Using carbon-fibre-reinforced polymer to strengthen concrete-filled steel tubular columns

Transcription

1 Proceedings of the Institution of Civil Engineers Structures and Buildings 170 December 2017 Issue SB12 Pages Paper Received 19/01/2016 Accepted 16/03/2017 Published online 17/05/2017 Keywords: columns/composite structures/rehabilitation, reclamation renovation ICE Publishing: All rights reserved G. R. Vijay Shankar BE, ME Professor, Department of Civil Engineering, Karpagam University, Coimbatore, Tamilnadu, India M. C. Sundarraja BE, ME, PhD Professor, Civil Engineering, Thiagarajar College of Engineering, Madurai, Tamilnadu, India Yun Yong Kim BS, MS, PhD Professor, Civil Engineering, Chungnam National University, Daejeon, Korea G. BE, ME, PhD Professor, Civil Engineering, KPR Institute of Engineering and Technology, Combatore, Tamilnadu, India (corresponding author: External bonding of carbon-fibre-reinforced polymer composites has been proposed as an innovative technique to strengthen steel structures. This paper discusses an experimental study carried out to investigate the feasibility of using carbon-fibre-reinforced polymer composite strips for strengthening concrete-filled steel. In this study, columns were strengthened using polymer composite strips 50 mm wide at two different spacings. They were then tested under compression until failure. The test results are discussed in terms of failure modes, stress strain behaviour, load-carrying capacity and ductility. The results show that externally bonding composite polymer strips is an effective approach to restraining axial deformation and enhancing the ultimate capacity of columns under compression. By bonding the strips, the deformation of the columns was restricted to a maximum of 141 2% and 69 75% compared to the reference column when set at spacings of 20 mm and 40 mm, respectively. The capacity of the columns increased by about 30% for a 20 mm spacing, whereas no significant increase in capacity was observed for a 40 mm spacing. It is therefore recommended to apply this technique for strengthening and rehabilitating concrete-filled steel and to use a 20 mm spacing. Notation A c area of concrete A concrete cross-sectional area of filled concrete A s area of steel A steel cross-sectional area of steel tube b frp width of carbon-fibre-reinforced polymer strips D diameter of concrete-filled steel tubular column d/t cross-sectional slenderness ratio of column f c compressive strength of concrete fc 0 cylinder compressive strength of concrete f ck compressive strength of concrete f frp tensile strength of fibre-reinforced polymer in the hoop direction f l lateral confining pressure f lcon lateral confining pressure exerted by the carbon-fibre-reinforced polymer strips f uncon unconfined compressive strength of concrete-filled steel tubular column f y yield stress of steel tube k effective confinement coefficient n number of carbon-fibre-reinforced polymer layers N pl,rd axial load-carrying capacity of bare concrete-filled member P o load carrying capacity of the bare CFST column P proposed load carrying capacity of a CFST circular column fully wrapped with CFRP sheets s frp t frp t s γ CFRP ε y ε 85% ε 75% ρ frp clear spacing between the carbon-fibre-reinforced polymer strips thickness of fibre-reinforced polymer confinement thickness of steel tube static design safety for carbon-fibre-reinforced polymer composite yield strain axial strain when load falls to 85% of ultimate load axial strain when load falls to 75% of ultimate load fibre-reinforced polymer volumetric ratio to concrete 1. Introduction In recent years, concrete-filled steel tubular (CFST) sections have been widely used as columns or piers in high-rise building structures. In addition, CFST sections are widely recognised for their excellent structural performance, and have been applied in the construction industry for more than 40 years. However, it has been reported that many CFST structures suffer from a variety of forms of deterioration, including cracking, yielding and large deformation. Such deterioration is caused by various factors, including fire, ageing, environmental degradation and corrosion. Above all, during an earthquake, many CFST structures, even if they do not collapse, are damaged to some extent. For this reason, these structures are armoured to support the designed load, or even renovated to 917

