Page 62

2015WinterShotcreteEMag

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 an architectural design firm that specifies For the area that is 6.25 ft (2 m) thick, should I be using extensive amounts of shotcrete to mimic rockwork in our multiple layers of reinforcement (or fiber reinforcement) to designs for large-scale animal exhibits. We often encounter prevent fallout? differing views on the need for galvanized or coated rein- forcing bar in our shotcrete work, especially when they are Answer: This is a very challenging potential installation and used as pool lining walls containing salt water. Can you tell there could be several potential approaches. For the shotcrete us your opinion on whether galvanizing or coating of rein- to bond to the existing concrete, the existing surface should be forcing bar is necessary if both integral and surface water- properly prepared, removing any unsound concrete, then proofing are specified? roughened and cleaned to allow for a good bonding interface. You mentioned using rock anchors or bolts. These should be Answer: Shotcrete is a process of placing concrete. The installed before any shotcrete. need to use galvanized or any other type of reinforcing is, For each 6 in. (152 mm) layer, a layer of welded wire rein- or should be, based on the needs of the project and the forcement or structural fibers should be used. These, in conjunc- preference of the structural design engineer. For liquid- tion with the rock bolts, should ensure the stability of each containing concrete structures, the designer may review ACI layer of the shotcrete from falling. 350-06, “Code Requirements for Environmental Engineering The surface reinforcement should not be installed before Concrete Structures,” which has specific concrete material, most of the area is within 6 to 8 in. (152 to 203 mm) of the cover, and reinforcing requirements for concrete exposed final surface. to a variety of chemical and saltwater exposures. (The There is currently similar thicknesses being placed on the aforementioned ACI Committee 350 publication may be East Side Access in New York City to build back the overbreak purchased from the American Concrete Institute at www. for the initial tunneling to the “A-line” or profile that was concrete.org.) intended for the mining. Question: I am lowering the roof of the draft tube on a hydro- Question: We’re designing two steel stacks to be located in power dam. The work is all overhead and has a slope to it. The Texas and subject to hurricane winds. The diameters are 9.19 new roof profile will be lowered from 2 in. (51 mm) (initial and 4.92 ft (3 and 1.5 m), respectively. Both stacks are 118 ft edge) to 6.25 ft (approximately 2 m) thick. The plan is to use (36 m) high. To reduce the vortex shredding oscillation effects, rock anchors to transfer the load to the existing concrete, and we want to increase the mass of the stacks by means of internal tie in a reinforcing bar grid at the lower section of the new roof gunite lining. Our calculation provides good results considering profile (4 in. 102 mm cover). Shotcrete is being planned for approximately 2 in. (51 mm) thick interior liner. the infill material. The traditional ACI 318 design method was For this scope, is it better to consider external or internal used for sizing reinforcing bar. lining? Would there be any advantage to applying an external The concern I have in the design is the application of the liner with respect to the interior solution? Which of these two shotcrete. The plan is to allow the contractor to install a wire is the most cost-effective solution? mat (or reinforcing bar mat) approximately 2 in. (51 mm) from the existing concrete roof, then apply shotcrete until Answer: Shotcrete would work well either for the stack exte- reaching the reinforcing bar location, up to 6 ft (1.8 m) thick, rior or as an interior lining. The 2 in. (51 mm) thickness could then apply the finish layers. I have concerns about how thick be easily applied on either the outside or inside surfaces. shotcrete can be applied overhead. I have received feedback For the 9 ft (3 m) diameter stack, it would be easier and from some shotcrete companies that one can apply up to 4 ft more cost-effective to scaffold and gun the inside of the stack. (1.2 m) thick overhead layers, yet others say to never apply The work could be done with less difficulty, from an interior more than 4 in. (100 mm) layers. I also have concerns of hanging platform suspended with sky climbers. There would delamination between the shotcrete and the existing concrete be less cost involved with rigging than there would be with during the installation process, and potential shotcrete falling scaffolding or rigging around the outside circumference of under its own weight in the thicker locations. the stack. Do you know of any situation where shotcrete was installed For the 4.92 ft (1.5 m) diameter stack, there is much less overhead to thicknesses of 6 ft (1.8 m)? How thick can the room to work inside, so unfortunately it may need to be gunned shotcrete be before additional reinforcement is required to on the outside. hold it for overhead applications? Is there a recommended maximum thickness for overhead application of shotcrete Question: I am building a perimeter overflow pool and need being placed before additional reinforcement is required? to know the best way to finish the trough. I have one shotcrete 60 Shotcrete • Winter 2015


2015WinterShotcreteEMag
To see the actual publication please follow the link above