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Position Statement #4 ASA Pool and Recreational Shotcrete Committee Watertight Shotcrete for Swimming Pools Sallows for the construction of a concrete pool shell of mixture should be selected with a proper aggregate grada-proper concrete mixture design is a prerequisite for successfulshotcrete application of concrete. A well-designed, shootablehotcrete is an ideal construction method for a concreteswimming pool. The versatility of shotcrete placement virtually any size and shape. Standards for shotcrete design tion, allowing it to pass through a shotcreting system. Special and applications can be found in American Concrete Institute attention should be given to the amount of fine aggregate (ACI) Committee 506 guides,1 specifications, and technical and cementitious paste necessary to cover the aggregate sur- notes, as well as ASA position papers.* Responsible shotcrete face area (minimum 4:1 cementitious-to-aggregate ratio, and contractors specializing in concrete pool construction must use ideally 3:1). Sufficient cementitious paste and a low water- appropriate design, quality materials, and construction tech- cementitious material ratio (w/cm) are necessary for producing niques to build a fully functional pool with long-term durabil- a dense, watertight final product. ity that is essentially watertight under normal service condi- Assuming the correct mixture design, compressive strength tions. Watertightness of the shotcrete material is a crucial of the concrete is generally indicative of the w/cm (lower durability and serviceability property of any properly con- w/cm equates to higher strength) and the potential watertight- structed water-holding shotcrete structure. Shotcrete placement ness of the shotcrete in the pool shell. Complete compaction that allows water to pass through the concrete of a pool shell of the pool shell concrete is a direct result of the velocity is a sign of flawed material or placement techniques. (typically 65 to 115 ft/s 20 to 35 m/s5) at which the mixture Pools are complex concrete structures with irregular shapes, is shotcreted onto the substrate. The high impact energy varying thicknesses, numerous shell penetrations, and variable produced by proper shotcrete equipment and placement soil conditions. The structural requirements for a pool shell techniques achieve maximum in-place encapsulation and must be evaluated by a professional engineer who is qualified compaction with minimum voids for a given concrete mixture. for structural evaluation. A proper design will include Compressive strength is measured through core samples taken specifications for subgrade preparation, concrete shell from in-place material or shotcrete test panels. Because thicknesses, concrete materials, reinforcing steel layout, and concrete compressive strength is affected by the same pipe and fixture penetration details. With an appropriate design, physical properties that affect watertightness, it is possible proper materials, and quality placement, the experienced to evaluate the quality of a shotcrete pool shell—both from builder creates a pool that meets loading conditions, provides a structural and a watertightness point of view—by considering long-term durability, and is essentially watertight. the compressive strength of the shotcrete produced. Thus, in The concrete material used to construct the pool shell must this context, compressive strength is an appropriate predictor meet watertightness requirements. Scientifically defined, of watertightness. watertightness of the concrete material is the result of complex Shotcrete made from properly graded aggregates, quality mass transport mechanisms (capillarity, permeability, and water cement, and potable water; properly placed by a qualified diffusion) that refer to various properties of concrete.2 A shotcrete crew; consolidated at the requisite velocity; and definition of watertightness is “impermeable to measurable properly cured will easily yield a 28-day compressive strength flow of water except when under hydrostatic pressure suf- of 4000 psi (28 MPa). In reality, 5000 psi (34 MPa) is desirable ficient to produce structural discontinuity by rupture.”3,4 A for enhanced durability and is routinely achievable by careful attention to materials and placement techniques. Table 4.2.2 of ACI 350-066 (refer to Fig. 1) has requirements for special *NOTE: “Shotcrete” and “concrete” are two distinct terms. Shotcrete is defined as “concrete placed by a high-velocity pneumatic projection from a nozzle.”3 exposure conditions. The table indicates concrete intended to In other words, concrete is the material; shotcrete is the placing process. The have low permeability when exposed to water should have a end result is a concrete structure built with the shotcrete process. However, maximum w/cm of 0.45 with a minimum 4000 psi (28 MPa) many members of the industry will use the terms interchangeably when the compressive strength. However, when the concrete is exposed reference to the shotcrete process is implied. We do the same in this Paper, to chlorides (brackish water, salts, and seawater), ACI 350 requires using “shotcrete” and “concrete” interchangeably. A “shotcrete pool shell” is understood to be comprised of concrete, and a “concrete pool shell” is—in a w/cm of 0.40 and compressive values of 5000 psi (34 MPa) this context—understood to have been placed using the shotcrete process. to provide protection for embedded reinforcement. Thus, lower Shotcrete • Winter 2015 69


2015WinterShotcreteEMag
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