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2015SpringShotcreteEMag

Shotcrete FAQs Question: I am specifying shotcrete to lower a draft tube roof exist, the design of the anchor spacing and size should in a dam. I like the bond characteristics shotcrete provides over be checked for sufficiency in pullout and shear.” Thus, traditional concrete for this application. I have two questions if the anchors are designed by an experienced engineer I am looking at resolving in the design process. The amount of to accept the expected loads, the spacing can be shotcrete could be approximated as a triangular wedge of 6.5 ft determined by the designer, as long as the supported deep by 30 ft long and 34 ft wide (2 x 9 x 10 m). The concrete reinforcing is stiff enough to not vibrate when shotcreted. will be roughed and cleaned prior to applying the shotcrete Many underground applications are designed to use larger and in good condition. anchors at a greater spacing. 1. Reference ACI 506R-05 Section 5.5, “Anchors,” indicates 2. The ACI 318 temperature and shrinkage requirements that overhead surfaces have anchor at 18 in. (457 mm) on allow wider crack widths in structural sections than ACI center (OC)-both ways. For the design loads, I am able to 350. ACI 350 requires more reinforcing to help control space anchors at 3.5 ft (1 m) distances and not approach crack widths and make the structures as liquid-tight as any loading issue. For most of the design, the dead weight possible. Thus, if your roof section is exposed to water, of the shotcrete is the driving factor. I intend to have the ACI 350 levels of temperature and shrinkage steel reinforcing bar (or wire reinforcement WRI) next to the would be advisable. roughed original concrete surface. I will require some small The spacing of the anchors wouldn’t normally be influ- anchors between the primer anchors to stiffen up the enced by the use of WRI wire or fiber, unless that was reinforcement (as required). Do you know what the driving considered by the designer in the ability of the sections to factor is behind the 18 in. (457 mm) spacing for anchors carry stresses between the anchors. However, use of steel for overhead application? I would prefer not having to fibers in the mixture at any significant dosage is not com- install so many anchors, and can see installation of that patible with encapsulation of reinforcing bars or mesh. quantity of anchor hanging down from the roof up to 5 ft The 18 in. (457 mm) spacing is a “rule of thumb”; the (1.5 m) long causing issue with applying shotcrete. I have intent is to have sufficient anchors to ensure that the rein- been researching tunnel lining and from all the photos I forcing bar or mesh is rigid and will not vibrate during the have seen, they did not follow that guidance. shotcrete placement. 2. Temperature and shrinkage issues: The best criteria on If there is a termination of the temperature and shrinkage designing for temperature and shrinkages has been from ACI reinforcement in sections experiencing internal tensile stresses 318 for slab design minimum reinforcement ratio and ACI due to shrinkage or temperature, the section would be more 350-06, Table 7.12.2.1, minimum shrinkage and temperature prone to cracking at the point the reinforcement is terminated. refinement for various spans greater than ACI 318 provides. However, geometry of the section can also contribute to the The temperature and shrinkage reinforcement is by far cracking potential. governing the amount of steel required in the shotcrete. Construction joints in shotcrete do not create planes of I am looking to specify steel fiber reinforcement or have weakness, but act as monolithic concrete, because shotcrete WRI wire reinforced with every layer of shotcrete to help has an excellent bond to previously shot sections. If you want with shrinkage issue. If I use WRI wire, would I need the to design to accommodate shrinkage and temperature volume anchor at 18 in. (457 mm) spacing for each layer? changes in the concrete, you should provide movement joints I also have a concern because the geometry varies from (contraction or expansion). 6.5 ft (2 m) thick down to 2 in. (51 mm) thick over 30 ft (9 m), which will require terminating WRI wire as the material thins. Do you know if at this termination there will be cracking issues from discontinuities in the reinforcement? The other item I am contemplating is to install construction joints for varying thicknesses to help reduce the shrinkage stresses, such as 2 to 12 in. (51 to 305 mm) and 12 to 30 in. (305 to 762 mm) and 30 in. to 6.5 ft (762 mm to 2 m). This is not the most desirable, but is doable. Answer: This is a great application for shotcrete. As you’ve indicated, proper surface preparation of the substrate is key to getting a durable shotcreted section. 1. ACI 506R-05 does provide guidance for most overhead applications using a maximum recommended spacing of 18 in. (457 mm) OC. This comes from the need to provide a reasonable value for most types of overhead work. However, ACI 506R does provide “If special conditions Shotcrete • Spring 2015 77


2015SpringShotcreteEMag
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