Overview
The continuous acoustic monitoring system SoundPrint was installed at the Ponte Moesa as part of the research project AGB2002/009. During the monitoring (May 2004 to February 2006) 14 spontaneous and two artificial wire breaks were detected. As a result of the monitoring and of two invasive inspections the owner decided to replace the bridge. The old one will be demolished in spring 2007. To evaluate the classification and localization of the wire breaks it is of substantial interest to open and examine parts of the bridge.
The examination will be part of the demolition and will be coordinated with the contractor. The part of the bridge, where most of the wire breaks occurred, will be cut off and examined separately on a yard not affecting the progress of the construction work. The sections will be 5 m long and 1 m wide.
A research assistant of the IBK will need one week to open the ducts and to document the wire breaks. TFB and requires one week to evaluate the corrosion. Since all examinations depend on the weather conditions a time frame of approximately three months is considered.
Acoustic Emission/SoundPrint
Inspecting the bridge by intrusive methods provides the chance to evaluate the reliability and the location accuracy of acoustic monitoring. Classification criteria of the events will be enhanced and fit for similar problems and applications. This will enable an effective use of acoustic monitoring in the future.
Insufficient grouting
One focus of the detailed examinations is to find out the reason and the extent of the insufficient grouting.
Corrosion of the prestressing wires
Age, character and cause of the wire breaks will be detected by the detailed examinations. The corrosion process and the extent of loss in the wire bundles can be evaluated for the modeling of similar problems.
Funding
Results
The diagnostic skills of the acoustic monitoring system were tested with blind tests. Artificial wire breaks as well as spontaneous wire breaks were detected, classified and localized.
The majority of the detected events were caused by construction works on the bridge. Ambient noises e.g. signals from expansion joints, bearings and traffic were detected classified and filtered out. The acoustic monitoring was compared with invasive inspections and half-cell potential measurement. Some of the events classified as spontaneous wire breaks could be verified with the invasive inspections. Also the results of the half-cell-potential measurement matched with the results of the acoustic monitoring. As a consequence of the monitoring and the further assessments, the owner decided to remove the bridge.
The demolition of the bridge provided the opportunity to examine the bridge with destructive methods. The examinations were planned in cooperation with the contractor of the removal. Two large-scale openings in the area of the middle pier were assessed. Most of the spontaneous wire breaks during the last period of monitoring clustered in this area. A second cluster of wire breaks was detected at the downstream side of the bridge close to the anchors of the cap tendons. Four sections of this region were cut out and examined independently from the demolition.
During the continuous acoustic monitoring 20 spontaneous and 15 artificial wire breaks were detected. 15 “new” and 63 “old” wire breaks were assessed and documented during the invasive inspections. The classification of the “old” and “new” wire breaks is based on evidences. Additionally, wide areas with severely corroded wires and even tendons consisting only of corrosion products were found.
The locations of the monitored and the assessed wire breaks were compared to each other. The possible correlations and accuracy of the localization in longitudinal and transverse direction were examined. The localization in longitudinal direction was more accurate than in transverse direction. The accuracy of the localization was good taking into account that almost all spontaneous wire breaks occurred in insufficient grouted or ungrouted tendons. Furthermore, several honeycombing was found in the surrounding concrete. Different factors influenced the accuracy of the localization at the Ponte Moesa. The anisotropy of the bridge deck due to the reinforcement had a great influence. Compared to the longitudina
Technical Implications
The range of the deterioration of the wires varied from completely corroded respectively
broken and corroded to completely intact. Significant loss of cross section was found above the pier but not at the soffit. In the case of honeycombing and insufficient grouting combined as found above the pier, the wires of some tendons were entirely corroded. For a structural evaluation it has to be assumed that only 95% of the cap tendons and half of the other tendons are still existent.
The post-tensioning system used at the Ponte Moesa proved to be very vulnerable to insufficient grouting. Since it was used in the pioneer era of post-tensioning, those bridges
nowadays have reached an age of about 50 years. Even if the ambient conditions are not
very aggressive, it is possible that severe corrosion took place. Those bridges have to be
carefully examined.
Hollow cores are obstacles for pouring the concrete. Compacting of a slender bridge with
hollow cores and many tendons is almost impossible. Honeycombs caused most of the
deterioration. When surveying the condition of a bridge and especially the ones with hollow
cores, it has to be done very carefully.
Policy implications
Continuous acoustic monitoring is always costly. To use a budget effectively all available
information has to be carefully considered beforehand. Since every structure or every
generation of structures has typical problems, those considerations enable an adequate
solution for the structure. Continuous monitoring is recommended for highly degraded
bridges like the Ponte Moesa. For structures in a critical condition or close to a collapse
without warning, acoustic monitoring provides the responsible structural engineer with
essential information. This can be an important factor for the owner to decide whether a
structure should be retrofitted or replaced.
Other results
Publication:
Fricker, S.; Vogel, T. (2006).
Feldversuche mit dem akustischen Überwachungssystem SoundPrint
FA AGB2002/009 ASTRA, VSS-Bericht in Vorbereitung.
Fricker, S.; Vogel, T. (2006).
Site Installation and Testing of a Continuous Acoustic Monitoring
Journal of Construction and Building Materials, 2006, in print.
Fricker, S.; Vogel, T. (2006).
Detecting wire breaks in a prestressed concrete road bridge with continuous acoustic monitoring
Proceedings, IABMAS’06, Porto, 16.-19. July 2006, accepted and in print.
Fricker, S.; Vogel, T. (2006).
Acoustic Monitoring of Post-Tensioned Bridges
Proceedings, Structural Faults & Repair-2006, Edinburgh, 13.-15. June 2006, in preparation.
Fricker, S.; Vogel, T. (2006).
Überwachung von Drahtbrüchen bei Stahlbetonbrücken
Proceedings, Fachtagung Bauwerksdiagnose, Praktische Anwendung Zerstörungsfreier Prüfungen, Berlin, 23.-24. Feb. 2006.