Estimation of Corrosion in Reinforced Concrete by Electrochemical Techniques and Acoustic Emission

Transcription

1 Journal of Advanced Concrete Technology Vol. 3, No., 37-47, February 5 / Copyright 5 Japan Concrete Institute 37 Scientific paper Estimation of Corrosion in Reinforced Concrete by Electrochemical Techniques and Acoustic Emission Veerachai Leelalerkiet, Toshimitsu Shimizu, Yuichi Tomoda 3 and Masayasu Ohtsu 4 Received 3 June 4, accepted 3 September 4 Abstract Deterioration of reinforced concrete caused by corrosion of reinforcing steel under chloride environment is experimentally studied. Onset of corrosion and nucleation of ing are estimated by acoustic emission (AE), comparing with the chloride content. Corroded areas are evaluated by the half-cell potential and the polarization resistance. To compensate the potentials, the inverse boundary element method (IBEM) is applied. Results show that AE parameters can effectively evaluate the onset of corrosion and the nucleation of ing, which are remarkably comparable to chloride concentration. The decrease in the half-cell potential is observed, following high AE activities. It is concluded that AE technique give an earlier warning of corrosion than the half-cell potential measurement. In addition, types at onset of corrosion and nucleation of ing are identified from AE parameters. Corroded areas estimated by IBEM solutions are in good remarkable agreement with those of visual inspection, although results of electrochemical techniques are marginally successful.. Introduction Evaluation of corrosion in reinforcing steel due to chloride attack has been a long-challenging problem in concrete engineering. Many nondestructive evaluation (NDE) techniques have been developed for detecting the corrosion before critical level (Dubravka et al. ; Yokota 999; Law et al. ; Elesner 3; Ohtsu & Yamamoto 997). So far, the half-cell potential measurement and the polarization resistance are practically applied, especially to on-site measurement. However, these methods are sensitive to environment conditions (Kyung & Ohtsu ), which often give misleading results. Recently, acoustic emission (AE) technique has been applied to detect corrosion s inside concrete (Ohtsu 3; Tanaka & Ohtsu 3; Idrissi and Limam 3; Yoon et al. ; Li et al. 998). Elastic waves due to micros on the surface of rebar or in the interface of steel/concrete are detected by AE sensor placed on the surface of concrete. In a dissimilar manner to electrochemical techniques, AE could evaluate the corrosion in the term of damages due to corrosion. Therefore, the effects of moisture content inside concrete and geometrical conditions are inconsequential. In chloride-induced corrosion process, it is wellknown that onset of corrosion is mainly controlled by Ph.D. student, Graduate School of Science and Technology, Kumamoto University, Japan. Master student, Graduate School of Science and Technology, Kumamoto University, Japan. 3 Technician, Kumamoto University, Japan. 4 Professor, Graduate School of Science and Technology, Kumamoto University, Japan. ohtsu@gpo.kumamoto-u.ac.jp ingress of chloride. Concrete provides the passive environment by formulating a protected film on the surface of embedded rebar. When the amount of chloride, however, exceeds the threshold limit (Clear 983; Kropp 995; Pettersson 99; JSCE ), the protected film is broken and corrosion is initiated. Subsequently, supply of water and oxygen in concrete urge the electrochemical process, and corrosion products on the surface of rebars grow with time. When expansive-tensile stress due to rust-growth is greater than tensile strength of concrete, nucleation of micro in the vicinity of rebar occurs. According to JSCE standard (), the critical level of chloride concentration is referred to as approximately.3 ~.6 kg/m 3 of concrete volume. When the chloride concentration become over. ~.4 kg/m 3, the corrosion incorporating with micro-s might be nucleated.. Objectives and scope This paper quantitatively estimates corrosion activities of embedded rebars in reinforced concrete slabs by applying NDE methods of acoustic emission (AE) and the half-cell potential measurement. A laboratory experiment is performed to study the corrosion process due to chloride attack under wet and dry condition. Onset of corrosion and nucleation of corrosion ing are investigated, comparing with the chloride content as prescribed by JSCE code. Chloride profiles inside concrete are determined by the titration method and numerically estimated by the error-function analysis. Corroded areas of rebars are identified by the half-cell potential and the polarization resistance. To compensate the potentials, the Inverse Boundary Element Method (IBEM) (Kyung & Ohtsu ; Leelalerkiet et al. 3; Kyung et al. 3) is applied, taking into account concrete resistivity.

