Launch Slideshow

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The return of vitrified clay pipe

The return of vitrified clay pipe

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    Impurities and imprecise firing methods caused voids and lamination in the terra cotta pipe placed in the late 1800s and early 1900s. Photos: National Clay Pipe Institute

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    Modern manufacturing is precisely controlled to quickly bring the clay from a drying room temperature up to 500º F before slowly raising it to 1,100º F to burn out impurities. Once the pipe body reaches 1,100º F the firing process can proceed quickly to 2,000º F, the point at which the pipe is vitrified — converted to a hard, glass-like state. To maintain dimensional stability, cooling also is controlled in steps.

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    CLAY VS. THE COMPETITIONSpecifiers tout vitrified clay as all-natural as well as corrosion-resistant. Other materials are available in longer lengths, such as 13.5 feet for plastics and more than 20 feet for fiberglass compared to 10 feet for clay. But as a blend of clay and shale with water added, only one chemical — hydrofluoric acid — attacks clay pipe compared to the 150 solvent-based chemicals that attack plastics. Source: Colleen Harold

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    Compression joints are factory applied and tested to eliminate leaks.

For some time, Portland's Bureau of Environmental Services had been interested in modern clay pipe. Each material offers advantages and disadvantages for different applications, but the long life cycle of vitrified clay was attractive.

The city, however, faced two major constraints: supply and experience. Without a local supplier, clay pipe hadn't been able to overcome added transportation costs to compete for the bureau's work. As a result, local contractors lacked experience and confidence in bidding the material.

Fortunately, communities in the Seattle area were building projects with vitrified clay pipe (VCP). A new supply hub began operating near Tacoma, which meant that pipe was now passing through Portland from California and other out-of-state suppliers on its way to the Seattle area.

To take advantage of this, the bureau contacted the National Clay Pipe Institute to find out what could be done to improve availability in Portland. Over the course of several months, institute experts spent several days in Portland training bureau employees who reworked the presentation into in-house training programs for specifying, educating contractors, and inspecting installations.

The bureau's design group was asked to create an opportunity in a $1.7 million sewer rehabilitation project that would give suppliers, contractors, and employees experience working with the material. To that end, part of the bureau's training program was visiting suppliers and familiarizing key employees with the material.

“Pallets from two prospective clay pipe suppliers for our area were ordered and delivered to our maintenance yard,” says Senior Engineering Associate Colleen Harold. “All design and construction staff, including inspectors and material testers, was invited to participate in becoming familiar with the pipe. They were able to ask questions, and observe cutting of the pipe, installing Inserta-Tees [a product of The Royal Group's Pipe Systems division], and handling and installing couplers.

“We learned how to work with pipe so that we were the experts, not the contractor. We found out, for example, that we couldn't use chains and nylon straps to handle the pipe because it was too slick and that grippers worked best.”

In March 2010, 384 feet of no-bell clay pipe was installed for rehabilitation; the rest of the project — a total of 6,208 feet of 12-inch bell-and-spigot clay mainline — was installed as new construction. All 205 laterals were replaced with 6-inch PVC.

“For contractors, clay compares best to concrete, not plastic,” Harold says. “The best part is that it comes in short lengths, so pipe can be installed right up to manholes without additional cutting of the pipe. This allows the contractors to lay more pipe in a day.”

Overcoming misperceptions

As the grandfather of modern sewer pipe, vitrified clay is overcoming a number of myths.

Until the 1950s the pipe was placed in hand-dug trenches without joints, or joints were fabricated in the field by the installer. Sometimes joints were made from cement mortar and sometimes from tar, both of which were readily available on jobsites. Thus, the earliest installations “were only as good as the guy in the trench on any given day,” says Jeff Boschert, civil engineer with the National Clay Pipe Institute. “Engineers considered infiltration and inflow (I&I) to be a benefit to the system because the lines were flushed and cleaned during rain events. But with the onset of wastewater treatment in the 1950's, I&I became undesirable due to increased treatment costs and environmental impacts.”

Manufacturing technology of the late 1800s and early 1900s produced pipe with both laminations and voids that compromised strength. Steam-driven extrusion equipment limited lengths to 3 feet. Today, laminations and voids in the pipe body are nonexistent due to the development of high densification extruding equipment.