National Academies Press: OpenBook

Service Life of Culverts (2015)

Chapter: CHAPTER EIGHT Conclusions

« Previous: CHAPTER SEVEN Life-Cycle Cost Analysis
Page 46
Suggested Citation:"CHAPTER EIGHT Conclusions." National Academies of Sciences, Engineering, and Medicine. 2015. Service Life of Culverts. Washington, DC: The National Academies Press. doi: 10.17226/22140.
×
Page 46
Page 47
Suggested Citation:"CHAPTER EIGHT Conclusions." National Academies of Sciences, Engineering, and Medicine. 2015. Service Life of Culverts. Washington, DC: The National Academies Press. doi: 10.17226/22140.
×
Page 47
Page 48
Suggested Citation:"CHAPTER EIGHT Conclusions." National Academies of Sciences, Engineering, and Medicine. 2015. Service Life of Culverts. Washington, DC: The National Academies Press. doi: 10.17226/22140.
×
Page 48
Page 49
Suggested Citation:"CHAPTER EIGHT Conclusions." National Academies of Sciences, Engineering, and Medicine. 2015. Service Life of Culverts. Washington, DC: The National Academies Press. doi: 10.17226/22140.
×
Page 49
Page 50
Suggested Citation:"CHAPTER EIGHT Conclusions." National Academies of Sciences, Engineering, and Medicine. 2015. Service Life of Culverts. Washington, DC: The National Academies Press. doi: 10.17226/22140.
×
Page 50

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

44 CHAPTER EIGHT CONCLUSIONS This chapter summarizes the key findings of this study, including the state of the practice for the required service life for culverts in varying conditions, the basis for deter- mining the service life, the range of processes that cause culverts to deteriorate and how they are controlled, the time for a particular material to reach the end of its useful service life, methods to allow the useful service life of culverts to be extended, and information on how the concepts of material service life and culvert failure limit states are correlated. This chapter also identifies gaps in current knowledge and implementation, and research needs. SUMMARY OF KEY FINDINGS Loss of Service Life Mechanisms Corrosion is the most commonly considered mechanism when predicting the rate of loss of service life of concrete and metal drainage pipes. The mechanisms of corrosion have been extensively studied and the causation factors are reasonably well defined for metal pipes. With reinforced concrete pipes, while the loss of serviceability mecha- nisms are understood, how and when they lead to a critical loss of pipe serviceability are less well defined. Abrasion is also considered in predicting rates of pipe degradation, but not to the same extent as corrosion as fewer, more geographically localized methods are available. The com- bined effects of corrosion and abrasion are generally not adequately considered. Mechanisms relating to the degradation of joints are also not often considered in practice, but research efforts in this area and toward this end are under way, and would be help- ful to advance the state of knowledge and practice. The use of various coatings and treatments act to delay the onset of the critical mechanisms leading to the loss of service life of the host pipe product. However, the coatings’ deterioration relate not only to the breakdown of the coating itself but also to how well it is bonded to the host pipe. The loss of service life mechanisms of thermoplastic pipes is not as well under- stood. Deterioration mechanisms such as slow crack growth and ultraviolet light–induced degradation have been studied, but how they may lead to loss of service life in the field is not understood. In general, loss of serviceability is defined in terms of some degree of physical degradation of the pipe material that can be identified by inspection or testing. However, these definitions are somewhat arbitrary (e.g., time to first perforation) and are not correlated with their effect on structural capacity or when they may lead to total collapse of the pipe system. No methods exist to predict when voids may develop along the outside of a pipe or under a pipe, or under what circumstances that could lead to catastrophic sinkhole formation. Service Life Prediction Methods and Models The majority of the available service life prediction mod- els for concrete pipes are largely empirical and not directly related to the physical mechanisms of degradation or when such degradation reaches a critical point. These methods also have not been recently updated or developed, and they focus predominantly on corrosion and do not consider other degradation mechanisms or joint performance. The use of culvert maintenance data to calibrate these methods to agency-specific conditions could be investigated. A range of methods exist for predicting the service life of metal pipes, and these methods are variations of a single well-established method. The corrosion rates for metal pipes are probably the best defined since there has been a long his- tory of applied research and the prediction models have been correlated with numerous field-performance studies. The service life prediction models have been particularly useful in identifying where certain metal pipe types are unsuitable and where upgraded coatings are needed. Recent research has shown that the basic corrosion models can be calibrated to agency-specific data with minor modifications. Other than for type 2 aluminized coatings, the add-on service life of most coatings and treatments are assigned somewhat arbitrarily. Predictive models for thermoplastic pipes have not yet been developed; however, research is being performed with these pipe materials. Reliable predictive models for joints, and for rehabilitation and repair methods, are not available. Correlations Between Service Life Degradation and Pipe Failure Modes Recent research has addressed some of the links between joint failure and service life degradation, but this area needs

