Mechanisms of Bond Failure Between Existing Concrete and Sprayed Polymer Cement Mortar with CFRP Grid by Acoustic Emission

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1 Mechanisms of Bond Failure Between Existing Concrete and Sprayed Polymer Cement Mortar with CFRP Grid by Acoustic Emission Junlei Zhang 1, Kentaro Ohno 2, Ryo Kikuchi 1, Tran Vu Dung 1, Kimitaka Uji 3 and Atsushi Ueno 4 1 Graduate Student of Dept. of Civil and Environmental Engineering, Tokyo Metropolitan University 2 Assistant Professor of Dept. of Civil and Environmental Engineering, Tokyo Metropolitan University 3 Professor of Dept. of Civil and Environmental Engineering, Tokyo Metropolitan University 4 Associate Professor of Dept. of Civil and Environmental Engineering, Tokyo Metropolitan University Abstract: Recently, retrofitting technique by using Carbon Fiber Reinforced Plastic (CFRP) grid and sprayed polymer cement mortar (PCM) has been adopted for deteriorated concrete structures because CFRP has such appropriate characteristics as high tensile strength and high corrosion resistance in concrete structures. In this study, focusing on shear crack generation area of shear span in reinforced concrete beam, plate specimens are prepared in a laboratory. Plate specimens consist of three materials of base concrete, sprayed PCM and CFRP grid. Specimens have several parameters of the sectional area of CFRP grid, grid interval and the use of primer on bond surface in base concrete because shear strengthening effect varies with those parameters. In the experiment, a longitudinal bar in CFRP grid is pulled out, shear force is brought in bond surface between base concrete and sprayed PCM. In addition, acoustic emission (AE) method is applied to investigate failure process of bond surface between base concrete and sprayed PCM with CFRP grid in the specimen. Failure processes of tests are estimated by SiGMA (simplified Green s functions for moment tensor analysis) procedure. Keywords: shear bond, retrofitting technique, CFRP grid, acoustic emission, SiGMA analysis. 1. Introduction Recently, a lot of existing concrete structures have been deteriorated by aging, natural disasters or several chemical attacks. Many restore and strengthen methods have been developed and used to extend the service life of existing concrete structures. Here, a retrofitting method is widely applied to repair existing concrete structures. A combination of sprayed polymer cement mortar (PCM) and carbon fiber reinforced plastic (CFRP) is used for this method to effectively strengthen existing reinforced concrete (RC) beams. The CFRP has good characteristics of high strength and high corrosion resistance. The shear capacity in RC beams increases by attaching CFRP grids and PCM to side areas of base concrete beams (Liang et al. (28)). However, strengthening effect for shear capacity in RC beams repaired by sprayed PCM and CFRP grids have not been clarified yet. It is well known that the shear bond strength between existing concrete and repairing mortar is varied with source roughness, soundness of existing concrete and the use of primer on bond surface (Momayez et al. (25)). The pull-out tests of element specimens are conducted in a laboratory (Uji et al. (27)), to investigate shear bond behavior between existing concrete and sprayed PCM with a CFRP grid. In the experiment, specimens have some parameters of the roughness of bond surface, the sectional area of CFRP grid, the grid interval of CFRP grid and the use of primer. As a result of this investigation, higher strengthening effect can be obtained in the case that specimen has highly rough bond surface, larger sectional area of CFRP grid, short grid interval of CFRP grid and using primer on bond surface. In addition, the bearing areas of shear bond stress are shift from bottom of the specimen which is near the loading point to upper part of the specimen (Uji et ai. (27) and Zhang et al. (212)). In this study, acoustic emission (AE) method is applied to the pull-out tests of element specimens to investigate bond failure process between existing concrete and sprayed PCM with CFRP grid. Locations of micro cracks are determined from detected AE signals, and crack motions are classified by SiGMA (simplified Green s functions for moment tensor analysis) procedure (Ohtsu (1991) and Ohno et al. (21)). 2. Experimental procedure 2.1 Materials and specimens The element specimens were prepared to simulate a part of bond surface at shear span in repaired RC beam which consists of three materials of base concrete, a CFRP grid and PCM as shown in Figure 1. Figure 2 shows the element specimen in pull-out test. Four types of specimens were prepared as

summarized in Table 1. Base concretes were cast into molds which have dimensions of 3 x 37 x 9 mm and 3 x 27 x 9 mm. They were cured in wet condition until 8 days.

