A Mix of Old & New

An ingenious combination of old concrete and new fiber-reinforced polymer (FRP) composite saved precious time while preventing a potentially serious waste water problem in metropolitan Denver.

- Ben R. Bogner

The old concrete started as a huge box culvert installed in 1977 to convey waste water from primary treatment to secondary treatment facilities of the Metro Waste water Reclamation District. The new composite is 500 ft of large-diameter pipe that Belco Manufacturing Co. made with corrosion-resistant Vipel F010 bisphenol A epoxy vinyl ester from resin-producer AOC. With an inside diameter of 11 ft, 2 in., the pipe is believed to be the largest composite sewer pipe ever made in North America.

The original 12- by 9-ft culvert was designed to be filled to capacity, but shortly after being put in service, the operational depth of the conduit was lowered. Waste water flow levels were typically 10 to 14 in. below the culvert ceiling—a situation that waste water engineers know can lead to a highly corrosive environment.

The corrosion problem started when naturally decomposing waste water components released hydrogen sulfide gas into the space between the water flow line and culvert ceiling. After bacteria above the waterline ingested the gas, secreted sulfuric acid aggressively attacked the culvert ceiling and non-submerged sidewall surface. Concrete was spalling; metal re bar was exposed; 35% of the original 14-in. thick roof was gone; and the roof’s structural integrity was in jeopardy.

Saving the remaining structure

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 in. of the upper sidewalls would be removed, leaving a U-shaped concrete channel.

The sewer line in need of repair conveys about 50 million gal of waste water on an average day. To keep flow moving through the treatment plant, engineers designed a bypass system using pumps and 3.8 miles of flexible thermoplastic pipe. To reduce costs and the potential for problems, engineers specified that the temporary bypass be in service for as little time as possible.

Composite pipe, designed by Belco Manufacturing 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, while cables held the pipe in place to keep the pipe from getting buoyant in the wet grout before it cured. The pipe was joined together using butt-and-strap composite joints that were applied internally.

Installation occurred over three months and required the use of a plastic tent around joint sites to shelter the newly laminated joints against cooler temperatures, which could retard cure. The resin, formulated by AOC technical service, performed well under these circumstances.

Longer lengths & lighter weight

“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 Corp., 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-ft-long sections. In contrast, the composite pipe was manufactured in 28-ft 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 Belco Manufacturing. The lighter weight of the composite made it easier to handle and allowed for the use of less expensive equipment, Gentry added. Composite density is about 120 lb/cu ft, and the nominal pipe wall thicknesses are only 1.375 in. thick. Concrete density is about 400 lb/cu ft, and wall thicknesses are typically 8 to 10 in. thick.

Corrosion resistance

Another advantage of composite made with Vipel F010 vinyl ester is its inherent long-term resistance to the corrosive effects of moisture and sewer gases.

“We understood the lightweight and corrosion-resistant benefits of FRP composites,” Knutzen said, “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. The pipe was made using the filament winding process in which resin-impregnated glass fiber reinforcement is wound in a computer-designed pattern around a metal mandrel. The resin cures into a permanently crosslinked state to create a fiber-resin composite cylinder with excellent mechanical and physical properties.

“The Vipel F010 resin processes very well,” Gentry said. “Our winder operators really appreciate the resin’s user-friendly characteristics. The resin wets well and has a predictable cure. AOC technical service is very good, too. Corrosion Specialist Geoff Ward is very helpful. We can always count on him if we have a question or need help.”