Behavior of Prestressed Concrete Beams Strengthened With CFRP Sheets Subjected to Pure Torsion

Transcription

1 Behavior of Prestressed Concrete Beams Strengthened With CFRP Sheets Subjected to Pure Torsion Mohamed N. Mahmood * and Akram Sh. Mahmood * Civil Engineering Department/ Engineering College/ University of Mosul Civil Engineering Department/ Engineering College/ University of Anbar akramsh@yahoo.com ABSTRACT: Strengthening of concrete structures with externally bonded carbon fiber reinforced polymers (CFRPs) has been a viable technique for at least a decade and it became interesting material and applied in strengthening reinforced and prestressed concrete beams. It has been found that CFRP sheets are very suitable; not only because of its strength, but also due to its ease of application in comparison to traditional strengthening systems. Eight medium-scale reinforced concrete beams (mmxmm) cross section and mm long were constructed pure torsion test. All beams have four strands have no eccentricity (concentric) at neutral axis of section. There are classified into two group according uses of ordinary reinforcements. Where four beams with steel reinforcements, for representing partial prestressing beams, while other four beams have not steel reinforcements for representing full prestressing beams. The applied CFRP configurations are full wrap, U-jacked, and stirrups with spacing equal to half the depth of beam along its entire length. The test results have shown that the performance of fully wrapped prestressed beams is superior to those with other form of sheet wrapping. All the strengthened beams have shown a significant increase in the torsional strength compared with the reference beams. Also, this study included the nonlinear finite element analysis of the tested beams to predict a model for analyzing prestressed beams strengthening with CFRP sheets by using ANSYS V. software. Key words: Torsion, CFRP sheets, Prestressed beams, Finite elements modeling INTRODUCTION The maturing of developed countries bridging infrastructure around the world has accelerated rapidly in the last century. However, increased service loading and traffic volumes, diminished capacity through aging and environmental degradation, more stringent updates in design code regulations, and the need for seismic retrofit in some parts of the world have necessitated the need for repair and rehabilitation of existing infrastructure []. The use of fiberreinforced polymer (FRP) has shown promise in this area, and has been used successfully in many applications around the world. Research into the use of FRP is very well developed in flexural and shear strengthening [ and ] as externally bonded reinforcement, but information on its applicability in torsional strengthening is limited. Many buildings and bridge elements are subjected to significant torsional moments that affect the design, and may require strengthening. Clearly, further research was needed to address this gap in knowledge. All previous torsional

2 strengthening investigations have focused on different strip layouts for ordinary RC beams [4,, 6, and 7]. In the present work the experimental results of medium scale eleven pre-tensioned prestressed concrete beams subjected to pure torsion, where each beam is prestressed using one, two, and strands and there are divided into. Eight prestressed beams have been strengthened with carbon CFRP configurations where these configurations were full wrap, U-jacked, and stirrups with spacing equal to half the depth of beam along its entire length, then tested under pure torsion and their torsional strength are compared with that of the three un-strengthened beams EXPERIMENTAL PROGRAM. Specimen details Eleven medium-scale reinforced concrete beams (mmxmm) cross section and mm long were constructed pure torsion test. A three groups are specified for each beam have number of prestressed strands. Where theses beams are divided in three groups of one, two, and four strands. All beams have no eccentricity ( concentric) at neutral axis of section. For each group have one beam as reference beam (i.e. without strengthening with CFRP), whereas the other three beams have full wrap, U-jacket, and ( mm width) stirrups have spacing (mm) along entire length of beam, except group which have two strands was test without U-jacketed specimen. The full wrap strengthening configuration has CFRP sheet on all four sides of beam, with overlap length more than mm [8], whereas U-jacket strengthening beam has three sides which are bonded with CFRP. All beams have conventional reinforcement where the ordinary reinforcement layout was designed for minimum torsional capacity to simulate a prestressed beam that is now torsionally deficient, with 6mm (4 MPa a yield strength) bars as stirrups have spacing of mm in the test zone for all specimens have ordinary reinforcements. The mm bars (49 MPa a yield strength) were used for longitudinal reinforcements. A summary of the specimen details can be found in table (). The dimensions of the beams and the reinforcement details shown in Fig.(). Instructions of naming tested beams are illustrated in the flowing example of beam has symbol (B4.ST.S) : B4 ST Beam has mm CFRP stirrups sheets. Beam has four prestressing strands Table Summary of specimen details Beam Symbol Details of beam No. of CFRP Strands configuration B.R.S Control Beam B.FW.S Specimen Full Wrap B.UJ.S Specimen U-jacket

3 B.ST.S Specimen Stirrups B.R.S Control Beam B.FW.S Specimen Full Wrap B.ST.S Specimen Stirrups B4.R.S Control Beam 4 B4.FW.S Specimen 4 Full Wrap B4.ST.S Specimen 4 Stirrups B4.UJ.S Specimen 4 U-jacket mm mm mm mm 4Strands. 7mm 4 mm 6@mm Section of tested beam with ordinary and prestressing reinforcements Strands. 7mm 4 mm 6@mm Details of -strands section of tested beam mm mm Strand. 7mm 4 mm 6@mm Details of -strand section of tested beam Figure Details of the Steel and prestressed reinforcements

