外文翻译---腐蚀和预应力混凝土桥梁(编辑修改稿)

外文翻译---腐蚀和预应力混凝土桥梁(编辑修改稿)内容摘要：

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 these structures. Podolny’s caution regarding placency was warranted, as were Poston’s concerns for lack of diagnostic nondestructive tools which would have provided a more reliable means for judging condition of post tensioned structures. In 1999, corrosionrelated failure of an external (unbonded) post tensioning tendon in Florida DOT’s 20year old Niles Channel Bridge focused attention on posttensioned bridges. Three other Florida bridges including the MidBay Bridge in Destin, Sunshine Skyway (St. Petersburg), and I75/I595 Sawgrass Interchange in Broward County were later investigated. Investigations of these structures highlighted the critical role that tendon grouting and plementary corrosion barriers take in successful construction. The root causes of reinforcement deterioration in these bridges were identified as grouting procedure ineffectiveness for controlling entrapment of bleed water in tendons, and poor tendon duct detailing creating access for aggressive solutions. These incidents prompted rigorous review of the durability issues related to corrosion of posttensioned concrete structures. Corrosion – Mechanisms and Threats Corrosion is defined as the degradation of a metal’s integrity and strength by interaction with its environment. Because the metals used in construction are refined from their naturallyoccurring oxides, the refined metal is less thermodynamically stable than its oxide. When corrosion occurs, the refined metal’s structure reverts to its naturallyoccurring state through an electrochemical reaction. Initiation and sustenance of the reaction require existence of an electrolytic cell. Each corrosion cell requires three elements。 • an anode ( the region of metal which corrodes, or reverts to an oxide), • an electrolyte (a corrosionenabling medium or environment which provides a path or environment for electrons or current to flow, which may be inherently aggressive or benign), and • a cathode ( the location where electrons are consumed or absorbed, and the metal is protected from corrosion). 5 Copyright ASCE 2020 Structures 2020 Structures Congress 2020 Metals in concrete, including mild steel reinforcement, prestressing steel encased in high quality concrete (pretensioned), and prestressing steel in fully grouted posttensioning ducts are known to be resistant to corrosion due to the beneficial effects of the highly alkaline environment that cementitious materials can provide. When these alkaline conditions are effectively sustained in an unbroken, continuous state, the metal’s surface, which would otherwise be unstable thermodynamically, is known to be passivated, or protected by a thin iron oxide film. Design of concrete structures for serviceability is predicated on this principle, and more than a hundred thousand US highway structures have performed adequately based on this design philosophy. However, if the highly alkaline environment provided by concrete or cementitious grout is absent (at voids and other defects)。 wherever the passivation is interrupted or breached。 or wherever the passivating environment is altered chemically, corrosion will occur. It is very important to note in any discussion on this subject matter that corrosion is never uniformly distributed. That kind of deterioration would ideally require that each atom on the metal’s surface be equally sensitive to the corrosion medium(electrolyte) and that the medium have uniform access to all surface atoms. The various forms and types of corrosion observed in reinforced and prestressed concrete are therefore manifestations of the nonuniformities inherent in materials, manufacturing defects, alteration of the concrete’s chemistry, and mechanical damage arising in m。