Feb 16, 2006

Good for the Long Run

esign and construction of pipelines, culverts and related drainage facilities are important areas of civil engineering in which decisions must be made regarding material and system selection. Material selection and development of appropriate design criteria are very involved undertakings requiring experience in material use and performance. Proper engineering design of any hydraulic structure requires consideration of many different but related fields, including planning, hydraulics, installation, durability, maintenance and economics.

Durability and economic aspects are generally not given proper consideration. For many projects, pipe materials or systems are selected on an initial- or capital-cost basis only. Lower initial costs, however, do not always result in the most economical product or system. To determine the most economical choice, the principles of economics must be applied through a life-cycle cost analysis (LCA).

Considering the future

Following the Intermodal Surface Transportation Equity Act of 1991, the Federal Highway Administration has supported and encouraged state DOTs to embrace LCA. In their LCA Primer they state: “In the face of increasing public sophistication and interest, transportation agency officials are expected to explain and justify decisions concerning the expenditure of taxpayer dollars. Documentation associated with the LCA process is a mechanism for transportation officials to demonstrate their good stewardship of the public’s transportation infrastructure investment. Decision makers have a record of their consideration of different assumptions and a formal analysis that supports the decision itself. These records are available to be revisited during or after the LCA process.”
Local and state governments are increasingly including some type of cost analysis in their material selection process. The importance of considering the future of a facility during the design phase has been made clear by the multitude of problems many authorities are facing as our infrastructure declines. In many instances, engineers and executive officers have to replace integral sections of infrastructure that have experienced premature degradation.

According to the U.S. Army Corps of Engineers, selection of all systems, components and materials for civil works projects are based on their long-term performance, including LCA. This design criterion is referred to as Regulation No. 1110-2-8159. The cost consideration in a project must be based on the long-term performance of the material being used, not solely on the initial cost of a product. It is Army Corps policy that design engineers are responsible for implementing life-cycle design concepts into the project development process.

Evaluating total cost

The American Society for Testing and Materials (ASTM) Committee C-13 on Concrete Pipe has developed and published ASTM Standard of Practice C 1131 for Least Cost (Life Cycle) Analysis of Concrete Culvert, Storm Sewer and Sanitary Sewer Systems. ASTM has also developed Practice A 930 for Least Cost Analysis of Corrugated Metal Pipe and Practice F 675 for Least Cost Analysis of Plastic Pipe. ASTM C 1131 covers procedures for using LCA techniques to evaluate alternative pipeline materials, structures or systems that satisfy the same functional requirement.

The LCA technique evaluates the present value constant dollar costs to install and maintain alternative drainage systems, including planning, engineering, construction, maintenance, rehabilitation and replacement, and cost deductions for any residual value at the end of the proposed project design life. Using the results of the LCA, the decision maker can readily identify the alternative with the lowest total cost based on the present value of all initial and future costs. First cost is only one of many factors that influence a proper economic analysis. It may be the least important factor if maintenance costs are high or pipe systems have to be replaced during the design life of the project. The other important factors are project design life and material service life.

Project design life

The National Cooperative Highway Research Program Synthesis of Highway Practice titled “Durability of Drainage Pipe” defines “service life” as the number of years of relatively maintenance-free performance. Based on synthesis recommendations, up to 50 years of relatively maintenance-free performance should be required for culverts on secondary road facilities and up to 100 years for higher-type facilities, such as primary and interstate highways and all storm and sanitary sewers.

Material services life

According to the U.S. Army Corps of Engineers, concrete pipe has a service life of 70 to100 years; corrugated metal pipe may obtain up to a 50-year service life with the use of coatings; and the designer should not expect a material service life greater than 50 years for any plastic pipe. High density polyethylene (HDPE) pipe shares the same characteristics as other plastic pipes in being lightweight and flexible. Service life of HDPE pipe greatly depends upon installation and composition of the surrounding soil of the embankment, which will add to the initial cost of the pipe. Other factors that affect the service life of HDPE pipe include flammability of polyethylene and ultra-violet light sensitivity. The U.S. Army Corps of Engineers states, “Long-term performance of aluminum pipe is difficult to predict, due to a short history of use. The designer should not expect a material service life of greater than 50 years.”

An extremely important report for the engineering profession is the Ohio Department of Transportation publication, “Culvert Durability Study.” Field surveys were completed, and an interim report presenting the data was published in 1972. The analysis of data and recommendations are presented in the final report published in 1982. The report evaluates the durability performance of both concrete pipe and corrugated steel pipe under the same environmental conditions and presents predictive equations and graphs for establishing service lives for both materials. The second issue of the American Concrete Pipe Association (ACPA) publication series, “Buried Facts,” reviews the Ohio Report and presents the procedures for evaluating service life.

Figure 2 is the predictive service life graph for concrete pipe, which relates pH and pipe slope to the number of years it takes for the pipe to reach a poor condition.

In evaluating pipe, the Ohio classification system rated concrete pipe poor if there was significant loss of mortar and aggregate, and the concrete was in a softened condition. Only nine concrete culvert pipes were rated poor, and they were being repaired to provide additional service. As indicated, concrete can be expected to provide a service life in excess of 100 years for all environments with a pH value above 4.0.

Figure 3 is the predictive service life graph for plain galvanized corrugated steel pipe, which predicts the amount of metal loss as related to pipe age, pH of the water and potential for abrasion.

The diagonal lines, representing the pH of the water, are solid when there is potential for abrasion and dashed when there is no potential for abrasion. For design purposes, the solid lines indicating a potential for abrasion should always be used, since during the 100, or even 50, years of required project design life, abrasion must be considered as a definite possibility.

Figures 4 and 5 are the predictive service life graphs for corrugated steel pipe with only bituminous coatings and bituminous coating with paving.

The Ohio classification system considered the rating good, even if the surface of the interior coating was completely cracked throughout, some of the interior coating was gone or the paving was eroded to the top of the corrugations. The predictive graphs, therefore, are liberal, and Ohio assigns an additional five to 10 years of service life to bituminous-coated pipe with paving, and no additional service life to only a bituminous coating. Since bituminous coating and paving have relatively short service lives, Ohio hydraulically sizes all CMP based on plain corrugated pipe.

Conclusion

Drainage pipes made from concrete, metal or plastic theoretically perform the same engineering function: transporting fluids. Therefore, all three may be considered “functionally equal” when designing drainage facilities. But are all three really “equal” for the specific application? By using LCA, engineers can better determine which pipes give the owner the best return on their investment. In addition, LCA can help engineers protect their professional reputations and shield themselves from tort-liability issues.

The American Concrete Pipe Association has used ASTM C 1131 to develop a comprehensive LCA practice, which eliminates unreliable assumptions, resulting in a readily usable and accurate design aid. This method is available from the ACPA in the form of PipePac software and is in the ACPA’s Design Data 25. For more information on life-cycle cost analysis, contact ACPA at www.concretepipe.org.

About the author

Childs is president of the American Concrete Pipe Association, Irving, Texas. He can be reached at 972/506-7216.

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