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2016SummerShotcreteEMag

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: Our company has been working on the design of a concrete pond for winery wastewater and the contractor proposed to replace the concrete liner with a geomembrane (canal 3) covered by shotcrete. Have you seen cases of this application being successful for wastewater holding? As an alternative, we are considering applying the shotcrete over a clay liner. Are there any concerns or recommendations for this approach? Answer: Shotcrete is a placement method for concrete. Shotcrete has been successfully used for over 70 years in thousands of industrial wastewater treatment/storage tanks, as well as in replacement linings of sewers and manholes. Thus, exposure of the shotcreted pond to wastewater should be as good or likely even better than the original cast concrete liner. Long-term durability of the shotcreted section will be dependent on the concrete mixture design. Many contractors use supplemental cementitious materials (SCMs) such as silica fume or fly ash to improve the pumping or shooting characteristics of the mixture. These SCMs also help to reduce permeability, increase strength, and thus make the concrete more durable. Fly ash also has the benefit of adding some sulfate resistance that would be beneficial in wastewater exposure conditions. Shotcrete is often shot on geomembranes or directly on the subgrade soils if they are stable enough to hold the impact and weight of the shotcrete. Question: We have a backwash tank on a wastewater treatment plant, which is made by a secant wall. The lower area for this structure will receive a shotcrete liner approximately 12 ft (3.7 m) tall on average; the interior perimeter of the structure includes 104 ft (32 m) of unreinforced and 440 ft (134 m) reinforced sections, which are a 12 in. (300 mm) minimum thickness. The drawings call for vertical control joints with a waterstop approximately every 30 ft (9 m). We don’t believe the control joints are necessary and could achieve the same desired performance with one monolithic installation of the shotcrete. Are the control joints really necessary when you are installing the shotcrete against a solid secant wall that does not contain any control joints? Answer: By “control joints,” we assume you mean contraction joints. Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Although shotcrete has a lower water-cementitious materials ratio (w/cm) than most form-and-pour concrete, it will still undergo shrinkage. In being shot on an existing concrete wall the shotcrete liner will be restrained by the bond to the substrate and the restraint of the horizontal volume change from shrinkage can create internal tensile stresses in the concrete. This is likely the reason the designer has specified contraction joints in the section. Spacing of 30 ft (9 m) between joints is common in new construction of concrete tanks. The question becomes whether the bond of the shotcrete to the existing substrate is high enough to restrain the volume change and prevent cracking along the hundreds of feet of wall you will be lining. The thickness of the lining, the type and duration of curing, the concrete mixture design, the strength of the concrete, the strength of the substrate, the quality of shotcrete application, proper surface preparation, and exposure to seasonal temperature changes will impact the effect of the volume change of the lining. With the many variables we’ve pointed out, you can see there isn’t a clear answer that covers all situations. We recommend you discuss your opinion with the designer or consult with a professional engineer experienced in shotcrete repairs to fully evaluate the specific structural sections you’re shotcreting. Question: What is the R-value per inch of shotcrete? Answer: Because shotcrete is simply a placement method for concrete, the R-value is the same as cast concrete. ACI 122R‑14, “Guide to Thermal Properties of Concrete and Masonry Systems,” would be a good reference. Question: I modified an existing pool and had a new 20 ft (6 m) wall built that was subsequently backfilled. The reinforcing bar was epoxied and tied into existing pool wall/floor. The wall is 4.5 to 6 ft (1.3 to 1.8 m) tall. Sixty days later, we have two vertical hairline cracks that run top to bottom. I watered the wall properly and there are no cracks in the other sections we shot (such as the spa). We backfilled 12 days after the wall was shot with hand equipment only. The sample test taken when shooting came back at 6500 psi (4.1 MPa). The original pool bottom is below the wall and has no issues. The wall appears to be 12 to 14 in. (300 to 350 mm) thick from top to bottom. My question is: If the wall was shot too thick, would the lack of additional reinforcing bar cause the wall to fail? And is the necessary course of action to demolish the entire wall and reinforce the reinforcing bar, then shotcrete again? Answer: There are many variables that can cause cracking. Vertical cracking is often the result of drying shrinkage of the concrete. You said you cured (watered) the wall properly, but don’t give any specifics. ASA recommends a minimum of 7 days of curing, with a wet cure preferred over a sprayapplied membrane. You should have a licensed engineer evaluate the structural sections, and determine if there were any problems with the amount or placement of reinforcement in your wall section. 78 Shotcrete • Summer 2016


2016SummerShotcreteEMag
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