4 specimens, CFRP grids were put on the bond surfaces of all specimens. The PCM was sprayed on the bond surface with 4 mm thickness.

Mechanical properties of hardened base concrete, PCM at pull-out test day (age is 46 days of base concrete) and CFRP grids are summarized in Table 4 and Table 5, respectively. Table 1.

2 summarized in Table 1. Base concretes were cast into molds which have dimensions of 3 x 37 x 9 mm and 3 x 27 x 9 mm. They were cured in wet condition until 8 days. The bond surfaces of all base concretes were treated by sand blast, then base concretes were cured in wet condition in 4 days. After that, primer coated on the bond surfaces of No.1, No.3 and No.4 specimens, CFRP grids were put on the bond surfaces of all specimens. The PCM was sprayed on the bond surface with 4 mm thickness. Table 2 and Table 3 show mix proportions of base concrete and PCM, respectively. Mechanical properties of hardened base concrete, PCM at pull-out test day (age is 46 days of base concrete) and CFRP grids are summarized in Table 4 and Table 5, respectively. Table 1. Parameters of specimens. No. CFRP grid Grid interval (mm) Primer Surface treatment Dimensions of base concrete (mm) Dimensions of PCM (mm) 1 CR13 1 Used Sand blast CR13 1 Not Used Sand blast CR8 5 Used Sand blast CR8 1 Used Sand blast Examination object Figure 1. Investigation area of repaired RC beam. Concrete PCM CFRP grid Concrete 9 4 Steel plate AE sensor PCM Strain gauge (a) No.1 and No.2 specimens. CFRP grid Block Figure 2. Conceptual diagram of the specimens. Table 2. Mix proportions of base concrete. (b) No.3 specimen. G max SL W/C s/a Air Unit content (kg/m 3 ) (mm) (cm) (%) (%) (%) W C S G Ad Table 3. Mix proportions of PCM. Pre-mix mortar Polymer suspension Water (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) Table 4. Mechanical properties of hardened concrete and PCM. (c) No.4 specimen. Figure 3. Location of the strain gauges. Compressive strength (N/mm 2 ) Tensile strength (N/mm 2 ) Young s modulus (kn/mm 2 ) Concrete PCM

