Page 46

2017SummerShotcreteEMag

Position Statement #6 ASA Pool and Recreational Shotcrete Committee Forming and Substrates in Pool Shotcrete Many factors lead to success in building strong, durable, and watertight pools. But what is success in constructing a pool shell? Structural integrity, proper shape, watertightness, and durability are key to success. Structural integrity requires meeting or exceeding the design strength for expected loading conditions with the desired thickness and shape. Providing the desired shape requires a sound, well-defined surface to accept the impact of the shotcrete placement. Thus, formwork or a prepared substrate is key to constructing a pool shell with the required structural section, desired shape, and aesthetics. Strength, watertightness, and long-term durability are essential to performance of all concrete swimming pools and other recreational water structures. Achieving these properties requires high-quality concrete materials, proper shotcrete equipment, and quality shotcrete placement. The receiving surface, whether erected formwork or a prepared subgrade, must meet certain performance criteria and local building codes. A solid, rigid, nonvibrating surface must withstand the high compaction energy produced by shotcrete placement, allowing maximum compaction for a watertight concrete pool shell and full encapsulation of embedded reinforcing bars. Reinforcing steel rests against, and is often attached to, the formwork or substrate. Thus, rigidity of the formwork is important for supporting the reinforcing steel and preventing excessive vibration. This is important to both durability and strength of the shotcreted shell. Reinforced concrete is a system, where the interaction of steel and concrete provide a combined compressive and flexural resistance to those respective forces. For the reinforcing steel to be effective, it must be fully encased by the concrete. A rigid form and well-secured reinforcing steel help facilitate proper encasement. Forming for shotcreted pool shells is one-sided, and falls into two general categories: “Against Soil” or “Installed Formwork.” “AGAINST SOIL” CONSIDERATIONS • The floors of most pools rest directly on the existing soil of the project. It is a basic but fundamental requirement that all organic materials that may decompose and reduce or expand in volume be removed with the remaining soil left in an undisturbed or properly compacted condition. • Many times, a layer of crushed stone is applied on top of the subgrade soils to provide a well-draining, stable, workable surface, as well as a clean, dense surface for the shotcrete. The crushed stone also serves as a drainage layer beneath the pool shell. An additional benefit of the crushed stone layer is it can be used to fill in voids or remove unevenness in the excavation process, which enables the shotcrete floor to meet the designed thickness as recommended in ACI 506R, “Guide to Shotcrete.” • In addition to being a “form” for the floor, the soil has structural significance. The soil must provide support in two directions: horizontal and vertical (that is, floors and walls). In most cases, the soil the pool shell rests on must support the combined weight of the shotcreted pool shell along with the weight of the contained water. These are very high loads and require competent soils to provide adequate support. Consider the soil loads for a pool with a 6 in. (150 mm) thick floor that goes from 3 ft (1 m) water depth to 10 ft (3 m) in depth. The vertical loads on the soil under the floor range from a low of 250 lb/ft2 to a maximum of 675 lb/ft2 (1200 to 3300 kg/m2). • The supporting soil subgrade should be evaluated by a geotechnical engineer for various properties when establishing its suitability for any project. These may include: type(s) of soil, variations in the soil composition, presence of groundwater, rock, expansive soils, sloping grades, slope stability, bearing capacity, and potential for differential settlement or sinkholes. The structural engineer will use the bearing capacity of the soil in the structural design of the pool. • When using the natural subgrade as a form, the soil must be compacted and stable before shotcreting. The geotechnical engineer should provide recommendations on methods needed to compact and test the soil for proper levels of compaction in the subgrade soils. During freezing weather, a frozen subgrade or one covered with frost must not be shotcreted upon. 44 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
To see the actual publication please follow the link above