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2013SprShotcreteEMag

And that was only the first challenge! CDOT required the repairs be performed sequentially, so that no more than four sections of load-bearing shoring could be in place at a time. This required ACR to chip and prepare the surface and place shotcrete in four phases. Considering the short time frame to complete the project, ACR proposed moving the shoring towers when the in-place shotcrete reached 75% of its design strength to accelerate the job. In addition, ACR recommended using epoxy-set “L-shaped” dowel bars to support the reinforcing steel rather than the specified expansion anchors. ACR documented the success of the proposed changes from successful use on past projects where time was of the essence. CDOT agreed to the proposed changes. They also allowed the shotcrete to be tested for compressive strength from 72 to 96 hours after placement instead of the specified 14 days. This allowed earlier resetting of the shoring as soon as 75% of the design strength was confirmed. ACR mobilized in mid-October and began the removal of the first segment of overhead repairs, averaging 6 in. (150 mm) in depth (Fig. 3). The repair areas needed to be cleaned and wire mesh needed to be installed prior to shotcrete place- ment. Special attention was given to the saw-cut edges because saw cutting leaves a polished surface that must be heavily blasted to ensure proper bonding. All existing reinforcing bars exposed by the concrete removal were coated with zinc-rich primer. The freshly sandblasted surface was prewetted to a saturated surface-dry (SSD) Fig. 2: Viaduct center left open for construction traffic condition. Due to the depth of the repairs, the areas needed to be shot in two lifts and the surface of In addition to the shotcrete operation, other the first lift was power-washed prior to placing trades were in close proximity performing their the second lift. The prepackaged, pre-blended work, including but not limited to crack injection, Illinois Department of Transportation (IDOT)- lighting, and formed concrete repairs. Because approved shotcrete material typically reached everyone was aware of the expedited schedule, it 75% of the design strength in 3 to 4 days. Once was important that the entire construction team complete, the repair areas were sounded by ACR work together to get the project done. As the to assure that all the shotcrete was fully bonded project was located in downtown Chicago, near to the substrate. After sounding was complete, the several large hotels, there were city noise ordi- ironworkers moved the shoring towers and ACR nances that had to be accommodated with the was able to proceed with the next section of workday schedules. The noise restriction only removals, followed by the surface preparation and allowed 12 hours of construction work per day. mesh installation. Along with the project time constraints and noise As late fall approached Chicago, the tempera- ordinances, the project was abandoned for a few ture became the next challenge. Work in the cold days due to 20 ft (6 m) waves flooding the under- temperatures causes equipment delays and inef- pass from a tremendous storm on Lake Michigan, ficient production. Additionally, the CDOT just 200 ft (60 m) away from the project. All the specification for shooting shotcrete required that contractors pulled together as a team and regular environmental conditions be a minimum ambient communication between the trades was a key temperature of 45°F (7°C), a 50°F (10°C) material element in making this project successful. temperature, and a 40°F (4°C) substrate tempera- In the four different phases of shotcrete, over ture. The experienced shotcrete contractor brought 300,000 lb (136,000 kg) of material was applied. in large heaters to raise the ambient and surface The shotcrete process, performed by the qualified temperatures. Heated water was used in the shot- shotcrete contractor, was given high praise for the crete mixture to keep it in compliance. completion of the project on schedule, even with Shotcrete • Spring 2013 11


2013SprShotcreteEMag
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