CHAPTER 2 REVIEW OF LITERATURE

Transcription

1 12 CHAPTER 2 REVIEW OF LITERATURE 2.1 GENERAL In construction industry, the selection of a building material is based on factors such as availability, structural strength, durability and workability. But the natural building material does not possess all these properties to the desired degree. It is, therefore, very important to select and combine appropriate materials to form a new element with desired properties resulting in a composite element. CFST member is an innovative idea, in which a steel element acts together with a concrete element, so that both elements resist the axial and flexural loads. For a variety of reasons, infrastructure concerned with concrete filled tubular structure and metallic structures becomes structural unsatisfactory and, in many cases, CFST structures are carrying loads far in excess of their design loads, but are still in service, because of their conservative design by modern standards. Hence it is necessary to restore or enhance the load carrying capacity and increase the life span of the structures. Many innovative applications have been explored for concrete structures whereas they were limited in the case of steel structures. This research is aimed at investigating the structural improvements of hollow square steel tubes filled with concrete and externally bonded with fiber reinforced polymer sheets and strips using a series of large scale experiments. The general review of literature on the reinforced concrete structures, steel hollow tubular members and concrete filled square steel

2 13 tubes, in which FRP used for strengthening purpose, is briefly presented and they were listed in the references at the end of the report. 2.2 REINFORCED CONCRETE MEMBERS Meier (1991) investigated the replacement of steel plates with FRP laminates for repairing and strengthening reinforced concrete beams. This paper encompassed externally bonding CFRP sheets to twenty-six concrete beams. Each beam was minimally reinforced with steel on top and bottom and included shear reinforcement. The maximum load increased over 100% compared to the control beam (unstrengthened) by applying a unidirectional CFRP laminate sheet to the tensile side of the specimens. Also, the deflection of the strengthened beam was 50% less than the control beam. The cracks in the repaired beams were small and closely spaced along the length of the member. This differed from the control beam, which showed a crack pattern of fewer and larger cracks. This study represented the first evidence that FRP laminates could help in the repair of deteriorated concrete beams. The failure modes related to FRP repaired beams were tensile failure of the CFRP sheets, classical concrete failure in the compressive zone, continuous peeling-off of the CFRP sheets due to an uneven concrete surface. Hamid Saadatmanesh et al (1994) presented a new technique for seismic strengthening of concrete columns. This technique requires wrapping thin flexible high-strength fiber composite straps around the column to improve confinement and thereby its ductility and strength. Analytical results were presented that quantify the gain in strength and ductility of concrete columns externally confined by means of high strength fiber composite straps. A parametric study was conducted to examine the effects of various design parameters such as concrete compressive strength, thickness and spacing of the straps and type of strap. The results indicated that the strength and

3 14 durability of concrete columns can be significantly increased by wrapping high-strength fiber composite straps around the columns. Tom Norris et al (1994) conducted an experimental program at the University of California at San Diego for the repair of earthquake damaged concrete members in high risk seismic zones. In this paper, an effective technique for repairing earthquake damaged concrete columns with FRP composites wraps was presented. E-glass uni-directional fibers were used. Four column specimens were tested to failure under reversed inelastic cyclic loading to a level that would occur in a severe earthquake. The columns were repaired with prefabricated FRP wraps and retested under simulated earthquake loading. The results of the repaired columns exhibited relatively larger lateral displacements at low load levels compared to original columns and it showed a significant impotents in the hysteresis loops of lateral load versus displacement. The results indicated that the proposed repair technique is highly effective. Both flexural strength and displacement ductility of repaired columns were higher than those of the original columns. M Bazaa (1996) presented a research work based on optimizing the length and orientation of the CFRP to increase beam strength and ductility. Eight beams were minimally reinforced with steel and over designed for shear to cause a flexural failure. One beam was used as a control while the others were bonded with three layers of CFRP. The sheets varied in length and orientation of the fibers. Four had unidirectional fibers with different lengths, and the other three had various fiber directions with regard to the longitudinal direction. The results of the experiment showed an increase in strength and stiffness and a decrease in deflection as compared to the control beam. All failures occurred at a load at least 57% higher than the control beam. The stiffness was similar until the cracking moment. At this point, less deflection was observed in the repaired beams. The load versus deflection

4 15 plots exhibited three different section modulus such as the start of the experiment to first crack, first crack to yielding of the steel began, and yielding of the steel to failure of the member. However, the off-axis CFRP provided an improved warning of failure due to cracking sounds. Tom Norris et al (1997) presented a paper and dealt with the results of an experimental and analytical study of the behavior of damaged or under strength concrete beams retrofitted with thin CFRP sheets. The CFRP sheets were epoxy bonded to the tension face and web of concrete beams to enhance their flexural and shear strengths. They concluded that CFRP sheets can provide increase in strength and stiffness to existing concrete beams when bonded to the web and tension face. Marco Arduini and Antonio Nanni (1997) presented a paper with experimental data obtained from strengthened, precracked, reinforced concrete specimens together with the results of material characterization. Strengthening was attained with the adhesion of carbon fiber-reinforced plastic sheets to the concrete surface. The CFRP was applied as in situ (i.e., working under the beam). Several variables were investigated, including: two types of CFRP material systems, two types of concrete surface preparations, two types of RC cross sections and the number and location of CFRP piles. For two specimens, the presence of applied load as well as external prestressing during the adhesion of the CFRP reinforcement, were investigated. It is shown that the effect of CFRP strengthening was considerable, but the effect of some of the tested variables was modest. An existing analytical model was extended to simulate the load-deflection behavior as well as the failure mode of the precracked RC specimens. Different failure mechanisms from ductile to brittle were simulated and verified.