2 38 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 3. Analytical background 3. Ingress of chloride ions Chloride ions penetrate from the environment and/or contaminate concrete-mixed materials. For one dimensional diffusion, chloride ions penetrate through the concrete cover to the reinforcement surface, which is governed by following Fick s second law, C( x, t) C( x, t) = D () t x where C(x,t) is concentration of chloride ions (kg/m 3 ) at a distance x (cm) from the surface and time of exposure t (second). D is the diffusion coefficient of chloride (cm /s). In the case that the initial surface concentration C o are known, a solution of Eq. () is obtained as the following error-function, x C( x, t) = Co ( erf ) () Dt where C o is concentration of chloride at the surface of concrete, and erf is error function. In this study, the diffusion coefficient is determined from JSCE standard (), log D = 3.9( W / C) + ( W / C).5 (3) where W/C is the water-cement ratio. The surface concentration C o is estimated form chloride concentrations at.5 cm.5 cm depths as the averaged value. 3. AE parameter analysis Acoustic Emission (AE) is a non-destructive evaluation technique to detect and characterize micro-ing inside solid materials (Ohtsu 996, 998). AE events are associated with ing and are detected by AE sensors as electrical signals, which are amplified, filtered, and processed as shown in Fig.. In the case of reinforced concrete, AE sources are caused by thermal ing, freezing and thawing, corrosion of reinforcing steel and so on (Ohtsu 998). An AE signal is characterized by employing AE parameters such as count, event, amplitude, rise-time and duration as illustrated in Fig.. In the present study, RA value and average frequency are defined in the following equations, RA value = Rise Time / Amplitude (4) Average frequency = Counts / Duration (5) According to the JCMS-III B576 standard (3), a type is classified from the relationship between RA value and average frequency as shown in Fig. 3. A ten- AE sensors Crack Steel Concrete Preamplifi Mainamplifier Filter Signal Condition & Event Detector Maximum amplitude Rise time Duration Threshold level Count Fig. AE measuremen. Fig. AE signal parameters. Average Freq. 3.5 Log N = logA Tensile Log N.5 Shear Fig. 3 Classification of type. RA values log A Fig. 4 Relationship between cumulative of and amplitude.

3 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 39 sile-type is referred to as AE signal with high average frequency and low RA value. In the other way, a shear-type is identified. The relationship between the number of cumulative, N(A), and the amplitude, A, is analyzed as, log N ( A) = a b log A (6) where a and b are empirical constants. b is named b- value and a correction factor of is introduced to correct it, because AE amplitudes are measured in decibels rather than the logarithmic peak amplitude (Ohtsu 998). Determination of b-value is illustrated in Fig IBEM analysis In the case that concrete is referred to as homogeneous, potential u (x) at boundary point x is obtained by solving the boundary integral equation (Brebbia 987), u G cu( x) = G x y y x y u y ds (, ) ( ) (, ) ( ) S n n (7) where c is the shape factor. y are the points located on the boundary S surrounding the concrete. G ( x, y) is the fundamental solution. Regarding to Eq. (7), internal potentials at the interface between concrete and rebar are calculated by, u G u( x) = G( x, y) ( y) ( x, y) u( y) ds S n n (8) Since the currents at the concrete surface are always equal to zero, Eq. (8) is simplified as, G u ( x ) = ( x, y ) u ( y ) ds S n (9) By taking into account the concrete resistivity, Eq. (9) is modified in discrete form as (Kyung & Ohtsu ), u i M G = j = n ij R j u j S () where R j is the relative coefficient of concrete resistivity. In the case that the distribution of the resistivity is inhomogeneous, R ave R j = () R p where R ave is the average concrete resistivity of a measured area and R p is the resistivity at each location. 4. Experimental procedure 4. Specimen Two reinforced concrete slabs were made. Specimen S has dimensions cm x 57 cm x cm, while specimen S is of dimensions 4 cm x 5 cm x cm. For both specimens, rebars of 6 mm diameter were embedded at.5 cm and 3. cm from the surface for transverse and longitudinal orientations, respectively. Configurations of specimens S and S are shown in Figs. 5 and 6 as well as their rebar arrangements. In specimen S, the lefthand side of specimen was mixed only water and another was mixed with NaCl solution. For specimen S, whole areas of specimen were mixed with NaCl solution. The amount of mixed NaCl was equivalent to the chloride Rebar 3 Rebar 4 Rebar 5 Rebar 6 Rebar Only Water NaCl Solution 5 5 Rebar AE sensors 5 5 Rebar AE sensors Without coating Dimension in mm Without coating Dimension in mm Fig. 5 Specimen S and rebar arrangement. Fig. 6 Specimen S and rebar arrangement.

