INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 1, 2012

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1 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 1, 2012 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN The new Steel-CFRP composite specimen (CFRP laminates sandwiched between two steel strips) and its Research Scholar, Department of Civil Engineering, Jamia Millia Islamia, New Delhi, India doi: /ijcser ABSTRACT This paper introduces new specimen of steel-carbon Fibre Reinforce Polymer (CFRP) composite developed in accordance with standard test method and definition for mechanical testing of steel (ASTM A370). Fifteen specimens were prepared and divided into five groups depending upon a number of the layers of CFRP. Uniaxial tensile tests were conducted to determine yield strength and ultimate strength of specimens. Test results showed that the stress-strain curves of the composite specimens were bilinear prior to the fracture of laminate. The tested composite specimens displayed a large difference in strength with ductility. Finally, the proposed model by (Gang et al. 2010) was modified in this paper to give better estimate of the ultimate load of specimen. The estimated values agreed very well with experimental ones. Keyword: Composite specimen, laminate, Uniaxial tension, steel strip. 1. 1ntroduction CFRP s high strength-to-weight ratio has played a significant role in creating interest of strengthening; repair and rehabilitation of steel structures (Miller. et al. 2001).In addition FRP products have certain deficiencies such as elastic behaviour until failure and big differences between longitudinal and transverse mechanical characteristics, as well as between tensile and compressive stresses. Main deficiencies of FRP, when compared to steel reinforcement, are their non-ductile behaviour and creep rupture (Zorislav et al. 2010). With these deficiencies, FRP has high strength, a low elastic modulus, poor ductility, good durability and lightweight while steel is the opposite (Rizkalla and Hassan 2002).Combining the advantages of the two, the new composite material is expected to have outstanding comprehensive properties such as high strength, a high elastic modulus, good ductility and low cost (Wu 2006; Wu et el.2009). (Gang et al. 2010) introduced a new reinforcing material of steel-fibre-reinforced (FRP) composite bar (SFCB). The composite bar consists of different amount of fibre (bundle) wrapped around steel rebar of 10 mm diameter. Test results of specimens under uniaxial tension showed that the stressstrain curve of the (SFCB) was bilinear before the fibre fractures and a post yielding stiffness achieved. In the present work, this new reinforcing material was developed that is a composite of linear elastic CFRP and elastic-plastic steel strip. Being a try to reduce the total dependence on the traditional steel bar reinforcement, a little amount of steel has been used to manufacture steel-carbon Fibre Reinforce Polymer (CFRP) composite specimens. The key goals of using a little amount of steel are as follows 1. To introduce ductility in the specimens after fracture of CFRP laminates. Received on June, 2012 Published on August

2 2. To provide stiffness so that it can be configured to any desirable form like a bends in anchorage zones, bends in the stirrups or hoop. Fifteen specimens were prepared including three samples of steel strips. Woven carbon fibre fabric (SikaWrap -300C) and 2-part epoxy impregnation resin (Sikadur -330) were used to prepare Steel-CFRP composite specimens. All these specimens were divided into five groups, three specimens each, depending on the numbers of CFRP layers being used. On all these specimens, uniaxial tensile tests were conducted. 2. Objective of study In this paper, the objective of the present work is to 1. Study the experimental behaviour of this new composite under uniaxial tension 2. Work out whether this can be configured to replace conventional flexural and shearing reinforcing steel bars. 3. Propose an improved model that gives better estimate of ultimate tensile load of this composite. 3. Material properties and experimental program 3.1. Material properties Three materials were used to prepare the specimens. These are CFRP, adhesive and steel strips. CFRP (SikaWrap -300C) with modulus of elasticity of 230 GPa and the nominal ultimate tensile strength is of 3900 MPa was chosen. The adhesive (sikadure-330) was used, it has tensile strength of 4500 MPa and tensile modulus 3800 MPa. Mild steel plates 200 mm long, 20 mm wide and 1.5 mm thick were cut in the test program. Finally, dimensions of steel strips were configured according to ASTM A370-02, figure 1. Table 1 shows the properties of three materials. All these values mentioned in table 1 are in accordance with manufacturer s specifications except the values of steel strips that were recorded after conducting tension tests. Figure 1: Final shape of steel plate International Journal of Civil and Structural Engineering 250