5 16 Spadea et al (1998) presented a paper to establish the structural behavior of reinforced concrete beam strengthened with externally bonded carbon-fiber-reinforced plastic sheets. Four beams, three with bonded CFRP plates on the tension face, and two of which were provided with carefully designed external anchorage at the ends of the plates and along the span, were tested under four-point bending over a span of 4.8m. The tests were carried out under displacement control. The results showed that bonding a CFRP plate on the tension face of a RC beam, without consideration of the endanchorage stresses and the bond slip between the plate and the concrete substrate, can lead to significant degradation in the structural response of the plated beam. Carefully designed external anchorage, on the other hand, can lead to preservation of composite action to almost the failure load, and increases in the load capacity of up to 70%, substantial regain of structural ductility, and the transformation of a brittle failure to a more ductile failure. Saadatmanesh and Malek (1998) presented a paper to give design guidelines for flexural strengthening of RC beams with FRP plates. The ultimate capacity of the strengthened beam is controlled by either compression crushing of concrete, rupture of the plate, local failure or concrete at the plate end due to stress concentration, or debonding of the plate. These failure modes have been considered for developing design guidelines for strengthening reinforced concrete beams using fiber composites plates. The effect of multistep loading of the beam, before and after upgrading, has been included in these guidelines. Limit state design concept has been followed in this paper. Terms, definitions, and notations compatible to design guidelines for ordinary reinforced concrete beams have been used. Demers and Neale (1999) presented results of an experimental investigation for 16 round reinforced concrete columns of 300mm diameter and 1200mm height. These columns were confined by means of carbon-epoxy

6 17 sheets and loaded concentrically in axial compression. The effects of various parameters on the structural behaviour of the confined concrete columns were investigated. These parameters included the concrete strength, longitudinal steel reinforcements, steel stirrups, steel corrosion, and concrete damage. The test results showed that composite confinement can considerably enhance the structural performance of the concrete columns, especially with regard to ductility. Mahmoud et al (2000) presented a paper on flexural behavior of reinforced concrete beams strengthened with externally bonded FRP laminates. A simple and direct analytical procedure for evaluating the ultimate flexural capacity of FRP strengthened reinforced concrete flexural members was presented. The procedure was derived from equilibrium equations and compatibility of strains and is applicable to both singly and doubly reinforced concrete rectangular sections, as well as flanged sections. The procedure was validated by comparisons with results of experimental data available in the literature. Upper and lower limits of FRP that may be bonded to a reinforced concrete cross section to ensure ductile behavior were established, and design nomographs to facilitate implementation of the procedure were presented. Xiao and Wu (2000) described axial compression test results of 27 concrete cylinders confined by carbon fiber reinforced polymer composite jackets. The experimental parameters included plain concrete compressive strength and the thickness of the composite jacket. It was found that the carbon fiber composite jacketing could significantly increase the compressive strength and ductility of concrete. The test results indicated that concrete strength and confinement modulus defined as the ratio of transverse confinement stress and transverse strain, are the most influential factors affecting the stress-strain behaviour of confined concrete. There is a

7 18 significant increase in strength and ductility of concrete which can be achieved by carbon fiber composite jacketing. Besides the material properties such as concrete strength, the performance of the confined concrete is dominated by the confinement modulus. Pantazopoulou et al (2001) presented the results of experimental parametric study of jacketing of bridge column piers by fiber reinforced composite sheets as a repair alternative for corroded structures. Several small size concrete columns with various reinforcement configurations were subjected to accelerated corrosion conditions in the laboratory. After a target level of steel loss was attained, the columns were repaired using a variety of repair alternatives. Most of the repair schemes considered included jacketing the damaged specimens with glass-fiber wraps, in combination with grouting the voids between the jacket and the original lateral surface of the specimen with either conventional or expansive grouts. To protect the glass fiber material from exposure to alkali activity of the fresh grout, and to reduce the supply of oxygen and water to the mechanism of corrosion, different types of diffusion barriers were considered in the study. The efficacy of each repair system was evaluated by (1) assessing the post-repair corrosion resistance of the specimens under repeated exposure to accelerated conditions; and (2) the mechanical strength and ductility enhancement under concentric compression loading. And they concluded that FRP wraps being strong and corrosionresistant, proved very effective as jacketing material. Bonacci and Maalej (2001) presented a paper and dealt with the performance of conventionally reinforced concrete beams strengthened in flexure with externally bonded fiber-reinforced polymers. One-third of the specimens with external reinforcement added and showed that the strength increases of 50% or more in combination with considerable deflection capacity. It was clear from the experimental studies that the procedures

