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data that reflects the recommended air-void spacing factor for dry-mix shotcrete. 2. Silica fume: Silica fume is a highly pozzolanic admixture that has been proven to improve both the plastic and hardened properties of concrete placed using the shotcrete process. The use of silica fume in shotcrete increases adhesion to the bonding surface and cohesion within the shotcrete, consequently allowing thicker placement of shotcrete before sloughing (especially in overhead applications). Although there is no standard ASTM or ACI test to measure attainable thickness in one pass, testing and field performance has proven that the benefits of silica fume from the perspective of overhead and vertical repairs are obvious. 3. Aggregates: Aggregates should meet the recommendations in ACI 506R, “Guide to Shotcrete,” and use Gradation No. 2 (the mixture with both fine and coarse aggregates), due to the thickness of the applications. To ensure optimum durability, including resistance to freezing and thawing and alkali-aggregate reaction, all concrete aggregates should also meet the minimum requirements outlined in ASTM C33. The use of larger (0.4 in. 10 mm) coarse aggregate also has a positive effect on the ability to pump and shoot the shotcrete mixture. The abrasion of coarse aggregate against the inside lining of the hose reduces the cement buildup and improves material flow. Consequently, a coarse aggregate gradation will allow the use of longer transportation hoses and reduce plugging. Significance of Shotcrete The greatest challenge faced by the STM management team and the contractor was, without a doubt, to complete the project in a timely manner while limiting disruption to the STM ridership. The engineering group not only had to design the structural elements of the repairs but also had to find a solution that would allow a fast turnaround time so the completion of the concrete repairs, crack injections, and electrical work would stay within the allocated 25 weekends. With that in mind, the decision to specify dry-mix shotcrete checked all the boxes for the STM engineering team. The nature of the shotcrete process allowed the crew to continually place concrete much quicker than form-and-pump placement methods. More importantly, the elimination of formwork resulted in reduced labor and allowed the contractor to mobilize, shoot, cut, and finish without having to transport, install, and later remove complex, curved forms. The result was not only an accelerated construction schedule but also a more cost-effective repair method (Fig. 2). Logistical Challenges Mobilization of the crew and transportation of the required materials and equipment to the worksite was extremely challenging. Access to the worksite by train was the only option available to the contractor, so Construction Interlag Inc. developed a system using two special flatbed train cars that were designed to carry all of the components, including the dry-mix shotcrete gun, pre-dampener, forklift, compressor, tool box, and enough bulk bags of shotcrete material to last the shift. This system allowed them to maximize the work hours during the weekend shutdowns of the metro line (Fig. 3). Fig. 1: The material specification called for a prepackaged, high-quality shotcrete mixture that would provide long-term durability Fig. 2: The STM engineering team identified approximately 6540 ft2 (600 m2) of overhead area requiring removal and replacement, most of which ranged in thickness from 18 to 36 in. (450 to 900 mm) Fig. 3: Construction Interlag Inc. developed a system using two special flatbed train cars which were designed to carry all of the components, including the dry-mix shotcrete gun, pre-dampener, forklift, compressor, tool box, and enough bulk bags of shotcrete material to last the shift Shotcrete • Winter 2016 31


2016WinterShotcreteEMag
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