3 Table 5. Mechanical properties of CFRP grid. Section area (mm 2 ) Tensile strength (N/mm 2 ) Young s modulus (kn/mm 2 ) CR CR Test method The pull-out test has been developed by according to the Test method for bond strength between reinforcing steel and concrete by pull-out test (JSCE-G53) to investigate shear bond behavior between base concrete and sprayed PCM with a CFRP grid. In the pull-out test, applied load was given by pulling out an appearing longitudinal CFRP grid as shown in Figure 2. Four blocks and four steel plates were arranged to prevent generation of eccentric load. Strains of the CFRP grid were measured by strain gauges mounted on the CFRP grid as illustrated in Figure 3. AE signals were detected by 12 AE sensors of 15 khz resonance frequency to investigate failure process of each specimen. Detected AE signals were amplified with 4 db gain by pre-amplifier, then they were recorded by µsamos AE system. The threshold level was set to 35 db and AE waveforms were recorded at 1 MHz sampling frequency. Before the pull-out test, pencil-lead break method was conducted to all specimens to obtain elastic wave in each specimen. The location of a micro-crack as an AE source can be localized based on obtained elastic wave velocity, locations of AE sensors and the differences of the arrival time of each AE wave at each AE sensor. 3. Results and discussion 3.1 Failure mode and Ultimate load Table 6 shows failure mode and ultimate load in the pull-out test for all specimens. Higher ultimate load is obtained in the case that specimen has larger sectional area of CFRP grid and primer is coated to the bond surface. In contrast, No.4 specimen which has smaller sectional area of CFRP grid and longer interval of grid is failed at lowest ultimate load. Crack patterns of the specimens are illustrated in Figure 4. Figure 5 shows CFRP grids after the tests. In No.1 and No.2 specimens which have larger sectional area (CR13) of CFRP, cracks generated on surface of the PCM. The PCMs in No.1 and No.2 specimens are removed from the base concrete at final stage of the test. In contrast, the PCMs are not removed from the base concrete in No.3 and No.4 specimens, regardless of what cracks generate on surface of the PCM. The joint parts of the tensioned CFRP reinforcement and horizontal CFRP reinforcements are broken in No.3 and No.4 specimens as shown in Figure 5. The bond strength between base concrete and the PCM might be higher than the bond strength between joint parts of the CFRP grid in No.3 and No.4 specimens. The larger cross section area of CFRP grid could be hold the integrity of the CFRP grid. In the case that the strength between joint parts of the CFRP grid becomes higher than the bond strength between the base concrete and the PCM, the PCM is removed from the base concrete in this test. From comparison between ultimate loads of No.1 and No.2 specimens, it is found that the primer plays a role in getting higher loading capacity. In addition, the ultimate load of No.3 specimen which has 5mm grid interval is higher than that of No.4 specimen which has 1mm grid interval. Therefore, higher ultimate load in the pull-out test could be obtained in specimen, which has lager cross section area of CFRP, shorter grid interval and the use of primer. Table 6. Results of the pull-out test. No. CFRP grid Section area of grid (mm 2 ) Grid interval (mm) Primer Failure mode Ultimate load (kn) 1 CR Used 63. Removing of PCM 2 CR Not Used CR Used Slippage of the tensioned CR Used CFRP reinforcement 28.12

(a) No.1 (b) No.2 (a) No.1 (b) No.2 (c) No.3 (d) No.4 (c) No.3 (d) No.4 Figure 4. Crack patterns of the specimens. Figure 5.

Strain distributions of the CFRP grid 3.2.

applied load in all specimens. At the final stage of failure in specimens, strain behaviors vary with types of specimens. In No.

height in Figure6(a)) dramatically decrease with increase of the applied load at around near the ultimate load.

This phenomenon might be explained that the bearing area shifts from lower part to upper part of the PCM because a lot of cracks generate at lower

However, strains of tensioned reinforcement at upper part of No.2 specimen are larger than those of No.

This difference might come from the difference whether the primer is used or not.

4 (a) No.1 (b) No.2 (a) No.1 (b) No.2 (c) No.3 (d) No.4 (c) No.3 (d) No.4 Figure 4. Crack patterns of the specimens. Figure 5. Appearance of the CFRP grid. 3.2 Strain distributions of the CFRP grid Strain distributions of the tensioned CFRP reinforcement Strain distributions of the tensioned longitudinal CFRP reinforcements in the specimens are shown in Figure 6. Strains of the lower part of the tensioned longitudinal CFRP reinforcement are larger than that of the upper part of the reinforcement at lower applied load in all specimens. At the final stage of failure in specimens, strain behaviors vary with types of specimens. In No.1 specimen which has 1 mm grid interval, CR13 grid and application of the primer, strains of the tensioned CFRP reinforcement at lower part (5mm height in Figure6(a)) dramatically decrease with increase of the applied load at around near the ultimate load. On the other hand, strains of the upper part of the reinforcement increase with increase of the applied load as shown in Figure 6(a). This phenomenon might be explained that the bearing area shifts from lower part to upper part of the PCM because a lot of cracks generate at lower part of the PCM near the ultimate load. This tendency is confirmed in results of No.2 specimen as shown in Figure 6(b). However, strains of tensioned reinforcement at upper part of No.2 specimen are larger than those of No.1 specimen when tensioned reinforcement is pulled-out same applied load (e,g; 4 kn). This difference might come from the difference whether the primer is used or not. Therefore, it is found that the application of the primer brings a good adhesion between the base concrete and the PCM. Comparing between strain distributions of No.1 and No.4 specimens, larger strain is obtained in No.4 specimen at the same applied load. The differences between No.1 and No.4 specimens are the sectional area of the CFRP reinforcement and the adhesion area of the PCM. From these results, larger sectional area of the CFRP reinforcement and large adhesion area are required to repair existing concrete beams. Strains of the tensioned CFRP reinforcement in No.3 specimen which has 5 mm grid interval are larger than those in No.4 which has 1 mm grid interval at 2 kn. From results of the strain distributions of No.3 and No.4 specimens, it is found that strain of the tensioned

