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Acoustic emission characteristics of concrete-piles
Construction and Building Materials 13 Ž1999. 73]85
Acoustic emission characteristics of concrete-piles Tomoki Shiotani a,U , Mitsuhiro Shigeishi b, Masayasu Ohtsu b a
Technological Research Institute, Tobishima Corporation, 5472 Kimagase, Sekiyado, Higashi-katsushika, Chiba 270-0222, Japan b Department of Ci¨ il Engineering and Architecture, Kumamoto Uni¨ ersity, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
Abstract Acoustic Emission ŽAE. characteristics due to microcracking are studied in full-scale prestressed concrete-piles. By applying AE techniques, fundamental study on the fracture mechanism of the piles under both cyclic and monotonic loads is made. The prestressed concrete-piles are subjected to bending and shear loads. Crack growth is monitored, and three-dimensional Ž3-D. AE source locations are conducted along with crack classification and crack orientation by SiGMA analysis. The results obtained are compared with those of the pile integrity test ŽPIT. and visual observation. To investigate AE characteristics of damaged piles, uni-axial load is applied to the damaged RC piles installed in an experimental pit of sand. AE is monitored directly in the RC piles. In addition, indirect monitoring is carried out by using a wave-guide nearby the pile. In both the direct and indirect monitorings, AE sources are located, applying one-dimensional source location. Locations estimated are compared with the real damaged zone. Finally, conditions of AE generation of the damaged RC piles are discussed with respect to crack width and crack orientation. Q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Acoustic emission; Prestressed concrete-piles; Structural integrity
1. Introduction The great Hanshin earthquake badly hit the Kobe area of Japan in 1995. After the earthquakes, it becomes important to investigate the soundness of structures in the disaster area. In this case, investigation of superstructure could be easily conducted by visual observation, whereas with substructure, especially pilefoundation installed in deep bearing strata, it is difficult to be estimated on the soundness. So far, there exists a few methods to examine the soundness of the piles. The pile integrity test ŽPIT., which is a kind of the impact-echo test, has been widely employed. Originally, the PIT was developed as a test for quality control of piles after completion to check the adequacy of the designed length. With the increase of pile length, instrumentation of PIT is modified for low frequency characteristics less than 1 kHz. The applicability of the
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Corresponding author. Tel.: q81 471 98 7551; fax: q81 471 98 7586.
PIT to investigate the soundness of the long piles is, therefore, not confirmed yet. Acoustic emission ŽAE. is known as elastic waves emitted directly due to fracture. As for the fracture process of concrete-piles, AE events due to both crack initiation and crack growth could be observed. In the case of concrete-piles damaged, it is also expected that secondary AE events due to fretting at existing crack surfaces would be generated. Paying attention to the secondary AE signals, an applicability of the AE technique to the diagnosis of concrete-piles has been examined w1x. This paper consists of two parts. In the first half, primary AE due to fracture of the prestressed concrete piles is discussed. A fundamental study on fracture mechanism of the concrete-piles under both cyclic and monotonic loads is carried out by applying AE techniques. Full-scale prestressed high-strength concrete ŽPHC. piles are subjected to two types of bending and shear loads. Growth of cracks is monitored by AE, and three-dimensional AE source locations are analyzed along with crack classification and crack orientation by SiGMA Žsimplified Green’s function for moment tensor
0950-0618r99r$ - see front matter Q 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 1 0 - 0
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analysis . w2,3x. These results obtained are compared with the PIT results. Thus, the applicability of AE to the structural integrity of the concrete-piles is studied. In the second half, secondary AE of RC concrete piles under uni-axial compression is studied. Lightly damaged piles due to bending are installed in an experimental pit of sand. AE is directly monitored in the piles. In addition, indirect monitoring is performed by employing a newly developed wave-guide w1x. In both cases, AE source locations are achieved by onedimensional source location. The locations estimated are compared with the zone really damaged. Finally, conditions of AE generation of damaged RC piles are discussed with crack width and crack orientation. Fig. 1. A cross-section of a PHC pile.
2. Fracture of prestressed concrete piles 2.1. Experiment 2.1.1. Specimens and fracture le¨ el Fracture tests were carried out on PHC piles under cyclic and monotonic loadings of both bending and shear w4x. The piles with two fracture levels of ‘light’ and ‘heavy’ were prepared. Note that the pile of light fracture was loaded up to the stage where cracks opened under loading would close under unloading. The pile of heavy damage was loaded up to the stage where cracks were opened even under no-load conditions. Table 1 summarizes concrete piles prepared. Fig. 1 shows a cross-section of the PHC pile. The pile is 7 m long with a 400-mm outside diameter and a 270-mm internal diameter. Ten steel tendons for giving pre-stress are spaced circumferentially at a 167.5-mm radius. 2.1.2. AE monitoring system and pile integrity test Fig. 2 illustrates bending and shear fracture tests and arrangements of AE sensors. Both the bending and the shear failure were generated by four-point loading. Here, the shear failure was conducted by making the shear span short. Thus, it was expected that the bending failure would occur in the bending span between the loading points, while the shear failure would apTable 1 Concrete piles tested Type of fracture
Loading pattern
Degree of damage
Name of piles
Bending
Cyclic loading Monotonic loading
Heavy Light
M1 M2 M3
Shear
Monotonic loading Cyclic loading
Heavy
S1 S2 S3
Light
pear in the shear span between the loading points and the supports. A six-channel AE system ŽLOCAN 320: PAC. was used for the measurement of AE parameters. AE signals detected by six sensors R6 Ž60 kHz resonance. were amplified by 40 dB in the preamplifier. The signals were further amplified by 40 dB in the main amplifiers. The signals which exceeded 50 dB were recorded. Since the moment tensor analysis requires a set of six AE waveforms recorded by six independent channels for each AE event, a six-channel AE system ŽTRA-212: PAC. was employed. A total of six AE sensors were properly placed on the pile surface as shown in Fig. 2. For bending tests, AE sensors were set in the zone of bending span, while AE sensors were placed in the shear span for shear tests. A pile integrity test ŽPIT. was performed by using IT-system ŽIFCO.. The transducer was set on the one end of the pile, and a hammer was hit at the other end. These locations are also shown in Fig. 1. Here, the coupling material between the transducer and the pile was putty. 2.2. Results and discussion 2.2.1. Kaiser effect in cyclic loading Fig. 3 shows relations between cumulative AE hits and cyclic loadings. In pile M2, five loading cycles were applied, and two loading cycles were applied in pile S3. Dotted lines in the figures show the loads where the surface cracks were visually observed. By using open circles, observed crack widths at the peak loads are indicated. In the case of pile M2, approximately 115 kN was the load where cracks started to appear. In contrast, the onset of visible cracks was over 500 kN in pile S3. From the relation between the previous maximum load and the load where AE starts to increase in each loading cycle, the Felicity ratios were determined and are given in Table 2, where ‘cumulative AE ratio’ is the ratio of the accumulated AE hits under loading to that
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Fig. 2. Illustrations of fracture tests and arrangements of AE sensors.
