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2014SumShotcreteEMag

Shotcrete FAQs As a service to our readers, each issue of Shotcrete will include selected questions and provide answers by the American Shotcrete Association (ASA). Questions can be submitted to info@shotcrete.org. Selected FAQs can also be found on the ASA website, http://shotcrete.org/pages/products-services/technical-questions.htm. Question: I am interested in any information or suggestions • http://shotcrete.org/media/Archive/2013Fal_Goin you may have regarding practical working space requirements Underground.pdf (concrete lining in tunnels) for shotcrete applications. Shotcrete is a common approach • http://shotcrete.org/media/Archive/2012Spr_Safety for sewer pipeline and storm-water culvert rehabilitation Shooter.pdf (fallouts) projects. My concern relates to the space requirements neces- • http://shotcrete.org/media/Archive/2012Spr_Under sary to best ensure a quality installation—for pipelines, this groundProject.pdf (incline tunnel) boils down to the question: What is the smallest diameter pipe • http://shotcrete.org/media/Archive/2013Fal_Shotcrete that can be used for this method? Technical specifications that Corner.pdf (reaching 20 MPa in 2 hours) I have come across call for a minimum of 3 ft (1 m) between • http://shotcrete.org/media/Archive/2013Spr_GoinUnder the surface being covered and the application nozzle. To me, ground.pdf (rapid-setting cement) this means that pipes that are much smaller than 6 ft (1.8 m) With properly qualified nozzlemen, a good shotcrete would create some difficulty. Similarly, for applications between mixture, and high-quality accelerator added at the nozzle, the vertical walls, how much room does a nozzleman need between re-entry time can be minimal—normally 24 hours. the wall receiving the shotcrete and the wall at his/her back? Are there robotic means or other methods in use that would Question: I know that there are many factors that affect the allow shotcrete applications without a hands-on nozzleman? distance that shotcrete can be pumped. For a dry-mix process, Are there any other workspace limitations or controls that is there a rule of thumb for a maximum recommended horizontal should be considered when determining feasibility of shotcrete pumping distance? application methods? Answer: The best information on this subject can be found in Answer: In the case of installing a lining inside of an existing ACI 506R-05, “Guide to Shotcrete,” and likely in past articles pipe, there are robotic methods available, such as spin lining, in Shotcrete magazine. The distance that can be pumped is a where the cementitious material is cast from a rotating head as function of too many parameters to fit a rule of thumb. The the carrier is moved along the pipe. For pipe smaller than 42 in. distance that can be pumped is influenced by the equipment (1067 mm) diameter, the spin lining is likely the best method. being used, the vertical lift, the available compressed air, and For pipe larger than 42 in. (1067 mm) and up to 6 or 8 ft other factors. We would suggest that you consult with one of (1.8 or 2.4 m) diameter, either hand shotcrete nozzling or spin our corporate members (http://shotcrete.org/pages/products- lining are applicable. For pipes much larger than 6 to 8 ft (1.8 services/Buyers-Guide/index.asp) in the area of the project to 2.4 m), hand nozzling is likely the best solution. and get their input. In the case of clearance between a wall to be shotcreted and an obstruction, 3 ft (1 m) is a good rule of thumb, but a qualified Question: We would like to place 4 in. (100 mm) thick shotcrete and experienced shotcrete contractor can use modified equipment reinforced with welded wire reinforcement and anchoring bolts to place quality shotcrete in tighter spaces. A recent article on in a water pressure tunnel. The water velocity would be between shotcreting in confined spaces can be found here: http:// 10 and 16.4 ft/s (3 and 5 m/s). We would like to know if there shotcrete.org/media/Archive/2013Sum_Sustainability.pdf. is a possibility of erosion or cavitation of the shotcrete at this range of velocity. Question: We have a vertical shaft that is (right now) 70 ft It is mentioned in our concrete manual that cavitation and (21.3 m) deep and we do blasting every 5 ft (1.5 m) after destructive erosion begin when water velocities reach about applying shotcrete to the vertical surface for protection. My 40 ft/s (12 m/s). Because the roughness of the shotcrete surface concern is that if we have less than 48 hours between succes- is higher than the concrete surface, is erosion more likely to sive blasting, is it allowable? How does one measure if the occur? Do you know what may be the maximum water velocity shotcrete reaches the required percentage of strength? acceptable for reinforced shotcrete? Answer: The best guidance on this subject can be found in ACI Answer: Shotcrete is a method of placing concrete and the 506.5, “Guide for Specifying Underground Shotcrete” (avail- surface finish can be as smooth as that of cast concrete. Even able through the ASA Bookstore), and some articles from past with a nozzle finish, shotcrete erosion or cavitation should issues of Shotcrete magazine might be of interest: not be an issue at the stated velocities. Examples of smooth • http://shotcrete.org/media/Archive/2006Spr_Keienburg. shotcrete surfaces can be found in many Shotcrete magazine pdf (support in tunnels) articles (http://www.shotcrete.org/media/archive/ • http://shotcrete.org/media/Archive/2006Sum_Keienburg. 2004Sum_Town.pdf) and in particular the article on the pdf (slope stabilization) Wachusett Aqueduct. Shotcrete • Summer 2014 67


2014SumShotcreteEMag
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