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The durability of prestressed concrete structures built between the 1950s and 1980s is becoming a growing problem. One reason is the sensitivity to stress corrosion cracking of the prestressing steel. Failure of prestressing wires can result in collapse of the entire structure without premature indication by transverse bending cracks or considerable deformation. The response of a structure to significant number of prestressing wire breaks was studied on a bridge in Brandenburg, Germany. Two thirds of the prestressing wires in concentrated tendons of two girders were cut before the bridge was demolished. Acoustic Emission Analysis was used to detect the wire breaks. Thus, the number of wire breaks was correlated with results of other measurement techniques, in particular strain measurements on the girders. In preparation of the measurements, the acoustic properties of the bridge were determined and the suitability of Schmidt hammer impacts as an acoustic reference source was validated.

A prestressed concrete cylinder pipe (PCCP) consists of a concrete core, a steel cylinder, prestressing wires, and a mortar coating. Most PCCP failures are related to the breakage of prestressing wires. It is thus expected that the load-bearing capacity of PCCP is significantly affected by the length of the prestress loss zone and the stress distribution in the broken wire. Based on a tri-linear bond-slip model, the length of prestress loss zone and the stress transfer mechanism between a broken wire and a mortar coating are analysed in this paper. During the breaking (unloading) process of a prestressing wire, the interfacial bondline exhibits the following three stages: elastic stage, elastic-softening stage, and elastic-softening-debonding stage. The closed-form solutions for the interfacial slip, the interfacial shear stress, and the axial stress in the broken wire are derived for each stage. The solutions are verified by the finite element predictions. A parametric study is presented to investigate the effects of the size of the prestressing wires, the prestressing level, the interfacial shear strength, and the residual interfacial shear strength on the interfacial stress transfer. For an example PCCP with an inner diameter of 4 m, the length of prestress loss zone increases from 500 mm to 3300 mm as the radius of prestressing wire increases from 1 mm to 7 mm. It increases from 2700 mm to 7700 mm when the interfacial shear strength reduces from 3.94 MPa to 0.62 MPa and reduces from 13,200 mm to 7300 mm as the residual interfacial shear stress factor increases from 0.1 to 0.9. [ABSTRACT FROM AUTHOR]

Experiences from the deconstruction of the Bridge Altstädter Bahnhof in the city of Brandenburg – Part 2: Damage Monitoring during Destructive Tests A routine inspection of the B1 bridge at the Altstädter Bahnhof in Brandenburg an der Havel in December 2019 revealed massive and rapidly progressing damage. The cause of the cracks along the longitudinal girders was suspected to be the failure of a significant portion of the prestressing wires of the concentrated tendons. The bridge was immediately closed to traffic and demolished in May 2021. Prior to the demolition, the Federal Ministry for Digital and Transport (BMDV) initiated further investigations into the damage pattern, which were coordinated and integrated into the deconstruction process by the Landesbetrieb Straßenwesen Brandenburg. The test concept was developed by the Hochschule für Technik und Wirtschaft Dresden (HTW) and is described in the previous article (Part 1). The article at present (Part 2) describes selected results of the extensive measurement and monitoring measures accompanying the tests. The Tests were carried out in cooperation with the HTW Dresden, Bilfinger Noell GmbH and the Bundesanstalt für Materialforschung und ‐prüfung (BAM). [ABSTRACT FROM AUTHOR]

