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AE monitoring of corrosion process in cyclic wet–dry test
Construction and Building Materials 24 (2010) 2353–2357
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
AE monitoring of corrosion process in cyclic wet–dry test Yuma Kawasaki a,*, Yuichi Tomoda b, Masayasu Ohtsu c a
Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan Faculty of Engineering, Kumamoto University, Japan c Graduate School of Science and Technology, Kumamoto University, Japan b
a r t i c l e
i n f o
Article history: Available online 1 June 2010 Keywords: Acoustic emission Corrosion of rebar Reinforced concrete AE parameter analysis SiGMA analysis
a b s t r a c t Recently, a number of deterioration of reinforced concrete (RC) structures due to salt attack have been reported. After corrosion of reinforcing steel-bar (rebar) is nucleated, expansion of corrosion products results in corrosion-induced cracks in RC. Thus, development of non-destructive evaluation (NDE) techniques is important for inspection of corrosion damage. It is reported that acoustic emission (AE) could identify the onset of corrosion in rebar and the nucleation of concrete cracking due to expansion of corrosion products in the corrosion process. In this study, AE techniques are applied to a cyclic wet–dry test of RC beams. It is confirmed that both the onset of corrosion and the nucleation of concrete cracking are clearly observed as two periods of high AE activity. Kinematics of micro-cracks are identified by SiGMA (Simplified Green’s functions for moment tensor analysis) analysis of AE. To compare with findings, crosssections of rebars are observed by SEM (Scanning Electron Micrograph). From these results, a great promise for AE techniques to monitor the corrosion process in RC structures is clarified. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Reinforced concrete (RC) structures are no longer maintenancefree. Especially, damage due to salt attack in RC structures is considered to be one of critical deteriorations in concrete engineering. In the concrete structures, reinforcing steel-bars (rebars) normally do not corrode because of a passive film nucleated on the surface of rebar in concrete of high PH. When chloride concentration at the level of rebar in concrete, however, exceeds the threshold value for corrosion, the passive film is destroyed and corrosion is initiated in rebar. The electro-chemical reaction continues with supplying oxygen and water. Then, due to expansion of corrosion products, corrosion-induced cracks are generated in concrete. Accordingly, development of non-destructive evaluation (NDE) techniques for detection of the corrosion in RC structures at early stage is urgently important. So far, electrochemical techniques of half-cell potential and polarization resistance are widely employed. These techniques estimate corroded conditions of rebars from electrical data. The relationship between deterioration and life-cycle of a concrete structure is standardized as shown in Fig. 1 [1]. Deterioration process of RC structure due to corrosion is divided into four stages as, dormant, initiation, acceleration and deterioration. Two transition periods are defined at the onset of corrosion and at the nucle-
* Corresponding author. E-mail address: [email protected] (Y. Kawasaki). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.05.006
ation of concrete cracking. The first is the transition period from the dormant stage to the initiation stage. In the former stage, penetration of chloride ions occurs. Following the transition, the electro-chemical reaction is initiated in the latter stage. Corrosion activity continues with supplying oxygen and water. Then, at the second transition, corrosion products are created on the surface of rebar, and corrosion-induced cracks start to be nucleated in concrete. It is normally considered that the electrochemical techniques are available to identify the corrosion process following the initiation stage. In previous research [2,3], it was reported that AE techniques could provide an earlier warning than the electrochemical techniques. Elsewhere, according to a phenomenological model of steel in marine environments [4], a typical corrosion loss during the corrosion process is illustrated in Fig. 2. At phase 1, the onset of corrosion occurs. The rate of the corrosion grow is controlled by the rate of transport of oxygen and water. As the corrosion products build up on the corroding surface of rebar, the flow of oxygen is eventually inhibited. Thus, the rate of corrosion loss decreases and is stabilized at phase 2. The corrosion process advances further, and the corrosion loss again increases as phases 3 and 4, due to anaerobic corrosion, where the corrosion penetrates inside the rebar and the expansion of corrosion products occurs. Whereas a monotonous increase is simply assumed in Fig. 1, a two-step corrosion process is modeled. Based on these findings, continuous AE measurement was conducted to monitor the corrosion process in RC specimens in a laboratory. The SiGMA analysis is applied to a cyclic wet–dry test of
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357
Durability
Deterioration
2354
of coarse aggregate was selected as 10 mm. Here, NaCl solution is employed as mixed-water. After the standard curing for 28 days in 20 °C water, chloride content was measured in one standard cylindrical sample of 100 mm diameter and 200 mm height and found to be 0.175 kg/m3 as lower than 0.3 kg/m3 in volume. Mechanical properties of hardened concrete are summarized in Table 2. A compressive strength of concrete at 28 days of the standard curing was 43.9 MPa, which was obtained as an averaged value from three cylindrical samples of 100 mm diameter and 200 mm height. Following the standard curing, all surfaces of the specimen were coated by epoxy, except for the bottom surface. To simulate the corrosion process in a typical seawater environment, cyclic wet–dry test was carried out as shown in Fig. 4. In the test, the specimens were submerged into NaCl solution up to the height of the rebar in the container as shown in Fig. 4a for a week, and subsequently taken out of the solution to dry under ambient temperature for another week as shown in Fig. 4b. AE measurement was continuously conducted, by using AE measurement system (DiSP, PAC). Six AE sensors (R15, PAC) of 150 kHz resonance were attached to the surface of the specimen. The coordinates and the locations of AE sensors are given in Table 3 and are illustrated in Fig. 3. The sensors were arranged as the target area was reasonably covered. The frequency range of the measurement was 10–2 MHz and total amplification was 60 dB gain. For event counting, the dead-time was set to 2 ms and the threshold level was set to 40 dB gain.
