Premature corrosion of steel tendons in post-tensioned pre-stressed concrete, cable stay and suspension bridges can cause them to fracture, compromising the strength, integrity and service life of the structure. There are many bridges of these types not only in the UK but worldwide, so how can infrastructure owners obtain information on the condition of their structures, whether they are deteriorating and, if so, the rate at which they are deteriorating?
Currently, to obtain information on their condition, these structures are subjected to in-situ special inspections which include intrusive drilling into a sample of the ducts of post-tensioned concrete bridges or, in the case of cable stay and suspension bridges, unwrapping the protective covering and undertaking a visual examination of the tendons.
While such inspections give an indication of the extent of the damage caused by corrosion, the risk of further fractures then has to be managed. Where there is deemed to be a risk of corrosion and fracture of tendons, interim measures such as lane closures or weight restrictions may be applied. However these measures can increase congestion, extend journey times and increase CO2 emissions.
An alternative is the use of asset management tools such as SoundPrint®. This is a non-destructive acoustic monitoring system originally developed by the Canadian company Pure Technologies Ltd to detect wire breaks in unbonded (ungrouted) tendons in the floor slabs of office buildings. When applied to bridges, this system can assist the bridge owner in developing a management strategy for the structure. Research and application by Pure Technologies Ltd, in conjunction with TRL, has seen the system being used as a structural monitoring tool in the United Kingdom since 1998.
Development of SoundPrint
In 1992, the Department of Transport (DoT) placed a moratorium on the construction of post-tensioned concrete bridges with internally bonded tendons and initiated a programme of special inspections of this type of structure. This followed concerns about the premature corrosion of the steel tendons.
In addition, the DoT, and subsequently the Highways Agency (HA), undertook a programme of research into methods for detecting corrosion and fracture of wires in post-tensioned tendons. As part of this research, TRL was commissioned to undertake trials to investigate the use of SoundPrint for detecting wire fractures in tendons in grouted ducts. The findings from the trials provided independent verification that the SoundPrint system could be used to detect and locate wire fractures in steel tendons in grouted and partially grouted post-tensioned concrete beams.
Supplementing the laboratory tests, the HA also funded a one-year field trial of the system which was implemented at the Huntingdon railway viaduct – a six span post-tensioned bridge in Cambridgeshire carrying the A14 (Cambridge to Kettering section) over the electrified East Coast main line and a local single carriageway. Initially, an array of 32 acoustic sensors was installed, predominantly on the northern cantilever span but also on the northern backspan of the bridge. Trials of the system, including blind trials using facsimile breaks and external wire breaks, proved that the system was working as intended.
The field trial proved so successful that HA commissioned TRL to continue monitoring the structure using SoundPrint to assist with the long term management of the viaduct.
In 2009 a major upgrade of the system was undertaken and the monitoring now comprises 112 acoustic sensors encompassing the north and south cantilevers and the north and south backspans. Monitoring of this structure is now in its 16th year.
Acoustic monitoring
For acoustic monitoring to be useful and effective, it has to provide information over many years and has to be continuous, effectively being able to capture critical data 24 hours a day. In addition to this, certain challenges have to be overcome and a number of important conditions must be assured, including:
- Independent verification of the applied techniques – SoundPrint is currently the only independently verified system in the UK;
- The ability to monitor operational structures and have a high system uptime;
- The ability to manage large volumes of data and diagnose system problems using state of the art techniques;
- Comparing data to known acoustic events;
- Timely reporting of the information to owners and engineers.
Within the structure, the post-tensioning strands contain a significant amount of potential energy in the form of prestress. When a wire in a strand breaks, the potential energy is converted into kinetic energy, which is suddenly injected into the structure, detectable as a dynamic response. The result is a series of multimodal vibrations of the concrete structure itself. These events are detected using an array of sensors and the resulting data continuously analysed using proprietary software/hardware to detect acoustic signatures similar to known wire breaks. Flagged events are compared to a database of over 2000 wire breaks, and confirmed by an experienced data processor. Active failures are reported to decision makers responsible for the structure.
The measures used to analyse the signals are manyfold, and are derivatives of time- frequency analysis popularised in electrical engineering disciplines over the past 30 years.
