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Nozzleman Knowledge external sources. Movement or large temperature tions within a “moist cabinet or room.” Specifica- fluctuations will influence panel integrity. Early- tions require a temperature of 70 to 77°F (21 to age shotcrete has not developed sufficient strength 25°C) and a relative humidity of not less than 95% to withstand handling or thermal stresses during be maintained to diminish curing variables that the first 24 hours or more. Test panels should may reduce strength development during storage. always be positioned and shot in a location where Unfortunately, panels kept within ideal conditions they can remain undisturbed and protected from at the lab will not reverse damage to the panel the elements for 24 hours or more. caused by improper handling or curing practices at the job site during the critical initial hours. Moisture’s Influence on the test Panel Summary Actions of the nozzleman and shotcrete crew Shotcrete does not harden by drying; it hardens have a powerful influence on the strength devel- by a complex chemical reaction called hydration. opment of specimens drawn from a shotcrete test Moisture is required for the hydration process to panel. Training and education are required to occur. With time, shotcrete will grow progressively diminish strength-test variables. A training pro- stronger if sufficient moisture is available for hydra- gram that draws attention to approved methods tion (refer to Fig. 4). If internal concrete moisture of creating, handling, and the proper care of drops to below roughly 80%, hydration will stop shotcrete test panels is absolutely necessary to and concrete will no longer continue to gain strength. attaining representative strength values. Using ACI and ASTM specifications require that techniques that assure a repeatable standard of “The test panel shall be kept moist and at 70 ± care for each test panel should be a component 10°F (21 ± 5°C) until moved to test laboratory. of every shotcrete worker’s education program. The panels shall be covered and tightly wrapped (as soon after fabrication as is safe to prevent References damage) with a sheet of material to prevent evapo- ACI Committee 506, 2005, “Guide to Shotcrete (ACI ration of water from the material.” 506R-05),” American Concrete Institute, Farmington Hills, Test panels left unprotected are subject to MI, pp. 31, 33, 34. excessive moisture loss that can disrupt hydration ACI CCS-4 (08), “Shotcrete for the Craftsman,” American and slow strength development. Low or erratic Concrete Institute, Farmington Hills, MI, p. 15. test values can occur anytime exposure allows Obtaining and Testing Drilled Cores and Sawed Beams ofASTM C42/C42M, 2013, “Standard Test Method for panels to lose a significant amount of moisture. Concrete,” ASTM International, West Conshohocken, PA, p. 3. Curing practices implemented by the nozzleman ASTM C94/C94M, 2004, “Standard Specification for and placement crew are crucial to the strength Ready-Mixed Concrete,” ASTM International, West Con- development of shotcrete test specimens. shohocken, PA, p. 8. After test panels are moved to a testing facility, Rooms, Moist Cabinets, Moist Rooms, and Water StorageASTM C511, 2009, “Standard Specification for Mixing they are stored under carefully controlled condi- Tanks Used in the Testing of Hydraulic Cements and Con- cretes,” ASTM International, West Conshohocken, PA, p. 2. 80 ASTM C1140/C1140M, 2011, “Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels,” ASTM International, West Conshohocken, PA, p. 3. w/cm = 0.31 60 Compressive Strength, MPa w/cm = 0.41 w/cm = 0.50 40 ACI Certified NozzlemanOscar Duckworth is an ASA and American Concrete Institute 20 (ACI) member with over 15,000 hours of nozzle time. He has worked as a nozzleman on over 0 2000 projects. Duckworth is56287310 Age, days currently an ACI Examiner for Fig. 4: Development of compressive strength of the wet- and dry-mix processes. He is also on the concrete with different w/cm (RHA content = 10%) ASA Board of Direction and Chair of the ASA (Originally from Fig. 2-4 of ACI 232.1R-00 Safety Committee. He continues to work as a Zhang and Malhotra) (Note: 1 MPa = 145 psi) shotcrete consultant and certified nozzleman. 50 Shotcrete • Summer 2013


2013SumShotcreteEMag
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