8 19 followed were most representative of member strengthening rather than repair. To assess the real potential of using FRP for expedient and economical field repair and strengthening of RC members, it was concluded that future research on the application of FRP to RC members should focus on conditions that are similar to what is observed in the field, including the effects of sustained load during repair/strengthening as well as corrosion and loadinduced damage. Lam and Teng (2002) presented a large test data base assembled from an extensive survey of existing studies, and data base employed to assess available axial strength models for confined concrete. The test data base was also deployed to examine the effect of various factors on performance of FRP confined concrete. This study showed that confinement effectiveness of FRP based on reported test results depends little on unconfined concrete strength, size, and length-to-diameter ratio of test specimens and FRP type, but depends significantly on the accuracy of the reported tensile strength of the FRP. The inherent variation of unconfined concrete strength also causes some scatter of data at low confinement s ratios. Using those test data with accurate FRP tensile strengths, only two of the nine existing models were found to give close predictions. A new simple model was finally proposed, based on the observation that a linear relationship exists between the confined concrete strength and the lateral confining pressure from the FRP. Arya et al (2002) presented a paper that reviews the contents of Concrete Society Technical Report 55 on strengthening concrete structures using externally bonded fiber composite materials. The report provides design guidance on flexural strengthening of beams and slabs, shear strengthening of beams and columns, and flexural and compressive strengthening of columns. A worked example of a beam strengthened with bonded FRP reinforcement is

9 20 presented here to illustrate the design procedure for flexural strengthening, this representing a majority of applications. Bonding of FRP to the tension face increases the flexural strength of concrete elements. Shamim et al (2002) presented a paper and dealt with the damaged specimens which were repaired with carbon and glass fiber-reinforced polymer (CFRP-GFRP) sheets and tested to failure. Companion control specimens were tested to failure without rehabilitation to provide a basis for comparison and evaluate the effectiveness of the repair techniques and results showed that fiber-reinforced polymers (FRPs) were effective in strengthening for flexure as well as shear. Over-reinforcing in flexure resulted in shifting the failure to shear mode, which in some cases may be undesirable. Retrofitting with FRPs provides feasible rehabilitation techniques for wall-slabs and beams. FRPs were effective in enhancing strength in both flexure and shear. Domingo and Pantelides (2002) presented a model for describing the compressive behavior of concrete members confined by FRP composite jackets. The proposed model was based on accepted concrete and FRP composite behaviour and fundamental principles of mechanics and applicable to both bonded and no bonded FRP confined concrete. The distinguishing feature of the proposed model was a variable strain ductility ratio, which was demonstrated to be a function of the stiffness of the confining FRP composite jacket and the extent of internal damage, rather than a constant, as is typically assumed for steel confined concrete. It was shown that the compressive behaviour of FRP-confined concrete can be separated into two components: (1) A strain softening component, which accounts for the nonlinear stress strain behaviour that results from unrestrained crack propagation near the peak compressive strength and strain of unconfined concrete; and (2) A bilinear strain-hardening component, which accounts for the increase in strength due to confinement provided by the elastic FRP jacket after the jacket

10 21 becomes effective in curtailing the dilation of the concrete core. An expression was obtained for predicting the ultimate compressive strength and strain of the FRP-confined concrete based on equilibrium. The ultimate compressive strength and strain of the FRP-confined concrete were found to be a function of the effective jacket stiffness, type of jacket construction bonded or no bonded and ultimate strain in the FRP jacket. Omar Chaallal et al (2003) proposed the results of a comprehensive experimental investigation on the behavior of axially loaded short rectangular columns that were strengthened with carbon fiber-reinforced polymer wrap. Six series, a total of 90 specimens, of uniaxial compression tests were conducted on rectangular and square short columns. The behaviour of the specimens in the axial and transverse directions was investigated. The parameters considered in this study were the concrete strength, the aspect ratio of the cross section; and the number of FRP layers. The findings of this research can be summarized as follows. The CFRP wrapping enhances the compressive strength and the ductility of both square and rectangular columns, but to a lesser degree than that of circular columns. The ultimate strength and the ductility of the CFRP confined concrete increase with the increasing number of confining layers. The increase in strength and ductility is more significant for lower strength concrete, representing poor or degraded concrete, than for normal-to-high strength concrete. The CFRP confining jacket must be sufficiently stiff to develop appropriate confining forces at relatively low axial strain levels. The gain in compressive strength obtained by the CFRP confined concrete depends mainly on the relative stiffness of the CFRP jacket to the axial stiffness of the column. Campione and Miraglia (2003) examined the analytical compressive behaviour of concrete members reinforced with fiber-reinforced polymer and also analysed the variation of shape of the transverse cross

