Watertight Shotcrete for Swimming Pools
POSITION STATEMENT #4
Shotcrete is an ideal construction method for a concrete swimming pool. The versatility of shotcrete
placement allows for the construction of a concrete pool shell of virtually any size and shape. Standards for
shotcrete design and applications can be found in American Concrete Institute (ACI) Committee 506 guides¹, specifications, and technical notes, as well as ASA position papers.* Responsible shotcrete contractors specializing in concrete pool construction must use appropriate design, quality materials, and construction techniques to build a fully functional pool with long-term durability that is functionally watertight under normal service conditions. Watertightness of the shotcrete material is a crucial durability and serviceability property of any properly con structed water-containing shotcrete structure. Shotcrete placement that allows water to pass through the concrete of a pool shell is a sign of flawed material or placement techniques.
Pools are complex concrete structures with irregular shapes, varying thicknesses, numerous shell penetrations, and variable soil conditions. The structural requirements for a pool shell must be evaluated by a professional engineer who is qualified for structural evaluation. A proper design will include specifications for sub-grade preparation, concrete shell thicknesses, concrete materials, reinforcing steel layout, and pipe and fixture penetration details. With an appropriate design, proper materials, and quality placement, the experienced builder creates a pool that meets loading conditions, provides long-term durability, and is functionally watertight.
The concrete material used to construct the pool shell must meet watertightness requirements. Scientifically defined, watertightness of the concrete material is the result of complex mass transport mechanisms (capillarity, permeability, and water diffusion) that refer to various properties of concrete². A definition of watertightness is “impermeable to measurable flow of water except when under hydrostatic pressure sufficient to produce structural discontinuity by rupture.”³
A proper concrete mixture design is a prerequisite for successful shotcrete application of concrete. A well-designed, shootable mixture should be selected with a proper aggregate gradation, allowing it to pass through a shotcreting system. Special attention should be given to the amount of fine aggregate and cementitious paste necessary to cover the aggregate surface area (minimum 4:1 cementitious-to-aggregate ratio, and ideally 3:1). Sufficient cementitious paste and a low water cementitious material ratio (w/cm) are necessary for producing a dense, watertight final product with full encapsulation of embedded reinforcing.
Assuming the correct mixture design, compressive strength of the concrete is generally indicative of the w/cm (lower w/cm equates to higher strength and lower permeability) and the potential watertightness of the shotcrete in the pool shell. Complete compaction of the pool shell concrete is a direct result of the high velocity (typically 60 to 80 mi/hr [100 to 130 km/hr])4 the concrete mixture is shotcreted onto the receiving surface. The high impact energy produced by proper shotcrete equipment and placement techniques achieve maximum in-place encapsulation and compaction with minimum voids for a given concrete mixture. Compressive strength is measured through core samples taken from in-place material or shotcrete test panels. Because concrete compressive strength is affected by the same physical properties that affect watertightness, it is possible to evaluate the quality of a shotcreted pool shell—both from a structural and a watertightness point of view—by considering the compressive strength of the shotcrete produced. Thus, in this context, compressive strength is an appropriate predictor of watertightness.
Shotcrete made from properly graded aggregates, quality cement, and potable water; properly placed by a qualified shotcrete crew; consolidated at the requisite velocity; and properly cured will easily yield a 28-day compressive strength of 4,000 psi (28 MPa). 5,000 psi (34 MPa) is desirable for enhanced durability and is routinely achievable by careful attention to materials and placement techniques.
Table 4.2.2 of ACI CODE 350-065 has specific code requirements for special exposure conditions. The table
indicates concrete intended to have low permeability when exposed to water should have a maximum w/cm of 0.45 with a minimum 4000 psi compressive strength. However, when the concrete is exposed to chlorides (brackish water, salts, and seawater), ACI 350 requires a maximum w/cm of 0.40 and compressive values of 5000 psi (34 MPa) to provide protection for embedded reinforcement. Thus, lower w/cm and higher compressive strength provide better protection for embedded reinforcement in water containing structures. Concrete technology and common-sense show that watertightness will be improved using a concrete with a low w/cm with sufficient paste content—that is, well consolidated by the placement techniques and properly cured and protected after placement—thus creating a dense functionally impermeable concrete.
There are many reasons why a shotcrete placement would not achieve specified minimum compressive strength. These include poor workmanship, defective equipment, unfavorable environmental conditions, or the inadequate selection of ingredients including poorly graded aggregates, soft or deleterious aggregates, expired cement, or contaminated water. In any case, compressive strength values less than 4,000 psi are, in most cases, the result of intentionally or unintentionally ignoring the rules of good practice 6, 7 and will result in pool shell shotcrete that is not watertight.
As with any other concrete structure, shotcrete pool shells must be properly designed—both from a structural and a concrete material perspective. Good quality shotcrete properly designed, manufactured, placed, and cured will produce the desired result—a durable and essentially watertight structure. Experienced pool builders pay attention to all the details of the shotcrete process and building pool shells with watertight shotcrete. Watertightness of shotcrete should be the expectation of all parties: the client, the designer, and the contractor.
*Note
“Shotcrete” and “concrete” are two distinct terms. Shotcrete is defined as “concrete placed by a high velocity pneumatic projection from a nozzle.”³ In other words, concrete is the material; shotcrete is the placing process. The end result is a concrete structure built with the shotcrete process. However, many members of the industry will use the terms interchangeably when the reference to the shotcrete process is implied. We do the same in this position statement, using “shotcrete” and “concrete” interchangeably. A “shotcrete pool shell” is understood to be comprised of concrete, and a “concrete pool shell” is—in this context—understood to have been placed using the shotcrete process.
References
1. ACI Committee 506, “Guide to Shotcrete (ACI PRC 506-16),” American Concrete Institute, Farmington Hills, MI, 2016, 52 pp.
2. Bolduc, L. S., and Jolin, M., “Shotcrete Boiled Water Absorption,” Shotcrete, V. 12, No. 1, Winter 2010, pp. 12-17.
3. ACI CT-20, ACI Concrete Terminology, American Concrete Institute, Farmington Hills, MI, 2020, 79 pp.
4. Ginouse, N., and Jolin, M., “Material Velocity at the Nozzle,” Shotcrete, V. 15, No. 4, Fall 2013, pp. 22-24.
5. ACI Committee 350, “Code Requirements for Environmental Engineering Concrete Structures (ACI 350 06) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2006, 487 pp.
6. Crom, T. R., “Dry Mix Shotcrete Nozzling,” Concrete International, V. 3, No.1, Jan. 1981, pp. 80-93.
7. “Shotcrete for the Craftsman CCS-4(20)” American Concrete Institute, Farmington Hills, MI, 2020, 92 pp.
Position Statements
ASA has produced position statements on the best practices for proper shotcrete placement. To date, seven position statements from our Pool & Recreational Shotcrete Committee, two from our Underground Committee, and one from our Board of Direction have been issued. These statements have also been published in Shotcrete magazine. Visit www.shotcrete.org.
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