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more diverse set of processes. Wet-mix was added in the second half of the century and we now have low-pressure sprayed mortar and the hybrid shotcrete/ place and vibrate processes. ICRI Technical Guideline 320.1R-92(96) very briefly covers wet-mix and dry-mix shotcrete. A future revision of the document is slated to include low-pressure sprayed mortar. All three will be included in “pneumatically applied methods.” So at this time, a specifier or owner simply requiring “pneumatically applied concrete” doesn’t necessarily get you high-velocity, highimpact force shotcrete. It may allow the lowpressure mortar spraying or a hybrid placement process. Low-pressure spraying by its inherent nature cannot provide high velocity, and thus has reduced ability to compact by impact and abrade the substrate to produce enhanced bond. Further, low velocity may well impair the ability of the mortar to flow around larger reinforcing bars and embeds. ACI CT-16 doesn’t define PAC. However, it does define “pneumatically applied mortar — see shotcrete.” This would seem to exclude lowpressure sprayed mortar because it is pneumatically applied but certainly not at the high velocity required of shotcrete. We will work with ACI to get their terminology aligned with the industry. So, in summary, what should we learn from this mix of terminology? • Specifiers and owners need to be aware that PAC is NOT always shotcrete. • Shotcrete requires high velocity for creating good bond, full compaction, complete encasement of reinforcing bars, and eliminating voids. • If high-velocity placement cannot be maintained in the structural section due to obstructions (heavy reinforcement or embeds), alternative methods for compaction must be available to densify the concrete mixture in-place. • Shotcrete has many ACI consensus documents directly addressing both dry-mix and wet-mix processes, including ACI 506.2-13 (Specification) and ACI 506R-16 (Guide), as well as a comprehensive set of ASTM standards for testing requirements. • There are no clearly established standards for low-pressure sprayed mortar or the hybrid application of shotcrete combining traditional shotcrete application with placing and vibrating. • Specifiers and owners should always specifically call out shotcrete when they desire placement at high velocity, high abrasion, compaction, and complete encasement of embedded reinforcement. • When specifiers or owners require shotcrete, they should verify the shotcrete contractor is using equipment (air compressors/delivery hose/nozzles) that produces enough air flow volume to propel the material at high speed. In closing, over the last hundred years, we’ve seen shotcrete dramatically improve in the variety of uses, equipment, and quality. As shotcrete has evolved, so has the term “pneumatically applied concrete,” as it now covers a wider variety of concrete placement technologies. Those of us in the industry simply need to be aware of this evolution, and when specifying application methods, be specific about the particular type of application we need for our project. Charles Hanskat is the current ASA Executive Director. He received his BS and MS in civil engineering from the University of Florida, Gainesville, FL. Hanskat is a licensed professional engineer in several states. He has been involved in the design, construction, and evaluation of environmental concrete and shotcrete structures for over 35 years. Hanskat is also a member of ACI  Committees 301, Specifications for Structural Concrete; 350, Environmental Engineering Concrete Structures; 371, Elevated Tanks with Concrete Pedestals; 372, Tanks Wrapped with Wire or Strand; 376, Concrete Structures for Refrigerated Liquefied Gas Containment; 506, Shotcreting; and Joint ACI-ASCE Committee 334, Concrete Shell Design and Construction. Hanskat’s service to the American Society of Civil Engineers (ASCE), the National Society of Professional Engineers (NSPE), and the Florida Engineering Society (FES) in over 50 committee and officer positions at the national, state, and local levels was highlighted when he served as State President of FES and then as National Director of NSPE. He served as a District Director of Tau Beta Pi from 1977 to 2002. He is a Fellow of ACI, ASCE, and FES and a member of ACI, NSPE, ASTM International, and ASCC. Shotcrete • Fall 2016 25


2016FallShotcreteEMag
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