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2017WinterShotcreteEMag

The software used for CAD simulation was RISA 3D Structural Engineering, which revealed the need for a highstrength watertight concrete (C30/37) that can withstand 145 psi (10 bar) of water pressure, made of high-grade Type I 52.5N cement, aggregates up to 3/8 in. (10 mm) in grain size, and water. No admixtures were used in this project. The scope of the rehabilitation project consisted of: • Laying a 5 ft (1.5 m) tall wall of dirt on top of the pipes to create enough gravity loading to prevent the pipe from floating after draining them; • Cleaning the pipe’s interior with high-pressure water jetting at roughly 36,000 psi (2500 bar); • Placing the 6.5 x 20 ft (2 x 6 m) wide, 0.25 in. (6 mm) thick steel-welded wire reinforcement on the inside wall of the pipes; and • Shotcreting a 3.15 in. (80 mm) concrete jacket over the interior surface of the pipes. EXECUTION OF SHOTCRETE A dry-mix, thin-flow shotcrete process was used throughout this project, chosen for three main reasons: • The very high early strength of the concrete required for shooting overhead in a thick layer; • The extreme weather conditions at surface, with temperatures ranging from 95°F (35°C) in mid-August to –4°F (–20°C) in January, which prevented the use of wet mix process; and • The need to push the concrete through delivery lines over large distances of up to 650 ft (200 m), thus minimizing the need for access points into the pipes. Crushed stone sands were supplied by a local gravel pit, with a grading curve that extended up to 3/8 in. (10 mm) grain size. The cement was supplied in silo trucks and stored on site in a 47 yd3 (36 m3) horizontal silo. The dry-mix process concrete was prepared using a 2.6 yd3 (2 m3) selfloading mobile batching plant and delivered underground to the shotcrete pumps through metal tubing placed in the air vents and access points. The rebound varied greatly with the type of aggregates (washed river or crushed stone aggregates) and position of the sprayed surface (overhead or bottom half of the pipe). Crushed stone generated 18% rebound on overhead surfaces, 14% on the vertical side parts of the pipes, and 9% on the floor, while washed river aggregates generally created 5% less rebound. On any given section of the pipes, the bottom half would be sprayed first, with a minimum of 24 hours passing before stepping onto the shotcreted concrete and applying the shotcrete overhead. The rebound material was removed using a pneumatic industrial vacuum pump and manually by shoveling. We aggressively undertook the work and completed the Simulation of nominal load stress for steel pipes project by working two crews per shift with two long shifts per day, with a 2-hour cleanup and maintenance break in between. A total of 1500 yd3 (1150 m3) of concrete were shot. Each shift’s crew consisted of three interchangeable skilled workers, three unskilled workers, one mechanic who was the operator for both pumps, and one highly-skilled Detail of placing steel-welded wire reinforcement www.shotcrete.org Winter 2017 | Shotcrete 43


2017WinterShotcreteEMag
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