5 Height (mm) Height (mm) Height (mm) Height (mm) CFRP reinforcement is widely distributed in the case that specimen has short grid interval. In this case, specimen bears for the applied load at all bond surface. 1kN 2kN 3kN 4kN 5kN 57kN 6kN 63kN kN 2kN 3kN 4kN 43.96kN 45.83kN (a) No.1 (CR13@1mm) (b) No.2 (CR13@1mm) 25 1kN 2kN 3kN kN 2kN 24kN Strain (µ) (c) No.3 (CR8@5mm) (d) No.4 (CR8@1mm) Figure 6. Strain distributions of the vertical CFRP reinforcement Strain distributions of the horizontal CFRP reinforcement Figure 7 shows the relation between the applied load and strains of horizontal CFRP reinforcements. In No.1 specimen, small strains of the horizontal reinforcements are obtained until about 55 kn. Here, as seen in Figure 6(a), the difference between strains of tensioned reinforcement at 5 kn and that at 57 kn is quite small at bottom of the specimen. Therefore, cracks might generate at bottom area of No.1 specimen at around 55 kn. After that, strains of lowest horizontal reinforcement (gauges No.3,4 and 5) suddenly increase, then the PCM is removed from the base concrete in No.1 specimen. In No.2 specimen, strain of gauge No.3 becomes larger than that of others from 1 kn. Cracks might generate at this applied load in No.2 specimen because AE sources generate at bottom of the specimen as shown in following section. Strains of horizontal reinforcements are not large in No.2 specimen comparing with other specimens. The application of primer does not affect the horizontal reinforcements, while the difference of the use of primer is represented with the strain behavior of the tensioned longitudinal reinforcement. The higher shear bonding strength in No.1 specimen brings the higher ultimate load, because primer is applied to the surface of the base concrete in this specimen. In No.3 specimen, large values of strain are obtained at gauges of No.3, 8, and 13 where they are located at near the tensioned reinforcement as shown in Figure 7(c). In addition, strains of the horizontal reinforcement decrease with increase of the distance from the tensioned reinforcement. The similar tendency is obtained in No.4 which has 1 mm grid interval. On the other hand, the values of strain in horizontal reinforcement at near the tensioned reinforcement increase in the case that specimen has short grid interval. Concerning results of strain distributions of the tensioned and the horizontal reinforcements, it is realized that specimen which has short grid interval bears for the applied load at all areas of bond surface.

6 3CH 4CH 5CH CH 4CH 5CH 7 6 9CH 1CH 11CH (a) No.1 (CR13@1mm) 9CH 1CH 11CH CH 16CH 17CH CH 16CH 17CH CH 4CH 5CH Strain (µ) 7 3CH 4CH 5CH (b) No.2 (CR13@1mm) 8CH 9CH 1CH Strain (µ) (c) No.3 (CR8@5mm) 9CH 1CH 11CH (d) No.4 (CR8@1mm) CH 14CH 15CH Strain (µ) 15CH 16CH 17CH Figure 7. Strain distributions of the horizontal CFRP reinforcement. 3.3 Results of the SiGMA analysis The SiGMA analysis is successfully applied to the pull-out test. The SiGMA analysis consists of the location analysis of an AE source and moment tensor analysis of the AE source. Results of the SiGMA analysis are brought locations of AE sources and AE generation modes of tensile, mixed-mode or shear The location of AE sources Figure 8 shows the results of the SiGMA analysis in all specimens. AE sources mainly generate at around the tensioned CFRP reinforcement in all specimens. In No.1 specimen, AE generation area is wider at bottom of the specimen by comparing with upper part of the specimen. Therefore, shape of the bearing area for the applied load might become triangle distribution. This shape is also obtained in No.3 specimen as shown in

