【正文】 d erected onsite. Posttensioned concrete for bridges predominately contains multiple strand or bar tendons. Tendons are stressed against builtin multipoint anchorages。s quality and its protection against aging and deterioration process are paramount to attaining functional and safety requirements of a concrete bridge structure. Design of prestressed members provides directly for several forms of durability measures such as dense concrete cover, corrosion mitigating grout systems, and innovative, proprietary full length corrosion barriers integrated with anchorage systems for post tensioned structures. Additionally, structural details and standards have been developed and introduced by AASHTO and various industry organizations to manage effects of prestressing reinforcement39。 ? 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)。allrightsreserved. quality, porous concretes and grouts. When the depth of concrete’s carbonation reaches the reinforcement, passivity is lost and metal’s corrosion can proceed unimpeded in the presence of moisture and oxygen. Concrete Cracking as Initiator. Cracks in the concrete or grout surrounding prestressing steel shorten the time to initiation of corrosion by acting as a path for introduction of aggressive environment, which can locally affect the steel surface. Locations where these cracks intersect reinforcement can also create localized corrosion and pitting. Attention to detailing of reinforcement for crack control is required in pretensioned structures. In post tensioned structures, cracking of tendon grout can bee problematical when it provides access points to reinforcement for electrolytic solutions in the event that other corrosion barriers (ducts and sheathing) are breached. Electrochemical (Macrocell) and Pitting Corrosion. Because the surface of steel reinforcement embedded in concrete is not always uniform electrochemically, separate corrosion cells can be created in regions of local differences in an otherwise benign, alkaline environment. Steel tendons in nonuniformly grouted ducts, for example are exposed to zones where different moisture and oxygen conditions occur. Similarly, strand in pretensioned ponents can be exposed to fluctuating depths of carbonation or concentrations of chlorides creating localized conditions favoring corrosion. In both these instances, macrocells are set up between the small anode area and a large cathodic region, when an electrolytic solution is present. In these cases, the macrocell effect can cause considerable corrosion in small regions, and high strength prestressing steels will exhibit severe pitting in these local areas at boundaries between different environmental conditions, ., grouted/ungrouted, wet/dry, carbonated/uncarbonated, etc. While total weight loss of the metal resulting from pitting may be small, the consequences for prestressing reinforcement can be severe, since corrosion rates at pits are relatively higher and will lead to fracture sooner than general loss of sectioncorrosion. Stress Corrosion Cracking and Embrittlement. This deterioration is a highly localized corrosion that produces cracking in the simultaneous presence of corrosion conditions and externally applied or residual tensile stresses. High strength, colddrawn reinforcing and prestressing steels are susceptible. Stress corrosion cracking induces sudden, brittle, unexpected failure as SCCinitiated cracks propagate across the ponent’s loadcarrying section. Most significantly, the process progresses at a far faster rate than the more monly recognized corrosion mechanisms, and manifests almost no signs of initiating and progress on the visible surfaces of the structure, such as concrete cracking, spalling, and delamination. Existence of surface flaws including longitudinal wire splits, wire seams created by overused/faulty drawing dies, and pits in wire from atmospheric corrosion created during storage and prior to grouting, are known to have provided crack initiation sites. Other locations susceptible to the phenomenon include locations where moisture perates to the steel in areas where steel is not 7 Copyright ASCE 2020 Structures 2020 Structures Congress 2020Downloadedfromascelibrary.orgbyTHREEGORGESUNIVERSITYon01/07/1opyrightASCorpersonaluseonly。te and B. Piwakowski (Time Frequency Analysis of ImpactEcho Signals: Numerical Modeling and Experimental Validation, ACI Materials Journal, V. 97, No. 6, . 2020, pp. 645657). This p