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2017SummerShotcreteEMag

• Properly stabilized soil or rock can be used as the vertical surfaces to be shotcreted on for the walls of the pool. This process can be quite effective, as it requires less time and materials spent excavating and forming, with little backfill. Loose, soft, or fractured soil or rock should be removed to give the shotcrete a stable, rigid receiving surface. Large voids in the soil can often be filled in with structural foam, or alternatively, formed so the shotcrete section is held to the designed thickness and in the desired shape. “INSTALLED” FORMWORK CONSIDERATIONS • Man-made materials are commonly used for the onesided forming of the walls. Forming may be required because the soil may be too rough, leaving large voids or undulating surfaces, or is unstable after excavation. The pool structure may need to have all or a portion of the pool built above the existing site grade, as is often the case in sloping sites or sites with very high groundwater conditions. On occasion, the pool may have been excavated without the shotcrete contractor being on site or even being consulted, and due to excessive excavation necessitate forms being built later. • If a forming material can conform to the desired shape, be durable, relatively rigid, and not be detrimental to the shotcrete process, it can be used to create a form surface. Rough-cut lumber, framing lumber, tempered hardboard, thin plywood, structural foam, and stay-inplace forms are common pool forming materials. Often, more than one material is used to create custom shapes or to add stability. • Forms must be constructed in a way that provides a rigid, stable, nonvibrating surface to receive high-velocity, high-impact shotcrete placement. With shotcrete typically being delivered at 60 to 80 mph (95 to 130 kph), the need for form rigidity is essential. If formed surfaces are weak or loose, the shotcrete crew may incorrectly reduce the force of shotcrete’s impact by reducing the volume of compressed air being delivered. In doing this, the reduced velocity of the shotcrete reduces the compaction, reduces the ability to fully encase the reinforcement, reduces the concrete’s strength, and increases the concrete shell’s permeability. Often sloping walls, bottoms of skimmers, and other areas require that forms be strong enough to carry both the impact and weight of the wet shotcrete. • Stay-in-place forms are often comprised of a thick-gauge welded wire covered with a heavy-duty fiberglass or water-resistant paper. This type of stay-in-place form is Position Statement #6 ASA Pool and Recreational Shotcrete Committee very useful for creating single- or double-curved profiles. However, if misused, it can create problems. An example of inappropriate use would be hanging the stay-in-place form from a wood form being used for the pool beam at the top of the pool wall, or attaching the stay-in-place forming material to the reinforcing bar cage with tie wire, but having no other support. During shooting, the impact and weight of the shotcrete pulls on the reinforcing bars and the wood forms. This can lead to many problems, and even catastrophic failure during the shoot. This type of stay-in-place form should be supported with an appropriate number and spacing of stakes, ribbing or other suitable bracing materials. • Sometimes soil or formwork may have extended exposure to weather and on occasion over the winter. Delayed shoots resulting from complexity or size of a pool, or because of coordination of other work on a project, may expose the soil or formwork for weeks and months prior to shotcrete placement. Other potentials for delay are pre-shotcrete placement inspection schedules and building department requirements. Under these circumstances, the exposed soil or formwork is exposed to the weather and potentially to swelling, shrinkage, cracking, or movement. In circumstances where longer-than-anticipated exposures occur, repairs must be made to restore the integrity of the soil subgrade or formwork prior to scheduling the shoot. In summary, whether using natural soil or installed formwork for pool construction, the shotcrete contractor must verify the receiving surface is stable, rigid, and nonvibrating; define and maintain the desired thickness and shape; and fully support any attached reinforcement during shotcreting. Full attention to the details of providing a proper surface for shotcrete placement will help to give the Owner a pool that is structurally sound, aesthetically pleasing, watertight, and will give decades of low-maintenance service. Contributing authors: Jamie Scott, Bill Drakeley, and Charles Hanskat Position Statements ASA has produced position statements on the best practices for proper shotcrete placement. To date, six position statements from our Pool & Recreational Shotcrete Committee and one from our Board of Direction have been issued. These statements have also been published in Shotcrete magazine. www.shotcrete.org Summer 2017 | Shotcrete 45


2017SummerShotcreteEMag
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