11 22 section. The bearing capacity and the increase in the maximum strain for the members having a cross section circular, square with round corners reinforced with FRP were determined. The proposed analytical model allows one to evaluate the confining pressure in ultimate conditions considering the effective confined cross-section and also allows one to determine the ultimate strain corresponding to FRP failure through a simplified energetic approach. Analytical results were then compared to experimental values available in the literature showing good agreement. Reza Esfahani and Reza Kianoush (2004) presented a study on the axial compressive strength of columns strengthened with FRP wrap. The experimental part of the study included testing six reinforced concrete columns in two series. The first series comprised three similar circular reinforced concrete columns strengthened with FRP wrap. The second series consisted of three similar square columns, two with sharp corners, and the other with rounded corners. Axial load and displacement of columns were recorded during the tests using a displacement control test set up. The test results were compared with the values calculated using proposed equations. It is shown that the FRP wrap increases the strength and ductility of circular columns, significantly. The equations proposed correlate well with the test results of circular columns. According to the test results, the FRP wrap did not increase the strength of square columns with sharp corners. However, the square column with rounded corners exhibited a higher strength and ductility compared to those with sharp corners. The equations proposed correlate well with the results of square columns with rounded corners. Mukherjee et al (2004) presented a study focusing on the mechanical response of the concrete columns confined with fiber-reinforced polymer composites. Practical columns were often derived from axisymmetric conditions due to noncircular cross section, geometric imperfections and

12 23 loading eccentricities. This paper discussed these complicating effects on the mechanical behaviour of columns confined with FRP. Experiments have been carried out to examine the effects of geometric and loading imperfections on columns of various shapes. A model originally developed for axisymmetric situations has been extended to include the complicating effects. And they concluded that, the FRP confinement was the most effective for columns of a circular cross section. For non-circular cross sections, although the stiffness of the columns remains close to that of circular ones, the ultimate strain of the columns decreases and as a result, the ultimate strength of the column decreases. The failure of the concentrically loaded non-circular columns takes place at the corners. The maintenance of minimum cover radius is of great importance for non-circular columns. A finite element study has been presented to determine the minimum cover radius that is necessary. It has been found that, at a 15 mm corner radius, the hoop stress concentration in FRP is negligible. The predicated results are in close agreement with present experimental results and those of other investigations. Michele Theriault et al (2004) carried out laboratory investigation on the compressive behaviour of FRP confined concrete columns using relatively small scale specimens. In this study, the influence of slenderness ratio and specimen size on axially loaded FRP confined concrete columns were investigated experimentally and the results were compared to theoretical models and experimental results gathered from the published literature. From the experimental and analytical results, they concluded that no significant variations occur in measured compressive strengths when FRP confined concrete cylinders and short FRP confined concrete column specimens are used. Furthermore, the effects of the confining pressure were seen to be independent of the slenderness ratio.

13 24 Severino Pereira Cavalcanti Marques et al (2004) presented a numerical model for evaluating the behaviour of axially loaded rectangular and cylindrical short columns of concrete confined by fiber-reinforced polymer FRP composites. The proposed formulation considers, for unconfined and confined compressed concrete, a uniaxial constitutive relation that utilizes the area strain as a parameter of measure of the material secant axial stiffness. For unconfined concrete, the model adopts an explicit relationship between axial strain and lateral strain, while for confined concrete, an implicit relation is considered. The model employs a simple iterative-incremental approach that describes the entire stress-strain response of the columns. The behaviour of the FRP is considered linear elastic until the rupture. To validate the model, a number of columns were analysed and the numerical results were compared with experimental values published by other authors. This comparison between experimental and numerical results indicated that the model provides satisfactory predictions of the stress-strain response of the columns. Dat Duthinh and Monica Starnes (2004) presented a paper to study the strength and ductility of concrete beams, in that test seven concrete beams were reinforced internally with varying amounts of steel and externally with precured carbon fiber reinforced polymer plates applied after the concrete had cracked under service loads were tested under four-point bending. Strains measured along the beam depth allowed computation of the beam curvature in the constant moment region. Results showed that FRP is very effective for flexural strengthening. As the amount of steel increases, the additional strength provided by the carbon FRP plates decreases. Compared to the beam reinforced heavily with steel only, beams reinforced with both steel and carbon have adequate deformation capacity, in spite of their brittle mode of failure. Clamping or wrapping at the ends of the precured FRP plate enhances the capacity of adhesively bonded FRP anchorage. Design equations

14 25 for anchorage, allowable stress, ductility, and amount of reinforcement were discussed. Savoia et al (2005) described the problem of long-term creep deformation of reinforced concrete tensile elements strengthened by external fiber reinforced plastic plates. Formation of discrete cracks in concrete under tension was taken into account. A kinematic model is used, where relative slips between concrete, steel bars, and FRP plates were considered, governed by viscous interface shear stress slip laws. Bazant s solidification theory and exponential algorithm were used to obtain incremental constitutive equations for concrete as well as for steel-concrete and FRP-concrete interface laws. Moreover, cohesive normal stresses across transverse cracks in concrete were considered. The incremental differential system of equations is transformed into a nonlinear algebric system by a finite difference discretization with respect to axial coordinate. It is shown that reinforcing by means of FRP plates or sheets has significant beneficial effects on the behavior of reinforced concrete elements under service loadings because it increases concrete tension stiffening effect and it strongly reduces crack width. 2.3 HOLLOW STEEL TUBULAR MEMBERS Jiao and Zhao (2004) conducted investigation on the behavior of carbon fibre reinforced plastics strengthened butt-welded very high strength (VHS) circular steel tubes. The VHS steel has a yield stress of 1350 MPa and an ultimate strength of 1500 MPa. Three types of epoxy resins with different lap shear strength were used. Tests were conducted to determine the lap shear strength between CFRP and VHS steel tubes. A total of 21 butt-welded VHS tubes strengthened with CFRP were tested in axial tension. Three kinds of failure modes, i.e. adhesive failure, fiber tear and mixed failure were observed. A significant strength increase was achieved using CFRP epoxy strengthening technique. The suitable epoxy adhesive for strengthening VHS