nucleation of cracking
onset of corrosion
deterioration stage dormant stage
initiation stage
Time acceleration stage
Corrosion loss
Fig. 1. Deterioration process due to corrosion.
0
1
2
3
4
3. AE analysis
Phases (Time)
3.1. Ib-value analysis
Fig. 2. Typical corrosion for steel in seawater immersion.
In addition to AE activity or occurrence, characteristic of AE signal are conventionally defined by such waveform parameters as rise time, maximum amplitude, counts and duration time. These are shown in Fig. 5. Here, in order to evaluate the size distribution of AE sources, the amplitude distribution of AE hits is taken into account. AE hit is the term to indicate that a given AE channel has detected and processed one AE transient signal. A relationship between the number of AE hits, N, and the maximum amplitudes, A, is statistically represented as,
RC beams, and kinematics of AE sources during the corrosion process are studied.
2. Experiments Reinforced concrete specimens of dimensions 100 mm 75 mm 400 mm were made. One deformed rebar of 13 mm diameter was embedded with 20 mm cover-thickness from concrete surface. Configuration of the specimen is illustrated in Fig. 3. The rebar was coated by epoxy except for a target area in the figure. Mixture proportion of concrete is given in Table 1. Because the cover-thickness was set to 20 mm, the maximum size
rebar
concrete
uncoated target area
Log10 N ¼ a bLog10 A
ð1Þ
where a and b are empirical constants. For the latter, so-called improved b value (Ib-value) is adopted for calculation, on the basis
coated
z 1CH 2CH
z 3CH
150
150
100 400
13 x
50 100
75 20 y [mm]
4CH
6CH y 5CH
x
Fig. 3. Sketch of RC specimen tested.
Table 1 Mixture proportion and properties of concrete. Maximum gravel size (mm)
Water to cement ratio W/C (%)
Weight per 1 m3 concrete Water (kg)
Cement (kg)
Sand (kg)
Gravel (kg)
Admixture (kg)
10
55
187
340
751
1113
1.632
Slump (cm)
Air (%)
8
5
2355
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357 Table 2 Mechanical properties of hardened concrete. Compressive strength (MPa)
Poisson’s ratio
P-Wave velocity (m/s)
43.9
0.2
4031
Fig. 6. Crack models in SiGMA analysis.
3% NaCl Solution
Container After solving Eq. (3) and locating AE source, the amplitudes of the first motion (P2) are substituted into the following equation.
(a) Wet Condition
AðxÞ ¼ C s
(b) Dry Condition
Fig. 4. Sketch of cyclic wet–dry test.
Table 3 The coordinate of AE sensor location. x (m)
y (m)
z (m)
0.010 0.100 0.090 0.020 0.095 0.010
0.030 0.070 0.000 0.000 0.100 0.100
0.075 0.075 0.045 0.030 0.033 0.055
Energy Threshold Level
Maximum Amplitude
4. Results and discussion 4.1. AE activity
Counts Rise Time Duration Fig. 5. AE waveform parameters.
of cumulative distribution [5]. For 100 AE hits, the Ib-value is defined, assuming averaged amplitude l and standard deviation r,
Ib ¼
ð4Þ
Here, A(x) is the amplitude of the first motion of P-wave and Cs is the calibration coefficient of the sensor sensitivity and material constants. The reflection coefficient Ref(t, c) is obtained as t is the direction of sensor sensitivity. DA is area of crack surface, and Mpq is the moment tensor. c is the direction vector of distance R from the source to the observation point x. Since the moment tensor Mpq is symmetric and of the second order, the number of independent unknown components of Mpq is six. Thus, to determine the moment tensor components, waveforms are to be detected at more than six sensors. The classification of a crack is performed by the eigen-value analysis of the moment tensor [6]. Eventually, microcracks are classified and visualized by employing the Light Wave 3D software (New Tek) as shown in Fig. 6. Here, an arrow vector indicates the direction of a crack motion vector, and a circular plate indicates the orientation of a crack surface, which is perpendicular to a crack normal vector.
