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2017SpringShotcreteEMag

| 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 are building an area of a park that is on an existing pier in Brooklyn, NY. We are researching using shotcrete to form contours on certain areas. Weighting of the pier is an issue. Our question is, can lightweight concrete, or cellular concrete, be used in shotcrete? If so, what are the weights? Answer: Shotcrete is a placement method for concrete. Lightweight concrete usually ranges from 90 to 115 lb/ft3 (1400 to 1800 kg/m3). In wet-mix shotcreting, lightweight concrete should use presoaked aggregate to make the mixture pumpable. When it’s pumpable, as with conventional concrete mixtures, it is then accelerated to a high velocity by air at the nozzle and projected onto the surface. Lightweight aggregates can also be used directly in dry- mix, and there you don’t need to worry about pumpability because the dry materials are conveyed through the delivery hose. Water is added at the nozzle. Here’s the specific reference on lightweight from ACI 506R-16, “Guide to Shotcrete”: 2.1.3.2 Lightweight aggregates—Lightweight aggregates should conform to ASTM C330/C330M if used in shotcrete. The aggregate should meet one of the gradations shown in Table 1.1.1. Wet-mix shotcrete with lightweight aggregate is seldom used and is difficult to pump because the aggregate absorbs water, which reduces the consistency of the mixture. Presaturating the lightweight aggregate before batching improves pumpability. Lightweight aggregate mixtures have been shot for wall and floor construction. Shotcrete is frequently employed for fireproofing structural steel members using lightweight aggregates in the mixture. We suspect that cellular concrete cannot be shot because it uses injection of a pre-formed foam into a cement slurry and is highly fluid. That would preclude any stacking of material to make a vertical surface, and would instead just be pumped in place like a high slump concrete. Question: Can we find an appropriate and easy way to evaluate the shrinkage performance of shotcrete? Answer: Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. Here’s a link to the archived PDF of the article (www.shotcrete.org/ media/Archive/2011Sum_Hanskat.pdf). The specific section on drying shrinkage tests says: “Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.” Question: Is there a way for me to find out what the standard or appropriate repair for exposed reinforcing bar in a shotcrete swimming pool shell? Answer: Because shotcrete is a placement method for concrete, standard concrete repair techniques are applicable. If the repair is being done by shotcreting new concrete on the existing concrete, the repair should include these steps: 1. Chip the poor or weak concrete back to sound concrete. 2. If reinforcing bars are exposed, they should be cleaned of any rust. 3. If more than half of the perimeter of a reinforcing bar is exposed, the concrete should be chipped back to provide at least a 0.75 in. (19 mm) space behind the bar to allow shotcrete to flow around the back of the bar. 4. Before shooting any additional shotcrete, the surface should be cleaned and then wetted to a saturated surface-dry condition (SSD). SUMMER IS ON ITS WAY! Get your ASA baseball cap today! ASA’s new logo adds a sharp look to this new baseball cap, available online now from the ASA Bookstore. www.shotcrete.org/bookstorenet 78 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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