CONTINUOUS ACOUSTIC MONITORING OF STRUCTURES

J.F. Elliott and M.W. Holley Pure Technologies Ltd.

Calgary, Alberta, Canada

ABSTRACT

This paper describes the development, testing and application of a continuous acoustic monitoring system to detect and locate corrosion-induced failures of tensioned steel elements in buildings, bridges and pipelines. The monitoring system utilizes acoustic sensors distributed about a structure. Data is processed on-site and transmitted over the Internet to a central processing facility where proprietary processing software is used to generate reports summarizing the time, location and classification of recorded events. Case studies describing the various applications and testing of the system are presented and discussed.

INTRODUCTION

The use of high-strength steel wire has contributed greatly to advances in design and structural performance over the last hundred years or so. Unfortunately, this increase in strength has not been accompanied by a corresponding increase in durability. In addition to conventional dissolution corrosion (or rusting), these high-strength steels are susceptible to failure through brittle fracture caused by stress corrosion, hydrogen embrittlement and fatigue.

These corrosion mechanisms cause a significant loss of ductility in the steel, and failure can occur without a gradual loss of cross-section in the wire (Figure 1). Loss of ductility is not always accompanied by a reduction in ultimate strength. Studies have shown that galvanized wire is more prone to embrittlement than non-galvanized wire when the galvanizing is damaged or locally depleted.

Copyright ©2000 by NACE International.Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in U.S.A.

It is important to note that, although chlorides and other contaminants can accelerate embrittlement, water and oxygen are the only ingredients necessary to initiate the process. For this reason, embrittlement has been found in post-tensioning strands in completely enclosed, climate controlled high-rise buildings where rainwater has entered the tendon system during construction.

The presence of corrosion in high-strength steel wire in bridges, prestressed pipelines and other structures can have serious consequences. The Ynys-y-Gwas bridge in the United Kingdom (U.K.) collapsed in 1985 as a result of corrosion of the post-tensioning system. A major program of inspection of the grouted post-tensioned bridge stock in the U.K. was then initiated. At least four other bridges were decommissioned and replaced because of serious corrosion. In 1992, the Highways Agency in that country banned the use of grouted post-tensioning in their bridges pending the development of improved detailing, grouting procedures and quality control measures l. Corrosion has also been found in post-tensioned bridges in France, Germany, Italy, Denmark and Japan. Severe corrosion and extensive wire failure has been discovered in the main cables and in the hanger systems of a number of suspension bridges in North America and Europe (Figure 2). This has resulted in expensive rehabilitation projects in many instances and replacement of the main cables. Corrosion-induced failures of stay cable wires have been documented in China, Argentina, Venezuela and Germany 2.

Prestressed Concrete Cylinder Pipe (PCCP) has been produced and installed throughout North America since 1942. Over 45,000 kilometers (28,000 miles) of PCCP is currently in service. While the percentage of PCCP failures has been low, the results can be catastrophic. Service interruptions for large pipe bursts are problematic, as often there is no system redundancy for big mains.

Beginning about 1970, a disturbing number of failures began to occur on PCCP, some in large diameter pipes with destructive results. During the fall of 1990, a meeting of concerned prestressed concrete pipe users was hosted in Denver, Colorado. Forty-six representatives from eighteen agencies/utilities across North America attended. Collectively, they represented organizations reporting 41 failures of PCCP that were produced by six different manufactures. As of the present, approximately 300 ruptures are known to have taken place.

In most cases, corrosion of high-strength steel wire in bridges, buildings and prestressed pipelines are not visually evident. Grouted post-tensioned tendons are obviously not visible. The circumferential wire wrapping on suspension cables and the steel or polymeric protective sheathing on stay cables preclude non-intrusive visual inspection. Prestressed pipelines are difficult to evaluate without dewatering the structure or excavating for external inspection.

Engineers have had to rely on intrusive investigations or on the use of available non-destructive evaluation techniques to provide some information about the condition of these components because of their inaccessibility. These methods have serious disadvantages in terms of sampling reliability, accuracy, cost and disruption.

INVESTIGATIVE TECHNIQUES

Grouted Post-Tensioned Bridges

Grouted post-tensioned bridges are usually inspected using a combination of intrusive and non- destructive techniques 3. Covermeters or other metal detectors can be used to determine the location of tendon ducts. Impact echo equipment can sometimes be used to detect voids in tendons, depending on their location and on the configuration of the concrete element. However, the method is slow and

requires expertise in data interpretation and evaluation. Impact echo can usually detect only the first discontinuity in a given cross-section. Radiography can be used to locate broken wires, although it is also slow, expensive and requires elaborate safety precautions, including the establishment of an exclusion zone around the survey location. Access to the concrete surface on opposite sides of the tendon duct is required.