½log10 Nðl a2 rÞ log10 Nðl þ a1 rÞ ða1 þ a2 Þr
ð2Þ
where N(l a2r) and N(l + a1r) represent the number of hits with the amplitudes higher than l a2r and l + a1r, respectively. In the case that the Ib-values are large, small AE hits are mostly generated. In contrast, the case where the Ib-values become small implies nucleation of large AE hits. 3.2. SiGMA analysis
Ri þ Riþ1 ¼ jxi x0 j jxiþ1 x0 j ¼ v p t i
ð3Þ
Here, vp is the velocity of P-wave and Ri represents the distance between AE source and observation point xi.
Stage 1
14000 12000 10000 8000 6000 4000 2000 0 0
Stage 2
AE events Cumulative AE hits
28 56 84 112 140 168 196 224 Time (day)
6 5 4 3 2 1 0
AE events
The SiGMA analysis consists of 3-D (three-dimensional) AE source location procedure and moment tensor analysis for AE source. Two parameters of the arrival time (P1) and the amplitude of the first motion (P2) are read from a waveform and applied to the analysis. In the location procedure, AE source x0 is located from the arrival time difference ti between the observation point xi and xi+1, by solving equations,
Generating behaviors of cumulative AE hits for all six channels and AE events during the cyclic wet–dry test are shown in Fig. 7. For AE events, which are reasonably located inside the specimen, the number of the hits for 1 h is indicated. Here, generating process of AE hits and AE events observed is classified into two stages, referring to the process of corrosion loss in Fig. 2. AE activity starts gradually to increase at the stage 1 during the first 126 days, and then acceleratedly increases at the stage 2 from 126 days to 238 days. Comparing these two stages with the curve of corrosion loss in Fig. 2, stage 1 could correspond to phase 1 to phase 2, because the first high activity is observed at around 28 days. The stage 2 corresponds to phase 3 to phase 4. So, AE activity due to the rebar corrosion is observed in the stage 1, and in the stage 2 corrosion-induced cracking in concrete is observed due to the expansion of corrosion products. Cumulative AE hits are compared with half-cell potentials in Fig. 8. The half-cell potentials start to decrease after 112 days elapsed at the stage 1. In the stage 2, the potentials keep more
Cumulative AE hits
1CH 2CH 3CH 4CH 5CH 6CH
Refðt; cÞ cp cq M pq DA R
Fig. 7. Cumulation AE hits and AE events during the test.
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357
Stage 2 Stage 1 14000 Half-cell Potentials Half-cell Potentials 12000 -350mV -350mV 10000 8000 6000 4000 2000 0 0 28 56 84 112 140 168 196 224
-500 -400 -300 -200 -100 0
Half-cell potential (mV)
Cumulative AE hits
2356
Time (day)
28 days elapsed
70 days elapsed
Fig. 10. Visual observation of rebars.
Fig. 8. Cumulation AE hits and half-cell potentials during the test.
Stage 1
Stage 2
0.25
Ib-value
0.2 0.15
Oxide film
Rust
0.1 0.05 0
µ 10µm
µm 100µ 0
28 56 84 112 140 168 196 224
Time (day)
28 days elapsed
70 days elapsed
Fig. 11. Distribution of ferrous ions on rebar surface.
Fig. 9. Variations of Ib-value.