45 additional research. The time to reach a defined level of sec- tion loss by corrosion in metal pipes can be predicted, but this definition of loss of serviceability does not explicitly address a critical pipe failure mode. With limited mainte- nance budgets, it may not be practical to use a very conser- vative definition of loss of serviceability as the trigger for major pipe rehabilitation intervention. DOTs have done significant work in developing com- prehensive pipe condition rating systems, which take into account a broad range of distress types and severities. It is the combination, severity, and extent of certain distresses that defines the overall pipe system condition. Thus, the challenge is to enhance the current pipe service prediction models to cater for these more realistic definitions of pipe condition and end of service life. Caution must be applied when evaluating new pipe mate- rial types with existing models or failure modes because current failure modes may not apply to newer pipe materials, which can have quite complex modes of failure. For example, it may be inadequate to consider only existing knowledge of HDPE pipe failure modes for steel reinforced high-density polyethylene pipes, given the differences in the structural performance and deflection mechanisms. A further limitation on the current models for predicting service life is their inability to account for variations in pipe material quality, installation quality, and the degree to which post-installation verification will be carried out. Effect of Regional Initiatives and Federal Policies Research efforts encompassing multiple agencies and a wide variety of conditions would assist in the development of service life prediction models with broad applicability and acceptance. State-specific research understandably tends to address the needs of the sponsoring state agency, but can limit the extent to which the research is applied outside of that state. Broader collaboration between agencies, initially between those with similar concerns or environmental con- ditions, is suggested in order to accelerate the improvement of service life estimates. MAP-21, Moving Ahead for Progress in the 21st Century, encourages state DOTs to extend their asset management efforts beyond pavement and bridges to include ancillary structures through the use of risk-based asset management plans. A recent FHWA study (FHWA 2014) presents a series of culvert management case studies. In the case of Ohio DOT, the collapse of a culvert on Interstate 480 in 2001 (Fig- ure 36) facilitated the launch of a statewide culvert manage- ment program. Fortunately, a district staff member noted a small dip in the pavement, which on investigation revealed that the traffic was being supported solely by the concrete pavement spanning the large void. FIGURE 36 Failed culvert under I-480 in 2001 (FHWA 2014). As part of the questionnaire responses for this synthesis, another agency reported the collapse of a 30-in.-diameter RCP under 18 ft of fill near an urban intersection (Figure 37). It caused a sinkhole about 20 ft in diameter in a major roadway. The pipe was believed to be 30 years old. FIGURE 37 Emergency repairs following collapse of 30-in.- diameter culvert near busy intersection. These and similar pipe failures highlight the need for both better pipe system service life prediction methods and bet- ter understanding of failure mechanisms (Figure 38). They also demonstrate the need for pipe asset inventory systems that can provide early warning of potential problems and feed into improvements in service life prediction models. In conjunction with a pro-active, ongoing inspection program, Ohio DOT now has in its inventory about 79,000 culverts and storm drains less than 10 ft in diameter. Case studies of seven culvert failures were presented at the 2004 Transportation Research Board Meeting (Perrin and Jhaveri 2004). The pipes were reported to be from 25 to 60 years old at the time of failure. One of the recommenda- tions from that paper was to set up a national database where culvert failures are documented using a “culvert accident

