Ductile Iron Pipe vs. Concrete
Concrete pipe cannot compare to Ductile Iron when considering their respective approaches to internal pressure design; and Ductile Iron Pipe is much easier to install and operate than concrete pipe.
Internal Pressure
The most important and interesting difference between Ductile iron pipe and BCCP is their designs for internal pressure. A careful examination of these materials’ approaches to this important aspect of pipeline specification, while complicated by the composite nature of the concrete pipe, brings to light striking deficiencies in the BCCP method.
The pipe is designated as a “pressure class” product, which means that the wall thickness is calculated taking into account both working and surge pressures that the pipeline will experience. “Pressure Class 350” Ductile iron pipe has a wall thickness that is calculated using a working pressure of 350 psi and an additional surge pressure of 100 psi with a nominal safety factor of 2.0, resulting in a design pressure of 900 psi.
BCCP is a composite product that is manufactured under the ANSI/AWWA C303 standard “Concrete Pressure Pipe, Bar-Wrapped, Steel-Cylinder Type.” It is classified by the concrete pipe industry as a “semi-rigid” pipe in that it does require ring deflection design for external load, but the limits of allowable deflection are very low to prevent cracking of the rigid protective cement-mortar coating.
The Cost of Corrosion Control
For external corrosion control, BCCP relies primarily on a rigid exterior cement-mortar coating. The hydrated cement mortar provides an alkaline environment with an initial pH of approximately 12.5 that is in contact with the steel bar and cylinder. This alkaline environment generates an oxide film on the steel, a process known as passivation.
In most environments, iron pipe has an inherent resistance to corrosion, as evidenced by the fact that it comprises the oldest pipelines in the world. For example, an iron pipeline in Versailles, France, served well over 300 years. In North America there are more than 570 utilities that received 100 or more years of service, and at least 18 have attained 150 years or more of service from their cast iron pipelines.
Ductile Iron Pipe's Ability to Perform
Many advantages exist for the owner, design engineer and contractor when Ductile iron pipe is specified. A conservative, straightforward design procedure that takes advantage of ductile iron’s tremendous strength gives an impressive factor of safety on a pipeline that is easy to construct. Field changes and adaptations further simplify the construction process while operational savings due to lower head losses and reliability of service provide the owner with a winning situation. Supplemental corrosion control, if needed, is effective and economically provided by polyethylene encasement, a simple passive system that requires no monitoring or maintenance.
Further, the owner has the satisfaction of knowing that any normal changes in operating conditions will not likely compromise Ductile iron pipe’s ability to perform. When pipes compete, they should do so on an equal basis and we feel that the standards should be raised, rather than lowered, to effect greater equality of performance. In order to compare with Ductile iron pipe, the competition must improve. But even then, we still find that all pipe materials are not equal. When comparing Ductile iron pipe to bar-wrapped concrete cylinder pipe, it is obvious that Ductile iron pipe is the right decision.