negative than 350 mV. The increase in AE activity during the corrosion process is in good agreement with the decrease in the halfcell potentials. Thus, identification of the corrosion process is practically promising by conducting both measurements. 4.2. Ib-value analysis Variation of the Ib-value is given in Fig. 9. Two sudden drops of the Ib-values are observed at 28 days and 70 days elapsed at the stage 1, when high activity of AE events is clearly observed in Fig. 7. This implies that these activities result from large AE events. AE generation is quite active in the stage 2, and Ib-values decrease consecutively. High activity after 168 days elapsed results from an abrupt decrease of Ib-values. Thus, it becomes evident that Ib-value is effective to detect the onset of corrosion in rebar and the nucleation of concrete cracking. Since Ib-values in the stage 2 are comparatively lower than those of the stage 1, large-scale cracks are considered to be actively generated as corrosion-induced cracks. According to the previous report [7], at the stage 2, AE events are associated with fairly large tensile cracks and could results from concrete cracking due to rebar expansion caused by corrosion products. 4.3. Visual observations by SEM During the cyclic wet–dry test, it was found that chloride concentration at rebar reached to the lower-bound threshold of 0.3 kg/m3 after 28 days. At approximately 70 days elapsed, the concentration became higher than 1.2 kg/m3 even at the stage 1. Accordingly, at 28 days elapsed and 70 days elapsed, rebars were removed from the specimen and visually inspected as shown in Fig. 10. In both figures, a little rust is observed. At 28 days elapsed, the oxide film at the surface is slightly exfoliated, while the surface of oxide film is almost exfoliated at 70 days elapsed. In Fig. 11, photos of SEM (Scanning Electron Microscope) are given. At 28 days,
no corrosion is identified from the cross-section, although a little exfoliation is observed at the surface in Fig. 10. At 70 days, rust and loss of the oxide film are clearly observed as the growth of corrosion is confirmed. Thus, at the stage 1, the oxide film on the surface of rebar was broken at around 28 days elapsed. Then, at 70 days, the oxide film was superficially lost, and rust started to be generated inside rebar. It is summarized that at 28 days, only the surface of rebar is corroded due to penetration of chloride ions. The corrosion penetrates and is nucleated inside at the 70 days. Comparing these SEM photos with AE activity and Ib-value analysis, it is summarized that at 28 days, the transition period from the dormant stage to the initiation stage is assigned. At 70 days, high AE activity and low Ib-values are associated with the transition from the initiation stage to the acceleration stage. 4.4. SiGMA analysis In the SiGMA analysis, AE event definition time (EDT) is set to 100 ls. EDT is applied to recognize AE waves occurring within the specified time from the first-hit wave and to classify them as part of the current event. Results of SiGMA analysis at the stage 1 and at the stage 2 are shown in Fig. 12. By employing the Light Wave 3D software (New Tek), a tensile crack is indicated with a 1yellow disk, direction an opening orientation with an arrow. A mixed-mode crack is denoted with a green disk and a shear crack is shown with a red disk. At the stage 1, only six AE events are determined. These events are located mostly at the top of the specimen, as it is realized that these events are not directly related with the onset of corrosion. This is because fairly large AE sources are able to be analyzed. At the stage 2, 49 AE events are determined. These events are located, surrounding the rebar, especially at the left portion of the specimen. This result suggests that the crack has progressed 1 For interpretation of color in Fig. 12, the reader is referred to the web version of this article.
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357
2357
(a) Stage 1
(b) Stage 2
(c) Total Fig. 12. Results of SiGMA analysis.
mostly at the left portion of the specimen. In addition, these cracks are located from the rebar toward the bottom of the specimen at around 154 days, of which AE sources are mostly classified into tensile cracks and mixed-mode cracks. This suggests that cracks extend outward from the rebar. After 154 days, shear cracks are dominantly observed. Concerning the mechanisms in the corrosion process, it can be summarized that tensile and mixed-mode cracks start to be generated around the rebar, and coalescing and connecting these cracks, shear cracks are nucleated. 5. Conclusion Continuous AE monitoring was conducted in reinforced concrete specimens under cyclic wet–dry conditions. Results are summarized, as follows: (1) From the SEM observation at the stage 1, it is found that a passive film on the surface of the rebar is destroyed. At 28 days, no corrosion is identified from the cross-section, although a little exfoliation is observed at the surface. At 70 days, rust and loss of the oxide film are clearly observed. (2) Comparing these SEM photos with AE activity and Ib-value analysis, it is summarized that at 28 days, the transition period from the dormant stage to the initiation stage is assigned. At 70 days, high AE activity and low Ib-values are associated with the transition from the initiation stage to the acceleration stage. It also becomes evident that Ib-value is effective to identify these two transition periods.
(3) At the stage 1, the number of AE events located is a few, and results of the SiGMA analysis are not directly related with the onset of corrosion. This is because fairly large AE sources are only able to be analyzed. (4) At the stage 2, kinematic of corrosion-induced cracks are clarified by the SiGMA analysis. Tensile cracks are generated around rebar, and then shear cracks are nucleated. These results imply that corrosion activity related with chlorideinduced cracking in concrete is readily detected the mechanisms can be identified by the SiGMA analysis of AE monitoring.
