Study of the bond behavior of carbon fibre reinforced polymer bars

Transcription

1 Study of the bond behavior of carbon fibre reinforced polymer bars S.B. Singh and Aditi Chauhan In this study, experiments were conducted to find the bond characteristics of three different carbon fibre reinforced polymer (CFRP) bars, i.e., DCI C-bars (manufactured by Diversified Composites Inc.), MIC C-bars (manufactured by Marshall Industries Composites Inc.) and Leadline C-bars (manufactured by Mitsubishi Functional Products Inc.) embedded in concrete. In addition to the bond characteristics of three different CFRP-bars, bond characteristics of steel bars were also evaluated. The bond characteristics of steel bars provided the basis for comparison of bond characteristics of CFRP bars. Load-slip relationships were plotted for the CFRP bars and the steel bars. The average bond strengths of DCI bars, MIC bars, Leadline bars and Steel bars are predicted to be 7.92 MPa, 6.89 MPa, 6.27 MPa and MPa, respectively. Keywords: Bond behavior; bond strength; CFRP; load-slip relationship. Bond between the fibre reinforced polymer (FRP) bars and concrete allow the transfer of stresses between them and effective utilization of the strength of rebars. Many factors contribute to the development of bond strength of the FRP bars. Studies show that with increase in the bond length, sufficient bond stresses are developed to attain full tensile strength of FRP bars [1]. It is also observed that the bond strength of glass fibre reinforced polymer (GFRP) splices, continuous braided aramid fibre rods, and carbon fibre rods subjected to static and fatigue loads is significantly affected by the variation of embedment length [2-3]. The effect of prestressing force on the bond strength of aramid fibre reinforced polymer (AFRP) and carbon fibre reinforced polymer (CFRP) bars were examined by Takagi et al [3]. Pullout tests conducted on GFRP bars demonstrate that the bond strength of GFRP bars is lower than that of steel bars. From the same tests, it is concluded The Indian Concrete Journal, , Vol. 89, Issue --, pp. - that hooked bars with larger bend radius provides larger stiffness [4]. Moreover, bond slip behavior of FRP bars is affected by the mechanical properties of resins. It has been observed the AFRP bars exhibit larger free end slip compared to steel bars, however bond creep effects in beams reinforced with carbon fibre twisted cables (CFTC) and AFRP bars are not significant [5]. Pullout bond tests conducted on deformed and smooth CFRP bars showed that deformed bars exhibit bond strength of 2 to 5 times that of smooth bars [6]. Also that the bond capacity is controlled by the strength and mechanical action on the surface of FRP rods rather than the adhesion and friction between the concrete and bars [7]. Tests were conducted by Achillides et al [8] and Baena et al [9] to study the effect of the type, shape and diameter of FRP bars and concrete compressive strength. They observed that the larger diameter bars result into lower bond strength and the shape of the FRP has a little effect on their bond The Indian Concrete Journal

2 strength. Thermal effects on the bond between GFRP bar and concrete reduce the bond strength more than that of steel. However, mechanical fatigue has no effect on the bond strength of GFRP bars, while it reduces the bond strength of steel bars by about 13% [1]. A comparison between the bond strength of different FRPs suggests that GFRP exhibits the highest bond strength, while CFRP and AFRP bars have no appreciable difference in their bond strengths. Also for the same configurations, bars with higher elastic modulus have higher bond strength [11]. Analytical analysis of FRP bars embedded in concrete suggests that the bond between the FRP and concrete depends on the surface configuration of bar and the manufacturing process [12]. Finite Element models with and without slip effect have been developed to study numerically the bond behavior of FRP bars [13]. The finite element analysis results agree well with those obtained from the experimental studies. To study the effect of cyclic actions/forces