2 resist possible higher loading. Over the past several decades, various methods of strengthening or rehabilitation techniques have been proposed to improve the performance of such structures. However, external bonding using steel plates creates some problems due to the addition of self-weight, corrosion of the steel plate and the requirement for skilled labour, as well as increased cost. In contrast, rehabilitation using fibre-reinforced polymer (FRP) composites does not involve any of these drawbacks. One of the main forces driving the development of external strengthening using FRP composite is the need to upgrade deteriorated members without significantly altering the appearance of the member. In addition, FRP composites are lightweight, durable, resistant to corrosion and having high tensile strength, stiffness and fatigue strength. In recent years, there many investigations have emerged on the use of FRP to strengthen steel structures, especially in the area of thin-walled steel structures. One of the earliest studies, by Sen and Liby (1994) was on a composite beam bonded with carbon-fibre-reinforced polymer (CFRP) laminates in the tension flange, and this was tested under a four-point bending test. High-strength steel bolts were used to prevent the delamination of the FRP plate, and a significant gain in strength was achieved. Tests on the application of FRP composites for strengthening of tubular structures subjected to various types of loading mainly tension, bending and compression, have been conducted over the past several years (Jiao and Zhao, 2004; Photiou et al., 2006; Seica and Packer, 2007). Choi and Xiao (2010), Narmashiri et al. (2012) and Kadhim (2012) experimentally and numerically investigated the structural behaviour of steel I-beams flexurally strengthened by CFRP composites. Wu et al. (2012), Al-Zubaidy et al. (2012) and Colombi and Fava (2012) studied the bond characteristics between CFRP laminates and steel under fatigue and impact tensile loads. More recently, research has been started on the strengthening of CFST members using FRP composites, and a number of studies are underway. Abdalla et al. (2013) found that the axial load-carrying and ductility capacities of the CFST column can be increased using glass-fibre-reinforced polymer (GFRP) confinement. A theoretical stress strain model to model the CFST column under cyclic and monotonic axial compression for future studies can be found in Teng et al. (2013) and Yu et al. (2014). An attempt made by Ho and Dong (2014) revealed that the elastic strength, elastic stiffness and ductility of the columns were increased with the confinement of the steel ring. Some of the research carried out so far ( and Sundarraja, 2013; Sundarraja and, 2012, 2013a, 2013b; Tao and Han, 2007; Tao et al., 2007) has investigated the behaviour of square CFST externally bonded by FRP composites. Even though some of the research has been carried out on the use of FRP composites as a strengthening material for CFST members, the studies have been focused on CFST columns strengthened with FRP composites in the form of the external surface fully confined by FRP composites. Thus studies are needed to derive the most favourable FRP wrapping scheme for strengthening of CFST members, because the FRP strengthening method is too expensive. In order to resolve this issue, the main objective of the present investigation is to investigate experimentally the feasibility of using CFRP circular strips in the strengthening of circular CFST columns, evaluating the effectiveness of an FRP wrapping scheme. The experimental parameters were the number of FRP layers and the spacing between the FRP strips. To eliminate galvanic corrosion between the steel tube and CFRP, a thin layer of fibre-glass mat was introduced between the steel and CFRP fabrics. 2. Material properties Commercially available, hollow, circular steel section conforming to IS 1161 (BIS, 1998) and with a diameter of mm was used in this study. The thickness of the hollow steel tube was 4 mm. The height of the tube was 600 mm, which is six times the cross-sectional dimension. The measured yield strength of the steel tube was 252 N/mm 2. Concrete with design strength of M30 and designed according to IS (BIS, 2009) was used in this study to infill the steel tube. The maximum size of the coarse aggregate was 10 mm. A test was performed to determine the 28 d compressive strength using 150 mm 150 mm 150 mm cubes. The average strength of the concrete obtained was about 38 5 N/mm 2. A low-modulus unidirectional carbon fibre called MBrace 240, which is fabricated by BASF India Inc., was used in this study to strengthen the column. Carbon fibre was selected for its superior mechanical and durability properties equivalent to steel. The stiffness and tensile strengths of the fibre were 240 kn/mm 2 and 3800 N/mm 2, respectively. The thickness of fibres was mm and the width was 600 mm. This is a fabric type of fibre and can be tailored into any desired shape. An epoxy saturant called MBrace saturant, supplied by BASF India Inc., was used in this study to make the bond between the steel and the CFRP. This is a two-part system that includes resin and hardener, and the mixing ratio was 100:40 (bonding agent (resin):hardener). 