Intrusive inspection involves the drilling or coring of holes in the concrete to provide access to the tendon duct. The non-destructive methods described above can be used to determine the most suitable locations for the access holes, although a drawing review combined with an awareness of where to look for high-risk areas may well be quicker and more cost effective. Methods such as endoscopy and pressure testing can then be used to determine the condition of the strands in the Vicinity of the access holes and to measure void volume and leakage.

Investigation of grouted post-tensioned bridges requires an holistic approach that combines the judicious use of non-destructive and intrusive techniques with an awareness of the probable construction practices and materials used on the structure, the quality control measures employed and the adequacy of design details relating to corrosion protection and durability.

Suspension Bridges

Investigation of the wires in suspension bridge main cables and suspender ropes usually involves intrusive inspection of the cable or removal of selected ropes for forensic testing. The selection of the locations for inspection depends, to a large extent, on the experience of the investigator based on a review process similar to that described above for grouted post-tensioned bridges. High-risk locations include anchorage zones and the sag points of the cables at mid-span and end-spans, although corrosion may not be present at, or limited to these locations. Inspection of the main cable involves the erection of work platforms, removal of the circumferential wrapping, wedging of the wires to facilitate partial internal inspection and testing of wire samples for strength and ductility. The cost of this process on a typical long-span bridge is rarely less than $1 million.

Post-Tensioned Buildings and Parking Structures

Investigation of the tendons in stuffed or paper wrapped monostrand post-tensioned structures usually involves intrusive inspection 4. Most inspections of these structures have relied on a 'poke and chip' methodology that involves removal of concrete at selected locations near the low point in a cable drape. Once excavated and exposed, tendons are tested for tension by driving a screwdriver between the wires of each tendon. If the head of a screwdriver can separate the individual wires fi:om the main tendon it is considered to be distressed. The selection of the locations for inspection depends, largely, on the experience of the investigator based on a review process of the structure and possible locations of risk. High-risk locations include anchorage zones and the first sag points of the cables near expansion joints or slab edges, although corrosion may not be present at, or limited to these locations.

Prestressed Concrete Cylinder Pipe (PCCP)

Over the last 20 years, many prestressed pipelines have been investigated in an attempt to delineate the potential problems. Due to the difficulty in evaluating the actual condition of the prestressing wire, most investigations have been limited to an internal walk-through of the pipeline with limited external evaluation. The internal investigation of the pipeline requires the structure be dewatered so that a survey team can safely enter the pipeline. While in the pipe visual information on the crack profile and location are recorded. In addition, a 'bonging' of the pipe wall is performed to

delineate any delaminations that have occurred as a result of corrosion of the steel liner or prestressing wire. This information can be useful in identifying locations for external evaluation of the pipeline. Until recently, this was the only way of investigating the condition of these structures.

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 called SoundPrint® which has been successfully applied to unbonded post-tensioned structures in North America since 19945.

There is thought to be over 150 million square meters of unbonded post-tensioned monostrand slab construction in North America. Corrosion of the steel strands in these systems 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 infrequency of visible manifestations of the problem.

While witnessing the removal of corroded strands, Peter Paulson, the inventor of the monitoring system, observed that wires cut during the de-stressing process generated an audible acoustic response. Paulson reasoned that, i f these events had frequency or energy characteristics sufficiently different ffi'om ambient acoustic activity in a structure, it would be possible to identify the events, as well as their location and time of occurrence, with an appropriate instrumentation, data acquisition and data analysis arrangement. This would permit the non-destructive identification of broken strands so that these strands could be replaced periodically as part of a long-term, cost-effective structural maintenance program.

A prototype monitoring system was installed in the 6,000 square-meter ground floor of a building in Calgary, Canada in February 19946 . The system consisted of an array of sensors (Figure 3) connected to an acquisition system (Figure 4) with coaxial communication cable. The sensors were broadband piezo-electric accelerometers which were glued to the underside of the concrete slab with cyano-acrylate adhesive. Sensor locations were chosen so that an event occurring anywhere on a slab could be detected by at least four sensors. The structure was divided into three acoustic zones delineated by expansion joints. A total of 60 sensors were used resulting in a density of one sensor per 100 square meters of slab area. A spatial multiplexing technique was employed to acquire data from the sensors using only 32 acquisition channels. For unbonded structures, sensor density varies between one per 35 to 100 square meters, depending on the geometry of the structure.

