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Sustainability concrete. Bond quality with shotcrete is derived from the very high energy imparted to the surface during shotcrete placement. Traditional casting placement methods rely on new material bonding to an existing substrate, essentially by contact. Conversely, shotcrete placement methods propel material to the substrate at a high velocity. This process significantly modifies the mixture’s proportions at the bond plane. As shotcrete material is initially placed, impact energy causes most of the mixture’s coarser components to bounce, rather than stick, to the substrate surface. Only the mixture’s smallest particles—the fine paste—can accumulate. As the paste layer builds, larger particles become embedded and rebound subsides. It is shotcrete’s high-velocity nozzle stream, through the tendency of fast-moving larger particles to ricochet off a hard surface, which produces a tight, well-compacted paste layer, Fig. 1: Exposed concrete ceiling element has deteriorated driven into the surface irregularities at the bond plane. This beyond its useful service life perfect material arrangement at the substrate surface facilitates an exceptionally strong crystalline connection—the primary element of a durable bond (refer to Fig. 3). Defining a Durable Bond Concrete can be designed to possess a very high compressive strength of 4500 to 7500 psi (30 to 50 MPa) or more. Its pull-apart resistance is comparably quite low at 145 psi (1 MPa). Bond strength values between new and existing concrete should be similar to the strength of the existing concrete. Therefore, for a bond to be considered durable, its bond strength should meet or exceed the pull-apart resistance of the underlying material. Typically, pneumatically applied shotcrete, when exposed to pull-apart or tensile loading, does not fail at the bond plane but, rather, within the substrate layer. This proves that the best possible bond has been attained. Structural designs requiring durable bonding typically Fig. 2: The bond characteristic of shotcrete is an important specify routine bond pulloff testing. Bond strength is commonly measured by coring through the shotcrete layer and into the issue for the engineer because it strongly affects repair substrate. A tensile load is applied to the surface of the core durability and then increased to the point of failure. The measured load failure divided by the core surface area provides a numerical bond strength. Both wet- and dry-mix shotcrete applications produce very good bond strength, typically 145 psi (1 MPa) or higher. Bond quality is a primary requirement for repair durability. Shotcrete’s unique material arrangement at the substrate surface enhances durability through improved bond strength. Note: Bonding agents are not recommended for shotcrete applications. Bonding agents interfere with shotcrete’s natural bond qualities and can create unreliable bonding. resources ACI CP 60(09), 2009, Craftsman Workbook, American Concrete Institute, 92 pp. Beaupre, D., 1999, “Bond Strength of Shotcrete Repair,” Fig. 3: Note fine materials developing a paste layer at Shotcrete, V. 1, Spring, pp. 12-15. leading edge of the nozzle spray pattern. Although larger Duckworth, O., 2011, “Can Nozzleman Skill Affect Bond particles have impacted this area, they cannot embed until a Quality?” Shotcrete, V. 13, Winter, pp. 30-33. sufficiently thick paste layer develops Shotcrete • Winter 2013 59


2013WinShotcreteEMag
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