3. Experimental study 3.1 Specimen fabrication Circular hollow tubes 600 mm high were machined from 6 m long hollow tubes. To obtain a level surface, both ends of the steel tube were levelled using a lathe. Before filling the tube with concrete, the inside portion was thoroughly wire brushed to remove any rust and loose debris that was present. The steel tube was then filled with concrete, layer by layer, and each layer was effectively compacted by a vibrator to ensure that the concrete was free from air gaps and flaws. During compaction, a steel plate was introduced at the bottom of the steel tube to eliminate any leakage of slurry, and then the concrete was allowed to cure for 28 d. After curing, all the CFST members were subjected to sand blasting using coarse sand to remove 918

3 the rust and to roughen the surface. Before the columns were strengthened with FRP composites, acetone was used to clean the surface to remove any contaminating materials, after which a thin GFRP surface mat was bonded to eliminate galvanic corrosion. Finally, the members were strengthened with CFRP composite strips using two different wrapping schemes; the different spacings are shown in Figure 1. During wrapping, a steel roller was used in the direction of the fibre to remove the air gaps and excess resin. 3.2 Experimental set-up A compression testing machine with a capacity of 2000 kn was used to test the column under axial compression. Each member was placed on the supports, and care was taken to ensure that its centreline was exactly in line with the axis of the machine. On all the columns, instruments were installed to measure the axial and lateral deformation using linear voltage displacement transducers (LVDTs); a 2000 kn load cell was used to monitor the load. Both load cell and LVDTs were connected to a 16-channel data acquisition system to store the data. Load was applied to the columns using a jack and the columns were tested to failure. The experimental observation recorded the nature of the failure, axial deformation and ultimate load. The axial loading set-up is shown in Figure 2. To identify the specimens easily, the columns were designated as CS T1, CS T2 CS T3, CS T1, CS T2 and CS T3. For example, the specimen name CS T3 indicates that it was strengthened by three (3) layers of CFRP circular strip (CS) 50 mm wide in the transverse direction (T) with a spacing of 40 mm. The reference columns are specified as CC1, CC2 and CC3. 4. Results and discussion 4.1 Failure modes In the unstrengthened columns (CC1, CC2 and CC3) a large deformation was observed in the initial stage due to the uneven surface at the top of the column, after which the deformation of the column was directly proportional to the applied load. Finally, the columns failed through outward buckling of the steel, which occurred at mid-height and was observed on all sides of the column; this is shown in Figure 3. From Figure 3 it can be confirmed that the concrete fill in the steel tube effectively stabilised the inward buckling of the column. In all reference columns, crushing of concrete was not observed in order that the applied load was decreased slowly after the failure load; furthermore, a favourable ductility response was observed due to the yielding of the steel tube (Tao and Han, 2007).The columns that were confined by one layer of strips with 20 mm spacing failed by yielding-cum-buckling of the steel tube alone, without any rupture of the fibre observed at the bottom of the column until the ultimate load of 989 kn, Ø Steel tube Concrete CFRP strips 50 mm width CFRP strips 20 mm spacing 40 mm spacing CS CS Figure 1. Wrapping scheme 919

4 Figure 2. Experimental set-up which is 6 57% higher than that of the reference column; this is shown in Figure 4. The failure mode is attributed to the small cross-sectional slenderness ratio (d/t) of the column. When the number of layers increased from one to two and three layers at a spacing of 20 mm, all columns failed through rupture of fibre, observed at the bottom and mid-height of the column for two and three layers, respectively, as shown in Figures 5 and 6. The change in the failure mode when the number of layers is increased is attributed to increases in the cross-sectional slenderness value of the steel tube. The increase in the slenderness value (d/t) by bonding CFRP composites to the steel tube delayed the yielding of the steel tube, and effectively increased the stability of the columns. In addition the bonded CFRP strip causes the steel tube near the CFRP strips to become more rigid owing to the greater confinement pressure exerted by the CFRP strips, and this counteracts the outward buckling of the steel tube. When the CFRP strips can no longer resist