PSI - Issue 28

Mihaela Iordachescu et al. / Procedia Structural Integrity 28 (2020) 39–44 Mihaela Iordachescu et al./ Structural Integrity Procedia 00 (2020) 000–000

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measures adopted for the bridge rehabilitation as well as for prevention and early detection of similar structural failures. The high-strength seven-wire strands used in structural engineering are generally manufactured from six cold drawn eutectoid steel wires that helically wound a central one. These have to withstand high-tensile loads when forming ties or tendons for respectively cable-stayed tensioned structures (DYWIDAG 2017, BBR 2009). Galvanizing of wires and sheathing in polyethylene sheaths of strands are additional measures commonly employed to increase their corrosion resistance to aggressive environments (prEN 10138-2 2009, FIB 2005). According to previous investigations related to the failure in-service of strand-tendons in different Spanish bridges (Álvarez J.A. et al. 2017, Parrondo-Rodríguez J., 2017), their rupture occurred in areas that are locally unprotected due to construction needs or human errors, with the origin being related to the exposure to aggressive environments that favor corrosion, or corrosion-assisted fatigue. The experimental program designed in this work for the analysis of failure included tensile and stress corrosion tests under constant load and under increasing load at low deformation rate on wire-samples extracted from the broken strands. The mechanical tests were complemented with the fractographic analysis of samples by using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Thus, from the analysis of the fractured areas and of the lateral surfaces of the wires broken in-service or during the laboratory tests a direct correlation between the environmentally induced damage mechanisms, the mechanical strength and the stress corrosion resistance of damaged wires was established. 1.1. Constructive details of the bridge and the failure description The cable-stayed bridge configuration, in which strand failure was detected, consists of a continuous concrete girder supported by inclined stay cables attached to two towers (two interior porticoed pylons), two intermediate porticoed piles and two other exterior pillars, as schematically shown in Fig. 1. The intermediate piles contribute to the longitudinal support of the girder as sliding bilateral supports. The bilateral support condition is materialized through a series of tie-down cables, of approx. 40 m length, mounted into the piles interior, which assure the girder anchoring at the piles base. Each tie-down cable is made of 12 parallel strands of 15.7 mm nominal diameter, additionally protected with grease and a polyethylene sheath of 2 mm thickness. The strands are made of seven galvanized high-strength strength eutectoid steel wires of 5.3 mm diameter. According to the bridge design, dating from the 1980s, the tie-down cables are bundles of 12 parallel strands fixed into the anchoring heads, so that the strands remained exposed to the environment action into the pile chambers. Due to constructive needs related to the anchoring process the polyethylene sheath had been removed from the lower end of the strands that emerged unprotected from the anchor heads about 0.5 m. Thus, in this peculiar zone, the strand protection was assured only by the remnant superficial grease layer and the wires Zn coating.

Fig. 1. Basic configuration of bridge and the tie-down cables failure zones.

The detected failures consisted of the rupture of two strands from distinct tie-down cables of opposite intermediate piles (Fig. 1), at only two days difference. The failures occurred at their lower ends, unprotected by the polyethylene sheaths. In one of them, the strands´ sheath was at approx. 30 cm from the rupture and in the other at less than 10 cm. Fig. 2a and Fig. 2b show half of the ruptures from the strands side that remained attached to the upper anchor heads.