The goal of continuous automated monitoring combined with low-cost, centralized data processing was central to the development of the technology. Original software consisted of a commercially available data acquisition package located at the site computer, and a proprietary data analysis and report generation package located at the processing facility. The data acquisition software was later replaced with more suitable proprietary software. As a partial strand replacement project was being undertaken coincident with the installation of the system, it was possible to acquire data fi'om many wire breaks. This information was used to train the data processing software to "recognize" wire breaks. When events possessed all the known properties of a wire break, they were classified as "probable wire breaks". Events possessing some of these properties were classified as "possible wire breaks". All other events were classified as "non-wire break events". By analyzing the time taken by

the energy wave caused by the break as it traveled through the concrete to arrive at different sensors, the software was able to calculate the location of the wire break, usually to within 300 - 600 turn of the actual location. From the beginning, the capability of the system to accurately identify and locate wire break events was remarkable. Independent testing showed the system to be 100% correct when spontaneous events classified as "probable wire breaks" were investigated. Figure 5 shows a typical acoustic response to an unbonded wire break at a sensor 10.0 m (32.8 ft.) from the break location. Figures 6 and 7 illustrate how the system locates events.

Initially, data transfer from the site to the processing centre was accomplished through the use of a direct dial-up modem connection. However, within nine months of system commissioning, automated transmission of data using Internet protocols was achieved. This was a major advance towards the goal of automated, cost-effective monitoring of multiple sites as no human intervention was needed for routine system operation and long-distance call charges were more or less eliminated.

Presently, over 300,000 square meters of unbonded post-tensioned slab in twenty structures are being simultaneously monitored. One technician manages all of these sites. The analytical software is capable of automatically generating reports summarizing the time and location of wire breaks and other significant events. The operating efficiency of the system over the monitoring period is also recorded on the reports.

MONITORING OF GROUTED POST-TENSIONED BRIDGES

In February 1997, the Highways Agency in the U.K. hired British engineering consultant Transport Research Laboratory (TRL) to evaluate the acoustic monitoring system for use on grouted post-tensioned bridges 7's. Initial tests were carried out on a free-standing bridge beam at the Laboratory's Crowthorne facility. The test protocol consisted of causing accelerated corrosion of grouted wires; as well as external wire breaks using a test rig designed to simulate fully-grouted and partially-grouted wire failures. Other events caused by impacts were also generated. The beam was monitored from Pure Technologies' processing center in Calgary, Canada, where reports were generated summarizing the event classifications and locations. In a combination of open and blind testing, the system correctly identified all 25 wire breaks generated.

To test the system under normal highway operating conditions, instnnnentation was installed on a section of a highway bridge at Huntingdon, 90 km north of London. This 24-year-old bridge crosses a railroad and a secondary highway. The bridge has cast-in-place 32-meter main spans with cantilever spans that support precast beams spanning the railway right-of-way. The cast-in place-spans are of grouted post-tensioned box girder construction. The bridge generates considerable acoustic activity caused by traffic hitting loose expansion joint fittings. Occasional construction activity also causes a significant amount of noise. The sensor arrangement used in the bridge is shown in Figure 8. Sensor density on the cantilever slab is approximately one per 25 square meters. Additional sensors were placed on the main span to provide information about signal attenuation.

The test rig used for the Crowthorne tests was used to simulate grouted and partially grouted post-tensioned breaks because it was not possible to generate corrosion breaks of post-tensioning wires within the structure. Other events were generated by using a rebound hammer and a 10 mm ball bearing attached to a flexible steel handle. Sophisticated software filters were designed to eliminate most spurious acoustic activity at the site-based acquisition system and only relevant events are transmitted to the Calgary processing center. The system correctly identified 41 out of 44 test events generated. The system will remain in place on this structure for the foreseeable future. The success of

the testing at Crowthorne and Huntingdon is likely to lead to the system being included in the Highways Agency's list of approved monitoring methods.

MONITORING OF SUSPENSION BRIDGES

Main Cable Monitoring

In October 1997, the monitoring system was tested on the Bronx Whitestone Bridge in New York City 9. This bridge, with a main span of 701 meters, was opened to traffic in 1939 and is owned and operated by MTA Bridges and Tunnels, an agency of the Metropolitan Transportation Authority of New York. The monitoring system was installed during a rehabilitation of the main cables. This work involved removal of the circumferential wire wrapping, repair of broken wires and the application of corrosion-inhibiting oil to the wires. Consequently, it was possible to cut wires in the cable to test the system's recognition and location capabilities. Single sensors were attached to six cable bands, each 12.2 m apart. An array of three additional sensors was placed around two of the cable bands to evaluate radial location capabilities.