under unloading. The Felicity ratios of smaller than one correspond to the damaged state of materials, while the ratios of greater than one indicate the sound state. In the case of pile M2, the Felicity ratio becomes smaller than one during the third loading. This implies that the pile was damaged at the second loading cycle. The cumulative AE ratio at the second loading cycle is 0.88, which means that the number of AE hit during the second unloading is compatible to that of the loading. Because AE activity under unloading indicates unstable states of materials, the damage estimated by the Felicity ratio agrees quite well with that of the cumulative AE ratio. Thus, it is noted that the cumulative AE ratio is also effective for the damage evaluation. The minimum crack width detectable by AE techniques seems to range from 0.08 to 0.30 mm. Following the third and the fourth loadings in the pile M2, the Felicity ratio and the cumulative AE ratio are similar to previous cycles. As for the pile S3, the minimum crack width detectable by the Felicity ratio seems to be within the range 0.15]0.20 mm. It agrees well with the result of the cumulative AE ratio in the first loading. Accordingly, it is concluded that the width for initiating unstable cracks is 0.08]0.30 mm for bending load, and 0.15]0.20 mm for shear load in PHC piles. As for the ultimate crack width w5,6x, it is reported that the Kaiser effect started to break down and high AE activities
were observed after the crack width exceeded 0.12]0.20 mm. These results of the reinforced concrete on the ultimate crack width are in good agreement with our results of prestressed concrete. As for characteristics of the PHC pile, it is known that a crack width smaller than 0.5 mm under loading would close due to unloading. This implies that it may be difficult to evaluate the soundness of the PHC piles from the surface crack width alone. By paying attention to the Felicity ratio and the cumulative AE ratio, the soundness of the PHC piles can be evaluated. 2.2.2. Moment tensor analysis To classify crack types and to determine crack orientations, the moment tensor analysis was applied for quantitative AE waveform analysis. Data sets of six-AE waveforms were recorded during the tests. A computer program called SiGMA was employed to analyze the set of the AE waveforms. Fig. 4 shows the results of AE source locations, crack classification and crack orientation with traced surface cracks at several stages of the bending tests. Fig. 5 shows those of the shear tests. In these figures, AE sources were projected onto the pile surface. For crack classification, tensile cracks were determined as AE events of the shear ratio smaller than 40%, mixed-mode cracks were determined as those greater than 40% and
Table 2 Chart of Felicity ratio and cumulative AE ratio in piles M2 and S3 Loading cycle
1st loading
2nd loading
3rd loading
4th loading
M2
Felicity ratio Cumulative AE ratio
] 0.14
1.00 0.88
0.92 0.83
0.90 0.69
S3
Felicity ratio Cumulative AE ratio
] 0.86
0.93 0.68
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Fig. 3. The relationship between cumulative AE hits and cyclic loadings.
smaller than 60%, and shear cracks were determined as those greater than 60%. The tensile cracks are indicated by the arrow symbol Žl., the orientation of which corresponds to the crack opening direction. The shear cracks and the mixed-mode cracks are denoted by the cross symbol Žq., two directions of which correspond to two vectors of crack motion and crack normal. For pile M1, badly damaged as a result of bending, microcracks were initially generated in the right side of the specimen as shown in M1 Ža. which leads to the visible crack as shown in M1 Žb.. Eventually cracks spread in whole area as shown in M1 Žc.. For a heavily damaged pile M2, microcracks were first observed around the bottom side of the specimen as shown in M2 Ža.. Then, the microcracks were intensively generated in the middle of the specimen together with visible cracks. Finally, it disperses. For a lightly damaged pile M3, the monotonic loading was applied, and cracks were observed throughout the specimen as shown in M3 Ža. and M3 Žb.. For a heavily damaged pile S1 under shear load, microcracks were initially observed in the shear span as shown in S1 Ža.. With the increase of load, the micro-
cracks spread in whole monitoring area as shown in S1 Žb. and S1 Žc.. As for the crack classification, the shear cracks were observed close to primary cracks in the center of the monitoring area, while the tensile cracks surround these shear cracks. For a heavily damaged pile S2, tensile cracks were firstly nucleated in the right side of the observed area as shown in S2 Ža.. Then shear cracks were observed in the left side as shown in S2 Žb.. In the case of heavily damaged piles under the shear load, shear cracks are macroscopically generated in the shear span. Actually, cracks due to bending were first observed directly below the loading points, and later the diagonal shear cracks appeared between the supports and the loading points. For a lightly damaged pile S3, the same trend as heavily damaged was confirmed, although cracks in the left side are not so active as those of heavily damaged. 2.2.3. Pile integrity test PIT was carried out at each loading level as shown in Figs. 6 and 7. Results of pile M2 heavily damaged by the bending are shown in Fig. 6. The left graphs exhibit crack traces observed at each loading level. The right
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Fig. 4. AE source locations, crack classification and crack orientation in the bending tests.
indicates records of velocity waves with time. The velocity waves are called ‘reflectogram’ in the PIT. Because the wave energy is easily attenuated with propagation, the signals are amplified by an exponential function. Additionally, these amplified signals are divided by the input wave velocity. As a result, the amplitude of the waves corresponds to the ratio of the relative velocity ŽRRV. as denoted in the graphs. In the PIT, after hitting the pile head with hammer, an elastic wave propagates through the pile. At the toe of the pile, the elastic wave partially reflects and propagates
back to the pile head as detected by a sensor. When the elastic wave propagates through the piles, other reflections occur at the locations of damage. As a result, the reflected signals at the damage spots are evaluated as negative phases of waveforms. From the arrival time of the reflected waves, the location of damage in the piles is estimated. Results are drawn by the vertical lines in the left graphs of the Figs. 6 and 7, where solid lines show clear reflections and dotted lines show ambiguous reflections. Fig. 6 Ž1. and Fig. 7 Ž1. show the results of PIT in
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Fig. 5. AE source locations, crack classification and crack orientation in the shear tests.
sound state. In Fig. 6 Ž2., clear reflection of the wave is not observed, although the crack of 0.04 mm width exists at the center. After the unloading, reflections of the wave were not observed either. From Fig. 6 Ž3. to Fig. 6 Ž7., clear reflections of the waves are still not found. At the fourth loading shown in Fig. 6 Ž8. at 170 kN and Fig. 6 Ž9. at 200 kN, reflected waves were evidently confirmed at 1.85 m and 4.26 m, and 2.19 m and 4.41 m, respectively. Although these locations estimated do not coincide with those of actual cracks, these results could indicate the approximate range of
the damage. After the unloading, the reflections of the wave disappear as shown in Fig. 6 Ž10.. In the fifth loading shown Fig. 6 Ž11., where the final fracture at 236 kN was reached, reflections of the waves are obviously observed. Even after the unloading in the pile once the final fracture was reached, distinguished reflections of the waves are still found as shown in Fig. 6 Ž12.. In the pile which was heavily damaged due to bending, it was found that rough evaluation of the integrity is possible by PIT, although precise estimation of crack locations is difficult.