4 In Fig.() shows the details of CFRP sheet configurations of strengthened beams. For this investigations, Sika Wrap-C CFRP sheets ( see Table ) and Sika Dur epoxy resin are used for strengthening the eghit beams in three groups. Where the CFRP sheets in all strengthening configurations have bonded along entire length except (mm) from ends, which is left without CFRP sheets for the support's clamps. mm mm.d spacing c/c Figure Details of the CFRP sheets configurations Table Specifications of the CFRP used in present study Properties CFRP SikaWrap-C Weight (g/m) Thickness (mm). Tensile strength (MPa) 4 Modulus of Elasticity (MPa). Test Setup The test setup is shown in Fig.(). Both ends of the specimen are rest on roller can be rotated freely with loading arm fixed at these ends to exert the torsional moment on specimen from vertical applied load. Both ends of beam were bolted to the loaded arm. So longitudinal and traverse movements is allowed in both ends. Torque was applied on the (.m) test zone through two lever arms by single kn capacity hydraulic actuator. 4

5 Figure Vertical system of applied load of test setup EXPERIMENTAL RESULTS. Torsional strength values and behavioral curves The measured torsional moment at cracking ( T cr ), the angle of twist per unit length at cracking ( cr ), the post cracking ultimate torsional moment ( T u ) and corresponding angle of twist per unit length ( Tu ) are presented in Table () The results of eight beams tested can be found into two figures; Fig.(4), and Fig.(). The results of four beams have not conventional reinforcements is shown in Fig.(4), where it is represented the plot of Torque-Twist relation of four beams. Also Fig.() shows results of four beams have conventional reinforcements. All beams exhibited elastic response with high torsional rigidity. Fully and completely wrapped beams with continuous CFRP sheets (specimens B4.FW, and B4.FW.S) showed a slight increase on the initial torsional rigidity and a significant increase on the torsional moment at cracking in comparison with other beams of same group. The strengthened beams with CFRP fabrics as external transverse reinforcement showed increased torsional strength and ameliorated performance with respect to the control specimens and steel reinforced beams with same volumn of stirrups. This improvement on the torsional response was significant in the rectangular beams, which were fully and completely wrapped with continuous CFRP sheets. The torsional capacity of these beams increased approximately (- )% times the capacity of the control beam for fibers ratio (.) but transverse steel ratio (.48 %) respectively.

6 Table : Experimental results of the tested beams. Beam Symbol. Cracking torque (kn.m) Cracking twist angle (Deg/m) Percent of increase in T cr (%) Ultimate torque (kn.m) Ultimate twist angle (Deg/m) Percent of increase in T u (%) B.R.S B.FW.S B.UJ.S B.ST.S B.R.S B.FW.S B.ST.S B4.R.S B4.FW.S B4.UJ.S B4.ST.S ( ) % increase with respect to (B.R.S) ( ) % increase with respect to (B.R.S) ( ) % increase with respect to (B4.R.S) 4 4 Torque (kn/m) B4.R B4.FW Tourqe (kn.m) B4.FW.S B4.ST B4.UJ B4.R.S B4.ST.S B4.UJ.S... Twist Angle( Deg/m) Figure 4: Experimental behavioral curves of beams without conventional reinforcements (4 strands) Figure : Experimental behavioral curves of beams with conventional reinforcements (4 strands). Crack patterns and failure modes Control specimens and steel reinforcements prestressed beams exhibited typical torsional failure modes with increase spiral diagonal cracks. Crack patterns at failure of typical strengthened beams are presented in Fig. (6, a-c). The failure of the wrapped beams with CFRP 6

7 stirrups was partially delayed in respect to the failure of the control specimens, but eventually diagonal torsional cracks occurred and widened in the unwrapped concrete part of the beams between the stirrups. Fully and U-jacketed wrapped beams with continuous since fibers inhibited the propagation of cracks. This is the reason that fully wrapped beams presented higher values of torque moment at cracking in respect to other beams. Subsequently, torsional failure occurred at high level of loading along with CFRP sheets. The failure started in most highly stressed fibers flowing rapidly by the rupture of the part of the CFRP sheet intersected by the main torsional crack. The U-jacketed strengthened beams exhibited premature debonding failure at concrete and CFRP adhesive interface, where concrete cover show in part of CFRP at failure area. a. Stirrups strengthened beam b. Full wrap strengthened beam c. U-jacketed strengthened beam Figure 6: Crack patterns of the tested presttressed beams 4 FINITE ELEMENT ANALYSIS OF TESTED BEAMS 4. Finite Element Mesh Due to the three dimensional nature of the torsional problems, the model was generated with three-dimensional element. Figure (7) illustrates the generated finite element model by ANSYS package for example (B4.ST.S), where solid6 brick elements have been used for concrete with xx mm elements size along the length of the beam, where this volume of elements have been found to give good results compared with the tested beams[9]. The loading lever arm was modeled also with 8-node isoparametric solid brick elements of the steel material having linear elastic properties. The CFRP laminates were modeled using 8-node Solid46 element. The concrete material model predicts the failure of brittle materials. Both cracking and crushing 7

