When South Korea was awarded the 2018 Winter Olympics, one significant project was building the PyeongChang Olympic Sliding Centre.
The Winter Olympics facility was required to host bobsleigh, luge and skeleton events. Construction of the long and winding sliding track was so unique that it was recognized with Honourable Mention 2016 as an Outstanding Shotcrete Project of the Year by the American Shotcrete Association.
Working behind the scenes to place a highly specialized concrete mix for the extremely complex design were Putzmeister Thom-Katt TK 10 and TK 20 trailer-mounted concrete/shotcrete pumps.
Setting the stage for the 2018 Winter Olympics
The centre, at an estimated $114.5 million project cost, occupies an area of 177,000 square metres. At the heart of it is the 2,018-metre-long and 1.4 metre wide sliding track that needed to accommodate racing speeds up to 135 km/hour. Extensive research was done before construction to develop a suitable concrete mix and placement methods that would pass stringent inspection tests.
To confirm the proposed mix and placement methods would work on the Winter Olympics track, a full-sized mockup with a reduced 5 metre length was built by simulating the transition curve section of the track.
The Thom-Katt TK 10 pump, at 4.6 cubic metres per hour output and a maximum 2,085 psi pressure, performed the shotcreting at two mockup trials which took place at Kangwon National University in South Korea.
Kangwon National University is the only specialized shotcrete research institute in Korea that owns shotcrete equipment. Dr. Kyong-Ku Yun, Professor of civil engineering at the university laboratory contains a Magnum combination mixer-pump, ideal for wet-process shotcrete and a Piccola for dry-process shotcrete
Professor Yun selected another Putzmeister model, the Thom-Katt TK 10. It proved to be an ideal unit for the mockup, as it could closely simulate the results that would be achieved with the larger TK 20 model, which would be used to shotcrete the longer final track. Two TK 20s, used by shotcrete contractor Daesang E&C, would efficiently complete shotcrete work of the final track and the artificial rock retaining walls in only six months.
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“The TK 20 is perfect for shotcrete applications. as it offers high outputs up to 17 cubic yards an hour and concrete pressures up to 2,000 psi said Bob Harmon, national sales manager for underground/shotcrete technology at Putzmeister America. “Plus, the model can pump difficult mixes far distances with almost no surge at the nozzle because of smaller cylinders than a concrete pump.”
Of major significance to the Winter Olympics project’s success was the design of a concrete mix that was appropriate for shotcrete placement and had excellent strength, as well as freezing and thawing, and surface-scaling resistance. This was necessary because bobsleds are known to crash. Plus, the track is continuously exposed to a cold environment to keep ice frozen on the surface.
The structural mix incorporated admixtures to achieve the needed high strength and high durability. Pumpability was improved by incorporating 10 to 15 per cent more air content before shooting. The air content was measured to be three to six per cent after shooting, providing the desired results.
The durability properties were improved by substituting silica fume for about nine per cent by weight of the cementitious content. The mix design also helped suppress early-age shrinkage cracks by substituting seven per cent of an expansive admixture by weight of the cementitious content. The concrete mix was economical because the fine aggregate was set at 75 per cent of the total aggregate weight to reduce the amount of rebound. Numerous tests (air content, slump, strength and durability) were performed at appropriate times to ensure performance criteria was consistently attained. A total 1,392 cubic metres of the specially designed mix was used for the final track.
The steps in the construction process were methodical. First, the support frame for the track was built to secure the space where the left and right walls of the sliding track were attached and defined the working environment. The support frame was manufactured using steel H-beams and designed to carry the load applied to the back of the wall.
The jig support frame was a critical support where all the reinforcement, cooling pipes, and stay-form were installed. Therefore, the frame’s installation required setting to an about 5 mm tolerance. Cooling pipes were installed in the frame, then reinforcing bars placed in the front and rear surfaces in an orthogonal layout.
The floor and head portions were mounted with a wooden form. However, because normal formwork or lining cannot be effectively used in creating multiple-curved surface structures, an inner wall stay-form and temporary screed pipe techniques were used. The stay-form reduced the rebound during shotcrete placement, and it also served to densely fill the interior.
Temporary screed pipes were used as guides for shotcreting at a uniform thickness. These pipes were mounted by taping a 28-mm plastic tube in place to get a 30 mm thickness.
The full-size sliding track was built without forming construction joints. The TK 20 pumps offered superb control in shotcreting monolithic sections throughout the final cross section of the track. When shooting the floor, 50 per cent of the arranged reinforcement was still visible after placing the first layer. During shooting, rebound had to be constantly removed using an air lance. The upper and lower head parts were shot with lower air pressure, and then the in-place concrete was compacted with a vibrator.
After shooting was complete, the freshly placed concrete was screeded to the 28 mm plastic tubes installed on the upper reinforcement as a temporary screed pipe. In the straight portions, it was precisely set to give the track a smooth surface and uniform thickness, necessary for the cooling pipe to create ice on the surface. The temporary screed pipes were removed after the surface was screeded, and the remaining void was filled with hand-applied concrete.
The final mockup inspection was performed at the project site and accepted after only one trial. This is exceptional compared to experience by others in building such a special track, as this is a first in Korea, and the 19th one in the world. However, Professor Yun had done his research and worked with key personnel such as Dr. D. R. Morgan, P.Eng., FACI of Canada.