Failure Mode of FRP Reinforced Concrete Beams

Transcription

1 Failure Mode of FRP Reinforced Concrete Beams Ana Lilia Orozco, Member ASCE, Professor, School of Engineering, Facultad de Ingenieria, Universidad Autonoma de Chihuahua, Mexico. 165 Sioux Dr, CH44(26), El Paso TX Arup Maji, Fellow ASCE, Professor, Dept. of Civil Engineering, Univ. of New Mexico, Albuquerque, NM Senior Research Scientist, Air Force Research Laboratory, AFRL/VSSV, Albuquerque, NM ABSTRACT A set of 24 concrete beams reinforced with different amounts of Carbon Fiber Reinforced Plastics (CFRP) reinforcement was subjected to static 3-point bending to investigate the modes of failure. These static tests showed a transition from bond failure to compression failure with increasing reinforcement ratio. Direct pull-out tests and hinged beam tests were conducted to investigate the bond of single 6.35-mm diameter Carbon/epoxy FRP rebars, and to determine the effect of the stress field on the bond strength. EXPERIMENTAL PROGRAM Manufacture of Specimens The concrete mix ratio used was 2.:1.8:1.:.4 (coarse aggregate, fine aggregate, cement and water/cement ratio respectively). The maximum size for the coarse aggregate was 12.7mm. The concrete strength used in these experiments was MPa. (675 psi). The Carbon/epoxy 6.35-mm diameter rods were manufactured with the pultrusion process, with AS4 Carbon fibers and Shell EPON Resin 945 with EPI-CURE Curing Agent 947. The theoretical tensile strength is 2.49 GPa (361 ksi.) and elastic modulus of the rebars is equal to GPa. (21 Msi). In order to improve bond the rebars were wrapped with a single tow of AS4 carbon fiber at 25.4mm pitch along the entire length using a room temperature cure epoxy. Table 1 shows the experimental program for the tests presented in this research. Table 1. Test Matrix Test Description No. Specimens Type of Reinforcement Cylinder Pull-Out 2 1 FRP embedded 25.4 cm Hinged-Beam Pull-out Tests 3 1 FRP 4 1 FRP 5 2 FRP 5 3 FRP 2 6 FRP Flexure Tests 2 1-#3 Steel 2 2-#3 Steel Pull-Out Tests Set Up Conventional cylinder pull-out tests were conducted on the 6.35mm diameter Carbon/epoxy rebar. The rebar was embedded 25.4cm inside a 15.24cm diameter by 3.48cm long concrete cylinders. A sleeve consisting of 6.35mm by 15.24cm of brass pipe with cap, filled with West System 15/26 epoxy, was used to grip the rebar. A Baldwin universal testing machine with 44.5kN (1kip) load cell was used to pull-out the specimens. In order to distribute stresses on the surface of the concrete cylinder, a 19mm steel plate and a 6.35mm thick hard rubber layer were used.

