Page 59

2017WinterShotcreteEMag

required. Sometimes the moves were at the last minute because wall sections had not been signed off on due to unanswered questions or new field discoveries dealing with existing conditions. With the minimal formwork required by shotcrete, the other trades were able to work above the walls until just before shotcrete operations started. This would not have been possible with a standard formand pour concrete operation. Often modifications of the reinforcement and required demolition would be completed immediately before shotcreting started on a particular wall. With shotcrete’s inherent flexibility, the schedule of the General Contractor was significantly less impacted by these changes than would have occurred with a form-and-pour operation because the formwork wouldn’t be able to be started until all issues were resolved or, even more timeconsuming, having to remove and replace the formwork when additional issues arose. One of the requirements of the project was the Owner’s desire for LEED certification. One of the ways that shotcrete was able to help improve the sustainability and in turn the LEED rating was by using recycled supplementary cementitious materials (SCMs), including slag and fly ash in the shotcreted concrete mixture. The design 28-day compressive strength of the shotcrete was 6000 psi (40 MPa) and the final concrete mixture used a 45% replacement of the cementitious material with 30% slag replacement and 15% fly ash replacement. Because of the high replacement of cement, the shotcrete mixture was very difficult to pump, but with the use of pumping aids, such as Rheomac VMA, they were still able to get a production rate of over 90 yd3 (69 m3) per shift through over 500 ft (150 m) of delivery pipe and hose. The Owner was also concerned about potential cracking in all of the concrete surfaces. This issue was resolved by using a shrinkage-reducing admixture (Masterlife SRA). Shrinkage tests were not performed on the project, so it is not known how much of a benefit the admixture provided, but the Owner was very satisfied with the results. The largest issue on the project that shotcrete was able to solve was with site logistics. The project was located in a very congested part of San Francisco with narrow streets, a lot of vehicle and pedestrian traffic, and numerous local businesses. When the project was bid, one of the conditions allowed use of both Sacramento Street (the long axis of the project) and Webster Street (the short axis of the project). However, by the time Dees Hennessey Inc.’s (DHI) portion of the work began, the project’s only allowable pumping location was from Webster Street, the narrower of the two streets. The General Contractor had numerous difficulties throughout the project with all of their concrete pours because of this narrowness of the street and numerous times had to perform off-hour pours due to space and traffic limitations. With the smaller footprint of a shotcrete operation, they were able to easily slide into the single parking lane that was available and shotcrete the entire project from the far end of the building. As mentioned previously, some of the pumping distances were up to eight stories high Fig. 3: Basement perimeter walls during shotcrete wire installation Fig. 4: Shotcrete installation for column encasement www.shotcrete.org Winter 2017 | Shotcrete 57


2017WinterShotcreteEMag
To see the actual publication please follow the link above