A portable acquisition system was set up at deck level (Figure 9) and the testing was done while construction work was in progress. Six wires were cut within the monitored section in a blind test. The system correctly classified the events and located them longitudinally with errors ranging from 0.0 m to 0.7 m. Radial location using all four sensors on a cable band was accurate to within 7.5 °. Acoustic events caused by steel chisels being driven between the wires were easily identified and filtered. Figures 10 and 11 show plots of the response of sensors to a wire break and construction activity respectively.

Analysis of the data generated during the test showed that sensors mounted on alternate cable bands would be able to provide information of sufficient quality to permit reliable identification and location of wire breaks. A complete system based on this configuration is presently being installed. The acquisition unit will be located in the Bronx anchorage house, and data wiU be transmitted from the sensors to the acquisition system through a coaxial trunk line attached to the existing messenger cable. Durability issues and ease of installation and maintenance were major factors in the design of the hardware. The sensor mounting brackets are designed to permit installation without modification to the cable band assembly and without damaging the paint system (Figure 12).

The acquisition system will include a conventional telephone as well as a back-up cellular service. Data will be transmitted over the Internet to the Calgary processing center, where it will be analyzed and archived. The data will also be routed from the Calgary center to a second data processing computer located at the bridge administration building. This will permit bridge operations personnel to review the data and reports using the same proprietary processing software in use at the processing center.

MONITORING OF PRESTRESSED CONCRETE PIPELINES (PCCP)

The monitoring system for pipelines differs from our conventional sensor configuration and layout. These structures use a hydrophone technology that is inserted into the flow of the pipeline (Figure 13) which 'listens' for wire failure in the fully operational pipeline. In fact, the hydrophones use the water column of the pipeline to transfer the acoustic event up and downstream of the origin.

Similar to the above-described systems, the arrival time of the acoustic event to each hydrophone is determined and the origin of the event located.

Recently, we have identified a relationship between pressure and rate of failure of the embedded prestressing wire in these pipelines l°. Transient pressure surges in these pipes can cause increased wire deterioration and subsequent failure. When possible, hydraulic testing (Figure 14) of a pipeline being acoustically monitored can improve and possibly reduce the monitoring intervals required to locate problem locations.

With valuable time and location information on the rate of deterioration of the prestressing wires in these pipes, the owner can now plan for proactive maintenance of the deteriorated pipe sections.

REPORT GENERATION AND PRESENTATION

Because the data acquisition and processing software permits continuous real-time data transmission and analysis, it is possible to make on-demand reports available to authorised users through a web page interface. Automatic e-mail notification alerts the user to the occurrence of a significant event.

Using password protection, the user can enter sites for which they have clearance and generate reports to their own specifications. User-defined parameters include reporting period, event classifications, event locations and temporal distribution of events. An example of a typical user- defined query is shown in Figure 14.

Once the parameters have been defined, presentation-quality reports can be generated in either Word or PDF format.

These capabilities provide users with immediate access to information about the performance of their structures or infrastructure. This addresses one of the main concerns with instrumentation systems, i.e. the collection of large amounts of data, which requires intensive manual post-processing to provide useful information.

NEW APPLICATIONS

The continuous acoustic monitoring principle has recently been adapted for damage surveillance of bridges and other structures. Using the acoustic system as a "trigger", video or other media records of events of interest can be acquired. The use of the acoustic trigger eliminates the need for continuous media recording and review tl.

This system can be used to detect and record instances of damage to bridges caused by over- height highway vehicles or ships. Once an event has been acquired, the data file can immediately be transmitted to a designated destination for review and appropriate response. Systems have recently been installed on two bridges in Alberta, Canada to detect and record truck impacts. Previous impacts on these bridges have generally gone unreported and the provincial transportation agency has been unable to recover the repair costs. The ability of the system to provide video evidence of the event will facilitate cost recovery for future impacts. This approach can also be used for seismic damage surveillance, where agencies are faced with the challenge of rapidly determining infrastructure damage,

possibly over a wide area, to a seismic event. The availability of remote surveillance information can help to prioritize the allocation of limited resources. The first such system was installed on a prestressed concrete water tank in California in June 1999. This system detected a wire break in the tank 5 hours after an earthquake occurred on August 17th, 1999. The earthquake had a magnitude of 5.0 (Richter) with an epicentre approximately 40 kms. from the tank location.

