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2015SummerShotcreteEMag

Shotcrete Corner The following article originally appeared in the Spring 2011 issue of ASA's Shotcrete magazine. Why Bonding Compounds are Not Recommended with Shotcrete The Question of Bonding Agents and Shotcrete Bcame up. As a shotcrete contractor for over response was overwhelming with nearly universalsomething. When the question was asked, theof bonding compounds with shotcrete By Ted Sofis ack when I first joined ASA, the question facturers, and suppliers, and we all agreed on 40 years, I had experienced all the problems agreement among the ASA membership that use you can imagine in trying to follow specifi - of bonding compounds with shotcrete was cations where the use of a bonding agent was actually detrimental to achieving a good bond to required with a shotcrete installation. ASA is a properly prepared substrate. I was not a lone made up of a wide range of people in the shotcrete voice in the wilderness. industry, including engineers, contractors, manu- With a good shotcrete repair, the deteriorated concrete is removed back to sound material, the concrete surface and existing reinforcing is either sand- or waterblasted to clean off scale from the reinforcing bars and create a textured profile on the concrete, the mesh or reinforcing is installed, and the repair area is washed with air and water to clean off any loose particulates and wet the concrete surface to create a saturated surface-dry (SSD) condition prior to the shotcrete placement. SSD refers to a surface that is wet without any standing water. The reason for wetting the concrete prior to placing the shotcrete is to create a better bond. A dry concrete substrate will draw the With good surface preparation and gunning moisture from the newly placed shotcrete, possibly practices, shotcrete provides an excellent bond leaving an inadequately hydrated material at the to the existing substrate point of contact between the existing concrete and the shotcrete repair. For this reason, wetting the repair area prior to shotcreting is an important step. When shotcrete impacts the hard concrete surface, a greater percentage of aggregate rebounds from the surface, leaving a thin layer of more cement-rich paste at the interface between the existing concrete and the new shotcrete. As the shotcrete material builds on itself in its plastic state, the rebound of the aggregate decreases. The velocity of the shotcrete process drives the new material in place, creating an excellent bond with the existing substrate. The use of a bonding The dry process of shotcrete being gunned on a compound interferes with this process and in many pier can provide a better bond without using cases actually creates a barrier or bond breaker. bonding compounds. The nozzleman is filling In addition, there are other problems with in the corners first and working toward the bonding compounds. With shotcrete, the rebound middle of the repair area so he doesn’t trap and overspray will stick to the adjacent areas where rebound in the corners a bounding compound has been applied. Rebound 30 Shotcrete • Summer 2015


2015SummerShotcreteEMag
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