on the bond behavior of FRP strips glued to concrete (quite common in seismic retrofitting of existing buildings), a theoretical model has been developed by Martinelli and Caggiano [14]. Bond performance of an FRP bar is dependent on the design, manufacturing process, mechanical properties of the bar itself, and the environmental conditions [15]. When anchoring the reinforcing bar in concrete, the bond force can be transferred by: Adhesion resistance of the interface, also known as the chemical bond. Frictional resistance of the interface against slip. Mechanical interlock due to irregularity of the surface. In FRP bars, it is postulated that bond force is transferred through resin to the reinforcement fibres, which may result in bond shear failure in the resin. Unlike steel reinforcement, bond of FRP bars is not significantly affected by compressive strength of concrete, provided adequate concrete cover exists to prevent longitudinal splitting. Pull-out test, cantilever test, and T-beam test are the usual bond tests to predict bond behavior of FRP rebars [15]. The principle objectives of the present paper are the following: To examine the bond characteristics of CFRP reinforcing bars / tendons surrounded with concrete using T beam specially fabricated for bond test. To develop load versus slip relationship for steel bars, DCI tendons, MIC bars and Leadline tendons. To find the average bond strength of steel and three types of CFRP bars, i.e., DCI tendons, MIC reinforcing bars and Leadline tendons. To compare the bond strength of CFRP tendons with that of steel tendons. Research Significance It is known that advanced fibrous composite materials such as CFRP can eliminate the problem of corrosion and substantially increase the strength and stiffness of the beams internally reinforced with CFRP bars. Additionally, CFRP offers excellent fatigue characteristics, electromagnetic neutrality, low axial coefficient of thermal expansion, and handle ability due to its lightweight. However, the inherent differences between steel reinforcement and CFRP reinforcement necessitate the development of appropriate design procedures. Current design codes dealing with reinforced concrete do not take into consideration the two most important differences between FRP and steel reinforcement. FRPs are anisotropic (directionally dependent elastic properties) and heterogeneous (composed of constituent materials having different properties). Recognizing that no standards exist for manufacturing of FRP bars, for example, the deformed bars include ribbed type bars, indented rods, twisted strands, and spiral glued type bars. Within each type, bars are made of different materials (different fibres and matrix), and have different geometry (fibre diameter, fibre orientation, spacing, and the size of ribs). These differences in physical and mechanical properties introduce randomness in the bond behavior of FRP bars, leading to complexities in the development of The Indian Concrete Journal

3 Table 1. Details of T-beam bond specimens Beam notation Number of specimens Bar Material Embedment Length, Nominal bar diameter, d (E f /E s ) f fu MPa f c MPa TD DCI 762 TD-61 3 DCI 61 TD DCI 457 TM MIC 762 TM-61 2 MIC 61 TM MIC 457 TL Leadline 762 TL-61 3 Leadline 61 TL Leadline 457 TS-51 2 Steel 51 TS-38 2 Steel 38 TS-25 2 Steel * Yield Strength; E f, E s, f fu, and f c represent the modulus of elasticity of CFRP bar and steel bar, ultimate strength of CFRP bar, and concrete strength, respectively * 48 a uniform quantitative bond-slip relationship. Therefore, determining the bond characteristics of commercial rebars is a fundamental requirement for their practical use, as this influences the mechanism of load transfer between reinforcement and concrete. Experimental investigation Leadline tendons provided by Mitsubhishi Functional Products Inc., Japan. The T-beams reinforced with a particular CFRP-bar were divided into groups of three different embedment lengths, i.e., 762 mm, 61 mm and 457 mm. In addition, six T-beams reinforced with steel bars were also fabricated and tested in order to Concrete having characteristic