15 26 tubes was recommended. A theoretical model was developed to estimate the load carrying capacity of butt-welded VHS tubes strengthened using CFRP. Yang Yong-xin et al (2005) carried investigation on bond behavior of CFRP to steel. There are several advantages of bonding carbon fibre reinforced polymer laminates to the deteriorated steel members over the traditional retrofit methods. The bonding of CFRP to steel affects the load transfer and the effectiveness of strengthening. Appropriate specimens were employed to test the bond strength and bond durability between CFRP and steel. Test results showed that the three adhesives chosen have excellent properties and are suitable for steel structures strengthened with CFRP. Photiou et al (2006) discussed the experimental results to investigate the effectiveness of an ultra-high modulus, and a high modulus, CFRP prepreg in strengthening an artificially degraded steel beam of rectangular cross-section under four-point loading. Four beams were upgraded of size 80x120x5 mm, two utilising U-shaped prepreg units, which extended up the vertical sides of the beam to the neutral axis height, whereas the other two beams used a flat plate prepreg. The composite containing the ultra-high modulus CFRP failed when the ultimate strain of the carbon fibre was reached in the pure moment region. The failure load exceeded the plastic collapse load of the undamaged beam, thus demonstrating the effectiveness of the proposed upgrading scheme. On re-loading the failed beams, the U-shaped hybrid upgrade continued to act compositely with the steel beam outside of a well confined region close to the original failure location, whereas the beams with the flat plate exhibited the typical response of a steel beam, owing to debonding having taken place over practically the entire length of the prepreg. The beams using the high modulus CFRP reached even higher ultimate loads and exhibited ductile response leading to very high deflections; neither fibre

16 27 breakage nor adhesive failure was observed in either the U-shaped or the flat plate strengthened beam. Teng and Hu (2006) investigated the behaviour of FRP-jacketed circular steel tubes and cylindrical shells under axial compression. In order to demonstrate the effect of FRP confinement on steel tubes, four steel tubes with or without a glass FRP jacket were tested. Three steel coupon tests were conducted according to BS18 to determine the tensile properties of the steel. The average values of the elastic modulus, yield stress, ultimate strength, and elongation after fracture from these tensile tests were GPa, MPa, MPa and 0.347, respectively. The compression tests were all conducted using an MTS machine with displacement control. Both the test and the numerical results showed that FRP jacketing is a very promising technique for the retrofit and strengthening of circular hollow steel tubes. Shaat and Fam (2006) conducted axial loading tests on CFRPretrofitted short and long HSS steel columns and they found that for short columns, transverse CFRP layers are effective in confining the outward local buckling. The major parameters varied in the testing program include the number of CFRP layers and the fibres orientation (transverse or the longitudinal). A layer of GFRP was installed on the steel surface to prevent direct contact between the carbon fibre and steel, which could lead to galvanic corrosion. The maximum load carrying capacity increased to 18% for short columns and 13% 23% for long columns. It was also suggested that a larger number of specimens to be tested to provide an average strength that could statistically compensate for the variability arising from imperfections. Xiao-Ling Zhao et al (2006) established a report on carbon fibre reinforced polymer (CFRP) strengthened rectangular hollow section (RHS) subjected to transverse end bearing force. They investigated the possibility of using the CFRP strengthening techniques to improve the web crippling

17 28 capacity of cold formed RHS. They adopted the strengthening techniques such as wrapping with CFRP sheets outside the RHS or applying CFRP plates outside or and inside the RHS. It was found that the CFRP strengthening significantly increased the web crippling capacity. The CFRP sheets possessed a nominal modulus of elasticity of 240 Gpa, nominal tensile strength of 3800 Mpa and thickness of 0.18mm, whereas CFRP plates were of 165 Gpa modulus of elasticity, 2700 Mpa tensile strength and 1.20mm thickness. Two layers of CFRP sheets were wrapped outside in RHS in one type specimen and were bonded to each side of RHS and on web in one specimen. In another type of specimen CFRP plates were bonded inside surface of RHS web and the next type specimen was the combination of type 3 and 4. The last type specimen was similar to the above mentioned type except horizontal CFRP plate bonded to inner surface of top flange. The orientations of fibres were kept perpendicular to longitudinal axis. All the tests were performed in a 500 KN Baldwin Universal Testing Machine. Web crippling was prevented by CFRP plates due to change of failure mode from web buckling to web yielding has been observed from the experimental investigation. Schnerch et al (2006) described the bond behavior of carbon fiber reinforced polymer (CFRP) strengthened steel structures. The research focused on surface preparation methods and means of galvanic corrosion prevention. The main aim of the investigation was to examine the proper techniques for the bond of CFRP to steel structures and the results were compared. Each specimen was strengthened by bonding CFRP of high modulus capacity at the bottom of tension flange. Four point bending test was used and compression strength was determined considerable variation in the development length and maximum CFRP strain at failure was found.

