32nd EWGAE 85

Czech Society for Nondestructive Testing32nd European Conference on Acoustic Emission TestingPrague, Czech Republic, September 07-09, 2016

AE DEFECT EVALUATION OF THE UPPER ANCHORAGE ELEMENTS OF A STAYED BRIDGE

Saúl CRESPO1, Francisco CARRIÓN1, Juan QUINTANA1, Francisco SEPÚLVEDA1, Jorge HERNÁNDEZ1, Héctor GASCA1

1 Structural Health Monitoring, Instituto Mexicano del Transporte; Querétaro, México Phone: +52 4422169777; e-mail: screspo@imt.mx.

Abstract

The Rio Papaloapan Bridge is a stayed structure in Mexico with a 204 m main span, 8 semi-harps with 14 cables each. In June 2015, the upper anchorage element of one cable failed due to a defective welding procedure which, in turn, generated micro cracking and eventual fracture due to fatigue after 20 years of service. Thence, concern arose for the structural integrity of the bridge, not only because of failure, but also for previous problems in the constitutive material of some elements that required replacement. Since these are almost fully embedded in concrete, direct inspection of the welding was not possible and it was necessary to develop an Acoustic Emission technique to evaluate the remaining 111 elements.

This study describes the development of the inspection method, from laboratory tests simulating real conditions for calibration, to field tests for method’s tuning. The AE inspection results are presented, where elements were identified with active emissions from defects that probably will develop to failure. To validate results, welding of two pre-selected elements were exposed for penetrant and ultrasonic inspection, for a more precise correlation between defects size and the acoustic emission for future monitoring and evaluation of the bridge.

Keywords: EA Inspection, bridge, structural element, welding, steel, structural health monitoring.

1. Introducción The Rio Paploapan Bridge is a cable stayed bridge located in the state of Veracruz in Mexico. Built in 1994, it has a main span of 203 meters and a total length of 407 meters with 112 cables distributed in 8 semi-harps (Figure 1.1).

Figure 1.1 - Semi-harps in the Rio Papaloapan Bridge

The special design of the upper anchoring system was proposed by Astiz [1] and it is made of one steel plate which is welded to the anchoring elements, which are cylindrical on one side and flat on the welded side (Figure 1.2). The cylindrical side is threaded to screw the collar that holds the cable in the upper side.

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(a) Upper anchoring system design (b) Anchoring system before installation

Figure 1.2 – Anchoring assembly used in the Rio Papaloapan Bridge Since commencement, the bridge has had two failures in the anchoring elements. The first took place in January 2000 and it was due to microstructural deficiencies of the steel and it occurred in the heat affected zone (HAZ). Despite it was an excellent quality AISI 1050 steel [4], it was an outcome of a deficient casting process that resulted in a low toughness brittle material with a microstructure with large grain size (ASTM 2) and a high content of pores and inclusions [2,3]. Consequently, the 111 remaining elements were inspected with an ultrasonic technique to detect internal defects and to evaluate the microstructural grain size and, finally, 16 elements were identified as structurally deficient and changed in subsequent maintenance stage [5]. As it can be seen in figure 1.3, the access area for ultrasonic inspection is very small and limited, though the UT inspection only possible for grain size identification from the attenuation factor and internal inspection was limited within a small region; weld inspection was practically impossible.

Figure 1.3 – Condition for ultrasonic inspection of the anchoring elements

The second failure took place in June 10th, 2015. In this case, it took place in the weld interface with the anchoring element. Analysis showed that an initial crack grew due to fatigue until it reached a size of almost 65% of the cross section area [6](figure 1.4) and where clearly two different zones can be identified; the first, which was is characteristic of fatigue growth, after failure it showed oxidation on its surface, indicating that it had sufficient time to seeped water into the crack. The second, characteristic to ductile fracture, corresponded to the final break due to overload [9].

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Figure 1.4 - Upper anchoring element of cable 1 semi-harp 5 [9].

Since the weld is fully embedded in concrete, direct inspection is not possible and concrete demolition, as shown in figure 1.5, is not feasible alternative. For these reasons, it was decided to inspect with acoustic emissions (AE) the 111 remaining elements in the bridge (including the previously rehabilitated).

Figure 1.5 – Anchoring element after demolition of concrete for direct inspection [7]

2. AE Inspection MethodologyThe inspection process was developed in two steps; the first, in laboratory conditions, was to characterize the AE signal from a defective weld, using the same materials used in the bridge. The second stage, in field conditions, planned to identify external emissions and noise to set filters and inspection parameters.

2.1 Laboratory Tests To characterize the acoustic emissions form a welded element, 10 specimens (figure 2.1) were used to obtain a typical AE signal; 5 specimens were perfectly welded to simulate optimal conditions, while other 5 specimens were intentionally defective welded, simulating the conditions of the failed element in the bridge. For this case, the specimens were made with AISI 1050 steel and the welding process and material were selected according to the design specifications used for the bridge. All specimens were ultrasonic inspected to verify the expected condition of the weld. The laboratory tests were made using an INSTRON servo-hydraulic testing machine with 10 tons capacity. During the tensile tests, each specimen was instrumented with two pairs of AE sensors (PK15i and PK30i), located on the upper and lower part of the specimen, as shown in figure 2.2.

