EXPERIMENTAL STUDY ON EFFECTIVENESS OF SHEAR STRENGTHENING OF RC BEAMS WITH CFRP SHEETS Yasuhiro Koda and Ichiro Iwaki Dept. of Civil Eng., College of Eng., Nihon University, Japan Abstract This research aims to clarify the effectiveness of shear strengthening of RC beams with externally bonded carbon fiber polymer (CFRP) sheets. In this experiment, the RC beams which were wrapped by CFRP strips were tested for shear strengthening. The experimental parameters included the existence of stirrups, the CFRP strip width ( and mm), and failure mode of RC beams. The experimental results showed that the failure load of RC beams was remarkably increased by strengthening with CFRP strips. Moreover, the failure patterns of RC beams and the strain behaviors on CFRP strips were influenced by the width of the CFRP strips. These results indicated that the effectiveness of shear strengthening of RC beams remarkably depended on CFRP strips. Key words: Shear capacity, CFRP sheet, Shear resistance behavior of CFRP strips, RC beams 1. INTRODUCTION The use of externally bonded carbon fiber reinforced polymer (CFRP) sheets to strengthen reinforced concrete structures has gained a high popularity as a retrofiting technique of RC structures. CFRP sheets are regarded as excellent construction materials because of their light weight, formability, and easy handling. As for shear strengthening of RC members with CFRP sheets, appropriate evaluation for shear resisting behavior of CFRP sheets is essential. However, the past researches on the behavior of RC beams strengthened in shear using CFRP sheets have not solved this problem yet. This research aims at the effectiveness of shear strengthening of RC beams with externally bonded CFRP strips. In this paper, the authors have conducted three experimental series: NS, S and B. The objective of NS and S series was to examine the shear resisting capacity of CFRP strips using the RC beams which were designed so that shear failure would occur before flexural failure. The objective of B series was to evaluate the shear resisting capacity of CFRP strips wrapped to prevent from peeling of CFRP sheet for flexural strengthening.
2. OUTLINE OF THE EXPERIMENT Nine RC beams having a cross-sectional dimension of 1mm 3mm and the ratio of shear span to effective depth (a/d) were 3.. These specimens are divided into three groups of NS, S and B series. Three specimens of NS series were without stirrups. For the flexural reinforcement in NS and B series, two D22(φ22.2mm) steel bars were used for tension and two D1(φ9.53mm) steel bars for compression. In the other three specimens of S series, two φ 26 deformed prestressing bars for tension and two D22( φ 22.2mm) steel bars for compression were prepared. In S and B series, the stirrups of D1 steel bars were used and were spaced at mm. The beams of NS and S series before unretrofitted by CFRP strips were designed so as to fail in shear while B series were designed to fail in flexure. For the shear strengthening of NS and S series, the CFRP strips with and mm width were used, and these strips were wrapped with one layer at a right angle to beam axis. The CFRP strips were spaced at mm. For the flexural strengthening of B series, the CFRP sheet of width 13mm on the bottom was bonded, and CFRP strips were wrapped for preventing from peeling of the CFRP sheet. All the beams to be tested were designed based on the design guidelines JSCE[1][2]. The details of the test specimens are shown in Fig. 1 and Table 1. The physical properties of steel bars and concrete used for the beams were obtained from laboratory experiments by the authors. These properties are shown in Table 2, and the physical properties of CFRP sheet are shown in Table 3. Four-point loading test was applied to a simply supported beam. The load was applied by a hydraulic jack and measured by a load cell. Deflection measurements were taken at midspan of the beams by a deflection transducer. The strain of stirrups and CFRP strips were measured by strain gages. The gages were mounted on the stirrups and on the top layer of CFRP strips. Strain gages on the CFRP strip, which is at the position expected to develop a shear crack (sheet, 3, 2, and 3 in Fig.1), were mounted at 2 and mm spacing for the accuracy measurement of a strain distribution. ÅyUnit ; mmåz C L :Strain gauge 4' 3' 2' 1' 1 2 3 4 D22 D1@ É 26 D22 D1 S series NS series (a) NS & S series Figure 1: Details of test specimens