18 29 Fawzia et al (2007) described the behaviour of very high strength circular steel tubes strengthened by carbon fibre reinforced polymer and subjected to axial tension. In this research, MBrace CF530 was used. It is a unidirectional tow sheet carbon fibre. The VHS steel tubes had a yield stress of 1350 MPa, an ultimate stress of 1500 MPa and the modulus of elasticity is about 200 GPa. A series of tests were conducted with different bond lengths and number of layers. The specimen was loaded in a Universal Testing Machine. The test was continued until failure of the specimen. The distribution of strain through the thickness of CFRP layers and along the CFRP bond length was studied. It was found that the strain was decreased along the CFRP bond length far from the joint and the strain through the thickness of the CFRP layers was also towards top layer. The effective bond length for high modulus CFRP was established. Finally empirical models were developed to estimate the maximum load for a given CFRP arrangement. Amr Shaat and Amir Fam (2007) carried investigation on nonlinear finite element analysis of axially loaded slender hollow structural section columns, strengthened using high modulus carbon-fiber reinforced polymer longitudinal sheets. The model was developed and verified against both experimental and other analytical models. Both geometric and material nonlinearities, which are attributed to the column s initial imperfection and plasticity of steel, respectively, are accounted for. Residual stresses have also been modeled. It was found that the axial strength in the experimental study was highly dependent on the column s imperfection. Consequently, no specific correlation was established experimentally between strength gain and amount of CFRP. The model predicted the ultimate loads and failure modes quite reasonably and was used to isolate the effects of CFRP strengthening from the columns imperfections. It was then used in a parametric study to examine columns of different slenderness ratios, imperfections, number of

19 30 CFRP layers, and level of residual stresses. The study demonstrated the effectiveness of high modulus CFRP in increasing stiffness and strength of slender columns. Michael V. Seica and Jeffrey A. Packer (2007) discussed the experimental results to investigate CFRP strengthening of an artificially degraded steel beam of circular cross-section under four-point loading. Round hollow sections were used in this investigation because offshore structures are mainly constructed with circular steel elements. Finally, the steel beam in bending and, although it was expected to reach the plastic moment, it only reached 94% of it. However, all CFRP wrapped specimens did reach the plastic moment and also exhibited increased ductility and rotation capacity. They had concluded that the use of CFRP composites to enhance the strength of tubular steel flexural members both in air and underwater is perfectly feasible. Xiao-Ling Zhao and Lei Zhang (2007) reviewed the properties of fibre reinforced polymer (FRP) strengthened steel structures such as the bond between steel and FRP, the strengthening of steel hollow section members (SHS) and fatigue crack propogation in the members. Different failure modes were observed based on the loading pattern. The strengthening of SHS were carried over an compression members subjected to four point bending, strengthened by carbon fibre reinforced polymer (CFRP) sheets. Regarding fatigue crack propagation, the influence of fatigue loading on the bond between steel and CFRP with normal modulus (240 GPa) and high modulus (640 GPa) were experimented and revealed that normal modulus CFRP bonded specimens were more sensitive to fatigue cycles, whereas high modulus CFRP bonded specimens were more sensitive to applied load ranges. Bambach and Elchalakani (2007) conducted investigation on plastic mechanism analysis of steel square hollow section (SHS) tubes

20 31 strengthened using externally bonded CFRP under quasi-static large deformation axial compression. The section dimensions of the SHS as per Australian Standard ranged from slender to compact sections according to the section designations used. The slenderness values ranged from 35 to 73, and all columns were of length equal to three times the width of the SHS. Highstrength CFRP was used and applied to the exterior of the SHS with Araldite 420 epoxy. The fold formation process of the stub column was such that the flat sides formed the well-known roof mechanism. The collapse proceeded progressively by folding about concentrated hinge lines and yielding of the four corners. An expression for the plastic collapse axial load was obtained by equating the total energy absorbed in bending and yielding to the external work carried out during deformation of the composite tube. The predicted instantaneous post-buckling and mean collapse loads were compared with the experimental results. Haedir et al (2010) discussed the principal values and reinforcing effect of externally bonded carbon fibre reinforced polymer in steel structures such as circular hollow sections (CHS). The investigation aimed to quantify the contribution of the external reinforcing system results enhancement of strength in thin walled sections. A total of 18 CHS beams were analyzed and tested under four point bending. The CFRP layers were of longitudinally oriented and circumferential (or) hoop direction termed as horizontally oriented. The number of layers varied from 1 to 4. The tensile yield stress of CHS was determined as 460 N/mm 2. The young s modulus (E) value of longitudinal CFRP was assumed to be same both in tension and compression, while the E value of hoop CFRP was assumed to be 14,000 N/mm 2 towards compressive modulus and 12,000 N/mm 2 towards tensile modulus respectively. The results of the analysis showed that the steel was fully yielded and only CFRP in tension contributed to the ultimate capacity of the composite beam. The stress distribution of the steel fibres in compression are

