【正文】 onard, P. C244。 consideration of each as an influence on optimal service life of a prestressed concrete bridge is needed in design and specification, construction, and bridge maintenance practice. Chlorides an Initiator. Chlorides have been introduced into concrete structures at the time of construction, although that practice is today forbidden by codes. More frequently, chlorides can be absorbed from the structure’s external environment (deicer solutions and sea water), or permeate from the environment through open cracks and joints. Presence of chlorides in high concentrations at the steel reinforcement’s location, has long been known to destabilize the passivating iron oxide film. Corrosion can then initiate and progress, even in the highly alkaline environment concrete provides. In posttensioned structures, chlorides can be introduced in tendon grout or percolate through structural joints and duct joints/discontinuities. Concrete Carbonation as an Initiator. The alkalinity of concrete and cementitious materials is disrupted with time in other ways. The process of carbonation affects all concretes and cementitious materials exposed to carbon dioxide in the atmosphere or to carbon dioxide dissolved in solutions in contact with steel. The process of concrete carbonation mences at exposed surfaces immediately upon exposure to carbon dioxide, and lowers the pH from about 13 to neutral conditions lower than a pH of 10. Carbonation rates increase for poor 6 Copyright ASCE 2020 Structures 2020 Structures Congress 2020Downloadedfromascelibrary.orgbyTHREEGORGESUNIVERSITYon01/07/1opyrightASCorpersonaluseonly。s highway infrastructure supported and highlighted need for design for serviceability. Findings of visual inspection/corrosion sensing/materials studies by Novokshchenov and Whiting et al. for pretensioned and posttensioned superstructures of various configurations throughout the full range of US environmental exposures, found no evidence of serious prestressing reinforcement corrosion or other systemic degradation. These studies and other similar findings of the highway transportation munity have led to remendations related primarily to minimization of the number of deck joints, more durable, protective concretes and coatings for anchorage ponents, improved deck and joint drainage systems to redirect passage of aggressive solutions from corrosion susceptible ponents near girder ends, and implementation of more effective deck joint maintenance programs. Some prestressed concrete structures have displayed troublesome premature performance impairment trends worldwide, discriminating this structural concept from others when one considers development of a rationale for assessing and maintaining bridge reliability and utility beyond the 50year age milestone. Szilard, in a 1969 survey performed on behalf of FIP, noted that corrosion of prestressing reinforcement was the predominant form of unexpected damage to structures, whether accidental or deterioration/aging based. In 1971, the Australian Water Resources Council published results of a wideranging study of the performance of prestressed concrete structures worldwide intended to assess the performance of Australian prestressed structures and pare it with that of structures elsewhere. The work was prompted by premature multiple failures of prestressed, wirewound pipelines and tanks, and helped establish improved specification for prestressing wire and concrete/mortar and other protective coatings. Citing the criticality of corrosion protection for prestressed concrete pressure vessels for nuclear reactor containments, Griess and Naus studied corrosion behavior of highstrength prestressing steels in 1978, reviewing several incidents of prestressing steel failures in nuclear pressure vassels in the ., France, and United Kingdom. Schupack conducted a performance survey covering the time period of 1950 to 1977, concluding that the noted failure incidence rate of 200 tendons out of an estimated worldwide prestressing steel consumption of 30,000,000 tons was negligible. However, an NCHRP study performed by a forensic engineering firm in 1982 reported that 50 structures with tendon corrosion were noted in the time period between 1978 and 1982. Ten of these reportedly displayed brittle fractures suggestive of environmental cracking phenomena. In 1992, Ciolko summarized structural evaluation and failure analysis data from . prestressed concrete pipeline failures, estimating based on industry and water utility records that more than 60 such pressure pipelines had failed in the US, principally due to accelerated corrosion of poorly protected prestressing wire. Common to all as a factor contributing to performance impairment was the heightened susceptibility of prestressing wire to accelerated corrosion phenomena. 4 Copyright ASCE 2020 Structures 2020 Structures Congress 2020 In 1992, Podolny warned designers of the danger of placency resulting from reliance on the some of the favorable conclusions from early surveys and reports of the condition of prestressed structures. He concluded that although the population of prestressed structures has greatly increased in the last two decades, it was only recently that we have begun to understand the role that environment and the many forms of corrosion have on reliability of prestressing reinforcement. In 1998, Poston and Wouters reported in an NCHRP study of durability of precast segmental (posttensioned) bridges, that no evidence of corrosion or other durability problems with these bridges existed, cautioning however, that the gathered information was based principally on visual inspections. The study cited that lack of improved nondestructive evaluation options hinders evaluation of