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2013WinShotcreteEMag

was required. The base contract volume was substrate, the fine sand in the mixture kept emerging approximately 4000 yd3 (3058 m3). to the surface. This resulted in the surface exhibiting In coordination with the concrete general an aesthetically unpleasing “dimpling” finish. contractor, JWG presented the owner with value After investigating the issue, JWG determined engineering proposals for the majority of the new that the root of the problem was the high content vertical cast-in-place concrete walls on the project. of fine sands. Subsequently, the mixture design By substituting the cast-in-place method with the was altered to include a blend of fine sands and shotcrete method, the owner attained significant fine aggregates, which increased the fineness cost savings while maintaining both quality and modulus and created a more desirable aggregate project schedule activity durations. gradation. Throughout the duration of the project, As a result, the shotcrete scope doubled in no further finishing issues were encountered. volume through the addition of the following work: retaining walls, shear walls, miscellaneous interior walls, and a new loading dock building structure adjacent to the existing stadium. Project challenges Like other shotcrete seismic retrofits, there were several challenges to the high-profile Cal Memorial Stadium retrofit project. First, because the project had a tight deadline schedule with a 6-day work week and double work shifts, the stadium was split into three work areas. Consequently, JWG had to coordinate multiple shotcrete crews working simultaneously in each area. Second, the mixture design composition was especially unique for a Bay Area shotcrete project. After testing the concrete material, JWG had to make some modifications to customize it specifically for project application. Third, the dense steel configuration in the shotcrete walls required close observation and quality control in the field. Preconstruction testing exemplified these congested areas to demonstrate that the nozzlemen were capable of performing the work. Fig. 3: Cores retrieved from Type 1 preconstruction test panel mixture Design troubleshoot The shotcrete mixture design was specified as a 7000 psi (48.3 MPa) mixture at 28 days. The original concrete mixture had a composi- tion of 940 lb (426 kg) of cementitious materi- als with 15% fly ash content (equivalent to 10 sacks of cement) and 30 to 70% coarse-to- fine-aggregate ratio. The slump was 3.5 in. (89 mm). After analyzing laboratory trial results and conducting preconstruction testing, JWG adjusted the mixture design to tailor it to the work at the stadium. When lab trial results proved that the mixture far surpassed the design strength, JWG was confident that the effects of a “hot mixture” could be reduced while still meeting the design strength. Thus, the cement content was reduced to 893 lb (405 kg), or 9.5 cement sacks. During preconstruction testing, the JWG crew found difficulty in achieving the required steel trowel finish. As the finishers were “working” the Fig. 4: Existing historic wall prior to shotcrete overlay application Shotcrete • Winter 2013 21


2013WinShotcreteEMag
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