7 Figure 8(c). On the other hand, only a few AE sources are generated at No.2 specimen. In this specimen, since the primer is not applied to the bond surface, adhesion force between the base concrete and the PCM is not effectively produce. As a result, lowest ultimate load is obtained in this specimen. In No.4 specimen, a lot of AE sources are obtained after the applied load reaches maximum load as shown in following section (Figure 9(d)). This is because the tensioned CFRP reinforcement is slipped in the specimen, a lot of AE sources generate due to friction between tensioned CFRP reinforcement and PCM or base concrete. TENSILE MIXED-MODE SHEAR TENSILE MIXED-MODE SHEAR x (m).9 z (m) x (m).9 z (m) (a) No.1 (CR13@1mm) (b) No.2 (CR13@1mm) TENSILE MIXED-MODE SHEAR TENSILE MIXED-MODE SHEAR x (m).9 z (m) x (m).9 z (m) (c) No.3 (CR8@5mm) (d) No.4 (CR8@1mm) Figure 8. The location of AE sources Relation between AE events and the applied load along with time The relations between height of AE source location and the applied load are summarized in Figure 9. In all specimens, AE sources shift from lower part to upper part of specimens with increase the applied load. These phenomena are suggested that bearing area for the applied load shifts from lower part to upper part of the specimens in these tests. In addition, the bearing area at lower part of the specimens becomes larger than that at upper part as mentioned above and shown in Figure 8. Grid priont Load TENSILE MIXED-MODE SHEAR Grid priont Load TENSILE MIXED-MODE SHEAR Time (s) 1 2 Time (s)

8 (a) No.1 (b) No.2 Grid priont 4 3 Load TENSILE MIXED-MODE SHEAR.25.2 Grid priont Load TENSILE MIXED-MODE SHEAR Time (s) Time (s) (c) No.3 (CR8@5mm) (d) No.4 (CR8@1mm) Figure 9. Relation between AE events and the applied load along with time. 4. Conclusions In this study, the pull-out tests of element specimens are conducted to investigate bond failure process between base concrete and sprayed PCM. As a result, failure mode varies with specimen conditions which are the sectional area of CFRP grid, grid interval and the use of primer. Higher loading capacity for shear bond is obtained in the case that specimen has large sectional area of CFRP grid, shorter grid interval and the use of primer. It is suggested from results of AE monitoring that the bearing area for the applied load shifts from lower part to upper part of the specimen. In addition, results of the SiGMA analysis imply that the shape of the bearing area for the applied load becomes triangle distribution. 5. Acknowledgement This research was supported by Asian Human Resources Fund from Tokyo Metropolitan Government. 6. References 1. Liang, J., Uji, K. et al., Shear strengthening of RC beam with CFRP grid and sprayed mortar, Proceedings of the Japan Concrete Institute, 3(2), 28, pp (in Japanese) 2. Momayez, A., Ehsani, M.R. et al, Comparison of methods for evaluating bond strength between concrete substrate and repair materials, Cement and Concrete Research, 35, 25, pp Ohno, K., Ohtsu, M., Crack Classification in Concrete based on Acoustic Emission, Construction and Building Materials, 24, 21, pp Ohtsu, M., Simplified moment tensor analysis and unified decomposition of acoustic emission source: Application to in situ hydrofracturing test, Journal of Geophysical Research, 96(B4), 1991, pp Uji, K., Satoh, K. et al., Effects of retrofitting method using CFRP grid on the shear behavior of existing concrete members, Proceedings of FRPRCS-8 (in-cd-rom), 27, Greece. 6. Zhang, J., Ohno, K. et al., Investigation of shear bonding behavior between base concrete and polymer-modified mortar with CFRP grid, Proceedings of the 3 RD ICCRRR (in-cd-rom), 212, Cape Town, South Africa.

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

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