Cullington, Bradbury and Paulson Page 1
CONTINUOUS ACOUSTIC MONITORING OF STEEL
TENDONS AND CABLES IN BRIDGES
D W CULLINGTON and T BRADBURY,
Transport Research Laboratory, Crowthorne, Berkshire, RG45 6AU
P O PAULSON,
Pure Technologies Ltd, Calgary, Canada, T2R 1L5
ABSTRACT
This paper describes laboratory trials and site installations of an acoustic monitoring system,
developed in Canada, for detecting the fracture of stressed high-tensile steel wires in
structures. The fracture of a stressed wire releases energy that can be detected as an acoustic
event by surface mounted sensors. Characteristics of the event provide information on the
position of the event and its cause. Trials at TRL have demonstrated that the system can
reliably detect wire fractures in grouted post-tensioned structures and hanger cables,
successfully rejecting other acoustic events. Installations on the Railway Viaduct in
Huntingdon in the UK and the Bronx-Whitestone suspension bridge in the USA have shown
the system to work in practice. Wire fractures can occur for a number of reasons. Acoustic
monitoring is useful because the wires are often inaccessible for visual examination and
fractures cannot generally be detected by non-destructive inspection techniques
1. INTRODUCTION
High tensile steel wires have many structural applications as individual wires, or in the form
of tendons, cables or ropes. Typical structures include pre-tensioned and post -tensioned
concrete bridges and buildings, pipelines, cable-supported bridges and anchors in ground
engineering. All of these structures have a common feature. Generally, the wires are difficult
to inspect because they are inaccessible. Post-tensioned tendons are located deep within a
bridge deck, and suspension cables are difficult to inspect non-destructively below the
surface laye rs. The problems are even greater in ground engineering applications. Wire
failures can occur in service due to corrosion, stress corrosion and fatigue. Very often the
loss of a few wires is not critical but eventually, for reasons of safety, repairs or replacement
of the cable or structure may be required. For the effective and economic management of
such structures, information is needed about the number and position of wires that have
fractured.
The Transport Research Laboratory has carried out evaluation trials of the SoundPrint®
acoustic monitoring system for detecting wire fractures in steel tendons and cables. The
system works by continuously listening for the characteristic acoustic events that accompany
the fracture. Most of the trials have been undertaken for the Highways Agency, as part of a
programme of research on post-tensioned bridges.
®
2. THE SOUNDPRINT ACOUSTIC MONITORING SYSTEM.
The SoundPrint® system was developed by the Canadian company Pure Technologies
originally to detect wire-breaks in unbonded (ungrouted) tendons in the floor slabs of office
buildings. Potentially it can be applied to any structure in which the integrity of steel cables is
Cullington, Bradbury and Paulson Page 2
difficult to appraise visually. Research has indicated that NDT is generally not able to detect
broken wires and monitoring is therefore a practical alternative.
The system has been described elsewhere (Halsall et al, 1996; Paulson, 1999). Briefly, when
highly stressed steel wires fracture there is a release of energy, which is transmitted through
the structure. Sensors (accelerometers) attached to the external surface of the structure are
used to detect these acoustic events. Each sensor is connected using coaxial cable to an on-
site data acquisition unit, such as the one shown in Figure.1. The system monitors
continuously but collects no data until triggered by an acoustic event lying within pre-set
limits. Software filters are then applied to reject events of no further interest. Events that
successfully pass these tests may be wire fractures. They are sent via the Internet to Canada
for the events to be classified and their position calculated.
Figure 1. 16-channel SoundPrint® acquisition unit.
Other events that might generate acoustic responses in a bridge include vehicles going over
discontinuities in the road surface, small objects such as stone chips striking the concrete and
expansion joint defects responding to trafficking.
Unless a monitoring system has previously been proved to work in a comparable location,
evaluation trials are desirable to:
• confirm that wire breaks can be detected in that particular application
• demonstrate they can be distinguished from the non-break events expected on the site
• show that they can be captured reliably in the presence of t he ambient noise.
