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Shotcrete FAQs that allows some shrinkage and thermal volume change movement. Your 10 in. (254 mm) of stone would serve to keep the new floor from bonding to the old and thus accomplish this, as well as preventing existing floor cracks from mirroring through the new concrete. It would seem the underdrain system should allow draining of water, so some kind of penetration through the old floor is advisable. Because the seat wall is a concrete wall that will undergo some shrinkage and thermal stresses, depending on the dimensions, it may need contraction joints with waterstops to accommodate shrinkage stresses. Your design engineer can consult ACI 350, “Code for Environmental Structures,” for guidance on joint spacing for temperature and shrinkage. Question 4—this is outside our expertise. Shotcrete finishes can vary from a rough as-shot to a smooth floated finish. Any coating introduces required maintenance because most coatings have a limited life span, especially in outdoor exposures with water wetting/drying cycles. If the shotcrete mixture is designed properly, and shot properly, then once the final concrete surface is set, there shouldn’t be any future imperfections. Question 5—we think casting on top of the stone base is a good way to address the rehabilitation. Question: Why is shotcrete not applied to general housing? It could be applied against outer insulation reinforced panels, forming both pillars and walls at the same time. The higher cost of shotcrete should be more than compensated by much shorter time and lower need of cranes. Answer: Shotcrete has been used for residential concrete construction, often for domed or other curved shapes. It has also been used to provide a structural skin over internal insulation panels in more rectangular layouts. Because shotcrete is concrete projected at high speed (between 60 and 80 mph 97 and 129 km/h), many insulation products do not withstand the impact and abrasion associated with shotcrete impact. In many cases, when shooting over an insulating foam panel that cannot withstand the high-velocity impact, shotcrete isn’t used, but a low-velocity plaster/grout mixture is spray-applied over the insulation. Here’s a link to the PCA website with a page on residential housing using a foam inner panel: www.cement. org/think-harder-concrete-/homes/building-systems/shotcrete. You can also find similar system information doing a web search for “shotcrete foam panel.” Question: I have a large aquatic project where the contractor would like to use gunite (dry-mix) versus wet-mix shotcrete. We have typically only allowed wet-mix on projects due to a bad experience in the past with using gunite. Would you have any reservations with the dry-mix process? We are being informed that they are taking the proper measures to ensure quality control—is that good enough or is there something we should reference or look for? Second question is in regard to aggregate in shotcrete. Per ACI 506R-05, there are the Grading limits—Grading 1 and Grading 2. They are submitting grading 1 for the dry-mix. I would like to suggest the use of Grading 2 aggregates with the benefits of the larger aggregate. There was also an ASA Technical Tip (Shotcrete, Winter 2011) supporting the use of larger aggregates of Grading 2. Is there reason not to use Grading 2 for the dry mix? I was not able to determine any reason not to. Answer: Properly mixed and applied by an experienced nozzleman, dry-mix can be equal to wet-mix shotcrete. The water-cementitious materials ratio (w/cm) is generally lower in dry-mix, so you will get less drying shrinkage. Areas where wet-mix is preferred would be thick sections needing a lot of volume because wet-mix has two to four times the production rate of dry-mix. Very congested reinforcement with large bars (No. 8 and above) may also be somewhat easier with wet-mix. ACI 506R Gradation 2 is certainly doable in dry-mix. Many pre-bagged dry-mix shotcrete materials use the larger aggregate. The contractor may not want to use the No. 2 gradation to avoid having a second aggregate on-site, if site batching manually. CCS is an official dealer for REED pumps! We sell all shotcrete/gunite supplies— hoses, pipe, nozzles, clamps, reducers, materials, etc. CCS can handle all your specialty shotcrete projects, and we have ACI Certified Nozzlemen on staff. Nebraska Office: (855) 752-5047 | Illinois Office: (618) 476-7224 E-mail: sales@ccsgrouponline.com Website: www.ccsgrouponline.com Shotcrete • Winter 2016 67


2016WinterShotcreteEMag
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