4 4 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 Table Mixture proportion of concrete. W/C (%) s/a (%) Weight of unit volume (kg/m 3 ) NaCl (kg) Slump (cm) Air (%) Comp. Strength Young s Modulus W C S G (MPa) (GPa) concentration at. kg/m 3 in concrete volume, which is slightly below the critical limit defined in JSCE standard. Mixture proportion and properties of concrete of both specimens are given in Table. In addition to these specimens, other two slabs SS- and SS- were made with the same dimension, mixture and condition as specimen S. These two slabs were employed for measuring the chloride contents at the different stages of corrosion. 4. Cyclic wet-dry test After 8-day curing in water, all surfaces of slabs were coated by epoxy, except the bottom surface to allow onedimensional ingress of chloride. In order to accelerate the corrosion process, all specimens are repeatedly tested weekly by drying on a support in ambient temperature and by putting into 3% NaCl solution tub as illustrated in Fig Electrochemical measurement After the standard cure, the half-cell potentials were weekly measured at the bottom surface of specimens S and S, by using a portable corrosion meter SRI-CM-II (Yokota 999). The bottom surfaces of the specimens were divided into 45 and meshes as illustrated in Figs. 8 and 9, respectively. The measurements were conducted at the center of each mesh. The measurement was finished when the average of potentials in dry condition reached to -35 mv (CSE). Concrete resistivities were simultaneously measured after the cyclic test by using SRI-CM-II. Results of the half-cell potentials and the polarization resistances were converted to a probability of corrosion and a corrosion rate by following ASTM C876 standard (99) and CEB recommendation (997). These are given in Table and Table 3, respectively. Results of concrete resistivities were introduced in IBEM analysis as described in Eq. (). In order to illustrate results of electrochemical measurements, the different colors were employed to symbolize the degrees of corrosion activities. As seen in the tables, white color refers as no corrosion, yellow color means uncertain or low corrosion rate and color indicates active corrosion or high corrosion rate. (a) Saturated in 3 % NaCl tub Fig. 7 Cyclic wet and dry condition. (b) dried in room temperature Coring Coring Fig. 8 Corrosion-measuring points and coring positions of specimen S. Fig. 9 Corrosion-measuring points and coring positions of specimen S.

5 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 4 Table ASTM C876 standard. Measured potential (mv vs CSE) Corrosion probability Color Resistance (kωcm ) Table 3 CEB recommendation. Icorr (µa/cm ) Corrosion rate - mv < E 9% no corrosion White 3 < R p 6.~. No corrosion or very low rate Low ~ middle rate -35 mv E - mv Uncertain Yellow 5 < R p 3.~.5 Middle ~ high rate E -35 mv 9% corrosion Red 6 < R p 5.5~. More than Very high rate R p 6. Color White Yellow Red 4.4 AE measurement AE measurements were continuously conducted by using LOCAN 3 (PAC). On top surface of each slab, two AE sensors were attached as shown in Figs. 5 and 6. AE sensors of 5 khz resonance (R-5) were used. Frequency range of the measurement was khz ~ MHz and total gain was 4 db. At the end of every week, AE measurement was temporarily stopped for the electrochemical measurement. 4.5 Chloride concentration Chloride profiles were determined at five periods. At first, initial concentration was measured by using a standard cylinder after 8 days cure. The second and third periods were conducted by coring slabs SS- and SS-, expecting the chloride concentrations higher than.3 kg/m 3 and. kg/m 3, respectively. The fourth and final steps were determined when the half-cell potential measured in specimens S and S reach to -35 mv. The coring positions of the specimens are shown in Figs. 8 and 9, respectively. After grinding core samples, chloride concentrations were determined by using the potentiometric titration method. 4.6 Visual inspection After the cyclic test, corroded areas in the both specimens S and S are visually determined. Concrete cover was removed and the corrosion was rated as three corrosion levels. Level indicates no visible rust on the surface. Level shows slight rust at the surface. Level 3 represents a heavy corrosion. These three levels were assigned as the same three colors in the electrochemical techniques defined in Tables and Results and discussion 5. Chloride concentration profile Chloride concentrations at.5 cm depth were determined by the potentiometric titration method at the initial stage (after curing), 56 days (8 weeks), 6 days (8 weeks) and the ends of tests of specimens S (8 days: 4 weeks) and S (38 days: 44 weeks). Chloride concentrations were calculated by Eq. (). From Eq. (3), diffusion coefficient D was determined as 6.5 x -8 cm /s. By inverse calculation of Eq.(), surface concentrations at 56 days, 6 days, 8 days and 38 days were obtained as 3.4 kg/m 3,.9 kg/m 3,. kg/m 3 and.7 Chloride concentration (kg/m3) Experiment ERF 5 days 4 days 8 days 4 days Cl -.4 kg/m 3 Cl -. kg/m 3 Cl -.6 kg/m 3 Cl -.3 kg/m Fig. Chloride concentration profile by the potentiometric and error-function methods.