Pure Technologies initially proved that wire breaks could be detected in office buildings with
unbonded tendons. The operation of the system on a bridge with grouted tendons was
believed to be more difficult than in buildings for two reasons. Grouted tendons would
Cullington, Bradbury and Paulson Page 3
probably release less energy on fracture than unbonded tendons, and non-break events and
background noise were likely to be more dominant.
3. LABORATORY TRIALS
TRL has undertaken a number of evaluation trials of the SoundPrint® system. Laboratory
trials are an integral part of the evaluation process because, compared with site, the
environment can be better controlled and wire breaks created for test purposes without
damaging a structure in service.
3.1 Trials on post-tensioned tendons
Corrosion is known to have caused the fracture of wires in post-tensioned concrete bridges,
and non-destructive testing has proved to be of limited value in detecting the presence of
broken wires (Cullington et al, 1996). Invasive inspection is overwhelmingly the method
adopted but this can be carried out at only a small number of locations for reasons of cost and
damage to the bridge.
For the first trials at TRL, the SoundPrint® system was used to detect trial wire breaks in the
tendons of two post-tensioned concrete members. One, the 30m long Bank Lane Unit,
contains fully-grouted ducts. The other, a 10m long beam, was specially constructed at TRL
and contains a combination of well-grouted and poorly-grouted ducts. Acoustic events were
created by cutting, grinding and corroding the wires to which access was gained by coring or
drilling into the ducts. Accelerated corrosion was obtained by means of anodic dissolution
using an electrode positioned close to the corrosion site and a small current imposed though a
saline electrolyte. In addition, other types of acoustic events were created by dropping,
throwing or rocking objects, dragging chains etc. This was done to test the system’s ability to
reject non-break events.
Two types of trial were carried out: open trials in which Pure Technologies were informed of
the tests being carried out, and blind trials in which details of the events were not revealed to
Pure Technologies until after their report was received.
The initial trials demonstrated that the SoundPrint® system could detect and locate ungrouted
and partially grouted wire breaks as well as some fully grouted breaks. It was also capable of
rejecting most non-wire break events. However, areas for improvement were identified in the
detection of low-energy, fully-grouted wire fractures and the rejection of small sharp impact
events some of which were occasionally incorrectly classified as wire breaks.
With data from the problem events supplied by TRL, Pure Technologies reconfigured the
system using new sensors, new software and a higher sensor density. It then reliably detected
fully-grouted wire breaks and rejected small sharp impact events. Following this success,
and a recommendation from Thorburn Colquhoun, the Highways Agency decided to install a
system on a bridge as described in Section 4 of this paper.
A further step on the Bank Lane unit was the design and development of an external wire-
break rig. This rig is fixed to the concrete surface and, as the name implies, can be used to
create wire breaks in a stressed strand held in the rig. The results of the final series of trails
using this rig are given in Table 1.
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3.2 Trials on a hanger cable
Hanger cables are subjected to dynamic loading from wind and traffic. As a result, they may
undergo a combination of corrosion and fatigue that eventually leads to wire fractures.
Detection of fractured wires is difficult, as only the wires on the outside of the cable bundle
are visible, and even this may require the removal of wrapping at the cable ends where
fractures are most likely to occur. Acoustic monitoring has the potential to detect fractures
anywhere in the cable, including within the sockets.
The trials used a 2.8m length of spiral-strand hanger cable 36mm in diameter. This was
installed in a hanger-cable fatigue rig at TRL. The rig has two servo controlled hydraulic
actuators, one to provide tension in the cable and the other to produce dynamic transverse
oscillations about the cable's midpoint, as shown in Figure 2.
Actuator
Bearing
Portal
Axial Tube Frame Reaction
Loading Socket Frame
Cylinder Restraint Cable Load
Cell
Location Socket Longitudinal Loading
Rings Adapter Members Disc
Figure 2. Diagram of hanger cable test rig.