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Fig. 6. Relfectogram of PIT and evaluated damage locations in heavily damaged pile M2.
Fig. 7 shows the results of pile S3 damaged lightly due to the shear load. In the figure, obvious reflections of the wave are not observed until the load reaches 642 kN as shown in Fig. 7 Ž6.. Although one crack location is evaluated, it does not agree with the locations of the actual cracks. After the unloading, there are no reflections of the wave as shown in Fig. 7 Ž8.. In the case of the lightly damaged pile due to the shear load, it is found that the application of the PIT to the diagnosis is not easy as it is similar to the case of lightly damaged pile due to the bending load as shown in Fig. 6 Ž10.. From the PIT results in the bending tests, the cracks are not successfully identified up to the loading level 236 kN in the fourth loading, where the crack width reaches 1.4 mm. Concerning AE results in the same pile, the loading level giving the unstable state of the
pile is found in the second unloading and the third loading where the maximum crack widths are 0.08 mm and 0.30 mm, respectively, as measured in the previous loading cycles. Thus, it is concluded that AE is more sensitive than the PIT. 2.2.4. Wa¨ e ¨ elocity In the case of the PIT, when the reflections of the wave from the toe of the piles are obtained, wave velocity could be estimated from the relationship between the pile length and the propagation time. Fig. 8 shows the relationship between the wave velocities and the measured crack widths in piles M1 and S2. For the bending pile M1, the velocity starts to drop at 0.08 mm crack width, and further decreases up to the crack width of approximately 0.2 mm. For the pile S2, the
Fig. 7. Reflectogram of PIT and evaluated damage locations lightly damaged in pile S3.
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Fig. 8. Wave velocity estimated by the PIT results in piles M1 and S2.
maximum crack width is measured smaller than that of the bending. However, onset of the velocity decrease is the same as the bending, and the width where the velocity steeply decreases is identical. In contrast, crack widths evaluated from the Felicity ratio and the cumulative AE ratio are 0.08]0.30 mm for the bending and 0.15]0.20 mm for the shear. It is noted that the AE results reflect quite well on those of the wave velocity.
3. Secondary AE of RC concrete-piles 3.1. Experiment 3.1.1. Piles and an experimental pit Three RC piles, 4 m long with a 300-mm diameter are prepared for the test. To apply the damage to the piles, bending load was applied prior to installing the piles in an experimental pit. The fracture due to bending resulted in the light level of the fracture. Then, the damaged piles were placed in a rubber-surrounded pit of 4.0 m = 8.0 m = 2.0 m as shown in Fig. 9. The rubber was adopted to delete undesired noise. After the pile installation, the pit was filled with river sand with compacting by a tamper in every 30-cm layer up to 3.7 m. The measured properties of the water contents and wet densities of sand were 8.8%, 1.652 gfrcm3 in the lowest layer, respectively; 4.0%, 1.520 gfrcm3 in the uppermost layer and 4.7%, 1.608 gfrcm3 on average, respectively. 3.1.2. AE monitoring AE monitoring was carried out on three piles denoted by A4, A5 and A6 as shown in Fig. 9. Fig. 10 demonstrates the arrangement of the AE sensors. The monitoring range for the piles is 2 m in length with 1 m below the head of the pile and 1 m above the end of the pile. To observe AE activity, direct and indirect monitoring were employed. In direct monitoring, four
Fig. 9. Pile arrangement in an experimental pit.
AE sensors of 60 kHz resonance were arrayed inside the pile. A wave-guide made of a polyvinyl chloride pipe of a 114-mm outside diameter and 107-mm inside diameter filled with water was used for indirect monitoring. It has been verified that the wave-guide could lead weak AE waves to the sensors efficiently. These AE waves were first radiated from the pile, propagated through the ground, and were then incident into the wave-guide w1x. The minimum distances between the outside of the wave-guides and the piles were 270 mm in pile A4 and A6, and 290 mm in pile A5. Detected AE signals at the sensors are amplified by 40 dB in sensor-embedded preamplifier, and the signals over 35 dB are measured by a 10-channnel AE monitoring
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3.1.3. Load conditions The piles are subjected to uni-axial load up to 294 kN, corresponding to approximately 15% of designed compression strength. After monitoring background noise for 3]5 min, uniaxial-load was applied to the pile head by a hydraulic jack. The load increases by 9.8 kN per min up to 294 kN, and remains constant until no AE activity was observed, and then it decreases by 49 kN per min. This load application was repeated three times on each pile. Settlements and lateral displacements of the pile head are measured by using four displacement gauges with an accuracy of 1r100 mm up to 100 mm. Two pairs of two gauges were placed at every two points on the head and on the side of the pile. 3.2. Results and discussion
Fig. 10. Arrangement of AE sensors.
system ŽMISTRAS: PAC. with 1 MHz sampling time and 2 kwords wavelength.
3.2.1. Crack obser¨ ation Fig. 11 sketches cracks observed after applying the bending load in piles A4, A5 and A6. Two views of observation from side ŽA. and ŽB. are illustrated. The distances Žm. between cracks and the pile head are indicated with crack widths Žmm. in the parentheses indicated by open circles. Pile A4 had only one crack of 0]0.6 mm width located 2.1 m from the pile head. Pile A5 had two cracks of 0.4]6.0 mm and 1.0]3.0 mm widths, located 1.3 m and 1.7 m away from the pile head, respectively. Pile A6 had three cracks of 0.1]4.0 mm, 0.4]1.2 mm and 0.2]4.0 mm widths located 1.2 m, 1.4 m and 3.0 m from the pile head, respectively.
Fig. 11. Sketch of cracks observed.
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3.2.2. Lateral displacement of pile head Fig. 12 shows the result of lateral displacement in Xand Y-directions with cyclic loads. In pile A4, elastic behavior between displacement and load is observed. In pile A5, the displacement apparently increased in the first loading cycle. As for pile A4 and pile A5, it is found that maximum displacements reached at the previous loading stage would not exceed their maximum displacement during unloading stages. In contrast, for pile A6 the displacement obviously increased not only during the first loading stage, but also during the unloading. The increase was also confirmed during the second and the third loading. Thus, this implies that the macroscopic fracture accompanied by plastic deformation would be generated in the pile A6. Remarkable AE activity from existing cracks is not expected due to elastic deformation in pile A4. AE gener-
ation due to noticeable deformation is generated in the first loading cycle in pile A5. 3.2.3. AE source location during first loading cycle Fig. 13 shows the results of AE source location during the first loading cycle. Open circles represent AE sources obtained by direct monitoring and gray circles correspond to results obtained from indirect monitoring with the wave-guides. Note that the diameter of the circle reflects the magnitude of the ringdown count of the AE source. As a result of pile A4, AE source locations at 1.0 and 1.5 m deep by direct monitoring do not correspond with the actual crack location. This implies that those invisible cracks may be produced when the initial damage is a result of bending. AE sources distribute by indirect AE monitoring a whole monitoring range, especially AE sources which
Fig. 12. Lateral displacement in the X- and Y-direction with cyclic loads.
