Failure Process in Shear Bonding Strength Tests between Existing Concrete and Repairing Material by Acoustic Emission Technique

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1 Failure Process in Shear Bonding Strength Tests between Existing Concrete and Material by Acoustic Emission Technique Kentaro Ohno 1, So Kurohara 1, Kimitaka Uji 1 and Atsushi Ueno 1 1 Dept. of Civil and Environmental Engineering, Tokyo Metropolitan University, Tokyo, Japan ABSTRACT: The reasonable and simple method is required to evaluate on interface of existing and repairing. In this study, four types of shear tests are carried out in a laboratory as a fundamental study on evaluation of the shear. In addition, interfaces on substrate s are prepared in three different es. Since fracture behavior of specimen is different by test methods and the interface, results of the shear in these tests vary widely. In order to investigate fracture process in these tests, acoustic emission (AE) method is applied to each test. Also, the center line expresses the interface. As a result, suitable and brief test method is suggested based on the number of failed specimens with highly rough interface and results of the AE-SiGMA analysis which demonstrate failure process and failure mode. 1 INTRODUCTION Since existing structures suffer from sever conditions due to corrosion of reinforcing bar, earthquakes and cyclic load, repairing of existing structures has become significant matter. A tight between existing and repairing is necessary for a sufficient repairing of the structure. Therefore, a reasonable standard method is required to evaluate the shear. It is known that the shear depends on the test method, and soundness of existing (Momayez et al (25)). In this study, four types of shear tests of bi- shear test, direct shear test, punching shear test (Uji et al (2)) and slant shear test are carried out as a fundamental study on evaluation of the shear. In addition, interfaces on substrate s are prepared in three different es. Since failure processes in these tests might be different, acoustic emission (AE) method is applied to these tests to investigate generation of micro cracks. Failure process of member can be investigated by applying SiGMA (simplified Green s functions for moment tensor analysis) procedure (Ohtsu (1991)). Result of each shear is compared each other, and failure process in each test is discussed based on results of the SiGMA analysis. 2 EXPERIMENTAL PROCEDURE Figure 1 shows four types of shear tests and dimensions of specimens in every test. Bi- shear test (BSST) and direct shear test (DST) can estimate the shear

under flexural stress working state. On the other hand, punching shear test (PST) and slant shear test (SST) bring the shear under compressive stress working state.

Premixed type repairing, which does not include polymer and fiber, is applied to the experiment.

Substrate is cured in 14 days, and then repairing is applied on the substrate. Each test is carried out after substrate with repairing mortar is cured in water until 28 days.

Cement paste or mortar on the interface of substrate is removed by wire brush in next day of casting substrate.

Five specimens are prepared for each on one shear test. In order to quantify the of the interface, it is traced by laser displacement transducer. The laser displacement transducer has.

2 under flexural stress working state. On the other hand, punching shear test (PST) and slant shear test (SST) bring the shear under compressive stress working state. Table 1 indicates the mixture proportion of substrate. Table 2 shows mechanical properties of hardened substrate and repairing. Premixed type repairing, which does not include polymer and fiber, is applied to the experiment. Compressive of repairing is higher than that of substrate, and elastic modulus of repairing is similar to that of substrate. Substrate is cured in 14 days, and then repairing is applied on the substrate. Each test is carried out after substrate with repairing mortar is cured in water until 28 days. Figure 2 shows three different es of substrate s in the experiment. The set retarder sheet is attached to the of substrate to make rough after hardening. Cement paste or mortar on the interface of substrate is removed by wire brush in next day of casting substrate. Then, the interface of substrate is prepared in three es such as without treatment (Figure 2(a)), medium in rough (Figure 2(b)) and highly (Figure 2(c)). Five specimens are prepared for each on one shear test. In order to quantify the of the interface, it is traced by laser displacement transducer. The laser displacement transducer has.5 mm resolution capability for height and sampling interval is set to.234 mm to slipping direction. In this study, AE method is applied to investigate failure process in each test method. AE signals are detected by six AE sensors (15 khz resonance frequency), and they are recorded by SAMOS AE system (PAC). The threshold level is 4dB. Detected AE signals are amplified with 4 db gain by a pre-amplifier and 2 db gain by main amplifier in the SAMOS. AE waveforms are recorded at 1MHz sampling frequency. Table 1. Mixture proportion of substrate Maximum Air Water unit content (kg/m 3 ) Slump aggregate size content cement ratio Water Cement Fine aggregate Coarse aggregate (%) w/c W C S G Table 2. Mechanical properties of hardened substrate and repairing BSST* DST** PST*** SST**** Compressive Substrate Tensile Elastic modulus (GPa) Compressive Tensile Elastic modulus (GPa) * : Bi- shear test, ** : Direct shear test, *** : Punching shear test, **** : Slant shear test (a) Smooth (b)medium (c) High Figure 2. Surface textures of substrate s substrate 4 substrate (a) BSST Substrate (b) DST (c) PST Figure 1. Test specimens and methods Carbon fiber sheet 1 6 substrate 1 1 (d) SST unit

