PSI - Issue 54

Maricely De Abreu et al. / Procedia Structural Integrity 54 (2024) 143–148 De Abreu M. et al / Structural Integrity Procedia 00 (2023) 000 – 000

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prestressed steel wires and strands favors assisted cracking previously described in Iordachescu M et al., 2015, among others. In order to confirm whether assisted cracking had occurred in the supplied strand samples, the in service ruptures of the central wire and of one of the peripheral wires of the strand 1 (Fig. 1b) were examined by SEM. The central wire exhibits the most ductile behavior in the four performed tensile tests (Fig. 2b). As shown by the images given in Fig. 4a, the fracture surfaces of the two wires remained long exposed to an aggressive environment able to produce severe corrosion, but with cross-section losses being quantitatively different. This is due to the protection and the consequent less exposure of the central wire, with it being helically wounded by the peripheral wires. The corrosion products have almost completely covered the fracture surface of this central wire, but the geometric configuration is a reminiscence of the cup and cone fracture occurred by the fast concatenation of overloading, ductile collapse and necking of the wire. The cup and cone configuration remains recognizable because the fracture surface was only exposed from the final failure of the entire strand, when loaded just above its decreasing bearing capacity. The fracture of the peripheral wire shows signs of cleavage, a micro-mechanism that requires small plastic deformation and usually indicates an environmentally assisted cracking of brittle nature. In prestressing steels, cleavage generally occurs when the cohesion between the microstructural components is weakened by the presence of precipitates or inclusions like Al 2 O 3 or MnS, or by the atomic hydrogen produced by a corrosion process and accumulated in the stress concentrators as reported by Iordachescu M et al., 2018, among others.

Fig. 4. a) In-service failures of a central wire and of a peripheral wire in one of the analyzed strands; b) longitudinal sections of the central and peripheral wires showing their corresponding damage condition, respectively due to corrosion and stress corrosion.

Fig. 4b illustrates the aggressive environmental action along the in-service breaking ends complementary to those shown in Fig. 4a. In both cases, corrosion generated pits on the wire surface which progressed parallel to the wire axis. However, in the peripheral wire this process evolved to assisted cracking because one of the penetration pathways deviated from the axial direction towards the axis of the wire generating a transversal secondary crack that stopped growing when an analogous one with faster growth caused the wire to break. The stress threshold required to activate the assisted cracking process is surpassed as a consequence of the loss of resistant cross-section as the external steel is corroded. 5. Conclusions The tensile tests and the analysis of the damage mechanisms carried out with samples of strands failed in an urban viaduct 50 years old show that the prestressing steel has a tensile behavior comparable to that of current high strength prestressing steels. In particular, its damage tolerance is as high as expected from an essential ingredient for prestressing concrete for which structural integrity is critical to preserve high levels of safety and performance. The damage condition of the supplied strand samples and the absence of any protective barrier inside the flexible steel-sheath intended to encapsulate the strands indicate that an aggressive medium soaked through concrete, accessed the strands and caused the wires to be severely corroded and, to a lesser extent, to undergo assisted