the lateral expansion of the steel tube, rupture of the CFRP occurs, followed by buckling of the tube. In all specimens, an abrupt reduction in load was observed after failure, and this is attributed to the rupture of fibre, resulting in the immediate absence of the confining pressure exerted by the CFRP strips. In addition, it has been noted that, in all of the specimens described above, no de-bonding of fibre was observed before their ultimate load was attained; thus it was confirmed that there was a perfect bond between the steel and the CFRP in all cases and it may be considered as a fully Figure 3. Failure mode of CC bonded steel CFRP composite plate (Tao et al., 2007; Teng et al., 2013). After increasing the spacing of the strips from 20 mm to 40 mm, the columns strengthened by one, two and three layers of CFRP failed by flexural buckling with single curvature. In addition, no rupture of the fibre was observed in any of the specimens, as shown in Figures 7 9. Close observation of Figures 7 9 clearly shows that the origin point of the buckling was the unbonded area. This is attributed to the fact that the increase in the CFRP thickness provides more confining pressure in the bonded area and shares the steel strain by providing confining pressure. At the same time, the unbonded area is subjected to more strain due to the absence of confining pressure, resulting in the buckling of steel occurring when the steel reaches its maximum strain. With further loading, the difference in the stress concentrations led to the curvature effect. Furthermore, crushing of concrete was observed in the compression zone and yielding of the steel tube was observed in the tension zone. However, control of axial deformation was observed when the number of layers was increased compared to the reference column. 920

5 Figure 5. Failure mode of CS T2(1) Figure 4. Failure mode of CS T1(1) 4.2 Axial stress strain behaviour Effects of number of CFRP strip layers The restraining effect against axial deformation when compared to the unstrengthened columns is summarised in Table 1, in the form of a percentage. In this study, the influence of the CFRP strip and the effect of the number of layers on the behaviour in terms of the restraining effect against axial deformation were investigated. As shown in Figures 10 and 11, at both spacings (20 mm and 40 mm), CFST columns that were externally confined with CFRP strips had a greater ability to withstand axial deformation. Moreover, an increase in the number of layers significantly increased the prevention of deformation of the column and enhanced the ultimate load. This enhancement may be caused by an increase in the confinement effect provided by the CFRP strips through confining pressure. The bonding of CFRP strips increases the composite plate s thickness, which decreases the cross-sectional slenderness value of the steel tube, causing buckling stress on the CFST columns to increase. Compared to the reference column, columns CS T1(2), CS T2(1) and CS T3(2) showed 46 61%, 69 73% and % increases in restraining Figure 6. Failure mode of CS T3(1) effect against axial deformation, respectively. Even if columns CS T1(3), CS T2(1) and CS T3(2) failed due to the curvature effect, they showed restraint against axial deformation and enhanced stress by 39 92%, 44 21% and 69 72%, respectively, and their deformation values were about 921

6 Figure 7. Failure mode of CS T1(2) Figure 8. Failure mode of CS T2(1) 7 92 mm, 7 57 mm and 6 71 mm at the respective ultimate loads of the reference column. In a few strengthened columns, a sudden drop in ultimate load was observed in the post-peak stage. This behaviour may have been caused by a rupture of the fibre, resulting in the immediate absence of the confining pressure exerted by the CFRP strips. According to Figures 10 and 11, although the restraining effects were increased with the increase in the number of layers, the enhancement was not proportional. This may be due to the development of cracks in the resin lying between the CFRP strips, resulting in a drop-off in the load transfer that occurred; and this may vary from layer to layer. With 20 mm spacing, the columns that had three layers showed a restraining effect that was increased by 64 23% and 92 43% when compared to the columns with one and two layers, respectively. Based on the above, it can be confirmed that an increase in the thickness of CFRP has a significant influence on the restraining effect against axial deformation; however, this behaviour is more efficient where strips have smaller spacing Effect of spacing The influence of strip spacing on stress strain behaviour is shown in Figure 12. It can be observed that the stress strain behaviour of columns CS T2(1) and CS T3(2) followed the same path as columns CS