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2015WinterShotcreteEMag

Shotcrete FAQs 2. ACI 506 defines “shadow” as any porous areas behind an of ours. STRUCTURAL has provided a steel reinforcement cage obstacle, such as reinforcement. Proper shotcrete application across each wall plane, which is tied to the existing CMU wall by an experienced nozzleman with adequate equipment and with a series of anchors running horizontally and vertically. crew will create shotcrete sections that have minimal Between this new shotcrete wall and existing CMU wall, we shadowing and voids. However, sometimes upon stripping are calling for a vapor barrier layer to retard moisture migra- of the backside forms there is a noticeably darker coloration tion from the exterior to the interior. At this point, we are of the concrete surface immediately behind reinforcing bars. considering going with a PROSOCO Cat-5 fluid-applied vapor This darker coloration often does not have porous areas, but barrier system over the CMU but were curious how this might is an indication that the reinforcing bar is close to the form interface with the shotcrete. and, although good, dense material has been placed, it has Have you dealt with this vapor barrier issue before, or seen a slightly higher cement paste content and thus appears similar installations? Would the steel reinforcing cage be ade- darker on the surface. Sounding the area of discoloration quate to support the shotcrete by itself during application, or should identify any porous areas that should be repaired, as should we be concerned about the surface of the backup wall? mentioned previously. It should be noted that this is not unique to shotcrete. Cast Answer: Shotcrete has been placed against fluid-applied concrete that is not properly placed and fully consolidated often waterproofing and other smooth surfaces, which would likely exhibits voids and rock pockets that need to be repaired upon be similar to the surface you are concerned with. An experi- stripping of the forms. enced shotcrete contractor should be able to install this shear wall, taking care to apply the shotcrete in a sequence which Question: We are the Architects of Record for a multi-family inhibits the material from sagging or sloughing. The key is rehabilitation here in Portland, OR. The existing building is hiring a good, well-experienced shotcrete contractor. three stories plus a half basement. It is a historic building and The reinforcing bar size and spacing can help provide support is also an unreinforced masonry building. As a result, it requires for building up the shotcrete. The spacing should be no greater significant seismic upgrades. than 12 in. (305 mm) on center each way and must be rigid. Our structural engineer is proceeding with a design that uses several 4 in. (102 mm) thick shotcrete walls as shear Question: I am currently working on a project that involves elements throughout the building. In most instances, these repairing the concrete walls of a sanitary sewer interceptor walls are being constructed adjacent to an existing wall to structure and line pipe and was wondering if you had any minimize their impact to the existing floor plans. reference information regarding shotcrete being used to repair This seems fine against the exterior masonry walls but we similar items and how well it has held up. Any information you have concerns where the shotcrete is to be installed directly send will be greatly appreciated. adjacent to a standard architectural partition (gypsum wall board and wood studs). We are planning on plywood sheathing Answer: Shotcrete, both wet- and dry-mix, have been used to to act as a one-sided form to prevent the shotcrete application repair and reline sewer structures for many decades and has from harming the existing stud wall. However, we were curious held up well as a lining or repair material. It is important to if we should also include a water barrier so that the application have the work done by a qualified shotcrete specialty contractor did not allow moisture to migrate into the existing walls during and to specify a durable concrete mixture design. The following the installation. Is this assumption correct? And if so, is there are links to articles which may also be of interest to you: a performance or product recommendation that you can offer • www.shotcrete.org/media/Archive/1999Spr_Snow.pdf, for this purpose? “Rehabilitation of Sanitary and Storm Sewers Using Shotcrete” • www.shotcrete.org/media/Archive/1999Win_Yoggy.pdf, Answer: Shotcrete has been used in similar structural upgrades “Shotcrete, Airplanes, and Automobiles” for decades quite successfully. There are many ways to ensure • www.shotcrete.org/media/Archive/2006Win_Morgan.pdf, that the moisture from the fresh shotcrete would not impact the “Advances in Shotcrete Technology for Infrastructure existing partitions. Methods for protection include use of green Rehabilitation” board (as used in a shower or bath), plywood, painted coatings, • www.shotcrete.org/media/Archive/2013Sum_Sustainability. moisture-resistant sheeting, and so on. pdf, “Ability to Access Restricted Space and Difficult-to- Shotcrete is placed with a low water-cement ratio (w/c) and Reach Areas, Including Overhead and Underground” the water is needed by the shotcrete to hydrate the cement and harden. As long as the existing surface or treated surface does Question: We have a project in the Los Angeles, CA, area that not actively absorb the moisture, the shotcrete will use the avail- we have designed as poured-in-place concrete. The contractor able internal moisture to hydrate the cementitious materials. would like to shotcrete all basement walls. However, we have columns that are integrated with the wall. A City of Los Angeles Question: We’re proposing to install a new shotcrete shear technical bulletin does not address shotcrete columns in a wall wall against existing 8 in. (203 mm) CMU walls for a project where the bar spacing varies from the wall. Do you have any 62 Shotcrete • Winter 2015


2015WinterShotcreteEMag
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