Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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THE USE OF ACOUSTIC MONITORING TO MANAGE CONCRETE STRUCTURES

Peter O. Paulson, Pure Technologies Ltd., Marcel de Wit, Advitam

ABSTRACT

Concrete and steel are widely used in civil structures. Both are excellent acoustic transmitters. In many structures tensioned wire elements are used. However, tensioned wire can be vulnerable to corrosion. To reduce the probability of corrosion sophisticated protection systems are used. To confirm that the design strength is available through time, extensive inspection and maintenance regimes are implemented.

Due to severe problems observed at later stages on several structures on a world-wide basis, technologies have been developed to monitor continuously prestressing failures and concrete cracking on key structures.

This paper presents two case studies of an acoustic monitoring technology which uses widely distributed sensors to detect and locate wire failures using the energy released at failure. The technology has been used on a range of structures including post-tensioned concrete bridges, suspension bridges, buildings, precast concrete cylinder pipelines (PCCP) and prestressed concrete containment vessels (PCCV), where it has increased confidence in structures and reduced maintenance costs.

Several projects have been completed o civil structures; several suspension bridges in the USA, prestressed bridges (internal and external prestressing) throughout Europe etc...

This paper describes this type of technologies, details of two cases showing, how the information given by this tool can be used as a management tool to mitigate significantly structural maintenance and repair costs.

One application is on a viaduct in the Netherlands the other is on a bridge in Belgium.

ACOUSTIC MONITORING SYSTEM

INTRODUCTION

Continuous acoustic monitoring has been used since 1994 to monitor failures in bonded and unbonded tendons in post-tensioned structures, where it has shown major benefits in confirming the performance of structures, increasing Client confidence and reducing maintenance costs. To extend the application of this technology to the monitoring of concrete cracking required that the effectiveness of the principles and methods was evaluated for each structural type.

For acoustic monitoring technology to function in a particular environment it must be shown that the signals generated by wire failure can be detected above general noise levels and distinguished from events which are not of interest. Furthermore, to assess the structural implication of each event it is generally important to be able to locate the source of each emission. Provided with high quality data of this type, the engineer can appraise a structure with knowledge of the actual failures in damaged elements, and their location, in the entire structure over the monitoring period. The alternative, to base the assessment on a physical inspection at a sample of locations, leads to uncertainty when for practical and economic reasons the number of inspection points is limited. Monitoring the entire structure may also reveal failures not detectable by a conventional investigation.

In many applications the acoustic data is transmitted over the Internet for processing and analysis. After processing and quality control checks, the data can be made available on a secure section of a web site, allowing owners rapid independent access to their database of results.

The technology is useful in providing cost-effective long-term surveillance of both unbonded and grouted post-tensioned structures. This paper shows the principle and some case studies of the technology.

DEVELOPMENT OF CONTINUOUS ACOUSTIC MONITORING

The principle of examining acoustic emissions to identify change in the condition of the structural elements is not new. However, until recently, continuous, unattended, remote monitoring of large structures was not practical or cost-effective. The availability of low-cost data acquisition and computing hardware, combined with powerful analytical and data management software, resulted in the development of a continuous acoustic monitoring system described hereafter, which has been successfully applied to unbonded post-tensioned structures in the world since 1994.

Corrosion of the steel strands in these post-tensioned structures has become a concern for designers and owners. As with grouted post-tensioned bridges, the extent of corrosion is not known, primarily because of the difficulty of identifying corrosion due to the inaccessibility of the corrosion sites, the lack of external evidence and the limited spatial coverage of intrusive inspections.

The continuous acoustic monitoring system uses the distinctive acoustic characteristics of wire breaks to separate them from other acoustic activity on a structure. Using a combination of instrumentation, data acquisition and data management, it is possible to identify events, as well to locate the failure and time of failure.

This concept allows the non destructive identification of broken strands, so that these strands can be replaced periodically as part of a long term cost effective structural health program. In addition, an understanding of the condition of the steel wire elements allows the life of the structure to be extended.

A typical system includes an array of sensors (Figure 1) connected to an acquisition system with coaxial communication cable. The sensors are broadband piezo-electric accelerometers fixed directly to the concrete slab. Sensor locations are chosen so that an event occurring anywhere on the slab can be detected by at least four sensors. Sensor spacing range from 1 per 60 square meters for fully grouted slabs up to 1 per 100 square meters for ungrouted tendons. Multiplexing techniques are able to acquire data from many hundreds of channels on 32 acquisition channels.