ÅyUnit ; mmåz C L 4' 3' 2' 1' :Strain gauge D1@ D22 D1 Specimen:B-F-S Specimen:B-F (b) B series Figure 1: Details of test specimens Table 1: Details of test specimens Series NS S B Shear reinforcing Flexural reinforcing Specimen Stirrup CFRP Strips CFRP Sheet name Type p s (%) Å 1 Usage Width(mm) p cf (%) Å 2 Usage Width(mm) NS - - Not used - - - - NS5 - -.6 - - Full-wrapping NS1 - -.11 - - S Not used - - - - S5.6 - - Full-wrapping S1 D1.11 - -.48 B- @ - - Not used - - B-F 3 layers 13 B-F-S Full-wrapping.6 at bottom Å 1 Å 2 Ratio of stirrup Shear reinforcement Ratio of CFRP Strips Table 2: Physical properties of steel bars and concrete Series NS&S B Type Yield strength (MPa) Steel bars Tensile strength (MPa) Modulus of elasticity (GPa) Compressive strength of concrete (MPa) D1 385 542 191 D22 381 567 194 28.5 É 26 Å 171 1199 229 D1 358 511 193 D22 335 193 26. Å Deformed prestressing bar Table 3: Physical properties of CFRP sheet Tensile strength (MPa) Modulus of elasticity (GPa) Fiber density (g/m 2 ) Thickness (mm) 417 231 313.167
3. TEST RESULTS 3.1 Load-deflection relationships The load-deflection relationships for NS and S series are shown in Fig. 2. As the shear strengthening by the CFRP strips increases, the failure load is increased. In the case of NS series, the failure loads of NS5 and NS1 are 92.8kN and 18kN higher than that of NS while NS5 and NS1 give larger deflections at the failure load than NS, which are 55mm and 77mm, respectively. Tension steel bars of NS5 and NS1 reached to yield strength under the loading point after 18kN. In the case of S series, the stirrups in S reached to yield point after kn, and reached the failure load at 33kN. On the other hand, the stirrups of S5 and S1 reached to yield point after kn. As the applied load increases, S5 and S1 failed by the rupture of CFRP strips at 449.5kN and 46.8kN, respectively. The rupture of CFRP strips in S5 was gradually occurred by the development of diagonal cracking in shear span. However, in S1, only CFRP strip (Fig. 1) suddenly ruptured. The load-deflection relationships for B series are shown in Fig. 3. B- specimen shows a large deflection after the yielding of tension bar at 17kN. The tension bars of B-F and B-F-S yielded at almost same load level, which is increased up to 25% in comparison with B-. Furthermore, the failure load of B-F followed by the peeling of CFRP sheet was 21kN, whereas in B-F-S, the failure load was improved to kn because the peeling of CFRP sheet was controlled by the CFRP strips wrapping specimen. 3.2 Failure patterns of specimens Failure patterns in the specimens are shown in Fig. 4. The failure mode of NS and S was a diagonal tension failure. However, the failure mode of NS5 and NS1 is changed to flexural failure due to higher resistance to the shear force by the CFRP strips. On the other hand, in case of S series, S5 failed by rupture of CFRP strips associated with diagonal cracking in shear span while S1 showed rapid collapse just after one of the CFRP strips ruptured suddenly. All specimens of B series showed flexural failure, but the flexural strengthened specimen, B-F, failed with crushing of concrete after the CFRP sheet was peeled off concrete. B-F-S failed with the separation of CFRP sheet after the rupture of CFRP strips. 3.3 Strain distributions of CFRP strips Typical strain distributions of CFRP strips of NS and S series are shown in Fig. 5. These strain distributions were measured at No. 2 and No. 3 CFRP strips (Fig. 1). The strain of CFRP strips is concentrated at diagonal crack across the CFRP strips. The strain of CFRP strips when debonding occurred was about μ, which is almost the same as the result indicated in [3]. In the case of S1, an even strain distribution is formed after debonding. Typical strain distributions of CFRP strips of B series are shown in Fig. 6. The strain of CFRP strips in the region of mm-mm from the bottom of B-F-S specimen after 214.6kN, which is almost the same as the failure load of B-F specimen, is remarkably increased. This means that the CFRP strips clearly contributed to preventing from peeling of CFRP sheet for flexural strengthening. Additionally, the strain of the CFRP strips near the diagonal crack around mm from the top is increased. This is due to the fact that the CFRP strips resist the applied shear force too.