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Figure 2.1 - Specimen design for laboratory tests of AE emissions from the weld

The test load was applied with increasing steps from 1 ton to 9 tons (figure 2.2). During these test it was possible to characterize the AE from the weld of the defective specimens and compared to those emissions from the healthy specimens. As expected, the AE from the flawed welds were different from the good ones, but in the first case the energy and wave characteristics were clearly identified and characterized (figure 2.3). The good specimens presented minimum or almost none AE emissions.

Figure 2.2 - Laboratory test son welded specimens

With these results, it was possible to define a typical result that could be expected in field measurements in the bridge and to set initial inspection parameters for acquisition, filtering and analysis.

Figure 2.3 - Typical signal output obtained from the specimens with defective weld

3

2

60°

DETALLE 1

ESCALA 2:1

40.00

97.50 225.00 97.50

450.00

50.00

4X R25.00

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2.2 Instrumentation Field Program AE field inspections for the anchoring elements were done using the Physical Acoustics Sensor Highway III System. This system has 16 active channels and, as in the laboratory tests, the sensor used were the PK-15i and PK-30i models with resonance frequency of 150 kHz and 300 kHz, respectively. Again, the measurements were done using the pair of sensors on each inspection point (as indicated in figure 2.4 and figure 2.6).

Figura 2.4 – Location of sensors anchoring element

Because of the channels limitation, simultaneous measurements were possible up to 8 anchoring elements. Measurements were planned for a 13 hours period from 6 pm to 7 am, since the highest traffic and load conditions on the bridge take place during the night in week days [8]. As a consequence, for each tower, 4 sets of measurements were planned as shown in figure 2.5. It was expected to complete a tower in a week period and the full bridge in a month period, but at the end, access limitations and rehabilitation works modified this initial program. At the beginning, the access to each measurement point was done using scaffolds, but later, it was decided to hire a company for height works, as shown in figure 2.6.

Figure 2.5 – Instrumentation arraignments for the inspection of anchoring elements in one tower

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2.3 Sensor Installation Sensors were fixed to each measurement point using grease as couplant and a heavy adhesive tape. To verify the installations, a pencil lead was broken to measure the sensors response according to the ASTM F2174-02 standard [10]. This test was done after installation and at the end of the measurement, after removal, to verify that there was no change in the sensors response. If change was detected, the measurement was repeated.

Figure 2.6 - Sensor installation works.

2.4 Acquisition Parameters As mentioned, data acquisition was done during 13 continuous hours, at night, where the traffic and loads are higher on the bridge. Taking the laboratory results, the AE measurements were set with 30 dB threshold, analogic filters between 20 kHz to 400 kHz, and digital filter between 40 kHz and 400 kHz. The sampling rate was set to 2 MHz, with a wave form length of 4000 µs. The Peak Definition Time (PDT) was set to 200 µs, the Hit Definition Time (HDT) to 800 µs, and the Hit Lockout Time (HLT) to 1000 µs, with a maximum length of 1000 ms for each acoustic event. 3. Results Two main parameters were used to evaluate the AE measurements. The first is the number of AE hits, which is a measurement of the acoustic activity in the inspected element. In figure 3.1, results are shown for the measurements of the anchoring elements in tower 3, upstream side, for the complete measuring period.

Figure 3.1 - Number of acoustic hits measured in elements in tower 3, upstream side.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

T1-S

7T1

-S8

T2-S

7T3

-S7

T3-S

8T4

-S7

T4-S

8T2

-S8

T5-S

7T5

-S8

T6-S

7T6

-S8

T7-S

7T7

-S8

T8-S

7T8

-S8

T9-S

7T9

-S8

T10-

S7T1

0-S8

T11-

S7T1

1-S8

T12-

S7T1

2-S8

T13-

S7T1

3-S8

T14-

S7T1

4-S8

Aco

ustic

Hits

/13

Hou

rs

Cable anchorage elements of tower 3

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The second parameter is the elastic energy measured in each element. Figure 3.2 shows the results for the same tower 3, upstream side. This parameter not only is related to the amount of activity in the inspected element, but also, is related to the type of activity.

Figure 3.2 – Accumulated AE in elements of tower 3, upstream side, for the measuring period.

To quantify the measurement of each element it is defined a severity index (Sr) according to the intensity analysis proposed by M. K. ElBatanouny et. al. [11].

𝑆𝑆𝑆𝑆𝑟𝑟𝑟𝑟 =1

50� 𝑆𝑆𝑆𝑆𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜

𝑜𝑜𝑜𝑜=50

𝑜𝑜𝑜𝑜=2

Where Soi is the signal strength of the i-th event. In this case, the severity index is the average of the 50 signals with the highest strength measured during the 13 hours period for each element. Figure 3.3 shows the final results for the calculated severity index for all the anchoring elements in the Rio Papaloapan Bridge.