Fig 1: Standard sensor for buildings, bridges and parking structures. Fig 2: Time domain and frequency spectrum plots of wire break detected by sensor 10.0 m. from event.

Using several characteristics of the acoustic events including frequency spectrum it is possible to classify wire breaks and to reject environmental noise.

By analyzing the time taken by the energy wave caused by the break as it travels through the concrete to arrive at different sensors, the software is able to calculate the location of the wire break, usually to within 300 - 600 mm of the actual location. Independent testing showed the system to be 100% correct when spontaneous events classified as "probable wire breaks" were investigated. Figure 2 shows a typical acoustic response to an unbonded wire break at a sensor 10.0 m from the break location. Figures 3 and 4 illustrate how the system locates events.


Fig 3:
a)Time domain plot showing relative arrival time of signal at different sensors. b)Time domain plot showing relative arrival time of signal at different sensors.
Fig 4:

The site acquisition systems download all data automatically using the Internet to the Paris processing center. This allows the cost of data transfer to be minimized. All data can be viewed by the owners team directly on the a secure web site. This allows the owner to review areas of concern in parallel with the generation of routine reports. Various levels of alarms can be triggered semi-automatically using e-mail, automatically voice activated phone alarms, etc.

MONITORING OF WIRE BREAKS IN GROUTED POST-TENSIONED BRIDGES

Acoustic monitoring has been used in a wide range of applications including suspension and cable stay bridges, and pipelines.

The technology has also been applied on many post-tensioned concrete bridges around the world.

Description of acoustic monitoring system on the viaduct in Breda the Netherlands.
Cadettenkamp viaduct in Breda is a viaduct on the A27 highway and crosses the railway connection between Breda and Tilburg. The A27 is highly used highway for cargo trucks.

It is a three span structure with 4 box girders and has a total length of 75 m and is 25 m wide.

The structure built in the 70's has classical internal grouted post-tensioned tendons in the walls of the boxgirders. Due to extension of the traffic capacity in terms of an extra lane. The viaduct will be strengthened by 4 external grouted post tensioned cables.

The old prestressed cables are not in very good condition due to their brittle character. The risk that the extra load will damage the old cables is still present after installing the 4 post-tensioned cables. To be able to follow the condition of the old cables under the new load characteristics, the client has decided to instrument the viaduct with the the acoustic monitoring system.

The system will safeguard the structure and identify and locate the damage on the tendons if it occurs.

106 acoustic sensors were placed on both sides of each box.. The sensors are connected to a 32 channel acquisition unit. The sensors were placed on the outside of the boxgirders. This together with a railway passage under the viaduct made the access very difficult. The installation above the railway were done in 2 night shifts during a very short time period where the railroad was out of service. Due to these difficulties the installation including tests and set-up covered 4 weeks.

Description of acoustic monitoring system on a bridge in Kortrijk Belgium.
The bridge over the canal Bosuit-Kortrijk is abridge on the E17 highway it is a three span structure with 4 box girders and has a total length of 210 m and is 40 m wide.

Fig 5: Fig 6: Test impact signal on divers sensors. Fig 7: Fig 8:

The structure built in the 60's is a cast-in-situ cantilever bridge and has classical internal grouted prestressed tendons in the walls of the boxgirders. Due to visual and detailed inspections question marks were placed on the condition of the tendons in the outer boxgirders that are heavily loaded with cargo traffic.

To obtain information on the condition of the tendons in these areas, the client has decided to instrument the bridge with the acoustic monitoring system.

The system will safeguard the structure and identify and locate the damage on the tendons if it occurs.

144 acoustic sensors were placed on both sides of one box and on only one side of the other box. The sensors are connected to a 32 channel acquisition unit. The sensors were placed inside the boxgirders.

Due to the easy access the installation including tests and set-up covered only 2 weeks.

The system will monitor the bridge for 1 year with an option for longer.

REFERENCES

  1. J.F. Elliott. Continuous Acoustic Monitoring of Bridges. International Bridge Conference, 1999, Pittsburgh, Pennsylvania IBC-99, pp. 70.
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