compressive strength of 48 MPa was used to cast the T-beams. Steel bars of yield strength 414 MPa were used as the reinforcement. Three types of CFRP bars/tendons were used as reinforcing bars: (i) DCI tendons, manufactured by Diversified Composites Inc., USA (ii) C-bars, manufactured by Marshall Industries Composites Inc. (MIC), USA and (iii) Leadline tendons manufactured by Mitsubishi Functional Products Inc., Japan. Details of material characteristics are shown in Table 1. Specimens, setup and testing equipments A total of twenty-five T-beams were fabricated and tested to examine the bond characteristics of CFRP reinforcing bars/tendons with concrete. Nine beams were reinforced with DCI tendons; six beams were reinforced with C- bars provided by Marshall Industries Composites Inc. (MIC), and remaining ten beams were reinforced with The Indian Concrete Journal

4 TECHNICAL PAPER of T-beam bond specimens and its stirrups, respectively. As shown in Figure 1, the width and depth of the flange of the T-beams were 254 mm and 76 mm, respectively. The total depth of each T-beam was 356 mm. The total and effective span of T-beams was 1,83 mm and 1,576 mm, respectively. Two CFRP bars (Figure 1) of 1,778 mm length were used to hold the reinforcing stirrups, while the reinforcing bar tested for bond strength was located at a distance of 38mm from the bottom (Figure 1). 89 mm 267 mm 38 mm (Radius) Instrumentation of T-beams Figure 2. Configuration of stirrups used in T-beam bond specimen compare the bond characteristics of CFRP tendons and steel bars. The embedment lengths for three different groups, each with two specimens, of steel reinforced Tbeam specimens TS-51, TS-38 and TS-25 were 51 mm, 38 mm and 25 mm, respectively. Details of all T-beam bond specimens are presented in Table 1. In the designation of beams, letters T,D,M,L and S stand for T-beam, DCI reinforcing bar, MIC reinforcing bar, Leadline bar, and steel bar, respectively. The number associated with each T-beam designation represents the embedment length of the bars in mm. Figures 1 and 2 show the configuration Two linear voltage displacement transducers (LVDTs) were mounted on the protruding portions of the bar at both ends of the T-beam (Figure 3) to measure the slippage of the bar from the concrete. Threaded cores of each LVDT were attached to the end of the concrete beam. Two strain gages were installed on the bar at midspan, i.e., within the notched portion (Figure 3) of T-beam to evaluate the tension force in the bar. A string pot mounted on a fixed strut with its string connected to the top surface of the beam at midspan measured the midspan deflection of the T-beams. Bond test setup and Testing T-beams using CFRP bars were tested for bar/tendon bond characteristics using four-point load system as shown in Figure 4. The beam was simply supported at both ends using hinge support at one end and a roller at the other end. The center of each support was located 127 mm from the corresponding beam end. A 1 x 1 x 12.5 mm steel box section loading beam was used to String pot TS-38-1 LVDT Strain gage Bonded length DCI tendon Figure 3. Instrumentation of bond test T-beam The Indian Concrete Journal Figure 4. Bond test setup of T-beam TD-457-2

5 The bond strength is calculated using Eq. (1). = π... (1) where d b is the bar diameter, l b is the bond length, and T is the tensile force acting on the bar. transfer the applied central load to the T-beam. Beams were loaded statically to failure using MTS actuator and pump that had a maximum capacity of 365 kn. A digital display unit connected to the actuator controlled the hydraulic pressure and the rate of loading. Tensile force (T) is calculated by equating the external and internal moments acting on the beam as expressed by Eq. (2). =... (2) where, P is the reaction at the support, d arm is the moment arm of the CFRP bar and a is the distance from the support to the upper end of the major crack, as shown in Figure 5. Tensile force (T) can also be calculated using the strain method as: = ε... (3) Table 2. Test results of T-beam bond specimens Beam notation Bar material Nominal bar diameter, d Failure load (kn) Bond length, l b Bond strength, u (MPa) Mode of failure TD-762* Bond TD Bond TD Bond TD Bond TD-61-2 DFI Bond TD Bond TD Bond TD Bond TD Compression TM Bond TM Bond TM Bond MIC 9.5 TM Bond TM Bond TM Bond TL Bond TL Bond TL Bond TL Bond TL Bond Leadline 1 TL Bond TL Bond TL Bond TL Bond TL Bond TS Bond TS Compression TS Bond Steel 9.5 TS Bond TS Bond TS Bond *Represents embedment of 762 mm for Beam TD All beams were fabricated with the concrete having strength of 48 MPa The Indian Concrete Journal