T1(2) and CS T2(1), respectively. This implies that, when the 40 mm spacing is used, there is a need for one more layer to achieve the behaviour of the column with 20 mm spacing. Column CS T1(3) had a higher axial deformation of 8 84 mm compared to column CS T1(2), which had an axial deformation of 8 11 mm; Figure 12 also illustrates that column CS T3(2) showed greater axial deformation (10 23 mm) than column CS T3(2), indeed this result was 54 25% higher than that of CS T3(2). This confirms that an increase in the restraint effect against axial deformation can be achieved by using a smaller spacing, and that this influence begins to decrease when the spacing is increased (i.e. for larger spacing). The decrease in the restraining effect is attributed to the phenomenon whereby, when the columns are strengthened with spacing, each unbonded steel tube may begin to act as a thin plate, and it is simultaneously subjected to axial compression and lateral force due to the expansion of the concrete. When the spacing between the strips is increased, this increases the height and cross-sectional slenderness value of the plate, with the result that buckling of the steel occurs even before the CFRP reaches its ultimate strain. From this it can be understood that the restraining effect against axial deformation of the column can be improved only by proper 922

7 spacing between the CFRP composite strips (Ho and Dong, 2014; Tao and Han, 2007). It is suggested that a column with strips at 20 mm spacing is more advisable for axial strengthening of the CFST column member than using a 40 mm spacing Ductility Generally, ductility is a very important property in CFST members, and as a result its application in earthquake-resistant structures has become widespread. Thus, the influence of CFRP strips on the ductility of the column has been investigated in this study. Based on Equation 1, as proposed by Tao et al. (2007) and Tao and Han (2007), the ductility index was evaluated for all of the columns. ε 85% is the axial strain when the load falls to 85% of the ultimate load; ε y is equal to ε 85% /0 75; and ε 75% is the axial strain when the load reaches 75% of the ultimate load. 1: DI ¼ ε 85% ε y Figure 9. Failure mode of CS T3(3) As shown in Figure 13, it was found that bonding of CFRP strips did not affect the ductility of the column and the presence of CFRP strips enhanced the ductility performance of the column; however, this enhanced performance was not significantly higher. The columns confined with three layers at both spacings showed the most beneficial effect on the ductility response compared to columns with one and two layers. The bonding of the CFRP strips acts like a composite plate and Table 1. Experimental results for all specimens Designation of columns Failure load: kn Percentage of reduction in axial deformation compared to CC1: % Percentage of increase in axial load-carrying capacity: % Theoretical P theo P theo /P exp Difference Δ ¼ P exp P theo 100: % P exp CC1 928 CC2 912 CC3 923 CS T1(1) CS T1(2) CS T1(3) CS T2(1) CS T2(2) CS T2(3) CS T3(1) CS T3(2) CS T3(3) CS T1(1) CS T1(2) CS T1(3) CS T2(1) CS T2(2) CS T2(3) CS T3(1) CS T3(2) CS T3(3)

8 Axial stress: N/mm Axial stress: N/mm CC1 CS T2(1) Axial strain CS T1(3) CS T3(3) CC1 20 CS T2(1) CS T1(3) CS T3(2) Axial strain CS T1(3) CS T3(3) CS T2(1) Figure 10. Axial stress strain behaviour of columns CS comparison Figure 12. Axial stress strain behaviour of all columns comparison 1 00 Axial stress: N/mm Ductility index, DI One layer Two layers Number of layers Figure 13. Ductility index of all columns comparision Three layers CC1 CS T2(1) Axial strain Figure 11. Axial stress strain behaviour of columns CS comparison CS T1(3) CS T3(2) enhances the capacity of the column without affecting its ductility. Even though a sudden drop in load was observed at peak level because of the brittle nature of the CFRP, this behaviour did not affect the ductility of the column. From this it can be understood that CFRP strip composites can be effectively used to enhance the capacity of the CFST column without affecting its ductility. 4.3 Load-carrying capacity The experimental load-carrying capacity of all columns is presented in Table 1, along with the enhancement in capacity compared to the reference column that results from the external bonding of CFRP strip composites. The enhancement in loadcarrying capacity is plotted in Figure 14 against the number of layers. It is clear from Figure 14 that a significant enhancement in load-carrying capacity of up to 30% more than that of reference column can be achieved by external bonding of CFRP strips. Furthermore, at both spacings, the increase in the number 924