21 32 elastic-plastic, with consideration of CFRP fibres in compression showed to under estimate the measured values. Those were evident in specimens with 3 or more CFRP layers. 2.4 CONCRETE FILLED STEEL TUBULAR MEMBERS Mohammad Shams and Ala Saadeghvaziri (1997) investigated the state of the art of concrete-filled steel tubular columns including experimental and analytical. The main parameters were cross sectional shape, aspect ratio, and column length on the behavior of CFT columns. Tests were conducted on 286 columns under concentric axial loads. Six sizes of circular, square, and octagonal steel tubing with different wall thicknesses were used during the test. It was found that CFT columns can fail in two modes. In the case of longer columns, general buckling and in shorter columns, crushing of concrete was observed. The ultimate strength of CFT columns was considerably affected by the slenderness ratio and the thickness of steel tubing, as well as the cross-sectional shape. A confining effect can be expected for circular columns, while for square columns there was no increase in axial strength due to triaxial effects despite small slenderness ratios and large wall thickness. From the results of bond effect, the bond strength has not significant effect on the flexural capacity of CFT columns, the flexural capacity considerably increases by increasing axial load, and steel tube has a significant effect on improving the compressive strength of concrete and preventing the brittle failure that is normally associated with unconfined high strength concrete. Stephen P Schneider (1998) presented an experimental study on the behavior of short, concrete filled steel tubular column concentrically loaded in compression to failure. Fourteen specimens were tested, three were circular, five were square, and six were rectangular steel tube shapes to investigate the influence of steel tube shape and wall thickness on the ultimate

22 33 strength of the composite column. The experimental parameters were thickness, cross section and length of the tubular section. Ultimate strength results were compared to specification governing the design of concrete filled steel tube column. Experimental results showed that circular tubes have a high post yield strength and stiffness compared to square and rectangular section. Weizi zhang and Bahram sharooz (1999) evaluated the strength of short and long concrete filled tubular (CFT) columns. Rectangular and square CFT columns were chosen. The applicability of American Concrete Institute (ACI) standard techniques for computing the capacity of short and slender CFT was investigated. The ACI standard approach fully predicted the capacity of short CFTs made with normal strength tubes filled with either by normal or high strength concrete, showed a close related values that arrived by employed a fibre analysis. Andrew and Vijaya Rangan (1999) conducted a test on high strength concrete filled steel tubular (CFST) columns. The primary parameters of testing were such as the load eccentricity and column slenderness. The objectives of CFST columns were therefore to study the behaviour of hollow section steel columns filled with high strength concrete. A total of 16 circular CFST columns were tested in eccentric compression producing single-curvature bending. All columns were tested in a 2500KN capacity universal testing machine. The trend showed that column slenderness had a marked detrimental effect upon column strength, that too for low slender columns specifically. Low slender columns possessed greater strength than very slender columns, therefore less ductile. Twenty five circular CFST columns were tested for double-curvature tests, as of for single-curvature test procedure. Consistent with the behaviour or reinforced concrete columns, the strength and flexural stiffness of a CFST column was enhanced when bended to double-curvature.

23 34 Amit varma et al (2002) experimentally investigated the behaviour of high strength square concrete filled steel tube (CFST) beam- columns. The parameters adopted for their investigation were the width to thickness ratio (b/t) ratio, yield stress (σ y ) of the steel tube and the axial load level. Several specimens of CFST beam-columns were tested under constant axial load and monotonically increased flexural loading. The test length of the beam-column specimen was taken as 4.0m. The axial load was maintained constant by 22,000KN in a universal testing machine. The flexural loading was applied by imposing monotonically increased rotations at the ends of specimens by two 670KN hydraulic rams and loading beams. The initial and serviceability load section flexural stiffness of CFST beam-columns were tracked by unwalled transformed and cracked transformed properties. Mohanad Mursi and Brian (2003) investigated the experimental and theoretical treatment of coupled local and global buckling of concrete filled steel columns. A series of three experiments were carried out to consider the effect of a slender cross-section on the overall buckling capacity of a concrete filled steel column. The plate slenderness of the cross-sections was 36.0, 46.4, and The tests were conducted on both hollow and concrete filled steel sections. All of the steel sections were fabricated from steel plate of 3mm wall thickness. Each of the tests was conducted in a selfstraining rig with a 5,000 kn capacity loading jack. From the axial loadlateral deflection it can be noted that the presence of concrete not only increases the ultimate load but the flexural rigidity as evidenced by the slope of the load deflection curves. And also, the local buckling effects depend on the slenderness of the component plates of the column and this plays a large role in considering the confinement effect provided by the concrete core. Kenji Sakino et al (2004) investigated the confining effect of steel tubes on concrete strength and the restraining effect of the concrete fill on

