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2015FallShotcreteEMag

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 have 18 ft high, 12 in. thick (5.5 m high, 205 mm specified and to what time frequency should a contractor thick) walls to shotcrete and need horizontal cold joints to place assemble them for quality control purposes? the shotcrete in three pours (three height sections). How do we create the joint? Answer: ACI 506.2-13, “Specification for Shotcrete,” is an excellent resource for answering your questions. ACI 506.2 Answer: Shotcrete is routinely used in creating retaining walls addresses both preconstruction panels and test panels used for or soil-nailed walls in this fashion. Designers and inspectors material quality control during construction. ACI 506.2, often confuse placement of multiple layers of shotcrete in Section 1.5.1.4, requires that when preconstruction panels are building out a section with cold joints experienced in cast-in- required, the Contractor shall “Construct test panels for each place concrete construction. Unlike cast-in-place concrete, proposed shotcrete mixture, each anticipated orientation, and shotcrete provides thorough consolidation and densification by each proposed nozzleman.” Preconstruction test panels vary high-velocity impact of fresh concrete material on the receiving in size to adequately represent the embedded reinforcement surface. The high-velocity impact of shotcrete on a hardened, and section thicknesses in the work to be done. previous shot layer (or existing concrete surface) provides a Test panels shot during construction for evaluation of strong abrasive blast to open up the surface, and then provides material properties are covered in ACI 506.2, Section 1.6.3.1, an immediate exposure of that hardened surface to fresh cement which states: “Construct a test panel for each mixture, each paste. As a result, shotcrete exhibits excellent bond to concrete nozzleman, and each work day or for every 50 yd3 (38 m3) and previously shot surfaces. Thus, the structural action placed—whichever results in the most panels. The face between the sections acts as a monolithic section without any dimensions of a test panel shall be a minimum of 16 x 16 in. weakened planes. (406 x 406 mm) with a minimum depth of 5 in. (127 mm). For In shotcrete construction, surface preparation between layers toughness testing in accordance with ASTM C1550, the face to provide full bond is important. ACI 506.2-13, “Specification dimension shall be 30.5 in. (775 mm) in diameter and 3 in. for Shotcrete,” specifically addresses this in the requirements (76 mm) thick. Shoot test panels in a vertical orientation only of Section 3.4.2.1 and 3.4.2.2 that require: unless otherwise specified.” 3.4.2.1 When applying more than one layer of shotcrete, use ACI 506.2-13 has extensive provisions for the submittals, a cutting rod, brush with a stiff bristle, or other suitable equip- testing, materials, and execution of shotcrete work by a shot- ment to remove all loose material, overspray, laitance, or other crete contractor. It cites many of the ASTM testing standards material that may compromise the bond of the subsequent layer appropriate for shotcrete construction. We recommend you of shotcrete. Conduct removal immediately after shotcrete review the document in its entirety to become familiar with reaches initial set. current industry standards. 3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface Question: We have an existing cut slope approximately 328 ft of all loose material, laitance, overspray, or other material that (100 m) high (3:1 vertical:horizontal) with cut benches and may compromise the bond of subsequent layers. Bring the need to apply shotcrete onto the slope surface at a height of surface to a saturated surface-dry condition at the time of approximately 230 ft (70 m) from the road level. Is it possible application of the next layer of shotcrete. to reasonably transfer and apply shotcrete mixture at such a height An experienced shotcrete contractor should routinely from the road level? What type of transfer hoses and equipment provide proper surface preparation between shotcreted sections, is preferable? Is dry-mix or wet-mix shotcrete preferable? and use skilled crews with ACI certified nozzlemen to place and cure the shotcrete placements. Answer: Yes, either dry-mix or wet-mix process can and have been used at this height. Because either process can be used in Question: I would like to know if there is any parameter for these conditions, you should use an experienced shotcrete test panel dimensions and inclination of proposed shotcrete contractor who will pick the best method based on their firm’s surfaces represented by the test panels. I also would like to shotcrete capabilities. Factors such as the shotcrete contractor’s know if there is any specification for frequency of making test specific shotcrete equipment, material availability, site panels during the shotcrete project duration. In general, what constraints, remoteness of the location, and crew experience is the specified type of panels and number of them to be will influence their choice. Shotcrete • Fall 2015 49


2015FallShotcreteEMag
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