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are intensively observed at the lower portion of the pile. As a result, this suggests that cracks were being closed at onset of the first loading, and subsequently, the stress was concentrated on the damaged region, nucleating invisible cracks. With regard to pile A5, AE source locations by direct monitoring were almost identical with the crack locations, though AE activity was not so active. Results of AE sources by indirect monitoring displayed a similar trend to that of pile A4. In the case of pile A6, a large number of AE sources were detected by direct monitoring, which agreed well with the crack locations. Agreement of the locations was also found in the following second and third loading cycles. The locations of AE sources by indirect monitoring were also coincident with crack locations. It was concluded here that there was no relation between AE sources detected by indirect monitoring and those due to fretting at existing crack surfaces, because AE sources by indirect monitoring were observed in the middle of the pile where there were no damages, no AE sources by direct monitoring were determined, and the differences of AE arrival times between AE waves by indirect and those by direct monitoring were out of
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the allowable time difference determined from wave propagation distances and wave velocities. 3.2.4. AE acti¨ ity based on Kaiser effect The number of AE events observed were 11, 2 and 0 in pile A4 and 8, 0 and 0 in pile A5, during the first, the second and the third loading cycles, respectively. It means that there were approximately no emissions in the second and the third loading. Based on the Kaiser effect, these piles are estimated as sound state. Here, the result of pile A6 is discussed, because a large number of AE events were obtained. Fig. 14Ža. exhibits the relationship between cumulative AE events and elapsed time by direct monitoring. Fig. 14Žb. shows results of indirect monitoring. From Fig. 14Ža., high AE activity during the first loading cycle and rapid AE generation at the load of 276.4 kN in the second and the third cycles are found. The loads where AE started to appear are lower than the previous maximum loads. Thus, the pile A6 is estimated as an unstable condition. On the contrary, AE activity in the first loading cycle is also observed during the second and the third loading in Fig. 14Žb.. It seems that there is no relation between
Fig. 13. AE source location during the first cycle.
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Fig. 15. Detailed results of crack observation.
tion. The locations of AE sources in pile A5 and pile A6 were almost identical to the locations of cracks. Additionally, AE sources obtained in pile A6 intensively appeared at the middle and the lowest cracks. Consequently, it is found that the secondary AE activity is strongly dependent on the inclination of crack rather than the crack width. 3.3. Conclusion Fig. 14. The relationship between cumulative AE events and elapsed time with repeated load. Ža. AE events by direct monitoring. Žb. AE events by indirect monitoring.
the AE activity by indirect monitoring, the previous maximum loads and AE activity of the direct monitoring. This result reflects the finding in a previous section that there was no relation between AE sources detected by indirect monitoring and those due to fretting at existing crack surfaces. 3.2.5. Relationship among crack widths, crack inclination and AE acti¨ ity Fig. 15 demonstrates the detailed results of crack observation in the piles. It was found that the pile A4 had a crack in the horizontal direction, pile A5 has two diagonal cracks the gaps of which are 20 and 70 mm. For pile A6, three cracks of 10, 50 and 170 mm gaps are observed. These characteristics of cracks mean that the crack would close in pile A4 due to homogeneous stress distribution. As for the piles A5 and A6, the load would concentrate on a small part of the crack surface due to tilting of the cracks. Particularly, the lower crack in pile A5 and the lowest and the middle cracks in pile A6 are steeply inclined. This implies that high AE activity would be produced. Examining the relationship between the AE activity by direct monitoring and crack properties, it is found that no AE activity in pile A4 was in good agreement with the effect of crack inclina-
To examine characteristics of primary AE on concrete piles, AE generated due to bending and shear fracture was studied in prestressed concrete piles. Concerning secondary AE in damaged RC piles, a uni-axial load was applied in the pit of sand. The results are summarized as follows: 1. The crack width of unstable failure evaluated by the AE technique was varied between 0.08 and 0.30 mm under the bending load, while it was 0.15]0.20 mm due to the shear load. These results were compatible with those of reinforced concrete. 2. Because the crack width smaller than 0.5 mm under loading, would close after unloading in PHC piles, it is difficult to evaluate the soundness of the piles from only the width of surface cracks. By paying attention to the Felicity ratio and the cumulative AE ratio, the soundness of the PHC piles could be evaluated. 3. In the heavily damaged piles due to bending, rough evaluation of the integrity is possible by PIT, whereas precise estimation of crack location is not easy. 4. In the lightly damaged piles due to both shear and bending loads, the PIT is not readily applicable to the structural integrity. To evaluate the damage of piles, AE was by far more sensitive than PIT. In addition, AE results reflected quite well with those of the wave velocity.
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5. AE radiation from cracks of lightly damaged RC piles was observed. The locations of AE sources by indirect monitoring accorded with crack locations, although there was no relation between AE sources detected by indirect monitoring and those due to fretting at existing crack surfaces. 6. Pile integrity is possibly evaluated by examining AE activity by direct monitoring based on the Kaiser effect. 7. The secondary AE activity is strongly dependent on the inclination of crack rather than the crack width.
Acknowledgements The experiments on RC piles were achieved by the joint research with PWRI ŽPublic Works Research Institute . of Ministry of Construction. The authors would like to express their gratitude to all those who were involved in this research.
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References w1x Shiotani T, Sakaino N, Ohtsu M, Shigeishi M. Damage Diagnosis of concrete piles after earthquakes by acoustic emission. Proceedings Fourth Far East Conference on Non-destructive Testing, KSNT, 1997:579]588. w2x Ohtsu M. Simplified moment tensor analysis and unified decomposition of AE source. J Geophys Res 1991;96:6211]6221. w3x Yuyama S, Okamoto T, Shigeishi M, Ohtsu M. Quantitative evaluation and visualization of cracking process in reinforced concrete by a moment tensor analysis of acoustic emission. Mater Eval ASNT 1994;53:751]756. w4x Shiotani T, Sakaino N, Shigeishi M, Ohtsu M, Asai Y, Hayashi T. AE Characteristics of full-scale concrete-piles under bending and shear load. ASNT, Paper Summaries of the 1998 Sixth International Symposium on Acoustic Emission from Composite Materials, 1998:163]172. w5x Yuyama S, Okamoto T, Shigeishi M, Ohtsu M. Acoustic emission generated in corners of reinforced concrete rigid frame under cyclic loading. Mater Eval ASNT 1994;53:409]412. w6x Yuyama S, Okamoto T, Shigeishi M, Ohtsu M. Cracking progress evaluation in reinforced concrete by moment tensor analysis of acoustic emission. J Acoust Emission 1995;13:s14]s20.
