete mixture. After finishing a layer, they would stop for 72 hours to let the mixture cure, assisted by the radiant piping.
As a side note, every single phase of the dam construction would have been a modern OSHA nightmare. Somewhere around 100 workers died, but that figure may be low. Carbon monoxide poisoning wasn’t well understood at the time and may have been written off as a nonconstruction-related illness. The spillway tunnel workers were especially susceptible to CO. Falling rocks and heat stroke were also significant hazards. However, during the Great Depression, many were willing to take the risk.
An urban legend of the dam construction is that there are a few bodies encased in concrete. It probably is not the case because each layer of the dam was constructed 2 to 6 inches at a time. If someone fell into the concrete, they would have to go unnoticed for at least 72 hours before the next pour.
Today, the process used in the Hoover Dam construction is called concrete thermal cooling. In non-dam applications, it allows workers to continuously pour large slabs. The difference between the Hoover Dam concrete thermal cooling and modern radiant heating and cooling systems is essentially the piping material.
The steel pipe used to circulate water in the Hoover Dam was filled with grout to prevent water from running through the blocks after the mechanical cooling was finished. It was also a potential structural concern if the pipe rusted away. With PEX, the networks don’t have to be abandoned after the concrete sets.
The Hoover Dam was the first man-made structure to exceed the Great Pyramid of Giza in masonry mass. Without the use of concrete thermal cooling, this dam may have washed down the Black Canyon years ago. In modern megastructures, the network of pipe could serve the dual purpose of concrete thermal cooling and radiant heating/cooling of the occupied building. These two systems could help builders move faster and deliver a premium heating and cooling system.
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