6 Table 3. Average bond strengths for bars and tendons Bar material Nominal bardiameter, d f c (MPa) Average bond strength (MPa) Ratio of bond strength of CFRP bars/tendons to the bond strength of steel bars DCI MIC Leadline Steel where, E f is the modulus of elasticity of CFRP tendons, e is the measured strain, and A f is the cross sectional area of the CFRP bar. This method is used to verify the values of tensile force (T) calculated from the bending moment method. Results and Discussions Bond capacity of CFRP bars/tendons such as DCI, MIC, Leadline bars, and steel bars are based on the maximum load carrying capacity of T-beam. Two types of failure modes were encountered during T-beam bond tests, i.e., bond failure and compression failure. Here, the bond failure is defined as the initiation of slippage of the free end of the bar, while compression failure is defined as the crushing of concrete at the top flange of the beam. Bond Characteristics of CFRP bars Details of failure load, effective bond length l b, bond strength u, and failure mode for each of the T-beams tested for bond characteristics of CFRP bars and steel bars are presented in Table 2. Test results characterizing the bond behavior of each kind of tendon and/or rebar are discussed in the following sections, while average bond strength of each type of tendon/bar is presented in Table 3. DCI bars The bond failure loads of nine T-beams corresponding to slips at ends A and B varied from kn to kn (Table 2 and Figure 6). It should be noted that bond length (l b ) was measured from the free end of the beam to the point where major crack intersected reinforcing bar. The bond length of DCI bars varied from mm to mm. It is observed from Figure 6 that the load carrying capacity of T beam does not increase significantly after the initiation of slippage of bars/tendons. It is observed from Table 2 that the bond strength of DCI bars varied from 6.55 MPa to 9.44 MPa. Two results for the DCI bond strength were found to be inconsistent with the other test results and were discarded. As presented in Table 3, the average value of the bond strength of DCI bars is 7.92 MPa, i.e.,.55 times that of steel bars. As shown in Table 2, all the T-beams reinforced with DCI bars failed due to bond failure except one beam (TD-457-3) which failed due to crushing of concrete. Figure 7 shows the typical failure of T-beam reinforced with DCI bars. The Indian Concrete Journal

7 TECHNICAL PAPER MIC bar 12 END B END A 1 8 Load, kn The load versus slip relationships for MIC-bar is shown in Figure 8. The failure load of six T-beams with MIC Cbar varied from kn to kn, while bond length varied from mm to 46.4 mm. The corresponding bond strength of MIC bars (Table 2) varied from 6.48 MPa to 8.34 MPa. As shown in Table 3, the average value of the bond strengths of MIC bars is 6.89 MPa, i.e.,.48 times that of steel bar. As in the case of DCI bars/tendons load capacity of T-beams reinforced with MIC bars does not exhibit significant increase after the initiation of slip. It is also noted from Table 2, that all the test Tbeams reinforced with MIC bars failed due to slippage of bars, i.e., bond failure. A typical failure mode of test T-beam with MIC bars is shown in Figure 9, where bond failure led to flexural shear crack eventually leading to compression failure. 