A SoundPrint® system was set up with two sensors mounted on the cable, one on each
socket. Only two sensors were required, because the location of acoustic events was needed
in one dimension - along the length of the cable. The test rig generates significant background
noise within which the monitoring system has to operate. Initially, therefore, a DAT sound
recording of the rig in operation, picked up by the two sensors, was sent to Pure Technologies
for examination.
To commission the system, the hardware filters were tuned by creating small impacts on the
cable at the ends and in the middle. A dxf geometry file was created and loaded onto the
system and the software filters were supplied by Pure Technologies. Calibration of the
system was straightforward and took only a matter of hours.
White rings were painted on the cable near the neck of the sockets and at the centre on both
sides of the central clamp. These were an aid in checking for breaks in the outside wires. A
fracture results in the wire moving due to the release of strain and displacing the white line.
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Values for axial strain and oscillation amplitude were based on those experienced by cable
hangers in service and from previous fatigue t rials at TRL. The axial load was initially set to
36 tonnes and the amplitude of the oscillations to ± 20mm with a frequency of 1Hz. Figure 3
shows the completed set up.
Figure 3. Photograph of hanger cable test rig
After 83000 cycles, the SoundPrint® system had registered 15 wire breaks. The experiment
was stopped and the cable was removed from the sockets, cut and dismantled to discover how
many wire-breaks were present and their location. These data were then compared wit h the
SoundPrint® report. Figure 4 shows a time domain plot of a wire break event captured
during the experiment. A TRL researcher heard this event (and later one other) as it
occurred, thus corroborating the fact that the system had correctly identified a break.
Figure 4. A one-second recording showing a hanger cable wire fracture and background noise.
Cullington, Bradbury and Paulson Page 6
On comparing the results from the destructive cable inspection with the events reported by
Pure Technologies, it was concluded that the SoundPrint® system had located the wire
fractures to within 300mm and generally within 100mm. A complete audit of the number of
wire breaks created and detected was not possible, because some breaks were present in the
cable from an earlier trial. However, the results as given in Table.1 indicate that none were
missed. It was also noted that the frequency and time domain plots were similar to those for
wire breaks in unbonded tendons in post-tensioned bridges.
3.3 On going Trials
Research is in progress at TRL into the measurement of concrete strain that occurs as wires
fracture. A post -tensioned test beam is being monitored using Vibrating Wire strain gauges
and a four -channel SoundPrint® system. A 20mm diameter hole was drilled into the
specimen to gain access to the tendon and a corrosion cell was set up to corrode the wires.
When a wire fractured, an operator who fortuitously was present heard it. At that moment,
SoundPrint® also recorded an event. The plot of this event and the data for the VW gauge
nearest to the site of corrosion are given in Figures 5 and 6. It can be seen that the VW gauge
shows a change in strain when the wire break occurred. Data evaluation is still in progress.
Figure 5. Time and frequency domain plots of a wire break heard by an operator
Microstrain against Time
00:00 00:30 01:00 01:30 02:00 02:30 03:00 03:30 04:00 04:30 05:00
-94
-96
-98
Wire break
Microstrain
-100 Wire
-102
-104
-106
-108
Time / Minutes
Figure 6. Graph from laboratory trials showing the change in microstrain due to the wire break as
recorded by the VW gauge nearest to the site of corrosion.
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4. SITE INSTALLATIONS AND TRIALS
Two site installations of the system are described in this section, one in England carried out
under the supervision of TRL and another in the USA carried out under the supervision of
Weidlinger Associates Inc of New York.
4.1 Railway Viaduct Huntingdon England
The post-tensioned A14 Railway Viaduct at Huntingdon was suitable for the first UK
installation of a monitoring system for several reasons. The structure had been the subject of
a Special Inspection that had indicated the presence of voids, water and chlorides in the
tendon ducts, but no significant corrosion of the strands. Further structural investigations
were in progress that would be supplemented by a clear indication of the presence or absence
of actively fracturing wires. The structural form contained features that lent themselves to
monitoring, in particular half-joints, which are difficult to inspect.
The fact that the prestressing system in the Railway Viaduct is apparently in a good condition
indicates that the structure has a long potential life and will require economic management
for the foreseeable future. The high volume of traffic using the route makes it essential to
maintain the structure in service with minimum interruptions and appropriate regard for
safety.