References [1] JSCE. Standard specification for concrete structures on maintenance. Japan Society of Civil Engineers; 2001. [2] Yoon DJ, Weiss WJ, Shah SP. Assessing damage in corroded reinforced concrete using acoustic emission. J Eng Mech ASCE 2000;126(3):273–83. [3] Idrissi H, Liman A. Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion. NDT&E International 2003(36):563–9. [4] Melchers RE, Li CQ. Phenomenological modeling of reinforcement corrosion in marine environments. ACI Mater J 2006;103(1):25–32. [5] Shiotani T, Ohtsu M, Ikeda K. Detection and evaluation of AE waves due to rock deformation. J Constr Build Mater 2001;15(5–6):235–46. [6] Ohtsu M. Simplified moment tensor analysis and unified decomposition of AE source: application to in situ hydrofracturing test. J Geophys Res 1991;96(B4):6211–21. [7] Ohtsu M, Tomoda Y. Phenomenological model of corrosion process in reinforced concrete identified by acoustic emission. ACI Mater J 2008;105(2):194–9.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
AE monitoring of corrosion process in cyclic wet–dry test Yuma Kawasaki a,*, Yuichi Tomoda b, Masayasu Ohtsu c a
Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan Faculty of Engineering, Kumamoto University, Japan c Graduate School of Science and Technology, Kumamoto University, Japan b
a r t i c l e
i n f o
Article history: Available online 1 June 2010 Keywords: Acoustic emission Corrosion of rebar Reinforced concrete AE parameter analysis SiGMA analysis
a b s t r a c t Recently, a number of deterioration of reinforced concrete (RC) structures due to salt attack have been reported. After corrosion of reinforcing steel-bar (rebar) is nucleated, expansion of corrosion products results in corrosion-induced cracks in RC. Thus, development of non-destructive evaluation (NDE) techniques is important for inspection of corrosion damage. It is reported that acoustic emission (AE) could identify the onset of corrosion in rebar and the nucleation of concrete cracking due to expansion of corrosion products in the corrosion process. In this study, AE techniques are applied to a cyclic wet–dry test of RC beams. It is confirmed that both the onset of corrosion and the nucleation of concrete cracking are clearly observed as two periods of high AE activity. Kinematics of micro-cracks are identified by SiGMA (Simplified Green’s functions for moment tensor analysis) analysis of AE. To compare with findings, crosssections of rebars are observed by SEM (Scanning Electron Micrograph). From these results, a great promise for AE techniques to monitor the corrosion process in RC structures is clarified. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Reinforced concrete (RC) structures are no longer maintenancefree. Especially, damage due to salt attack in RC structures is considered to be one of critical deteriorations in concrete engineering. In the concrete structures, reinforcing steel-bars (rebars) normally do not corrode because of a passive film nucleated on the surface of rebar in concrete of high PH. When chloride concentration at the level of rebar in concrete, however, exceeds the threshold value for corrosion, the passive film is destroyed and corrosion is initiated in rebar. The electro-chemical reaction continues with supplying oxygen and water. Then, due to expansion of corrosion products, corrosion-induced cracks are generated in concrete. Accordingly, development of non-destructive evaluation (NDE) techniques for detection of the corrosion in RC structures at early stage is urgently important. So far, electrochemical techniques of half-cell potential and polarization resistance are widely employed. These techniques estimate corroded conditions of rebars from electrical data. The relationship between deterioration and life-cycle of a concrete structure is standardized as shown in Fig. 1 [1]. Deterioration process of RC structure due to corrosion is divided into four stages as, dormant, initiation, acceleration and deterioration. Two transition periods are defined at the onset of corrosion and at the nucle-
* Corresponding author. E-mail address: [email protected] (Y. Kawasaki). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.05.006
ation of concrete cracking. The first is the transition period from the dormant stage to the initiation stage. In the former stage, penetration of chloride ions occurs. Following the transition, the electro-chemical reaction is initiated in the latter stage. Corrosion activity continues with supplying oxygen and water. Then, at the second transition, corrosion products are created on the surface of rebar, and corrosion-induced cracks start to be nucleated in concrete. It is normally considered that the electrochemical techniques are available to identify the corrosion process following the initiation stage. In previous research [2,3], it was reported that AE techniques could provide an earlier warning than the electrochemical techniques. Elsewhere, according to a phenomenological model of steel in