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2016WinterShotcreteEMag

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: We are working on a project in the United States, in Chicago, IL, and we are investigating the feasibilities involving shotcrete façade. The project involves large elements (50 x 50 ft 15 x 15 m) with double curvatures (average radius 50 ft 15  m, smaller in some areas). The concrete would have an architectural finish and should be in white color (cemento bianco). We are thinking about a reinforcement grid of 4 x 4 in. (100 x 100 mm); the diameter has still to be determined. Do you have any recommendations/specifications concerning the conception of this type of façade? We saw that it is possible to use reinforcement in glass fiber, epoxy reinforcing bars, or steel reinforcing bars. What are the advantages of each? Also, we have some more specific questions: 1. What is the durability of the product for this type of application—for example, estimated design lifetime? 2. Is on-site cold bending possible? To which radius of curvature? 3. Could there be a problem with steel staining the concrete? 4. Could there be a problem with concrete aggregation on the mesh when the concrete is shot? The coating doesn’t get away? 5. Do you have an estimation of the cost per m2? Answer: The double-curved concrete shape you mentioned is an ideal application for shotcrete. Because the shotcrete surface is able to be finished by hand once the sections are shot, a wide variety of architectural finishes are readily available. Because shotcrete is a placement method for concrete, the structural reinforcement is determined by conventional concrete design requirements. Reinforcing steel bars are typically conventional mild steel bars or epoxy-coated bars. Micro- or macro-fibers can also be added to the concrete mixture to provide additional reinforcement, although they generally supplement conventional reinforcing bars. Synthetic fibers are preferable over steel fibers in sections that will be exposed to view, as steel fibers can exhibit rust stains at the surface. The structural engineer should determine the proper amount of reinforcement for the structural sections. Answering your specific questions: 1. Durability—because shotcrete is a placement method for concrete, it will have the same durability as cast concrete. The shotcrete mixture can have air-entraining admixtures for increased freezing-and-thawing resistance. Also, shotcrete mixtures often use silica fume that helps to increase strength and reduce permeability of the concrete, giving the concrete enhanced durability. 2. Sure. Because shotcrete is concrete, all bending methods used in conventional reinforcement are applicable. ACI 318- 14, Section 25.3.2, has minimum diameters for bends. Code Section 26.6.3.1 requires cold bending unless permitted by the engineer. 3. With proper concrete cover and close attention to tie wire projections, there should not be any significant staining of embedded steel reinforcement on the surface. 4. Shotcrete impacts the surface with a material stream velocity from 60 to 80 mph (97 to 129 km/h). It compacts and consolidates the concrete around the embedded reinforcement. With proper shotcrete application techniques, there should be no buildup of aggregate on the reinforcement. Epoxy-coated reinforcement is used extensively in shotcreted work. For more detailed information about the use of epoxy-coated reinforcement, you may want to consult the Epoxy Interest Group of the Concrete Reinforcing Steel Institute (www.epoxyinterestgroup.org). 5. Cost is determined by the geographic location, size, shape, and accessibility of your structure. You may want to consult our ASA Buyers Guide to find experienced shotcrete contractors who work in the Chicago area (www.shotcrete. org/pages/products-services/Buyers-Guide/index.asp). Question: I am exploring ways to renovate an existing fountain, the mechanics (plumbing and so on) of which have been/will be abandoned. There are existing cracks running straight across the reservoir, perhaps because the original drawings did not include a proper crushed stone base down to the frost line. We are raising the fountain base and water line up 8 to 12 in. (203 to 305 mm). I hope we can place the water, electrical, and drain lines directly on top of the existing reservoir (opposed to trenching down through it). Budget is a huge concern. My questions/concerns: 1. Can/should we provide a ±10 in. (±254 mm) layer of crushed stone or gravel in between the existing concrete reservoir and new, raised reservoir, which could be a 9 to 10 in. (229 to 254 mm) layer of gunite? I’m hoping this may provide some buffer for any future heaving of the existing reservoir and hopefully keep costs down. 2. Should we then core drill the existing concrete to provide drainage—that is, 1/2 in. (13 mm) weep holes—in case any water finds its way there? 3. Should there be any water stop(s) in the new gunite reservoir, which also acts as a seat wall along the perimeter? 4. Can you recommend any low-cost “flexible” products for the finished surface that would “hide” any (heaven forbid) future imperfections? Perhaps an epoxy paint? 5. If you recommend we instead pour directly on top of the existing gunite (instead of the stone buffer), are there any other recommendations beyond patching the cracks and sandblasting/cleaning the existing concrete? Answer: Questions 1, 2, and 3—in liquid-containing structures, it is generally preferred to keep the concrete floors on a subbase 66 Shotcrete • Winter 2016


2016WinterShotcreteEMag
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