9 Ultimate load: kn mm spacing 40 mm spacing One layer of CFRP layers further enhances the load-carrying capacity of the column. Columns CS T3(2) and CS T3(2) had their load carrying capacity enhanced by 30 13% and 12 72%, respectively, compared to the reference column. This is because the addition of CFRP decreases the slenderness value of the steel, increasing the capacity of the column. It is clear from Figure 14 that the increase in the capacity of the columns is proportionate to the increase in the number of layers; moreover, the exertion of confining pressure between the layers is decreased when the number of layers is increased. This may be due to cracking in a thin layer of resin between the CFRP layers. Compared to columns CS T1(2) and CS T2(1), column CS T3(2) has a capacity increased by 22 24% and 12 46%, respectively. However, column CS T3(2) has its capacity increased only by 10 10% and 6 19% compared to the columns strengthened by one and two layers, respectively. From this, it can be understood that the enhancement in the capacity of the column mainly depends upon the proper spacing between the CFRP strips. The increase in spacing affects the load-carrying capacity of the column. Figure 14 clearly shows that, for one, two or three layers, the columns strengthened at a spacing of 40 mm showed inferior performance when compared to the columns strengthened using 20 mm spacing. The results demonstrated that, while CFRP strips can be effectively used for strengthening of CFST circular members. Therefore, it is concluded that, for columns externally bonded with CFRP composite strips, the performance of the CFST column member is enhanced. In addition, it was found that, for columns with a 20 mm strip spacing, a higher increase in the capacity of the column could be achieved, whereas external bonding of the columns at a 40 mm spacing does not provide a significant increase in strength and, as such, this practice is not advisable for strengthening of CFST members. 5. Analytical study Two layers Number of layers Figure 14. Ultimate load for all columns comparison Three layers 5.1 Prediction of the axial strength of FRP-confined CFST column Existing confinement models The Eurocode 4 (BSI, 2005) proposed the following Equation 2, to calculate the axial load-carrying capacity (N pl, Rd ) of the bare CFST column member. This is a simple equation, simply summing up the strength properties of the in-filled concrete and steel tube. 2: N pl; Rd ¼ A s f y þ A c f ck where f y is the yield stress of the steel tube; f ck is the compressive strength of the concrete; and A s and A c are the crosssectional areas of the steel tube and the filled concrete, respectively. Eurocode 4 restricted this to composite columns with concrete cylinder strength and steel yield stress not greater than 50 and 355 MPa, respectively. Hajjar and Gourley (1995) proposed Equation 3, to predict the load-carrying capacity (P o ) of a bare CFST column under axial compression 3: P o ¼ A steel f y þ A concrete f 0 c where f y is the yield stress of steel tube and fc 0 is the cylinder compressive strength of concrete. A steel and A concrete are the cross-sectional areas of steel tube and filled concrete, respectively. When the FRP-strengthened CFST columns are subjected to axial compression, the concrete core of the CFST column expands laterally, while in the meantime the steel tube and FRP lie in the outer limits restraining the lateral expansion and are subjected to hoop tension. In order to predict the confining pressure exerted by the FRP, Equation 4 has been used in many studies ( and Sundarraja, 2013; Lam and Teng, 2002; Sundarraja and, 2012, 2013a, 2013b; Tao and Han, 2007). 4: f l ¼ 2f frpt frp D where D is the diagonal length of the square cross-section; f frp is the tensile strength of the FRP in the hoop direction; and t frp is the thickness of the FRP confinement. Based on the aforementioned models, very recently, Abdalla et al. (2013) proposed a new model (Equation 5) to predict the loadcarrying capacity of a CFST circular column fully wrapped with CFRP sheets, and this model was developed using the simple approach followed by Liu and Lu (2010). The lateral confinement pressure provided by both steel tube and fibre were accounted for by the product of k. 5: P proposed ¼ A c f c þ A s f y þ k 2f yt s D þ 2f frpt frp D where D and t s are the diameter of the column and the thickness of the steel tube Proposed model Based on confinement models proposed in the literature (Abdalla et al., 2013; and Sundarraja, 2013; 925