6 4 End A End B Slip, mm Figure 8. Typical load versus slip relationship for MIC bars Leadline tendons Figure 1 shows the typical load versus slip relationships for a Leadline tendon. Based on the failure of ten test Tbeams with different embedment lengths, it is observed that the failure load varied from 71.2 kn to 12.8 kn. Bond lengths of Leadline tendons varies from mm to mm. The corresponding bond strength varies from 5.1 MPa to 8.2 MPa. One of the test results of Leadline tendon was found to be inconsistent, i.e., bond strength of 1.34 MPa for Leadline was discarded. As presented in Table 3, the average value of the bond strengths of Leadline tendon is 6.27 MPa which equals to.43 times Flexure/shear crack TM-61-1 MIC bar Figure 9. Flexural/shear failure of bond test T-beam with MIC bar END B END A Load, kn 7 TD Flexural crack End B End A 2 1 DCI tendon Figure 7. Flexural failure of bond test T-beam with DCI bars Slip, mm Figure 1. Typical load versus slip relationship for Leadline tendons The Indian Concrete Journal

8 TECHNICAL PAPER that of steel bars. The load capacity of T-beams with Leadline tendons decreased significantly after the peak load. It is observed that all the T-beams reinforced with Leadline tendons failed due to bond failure (Table 2). A typical failure mode of T-beam reinforced with Leadline tendons is shown in Figure 11, where bond failure led to shear crack eventually leading to crushing of concrete. lengths. Thus it may be recommended that embedment length for steel bars should not be less than 38 mm, as bond strength values for 38 mm and 51 mm embedment length values are of the same order of magnitude. Unlike CFRP bars, the load versus slip relationship exhibit drastic reduction in load capacity immediately after the initiation of slip. Bond characteristics of steel bars Conclusions In addition to the bond characteristics of three different CFRP bars, bond characteristics of steel bars were also evaluated, which provided the basis for comparison of bond characteristics of CFRP bars. Based on the six test T-beams with steel bars, failure load was predicted to vary from kn to kn. The embedment of steel bars was smaller than those of CFRP bars. It was observed that one of the beams having embedment length of 51 mm failed in compression, while the other beams experienced bond failure. The typical load versus slip relationships for steel bar is shown in Figure 12. Bond length was observed to be equal to the full embedment length in all the beams and the calculated bond strength varied from MPa to MPa. As shown in Table 3, the average value of the bond strength of steel bar is MPa. While calculating the average bond strength value, two loads (34.8 and kn) were discarded as they overestimate the bond strength values. It is noted from Table 2 that for test T-beam with smallest embedment length of 25 mm results into very high value of bond strength which is not consistent with bond strength values for other embedment The bond characteristics of carbon fibre reinforced polymer (CFRP) DCI bars, MIC bars, Leadline tendons, and steel bars were experimentally predicted using Tbeam tests. The primary mode of failure of test beams was bond failure causing flexure/ shear cracks which eventually led to the crushing of concrete. The average bond strength of DCI, MIC, and Leadline bars are equal to.55,.48 and.43 times the bond strength of steel bars, respectively. In the CFRP bars, the DCI bars are observed to have superior bond characteristics to that of MIC Cbars and Leadline tendons. It is noted that a very small embedment length of the order of 25 mm may lead to high value of bond strength of steel bars and hence it is recommended that minimum embedment length for steel bars may be taken as equal to 38 mm, which is giving consistent results with those corresponding to the higher embedment length. The load versus slip relationship for CFRP bars could be modeled as a linear curve. Unlike CFRP bars, load versus slip relationship for steel bars shows a drastic decrease in the load capacity immediately after the slip of bars. 3 END B END A 25 TL Leadline tendon Shear crack Load, kn End A End B Slip, mm Figure 11. Shear failure of bond test T-beam with Leadline tendons The Indian Concrete Journal Figure 12. Typical load versus slip relationship for steel bars 3

