EXPERIMENTAL INVESTIGATION OF CFRP STRENGTHENED DAMAGED STEEL PLATES SUBJECTED TO STATIC LOADING

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1 EXPERIMENTAL INVESTIGATION OF CFRP STRENGTHENED DAMAGED STEEL PLATES SUBJECTED TO STATIC LOADING T. Bai, X.L. Zhao and R.Al-Mahaidi Department of Civil Engineering, Monash University, Clayton, VIC, Australia ABSTRACT This paper presents an experimental investigation on the static behaviour of damaged steel plates strengthened with Carbon Fibre Reinforced Polymers (CFRP). Key parameters varied in the test program include steel plate thickness, damage degree of steel plates and elastic modulus of CFRP. The load carrying capacity of strengthened members significantly increased by up to 117% for specimens strengthened with normal modulus CFRP and 78% increase for those with High modulus CFRP, compared with those non-strengthened damaged members. Ductile behaviour is observed for specimens strengthened with normal modulus CFRP and even those strengthened with High Modulus CFRP sheets, given the damage degree of base steel plates not exceeding %. It is also found that there is little effect of damage degree on bond strength for specimens with Normal modulus CFRP, but damage degree is more critical on bond strength for specimens with High Modulus CFRP. KEYWORDS CFRP, Damaged steel plate, Static loading behaviour INTRODUCTION There is an increasing need for strengthening or repair of existing structures around the world every year. This is mainly due to corrosion, fatigue damage, error in the stage of design or construction, and demand for increased load carrying capacity. Conventional strengthening methods are based on the use of steel plates to strengthen deficient members via bolting, riveting or welding. However there are some significant drawbacks for these methods including difficult on-site handling, additional self-weight and susceptiblity to corrosion and fatigue damage. Therefore, it is of great significance to develop more advanced methods for structural rehabilitation. The use of fibre reinforced polymers (FRP) to strengthen/repair existing structures offers an attractive solution due to their extraordinary properties including high tensile strength and stiffness, light weight, high fatigue and corrosion resistance combined with their superior environmental durability. Extensive research has been conducted on the use of CFRP to strengthen timber, masonry and concrete structures, yet the research on the application to steel structures is comparatively very limited. Within the existing research on FRP-to-steel, the majority is focused on the repair of bridge girders (Phares et al 3, Schnerch et al 5), tubular sections (Fawzia et al 7, Jiao and Zhao 4, Seica and Packer 7) and steel plates (Al-Emrani and Kliger 6, Fawzia et al 5). This paper reports on a series of tests aimed at investigating the bond behaviour of CFRP strengthened damaged steel plates under monotonic loading. Steel plate damage is introduced by cutting notches on both sides of steel plate to simulate member cross section loss. Key parameters varied in this test program included damage degree (different cut length), thickness of damaged steel plate and different type of CFRP sheets. TEST PROGRAM Material properties Mild steel plates with a thickness of 3mm and 4mm are used in the test. The dimension is identical to all steel plates with the length of mm and width of mm. Cracks are introduced on both sides of steel plates with notch lengths of 5mm, mm and 15mm. Coupon tests showed that the yield stress of the steel was 445MPa for the 3mm thick plates and 422MPa for 4mm plates. 5

