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2013SprShotcreteEMag

Technical Tip Fig. 2: High-pressure water blasting the concrete substrate after concrete removal with a jackhammer starts, even when hydrodemolition is used. Although this procedure may seem redundant, it is a crucial step to ensure good quality bond between the substrate and the shotcrete repair by removing any microfractured concrete, dust particles, debris, and loose sand. This procedure is specified by the Ministry of Transportation of Quebec.2 In addition to the cleaning procedures, it is also recommended that adequate prewetting of the concrete substrate is performed before shotcreting (refer to Fig. 3). This procedure has been described by Dufour et al.3 Concrete substrates should be in a saturated surface-dry (SSD) condition immediately prior to the shotcrete application. Bonding agents are never recommended when using the shotcrete process. Firstly, it is not neces- sary, as the shotcrete process provides excellent bond by itself. Secondly, if the bonding agent is not installed properly or the shotcrete material placement is delayed and the bonding agent dries out before the repair material is placed, the bonding agent will act as a bond breaker. Thirdly, it is another step added to the repair process. The more steps one adds to the repair process, the greater an opportunity for mistakes to happen. Finally, it will also create two layers where there could be potential for debonding instead of only one, which also increases the risk of failure. These comments on bonding agents are also reported in the Report Number MERL 12-17.4 Surface Preparation Testing Evaluating the quality of surface preparation and ultimately the durability of bond is a critical factor in determining the quality of a repair. At the beginning of a major project, a qualification test of the repair method should be conducted. A representative surface area should be prepared with the selected technique and repaired with the chosen repair method. After a certain period of Fig. 3: A worker prewetting the concrete substrate Shotcrete • Spring 2013 29


2013SprShotcreteEMag
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