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2014SumShotcreteEMag

Shotcrete Corner followed ASTM C1604/C1604M, which involved assessed through the use of the end beam testing shooting a 2 ft x 2 ft x 4 in. (0.6 m x 0.6 m x apparatus, which allowed the engineers to prop- 100 mm) wooden panel and then coring the panel erly quantify the results. to retrieve specimens for traditional compressive Because there are many factors that can affect strength testing. The problem with this method set time and rate of strength gain, including was that the panel must remain in the position it water-cement ratio, water temperature, air tem- was shot until the material had reached final set. perature, and product temperature, the company, In an underground environment, this usually to maintain their due diligence, completes fre- meant leaving the panel underground for an extra quent testing of their backfill barricades with the shift before transporting it to the surface, where end beam testing apparatus. it was then shipped to a certified lab where the Within the mining and tunneling industries, actual testing could begin. This process was time- the popularity and diversity of shotcrete continues consuming and cumbersome, resulting in the to grow. Engineers understand that the shotcrete earliest available compressive strength values process allows for the elimination of labor- being 24 to 36 hours. intensive forming activities and the costly trans- The end beam tester allowed a nozzleman to portation of forming materials underground. The shoot directly into molds that were designed to use of the welded wire reinforcement panels as a be quickly and easily disassembled. After disas- receiving surface for shotcrete has dramatically sembling, each end of the beam could be broken reduced the labor content and cost of projects, on-site, using a hydraulic hand pump so that a such as the construction of backfill barricades. compressive strength value could be determined Ventilation bulkheads, garage doors, refuge sta- at almost any stage of the hydration process. tion walls, and many other projects that require One company’s barricade and backfill proce- concrete placement are now also completed using dures specified that the empty stope could not be the shotcrete process. It is now understood that backfilled until the shotcrete reached a minimum the shotcrete process is merely an alternative compressive strength of 2900 psi (20 MPa) which, method for placing concrete, so with proper based on the available test methods, meant design and engineering, the use of shotcrete 48 hours after the shotcrete was placed. Using the underground is only limited by the imagination early-age compressive strength testing equipment, of the design engineer. the company was able to determine that the shot- crete material reached the minimum 2900 psi (20 MPa) value only 24 hours after placement. This information allowed the mine to change their Craig McDonald is a Sales procedures so that the stope could be filled just Manager for the Mining Mar- 24 hours after placement, translating into a full kets Division of King Packaged day’s reduction in cycle time (Fig. 4). Although Materials Co. Equipment. The this decision was based on the use of a special wet- and dry-mix shotcrete pro- shotcrete mixture that provided high-early- cess, as well as the mixing and strength gain, the early-age compressive strength placing of dry-mix concrete and of the in-place shotcrete could only be properly grout products, are his areas of expertise. He is a graduate of the Mechanical Engineering Technology program at Canadore College, North Bay, ON, Canada. Joe Hutter is the Vice Presi- dent, Sales, for King Packaged Materials Company, Burlington, ON, Canada. He has more than 25 years of experience in the cement/shotcrete industry. He is a former President and an active member of ASA and has Fig. 4: The end beam tester provided one mine with the information required been Chair of the ASA Marketing/Membership to change their procedures so that the stope could be filled just 24 hours Committee since its inception. Hutter is also a after placement, translating into a full day’s reduction in cycle time member of the American Concrete Institute. 40 Shotcrete • Summer 2014


2014SumShotcreteEMag
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