A prestressed concrete cylinder pipe (PCCP) consists of a concrete core, a steel cylinder, prestressing wires, and a mortar coating. Most PCCP failures are related to the breakage of prestressing wires. It is thus expected that the load-bearing capacity of PCCP is significantly affected by the length of the prestress loss zone and the stress distribution in the broken wire. Based on a tri-linear bond-slip model, the length of prestress loss zone and the stress transfer mechanism between a broken wire and a mortar coating are analysed in this paper. During the breaking (unloading) process of a prestressing wire, the interfacial bondline exhibits the following three stages: elastic stage, elastic-softening stage, and elastic-softening-debonding stage. The closed-form solutions for the interfacial slip, the interfacial shear stress, and the axial stress in the broken wire are derived for each stage. The solutions are verified by the finite element predictions. A parametric study is presented to investigate the effects of the size of the prestressing wires, the prestressing level, the interfacial shear strength, and the residual interfacial shear strength on the interfacial stress transfer. For an example PCCP with an inner diameter of 4 m, the length of prestress loss zone increases from 500 mm to 3300 mm as the radius of prestressing wire increases from 1 mm to 7 mm. It increases from 2700 mm to 7700 mm when the interfacial shear strength reduces from 3.94 MPa to 0.62 MPa and reduces from 13,200 mm to 7300 mm as the residual interfacial shear stress factor increases from 0.1 to 0.9.

Currently, there is no automation and protection equipment to detect wire breaks in 6 – 10/0.38 kV electric networks. A protection system based on microprocessor electricity meters installed in a 0.38 kV network for the automatic detection of a damaged section of 0.38 and 6 – 10 kV overhead lines is proposed. A functional diagram of the protection system is described. [ABSTRACT FROM AUTHOR]

Magnetic nondestructive testing (NDT) for monitoring the equipment of overhead power lines (OHL) such as conductors, ground wires, and guy wires is discussed. Magnetic NDT allows effective testing of steel ground wires, guy wires, and steel cores of bimetallic conductors. The results of monitoring the conductors and ground wires of 35 – 220-kV OHLs in Rosseti networks are illustrated. The residual load-carrying capacity (strength) of the tested conductors and wires is estimated based on test data. It is concluded that magnetic detectors are effective in testing ground wires, guy wires, and steel cores of bimetallic conductors. [ABSTRACT FROM AUTHOR]

In electrical networks, constant monitoring of the electrical equipment operation is necessary. To accomplish this task, current transformers and voltage transformers (VT) are used, on which the accuracy of electrical measurements, electricity metering, and the reliability of the emergency control system depend. The results of the analysis of the conditions for forming the magnetization of a voltage transformer in the mode with unilateral connection of overhead transmission lines, as well as variants of violations in the 110 kV network that may exist for a relatively long time have been presented. It should be noted that the considered network has its own peculiarities: the overhead transmission lines connected in series have a significant length and a low level of load, which may be a condition for the appearance of a transmission lines capacitive effect (the Ferranti effect), the latter can cause overvoltage. Overvoltage occurs when the operating modes are temporary in terms of operation, adverse combinations of network parameters take place, and can continue until the circuit and network modes change. Most often they appear in asymmetric modes, with short circuits to ground, etc. Asymmetric modes caused by incomplete-phase commutations and wire breaks on the overhead line, as well as wire breaks with subsequent disconnection of power transformers that can affect the voltage level on the VT have been considered. The possibility of VT magnetizing with a prolonged voltage rise in symmetric modes, namely, the manifestation of the Ferranti effect with various loads at substations up to complete switching-off and power transformers switching off has been investigated. The analysis of the measuring voltage transformer operating conditions has been based on materials provided by the "Kolenergo" services.

The development of corrosion as a result of tendon exposure to moisture or corrosive media often leads to wire fractures. Such fractures in unbonded tendons are expected to induce global strand and anchor response through the progression of prestress loss from the break to the end anchors. Radial pressure, interwire friction, and lateral confinement, however, affect the magnitude of prestress loss carried to the anchors and have important implications on breakage detectability by tendon monitoring methods that rely on anchor response. This paper presents an experimental investigation of tendon behavior after successive wire cuts in confined and unconfined conditions and demonstrates the effects of confinement on stress recovery. In addition to analysis of strand prestress loss and axial strain along individual wires, the strand's dynamic postbreakage response was examined. Moreover, the effects of deviators on stress recovery were tested with a multistrand tendon specimen. A finite-element model of a seven-wire strand was developed and experimentally validated. Both experimental and numerical results showed significant prestress loss even at distant locations from wire breaks and confirm a correlation between wire breakage and modal properties, which can be a useful damage indicator. [ABSTRACT FROM AUTHOR]