2 There is a variety of CFRP sheets available in the market with elastic modulus ranging from -GPa (Hollaway and Head, 1). In this test, CF1 and CF5 sheets are used. Elastic modulus of CF1 is 2 GPa and is called Normal Modulus CFRP while CF 5 is called High Modulus CFRP, which has elastic modulus of 6 GPa. Detailed properties of the two CFRP sheets are listed in table 1. Araldite 4 is chosen to bond CFRP sheets to the steel plate. It is a two part epoxy adhesive of high strength and toughness. It is suitable for a variety of metal, honeycomb and fibre reinforced composites bonding applications. It has very high strength even at temperatures of up to C and good peel strength. Elastic modulus of araldite is 1.9 GPa and the tensile strength is 32 MPa. (Fawzia et al 5) Specimen preparation and test set up Table 1 Material properties of CFRP (Manufacturer s data) CFRP Thickness(mm) Tensile strength Elastic modulus CF1.165 MPa 2 GPa CF MPa 6 GPa The surfaces of the steel plate were carefully sandblasted to remove all rust, tarnish or mill scale. CFRP sheets were cut into desired strips for use. Immediately prior to the bond application, the steel plate surface was cleaned with acetone. Then the first layer of araldite is applied using a soft brush, followed by the first layer of CFRP sheet. Ribbed roller was used to apply uniform pressure to make sure the CFRP sheet is fully soaked in the adhesive layer. A total of three layers of CFRP sheets were bonded on one side of steel plate. Using the similar method three layers CFRP were bonded onto the other side of steel plate. Specimens were then cured at room temperature for 7 days and post cured for another 24 hours at C. Foil strain gages were attached on the outer face of CFRP sheet at mm intervals. String pots were used to measure the axial deformation. A schematic view of a typical test specimen is shown in Figure 1. 3 layer CFRP sheets steel plate 3mm (4mm) pre-cut crack strain gages mm Figure 1 Schematic view of test specimen Test setup Tests were conducted on Baldwin universal testing machine at Monash University as shown in Figure 2. All the specimens are subjected to monotonic tensile loading until failure. Load was applied under deformation control at 2mm per minute rate for all the specimens. TEST RESULTS Important test results are summarized in Table 2. The table lists the results of 12 specimens strengthened with CFRP sheets along with their unstrengthened pairs. Specimens are distinguished by CFRP type, thickness of steel plate and cut length. For example, the first letter N stands for normal modulus CFRP sheet CF1, H stands for High modulus CFRP sheet CF5, the first number means the thickness of steel plate, while the last numbers stands for the cut length. For the purpose of comparison, test results of non strengthened damaged steel plates (DP) are also summarized in Table 2. 6

3 Figure 2 Typical test setup Table 2 Summary of test results Member Crack length (mm) Loss of cross section (%) CFRP modulus (GPa) Peak load (kn) Strengthening ratio Failure mode NSF Debonding NSF Debonding NSF Debonding NSF Debonding NSF Debonding NSF Debonding HSF Debonding HSF Fibre break HSF Fibre break HSF FB+DB HSF Fibre break HSF Fibre break DP DP DP DP DP DP EFFECT OF DAMAGE The degree of damage of the steel plates strengthened with CFRP is depicted in Figure 3. As shown in the figure, all the specimens showed significant increase in load carrying capacity compared with unstrengthened members. The larger the damage degree is, the more evident strengthening effect is. The ultimate strength of specimen NSF 3-15, which has % damage degree, has been increased by 117%, which is over two times the strength of non strengthened damaged steel plate. Similarly, the ultimate strength of specimen NSF 4-15 has been increased by 2%. Therefore good strengthening effect has been achieved. It is also shown in Figure 3 that there is virtually no difference in load carrying capacity for specimens with normal modulus CFRP (CF 1). However for the specimens with high modulus CFRP (CF5), there is a significant drop on load carrying capacity when damage degree exceeds % (mm cut length). 7

4 Damage degree 3mm steel plate/ CF1 3mm steel plate/ CF5 3mm steel plate only Damage degree 4mm steel plate/ CF1 4mm steel plate/ CF5 4mm steel plate only (a) (b) Figure 3 Load carrying capacity of test specimen. (a) 3mm steel plate (b) 4mm steel plate BOND BEHAVIOUR OF SPECIMENS UNDER MONOTONIC LOADING Monotonic loading behaviour of 3mm and 4mm steel plates strengthened with CFRP are plotted in Figures 4 and 5. Specimens strengthened with normal modulus CFRP sheets CF 1 exhibited very close behaviour independent of the different initial crack length. Ductile behaviour is evident for all the specimens. However, for the specimens strengthened with high modulus CFRP sheets CF5, different load deformation behaviour was observed, e.g.specimen HSF3-15 and HSF3-5. For specimens with large initial crack length (e.g. % crosssection loss) high modulus CFRP strengthening seems less efficient. Failure is much more brittle and catastrophic for specimens with larger initial crack compared with those with shorter initial crack length. Load(kN) NSF3-5 NSF3- NSF Deformation(mm) HSF3-5 HSF3- HSF Deformation(mm) Figure 4 Behaviour of 3mm steel plates strengthened with CF1 and CF5 9 NSF4-5 NSF4- NSF HSF4-5 HSF4- HSF Figure 5 Behaviour of 4mm steel plates strengthened with CF1 and CF5 8