marine environments [4], a typical corrosion loss during the corrosion process is illustrated in Fig. 2. At phase 1, the onset of corrosion occurs. The rate of the corrosion grow is controlled by the rate of transport of oxygen and water. As the corrosion products build up on the corroding surface of rebar, the flow of oxygen is eventually inhibited. Thus, the rate of corrosion loss decreases and is stabilized at phase 2. The corrosion process advances further, and the corrosion loss again increases as phases 3 and 4, due to anaerobic corrosion, where the corrosion penetrates inside the rebar and the expansion of corrosion products occurs. Whereas a monotonous increase is simply assumed in Fig. 1, a two-step corrosion process is modeled. Based on these findings, continuous AE measurement was conducted to monitor the corrosion process in RC specimens in a laboratory. The SiGMA analysis is applied to a cyclic wet–dry test of
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357
Durability
Deterioration
2354
of coarse aggregate was selected as 10 mm. Here, NaCl solution is employed as mixed-water. After the standard curing for 28 days in 20 °C water, chloride content was measured in one standard cylindrical sample of 100 mm diameter and 200 mm height and found to be 0.175 kg/m3 as lower than 0.3 kg/m3 in volume. Mechanical properties of hardened concrete are summarized in Table 2. A compressive strength of concrete at 28 days of the standard curing was 43.9 MPa, which was obtained as an averaged value from three cylindrical samples of 100 mm diameter and 200 mm height. Following the standard curing, all surfaces of the specimen were coated by epoxy, except for the bottom surface. To simulate the corrosion process in a typical seawater environment, cyclic wet–dry test was carried out as shown in Fig. 4. In the test, the specimens were submerged into NaCl solution up to the height of the rebar in the container as shown in Fig. 4a for a week, and subsequently taken out of the solution to dry under ambient temperature for another week as shown in Fig. 4b. AE measurement was continuously conducted, by using AE measurement system (DiSP, PAC). Six AE sensors (R15, PAC) of 150 kHz resonance were attached to the surface of the specimen. The coordinates and the locations of AE sensors are given in Table 3 and are illustrated in Fig. 3. The sensors were arranged as the target area was reasonably covered. The frequency range of the measurement was 10–2 MHz and total amplification was 60 dB gain. For event counting, the dead-time was set to 2 ms and the threshold level was set to 40 dB gain.
nucleation of cracking
onset of corrosion
deterioration stage dormant stage
initiation stage
Time acceleration stage
Corrosion loss
Fig. 1. Deterioration process due to corrosion.
0
1
2
3
4
3. AE analysis
Phases (Time)
3.1. Ib-value analysis
Fig. 2. Typical corrosion for steel in seawater immersion.
In addition to AE activity or occurrence, characteristic of AE signal are conventionally defined by such waveform parameters as rise time, maximum amplitude, counts and duration time. These are shown in Fig. 5. Here, in order to evaluate the size distribution of AE sources, the amplitude distribution of AE hits is taken into account. AE hit is the term to indicate that a given AE channel has detected and processed one AE transient signal. A relationship between the number of AE hits, N, and the maximum amplitudes, A, is statistically represented as,
RC beams, and kinematics of AE sources during the corrosion process are studied.
2. Experiments Reinforced concrete specimens of dimensions 100 mm 75 mm 400 mm were made. One deformed rebar of 13 mm diameter was embedded with 20 mm cover-thickness from concrete surface. Configuration of the specimen is illustrated in Fig. 3. The rebar was coated by epoxy except for a target area in the figure. Mixture proportion of concrete is given in Table 1. Because the cover-thickness was set to 20 mm, the maximum size
rebar
concrete
uncoated target area
Log10 N ¼ a bLog10 A
ð1Þ
where a and b are empirical constants. For the latter, so-called improved b value (Ib-value) is adopted for calculation, on the basis
coated
z 1CH 2CH
z 3CH
150
150
100 400
13 x
50 100
75 20 y [mm]
4CH
6CH y 5CH
x
Fig. 3. Sketch of RC specimen tested.
Table 1 Mixture proportion and properties of concrete. Maximum gravel size (mm)
Water to cement ratio W/C (%)
Weight per 1 m3 concrete Water (kg)
Cement (kg)
Sand (kg)
Gravel (kg)
Admixture (kg)
10
55
187
340
751
1113
1.632
Slump (cm)
Air (%)
8
5
2355
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357 Table 2 Mechanical properties of hardened concrete. Compressive strength (MPa)
Poisson’s ratio
P-Wave velocity (m/s)
43.9
0.2
4031
Fig. 6. Crack models in SiGMA analysis.
3% NaCl Solution
Container After solving Eq. (3) and locating AE source, the amplitudes of the first motion (P2) are substituted into the following equation.
(a) Wet Condition
AðxÞ ¼ C s
(b) Dry Condition
Fig. 4. Sketch of cyclic wet–dry test.