An important parameter used to discern between ambient noise and wire breaks is an autocorrelation of the time-frequency- energy representation of a signal to known wire breaks captured on similar structures. Another useful parameter that can be calculated using time-frequency analysis is the absolute amount of energy captured, which is proportional to the size of the wire break.
Where the structure remains ‘quiet’ (low number of wire breaks) it gives an assurance to the bridge owner/operator that it is not deteriorating rapidly. If failures are detected, and taking structures out of service immediately would cause large- scale disruption, continuous monitoring will allow structures to remain in-service while maintenance or replacement options are developed and budgeted.
Huntingdon trials
The Huntingdon railway viaduct was constructed in 1975 and crosses a local single carriageway, the electrified East Coast main line and part of the passenger platform at Huntingdon Station. It is a six-span structure which forms part of the Cambridge to Kettering section of the A14 dual carriageway. Spans 1, 2 and 6 are simply supported reinforced concrete beams and spans 3 to 5 are of post-tensioned beam construction. The main span (span 4) consists of a 32 metre-long suspended span which sits on half-joints formed at the end of two 16m long cantilevers extending from the adjacent piers. The remaining five spans are 32.3 metres in length.
A special inspection undertaken in 1994/5 discovered the presence of voids, water and chlorides in the tendon ducts of the cantilevers (span 4) and back spans (spans 3 and 5) of the viaduct but no significant corrosion of the strands. There was concern that such conditions could allow the corrosion of the post-tensioning strands to develop to such an extent that they would break. However, as the prestressing system was apparently in a good condition, the structure had a long potential life but required careful management. The high volume of traffic using the route made it essential to maintain the structure in service with minimum interruptions whilst maintaining an appropriate level of safety.
Monitoring
The half-joint was the region considered most likely to benefit from an assurance about tendon condition as half-joints are difficult to inspect. Also, the two cantilevers contained no geometrical redundancies in respect of shear failure in the half-joint and failure over the pier in hogging flexure. It was therefore decided that early detection of wire fractures, or evidence that none were occurring, would assist the long-term management of the viaduct.
The western cantilever was preferred to the eastern cantilever for monitoring as access to the cantilever section and half-joint only required lane closures on the local carriageway and a hydraulic access platform. This was considerably more straightforward than obtaining access to the eastern cantilever which partially spans the railway line.
A SoundPrint acoustic monitoring system was installed on the viaduct in mid-1998. As this was the first installation of the SoundPrint acoustic monitoring system in the UK, one of the objectives was to assist in the evaluation of the system as well as providing data on the condition of the viaduct. It was anticipated that the presence of the noisy expansion joints at the two half-joints of the viaduct would provide a great many acoustic events and present a considerable challenge to data collection and analysis. This was investigated prior to installation of the system on the bridge using a sensor installed temporarily on the deck. A Schmidt hammer impact was recorded and played back through the acoustic monitoring equipment, and it was found that the impact could be readily discerned from the background noise. In addition a number of facsimile wire break events were created as part of the commissioning trials.
In total, 36 sensors were installed on the western cantilever and anchor span. The sensor array was designed with the objective of detecting wire breaks in partially grouted tendons at the half-joint and over the cantilever span, but nevertheless was expected to detect wire breaks in fully grouted ducts.
The monitoring system was extended in 2009. Sixteen additional sensors were installed on the anchor span of the western cantilever and the sensor array was replicated on the eastern cantilever and anchor span.
The structure is still in service 15 years after the installation of the system. For the period between its installation in 1998 and 2005 no wire breaks were detected.
This provided a reasonable level of certainty that prior to the installation of the system in 1998 no wire breaks had occurred in the area covered by the system. Since 2005 a small number of wire breaks have been detected but not enough to cause concern about the integrity of the structure.
Alternatives
If the acoustic monitoring system had not been available, it would have been necessary to have installed a broader regime of strain measuring devices around the structure to detect changes in behaviour that reflect losses of post-tensioning. It may also have been necessary to take routine x-rays of key areas of the structure to try to detect wire breaks.
Such an approach would not have provided the same level of confidence as the acoustic monitoring system and would have required a number of closures of the structure.
Report by Kevin Barker, senior project engineer , TRL