5 COMPARISON OF CFRP STRENGTHENED SPECIMENS WITH BARE STEEL SAMPLES The static behaviour of specimens are also compared with bare steel samples as shown in Figure 6. It is clearly shown in the figure that for the strengthening with normal modulus CFRP, both ultimate strength and ductility significantly increased. While for the specimens strengthened with high modulus CFRP, although strength is significantly increased, the decrease of ductility is evident when damage degree is greater than %. DP 3-5 NSF 3-5 HSF DP 3- NSF 3- HSF DP 3-15 NSF 3-15 HSF DP 4-5 NSF 4-5 HSF DP 4- NSF 4- HSF DP 4-15 NSF 4-15 HSF Figure 6 Comparisons of load-deformation curve FAILURE MODES Failure modes are also summarized in Table 2. It can be seen that all the specimens strengthened with CF1 showed typical debonding failure, specimens strengthened by CF5 with initial crack length of mm and 15mm failed by fibre breakage. The failure modes agree well with those reported in (Fawzia et al 5). However, it is worth noting that specimens with initial crack length of 5mm showed a mixed failure mode, with fibre break occurring on one side of specimen and bond failure at the end of specimen. The reason for this mixed failure mode is believed to be due to the interfacial strength being very close to the tensile strength of CFRP. Typical failure modes are shown in Figure 7. CONCLUSIONS (a) (b) (c) Figure 7 typical failure modes in the test A total of 12 steel plates with different initial crack length have been strengthened with CFRP sheets and tested under monotonic loading up to failure. The bond performance is compared with those of damaged steel plates. Based on the limited test results, the following conclusions are made: 9

6 Load carrying capacity significantly increased by a maximum 117% for normal modulus CFRP bonding application and 78% for those with high modulus CFRP. This is due to the higher tensile strength of the normal modulus CFRP. Results showed that the degree of damage did not affect the strength of specimens with normal modulus CFRP. Degree of damage, however, played a significant role in the strength results of specimens with high modulus CFRP. For specimens with loss of cross section limited to %, ductile behaviour could be obtained even in the specimens strengthened by high modulus CFRP. ACKNOWLEDGEMENTS The financial support from the Australian Research Council (ARC) Discovery Grant is gratefully acknowledged. The authors wish to thank Mr Long Kim Goh and Mr Kevin Nievaart of the Civil Engineering Laboratory at Monash University for their assistance in the experimental works. REFERENCES Al-Emrani, Mohammadand and Kliger, Robert (6). Experimental and numerical investigation of the behaviour and strength of composite steel-cfrp members, Advances in Structural engineering, 9(6), Fawzia, S., Zhao, X.L., Al-Mahaidi, R. and Rizkalla, S. (5). Bond characteristics between CFRP and steel plates in double strap joints, Advanced steel construction- an international journal, 1(2), Fawzia, S., Al-Mahaidi, R., Zhao, X.L. and Riakalla, S. (7). Strengthening of circular hollow steel tubular sections using high modulus CFRP sheets, Construction and building materials, 21(4), Hollaway, L.C. and Head, P.R. (1).Advanced polymer composites and polymers in the civil infrastructure, Elsevier, Oxford, UK Jiao, H. and Zhao, X.L. (4). CFRP strengthened butt-welded very high strength (VHS) circular steel tubes, Thin-Walled Structures, 42(7), Phares, Brent, Wipf, T, Klaiber, W., Abu-Hawash, A. and Lee, Y. S. (3). Strengthening of steel girder bridges using FRP, Proceedings of the 3 Mid-Continent Transportation Research Symposium, Ames, Iowa Schnerch, D. Stanford, K., Sumner, E. and Rizkalla, S. (5). Bond behaviour of CFRP strengthened steel bridges and structures, Proceedings of International Symposium on Bond Behaviour of FRP in Structures, Hong Kong, Seica, M. and Packer, J.A. (7). FRP materials for the rehabilitation of tubular steel structures for underwater applibations, Composite Structures, (3), 4-4 1