46 report” form. In one of the case studies, it was noted that whereas the actual cost of emergency repair was more than $4 million, when user-delay costs were factored in, the cost exceeded $8 million. SUMMARY OF KNOWLEDGE GAPS Knowledge gaps constitute areas where the state of knowledge has not reached maturity or where consensus has not been reached about the appropriate approach to a given design prob- lem or an evaluation of a particular aspect of performance. To date, the following critical knowledge gaps that affect evalua- tion of culvert system service life have been identified: • Fundamental Pipe Failure Models—Although culvert research is an active area and progress has been made in understanding pipe deterioration mechanisms, still no comprehensive deterioration models have been developed that consider the combined effects of all critical parameters for the major pipe types and define when end of service life occurs or when total failure will occur. • Design Service Life—Standard (universal) and objec- tive guidelines for defining service life requirements for various drainage pipe system applications are not defined by AASHTO. • Time-Dependent Performance Data—In general, there is a lack of statistical data of long-term field perfor- mance for the full range of drainage system and service conditions. • Pipe Joint Evaluation—The evaluation of structural and hydraulic performance impacts from various pipe joint systems results in both knowledge and implemen- tation gaps. • Installation Quality—A clear and universally accepted methodology to quantify the impacts of installation quality on drainage system performance is not known to exist, and sufficient performance data to generate such an evaluation system may not exist for all pipe systems and installation conditions. It is noted that the hydraulic and structural design of new (virgin) drainage pipe systems is generally well under- stood and the methodologies presented in reference docu- ments [e.g., LRFD Bridge Design Specifications, Chapter 12 (AASHTO 2013)] are well accepted as appropriate. How- ever, the methodologies and procedures to estimate durabil- ity and service life of culverts are not as mature and do not cover all of the material types in use. Fundamental Pipe Failure Models This report provides an overview of the current state of knowledge with respect to deterioration mechanisms of vari- ous pipe types under a range of field conditions and applica- tions. The current service prediction models are generally based on a selected end-of-service-life indicator and only consider one distress mode, typically corrosion, to predict expected service life. Where combined abrasion and corro- sion are present, the model no longer applies. Thus, to pro- long service life resulting from corrosion, coatings can be considered; however, at what stage is invert paving required and what are the economics of selecting various invert paving options? The current deterioration models, while providing broad guidance on pipe type suitability, are not sufficiently developed to allow a meaningful comparison of alternatives. A further limitation is the inability to relate a defined end- of-service-life indicator to the ultimate failure of the pipe system. For example, how much time is available between the first perforation of a metal pipe and the risk of complete pipe failure? This type of information would allow agency engineers to decide whether a deteriorated pipe can be left FIGURE 38 Failure of road embankment slope above a 12-ft-diameter structural plate corrugated steel culvert that had been extended at both ends. Perforation along bolt lines and 18-in. deflection at obvert of pipe.

47 in place for a further 5 years until road rehabilitation is required. Clearly, deferral of pipe rehabilitation to coincide with road rehabilitation can be cost-effective, provided the risk can be managed. Ideally, pipe deterioration models need to be able to model the progressive loss of pipe condition from installation to final failure. With this type of model, it would be possible to evaluate the cost-effectiveness of maintenance activities, rehabilitation options, and full pipe replacement, and assist in establishing when these interventions are needed. Such a deterioration model would have to consider the potential for changes in system material properties (pipes and surrounding materials) over time (durability) because these changes affect all aspects of drainage system per- formance. The process is further complicated in practice because most prediction models assume the pipe system is correctly installed and are invalidated if this is not the case. Ideally, a deterioration model could have some optional risk factors defined related to installation quality. One reason most pipe protective coatings are given very conservative add-on lives is the concern about damage to the coating during installation. If this risk could be properly quantified, then the cost-effectiveness of protective coatings would be better demonstrated. The knowledge gaps related to pipe durability are well known and reported in a wide range of reference documents including MTO (2007) and NCHRP Synthesis 254 (1998), and significant ongoing research being conducted at universi- ties, within state DOTs, and by pipe manufacturers and trade associations aims to improve understanding of this subject. To date, the significant research progress that has been made over the past 15 years in understanding the various deterio- ration mechanisms for a range of pipe types has yet to be incorporated into improving the overall pipe deterioration models. Although this research has improved pipe material selection methods and refined pipe material specifications, it is yet to be integrated into a more comprehensive model of pipe failure mechanisms. A lack of comprehensive failure models exists for all pipe types, although metal and concrete pipes have initial limited working models. However, with the continuing growth of new pipe products, especially those involving composite materials, and the rapid increase in the use of trenchless technologies for pipe rehabilitation, signifi- cant research and development work still needs to be done. Design Service Life Establishing the life expectancy at a minimum required level of service for a pipe system is a basic necessity to allow a comparison of alternative pipe systems at the design stage. Although some DOTs and industry have guidelines on defin- ing design service life for various highway applications, a standard approach for this process does not exist. On a sim- ple level, most agencies relate design service life to the high- way classification or the strategic importance of the route. Thus, design service lives of 25, 50, 75, or 100 years can be assigned. Other factors that need to be considered are the ease of replacement of a particular pipe system. For example, if a cross culvert is at the base of a high rockfill embankment, and replacement would require the construction of a temporary highway detour, the design ser- vice life may need to be increased irrespective of the road classification. The authors are not aware that any compre- hensive life-cycle costing studies have been done on the dif- ferential between a 25-year pipe design and a 75-year pipe design. The study by Perrin and Jhaveri (2004) provides the most thorough analysis; it indicates that longer design ser- vice life requirements will likely result in overall savings. With in situ rehabilitation technologies becoming almost routine for many DOTs, the notion of initial pipe design service life becomes less rigid and enhanced life-cycle costing tools could play a bigger part in helping agencies get the best value for money in terms of drainage infrastructure investments. Time-Dependent Performance Data In general, additional evaluations of time-dependent perfor- mance data on all drainage systems are needed. Drainage systems and pipe products that have longer histories have significantly more data available, but often these collections of data are potentially biased because they are presented by industry trade organizations or they do not cover the full range of installation conditions. For newer pipe products and systems, the lack of both evaluation and unbiased compilation of performance data leads to the exclusion of newer pipe products in some juris- dictions. The need for continued and additional studies to collect and analyze drainage system performance data is well established. As more DOTs adopt comprehensive drainage pipe inventory and condition rating systems, the source data for a greater understanding of pipe performance through its life cycle becomes available. Pipe Joints The performance of many joint systems is strongly depen- dent on the quality of installation (i.e., proper versus improper installation), and the performance of improperly installed joints is in general not well documented or quan- tifiable. Instances where joint performance data or evalua- tion tools are not available in the literature (even if they are available internally within pipe manufacturer’s literature) are considered knowledge gaps in the current study. The knowledge gaps related to pipe joint systems are evident by the proportionally large percentage of failures

