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often lead to questions about the ability of the testing laboratory to properly conduct the test. A baseline BWA for the concrete mixture (not shot) should be conducted before testing shotcrete cores. Bond strength—The bond strength of shotcrete continues to be one of shotcrete’s main attributes. Because shotcrete is physically driven into the receiving surface by the high-velocity impact of the fresh concrete particles, excellent bond is achieved. Studies focusing on the bond qualities of shotcrete have proven that highvelocity placement to a sound substrate surface with adequate roughness provides durable bond. Multiple layers—This section has been added to help inform engineers who often confuse placement of multiple layers of shotcrete with the cold joints experienced with form-and-pour concrete. Shotcrete provides excellent bond between layers due to the consolidation and densification by high-velocity impact of fresh concrete onto a properly prepared concrete substrate. Studies of bond between multiple layers of shotcrete have proven shotcrete achieves excellent bond between layers, and provides a structural section that acts as if placed monolithically. Finishing—The Guide has expanded the section on finishes. The preferred finish is still a “gun” or “natural as-shot” finish. However, to compete with form-and-pour concrete, some owners want a smooth trowel finish which, for shotcrete, requires a two- or three-step procedure. Tolerances—The tolerance section has been expanded. Because shotcrete permits a wider variety of applications and surface finishes than form-andpoured concrete, ACI 117, “Specification for Tolerances for Concrete Construction and Materials,” specifically excludes shotcrete. ACI 117 provides excellent guidance for reinforcement placement and cover. The Guide gives the shotcrete project specifier criteria for specifying tolerances. Repair—A section on shotcrete repair was added to provide commentary to the ACI 506.2-13 repair section. Sustainability—In recent years, ACI has requested that new documents address sustainability. Shotcrete shares not only concrete’s durability, but because of its unique characteristics, also enhances concrete’s sustainability. Shotcrete promotes sustainability in many ways, including but not limited to: • A repair material that extends a structure’s life; • Formwork reduction, which saves resources; • Reduction of equipment needs on a project; • Reduction of the time for construction; and • Promotes creativity due to the ease of construction of curved sections. Safety—Early in preparation of the Guide, a chapter on safety was compiled. Traditionally, however, ACI has not produced safety documents. As we were developing the Guide, the American Shotcrete Association (ASA) put together a safety document far more encompassing than what was planned for the guide so the safety chapter was discarded. Summary Shotcrete has come a long way. The new ACI 506R-16, “Guide to Shotcrete,” builds on the original 1966 ACI Standard ACI 506-66, “Recommended Practice for Shotcreting,” and has been reorganized to serve as commentary to ACI 506.2- 13, “Specification for Shotcrete.” A section, “Preconstruction testing by contractor,” was added to provide guidance of when to include and what to include preconstruction testing. Also, “Testing during construction,” which is QA/QC guidance, was expanded. The QA/QC section defines the different types of shotcrete panels for testing or evaluation. The section on admixtures has been updated. The new Guide continues to emphasize the superior bond strength shotcrete achieves and explains why multilayered shotcrete should not be considered multiple cold joints. Lastly, shotcrete enhances the sustainability properties of concrete. The new Guide, like the first guide, is a consensus document compiled by volunteers with the goal of improving the quality of shotcrete projects. The volunteers, to be sure, have differences of opinion most often driven by different experiences in different regions. Thank you to all the Guide volunteers who devoted many, many hours of their time. References Rodriquez, L., From Elephants to Swimming Pools: Carl Akeley, Samuel W. Taylor, and the Development of the Cement Gun, Canal History and Technology Press, Easton, PA, 2006, 150 pp. Lars Balck is a concrete consultant and ASA/ACI Nozzleman Examiner. He recently retired from CROM, LLC, as a Senior Vice President. He has been involved in the design and construction of prestressed concrete tanks built with shotcrete for over 40 years. He received his bachelor’s degree in civil engineering from the University of Florida and served with the U.S. Army as First Lieutenant in Vietnam as a Combat Engineer. Balck is a Past President of ASA. He is Chair of ACI Subcommittee 506-C, Shotcreting-Guide; a past Chair and current member of ACI Committee 506, Shotcreting; and member of ACI Committees 376, Concrete Structures for Refrigerated Liquefied Gas Containment; 563, Specifications for Repair of Structural Concrete in Buildings; and C660, Shotcrete Nozzleman Certification. Shotcrete • Summer 2016 19


2016SummerShotcreteEMag
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