24 35 local buckling of the steel tube wall, and evaluated the ultimate load and load deformation relationships. The main parameters of this tests were tube shapes (circular and square), tube tensile strength (400, 600, 800 MPa), tube diameter width-to-thickness (D/t or B/t) ratio, and design concrete strength (20, 40, 80 MPa). An analytical model is also developed to estimate the ultimate strength of CFT short columns. A total of 114 specimens were tested in the experimental investigations on centrally loaded hollow and CFT short columns. The capacity reduction factor due to local buckling of the square steel tube wall was empirically derived based on the test results of hollow square steel tube columns with a thin wall, and then modified applicable to the steel tube in square CFT columns by considering the restraining effect of filled concrete on local buckling of the steel tube wall. Amir Fam et al (2004) carried out experimental work and analytical modeling for concrete-filled steel tubes subjected to concentric axial compression and combined axial compression. The main objective of the study is to evaluate the strength and ductility of CFST short columns and beam-column members under different bond and end loading conditions. Both bonded and unbonded specimens were tested. For the unbonded columns, the bond between the steel and concrete was prevented by a layer of asphalt applied to the inside surface of the steel tube. A length-to-diameter ratio of 3 was selected for the CFST short columns in order to ensure short column behavior. The short columns were tested using a 600 kips Universal Testing Machine. The test result shows that the axial strength capacity of CFST short columns occurred at axial strain ranges from to 0.012, the strength was 65 to 75% higher than the strength of the composite section based on unconfined concrete strength. The maximum and residual axial load capacities of unbonded CFST short columns were slightly higher than those of bonded specimens, due to confinement effect.

25 36 Dalin-Liu and Wie-Min Gho (2005) conducted experimental investigation into the axial load behavior of rectangular concrete filled steel tubular (CFT) stub columns, subjected to concentric loading. A total of 26 specimens were tested under concentric compression. Material strength f c =55 and 106 MPa, f y =300 and 495 MPa were taken as primary parameters. Cross sectional aspect ratio lied between 1.0 and 2.0. Axial load was applied by 5000KN capacity Instron Testing Machine. They compared the experimental data with the design codes (EC4, ACI, AISC), which gave safe estimation by 7 and 8%. Favorable ductility performance was observed for all specimens during the tests. It was observed that EC4 seen to be unsafe to predict the ultimate capacity of CFT columns fabricated from mild steel and high strength concrete. On the other hand ACI, AISC codes estimated the failure loads of specimens by 7, 8 and 2% respectively. Yan Xiao et al (2005) discussed the confined concrete-filled tubular columns (CFT) in controlling the local buckling of the steel tube and confining the concrete in the potential plastic hinge regions of a CFT column. In order to achieve the objective, severe efficient transverse confinement were proposed and carbon fibre reinforced polymer (CFRP) as additional confinement of CFT columns. The specimens were examined through experimental testing. Thirteen cylinder specimens were tested under monotonic axial compression using 500T capacity high stiffness compression testing machine. Testing parameters for CCFT columns were the number of layers of CFRP wraps ans with or without the gap between the steel tube and the CFRP confinement. CCFT specimens directly wrapped with CFRP exhibited similar bilinear behaviour as that of concrete cylinders confined by FRP prior to rupture of CFRP. The local buckling and subsequent rupture of the steel tube were effectively delayed compared with the counterpart CFT specimens.

26 37 Lin-Hai Han and You-Fu Yang (2006) analyzed a model study on cyclic performance of concrete filled steel circular hollow sections (CHS) under flexure loading. The parameters in that study included the concrete strength (fas) and axial load level (n). Eight concrete filled steel CHS specimens were tested to constant axial load and cyclically increased flexural loading. Two empty columns were also carried out the same test procedure for comparison purpose Testing was conducted until either the specimen failed due to fracture of the steel tube (or) lateral load resistance had deteriorated to 50 % of lateral load capacity. The following observations and conclusions were made based on the limited research work. Because of infill concrete, the circumferential deformation and ovalization of composite c/s were prevented, resulting in higher stiffness, larger bearing capacity and richer ductility. Andrew wheeler and Russell Bridge (2006) investigated the behavior of circular concrete filled thin walled steel tubes in flexure. A series of flexural tests were carried out on full-scale tube specimens both filled and unfilled. Concrete filled tubes utilized the compressive strength where as steel tube counteracted the tensile strength, when subjected to pure bending and observations made on behavior of section, its ultimate capacities, such as, flexural stiffness, with modes of failures. Larger sections were used that have significant effect on the behavior and the specimens were tested under displacement control. Regarding bare steel sections loading was continued until either a buckle formed or a significant loss in load was observed. Concrete filled specimens were tested to its full capacity. Flexure tests were carried out on 406mm and 456mm diameter tubes. It was observed that the flexural strength and ductility of a circular hollow section was increased due to the in-filled concrete, which resulted in prevention of local buckling. Ultimate moment of concrete filled tubes were exceeded the nominal moment capacities. The slip was measured at the ends of the sections with minimum