In addition to acquiring video and acoustic records, the system can be configured to sample structural properties, such as natural frequency response, before and after an event. This kind of information could provide evidence of changes in the structural performance of the element or structure. Additional structural instrumentation can be integrated with the monitoring system for this purpose.

The application of continuous remote acoustic monitoring to the detection of fatigue crack development in steel bridges is presently being investigated. The frequencies of interest for this type of deterioration are much higher (>150 kHz) than those for prestressing wire breaks and for concrete cracking (4 to 25 kHz). Because these higher frequencies attenuate very rapidly through the acoustic medium, global monitoring of bridges would probably not be cost effective with conventional piezo- electric technology because of the sensor spacing required. However, localized monitoring of high-risk areas would be practical.

SUMMARY

The development of fast, inexpensive computing and data acquisition hardware, combined with the availability of low-cost Internet data transmission has led to development of a dependable remote continuous acoustic health monitoring system for bridges and other structures. The ability of the system to identify and locate events of interest in noisy environments has been verified for unbonded and grouted structures as well as for suspension bridges.

The information provided by the system can be used to accurately identify localized areas of deterioration in very large structures using widely distributed sensors. Systems installed prior to intrusive inspection of the structure can help to determine where to focus the inspection. Systems installed after inspection or repair can ensure that the long-term durability performance of the entire structure can be quantified. The ability of the system to determine the time and location of significant events permits confident statistical modeling of deterioration.

The adaptation of the system to provide information from other instrumentation or recording media, using the acoustic data as an acquisition "trigger", has resulted in its use as a continuous surveillance system for impact and seismic damage.

Real-time, user-defined, on-demand generation of reports using a web interface allows users immediate access to useful information from the instrumentation system.

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REFERENCES

Cullington, D.W., Hill, M.E., Woodward, R.J. and Storrar, D.B., "Special Inspections on Post- Tensioned Bridges in England: Report on Progress", FIP Symposium, London, England, September 25-27, 1996.

. Saul, R., Svensson, H.S. and Humpf, K., "Inspection and Maintenance of Cable-Stayed Bridges - German Experiences", American Concrete Institute, 1996 Fall Convention, New Orleans, Louisiana, November 3-8, 1996.

3. Woodward, R.J., Hill, M.E. and Cullington, D.W., "Non-Destructive Methods for Inspection of Post-Tensioned Concrete Bridges", FIP Symposium, London, England, September 25-27, 1996.

. Paulson, P.O., "Continuous Acoustic Nondestructive Evaluation of Unbonded Post-Tensioning Strands", Third Conference on Nondestructive Evaluation of Civil Structures and Materials, Boulder, Colorado, September 1996.

. Elliott, J.F., Schupack, M., and Greenhaus, S., "Investigation, Repair and Monitoring of Unbonded Post-Tensioned Tendons", Building to Last, Proceedings of Structures Congress XV, pp. 334 - 338, American Soc. Of Civil Engineers, New York, New York.

6. Elliott, J.F., "Monitoring Prestressed Structures" Civil Engineering, American Society of Civil Engineers, July 1996.

7. Reina, Peter, "Sensors Monitor U.K. Bridge", Engineering News Record, p. 15, McGraw Hill, New York, New York, February 15, 1999.

8. "TRL Evaluates Tendon Monitoring System", Research Focus, Issue No. 36, p.4, Institute of Civil Engineers, London, England, February 1999.

9. Paulson, P.O., "Practical Continuous Acoustic Monitoring of Suspension Bridge Cables", Transportation Research Board 78th Annual Meeting, Washington D.C., January 10 - 14, 1999.

10. Holley, M.J., Rittenhouse S., "Corrosion of Wire in Prestressed Concrete Cyclinder Pipe" ASCE Pipeline Conference, Denver, Colorado, September 18-23, 1999.

11. Elliott, J.F., "Continuous Acoustic Monitoring of Bridges", International Bridge Conference, Pittsburgh, Pennsylvania, July 13 - 16, 1999.

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FIGURE 8 - Sensor layout on grouted post- tensioned span.

FIGURE 9 - Testing of monitoring system on the Bronx-Whitestone Bridge.

FIGURE 10 - Time domain and frequency spectrum plots of wire break detected by sensor 5.0 m. from event.

FIGURE 11 - Sensor response to steel chisel FIGURE 12 - Sensor with mounting bracket. impact 6.0 m distant.

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