9 References Makitani, E., Irisawa, I., and Nishiura, N. (1993), Investigation of Bond in Concrete Member with Fibre Reinforced Plastic Bars, American Concrete Institute, ACI SP-138, pp Benmokrane, B., Tighiouart, B., and Theriault, M. (1997), Bond Strength of FRP Rebar Splices, Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 2, Sapporo, Japan, pp Takagi, N., Kojima, T., Iwamoto, K., and Inoue, S. (1997), A Study on Bond of Continuous Fibre Rods Embedded in Beam Specimen, Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 2, Sapporo, Japan, pp Ehsani, M. R., Saadatmanesh, H., Tao, S. (1993), Bond of GFRP Rebars to Ordinary Strength Concrete, American Concrete Institute, ACI SP-138, pp Hattori, A., Inoue, S., Miyagawa, T., and Fujii, T. (1995), A Study on Bond Creep Behavior of FRP Rebars Embedded in Concrete, Non- Metallic (FRP) Reinforcement for Concrete Structures, Proceedings of the Second International RILEM Symposium (FRPRCS), Ghent, Belgium, August 23-25, pp Jerrett, C. V., and Ahmad, S. H. (1995), Bond Tests of Carbon Fibre Reinforced Plastic (CFRP) Rods, Non-Metallic (FRP) Reinforcement for Concrete Structures, Proceedings of the Second International RILEM Symposium (FRPRCS), Ghent, Belgium, August 23-25, pp Nanni, A., Al-Zaharani, Al-Dulaijan, S. U., Bakis, C. E., and Boothby, T. E. (1995), Bond of FRP Reinforcement to Concrete - Experimental Results, Non-Metallic (FRP) Reinforcement for Concrete Structures, Proceedings of the Second International RILEM Symposium (FRPRCS), Ghent, Belgium, August 23-25, pp Achillides, Z., Pilakoutas, K., and Waldron, P. (1997), Bond Behavior of FRP Bars to Concrete, Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 2, Sapporo, Japan, pp Baena, M., Torres L., Turon A., Barris C. (29), Experimental Study of Bond Behavior between Concrete and FRP Bars using Pull-out Test, Composites Part B Eng., Vol. 4, pp Shield, C., French, C., and Retika, A. (1997), Thermal and Mechanical Fatigue Effects on GFRP Rebar-Concrete Bond, Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 2, Sapporo, Japan, pp Wang, Z., Goto, Y., and Joh, O. (1997), Bond Characteristics of FRP Rods and Effect on Long-term Deflection of Concrete Beams, Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 2, Sapporo, Japan, pp Cosenza, E., Manfredi, G., and Realfonzo, R. (1995), Analytical Modeling of Bond between FRP Reinforcing Bars and Concrete, Non- Metallic (FRP) Reinforcement for Concrete Structures, Proceedings of the Second International RILEM Symposium (FRPRCS), Ghent, Belgium, August 23-25, pp Lin, X., and Zhang, Y.X. (213), Bond Slip Behavior of FRP Reinforced Concrete Beams, Construction and Building Materials, University of New South Wales, Australia, Vol. 44, pp Martinelli, E., and Caggiano, A. (214), A Unified Theoretical Model for the Monotonic and Cyclic Response of FRP Strips Glued to Concrete, Polymers (ISSN ), pp Singh, S.B. (214), Analysis and Design of FRP Reinforced Concrete Structures, McGraw Hill Education Pvt. Ltd., New Delhi, pp Dr. S.B. Singh holds a professional engineering (P.E.) license in civil engineering from State of Michigan, USA and PhD in Structural Engineering from IIT Kanpur. He also has a postdoctoral fellowship from Lawrence Technological University, Southfield, Michigan, USA. Dr. Singh is a Professor and Former Head of Civil Engineering Department, BITS Pilani, Rajasthan. He is an associate member of ACI 44 Committee on Fibre Reinforced Polymer System and life member of Indian Society for Technical Education. He has also been selected as Fellow of Indian Institution of Engineers. His areas of research are development of design guidelines for fibre reinforced polymer, reinforced prestressed concrete structures in particular and composite structures in general including nonlinear finite element modeling. Aditi Chauhan received her B.Tech in Civil Engineering from National Institute of Technology Hamirpur and is pursuing her M.E. (Structural Engineering) at BITS Pilani, Rajasthan. She is also a Teaching Assistant at BITS Pilani, Rajasthan. Her areas of interest are earthquake engineering and application of fibre reinforced polymers (FRP) in blast resistant design of structures. The Indian Concrete Journal