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2015SpringShotcreteEMag

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: I recently hired a pool contractor to build a resi- Answer: It is difficult to make an assessment of a situation like dential pool. The contractor has been in business for more than this with a few photos and the description given. Based on your 30 years and has a great reputation. The progress so far is that statement that the contractor has an excellent 30-year reputa- the pool has been installed using shotcrete. The shotcrete has tion, we’d suggest you discuss these concerns with the con- been curing for the last 9 days. Within the last 9 days, it has tractor. If his response does not give you a full explanation, we rained heavily twice. On the second rainy day, immediately suggest you engage an independent professional familiar with after the rain finished, I walked outside to see the amount of shotcrete installations and swimming pools to give you an water that had collected inside the pool. I noticed that the water opinion. You can use ASA’s Buyers Guide at www.shotcrete.org was muddy looking. Upon closer inspection, as the rest of the to find a consultant. pool was dry, except for the deep end, there were two trails of There certainly is reason to question the quality of the instal- water coming from the shallow end and running into the pool lation based on the description. However, it is not unusual for of water in the deep end. the pool shell contractor to leave a temporary opening in the After getting down into the pool, I noticed that these trails shell to relive potential groundwater pressure and prevent of water were from water bubbling up through the shotcrete floating of the empty pool shell. These holes are, or should be floor on the slope closer to the shallow end. The bubbling was done, in a professional manner to allow complete watertight like a small stream of water coming up out of the shotcrete in sealing when filled. In some cases, the openings may include two places. I suppose it is from the hydrostatic pressure from a pressure relief valve. the groundwater under the concrete? My question is should this be concerning? The plaster has not been installed. How Question: We are proposing a vertical support of excavation should these holes be filled? The holes certainly do not look structure using reinforced shotcrete to retain a 10 ft (3 m) like they were intentional, as you can’t really even see them, high sandy soil. What are the design criteria to choose the except for the water coming out of them. Is there a problem reinforcement and the thickness of the shotcrete? Also, what with the shotcrete installa- is the minimum reinforcement and shotcrete thickness you tion? Does this mean that would recommend? my pool will leak when it is filled with water? I would Answer: The shoring design should be done by a competent image that if water can licensed professional engineer who specializes in earth reten- come up through the shot- tion systems. This is not a question that can properly be crete, the water can also go answered by ASA. You can search for such a professional in down through the shotcrete, our Buyers Guide section of the ASA website (www.shotcrete. resulting in erosion of the org). Another resource is the FHWA Manual for Design and soil under the pool? Before Construction of Soil Nail Walls. the shotcrete was installed, there was no groundwater Question: I am designing the resurfacing of the downstream present and the dirt was dry. face of a concrete gravity/mass concrete dam. I am calling for the addition of 9 in. (229 mm) of new concrete, reinforced with a grid of reinforcing bar, and anchored with dowels into the dam. I have specified wet-process shotcrete based on conven- tional knowledge (and Army Corps Engineering Manual EM 1110-2-2005) that wet-process shotcrete can be air-entrained and therefore provide better resistance to freezing-and-thawing damage, which is a concern for this application on a dam in New England. Contractors are requesting substitution of wet- process for dry-process due to the difficult site conditions; that is, working on a near-vertical surface over water with long distances (500 to 700 ft 152 to 213 m) to pump the wet-mix shotcrete. My concern with dry-process shotcrete, in addition to no air entrainment and poor freezing-and-thawing resis- tance, is the low permeability of dry-process shotcrete. My concern is that a low/nonpermeable covering on the down- stream face of the dam will trap the moisture that permeates 74 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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