6 4 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, days 4 days 8 days 4 days ASTM Half-cell potential -5-9 % No corrosion Potential (mv CSE) Uncertain % corrosion Fig. Results of AE activity and the half-cell potentials in specimen S. 5 5 days 4 days 8 days 4 days Half-cell potential -5 - ASTM 9 % No corrosion Peak Potential (mv CSE) Uncertain % corrosion Fig. Results of AE activity and the half-cell potentials in specimen S. kg/m 3. By employing these data, the chloride concentrations at the cover depth are numerically calculated as shown in Fig.. It is observed that the chloride concentrations estimated by Eq. () are in good agreement with experimental results, where chloride concentrations pass.3 kg/m 3,.6 kg/m 3,. kg/m 3 and reaches.4 kg/ m 3 at 5 days, 4 days, 8 days and 4 days, respectively. 5. AE monitoring Onset of corrosion and nucleation of ing are studied by AE technique, compared with the half-cell potentials. Results of averaged from two sensors and averaged half-cell potentials in specimens S and S are plotted in Figs. and, respectively. Concerning AE results, it is found that the first high AE activities in both specimens are observed approximately at 3 days in which the chloride concentration reaches to the critical level.3 ~.6 kg/m 3 as found in Fig.. During 8 ~ 85 days, the second high activities of are found, corresponding to the chloride concentration reaching. kg/m 3 (see Fig. ). Between 8 days and 4 days, high AE activities are again observed in both specimens of which chloride content are in the range. kg/m 3 to.4 kg/m 3. High AE activities in specimen S are observed continuously until 85 days. According to AE results, it is concluded that onset of corrosion and nucleation of

7 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, days 5 days 8 days days RA Avg.Freq days 5 days 8 days days b-value b-value days 5 days 8 days 9 4 days RA Avg. Freq 3 4 days 5 5 days 8 days 4 days b-value b-value (b) Specimen S (b) Specimen S Fig. 3 RA value and average frequency. Fig. 4 Variation of b-value. corrosion ing started approximately at 4 to 5 weeks and 3 to 4 weeks, respectively. As compared to AE results in Figs. and, during the first and the second AE activities, no sign of corrosion is observed from the half-cell potentials in both specimens. In specimen S, the potentials increase gradually and reach to - mv around to days and then decrease with a constant rate until the end of test. Similarly, the increase in the potentials of specimen S is observed around 4 days and then the potentials fluctuate within range - to - mv until 4 days. Subsequently, the potentials drop sharply to -3 mv and continue decrease toward the end of the test with a constant rate. Hence, it is concluded that onset of corrosion and nucleation of ing are not practically identified by the half-cell potential measurement. However, it is noted that the potentials in specimens S and S start to decrease after high AE activities approximately at days and days, respectively. These imply that halfcell potentials are sensitive after micros reach to a remarkable level. On the other hand, it means that AE technique detects the onset of corrosion earlier than the half-cell potential measurement. Onset of corrosion and nucleation of ing can be distinguished by using AE parameters of RA value, average frequency and b value. As similar to, RA value and average frequency significantly changes at 4 ~ 5 weeks and 3 ~ 4 weeks in both specimens. At the first high AE activity, RA values become high and the average frequencies become low, while those values are reverse at the second high AE activity as demonstrated in Fig. 3. Variation of b-value is compared to that of averaged in Fig. 4. It is clearly seen that b-value is high at the first AE activity and is low at the second in both specimens. 5.3 Classification of type Based on JCMS-III B576, types are classified by using the relation between RA values and average frequencies as shown in Fig. 5. AE activities of 4 ~ 5 weeks and those of 3 ~ 4 weeks, which are referred to as the onset of corrosion (at first high AE activity) and nucleation of ing (at second high AE activity), are in focus. In both specimens, it is found that shear-type s are dominantly at 4 ~ 5 weeks, while tensile-type s are actively generated at 3 ~ 4 weeks. It is concluded that shear-type s are actively nucleated at the onset of corrosion and tensile-type s are gener-

