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Technological Innovations in Concrete

Despite the lack of significant engineering or design "firsts" attributed to Bluestone Dam or reported in contemporary engineering journal articles on the project, there are several noteworthy aspects of the dam's design and engineering. The dam was built in an era of marked advancements in concrete technology. In the 1940s, for example, construction specialists were beginning to understand the value of air entrainment, a technique in which the strength and durability of concrete was enhanced by trapping air bubbles in the mix. This technique was not used in the first construction phase of Bluestone Dam, but it was adopted soon after the second construction phase began in 1946. Bluestone Dam, like Tygart before it, is a fairly early example of the use of hydraulic model testing to assist in the design of the spillways and other parts of the dam related to water flow. Bluestone Dam's construction reflects technological changes that occurred from the mid-1930s to the late 1940s.

The vast majority of Bluestone Dam's mass consists of concrete, some of it reinforced with steel rods (or rebar). Transporting and pouring of the concrete appears to have been fairly typical for the time. Bottom-dump concrete buckets, each with a capacity of two to three cubic yards, hauled the concrete to flat cars on the dinkey railroad, which then moved the buckets around as needed. Whirler cranes picked up the concrete dump buckets, positioned them above the spot where the concrete needed to be poured, and then the buckets were emptied. Workers inserted a vibrator into the liquid concrete to settle and consolidate the material, and to remove air and water pockets that formed during pouring. Next came tamping and leveling of the surface of the concrete, followed by finishing with wooden floats or a steel trowel.

The 1949 Final Report for Bluestone Dam indicates that all concrete poured on the dam was cured with either water or steam, depending on the weather conditions. The curing, or hardening, process is actually a chemical change that takes place within the concrete, and is not a matter of the concrete simply "drying out." The exposed portions of the concrete at Bluestone had to be kept wet during the curing process, and during warm months a hose and sprinkler system sprayed water on the exposed concrete surfaces and wooden forms. In colder months when freezing presented problems, curing was done using steam pumped onto the concrete's exposed surface. The steam had the dual purpose of heating the concrete while also keeping the surface of the concrete wet. In extremely cold temperatures, tarps were placed over the concrete's surface to retain the steam heat. Steam curing generally took about five days, but could take longer during very cold conditions.

A more complex problem faced by the dam builders was the maintenance of an acceptable internal temperature inside the curing concrete. The U.S. Army Corps of Engineers learned from experience the importance of avoiding temperature extremes inside large masses of uncured concrete. One way to avoid excessively high temperatures was to chill the water used in mixing the concrete. Refrigerated mixing water may have been used as early as 1939 at Hiwassee Dam to lower the temperature of concrete as it cured. The Corps further developed water-chilling techniques for the Norfolk Flood Control Dam in Arkansas (1941-1945) by dumping crushed ice into the concrete mixing water during warm months. This successfully reduced the temperature of the freshly mixed concrete by about 10 degrees Fahrenheit. Bluestone Dam's answer to cooling the concrete was the installation of a water-cooling unit in the original concrete plant that was built on the site in 1942. A York ice machine with two ammonia compressors powered by a pair of 100-horsepower motors chilled 100 gallons of river water per minute, from its original temperature of 85 degrees down to 35 degrees. If the water temperature before chilling was as low as 60 degrees, the plant could process up to 200 gallons per minute.

Another interesting technological aspect of the Bluestone Dam's construction is the modification of the concrete mixture as the construction progressed. The concrete mixture appears to have remained consistent throughout the early construction campaign, but in late 1946, detected problems encouraged two changes. First came the revelation that the fine aggregate contained an insufficient amount of natural and stone sand. A significant increase in the amount of fine sand improved the concrete's quality by reducing air and water pockets and other unsightly pockmarks on the surface of the concrete. This technique mainly improved the appearance of the concrete, but not its strength or durability. Next came a major change in the composition of Bluestone Dam's concrete. Beginning in the mid-twentieth century, a technique known as air entrainment was developed to provide more durable, workable concrete. Air-entrained concrete often possesses a more consistent appearance than regular concrete, and it is highly resistant to harsh climate conditions. In the 1940s, the air entrainment technique generally involved adding a mixture that trapped small air bubbles in the concrete. Air entrainment eventually became standard practice in concrete construction, but it was still in its infancy in the early 1940s. The original 1941 construction specifications for Bluestone Dam do not mention air entrainment, and the technique was not used during the 1942-1944 construction phase. A change order of March 20, 1947 explained, "When the original plans/specs were prepared, the advantage of the use of air entraining agents in the concrete had not yet been adequately determined. Higher authority has subsequently recommended the use of air entraining admixtures in concrete structures of this type."

In a series of experimental concrete pours at Bluestone Dam in 1946, a variety of test materials were added to the concrete to improve its workability, durability, consistency, and appearance. One air-entraining agent, Darex AEA (a triethanolamine salt of a sulfonated hydrocarbon), proved to be the best choice to "produce a more workable mix and a more durable concrete."

The concrete of the lower sections of the dam built in 1942-1944 still retains a darker, ochre-colored hue, and there are many areas of moss growth and discoloration. The concrete of the upper portions of the dam is of a slightly different color and is cleaner and smoother, with relatively few moss growths, pockmarks, or other surface irregularities. This may be evidence of the greater durability of the post-1946 concrete poured at the dam, a concrete produced with a higher percentage of fine sand and using the process of air entrainment.

According to Nicholas Schnitter's A History of Dams: The Useful Pyramids, air-entraining agents were first used in connection with concrete dam construction at Angostura Dam, completed by the U.S. Bureau of Reclamation in South Dakota in 1949. Raymond E. Davis states in "Historical Account of Mass Concrete" that around 1945 air entrainment became standard practice for pouring concrete that was to be exposed to severe weather conditions. However, he gives no further details. Depending on when air entrainment was first used in dam construction, Bluestone may have been one of the first concrete gravity dams built in the United States with air-entrained concrete.