2 Hinged-Beam Pullout Tests Since there is extensive evidence in the literature on how the pull-out tests from cylinders provide an overestimation of the bond strength, due to the compressive stress field and the limited embedment length [1]. A new method based on Makitani et al. [2] was used to create the tensile stress field that is more representative of the actual stresses in flexure tests. These specimens were 16.7cm long and 1.2cm square beams that were cast in wood forms. The FRP rebar was supported in slots at the two ends of the formwork. The beams were cast with a gap of 2.5cm (1 ) at midspan made with removable Styrofoam inserts. A steel hinge was placed in the gap at the top of the beam. This allows the transfer of compressive stresses while the beam rotates about the hinge, and allows the FRP rebar to pull-out. The hinge was sufficiently stiff to prevent buckling of the hinge itself prior to rebar pull-out. The beams were statically tested in four point bending with 15.24cm space between the loads. The loading was applied through rollers with a simply supported span of 11.6 cm. Tests were conducted on a Baldwin universal-testing machine with a 44.5kN load cell. The beams were supported on a 182.9cm (6 ) long W 8X8 steel beam in order to extend the table of the loading fame. Two strain gages (CEA-6-25UW-35 Option P2 from Measurements Group, Inc.) were located at ½ span and ¼ span and a.25mm (.1 ) sensitivity dial gage was used across the opening at mid-span adjacent to the rebar to measure the pull-out of the rebar. Flexure Tests on Beams with FRP Rebars Reinforced concrete beams of the same size as the hinged-beams were cast with different quantities (1,2,3, and 6) of FRP rebars in each set of beams. Beams with equivalent amount of steel rebar were also tested. The beams were statically tested under three point bending using the same test setup as the hinged beams (Figure 1). Vertical deflections were measured at midspan using a.25mm (.1 ) sensitivity dial gage. Figure 1. Flexural Tests EXPERIMENTAL RESULTS AND ANALYSES Bond Strength Test results for the 2 direct pull-out tests are given in Table 2. For the hinged-beam bond tests the tensile force acting on the reinforcement was calculated form the known bending moment at the open end of the bars (1.3cm, 1/2 from mid-span). This tensile force is obtained from the relation T=M/(jd). The lever arm (jd) is the distance between the resultant tensile and compressive forces. Since the location of the hinge is known, this distance can be measured directly on the specimen (9.9cm). Defining the bond stress as the tensile force divided by the surface area of bars and defining pull-out as the magnitude of slip, the relationship between the bond stress and slip of the rebar is obtained as shown in Figure 2. The magnitude of the observed slip is comparable to the values found by Makitani et al. [2]. Consenza et al. [3] discuss the analytical modeling of bond using three theoretical bond-slip relationships. Focacci at al. [4] presented a numerical method to calibrate parameters of a given local bond-slip relationship using experimental results of pull-out tests.

3 The average bond strength obtained from the flexural (hinged beams) bond tests ( µ AVE = 2.13Mpa) was 43% lower than the bond strength obtained from the cylinder pull-out experiments ( µ AVE = 3.76 Mpa). The concrete in compression in the pull-out test restrains the tendency to fail by splitting, therefore giving higher bond-strength values, while flexural bond failures involve splitting in the tensile zone of the concrete beam. The maximum tensile force in the rebar corresponding to this bond failure is 21.13kN. This force will be used later to determine bond strength in defining failure envelopes. Table 2. Cylinder Pull-Out Tests Results Test Load Tensile stress Bond stress Average bond stress (kn) 1 - a b Predicting Compression and Bond Failure Provided that perfect bond is attained, determination of strength using the equations provided by the ACI Code for steel reinforced beams [5] require the yield strength of the rebars. Since FRP rebars are elastic up to failure, researchers have proposed some modifications based in the ACI Code equations. ([6], [7], [8], [9], [1], [11]) The moment at which the maximum compressive stress in these beams reach the compressive strength of the concrete f c (strain at failure equal to ε o=.22) was calculated using the initial parabolic region of the Hognestad s stress-strain relation for concrete [12]. Assuming elastic response of the rebars, the moment capacity was calculated based on equilibrium and compatibility, as in conventional reinforced concrete design. The results of these calculations for each of the beams are presented in Table 3. It can be seen that compressive failure is expected to occur long before the rebar tensile strength (2.49 GPa) is reached. Figure 3 shows the comparison of the compression failure prediction based on this theoretical model with the experimental results. It can be noted that most of the beams failed before compressive failure could occur (due to bond failure). Only the beams reinforced with 6 FRP rebars failed in compression at a load close to the predicted value. 2.5 Bond Stress-Slip Bond Stress Slip (mm) Figure 2. Average Bond Stress versus Slip Based on the hinge-beam bond tests, bond failure occurred at an average pull-out force of KN. This corresponds to a tensile stress of 667 MPa in each rebar. The rebar stress and concrete strain corresponding to the actual failure loads were calculated based on compatibility and equilibrium (Table 4). It can be seen that for the beams with 1,2, and 3 FRP rebars, the tensile stress in the rebars at failure were very close to the rebar pull-out stress of 667 MPa. The load at which bond failure is expected was analytically determined based on a rebar tensile force of KN. and the previously mentioned stress-strain response of concrete. In Figure 4, it can be seen that bond failure is likely in beams with one, two and three FRP rebars, but not for the beam with six FRP rebars Table 3. Flexural Strength for Concrete Strain ε c =.22 No.of Rebars ρ ε s f s M n (N-m)