8 44 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, Tensile y = -.7x +.73 R =.43 Shear Tensile y = -.x R =.73 Shear (a) 4 and 5 weeks of specimen S (b) 4 and 5 weeks of specimen S Tensile y = -.593x + 5. R =.38 Shear 5 5 Tensile y = -.936x R =.6 Shear (c) 3 and 4 weeks of specimen S (d) 3 and 4 weeks of specimen S Fig. 5 Relationship between RA value and average frequency in specimens S and S. ated at the nucleation of ing. Regarding to the variation of b-value in Fig. 4, b- value is high at the first high AE activity, corresponding small shear-type s. At the second high AE activity, low b-value implies fairly large tensile-type s. Accordingly, it is summarized that AE parameters are capable to classify types at the onset of corrosion and nucleation of ing. 5.4 Electrochemical measurements After the cyclic tests, corroded areas of both specimens were evaluated from the half-cell potentials and the polarization resistances. Measured values are classified by Tables and 3 as shown in Figs. 6 and 7. Regarding to the half-cell potentials, in specimen S, most areas of rebars in NaCl-mixed part have high probability of corrosion as well as some portions of rebars 3 and in water-mixed part. The other rebars are in uncertain level as shown in Fig. 6 (a). For specimen S, all rebars are referred to as high corrosion activity in Fig. 6 (b). Concerning the polarization resistances, corrosion activities of all rebars in specimen S are in intermediate level, except a very low-level portion on rebar 4 as shown in Fig. 7 (a). In specimen S, no-corrosion is observed on meshes No. ~ 3 as well as 5, while the other parts are in intermediate level as illustrated in Fig. 7 (b). Results of visual inspection are shown in Fig. 8. In specimen S, heavy corrosion is mainly observed on rebar 6 and one portions of rebar 3. Minor corrosion is found on rebars 5, and in the NaCl-mixed part. The other rebars in water-mixed part keep the passive state as shown in Fig. 8 (a). For specimen S, high corrosion activities are seen on meshes No., 5 and. Nocorroded areas are observed on meshes No. ~ 3, while the others are in middle level as illustrated in Fig. 8 (b). Comparing to the actual corroded areas, the half-cell potentials give overestimations, while the polarization resistances underestimate. In IBEM analysis, concrete resistivities are taken into account as relative coefficient. Concrete resistivities measured in both specimens after the cyclic tests are given in Fig. 9. Distributions of concrete resistivities are fairly inhomogeneous. The averaged values R ave of specimens S

9 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 45 Rebar 3 Rebar 4 Rebar 5 Rebar 6 Rebar Rebar Water Nacl (b) Specimen S Fig. 6 Half-cell potentials measured on the surface of concrete (mv, CSE). (b) Specimen S Fig. 7 Polarization resistances on the surface of concrete (kω.cm ). and S were obtained as.6 kω and.49 kω, respectively. Then, half-cell potentials were compensated by IBEM, applying the relative coefficients of concrete resistivities. Results of IBEM solution are illustrated in Fig.. In specimen S, corroded areas estimated by IBEM show remarkable agreement with the actual corrosion areas in Fig. 8 (a), particularly on the passive state on rebar 4 and high activity of corrosion on rebars 6 and 3. Results in specimen S are not completely identical to the actual corrosion in Fig. 8 (b), but improvement is fairly observed. Accordingly, it is demonstrated that the half-cell potential measurement is practically improved by IBEM. 6. Conclusion AE technique is applied to evaluate the onset of corrosion and the nucleation of corrosion in RC slabs, comparing with electrochemical measurements. Corroded areas are estimated by IBEM analysis, compensating the half-cell potentials. The results are summarized as follows: () According to AE results, it is found that onset of corrosion initiates after chloride concentration becomes above the critical level of.3 ~.6 kg/m 3. Nucleation of corrosion ing starts actively, when the chloride content exceeds. kg/m 3. () Half-cell potentials measured start to decrease after the second high AE activities. This implies that the half-cell potentials vary after micros are nucleated. In other words, AE technique can identify onset of corrosion and give warning of corrosion earlier than the halfcell potential measurement. (3) According to AE parameters of RA value, average frequency and b-value, it is found that onset of corrosion on rebars corresponds to rust break of shear-type ing. Nucleation of concrete ing is observed as tensile s. (4) Regarding to the half-cell potentials and the polarization resistances, corroded areas are not readily identified. In order to compensate the half-cell potentials, IBEM analysis is applied. It is found that corroded areas estimated by IBEM are in remarkable agreement with the actual corrosion.