3 Table1: Material properties Properties CFRP Steel strips Adhesive Tensile modulus (MPa) Tensile strength (MPa) Yield stress (MPa) Elongation at break 1.5% 30% 0.9% Poisson s ratio Experimental program A total of fifteen specimens were prepared with CFRP laminates. All steel strips have a dimension of 200 mm in length and 20 mm in width and 1.5 mm thickness. A reduction in specimens width starts at 50 mm from strip s edges was done. Rust, paint and primer were removed from steel strips using grinder. The surfaces were cleaned with acetone to achieve a good bond between the metal and the CFRP. Single- layered to five- layered strips three each were prepared by applying CFRP laminates on one side of a steel strip that configures the specimen. The final stage of preparing the specimens was to put the second steel strip. Parallel clamps were tightened to hold the sandwiching CFRP with steel specimens together and to ensure removal of any air that might be entrapped between CFRP laminate and the second strip. Each specimen was tested in tension in a 500kN capacity Zwick Roell universal testing machine with a loading rate of 2 mm/min. The test continued until the fracture of steel strip of the composite specimen. Figure 2 illustrates used parallel clamps. Figure 2: Parallel clamps to ensure good bond 4. Behavior of specimens under uniaxial tensile load and test results 4.1 Behavior of specimens under uniaxial tensile load Figure 3 show the load-displacement curves of one specimen for each type under tensile load while figure 4 shows the stress-strain curves of the uniaxial tensile test on specimens. At the initial stage of loading, the load was shared by steel and CFRP laminates. The specimen appeared to yield when the strain reached about Because the steel strips had already International Journal of Civil and Structural Engineering 251

4 yielded, they were not available to share additional load. As the load capacity of specimen reached to its peak value where the CFRP laminate fractured, the load decreased rapidly and the steel strips carried the entire load until it fractured. The specimen showed the ideal failure mode of steel yielding first, followed by CFRP laminate fracture and, finally, followed by tensile failure of steel. Point of fracture of CFRP laminate Point of fracture of CFRP laminate Point of fracture of CFRP laminate International Journal of Civil and Structural Engineering 252

5 Point of fracture of CFRP laminate Figures 3 (a), (b), (c), (d): Load vs. Displacement curves 4.2. Test results Table 2 shows the values of yielding and ultimate load of specimens. The values represent the average of values of three specimens. Table 2: The strengthening effects of bonding CFRP laminate to the steel strips. Specimens Yielding load ( N) Steel (two trips) With laminate of two layers With laminate of three layers With laminate of four layers With laminate of five layers Ultimate load N) International Journal of Civil and Structural Engineering 253

6 From table above, Figs. 3 and Fig. 4 following observations can be drawn 1. The yield and ultimate loads increased with the increase of the number of layers of CFRP. 2. The stress-strain curve for all the specimens was bilinear before the laminate fractures. 3. After fracture of laminate the steel carries on displaying its intrinsic ductility but with its yield load. 4.3 The modified model and its relationship with the other models 4.4 The empirical strain distribution formula (Fawzia S.R. et al. 2007) proposed an empirical strain distribution formula to estimate the strain at the ultimate state for multilayer CFRP bounded to circular hollow steel tubular section. Axial tension Tests were conducted for very high strength steel tubes strengthened with CFRP. Five layers of CFRP were wrapped around steel tube.two parts of steel tube were jointed together by applying adhesive at the cross sectional surface of the tube. To determine the distribution of strain across the layers, strain gauges were placed on the 1st (layer closest to steel surface), 3rd and 5th layers of CFRP at the same distance from the joint. As load progressively increased, the trend of strain variation across the layers was noticed and the strain values decreased from bottom to top. To estimate the distribution of strain at ultimate state, Expression of strain in terms of layer numbers was derived by (S. Fawzia R. et al. 2007). The strain ultimate state is as follow Where represent the layer number (, is the ultimate strain in the th layer, is the ultimate strain in the first (bottom ) layer and its value represents the maximum strain obtained in tensile tests of CFRP with epoxy.the corresponding stress in the th is Where represents the Young s modulus of CFRP 4.5 Theoretical model of steel-fiber reinforced polymer composite bar (SFCB) (Gang et al. 2010) presented theoretical model for steel-fiber-reinforced polymer composite bar (SFCB) under a uniaxial tensile load. Uniaxial tensile test and cycle tensile test were conducted to determine the initial elastic modulus, post yield stiffness, yield strength, ultimate strength, unloading stiffness and residual deformation. Test results showed that the stress-strain curve of the SFCB was bilinear before the fiber fractures. Detailed explanation of manufacturing of SFCB can be found in (Gang et al. 2010). According to mixture rule, (Gang et al. 2010) explained that the value of tensile property of SFCB could be obtained from those of steel and fiber. Depending on the model, the total strain was divided into three intervals as shown in figure 5.The strain interval. Strain interval ( ɪ )was from zero to the yielding of the steel, the equation for tensile stress is as follows: International Journal of Civil and Structural Engineering 254