10 Lam and Teng, 2002; Sundarraja and, 2012, 2013a, 2013b; Tao and Han, 2007), a new analytical model for predicting the axial load capacity of a CFRP confined CFST column is proposed herein. These models are simple ones; additional developments such as the effect of concrete strength, yield strength of steel tube and height of columns need to be taken into account. When a uniform axial load is applied on the FRP strengthened column, the steel tube lying in the outer limit started to expand laterally due to the expansion of the concrete. The CFRP composites in the outer limits counteract the steel tube s expansion by providing a confining pressure as they are subjected to hoop tension. The confining pressure exerted by the CFRP is directly proportional to the lateral pressure ( f lcon ), and the model for this is shown in Figure 15. Considering the equilibrium, the lateral confining pressure ( f lcon ) provided by the CFRP strips is calculated by Equation 6 below, for the general case of a CFST column with the number of CFRP layers corresponding to 1 n 4. 6: f lcon ¼ ρ frpf frp n n þ 1 where f lcon is the lateral confining pressure exerted by the CFRP strips; n is the number of CFRP layers (n =1,2,3,4); and the static design safety for CFRP composite (γ CFRP ) is taken as 1 2. ρ frp is the FRP volumetric ratio to concrete for the column wrapped with CFRP strips (Figure 1) and can be determined by the following Equation 7. t frp b frp 7: ρ frp ¼ Db frp þ s frp where b frp and t frp are the width of CFRP strips and the thickness of the FRP confinement, respectively; D and s frp are the diameter of the column and the clear spacing between the CFRP strips. For a column confined with normal modulus CFRP composites, the compressive strength of the CFRP f frp t confined CFST circular column f 0 ccon, can be calculated using Equation 8. 8: f ccon 0 ¼ 1 þ k f lcon f uncon f uncon where f uncon and k are the unconfined compressive strength of the CFST column and the effective confinement coefficient, respectively. The proposed effective confinement coefficient (k) values for a column confined by CFRP strips at a spacing of 20 mm and 40 mm are 3 and 1 5, respectively. The following Equation 9 proposed by Mander et al. (1998) can be used to determine the axial load-carrying capacity of the unconfined CFST member. This simple equation was developed by considering the confining pressure provided by the steel tube, which embodies the effective lateral confining stress on the concrete. 9: f unconðmandþ ¼ A c f cc þ A s f y where f y is the yield stress of the steel tube and f cc is the lateral confining compressive stress of concrete, which can be defined as follows (Equation 10). ( " s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! # ) 10: f cc ¼ f c 1254 þ þ 794f l f c 2 f l f c The calculated axial load-carrying capacity of CFRP confined CFST columns is listed in Table 1 along with the failure load obtained from the experiments. The average percentage of the difference between calculated and experimental values is only ±5%. 6. Conclusion This paper has presented the experimental and analytical results for circular CFST column members strengthened by CFRP composite strips at two different spacings. Based on the test carried out on 21 specimens, the following conclusion can be drawn. f l D f frp t Figure 15. Confinement pressure provide by the CFRP strips Columns strengthened with CFRP composite strips at the closer spacing (20 mm) failed due to rupture of fibre in all three layers; however, all the columns that were strengthened with CFRP composite strips at a spacing of 40 mm failed due to buckling of the steel tube alone, which occurred in the unbonded area because of the absence of the confining pressure provided by CFRP strips. At both spacings (20 mm and 40 mm), the columns showed a stronger ability to withstand axial deformation than the reference column in all layers; moreover, increases in the number of layers significantly prevented the deformation of the column and enhanced the capacity of the column. Columns CS T3(2) and CS T3(2) showed an increased resistance to axial deformation of % and 69 72% when compared to the reference column. 926

11 The bonding of FRP strips did not affect the ductility of the columns, and the strips acted like a composite plate to simply enhance the capacity of the column. The bonding of FRP strips led to a significant enhancement in load-carrying capacity specifically a maximum of 30 13% and 12 06% for spacings of 20 mm and 40 mm, respectively, when compared to the reference column. Based on the experimental results obtained, simple equations were proposed to predict the confining pressure exerted by the CFRP composite strips. The column strengthened at a spacing of 40 mm showed inferior performance when compared to the 20 mm spacing. It is proposed that the CFRP composite strips can be effectively used to improve the structural performance of the CFST columns; in addition for axial strengthening of CFST columns, the column confinement at a 20 mm spacing is more suitable than a 40 mm spacing. 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