Search

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]

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.

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.