Table 3 The coordinate of AE sensor location. x (m)
y (m)
z (m)
0.010 0.100 0.090 0.020 0.095 0.010
0.030 0.070 0.000 0.000 0.100 0.100
0.075 0.075 0.045 0.030 0.033 0.055
Energy Threshold Level
Maximum Amplitude
4. Results and discussion 4.1. AE activity
Counts Rise Time Duration Fig. 5. AE waveform parameters.
of cumulative distribution [5]. For 100 AE hits, the Ib-value is defined, assuming averaged amplitude l and standard deviation r,
Ib ¼
ð4Þ
Here, A(x) is the amplitude of the first motion of P-wave and Cs is the calibration coefficient of the sensor sensitivity and material constants. The reflection coefficient Ref(t, c) is obtained as t is the direction of sensor sensitivity. DA is area of crack surface, and Mpq is the moment tensor. c is the direction vector of distance R from the source to the observation point x. Since the moment tensor Mpq is symmetric and of the second order, the number of independent unknown components of Mpq is six. Thus, to determine the moment tensor components, waveforms are to be detected at more than six sensors. The classification of a crack is performed by the eigen-value analysis of the moment tensor [6]. Eventually, microcracks are classified and visualized by employing the Light Wave 3D software (New Tek) as shown in Fig. 6. Here, an arrow vector indicates the direction of a crack motion vector, and a circular plate indicates the orientation of a crack surface, which is perpendicular to a crack normal vector.
½log10 Nðl a2 rÞ log10 Nðl þ a1 rÞ ða1 þ a2 Þr
ð2Þ
where N(l a2r) and N(l + a1r) represent the number of hits with the amplitudes higher than l a2r and l + a1r, respectively. In the case that the Ib-values are large, small AE hits are mostly generated. In contrast, the case where the Ib-values become small implies nucleation of large AE hits. 3.2. SiGMA analysis
Ri þ Riþ1 ¼ jxi x0 j jxiþ1 x0 j ¼ v p t i
ð3Þ
Here, vp is the velocity of P-wave and Ri represents the distance between AE source and observation point xi.
Stage 1
14000 12000 10000 8000 6000 4000 2000 0 0
Stage 2
AE events Cumulative AE hits
28 56 84 112 140 168 196 224 Time (day)
6 5 4 3 2 1 0
AE events
The SiGMA analysis consists of 3-D (three-dimensional) AE source location procedure and moment tensor analysis for AE source. Two parameters of the arrival time (P1) and the amplitude of the first motion (P2) are read from a waveform and applied to the analysis. In the location procedure, AE source x0 is located from the arrival time difference ti between the observation point xi and xi+1, by solving equations,
Generating behaviors of cumulative AE hits for all six channels and AE events during the cyclic wet–dry test are shown in Fig. 7. For AE events, which are reasonably located inside the specimen, the number of the hits for 1 h is indicated. Here, generating process of AE hits and AE events observed is classified into two stages, referring to the process of corrosion loss in Fig. 2. AE activity starts gradually to increase at the stage 1 during the first 126 days, and then acceleratedly increases at the stage 2 from 126 days to 238 days. Comparing these two stages with the curve of corrosion loss in Fig. 2, stage 1 could correspond to phase 1 to phase 2, because the first high activity is observed at around 28 days. The stage 2 corresponds to phase 3 to phase 4. So, AE activity due to the rebar corrosion is observed in the stage 1, and in the stage 2 corrosion-induced cracking in concrete is observed due to the expansion of corrosion products. Cumulative AE hits are compared with half-cell potentials in Fig. 8. The half-cell potentials start to decrease after 112 days elapsed at the stage 1. In the stage 2, the potentials keep more
Cumulative AE hits
1CH 2CH 3CH 4CH 5CH 6CH
Refðt; cÞ cp cq M pq DA R
Fig. 7. Cumulation AE hits and AE events during the test.
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357
Stage 2 Stage 1 14000 Half-cell Potentials Half-cell Potentials 12000 -350mV -350mV 10000 8000 6000 4000 2000 0 0 28 56 84 112 140 168 196 224
-500 -400 -300 -200 -100 0
Half-cell potential (mV)
Cumulative AE hits
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Time (day)
28 days elapsed
70 days elapsed
Fig. 10. Visual observation of rebars.
Fig. 8. Cumulation AE hits and half-cell potentials during the test.
Stage 1
Stage 2
0.25
Ib-value
0.2 0.15
Oxide film
Rust
0.1 0.05 0
µ 10µm
µm 100µ 0
28 56 84 112 140 168 196 224
Time (day)
28 days elapsed
70 days elapsed
Fig. 11. Distribution of ferrous ions on rebar surface.
