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2016FallShotcreteEMag

Pneumatically Applied Concrete: A Confusion of Terminology In recent years, we’ve heard and seen the term “pneumatically applied concrete” (PAC) used in many discussions and specifications. The term seems straightforward, as it simply describes the act of placing concrete (or mortar) by flow of air through a nozzle. However, we are now seeing specifiers and owners specifying PAC, and apparently not fully understanding the variety of pneumatically placed methods in modern concrete construction and repair. Evolution of Terminology According to ACI 506R-16, “Guide to Shotcrete,” the term “pneumatically applied mortar or concrete” (PAM or PAC) evolved as a rather generic term early in the 1900s after the proprietary term “gunite” was first established for what we now refer to as the dry-mix shotcrete process. In the 1930s, the American Railway Engineering Association (now known as AREMA) created the term “shotcrete” to replace the proprietary term “gunite” in railroad concrete works. In 1951, the American Concrete Institute (ACI) adopted the AREMA term “shotcrete” because use of tradenames in technical documents was frowned upon. As the wet-mix process developed soon By Charles Hanskat thereafter, it too adopted the term “shotcrete.” “Sprayed concrete” is often used in lieu of the term “shotcrete” in other countries around the world. As concrete repair materials became more specialized than the original sand and cement mortars, a new process, “low-pressure spraying,” was introduced as a method of application to replace hand troweling of spalls. Concrete spall repair sections that the method were designed for were thinner and generally didn’t require a high production rate of material placement. Although these spalled sections could certainly be shotcreted, the low-pressure sprayers required less equipment investment, less nozzleman expertise, and due to lower pressure, could be used in tighter spaces with less rebound. However, successful application with low-pressure spraying generally required proprietary, prepackaged cementitious products. Shotcrete As we look at the variety of application methods that may be considered PAC, let’s first Fig. 1: Dry-mix gun in early 1900s Fig. 2: Graco low-pressure mortar sprayer 22 Shotcrete • Fall 2016


2016FallShotcreteEMag
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