Pumped for 2018 Winter Olympics
Pumped for 2018 Winter Olympics
When South Korea was awarded the 2018 Winter Olympics, one significant project was building the PyeongChang Olympic Sliding Centre to host bobsleigh, luge and skeleton events. Construction of the long and winding sliding track was so exceptionally unique that it was recognized with Honorable 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.
The Centre, at an estimated $114.5 million project cost, occupies an area of 44 acres (177,000m2). At the heart of it is the 6,620 foot (2,018m) long and 56 inch (1.4m) wide sliding track that needed to accommodate racing speeds up to 85 mph (135 kph). 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, a full-sized mockup with a reduced 16 foot (5m) length was built by simulating the transition curve section of the track. The Thom-Katt TK 10 pump, at seven cubic yards an hour (4.6m³/hr) output and a maximum 2,085 psi (144 bar) pressure, performed the shotcreting at two mockup trials which took place at Kangwon National University, South Korea.
Kangwon National University carries out technical research on shotcrete material and construction methods and is the only specialized shotcrete research institute in Korea that owns shotcrete equipment. Dr. Kyong-Ku Yun, Professor of civil engineering at the university, is very familiar with Putzmeister equipment. His laboratory contains a Magnum combination mixer-pump, ideal for wet-process shotcrete and a Piccola for dry-process shotcrete
Highly satisfied with the reliable performance of the equipment at the lab, 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.
"The TK 20 is perfect for shotcrete applications. as it offers high outputs up to 17 cubic yards an hour (13m3/hr) and concrete pressures up to 2,000 psi (138 bar)," said Bob Harmon, National Sales Manager - 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. In addition, an adjustable end control allows the operator to increase or decrease the pump to achieve the exact amount of material output desired."
Professor Yun adds, "The Thom-Katt TK 10 and TK 20 machines performed very well in their respective roles on the project. The pumps are robust, very simple to operate and need little maintenance. And Bob Harmon at Putzmeister has always been very helpful whenever there has been pumping questions."
Of major significance to the 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 collisions happen frequently in bobsleigh races. Plus, the track is continuously exposed to a very 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 percent more air content before shooting. The air content was measured to be three to six percent after shooting, providing the desired results. The durability properties were improved by substituting silica fume for about nine percent by weight of the cementitious content. The mix design also helped suppress early-age shrinkage cracks by substituting seven percent of an expansive admixture by weight of the cementitious content. The concrete mix was economical because the fine aggregate was set at 75 percent of the total aggregate weight to reduce the amount of rebound. Numerous tests (air content, slump, strength and durability) were diligently performed at appropriate times to ensure performance criteria was consistently attained. A total 1,820 cubic yards (1,392m³) 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 a one-fifth inch (±5mm) 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 1.1 inch (28 mm) plastic tube in place to get a 1.2 inch (30mm) 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 percent of the arranged reinforcement was still visible after placing the first layer. During shooting, rebound had to be constantly removed using an air lance (blow pipe). 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 1.1 inch (28mm) 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. After the final surface finish, the concrete surface was wet-cured with a wetted woven fabric that was continuously watered. No curing membranes were applied.
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, 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 (previous host country of the Winter Olympics); Yong-Gon Kim, CEO of Daesang E&C; and Kyu-Woon Lee, a Director General of Construction at Gangwon Headquarters for the 2018 Winter Olympics. The completed center is currently hosting test events until the Winter Olympics are held from February 9-25, 2018.
Project Owner: Gangwon-do Province
General Contractor: Daelim Co. — Seoul, Korea
Shotcrete Contractor: Daesang E&C — Seoul, Korea
Equipment: Putzmeister Thom-Katt® TK 10 and TK 20 trailer-mounted concrete/shotcrete pumps