Fig. 9. Variations of Ib-value.
negative than 350 mV. The increase in AE activity during the corrosion process is in good agreement with the decrease in the halfcell potentials. Thus, identification of the corrosion process is practically promising by conducting both measurements. 4.2. Ib-value analysis Variation of the Ib-value is given in Fig. 9. Two sudden drops of the Ib-values are observed at 28 days and 70 days elapsed at the stage 1, when high activity of AE events is clearly observed in Fig. 7. This implies that these activities result from large AE events. AE generation is quite active in the stage 2, and Ib-values decrease consecutively. High activity after 168 days elapsed results from an abrupt decrease of Ib-values. Thus, it becomes evident that Ib-value is effective to detect the onset of corrosion in rebar and the nucleation of concrete cracking. Since Ib-values in the stage 2 are comparatively lower than those of the stage 1, large-scale cracks are considered to be actively generated as corrosion-induced cracks. According to the previous report [7], at the stage 2, AE events are associated with fairly large tensile cracks and could results from concrete cracking due to rebar expansion caused by corrosion products. 4.3. Visual observations by SEM During the cyclic wet–dry test, it was found that chloride concentration at rebar reached to the lower-bound threshold of 0.3 kg/m3 after 28 days. At approximately 70 days elapsed, the concentration became higher than 1.2 kg/m3 even at the stage 1. Accordingly, at 28 days elapsed and 70 days elapsed, rebars were removed from the specimen and visually inspected as shown in Fig. 10. In both figures, a little rust is observed. At 28 days elapsed, the oxide film at the surface is slightly exfoliated, while the surface of oxide film is almost exfoliated at 70 days elapsed. In Fig. 11, photos of SEM (Scanning Electron Microscope) are given. At 28 days,
no corrosion is identified from the cross-section, although a little exfoliation is observed at the surface in Fig. 10. At 70 days, rust and loss of the oxide film are clearly observed as the growth of corrosion is confirmed. Thus, at the stage 1, the oxide film on the surface of rebar was broken at around 28 days elapsed. Then, at 70 days, the oxide film was superficially lost, and rust started to be generated inside rebar. It is summarized that at 28 days, only the surface of rebar is corroded due to penetration of chloride ions. The corrosion penetrates and is nucleated inside at the 70 days. Comparing these SEM photos with AE activity and Ib-value analysis, it is summarized that at 28 days, the transition period from the dormant stage to the initiation stage is assigned. At 70 days, high AE activity and low Ib-values are associated with the transition from the initiation stage to the acceleration stage. 4.4. SiGMA analysis In the SiGMA analysis, AE event definition time (EDT) is set to 100 ls. EDT is applied to recognize AE waves occurring within the specified time from the first-hit wave and to classify them as part of the current event. Results of SiGMA analysis at the stage 1 and at the stage 2 are shown in Fig. 12. By employing the Light Wave 3D software (New Tek), a tensile crack is indicated with a 1yellow disk, direction an opening orientation with an arrow. A mixed-mode crack is denoted with a green disk and a shear crack is shown with a red disk. At the stage 1, only six AE events are determined. These events are located mostly at the top of the specimen, as it is realized that these events are not directly related with the onset of corrosion. This is because fairly large AE sources are able to be analyzed. At the stage 2, 49 AE events are determined. These events are located, surrounding the rebar, especially at the left portion of the specimen. This result suggests that the crack has progressed 1 For interpretation of color in Fig. 12, the reader is referred to the web version of this article.
Y. Kawasaki et al. / Construction and Building Materials 24 (2010) 2353–2357
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(a) Stage 1
(b) Stage 2
(c) Total Fig. 12. Results of SiGMA analysis.
mostly at the left portion of the specimen. In addition, these cracks are located from the rebar toward the bottom of the specimen at around 154 days, of which AE sources are mostly classified into tensile cracks and mixed-mode cracks. This suggests that cracks extend outward from the rebar. After 154 days, shear cracks are dominantly observed. Concerning the mechanisms in the corrosion process, it can be summarized that tensile and mixed-mode cracks start to be generated around the rebar, and coalescing and connecting these cracks, shear cracks are nucleated. 5. Conclusion Continuous AE monitoring was conducted in reinforced concrete specimens under cyclic wet–dry conditions. Results are summarized, as follows: (1) From the SEM observation at the stage 1, it is found that a passive film on the surface of the rebar is destroyed. At 28 days, no corrosion is identified from the cross-section, although a little exfoliation is observed at the surface. At 70 days, rust and loss of the oxide film are clearly observed. (2) Comparing these SEM photos with AE activity and Ib-value analysis, it is summarized that at 28 days, the transition period from the dormant stage to the initiation stage is assigned. At 70 days, high AE activity and low Ib-values are associated with the transition from the initiation stage to the acceleration stage. It also becomes evident that Ib-value is effective to identify these two transition periods.
(3) At the stage 1, the number of AE events located is a few, and results of the SiGMA analysis are not directly related with the onset of corrosion. This is because fairly large AE sources are only able to be analyzed. (4) At the stage 2, kinematic of corrosion-induced cracks are clarified by the SiGMA analysis. Tensile cracks are generated around rebar, and then shear cracks are nucleated. These results imply that corrosion activity related with chlorideinduced cracking in concrete is readily detected the mechanisms can be identified by the SiGMA analysis of AE monitoring.
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