48 (or other service impacts) related to pipe joints. Joints can affect the pipe system’s hydraulic and structural perfor- mance through leaks that can degrade or erode bedding and embedment materials. Infiltration of soil particles into pipes can also increase abrasion. Objective data are needed on the relative merits of alternative joint types and how they impact overall pipe performance and service life. RESEARCH NEEDS Based on the literature reviews, additional research into dura- bility and the development of additional durability evaluation models would benefit the practice. Future research on durabil- ity evaluation models would benefit from the following: • Develop a more global fundamental understanding of overall pipe deterioration and failure mechanisms that includes all contributions to deterioration as well as combined and consequential effects. – Develop models that account for the combined and coupled effects of corrosion and abrasion (not sim- ply additive). – Develop models that account for the combined and coupled effects of structural loading–induced pipe stress with respect to corrosion. • Investigate the relative importance of soil-side corro- sion compared with water-side corrosion in predicting pipe failure owing to corrosion. • Use recognized and measurable engineering param- eters in the development of future models. • Develop statistical and probabilistic models and include variations in construction quality. • Estimate the accuracy of the existing models and work toward defining the accuracy of new models. • Use predictive models to back-analyze pipe failures and suggest modifications to the regression equations. • Use the results of actual pipe condition survey data to improve understanding of deterioration throughout the complete pipe life cycle. • Conduct a cost-benefit study that quantifies the effect of increasing the frequency and extent of performing post-installation inspections. • Conduct additional research on developing structural and durability design methods for in situ pipe rehabili- tation technologies. • Analyze the costs associated with waiting until failure to replace a pipe, rather than replacing a pipe at the end of a defined service life, prior to the risk of emergency replacement. • Develop material abrasion prediction models based on the physical mechanisms of abrasion on different materials. • Develop best-practice guidelines for environmental sampling of soil and water to obtain representative values for use in culvert durability assessment. In par- ticular, the timing (summer, winter, etc.), number, and location of sampling should be addressed. • Investigate the use of, and augmentation of, existing maps of environmental parameters (from the Natural Resources Conservation Service or similar organiza- tions) in culvert durability assessments. • Investigate the mechanisms of joint separation in con- crete pipes, especially how the freeze-thaw process affects joints. In addition to the development of more and improved durability evaluation methods and models, the continued collection and evaluation of field performance data and case histories will provide significant benefits to the accurate pre- diction and evaluation of durability during design. Increas- ing the database of available field performance data will be especially critical for new products and those with shorter service life histories than for more established pipe products.

Next: REFERENCES »
Service Life of Culverts Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 474: Service Life of Culverts explores the time during which a culvert is expected to provide a desired function with a specified level of maintenance established at the design or retrofit stage.

This study is an update of NCHRP Synthesis 254: Service Life of Drainage Pipe (1998), which itself was an update of NCHRP Synthesis 50: Durability of Drainage Pipe (1978). Developments in plastic pipe, fiber-reinforced concrete pipe, polymeric-coated metal pipe, recycled materials, larger and more diverse structures, and sophisticated analytical soil-structure interaction modeling within the past 15 years led to the development of this report.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!