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Guide to Shotcrete In 1910, naturalist Carl Akeley introduced a machine he invented to build mortar models of animals at the cement show in New York. Shortly after, the Cement Gun Product Company was formed and the term “gunite”—what we now call dry-mix shotcrete—was coined. The cement gun was a breakthrough for concrete construction. Mortar could now be conveyed long distances and produce high-strength concrete (shotcrete). By 1916, however, manufacturing problems, the failure of a test application on the Panama Canal, and bickering among the original partners put the Cement Gun Product Company on the verge of bankruptcy. Samuel Taylor, a munitions and mining equipment manufacturer, bought the Cement Gun Company later the same year. He recognized both the potential of the cement gun and knew that the poor reputation gunite had developed impeded its success. To turn things around, Taylor consolidated control of the company and became the sole manufacturer. He then put together an experienced team and organized a contracting company to specialize in the placement of gunite. Engineering articles in the Cement Gun Company Bulletin were produced and reprinted in a number of engineering periodicals. These articles documented many of the merits of using gunite, including producing compressive strengths as high as 10,000 psi (69 MPa). Those strengths were extremely impressive for that time. Mixture proportions of 1-2-3 concrete (one shovel of cement, two shovels of sand, and three of large aggregate) were customary for site-mixed concrete and generally only achieved strengths of 3000 psi (21 MPa). By Lars Balck Between 1916 and 1920, Taylor improved gunite’s reputation and reversed the cement gun sales decline. Everyone wanted a cement gun, and many were sent overseas. Of course success also attracted imitators. By 1950, with no standards for equipment, a variety of manufacturers around the world produced inferior equipment that impeded the proper application of gunite. On top of that, inexperienced contractors with no idea of the details required for good gunite field application produced poor-quality gunite on many projects. Once again, gunite developed a bad reputation. Although the American Concrete Institute (ACI) was organized in 1913, the Institute didn’t establish a shotcrete technical committee until 1960. The term “shotcrete” was adopted by ACI because the original “gunite” was a registered tradename. The new committee was charged with revising ACI Standard 805-51, “Recommended Practice for the Application of Mortar by Pneumatic Pressure.” In 6 years, the committee made up of experienced shotcrete contractors, owners (including the U.S. Army Corps of Engineers), and testing laboratories published the ACI Standard, “Recommended Practice for Shotcreting (ACI 506-66).” This was essentially the first version of the document we now call the “Guide to Shotcrete.” The purpose of the Recommended Practice was to educate engineers, owners, and contractors about shotcrete and to provide practice standards to improve the quality of shotcrete projects. Much of the content in the early ACI 506-66 document is still contained in the present Guide. Updated versions were published in 1985, 1990, 1995, and 2005. ACI 506R-85 ACI 506R-90 16 Shotcrete • Summer 2016


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