3 3 RESULTS AND DISCUSSIONS 3.1 Relation between index and the shear The shear significantly depends on friction and aggregate interlock factors even if same shear test method is carried out. Therefore, quantitative index are required to evaluate interface of substrate. In this study, interface in substrate is measured by laser displacement transducer. Figure 3 shows the results of interfaces in substrate s with different es. It is found that maximum difference between smooth interface and highly rough interface is within about 2. mm. As for the index, a standard method has not been adopted for evaluation of on interface in. In order to quantify the of substrate, center line is applied to index. The center line is calculated by following equation, 1 H ave f ( x) dx L where, H ave is center line, f(x) is height from center line, L is the number of sampling data as shown in Figure 4. Table 3 shows results of the center line for all specimens. It is found that differences in substrate s are quantitatively represented by the center line. Therefore, it is suggested that the center line can be adopted as index for interface of substrate. The relation between the index and the shear for all specimens is given in Figure 5. The index correlates with the shear as shown in Figure 5, the shear increases in all test methods with increases of the index. The results of the BSST are similar to those of the DST because both methods Height of f(x) Middle Horizontal distance Figure 3. Measurement results of interfaces in substrate s. f(x) Figure 4. The center line. R² =.741 R² =.696 index and the shear. Table 3. Calculation results of the center line R² =.878 R² = Roughness index Roughness Smooth Medium High Specimen S-1 S-2 S-3 S-4 S-5 M-1 M-2 M-3 M-4 M-5 H-1 H-2 H-3 H-4 H-5 BBST DST PST SST Average High Shear (N/mm 2 ) Smooth BSST DST PST SST Figure 5. The relation between the (1)

4 are carried out under the flexural moment. On the other hand, shear s of the PST and the SST are higher than those of the BSST and the DST. This is because these tests are carried out under the compressive loading, and slip motion on interface is restrained by carbon fiber sheet in the PST and loading plates in the SST. 3.2 Failure Process in Shear Bonding Strength Tests Bi- Shear Test The shear in the BSST is summarized in Table 4. Specimens which have smooth interface and medium in rough interface failed at the interface. Straight failure planes are observed for smooth interface specimens and some coarse aggregate in substrate are removed by repairing in highly rough interface specimens (H-1 and H-4 specimens). By applying SiGMA analysis to obtained AE signals, fracture process zone is visually found as shown in Figure 6. It is confirmed that shear tests are successfully carried out for specimens, which have smooth interface and medium in rough interface. However, only two specimens with highly rough interface failed at, the others failed at bending span in the BSST. Figure 7 shows the relation between applied load and the number of AE events identified by the SiGMA analysis along with time in H-1 specimen. In stage1, micro-cracks which are classified as shear-mode localize at bottom of interface as shown in Figure 8(a). When the applied load decreases in stage 2, a lot of tensile cracks are generated at around bending span. After decreasing the applied load, failure plane suddenly occurs at interface in stage 3. From results of the SiGMA analysis in H-1 specimen, tensile cracks mainly occur under flexural stress, while shear cracks intensively generate under shear stress. It is difficult to generate failure for specimens with highly rough interface Specimen Table 4. Results of the BSST BBST S S S S S M M M M M H H H H H Stage1 Stage2 Stage Time (s) Figure 7. The relation between the applied load and AE events in H-1 specimen. 6 4 Load (kn) Figure 6. Results of the SiGMA analysis in M-2 specimen (a) Stage (b) Stage (c) Stage 3 Figure 8. Results of the SiGMA analysis in H-1 specimen.

5 in the BSST. In addition, it is realized that failure mode would change from failure to flexural failure with increasing index Direct Shear Test In the DST test, except for H-2 specimen, all specimens failed at interface as shown in Table 5. However, it is clearly found that cracks concentrate at bottom of the substrate s with medium in rough and highly rough interfaces as given in Figure 9. Since a lot of shear cracks generate in this area, shear stress might works mechanically at the bottom of the substrate. In H-2 specimen as shown in Figure 9(c), since maximum flexural moment generates at around center of the bending span, flexural failure occurs. In this case, a small amount of misalignment between loading position and interface might introduce bending crack. Therefore, shear stress does not work effectively at interface between substrate and repairing. Figure 1 shows the relation between the number of AE events and length of specimen. Dominant movement of micro-cracks on failure plane is shear crack in M-2 and H-1 specimens, while tensile cracks mainly generate at around bending span in H-1 specimen. On the other hand, a lot of shear cracks generate at center of the bending span in H-2 specimen. This phenomenon is explained that tensile crack firstly generate at upper of the bending span, and then a lot of shear cracks follow as shown in Figure 11. Table 5. Results of the DST DST S M H S M H S M H S M H S M H (a) M-2 specimen (b).5 H-1 specimen (c) H-2 specimen Figure 9. Results of the SiGMA analysis in the DST. Left Interface (a) M-2 specimen Right Length(m) Left Interface Length(m) Left (b) H-1 specimen Right Interface Right Length(m) (c) H-2 specimen Figure 1. for specimen length.