4 NS NS5 NS1 S S5 S1 Load (kn) S NS S5 S1 NS1 NS5 2 4 6 8 Deflection (mm) Figure 2: Load-deflection relationships of NS & S series Load (kn) B- B-F B-F-S Design shear capacity of B series 2 4 6 8 Deflection (mm) Figure 3: Load-deflection relationships of B series
NS S B NS5 S5 B-F NS1 S1 B-F-S Figure 4: Failure patterns of specimens NS5 Debonding S1 Distance from top (mm) 1 Load (kn) CFRP 12 114 1 19 Strips 3 Strain (É ) CFRP Strips Load (kn) 282 314 385 456 Figure 5: Typical strain distributions of CFRP strips of NS&S series Distance from top (mm) 1 kn 198kN (Yielded at tension bar) 214.6 kn (P max : B-F) 251kN(P max) Specimen:B-F-S CFRP Strips: Cracking 3 4 Strain (É ) Figure 6: Typical strain distributions of CFRP strips of B series
3.4 Shear resisting behavior of CF strips Fig. 7 shows the relationships between the applied shear force and the resisted shear force by CFRP strips in NS and S series. The resisted shear force by CFRP strips is obtained by the average strain of CFRP strips (No. 2, No. 3). Thus, the value, V cf is given by V cf = ε cf E cf A cf (1) where, ε cf is the average CFRP strips strain, E cf is elastic modulus of CFRP sheet, A cf is cross section of CFRP sheet. The resisted shear force by CFRP strips increases according to the increment of applied shear force. The total resisted shear force at failure load was 116kN for S5 and 156kN for S1, respectively. Hence, the resisted shear force of CFRP strips increased according to increment of CFRP strips width. Fig. 8 shows the test results with the failure load in B series. Compared with B-, the failure load of B-F gives a 24% increase by flexural strengthening effect. Further, the failure load of B-F-S increased 45% more than that of B- by the peeling control effect. Especially, differential force at failure load between B-F and B-F-S was about 4kN. Thus, even the CFRP strips for peeling control obviously contribute to resisting of the shear force. 4. CONCLUSIONS (1) The failure load of RC beams was remarkably increased by shear strengthening with full-wrapped CFRP strips. However, the effect of the CFRP strip width on the failure load was not obviously found in this research. (2) The failure patterns of RC beams and the strain behaviors on CFRP strips were influenced by the width of CFRP strips. (3)The CFRP strips wrapped to control the peeling of CFRP sheet remarkably contributed to the shear strengthening of RC beams. Applied shear force (kn) NS Applied shear force (kn) 1 1 CFRP strips CFRP strips NS5 S5 NS1 S1 2 4 6 2 4 6 Shear resisting force of CFRP strips (kn) Figure 7: Relationships between of applied shear force and shear resisting force by CFRP strips in NS& S series S
Failure load (kn) 1 Δ4kN:shear resisting force of CFRP strips Peel ing control by wrapped CFRP strips Flexural strengthening with CFRP sheet B- B-F B-F-S Specimens of B series Figure 8: Compare the failure load of B series ACKNOWLEDGEMENTS This research was partially supported by a Grant from the Ministry of Education, Culture, Sports, Science, and Technology to promote multidisciplinary research projects on Study of ecological life cycles in local cities and middle grade mountain areas and the information and communication technology indispensable for their support at Nihon University, College of Engineering (Director: Prof. Motohisa Onozawa). REFERENCES [1] Japan Society of Civil Engineers, 'Standard Specifications for Concrete Structures-2 (Structural Performance Verification)', JSCE (2) 58-93 [2] Japan Society of Civil Engineers, 'Recommendations for Upgrading of Concrete Structures with Use of Continuous Fiber Sheet', Concrete Engineering Series 41, 1 [3] Abe, H. et al, 'The effect of shear strengthening and its resistance mechanism of RC beams with carbon fiber sheets', Journal of Structural Engineering, Vol.51A, March, 5(Japan Society of Civil Engineers)1291-138.