The objective of this study was to update the 1998 NCHRP Synthesis 254: Service Life of Drainage Pipe, which in turn updated the 1978 NCHRP Synthesis 50: Durability of Drainage Pipe. In the past 18 years, the culvert pipe industry and research community has made numerous developments in pipe materials, sophisticated analytical soil-structure interaction modelling techniques, greater use of in situ pipe rehabilitation, and the introduction of larger and more diverse structures. As such, the AASHTO subcommittees on culverts determined that a new synthesis study of the service life of culverts was needed. The study approach consisted of two primary thrusts. First, a survey of North American transportation agencies was performed to determine the current state of practice. Second, a literature review was performed to assess both the state of practice and the state of the art with regard to the subject topic. Forty-one U.S. departments of transportation (DOTs) and seven agencies in Canada responded to the survey. The results showed a diverse range of pipe types in common usage, with concrete, corrugated galvanized steel, and high-density polyethylene (HDPE) pipes as the most common pipe types, followed by galvanized structural plate and polyvinyl chloride (PVC). Almost half of agencies gather site-specific environmental data on drainage projects, indicating a broad appreciation of the importance of selecting the durability of materials to match site conditions. Less than a quarter of respondents indicated that they had developed or improved pipe durability prediction models. Those that have developed prediction models include DOTs that are subject to extremely variable or extreme environmental conditions. There was little consistency in definitions of end of service life, but there appears to be a trend toward using the results of pipe inspection rating systems to set threshold values that trigger maintenance, rehabilitation, or replacement. The majority of respondents indicated that quality of pipe installation has a significant influence on culvert pipe performance. HDPE and PVC were identified as the pipe types where the relationship between pipe performance and installation quality were strongest. Less than 40% of agencies had a formal culvert asset management system in place. In situ pipe rehabilitation is becoming routine, with only two agencies indicating that they have not used it. Slip-lining was the most common technology in use. Agencies are developing methods for predicting the service life of culverts, but developments are generally concentrated within a core group of agencies where this topic is regarded as a high research priority. Within the past 15 years, much advancement has occurred in understanding the mechanisms of pipe degradation in service. Significant work has been done with respect to corrosion and the main factors that influence its development in concrete and metal pipes; that is, pH, resistivity, chloride, and sulphates. This work has led to studies about how to retard or prevent corrosion through the use of thicker walls, better materials, coatings, and liners. Advances have also been made in understanding abrasion and how it enhances the rate of degradation from corrosion and how its damaging effects can be mitigated. Effective research has also been undertaken in understanding time-dependent changes in the mechanical properties of thermoplastic pipe, particularly slow crack growth and oxidative/ chemical failure and how they can be controlled. Florida DOT and several other select agencies have sponsored significant research in the area of pipe degradation, and this research can form the basis for better service life prediction models in the future. The schematic degradation models for metal-reinforced concrete and thermoplastic pipe materials (Figure 7 in chapter two, Figure 11 in chapter three, and Figure 20 in chapter four) indicate the trend of an initial relatively stable condition followed by a more rapid deterioration. Little recent advancement has been made in refining pipe service prediction models, even for the more common pipe types. However, with research on degradation mechanisms and a better understanding of the progression of deterioration, combined with greater sources of pipe performance data from agency pipe inventories, more rapid future progress in improving these models should be possible. Survey results indicate that, in practice, a majority of agencies predict service life using case studies, internal research results, or default estimated service life values holistically or categorized by local environmental conditions, rather than published models. This study has confirmed rapid growth in the use of in situ pipe rehabilitation or trenchless technologies for extending the life of culverts. This trend will continue as technologies improve and more contractors can offer the service. This trend will increase the demand from agencies for better methods for predicting pipe durability so that a broader range of pipe strategies can be evaluated and best value for money in delivering highway drainage systems can be achieved. This report provides an overview of the current state of knowledge with respect to deterioration mechanisms of various pipe types under a range of fild conditions and applications. The current service prediction models are generally based on a selected end-of-service-life indicator and consider only one distress mode–typically corrosion–to predict expected service life. Where there is combined abrasion and corrosion, the models no longer apply. 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 defied end-of-service-life indicator to ultimate failure of the pipe system. 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 to assist in establishing when these interventions are needed. (Author/publisher)
Samenvatting