7 )/, 0 (3) Where and =elastic modulus, cross section area and yield strain of steel, respectively; and =elastic modulus and cross section area of fibre; and, where is the area of resin in SFCB. Strain interval (ɪɪ) was from the yielding of steel to fracture of fibber. The equation for the tensile stress as follows: )/, (4) Where =yield of stress of steel and =fracture strain of the FRP. Finally, strain interval (ɪɪɪ) was from the fracture of fibre to the fracture of steel. Without considering the strengthening effect of steel, equation for the tensile stress is as follow: /, (5) Where =fracture strain of steel Figure 5: Stress-strain relationship of SFCB 4.6 Modified model for steel-cfrp composite (CFRP sandwiched between steel plate) The theoretical model of SFCB presented above was derived for steel-frp composite bar. Crescent-rib rebar with a diameter of 10 mm was used as inner steel bar. Different fiber amounts (bundles) were wrapped around the inner rebar. The stress-strain behavior of all specimens was bilinear before the fiber fractured. The specimen showed the ideal failure mode of steel yielding first, followed by the outside fiber fracture and, finally, followed by tensile failure of steel in region near the fractured fiber. To derive the theoretical model for steel-cfrp, (Gang et al. 2010) assumed that the outside fiber has fine interface bonding and harmonious deformation with the inner steel bar in the process of loading-that is, they have same strain in one section. As the results of the comparisons between calculated values and test results, (Gang et al. 2010) explained, that the theoretical model had fine precision except for several specimens. Also (Gang et al. 2010) explained that the errors may result from the assumption that there is no initial flexural of FRP and no slip on the interface between the fiber and steel bar, that is, the steel bar and FRP strain are the same at a one cross section. (S. Fawzia R.et al. 2007) presented the empirical strain distribution formula (Eq. 1) to determine the distribution of strain across the layers of CRRP for circular hollow steel tubular section under tensile load. (S. Fawzia R.et al. 2007) found experimentally that the strain through the thickness of CFRP layers decreases from the bottom to top layers. International Journal of Civil and Structural Engineering 255

8 To take the distribution of strain across the layers into consideration in this present study, the strain in Eq. (4) was replaced with ultimate strain of Eq. (1). The new model represents the modified model to predict the ultimate strength of steel-cfrp composite (CFRP sandwiched between steel plates) under Uniaxial tensile load. Therefore, the ultimate load using modified model can be written as: In addition, the ultimate stress can be written as: To validate these formulas, a comparison between the experimental and theoretical results was done. Figures 6a, 6b show the convergence between the experimental and theoretical results. The results obtained after using the model of SFCB proposed by (Gang et al. 2010) are also shown for comparison. The same results are listed in table. 3. Table 3: Ultimate tensile load; experimental, Gang s model and modified Gang s model Specimen Area Experimental results of Steel-CFRP *Theoretical results of modification Gang s model Theoretical results by Gang s model With laminate of two layers With laminate of three layers With laminate of four layers With laminate of five layers (kn) (MPa) (kn) (MPa) (kn) (MPa) International Journal of Civil and Structural Engineering 256

9 Load kn Number of layers Experimental results Modified modle of present study Modle of SFCB Figure 6(a): Load vs. CFRP layer numbers Load kn Number of layers Experimental results Modified modle of present study Modle of SFCB 5. Conclusion and summary Figure 6(b): Stress vs. CFRP layer numbers In this paper, the experimental results of the tensile capacity of sandwiched specimens of steel-cfrp composite have been presented. A new type of specimen has been developed depending on standard test method and definition for mechanical testing of steel (ASTM A370). Through the uniaxial tensile test, yield load, ultimate load were studied. Test results showed that the stress-strain curve of sandwiched specimen was bilinear before laminate fractures. A ductility after fracture of laminate was observed but with lower load than the ultimate load. Modified formula of strength carrying capacity based on strain distribution International Journal of Civil and Structural Engineering 257

10 across layers is proposed and validated by experimental results.. The predicted values agreed well with experimental ones. 6. References 1. American Society for Testing Materials (ASTM), (2002), Standard Test Methods and Definitions for Mechanical Testing of Steel Products, ASTM, A Fawzia S.R., Al-Mahaidi X. L., Zhao S. Rizkalla., (2007), Strengthening of circular hollow steel tubular sections using high modulus CFRP sheet, Construction and building Materials, 21, Gang, Wu1, Zhi-Shen, Wu, M ASCE. Yun-Biao Luo, Ze-Yang Sun, and Xian-Qi Hu, 2010, Mechanical properties of steel-frp composite bar under Uniaxial and cyclic tensile loads, Journal of materials in civil engineering, ASCE, 22(10), Miller TC, Chajes MJ, Mertz DR, Hastings JN., (2001), Strengthening of a steel bridge girder using CFRP plates, Journal of Bridge Engineering, ASCE,6(6), Rizkalla, S. and Hassan, 2002, The effectiveness of FRP for strengthening concrete bridges, Journal of the International Association for Bridge and Structure Engineering, 12(2), Wu, Y. F., 2006, New Avenue of achieving ductility for reinforced concrete members Journal of Structure Engineering, 132(9), Wu, G., Wu, Z. S., Luo, Y. B., and Wei, H. CA., 2009, New reinforcement material of steel fibre composite bar (SFCB) and its mechanics properties, Proc., 9th International. Symp. on Fibre Reinforced Polymer Reinforcement for Reinforced Concrete Structures (FRPRCS-9), Sydney, Australia. 8. Zorislav Soric, Tomislav Kisicek, Josip Galic, 2010, Deflections of concrete beams reinforced with FRP bars. Journal of Materials and Structure, 43, International Journal of Civil and Structural Engineering 258