Figure 3.3 – Severity index for the inspected anchoring elements [12]

From figure 3.3, it is possible to see that the elements with the highest severity index value are those associated to cables T6-S4, T6-S3, T6-S5 and T2-S8. Specific analysis of the Duration-Amplitude of the signals measured in the corresponding elements of cables T2-S8 and T6-S5 is shown in figure 3.4. It is also important to note that the elements that were

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rehabilitated show almost none AE activity, which is expected for this case since the rehabilitation was done with a strict quality control.

Figure 3.4 – Duration-Amplitude for anchoring elements T2-S8 and T6-S5 [12]

From figure 3.4, the acoustic emissions of both elements have events over 60 dB (Zone II), which are related to events of high amplitude and short time, which can indicate micro-crack growth. At the same time, in both cases take place events with a short time growth and low energy (Zone I), which indicate evolution of small size defects in the element. 5. Conclusions The inspection through the acoustic emissions technique to evaluate the weld of the anchoring elements of the Rio Papaloapan Bridge, identified 4 critical elements with the highest severity index and which are most probably related to micro-crack growth. Since these elements will be inspected directly through ultrasonic inspection and penetrant, it will be possible to corroborate the results and to calibrate the severity index to the actual condition of the weld. These results will be valuable to define a rehabilitation strategy for the bridge and to prevent future failures through regular monitoring of the AE activity. The AE technique has shown the capacity to distinguish the weld condition of all the anchoring elements and to identify those with potential damage. The almost null AE activity measured from the rehabilitated elements is a significant proof of the effectiveness of this technique to evaluate the condition of this type of elements. 5. Acknowledgments The authors would like to thank Mr. Hector Hernandez of CAPUFE, and Mr. Luis Rojas and Mr. Martín Sandoval of Freyssinet de México, S. A. de C. V., for their support and trust for this project.

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6. References 1. Astiz M. A., “Composite Construction in Cable-Stayed Bridge Towers”, International Conference on

Composite Construction – Conventional and Innovative, Conference Report, 16-18 September, Innsbruck, Austria, pp 127-132, 1997

2. Aguirre A., Carbajal J., “Análisis en el Tirante 11 del Puente Papaloapan”, Corporación Mexicana de Investigación de Materiales S. A. de C. V., Reporte Interno AF-IFT/00087, Saltillo, Coahuila, México, 2000 (In Spanish).

3. López A., Poblano C., “Análisis de falla y Pruebas de Fatiga del Anclaje Desprendido del Tirante 11, Lado Agua, Torre 3, del Puente Río Papaloapan”, Instituto Mexicano del Transporte, Reporte Interno, EQ001/00, Sanfandila, Querétaro, México, 2000 (In Spanish).

4. ASTM, “Standard Specification for Steel Castings, High Strength, for Structural Purposes”, ASTM designation, A148/A 148M-93B (rev 98), West Conshohocken, PA, U. S. A., 2005.

5. Carrion, F. J., Lomelí, M. G., López, J. A., Pérez, J. T., Terán, J., y Jiménez, R., “Estudio para la evaluación de los dispositivos de soporte superior (botellas) de los anclajes de los tirantes del puente Río Papaloapan”, Instituto Mexicano del Transporte, Informe Final de Investigación ET-81/20013, Sanfandila, Qro., 2003 (In Spanish).

6. Terán, J., y Martínez, M., “Análisis de falla del elemento de anclaje del tirante 1 de la semi-arpa 5”, Informe de Servicios Tecnológicos E-010/2015, Instituto Mexicano del Transporte, 2015 (In Spanish).

7. López J. A., Carrión F. J., Quintana J. A., Samayoa D., Lomelí M. G., Orozco P. R., “Verification of the Ultrasonic Qualification for Structural Integrity of partially Embedded Steel Elements”, Advances Materials Research, 65, pp. 69-78, 2009.

8. Crespo Sánchez S.E., “Inspección por emisiones acústicas de elementos de anclaje”, Procedimiento 3: No. EE 23/15: “Análisis de falla del tirante 1 semi-arpa 5 y evaluación estructural de los elementos de anclaje del puente Río Papaloapan”, Instituto Mexicano del Transporte, 2016 (In Spanish).

9. Crespo Sánchez S.E., “Inspección por emisiones acústicas de elementos de anclaje”, Informe Parcial 1 Etapa 6: No. EE 23/15: “Análisis de falla del tirante 1 semi-arpa 5 y evaluación estructural de los elementos de anclaje del puente Río Papaloapan”, Instituto Mexicano del Transporte, 2016 (in Spanish).

10. ASTM F2174-02, “Standard practice for verify acoustic emission sensor response”, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States, 2015.

11. ElBatanouny M. K., Ziehl P. H., Larosche A., Mangual J., Matta F., & Nanni A., “Acoustic emission monitoring for assessment of prestressed concrete beams”, Construction and Building Materials (58), 2014.

12. Crespo Sánchez S.E., “Inspección por emisiones acústicas de elementos de anclaje”, Informe Parcial 7 Etapa 6: No. EE 23/15: Análisis de falla del tirante 1 semi-arpa 5 y evaluación estructural de los elementos de anclaje del puente Río Papaloapan, Instituto Mexicano del Transporte, 2016 (in Spanish).

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