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An ingenious combination of old concrete and new fibre-reinforced polymer (FRP) composite saved precious time while preventing a potentially serious wastewater problem in metropolitan Denver.
The old concrete started as a huge box culvert installed in 1977 to convey wastewater from primary treatment to secondary treatment facilities of the Metro Wastewater Reclamation District. The new composite is 500 feet (152 meters) of large diameter pipe that Belco Manufacturing Company made with corrosion-resistant Vipel F010 bisphenol A epoxy vinyl ester from AOC. With an inside diameter of 11 feet, 2 inches (3.4 meters), the pipe is believed to be the largest composite sewer pipe ever made in North America.
The original 12 feet wide by 9 feet high (3.7 by 2.7 meters) culvert was designed to be filled to capacity; but shortly after being put in service, the operational depth of the conduit was lowered. Wastewater flow levels were typically 10 to 14 inches (25 to 36 centimetres) below the culvert ceiling – a situation that wastewater engineers know can lead to a highly corrosive environment.
The corrosion problem started when naturally decomposing wastewater components released hydrogen sulphide gas into the space between the water flow line and culvert ceiling. After bacteria above the waterline ingested the gas, they secreted sulphuric acid that, in turn, aggressively attacked the culvert ceiling and non-submerged sidewall surface. Concrete was spalling, metal rebar was exposed, 35% of the original 14-inch (36-centimeter) thick roof was gone, and the roof’s structural integrity was in jeopardy.
Due to very limited construction space, District engineers sought a solution that would keep the undamaged lower portion of the concrete culvert in place. The roof and about 4 inches (10 centimeters) of the upper sidewalls would be removed, leaving a U-shaped concrete channel. The sewer line in need of repair conveys about 50 million gallons (189,270 kiloliters) of wastewater on an average day. To keep a 24/7 flow moving through the treatment plant, engineers designed a by-pass system using pumps and 3.8 miles (6 kilometres) of flexible thermoplastic pipe. To reduce costs and the potential for problems, engineers specified that the temporary by-pass be in service for as little time as possible.
Composite pipe designed by Belco specifically for the project offered the best solution for speed of installation. The custom pipe fit precisely into the U-shaped channel which served as a trench. A concrete grout was injected between the trench inner wall and pipe outer wall. Cables held the pipe in place to keep the pipe from floating in the wet grout before it cured.
The pipe was joined together using butt-and-strap composite joints that were applied internally. Installation during October through December required the use of a plastic tent around joint sites to shelter the newly laminated joints against cooler temperatures which could retard cure.
“We considered topping the remaining concrete with a new precast concrete ‘cover’ that would be PVC-lined to protect the concrete from corrosive attack,” said Ted Knutzen, engineer for URS Corporation, the engineering design firm for the project. “But because composite pipe could be made in longer lengths, the composite provided the opportunity to reduce installation time.”
New precast concrete would have been made in 8-foot (2.4-metre) long sections. In contrast, the composite pipe was manufactured in 28-foot (8.5-meter) sections. “With one composite length for every three to four concrete lengths, composite piping reduced the number of joints, resulting in less installation time,” said Guy Gentry, Engineering Manager for pipe manufacturer Belco Manufacturing. The lighter weight of the composite also made it easier to handle and allowed for the use of less expensive equipment, added Gentry.
Pipes installed in the concrete culvert
General contractor Stanek Constructors, Golden, Colorado, completed the composite pipe installation within the 52 days specified for the by-pass. The pipe was supplied through Process Equipment Corporation, a Belco Manufacturing representative located in Boulder, Colorado.
“We understood the lightweight and corrosion-resistant benefits of FRP composites,” said Knutzen, “but many of us in civil engineering are more familiar with concrete. It is easier for us to recognize quality in concrete manufacturing and installation. To help us better understand composites, Guy Gentry and the people at Belco were very helpful in providing information, data and photographs.” Belco manufactured the composite pipe in accordance with the standards of AWWA M-45 and ASME RTP-1, made using the filament winding process in which resin-impregnated glass fibre reinforcement is wound in a computer-designed pattern around a metal mandrel. The resin cures into a permanently crosslinked state to create a fibre-resin composite cylinder with excellent mechanical and physical properties.
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