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Shotcrete FAQs through the dam from the head pond and either the static head have been demonstrated, even if the substrate concrete con- pressure (from the head pond) or repetitive freezing and thawing tinues to experience frost action.2-4,8 of the trapped moisture will deteriorate the existing and/or new concrete and/or pop the new shotcrete off the dam. So my ques- References tions include: 1. Heere R.; Morgan D.R.; Banthia, N.; and Yogendran Y., “Evaluation of 1. Is air entrainment of dry-process shotcrete a proven and Shotcrete Repaired Dams in British Columbia,” Concrete International, V. 18, No. 3, Mar. 1996, pp. 24-29. dependable process that will provide comparable resistance 2. Zhang, L.; Ezzet, M.; Shanahan, N.; Morgan, D.R.; and Sukumar, A.P., to freeze-thaw to that of wet-process shotcrete, as suggested “Ruskin Dam Spillway Shotcrete Assessed,” Concrete International, V. 33. by J. F. Dufour in Shotcrete, Fall 2008 issue? No. 2, Feb. 2011, pp. 37-43. 2. What is the permeability/hydraulic conductivity of both wet- 3. Gilbride, P.; Morgan D.R.; and Bremner, T., “Deterioration and Reha- and dry-process shotcrete? Are there published numbers or bilitation of Berth Faces in Tidal Zones at the Port of Saint John,” Concrete in Marine Environment, SP-109, V.M. Malhotra, ed., American Concrete In- ranges that can be used to compare the two methods to each stitute, Farmington Hills, MI, 1988, pp. 199-227. other, and to traditional cast-in-place concrete? 4. Gilbride, P.; Morgan, D.R.; and Gilbride, P., “Performance of Shotcrete 3. What admixtures, if any, do you recommend to 1) entrain Repairs to Berth Faces at the Port of Saint John,” Third CANMET/ACI Inter- air; and 2) increase permeability in dry-process shotcrete? national Conference on Performance of Concrete in Marine Environment, St. Andrews-by-the-Sea, NB, Canada, Aug. 4-9, 1996, pp. 163-174. 5. Morgan D.R.; Kirkness A.J.; McAskill, N.; and Duke, N., “Freeze- Answer: The downstream face of the dam could be relined Thaw Durability of Wet-Mix and Dry-Mix Shotcrete with Silica Fume and with either wet-mix or dry-mix shotcrete. Both methods have Steel Fibers,” Cement, Concrete and Aggregates, V. 10, No. 2, Winter 1988, been successfully used for this purpose on a number of dams pp. 96-102. and marine structures in North America and elsewhere. See for 6. Vézina, D., “Development of Durable Dry-Mix Shotcrete in Quebec,” example the references that follow.1-4 However, from a produc- Shotcrete, V. 3, No. 2, Spring 2001, pp. 18-20. 7. Dufour, J­F.; Reny, S.; and Vézina, D., “State­of­the­Art Specification tivity perspective, wet-mix shotcrete is usually the preferred for Shotcrete Rehabilitation Projects,” Shotcrete, V. 8, No. 4, Fall 2006, process, as it can be applied at about four times more volume pp. 4-11. per hour as compared to dry-mix shotcrete. Also, there is less 8. Forrest, M.P.; Morgan, D.R.; Obermeyer, J.R.; Parker, P.L.; and rebound in wet-mix shotcrete. LaMoreaux, D.D., “Seismic Retrofit of Littlerock Dam,”Concrete Inter- In response to your specific questions, we comment as follows: national, V. 18, No. 3, Mar. 1996, pp. 24-29. Note: References 1, 3, 5, and 8 can be found in the book Shotcrete, A 1. With the addition of an air-entraining admixture to dry-mix Compilation of Papers, by D.R. Morgan, published by the American Shotcrete shotcrete (typically added as a dry-powdered admixture Association, 2008, 424 pp. during batching in dry-bagged premix shotcrete, or sometimes as a liquid added to the mix water added at the Question: Is it possible to apply shotcrete on glass? Can shotcrete nozzle), the dry-mix shotcrete can develop an air shotcrete or concrete bond with glass? If it is an issue due to voids system which provides good freezing-and-thawing the smooth surface properties of glass, will sandblasting help? durability.5-7 2. Research studies currently being conducted by LZhang Answer: We are not aware of any specific applications of Consulting & Testing in Canada show that the values of shotcrete to glass. The glass would have to be tough or strong boiled absorption and volume of permeable voids (as enough to withstand the sprayed application. We have seen measured in the ASTM C642 test), for cast concretes and shotcrete stick to smooth surfaces like glass and glossy paint, wet-mix shotcretes made with the same cementing materials but have not seen any data on the bond. In general terms, better and aggregates and the same water/cementing materials ratio bond is achieved with roughened surfaces so sandblasting the are quite similar. Values tend to be slightly lower in well- glass would likely improve the bond. applied dry-mix shotcrete. 3. With respect to air-entraining admixtures for dry-mix Question: There is a pre-stabilized steep slope (nailing is used shotcrete, contact the major chemical admixture supply along with shotcrete). The client wishes to benefit from the max- companies to get recommendations for brand name products imum achievable area in plan and is asking for excavation of the and recommended dosage rates. stabilized slope to shift back the wall face around a few meters. In summary, air-entrained dry-mix shotcretes with good The new stabilization plan shall include a safe gradual freezing-and-thawing durability are routinely used with good excavation of the existing wall from top to bottom along with results in aggressive freezing-and-thawing exposure environ- the destruction of the existing shotcrete face, reinforcement, ments.6,7 Dry-mix shotcretes have a long history of successful and the installed nails. What is the most common destruction performance on the downstream face of concrete dams.1,2 Wet- method for the existing shotcrete? At the same time, I think mix air-entrained shotcretes have been successfully used to pulling out the nails using hydraulic jacks would be applicable. resurface both the upstream and downstream faces of concrete dams and marine structures. With proper design of anchorage Answer:The existing shotcrete can be removed by many methods, systems tying the shotcrete facing system back into the substrate such as using a Hoe-Ram. The existing soil nails could be removed concrete at depth, many decades of satisfactory performance as suggested or left in place and trimmed back to some distance Shotcrete • Spring 2015 75


2015SpringShotcreteEMag
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