Following a commission from Thorburn Colquhoun, who are responsible for the Railway
Viaduct as part of the Area 8 maintenance contract for the Highways Agency, a SoundPrint®
acoustic monitoring system, was installed on the viaduct in mid 1998 (Cullington et al 1999).
It comprises 32 channels positioned over a 48m length. Since the commissioning, numerous
trials have been carried out on site. These mainly comprised facsimile wire break events
created by a Schmidt hammer or spring impactor applied to the soffit, and external wire
breaks as mentioned in section 3.1, created on the internal web of a cell of the structure.
Table 1 contains a summary of the results. Continuous uninterrupted operation is important
for this type of monitoring. After an initial settling in period, the system uptime has been
over 99%. No naturally -occurring wire breaks have been detected.
4.2 Bronx -Whitestone Suspension Bridge USA.
A SoundPrint® system was installed on the main cable of this bridge in northern USA.
Acoustic sensors were attached to six consecutive cable bands on the main cable monitoring a
length of approximately 84m. The SoundPrint® data acquisition unit was located at deck
level. The cable itself was undergoing maintenance and the outside wrapping had been
removed revealing the high tensile steel wires. This enabled blind trials of the system to be
carried out by cutting six wires, the results of which are summarised in Table 1.
5. CONCLUDING COMMENTS
The SoundPrint® system has proved to be an effective way of monitoring the fracture of
stressed high tensile steel wire in structures. There are many possible applications of this
technology to the safe and efficient management of structures containing these elements. The
system is already fully functional but remains responsive to new challenges. TRL is actively
pursuing the application of SoundPrint® technology to other areas not yet evaluated
elsewhere. Pure Technologies are co-operating with this work.
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Location Nature of trials Outcome
Bank Lane Unit External wire breaks 25 breaks, five blind, all detected, 22 located
within 0.2m and 3 within 0.5m
Hanger cable rig Fatigue test fractures End A1, breaks count exact (3 new); End A0, 6 old
+ 6 new breaks exceeds actual by 2.
Strain gauged beam Corrosion fracture of internal Breaks detected at TRL; data analysis in progress.
wires
Railway Viaduct External wire breaks and 44 wire breaks and facsimiles, 18 blind; 43
facsimiles detected 18 within 0.2m and rest within 0.5m, 41
events correctly classified.
Bronx Whitestone main Wires cut mechanically 6 wires cut blind, 5 detected within 0.22m and one
cable within 0.7m
Table 1 . Results of trials at various locations
6. ACKNOWLEDGEMENTS
The paper has been written with the permission of TRL and the Department of Transport
Environment and the Regions. The views expressed are those of the authors and not
necessarily those of the sponsors. The authors wish to thank the many individuals and
organisations who helped with the gathering of these data, including Donald MacNeil of TRL
David Ball of HA, Tony Wakeman of Thorburn Colquhoun and John St Ledger of WS
Atkins.
7. REFERENCES
Cullington, D W, M E Hill, R J Woodward and D B Storrar. Special inspections on post-
tensioned bridges in England: Report on progress. FIP Symposium 1996: Post -tensioned
concrete structures. The Concrete Society, 1996, pp 482-491.
Cullington, D.W., D MacNeil, P O Paulson and J Elliott, (1999), Continuos Acoustic
Monitoring of Grouted Post-Tensioned Concrete Bridges. Paper Presented at the 8th
International Structural Faults & Repair Conference, London, UK.
Halsall A.P, W E Welch and S M Trepanier. Acoustic monitoring technology for post-
tensioned structures. FIP Symposium 1996: Post-tensioned concrete structures. The
Concrete Society, 1996, pp 521-527.
Paulson, P.O. (1999), Practical Continuous Acoustic Monitoring of Suspension Bridge
Cables. Transportation Research Board 78th Annual Meeting January 10-14 1999.
Washington DC. TRB, 2101 Constitution avenue NW Washington 20418.