Acoustic emission characteristics of concrete-piles Tomoki Shiotani a,U , Mitsuhiro Shigeishi b, Masayasu Ohtsu b a
Technological Research Institute, Tobishima Corporation, 5472 Kimagase, Sekiyado, Higashi-katsushika, Chiba 270-0222, Japan b Department of Ci¨ il Engineering and Architecture, Kumamoto Uni¨ ersity, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
Abstract Acoustic Emission ŽAE. characteristics due to microcracking are studied in full-scale prestressed concrete-piles. By applying AE techniques, fundamental study on the fracture mechanism of the piles under both cyclic and monotonic loads is made. The prestressed concrete-piles are subjected to bending and shear loads. Crack growth is monitored, and three-dimensional Ž3-D. AE source locations are conducted along with crack classification and crack orientation by SiGMA analysis. The results obtained are compared with those of the pile integrity test ŽPIT. and visual observation. To investigate AE characteristics of damaged piles, uni-axial load is applied to the damaged RC piles installed in an experimental pit of sand. AE is monitored directly in the RC piles. In addition, indirect monitoring is carried out by using a wave-guide nearby the pile. In both the direct and indirect monitorings, AE sources are located, applying one-dimensional source location. Locations estimated are compared with the real damaged zone. Finally, conditions of AE generation of the damaged RC piles are discussed with respect to crack width and crack orientation. Q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Acoustic emission; Prestressed concrete-piles; Structural integrity
1. Introduction The great Hanshin earthquake badly hit the Kobe area of Japan in 1995. After the earthquakes, it becomes important to investigate the soundness of structures in the disaster area. In this case, investigation of superstructure could be easily conducted by visual observation, whereas with substructure, especially pilefoundation installed in deep bearing strata, it is difficult to be estimated on the soundness. So far, there exists a few methods to examine the soundness of the piles. The pile integrity test ŽPIT., which is a kind of the impact-echo test, has been widely employed. Originally, the PIT was developed as a test for quality control of piles after completion to check the adequacy of the designed length. With the increase of pile length, instrumentation of PIT is modified for low frequency characteristics less than 1 kHz. The applicability of the
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Corresponding author. Tel.: q81 471 98 7551; fax: q81 471 98 7586.
PIT to investigate the soundness of the long piles is, therefore, not confirmed yet. Acoustic emission ŽAE. is known as elastic waves emitted directly due to fracture. As for the fracture process of concrete-piles, AE events due to both crack initiation and crack growth could be observed. In the case of concrete-piles damaged, it is also expected that secondary AE events due to fretting at existing crack surfaces would be generated. Paying attention to the secondary AE signals, an applicability of the AE technique to the diagnosis of concrete-piles has been examined w1x. This paper consists of two parts. In the first half, primary AE due to fracture of the prestressed concrete piles is discussed. A fundamental study on fracture mechanism of the concrete-piles under both cyclic and monotonic loads is carried out by applying AE techniques. Full-scale prestressed high-strength concrete ŽPHC. piles are subjected to two types of bending and shear loads. Growth of cracks is monitored by AE, and three-dimensional AE source locations are analyzed along with crack classification and crack orientation by SiGMA Žsimplified Green’s function for moment tensor
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analysis . w2,3x. These results obtained are compared with the PIT results. Thus, the applicability of AE to the structural integrity of the concrete-piles is studied. In the second half, secondary AE of RC concrete piles under uni-axial compression is studied. Lightly damaged piles due to bending are installed in an experimental pit of sand. AE is directly monitored in the piles. In addition, indirect monitoring is performed by employing a newly developed wave-guide w1x. In both cases, AE source locations are achieved by onedimensional source location. The locations estimated are compared with the zone really damaged. Finally, conditions of AE generation of damaged RC piles are discussed with crack width and crack orientation. Fig. 1. A cross-section of a PHC pile.
2. Fracture of prestressed concrete piles 2.1. Experiment 2.1.1. Specimens and fracture le¨ el Fracture tests were carried out on PHC piles under cyclic and monotonic loadings of both bending and shear w4x. The piles with two fracture levels of ‘light’ and ‘heavy’ were prepared. Note that the pile of light fracture was loaded up to the stage where cracks opened under loading would close under unloading. The pile of heavy damage was loaded up to the stage where cracks were opened even under no-load conditions. Table 1 summarizes concrete piles prepared. Fig. 1 shows a cross-section of the PHC pile. The pile is 7 m long with a 400-mm outside diameter and a 270-mm internal diameter. Ten steel tendons for giving pre-stress are spaced circumferentially at a 167.5-mm radius. 2.1.2. AE monitoring system and pile integrity test Fig. 2 illustrates bending and shear fracture tests and arrangements of AE sensors. Both the bending and the shear failure were generated by four-point loading. Here, the shear failure was conducted by making the shear span short. Thus, it was expected that the bending failure would occur in the bending span between the loading points, while the shear failure would apTable 1 Concrete piles tested Type of fracture
Loading pattern
Degree of damage
Name of piles
Bending
Cyclic loading Monotonic loading
Heavy Light
M1 M2 M3
Shear
Monotonic loading Cyclic loading
Heavy
S1 S2 S3
Light
pear in the shear span between the loading points and the supports. A six-channel AE system ŽLOCAN 320: PAC. was used for the measurement of AE parameters. AE signals detected by six sensors R6 Ž60 kHz resonance. were amplified by 40 dB in the preamplifier. The signals were further amplified by 40 dB in the main amplifiers. The signals which exceeded 50 dB were recorded. Since the moment tensor analysis requires a set of six AE waveforms recorded by six independent channels for each AE event, a six-channel AE system ŽTRA-212: PAC. was employed. A total of six AE sensors were properly placed on the pile surface as shown in Fig. 2. For bending tests, AE sensors were set in the zone of bending span, while AE sensors were placed in the shear span for shear tests. A pile integrity test ŽPIT. was performed by using IT-system ŽIFCO.. The transducer was set on the one end of the pile, and a hammer was hit at the other end. These locations are also shown in Fig. 1. Here, the coupling material between the transducer and the pile was putty. 2.2. Results and discussion 2.2.1. Kaiser effect in cyclic loading Fig. 3 shows relations between cumulative AE hits and cyclic loadings. In pile M2, five loading cycles were applied, and two loading cycles were applied in pile S3. Dotted lines in the figures show the loads where the surface cracks were visually observed. By using open circles, observed crack widths at the peak loads are indicated. In the case of pile M2, approximately 115 kN was the load where cracks started to appear. In contrast, the onset of visible cracks was over 500 kN in pile S3. From the relation between the previous maximum load and the load where AE starts to increase in each loading cycle, the Felicity ratios were determined and are given in Table 2, where ‘cumulative AE ratio’ is the ratio of the accumulated AE hits under loading to that
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Fig. 2. Illustrations of fracture tests and arrangements of AE sensors.
