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Shotcrete FAQs Finally, the efflorescence caused by exposure to rainwater Supplemental cementitious materials including microsilica, is generally only a visual defect and doesn’t affect the long-term fly ash, and slag will generally reduce the porosity of the structural integrity or durability of the tank. Many owners hardened concrete. Microsilica is used in many shotcrete tolerate efflorescence on the tanks and simply clean it off when mixtures, as it helps to reduce rebound, as well as gives the it becomes objectionable. fresh concrete better adhesion and cohesion that can allow for thicker or overhead placements. Question: We have a large project involving shotcreting soffits in an underground parking garage. The shotcreting overhead Question: I have a client with an old, soft-stone masonry is not the problem; the problem is properly screeding the excess building of approximately 150 years of age. The mortar is badly shotcrete from the ceiling leaving a semi-smooth finish. deteriorated and the stone is quite friable. I am advocating the use of shotcrete as an application to the interior face of the Answer: Properly screeding and finishing overhead shotcrete walls that will restore both in-plane and out-of-plane strength is very challenging. The contractors who do this type of work to the building walls. properly have very well-trained and skilled tradesmen My client has expressed concern that there may be throughout the crew, including the nozzlemen, rodman, and incompatibility issues between the stone masonry and the finishers. Shotcrete that is not screeded and finished properly shotcrete both from a structural stiffness perspective as well will likely suffer bonding and other issues. as from a moisture intrusion perspective. (We have successfully used shotcrete over stone masonry in the past.) Question: We are involved in the design of a hydro project in Do you have any information you can share with me on this a section of a water-conveyance power tunnel; we are consid- topic? Do you have either examples of incompatibility or ering using shotcrete reinforced with welded wire reinforcement successful use of shotcrete over stone masonry? as a final liner. In this particular section, the tunnel is under an internal water pressure of 189 psi (1.3 MPa) and water Answer: As you have noted, shotcrete has been used exten- velocities in the range of 16.4 ft/s (5 m/s) can be expected. We sively to reinforce unreinforced or under-reinforced masonry have not found any examples of such a design/use at this water walls and rock walls. It has been used on the Crater Lake Lodge velocity and are concerned about long-term durability and to strengthen and stabilize a rock wall foundation and any potential erosion of the shotcrete and entrainment of fragments number of other projects. In California, shotcrete has been used into the turbine/powerhouse. to strengthen or repair walls since the 1933 Long Beach Earth- Would you have any information regarding the ability of quake. It was used to strengthen the California State Capitol shotcrete to resist water erosion, particularly at 16.4 ft/s (3 ft 0.9 m thick brick walls) in the late 1970s and all of the (5 m/s)? (Any examples would be appreciated.) What additive older unreinforced masonry walls for the San Francisco School can be used to reduce the porosity of the projected mixture? District. To the best of our knowledge, there have been no failures of shotcrete strengthening on the West Coast in the past Answer: Shotcrete is simply a placement method for concrete, 80 years. so characteristics of concrete that are resistant to erosion are equally applicable to shotcrete. ACI 210R-93, “Erosion of Con- Question: Why is there not more extensive use of fiberglass crete in Hydraulic Structures,” has guidance on flow character- reinforcing bars? It seems like it would be a natural choice for istics that lead to erosion of concrete. Also, ACI 350-06, “Code most projects involving shotcrete in wet applications, as well Requirements for Environmental Engineering Concrete Struc- as conventionally placed concrete, especially in the types of tures and Commentary,” Sections 4.6.2 and 4.6.3, also provide jobs we do, such as the rehabilitation of existing concrete guidance on concrete mixture characteristics helpful for pro- channels that usually contain acidic waters. I understand that tecting against cavitation erosion. Properly designed shotcrete anything other than steel is more expensive, but isn’t prevention mixtures can easily meet the ACI 350 4.6.3 concrete requirements. now cheaper than remediation later? In 2000, Rusty Morgan compiled a list of some 37 water supply tunnels that had been lined with shotcrete (a copy of Answer: Although similar in dimensions, fiberglass reinforcing the data sheet can be supplied upon request). Shotcrete was not has distinctly different structural properties when compared to the final lining in all of these tunnels and not all the inverts conventional steel reinforcement. This is a question better were lined with shotcrete. The evaluation does not document answered by the fiberglass reinforcing industry or the structural the water velocity in these tunnels, but could be ascertained by engineering community. As the American Shotcrete Associa- contacting the project owners. tion, we do not get involved in the engineering design of It should be noted, however, that the 16.4 ft/s (5 m/s) water flow structural sections. However, it should be pointed out that rate is not particularly fast. The water velocity needs to be in excess properly designed and applied shotcrete provides a very cor- of 39.3 ft/s (12 m/s) before cavitation erosion can be expected rosion-resistant environment around embedded steel reinforce- (refer to A. M. Neville, Properties of Concrete) and cavitation ment, providing excellent long-term durability in normal would be the most likely cause of erosion of the concrete surface. exposure conditions. Shotcrete • Summer 2014 69


2014SumShotcreteEMag
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