The use of post-tensioning tendons with flexible filler is expected to increase in U.S. bridges in the coming years as an alternative to grouted tendons. Flexible filler results in purely unbonded behavior when used with either internal or external tendons, which creates opportunities for the application of tendon monitoring techniques that were not possible with grouted tendons. One such technique is the detection of wire breakage based on the strain response of the post-tensioning anchorage. The encouraging results from simplified laboratory experiments and analytical models in support of the proposed method motivated the full-scale experiments reported in this paper. Tests were conducted with flexible filler in both external and internal tendons in I-girder specimens having deviated multi-strand tendon profiles, allowing the investigation of the effects of deviators on stress recovery and anchor strain response. The strands of the post-tensioned girders were detensioned by individual wire cuts and the resulting changes in anchor strain distribution were captured. Finally, a broken strand identification algorithm developed for automated damage detection was applied to the experimental results. In addition to instrumenting the anchors with strain gauges, an acoustic sensing system was employed to independently monitor and confirm the breakage events. The findings reported in this study suggest that the wire breakage detection method based on anchor strain measurement can be a reliable and cost-effective approach for in-field bridge monitoring applications. [ABSTRACT FROM AUTHOR]

With steel prestressing wires tensioned around core, Prestressed Concrete Cylinder Pipe (PCCP) is a high-strength composite pipe designed for large-diameter water propagation and distribution. However, some PCCPs may fail after a certain time of service due to corrosion and deterioration. Real-time health monitoring of wire breaks is essential to assess the condition and avoid catastrophic failures of PCCPs. In this paper, a novel approach based on Fibre Bragg Grating (FBG) sensing technology, is presented to monitor and locate wire breaks in PCCP lines. FBG vibration sensors are utilised to capture the signals of wire break activities in the pipeline and obtain their locations. The online monitoring system and localization principle of wire break activities are described. Preliminary experiments were carried out on PCCPs with an inner diameter of 2 m, which were filled with pressurized water. Experimental results indicate that the wire break activity can be detected and located effectively by FBG sensors. The detection error of wire break locations is analysed. With unique features, the proposed non-destructive method is expected to be used for the online monitoring of wire break activities in long term for the condition assessment and performance prediction of PCCP lines. [ABSTRACT FROM AUTHOR]

Abstract: Failures in high-strength steel tendons of prestressed concrete structures are becoming a serious problem, as the deterioration of structures progresses due to aging. The failure is mainly attributed to the corrosion induced in severe environment as salt attack. Intensive studies were made in a laboratory and fields to investigate an applicability of acoustic emission (AE) technique to detect and locate the corrosion-induced failure. Three types of beams post-tensioned by steel bar, strand, and parallel wire cable with three different grout conditions (unbonded, partially grouted and fully grouted) were prepared in the laboratory tests. Failure was introduced by artificial corrosion, charging anodic current to the tendon. It was found that significant AE signals with extremely high amplitudes were produced by failures of steel wires and bars. Linear source location of the events was performed by three AE sensors placed in the center and the two ends of the beams. Reliability was shown to be 82–86% in terms of delectability of the failures. Continuous AE monitoring was carried out in two high-way bridges in service for 24 days. Eighteen AE sensors were placed at strategically selected positions on the post-tensioned beams of these bridges. Artificial AE signals were indirectly detected from the beams due to artificial failures by corrosion in small post-tensioned specimens bonded to the beams. Intensive analysis of the detected AE signals showed that meaningful AE events from the failures are clearly discernable from other sources as traffic noises and hammering. It has been proven that AE is a very useful technique to detect and evaluate failures of high-strength steel tendons in prestressed concrete bridges. [Copyright &y& Elsevier]