under unloading. The Felicity ratios of smaller than one correspond to the damaged state of materials, while the ratios of greater than one indicate the sound state. In the case of pile M2, the Felicity ratio becomes smaller than one during the third loading. This implies that the pile was damaged at the second loading cycle. The cumulative AE ratio at the second loading cycle is 0.88, which means that the number of AE hit during the second unloading is compatible to that of the loading. Because AE activity under unloading indicates unstable states of materials, the damage estimated by the Felicity ratio agrees quite well with that of the cumulative AE ratio. Thus, it is noted that the cumulative AE ratio is also effective for the damage evaluation. The minimum crack width detectable by AE techniques seems to range from 0.08 to 0.30 mm. Following the third and the fourth loadings in the pile M2, the Felicity ratio and the cumulative AE ratio are similar to previous cycles. As for the pile S3, the minimum crack width detectable by the Felicity ratio seems to be within the range 0.15]0.20 mm. It agrees well with the result of the cumulative AE ratio in the first loading. Accordingly, it is concluded that the width for initiating unstable cracks is 0.08]0.30 mm for bending load, and 0.15]0.20 mm for shear load in PHC piles. As for the ultimate crack width w5,6x, it is reported that the Kaiser effect started to break down and high AE activities
were observed after the crack width exceeded 0.12]0.20 mm. These results of the reinforced concrete on the ultimate crack width are in good agreement with our results of prestressed concrete. As for characteristics of the PHC pile, it is known that a crack width smaller than 0.5 mm under loading would close due to unloading. This implies that it may be difficult to evaluate the soundness of the PHC piles from the surface crack width alone. By paying attention to the Felicity ratio and the cumulative AE ratio, the soundness of the PHC piles can be evaluated. 2.2.2. Moment tensor analysis To classify crack types and to determine crack orientations, the moment tensor analysis was applied for quantitative AE waveform analysis. Data sets of six-AE waveforms were recorded during the tests. A computer program called SiGMA was employed to analyze the set of the AE waveforms. Fig. 4 shows the results of AE source locations, crack classification and crack orientation with traced surface cracks at several stages of the bending tests. Fig. 5 shows those of the shear tests. In these figures, AE sources were projected onto the pile surface. For crack classification, tensile cracks were determined as AE events of the shear ratio smaller than 40%, mixed-mode cracks were determined as those greater than 40% and
Table 2 Chart of Felicity ratio and cumulative AE ratio in piles M2 and S3 Loading cycle
1st loading
2nd loading
3rd loading
4th loading
M2
Felicity ratio Cumulative AE ratio
] 0.14
1.00 0.88
0.92 0.83
0.90 0.69
S3
Felicity ratio Cumulative AE ratio
] 0.86
0.93 0.68
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Fig. 3. The relationship between cumulative AE hits and cyclic loadings.
smaller than 60%, and shear cracks were determined as those greater than 60%. The tensile cracks are indicated by the arrow symbol Žl., the orientation of which corresponds to the crack opening direction. The shear cracks and the mixed-mode cracks are denoted by the cross symbol Žq., two directions of which correspond to two vectors of crack motion and crack normal. For pile M1, badly damaged as a result of bending, microcracks were initially generated in the right side of the specimen as shown in M1 Ža. which leads to the visible crack as shown in M1 Žb.. Eventually cracks spread in whole area as shown in M1 Žc.. For a heavily damaged pile M2, microcracks were first observed around the bottom side of the specimen as shown in M2 Ža.. Then, the microcracks were intensively generated in the middle of the specimen together with visible cracks. Finally, it disperses. For a lightly damaged pile M3, the monotonic loading was applied, and cracks were observed throughout the specimen as shown in M3 Ža. and M3 Žb.. For a heavily damaged pile S1 under shear load, microcracks were initially observed in the shear span as shown in S1 Ža.. With the increase of load, the micro-
cracks spread in whole monitoring area as shown in S1 Žb. and S1 Žc.. As for the crack classification, the shear cracks were observed close to primary cracks in the center of the monitoring area, while the tensile cracks surround these shear cracks. For a heavily damaged pile S2, tensile cracks were firstly nucleated in the right side of the observed area as shown in S2 Ža.. Then shear cracks were observed in the left side as shown in S2 Žb.. In the case of heavily damaged piles under the shear load, shear cracks are macroscopically generated in the shear span. Actually, cracks due to bending were first observed directly below the loading points, and later the diagonal shear cracks appeared between the supports and the loading points. For a lightly damaged pile S3, the same trend as heavily damaged was confirmed, although cracks in the left side are not so active as those of heavily damaged. 2.2.3. Pile integrity test PIT was carried out at each loading level as shown in Figs. 6 and 7. Results of pile M2 heavily damaged by the bending are shown in Fig. 6. The left graphs exhibit crack traces observed at each loading level. The right
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Fig. 4. AE source locations, crack classification and crack orientation in the bending tests.
indicates records of velocity waves with time. The velocity waves are called ‘reflectogram’ in the PIT. Because the wave energy is easily attenuated with propagation, the signals are amplified by an exponential function. Additionally, these amplified signals are divided by the input wave velocity. As a result, the amplitude of the waves corresponds to the ratio of the relative velocity ŽRRV. as denoted in the graphs. In the PIT, after hitting the pile head with hammer, an elastic wave propagates through the pile. At the toe of the pile, the elastic wave partially reflects and propagates
back to the pile head as detected by a sensor. When the elastic wave propagates through the piles, other reflections occur at the locations of damage. As a result, the reflected signals at the damage spots are evaluated as negative phases of waveforms. From the arrival time of the reflected waves, the location of damage in the piles is estimated. Results are drawn by the vertical lines in the left graphs of the Figs. 6 and 7, where solid lines show clear reflections and dotted lines show ambiguous reflections. Fig. 6 Ž1. and Fig. 7 Ž1. show the results of PIT in
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Fig. 5. AE source locations, crack classification and crack orientation in the shear tests.
sound state. In Fig. 6 Ž2., clear reflection of the wave is not observed, although the crack of 0.04 mm width exists at the center. After the unloading, reflections of the wave were not observed either. From Fig. 6 Ž3. to Fig. 6 Ž7., clear reflections of the waves are still not found. At the fourth loading shown in Fig. 6 Ž8. at 170 kN and Fig. 6 Ž9. at 200 kN, reflected waves were evidently confirmed at 1.85 m and 4.26 m, and 2.19 m and 4.41 m, respectively. Although these locations estimated do not coincide with those of actual cracks, these results could indicate the approximate range of
the damage. After the unloading, the reflections of the wave disappear as shown in Fig. 6 Ž10.. In the fifth loading shown Fig. 6 Ž11., where the final fracture at 236 kN was reached, reflections of the waves are obviously observed. Even after the unloading in the pile once the final fracture was reached, distinguished reflections of the waves are still found as shown in Fig. 6 Ž12.. In the pile which was heavily damaged due to bending, it was found that rough evaluation of the integrity is possible by PIT, although precise estimation of crack locations is difficult.