10 46 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 Rebar 3 Rebar 4 Rebar 5 Rebar 6 Rebar Rebar (b) Specimen S Fig. 8 Results of visual inspection (b) Specimen S Fig. 9 Contour of concrete resistivities on the surface of concrete (kω). Rebar 3 Rebar 4 Rebar 5 Rebar 6 Rebar Rebar Water Nacl (b) Specimen S Fig. Potentials on rebar by IBEM (mv, CSE).

11 V. Leelalerkiet, T. Shimizu, Y. Tomoda and M. Ohtsu / Journal of Advanced Concrete Technology Vol. 3, No., 37-47, 5 47 References ASTM C876 (99). Standards test method for halfcell potentials of uncoated reinforcing steel in concrete. Annual book of ASTM standard, Philadelphia. Brebbia, C. A. (987). Topics in boundary element research. Vol. 3, Heidelberg, Springer-Verlag. CEB, Working Party V/4. (997). Strategies for testing and assessment of concrete structures affected by reinforcement corrosion (draft.4). BRI-CSTC- WTCB, CEB Bulletin no. 43. Clear, K. C. (983). Chloride at the threshold. Report of Kenneth C. Clear Inc., March,. Dubravka, B., Dunja, M. and Dalibor, S. (). Nondestructive corrosion rate monitoring for reinforced concrete structures. 5 th WCNDT, Rome 5- October. Elesner, B. (3). Half-cell potential measurements potential mapping on reinforced concrete structures. Materials and Structures, 36, Idrissi, H. and Limam, A. (3). Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion. NDT&E international, 36, JCMS-III B576 (3). Monitoring method for active s in concrete by AE. Tokyo: Japan Construction Material Standards. JSCE (). Standard specification for concrete structures-maintenance. Working Group on Standard Specification for Concrete Structures-, -63. Kropp, J. (995). Chloride in concrete, performance criteria for concrete durability. RILEM report, Kyung, J. W. and Ohtsu, M. (). Study on half-cell potential measurement for NDE of rebar corrosion. Structural Faults and Repair, London 4-6 July. Kyung, J. W. and Ohtsu, M. (). Inversion by BEM of half-cell potential measurement for NDE of rebar corrosion. The first FIB Congress Concrete Structures in the st Century, Osaka 3-9 October, Paper no. S5--5. Kyung, J. W., Yokota, M., Leelalerkiet, V. and Ohtsu, M. (3). Practical use of half-cell potential method for NDE of reinforced concrete structure of corrosion. th APCNDT, Jeju 3-7 November, Paper no. A63. Law, D. W., Millard, S. G. and Bungey, J. H. (). Linear polarization resistance measurements using a potentiostatically controlled guard ring. NDT&E International, 33 (), 5-. Leelalerkiet, V., Kyung, J. W., Ohtsu, M. and Yokota, M. (3). Analysis of half-cell potential measurement for corrosion of reinforced concrete. Construction and Building Material, 8 (3), Li, Z., Li, F., Zdunek, A., Landis, E. and Shah, S. (998), Application of Acoustic Emission Technique to Detection of Rebar Corrosion in Concrete. ACI Materials Journal, 95 (), Ohtsu, M. (996). The history and development of acoustic emission in concrete engineering. Magazine of Concrete Research, 48 (77), Ohtsu, M. (998). Basics of acoustic emission and applications to concrete engineering. Materials Science Research International, 4 (3), 3-4. Ohtsu, M. (3). Detection and identification of concrete ing in reinforced concrete by AE. Review of progress in quantitative NDE. AIP conference 3, Proc. 657, B, Ohtsu, M. and Yamamoto, T. (997). Compensation procedure for half-cell potential measurement. Construction and Building Material, (7-8), Pettersson, K. (99). Corrosion threshold value and corrosion rate in reinforced concrete. CIB report :9. Tanaka, M. and Ohtsu, M. (3). Monitoring corrosion damage in reinforced concrete by acoustic emission. AEWG-46, Oregon 4-6 August, Session 4. Yokota, M. (999). Study on corrosion monitoring of reinforcing steel bars in 36-year-old actual concrete structures. Concrete library of JSCE, 33, Yoon, D. J., Weiss, W. J. and Shah, S. P. (). Assessing damage in corroded reinforced concrete using acoustic emission. Journal of Engineering Mechanics, March,