3.2.3 Punching Shear Test The shear in the PST as shown in Table 6 is higher than those of the BSST and the DST because the PST is carried out under the compressive loading.

6 3.2.3 Punching Shear Test The shear in the PST as shown in Table 6 is higher than those of the BSST and the DST because the PST is carried out under the compressive loading. Similar failure mode is obtained both specimens with medium in rough and highly rough interfaces as shown in Figure 12. A lot of micro-cracks are localized at specific area. In specimens with medium in rough and highly rough interfaces, repairing and substrate suddenly split off after the applied load reached maximum load as illustrated in Figure 13. The failure process is estimated that failure area is not all interfaces but a specific area of interface in the PST. Therefore, it is difficult to estimate failure mode whether shear failure on interface or splitting of repairing and substrate. Since the orientation of the interface is significant on the failure mode and the shear, it requires that interface between substrate and repairing is accurately-parallel to loading direction in the PST Slant Shear Test The shear in the SST becomes much higher than other test methods as summarized in Table 7. AE sources are concentrated at interface in specimen with medium in rough interface as shown in Figure 13(a). It is found that crack distribution widely expands comparing shear tests under the flexural stress. In specimens with highly rough interface, since the applied load reached maximum load of substrate, a true shear cannot obtained, and a few AE sources are observed as illustrated in Figure 13 (b). As increasing index of the interface, it is required high interface angle from horizontal axis in Figure 1 (Austin et al., 1999). With increasing index, failure mode would change from failure to compressive failure of repairing or substrate Time (s) Figure 11. AE event generateion behavior in H-1 specimen under the DST Load(kN) (a) M-1 specimen (b) H-5 specimen Figure 12. Results of the SiGMA analysis Figure 13. Splitting crack in H-5 specimen. in the PST. Table 6. Results of the PST PST S M-1.34 (6.8) H-1.37 (7.46) S M-2.32 (6.29) H-2.66 (7.12) S M-3.39 (6.53) H-3.72 (7.59).63 (6.95) S M H-4.64 (5.44) S M H-5.76 (7.14) () represents the split failure of substrate or/and repairing

7 Specimen Table 7. Results of the SST SST S S S S S M M M M M H H H H H (a) M-5 specimen (b) H-1 specimen Figure 13. Results of the SiGMA analysis in the SST. 4 CONCLUSIONS In order to estimate shear between substrate and repairing, four types of shear tests are carried out. Results can be summarized as follow; (1) Surface in substrate is quantitatively estimated by applying the center line to detected waveform by laser displacement transducer. It is confirmed that the shear correlate with the center line for all test methods. (2) The shear in the BSST is similar to that of the DST, while higher shear in the PST and the SST are obtained. These differences are concluded that the BSST and the DST are carried out under the flexural moment, compressive stress effects for the PST and the SST. (3) Failure mode is investigated by applying AE-SiGMA analysis for all test methods. As a result, the number of failure specimens with highly rough interface is different by test methods. It is difficult to fail at interface of specimens with highly rough interface in the PST and the SST under the compressive loading. (4) The reasonable and easy shear test method is not conducted under the compressive stress but a procedure of which displacement of interface is not restrained such as the BSST and the DST. However, it requires a technique to reduce flexural moment in the BSST and the DST. ACKNOWLEDGMENT This work was supported by KAKENHI ( ) of JSPS. REFERENCES Austin, S., Robins, P. and Pan, Y. (1999) Shear bond testing of repairs, Cement and Concrete Research, 29: Uji, K., Satoh, K. and Kobayashi, A. (27) Effects of Retrofitting Method using CFRP Grid on The Shear Behavior of Existing Concrete Members, Proceedings of FRPRCS-8, (in CD-ROM) Momayez, A., Ehsani, M.R., Ramezanianpour, A.A. and Rajaie, H. (25) Comparison of Methods for Evaluating Bond Strength between Concrete Substrate and Repair Materials, Cement and Concrete Research, 35: Ohtsu, M. (1991) Simplified Moment Tensor Analysis and Unified Decomposition of AE Source, Journal of Geophysical Research, 96(B4):

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