PSI - Issue 62

Edoardo Proverbio et al. / Procedia Structural Integrity 62 (2024) 285–298 Author name / Structural Integrity Procedia 00 (2019) 000–000

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concrete (e.g., galvanized ducts, galvanized reinforcing steel, galvanized fastenings/dowels, etc), unfavourable conditions exist when chloride penetration causes corrosion of galvanized components. In this regard, worthy of note is the case of the San Francisco-Oakland Bay Bridge (Gorman et al., 2015). The east span of the San Francisco–Oakland Bay Bridge, originally built between 1933 and 1936, was damaged during an earthquake in 1989 and was replaced by a new structure completed in 2013 after eleven years of construction and a cost of 6,4 billion dollars. The new east span features a self-anchored suspended structure (SAS). The span is supported by 2300 hot-dip galvanized (HDG) high-strength steel anchor bars, with both ends threaded. The bars were made of UNS G41400 steel, conforming to ASTM A354 Grade BD with a maximum hardness of 39 HRC. In 2013, after a prolonged period of exposure to the environment, and just 6 months before the scheduled opening of the new east span, 32 of the hot-dip galvanized anchor bars failed due to hydrogen embrittlement following pre tensioning prior to final commissioning. All fractures occurred in lower threads that had been inadvertently submerged in rainwater pools for over four years prior to pre-tensioning. Several fracture analysis reports (Chung, 2014) (Ocel & Provines, 2015) have stated that the failure occurred due to hydrogen embrittlement, prior to pre tensioning and grout clogging of the tie rod seats, following oxidation and cracking of the protective zinc layer which allowed the diffusion of the hydrogen, generated following steel contact with stagnant water. Prestressing steel production has changed over the years. At first the changes did not always result in an improvement in quality. For example, the introduction of patenting in air (or bainitic hardening carried out on harmonic steel wires to obtain fine, easily drawable pearlite) as a replacement for patenting in a smelting lead bath, led in the early years in some cases to serious problems (Bujňáková, 2020). The use of hardened and tempered oval bars obtained in the 1950s with the Baur-Leonhardt process has in some cases resulted in severe damages (Hunkeler et al., 2005). As strand tensioning methods developed in the 1960s, 7-strand strands were increasingly used. From the mid-1960s, threaded rods or plain rods (usually hot-rolled), were often used instead for shorter tendons, e.g., for transverse prestressing. In some countries, this material proved sensitive to stress corrosion cracking and hydrogen embrittlement and had to be withdrawn from the market after numerous failures occurred. There are basically three types of steel used for prestressed concrete structures (Manuel Elices et al., 2003):  hot-rolled strained and tempered (air cooled) steel: Y1100H, Y1030H and Y1230H grade (CEN, 2000b)  quenched and tempered steel: 1420/1570 grade (ISO 6934-3:1991 Steel for the Prestressing of Concrete Part 3: Quenched and Tempered Wire, 1991))  heat treated, cold drawn, tempered (stabilized) steel: Y1570C, Y1670C, Y1770C and Y1860C grade for wire (CEN, 2000a) and Y1860S7G, Y1960S7, Y2060S7 and Y2160S7 grade for strand (CEN, 2006). Quenched and tempered steels - currently produced in modest quantities only in Germany and Japan - are the most susceptible to embrittlement. In fact, the martensitic microstructure induced by quenching presents internal stresses that are not always completely removed by tempering and a relatively small number of dislocations, so the quantity of hydrogen necessary to interfere with their movement and cause embrittlement is low. An increase in chromium and silicon content and a corresponding reduction in manganese together with a fine microstructure made it possible to obtain steels with an ultimate strength of 1650 MPa and good resistance to hydrogen embrittlement (Manuel Elices et al., 2003). Hot rolled and air-cooled prestressing steels includes hot rolled high carbon pearlitic steels melted with carbon, manganese and silicon as basic alloying elements. The prestressing rods used at the beginning were mostly hot rolled and air cooled with nominal ultimate strengths about 900 MPa but they resulted susceptible to brittle failure in 4. Steel composition, microstructure and hydrogen susceptibility