4 Table 4. Experimental Results at Failure Beam M u (N-m) ε cu f su 1FRP F-1d F-1c FRP F-2c F-2d FRP F-3c F-3e FRP F-6b F-6a Compression Failure Load (kn) εcu=.22 Tests ρ Figure 3. Strength Prediction for Compression Failure Bond Failure Load (kn) ρ T = kn Tests Figure 4. Strength Prediction for Bond Failure

5 CONCLUSIONS Progressive bond failure is a likely and desirable failure mechanism for FRP reinforced concrete beams, since tensile FRP rebar failure is catastrophic. Prediction of failure due to bond and compression and the transition from bond failure to compressive failure was discussed. A new test method to determine bond strength under a tensile stress field was found to provide data significantly different from conventional pull-out bond strength. This bond strength was also accurate in predicting bond failure. A simple bond-slip model was investigated and validated with strain-gage data. ACKNOWLEDGEMENT The manufacture of FRP rebars was made possible by the Air Force Research Laboratory, Space Vehicles Directorate, (AFRL/VS) Albuquerque, NM. REFERENCES 1. Sakai T., Kanakubo T., Yonemaru K., and Fukuyama H. Bond Splitting Behavior of Continuous Fiber Reinforced Concrete Members, in Fiber Reinforced Polymer for Reinforced Concrete Structures, ACI-SP 188, (1999). 2. Makitani, E., Irisawa, I., Nishiura, N. Investigation of Bond in Concrete Member with Fiber Reinforced Plastic Rebars, Fiber-Reinforced-Plastic Reinforcement for Concrete Struct.-Int. Symp. ACI SP-138, (1993) 3. Cosenza, E., Manfredi, G.and Realfonzo, R. Analytical Modeling of Bond between FRP Reinforcing Bars and Concrete. Non-metallic (FRP) Reinforcement for Concrete Struct. RILEM, (1995). 4. Focacci F, Nanni A, and Bakis C. E. Local Bond-slip Relationship for FRP Reinforcement in Concrete, J. of Compos. for Construction, ASCE, 4 (1), (2). 5. American Concrete Institute (ACI) Building Code Requirements for Reinforced Concrete and Commentary. ACI Alkhrdaji, T., Mettemeyer, M., Nanni, A. and Belarbi, A. Flexural Behavior of FRP-Reinforced Concrete Members. Center for Infrastructure Engineering Studies. University of Missouri-Rolla. CIES (1999). 7. Faza, S. and GangaRao, H.V.S. Theoretical and Experimental Correlation of Behavior of Concrete Beams Reinforced with Plastic Rebars. Fiber-Reinforced-Plastic Reinforcement for Concrete Struct.-Int. Symp. ACI SP-138, (1993). 8. GangaRao, H. and Vijay, P.V. Design of Concrete Members Reinforced with GFRP Bars, Proc. 3 rd. Int. Symp. on Non- Metallic (FRP) Reinforcement for Concrete Structures (FRPCS-3), Japan Concrete Institute, Vol. 1, (1997). 9. Mutsoyoshi, H., Uehara, K. and Machida, A. Mechanical Properties and Design Method of Concrete Beams Reinforced with Carbon Fiber Reinforced Plastics, Transaction of the Japan Concrete Institute, Japan Concrete Institute, Vol. 12, (199). 1. Nanni, A., Flexural Behavior and Design of Reinforced Concrete Using FRP Rods, J. of Struct. Engrg, ASCE, 119 (11), (1993). 11. Nawy, E.G., Neuwerth, G.E. Fiberglass Reinforced Concrete Slabs and Beams. J. of the Struct. Div. Proc. of ASCE. 13, pp (1977). 12. Hognestad, E., Hansen, N. M., Mc Henry, D. "Concrete Stress Distribution in Ultimate Strength Design", ACI Journal, Proc. V. 52 (4), (1955).