DIPRA Studies Examine Polyethylene Encasement

Nov. 1, 1999
Editor?s note: This article was adapted with permission from Ductile Iron Pipe News, Spring/Summer 1999. For more information on DIPRA, visit the web site at www.dipra.org.

Editor?s note: This article was adapted with permission from Ductile Iron Pipe News, Spring/Summer 1999. For more information on DIPRA, visit the web site at www.dipra.org.

The Ductile Iron Pipe Research Association (DIPRA) first investigated polyethylene-encased pipe in an operating system 36 years ago. Since then, the organization has conducted investigations across the United States and Canada in every kind of corrosive soil imaginable, inspecting iron water mains and conducting soil surveys to assess corrosion protection.

The results have shown that polyethylene encasement is effective to protect cast and ductile iron pipe. At the same time these investigations have shown the importance of properly installing and handling polyethylene encasement.

DIPRA recommends polyethylene encasement because it is economical and has proved effective in a broad range of soils. It is not universally applicable in all situations warranting corrosion protection, but neither are cathodic protection or bonded coating systems. Utility managers should not be deceived into thinking any system of corrosion protection is perfect.

Since its introduction into the marketplace in 1955, ductile iron pipe has become an industry standard for water and wastewater systems. In 1963, DIPRA began a program of polyethylene encasement investigations. Since then, DIPRA has conducted 122 such investigations across the United States and Canada in a wide variety of corrosive soils, and continues to supplement that number every year.

Engineers inspected and evaluated the condition of polyethylene-encased cast and ductile iron water mains in operating systems to assess the effectiveness of polyethylene encasement as a means of corrosion protection. Often, DIPRA performed these investigations after the mains had been in prolonged service.

DIPRA also has investigated numerous in-service pipelines, including many that were protected by bonded coatings and/or cathodic protection systems. The continuing opportunity to examine in-service pipelines provides a better understanding of the corrosion phenomenon.

DIPRA has conducted extensive tests since 1951, both in DIPRA?s Research Lab in Birmingham, Ala., and in 24 test sites around the nation having soils ranging from moderately to severely corrosive. Currently, studies continue at 15 of these test sites. Projects include research on several new types of encasement materials and 10 different types of bonded coatings, including factory-applied zinc coatings manufactured in some European countries, to determine if a better method of protection exists.

Other DIPRA research includes investigating the effects of stray direct current on polyethylene-encased pipelines, cathodic protection, corrosion rate studies on ductile iron and other materials, measuring electrical resistance afforded by rubber-gasketed joints used on ductile iron pipe, microbiological corrosion, strength of corrosion products, and various methods of mitigating bimetallic corrosion.

Conducting polyethylene investigations is one of many services offered by DIPRA. The organization?s regional engineers, 10 in the United States, two in Canada, also perform soil surveys for proposed pipelines to determine if soils are considered corrosive to iron pipe and whether corrosion protection is needed. DIPRA works closely with water utilities to perform these investigations.

?These free DIPRA services can help ensure that a utility?s investment in ductile iron pipe truly becomes an investment for a lifetime ? even in the most corrosive soils,? said DIPRA President Troy Stroud.

In the vast majority of cases, the polyethylene-encased iron pipe inspected and evaluated by DIPRA was in good condition. In the few situations where that was not the case, DIPRA has found that the problem was not the system itself, but rather in the way the polyethylene encasement was installed or handled.

According to DIPRA, encasing iron pipe with polyethylene during installation is not difficult. Neither is taking care of the polyethylene while making service taps. Most problems, in Stroud?s opinion, result from inadequate training and supervision/ inspection during installation.

In 1992, for example, a utility in Connecticut installed an 8-inch polyethylene-encased ductile iron water main in corrosive soil. Six years later, the pipe experienced a corrosion failure at a service connection. When the pipe was uncovered, the crew discovered that a 3-foot section of polyethylene had been removed by the original tapping crew and had not been replaced prior to backfilling.

To determine if moisture was migrating under the polyethylene to sites remote from the unrepaired polyethylene, the utility requested that a section of pipe 25 feet from the service connection be inspected. The inspection results indicated that the portion of the pipeline away from the service tap was in excellent condition.

In Colorado, a utility reportedly had experienced 11 failures on an eight-inch polyethylene-encased ductile iron pipeline since its installation in corrosive soils in 1985. Working with the utility, DIPRA inspected the pipeline at four locations. In three of those locations the pipe was found to be bare. Polyethylene encasement thought to be in place at those three spots had never been installed. At the fourth location, the polyethylene was in place and providing corrosion protection.

In another case in California, an investigation revealed that the iron pipe had not been encased in polyethylene as reported. Rather, the polyethylene simply had been laid over the top of the pipe. Even then, it had protected the top of the pipeline, while corrosion had occurred on the bottom where it was exposed to corrosive soil.

In Latham, NY, six-inch cast iron pipe encased in loose polyethylene was installed in 1962 near a brick plant in saturated soil conditions consisting of a dark brown, stiff clay. To determine the condition of pipe and encasement approximately eight feet of trench was excavated to pipe depth and then exposed by shovel and hand to avoid damage to the polyethylene.

The polyethylene was clear, undamaged, and appeared to be in excellent condition. It appeared that recommended installation procedures had been followed closely as the polyethylene was folded and taped on top of the main.

Approximately 4 feet of pipe was exposed by removing the polyethylene encasement. Moisture was observed under the wrap with evidence of superficial oxidation on the surface of the pipe. The oxidation was easily removed with a wire brush and damp cloth. The pipe was found to be free of any pitting or graphitization resulting from galvanic corrosion.

The DIPRA inspector concluded that it was apparent from this examination that the polyethylene encasement on the subject main had provided excellent corrosion protection for the past 36 years.

Although polyethylene encasement is not a water-tight system, once installed the weight of the earth backfill and surrounding soil prevents any significant exchange of groundwater in the space between the wrap and the pipe. Typically, some seepage of groundwater beneath the wrap will occur, but the corrosive characteristics of the water are soon depleted, usually by the action of initial oxidation. Once the available oxygen in the moisture film beneath the wrap has been consumed, further corrosion activity is effectively halted and a uniform environment exists around the pipe. Additionally, the polyethylene film retards the diffusion of additional dissolved oxygen to the pipe surface and also retards the migration of corrosion products away from the pipe surface.

The Ductile Iron Pipe Research Association is a non-profit association supported by ductile iron pressure pipe manufacturers in the United States and Canada. Member companies include American Cast Iron Pipe, Atlantic States Cast Iron Pipe, Canada Pipe, Clow Water Systems, Griffin Pipe Products, McWane Cast Iron Pipe, Pacific States Cast Iron Pipe, and the United States Pipe and Foundry. Further information about iron pipe or encasement installation can be found at the organization?s web site at http://www.dipra.org/pub.html.

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