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Fig. 6. Relfectogram of PIT and evaluated damage locations in heavily damaged pile M2.
Fig. 7 shows the results of pile S3 damaged lightly due to the shear load. In the figure, obvious reflections of the wave are not observed until the load reaches 642 kN as shown in Fig. 7 Ž6.. Although one crack location is evaluated, it does not agree with the locations of the actual cracks. After the unloading, there are no reflections of the wave as shown in Fig. 7 Ž8.. In the case of the lightly damaged pile due to the shear load, it is found that the application of the PIT to the diagnosis is not easy as it is similar to the case of lightly damaged pile due to the bending load as shown in Fig. 6 Ž10.. From the PIT results in the bending tests, the cracks are not successfully identified up to the loading level 236 kN in the fourth loading, where the crack width reaches 1.4 mm. Concerning AE results in the same pile, the loading level giving the unstable state of the
pile is found in the second unloading and the third loading where the maximum crack widths are 0.08 mm and 0.30 mm, respectively, as measured in the previous loading cycles. Thus, it is concluded that AE is more sensitive than the PIT. 2.2.4. Wa¨ e ¨ elocity In the case of the PIT, when the reflections of the wave from the toe of the piles are obtained, wave velocity could be estimated from the relationship between the pile length and the propagation time. Fig. 8 shows the relationship between the wave velocities and the measured crack widths in piles M1 and S2. For the bending pile M1, the velocity starts to drop at 0.08 mm crack width, and further decreases up to the crack width of approximately 0.2 mm. For the pile S2, the
Fig. 7. Reflectogram of PIT and evaluated damage locations lightly damaged in pile S3.
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Fig. 8. Wave velocity estimated by the PIT results in piles M1 and S2.
maximum crack width is measured smaller than that of the bending. However, onset of the velocity decrease is the same as the bending, and the width where the velocity steeply decreases is identical. In contrast, crack widths evaluated from the Felicity ratio and the cumulative AE ratio are 0.08]0.30 mm for the bending and 0.15]0.20 mm for the shear. It is noted that the AE results reflect quite well on those of the wave velocity.
3. Secondary AE of RC concrete-piles 3.1. Experiment 3.1.1. Piles and an experimental pit Three RC piles, 4 m long with a 300-mm diameter are prepared for the test. To apply the damage to the piles, bending load was applied prior to installing the piles in an experimental pit. The fracture due to bending resulted in the light level of the fracture. Then, the damaged piles were placed in a rubber-surrounded pit of 4.0 m = 8.0 m = 2.0 m as shown in Fig. 9. The rubber was adopted to delete undesired noise. After the pile installation, the pit was filled with river sand with compacting by a tamper in every 30-cm layer up to 3.7 m. The measured properties of the water contents and wet densities of sand were 8.8%, 1.652 gfrcm3 in the lowest layer, respectively; 4.0%, 1.520 gfrcm3 in the uppermost layer and 4.7%, 1.608 gfrcm3 on average, respectively. 3.1.2. AE monitoring AE monitoring was carried out on three piles denoted by A4, A5 and A6 as shown in Fig. 9. Fig. 10 demonstrates the arrangement of the AE sensors. The monitoring range for the piles is 2 m in length with 1 m below the head of the pile and 1 m above the end of the pile. To observe AE activity, direct and indirect monitoring were employed. In direct monitoring, four
Fig. 9. Pile arrangement in an experimental pit.
AE sensors of 60 kHz resonance were arrayed inside the pile. A wave-guide made of a polyvinyl chloride pipe of a 114-mm outside diameter and 107-mm inside diameter filled with water was used for indirect monitoring. It has been verified that the wave-guide could lead weak AE waves to the sensors efficiently. These AE waves were first radiated from the pile, propagated through the ground, and were then incident into the wave-guide w1x. The minimum distances between the outside of the wave-guides and the piles were 270 mm in pile A4 and A6, and 290 mm in pile A5. Detected AE signals at the sensors are amplified by 40 dB in sensor-embedded preamplifier, and the signals over 35 dB are measured by a 10-channnel AE monitoring
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3.1.3. Load conditions The piles are subjected to uni-axial load up to 294 kN, corresponding to approximately 15% of designed compression strength. After monitoring background noise for 3]5 min, uniaxial-load was applied to the pile head by a hydraulic jack. The load increases by 9.8 kN per min up to 294 kN, and remains constant until no AE activity was observed, and then it decreases by 49 kN per min. This load application was repeated three times on each pile. Settlements and lateral displacements of the pile head are measured by using four displacement gauges with an accuracy of 1r100 mm up to 100 mm. Two pairs of two gauges were placed at every two points on the head and on the side of the pile. 3.2. Results and discussion
Fig. 10. Arrangement of AE sensors.
system ŽMISTRAS: PAC. with 1 MHz sampling time and 2 kwords wavelength.
3.2.1. Crack obser¨ ation Fig. 11 sketches cracks observed after applying the bending load in piles A4, A5 and A6. Two views of observation from side ŽA. and ŽB. are illustrated. The distances Žm. between cracks and the pile head are indicated with crack widths Žmm. in the parentheses indicated by open circles. Pile A4 had only one crack of 0]0.6 mm width located 2.1 m from the pile head. Pile A5 had two cracks of 0.4]6.0 mm and 1.0]3.0 mm widths, located 1.3 m and 1.7 m away from the pile head, respectively. Pile A6 had three cracks of 0.1]4.0 mm, 0.4]1.2 mm and 0.2]4.0 mm widths located 1.2 m, 1.4 m and 3.0 m from the pile head, respectively.
Fig. 11. Sketch of cracks observed.
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3.2.2. Lateral displacement of pile head Fig. 12 shows the result of lateral displacement in Xand Y-directions with cyclic loads. In pile A4, elastic behavior between displacement and load is observed. In pile A5, the displacement apparently increased in the first loading cycle. As for pile A4 and pile A5, it is found that maximum displacements reached at the previous loading stage would not exceed their maximum displacement during unloading stages. In contrast, for pile A6 the displacement obviously increased not only during the first loading stage, but also during the unloading. The increase was also confirmed during the second and the third loading. Thus, this implies that the macroscopic fracture accompanied by plastic deformation would be generated in the pile A6. Remarkable AE activity from existing cracks is not expected due to elastic deformation in pile A4. AE gener-
ation due to noticeable deformation is generated in the first loading cycle in pile A5. 3.2.3. AE source location during first loading cycle Fig. 13 shows the results of AE source location during the first loading cycle. Open circles represent AE sources obtained by direct monitoring and gray circles correspond to results obtained from indirect monitoring with the wave-guides. Note that the diameter of the circle reflects the magnitude of the ringdown count of the AE source. As a result of pile A4, AE source locations at 1.0 and 1.5 m deep by direct monitoring do not correspond with the actual crack location. This implies that those invisible cracks may be produced when the initial damage is a result of bending. AE sources distribute by indirect AE monitoring a whole monitoring range, especially AE sources which
Fig. 12. Lateral displacement in the X- and Y-direction with cyclic loads.