8 failure modes are accounted for concrete accesses this material model, which is available with the reinforced concrete element SOLID6.The criterion for failure of concrete subjected to a multiaxial stress state, using Willam and Warnke criteria[]. Loading of the beam is applied incrementally. Non Prestressed Reinforcement Element (LINK8) Prestressed Reinforcement Element (LINK8) D Concrete Element (Solid 6) D Steel Element (Solid 4) D CFRP Element (Solid 46) Figure 7: Finite element modeling of beam B4.ST.S 4. Boundary and Symmetry Conditions For the end supports, the center of the section was considered as a pivot point at the loaded end. This point is fixed in the lateral and vertical directions (i.e., Δy = Δz = ), but unrestrained in the longitudinal direction (X-axis) to allow elongation of the beam under the applied torque. To control the local concrete crushing at the ends of the beam (which did not appear in the experimental tests), the concrete brick elements around the support were assigned elastic properties of the concrete material (i.e. no crushing occur at supports). At mid-span, the beam cross section s surface was restrained against movement in axial, vertical and lateral directions (i.e., Δx = Δy = Δz = ) at the selected nodes on symmetric axes of section to simulate the symmetry conditions and to allow the cross section to warp in the longitudinal direction as shown in figure(8). The nonlinear finite element analysis used in simulating the response of strengthened prestressed beams in this study, where it is include a criterion to terminate the analysis when failure of the beam is reached Restrained section at midspan Figure 8: Boundary and symmetry conditions imposed on finite element model 8

9 4. Verification Torque-Twist Relationships of the Adopted Model From Fig.(9) the torque-twist relation by finite elements has a good agreement in linear range but when concrete is cracked, the poor representation of tension stiffing condition in finite element model leads to divergent the finite element results from experimental results. figures (-) for the torque- twist relation of tested beams have a good agreements till ultimate torque. The finite element results in Fig.(-6) for tested beams have stiffer results when comparing with experimental results that occur due to poor representation of tension stiffening condition in concrete constitutive material. Also there are several factors that may cause the higher stiffness in finite element models, micro cracks produced by drying shrinkage and handling are present in the concrete to some degree. These would reduced the stiffness of the actual beams, while the finite element models do not include micro cracks. Perfect bond between the concrete, steel reinforcements, strands is lost. Thus, the overall stiffness of the actual beams could be lower than what the finite element model predicted, due to factors that not incorporated into the model (kn.m) FEA_B4.R EXP_B4.R FEA_B4.FW EXP_B4.FW Figure 9: Torque-Twist Behavior of Beam B4.R... Figure : Torque-Twist Behavior of Beam B4. FW FEA_B4.U EXP_B4.U 6 4 FEA_B4.ST EXP_B4.ST Figure : Torque-Twist Behavior of Beam B4. UJ Figure : Torque-Twist Behavior of Beam B4.ST 9

10 4 4 (kn.m) EXP_B4.R.S EXP_B4.FW.S FEA_B4.R.S FEA_B4.FW.S Figure : Torque-Twist Behavior of Beam B4.R.S Figure 4: Torque-Twist Behavior of Beam B4. FW.S Torque (kn.m) FEA_B4.UJ.S EXP_B4.UJ.S EXP_B4.ST. S FEA_B4.ST. S.. Twist Angle (Deg/m). Figure : Torque-Twist Behavior of Beam B4. UJ.S... Twist Angle (Deg/m) Figure 6: Torque-Twist Behavior of Beam B4. ST.S. 4 CONCLUSIONS. The initial crack which developed is observed at the center of wider face for all full wrap strengthened and unstrengthed prestressed specimens, while the initial crack developed at the unstrengthened face of beam in U-jacketed strengthened specimens.. Although the volumetric ratio of used CFRP in 4 sides/ sides is (.), the ratios of cracking and ultimate torque of 4 sides to sides strengthened beams for both reinforced and unreinforced prestressed concrete beams are in range (.6-.) times.. Ductility and energy absorption capacity of the beam increases considerably when the beam strengthened with continuous CFRP sheets. 4. Beams having conventional reinforcements have shown more torsional capacity than unreinforced specimens. The increase in ultimate torque is about (79%, 74%, and 68%) for beams having full wrap, U-jacketed, and stirrups strengthen configurations, respectively. Comparisons of the FEM results with the experimental data show that maximum difference in ultimate torque for most of the tested beams was less than %. 6. The adopted concrete model proved to be capable of providing good estimates of strength and deformations for concrete elements subjected to multiaxial stresses.

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