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variety of applications, including underground, foundation walls, ground support, repair or repurposing, domes, cylindrical tanks, pools, and skate parks. Shotcrete is likely one of the biggest users of silica fume and fibers. We also regularly use hydration control, shrinkage-reducing admixtures, and accelerating admixtures. We’re even moving into the use of nanoparticle technology to enhance the application and performance of our shotcreted projects. Shotcrete has many ACI consensus documents directly addressing both dry-mix and wet-mix processes, including ACI 506.2-13, “Specification for Shotcrete,” and ACI 506R-16, “Guide for Shotcrete.” There is also a comprehensive set of ASTM Standards (C1140, C1385, C1480, and C1604) clearly delineating testing requirements. Additionally, ACI has a well-defined and comprehensive certification for shotcrete nozzlemen in both wet and dry processes and in both vertical and overhead orientations. Low-Pressure Mortar Spraying According to ACI Repair Application Procedures, “Spall Repair by Low-Pressure Spraying” (RAP-3), “Similar to wet-mix shotcrete but sprayed at a much lower velocity, low-pressure spall repair spray comes in the form of prepackaged mortar. The spray is applied using small concrete pumps or heavy-duty grout pumps to force the low-slump mortar through a hose. Air is added at the nozzle to impel the mortar. Bond with the prepared substrate is achieved through a combination of proper surface preparation, lowvelocity impact, and the material properties of the prepackaged mortar.” The pumped mortar is at a lower pressure than wet-mix shotcrete (200 to 500 lb/in.2 13.7 to 34.5 bar), and uses very low air flow as compared to either wet-mix or dry-mix shotcrete (10 to 15% of the flow rate). As evidenced by the title of the RAP-3 document, the application method was primarily developed to replace hand troweling of repair material in areas of spalled concrete. So how does this method compare to shotcrete? There is much less compaction force and surface abrasion of the substrate. Bond depends on the quality of the prepared substrate and mostly on the adhesive properties of the repair mortar. ACI RAP-3 indicates that all low-pressure, sprayapplied repair materials are proprietary, prepackaged cementitious products. Finally, with such low pressure and velocity, it is difficult to produce the differential pressure between the air flow and the space behind reinforcement to produce an active flow of material around larger reinforcement. Thus, much of the low-pressure sprayed work is very lightly reinforced with wire mesh or very-small-diameter bars. It should also be noted there are no clearly established technical standards for materials, equipment, and placement of low-pressure sprayed mortar. ACI RAP-3 appears to be the only document addressing low-pressure spraying but has minimal technical provisions in its less-thanfour page, non-mandatory language overview of low-pressure spraying. A future revision of International Concrete Repair Institute (ICRI) Technical Guideline No. 320.1R, “Guide for Selecting Application Methods for the Repair of Concrete Surfaces,” will include more information, but the document is still under development. There is no equivalent to the ACI Shotcrete Nozzleman Certification, and there doesn’t appear to be any directly applicable ASTM testing standards. A “Hybrid” Placement Process As shotcrete placement was evaluated for thicker, structural sections with dense reinforcement, a hybrid placement process has evolved. Sections with two or three layers of reinforcement, several feet thick, and with reinforcing bars up to No. 11 (No. 36M) have been successfully shotcreted by experienced shotcrete contractors. With dense reinforcement placed in multiple layers, the velocity of the shotcrete (once it reaches the back of the shotcreted section) may be significantly reduced due to impact with the reinforcing bars in the outer layer(s). With the reduced impact force from lowered velocity, the concrete may not be completely consolidated and supplemental consolidation must be supplied. This is usually achieved with a small pencil vibrator to keep the very low-slump concrete from shifting to the front of the section. In these thick sections, the outer layer of reinforcement may very well be properly encased by normal shotcrete techniques. The shooting of these types of sections has been very successful, and due to the benefits of shotcrete placement including reduced formwork, efficient scheduling, flexibility in placement of the delivery hose, and overall improved sustainability. Being a hybrid process, some have used the term PAC to denote this type of work because the concrete is being placed pneumatically. As this is a developing placement technique, there are no clearly established standards for the hybrid application of shotcrete combining traditional shotcrete application with placing/vibrating. In the future, perhaps ACI 506 will address this hybrid system. Sorting Out the Confusion of Terms As you can see, “PAC” is a term applied (for right or wrong) to a variety of processes. PAC in the early 1900s was gunite, where there were no other possibilities. Today’s PAC includes a much 24 Shotcrete • Fall 2016


2016FallShotcreteEMag
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