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are intensively observed at the lower portion of the pile. As a result, this suggests that cracks were being closed at onset of the first loading, and subsequently, the stress was concentrated on the damaged region, nucleating invisible cracks. With regard to pile A5, AE source locations by direct monitoring were almost identical with the crack locations, though AE activity was not so active. Results of AE sources by indirect monitoring displayed a similar trend to that of pile A4. In the case of pile A6, a large number of AE sources were detected by direct monitoring, which agreed well with the crack locations. Agreement of the locations was also found in the following second and third loading cycles. The locations of AE sources by indirect monitoring were also coincident with crack locations. It was concluded here that there was no relation between AE sources detected by indirect monitoring and those due to fretting at existing crack surfaces, because AE sources by indirect monitoring were observed in the middle of the pile where there were no damages, no AE sources by direct monitoring were determined, and the differences of AE arrival times between AE waves by indirect and those by direct monitoring were out of
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the allowable time difference determined from wave propagation distances and wave velocities. 3.2.4. AE acti¨ ity based on Kaiser effect The number of AE events observed were 11, 2 and 0 in pile A4 and 8, 0 and 0 in pile A5, during the first, the second and the third loading cycles, respectively. It means that there were approximately no emissions in the second and the third loading. Based on the Kaiser effect, these piles are estimated as sound state. Here, the result of pile A6 is discussed, because a large number of AE events were obtained. Fig. 14Ža. exhibits the relationship between cumulative AE events and elapsed time by direct monitoring. Fig. 14Žb. shows results of indirect monitoring. From Fig. 14Ža., high AE activity during the first loading cycle and rapid AE generation at the load of 276.4 kN in the second and the third cycles are found. The loads where AE started to appear are lower than the previous maximum loads. Thus, the pile A6 is estimated as an unstable condition. On the contrary, AE activity in the first loading cycle is also observed during the second and the third loading in Fig. 14Žb.. It seems that there is no relation between
Fig. 13. AE source location during the first cycle.
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Fig. 15. Detailed results of crack observation.
tion. The locations of AE sources in pile A5 and pile A6 were almost identical to the locations of cracks. Additionally, AE sources obtained in pile A6 intensively appeared at the middle and the lowest cracks. Consequently, it is found that the secondary AE activity is strongly dependent on the inclination of crack rather than the crack width. 3.3. Conclusion Fig. 14. The relationship between cumulative AE events and elapsed time with repeated load. Ža. AE events by direct monitoring. Žb. AE events by indirect monitoring.
the AE activity by indirect monitoring, the previous maximum loads and AE activity of the direct monitoring. This result reflects the finding in a previous section that there was no relation between AE sources detected by indirect monitoring and those due to fretting at existing crack surfaces. 3.2.5. Relationship among crack widths, crack inclination and AE acti¨ ity Fig. 15 demonstrates the detailed results of crack observation in the piles. It was found that the pile A4 had a crack in the horizontal direction, pile A5 has two diagonal cracks the gaps of which are 20 and 70 mm. For pile A6, three cracks of 10, 50 and 170 mm gaps are observed. These characteristics of cracks mean that the crack would close in pile A4 due to homogeneous stress distribution. As for the piles A5 and A6, the load would concentrate on a small part of the crack surface due to tilting of the cracks. Particularly, the lower crack in pile A5 and the lowest and the middle cracks in pile A6 are steeply inclined. This implies that high AE activity would be produced. Examining the relationship between the AE activity by direct monitoring and crack properties, it is found that no AE activity in pile A4 was in good agreement with the effect of crack inclina-
To examine characteristics of primary AE on concrete piles, AE generated due to bending and shear fracture was studied in prestressed concrete piles. Concerning secondary AE in damaged RC piles, a uni-axial load was applied in the pit of sand. The results are summarized as follows: 1. The crack width of unstable failure evaluated by the AE technique was varied between 0.08 and 0.30 mm under the bending load, while it was 0.15]0.20 mm due to the shear load. These results were compatible with those of reinforced concrete. 2. Because the crack width smaller than 0.5 mm under loading, would close after unloading in PHC piles, it is difficult to evaluate the soundness of the piles from only the width of surface cracks. By paying attention to the Felicity ratio and the cumulative AE ratio, the soundness of the PHC piles could be evaluated. 3. In the heavily damaged piles due to bending, rough evaluation of the integrity is possible by PIT, whereas precise estimation of crack location is not easy. 4. In the lightly damaged piles due to both shear and bending loads, the PIT is not readily applicable to the structural integrity. To evaluate the damage of piles, AE was by far more sensitive than PIT. In addition, AE results reflected quite well with those of the wave velocity.
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5. AE radiation from cracks of lightly damaged RC piles was observed. The locations of AE sources by indirect monitoring accorded with crack locations, although there was no relation between AE sources detected by indirect monitoring and those due to fretting at existing crack surfaces. 6. Pile integrity is possibly evaluated by examining AE activity by direct monitoring based on the Kaiser effect. 7. The secondary AE activity is strongly dependent on the inclination of crack rather than the crack width.
Acknowledgements The experiments on RC piles were achieved by the joint research with PWRI ŽPublic Works Research Institute . of Ministry of Construction. The authors would like to express their gratitude to all those who were involved in this research.
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References w1x Shiotani T, Sakaino N, Ohtsu M, Shigeishi M. Damage Diagnosis of concrete piles after earthquakes by acoustic emission. Proceedings Fourth Far East Conference on Non-destructive Testing, KSNT, 1997:579]588. w2x Ohtsu M. Simplified moment tensor analysis and unified decomposition of AE source. J Geophys Res 1991;96:6211]6221. w3x Yuyama S, Okamoto T, Shigeishi M, Ohtsu M. Quantitative evaluation and visualization of cracking process in reinforced concrete by a moment tensor analysis of acoustic emission. Mater Eval ASNT 1994;53:751]756. w4x Shiotani T, Sakaino N, Shigeishi M, Ohtsu M, Asai Y, Hayashi T. AE Characteristics of full-scale concrete-piles under bending and shear load. ASNT, Paper Summaries of the 1998 Sixth International Symposium on Acoustic Emission from Composite Materials, 1998:163]172. w5x Yuyama S, Okamoto T, Shigeishi M, Ohtsu M. Acoustic emission generated in corners of reinforced concrete rigid frame under cyclic loading. Mater Eval ASNT 1994;53:409]412. w6x Yuyama S, Okamoto T, Shigeishi M, Ohtsu M. Cracking progress evaluation in reinforced concrete by moment tensor analysis of acoustic emission. J Acoust Emission 1995;13:s14]s20.