STRENGTHENING STEEL-CONCRETE COMPOSITE GIRDERS USING FRPs: STATE-OF-THE-ART

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1 Asia-Pacific Conference on FRP in Structures (APFIS 2007) S.T. Smith (ed) 2007 International Institute for FRP in Construction STRENGTHENING STEEL-CONCRETE COMPOSITE GIRDERS USING FRPs: STATE-OF-THE-ART N. Ragab, R. El-Hacha *, and M. Aly Department of Civil Engineering, University of Calgary, Canada. ABSTRACT More than 40% of the bridges in North America built in the fifties and sixties are in serious and urgent need of repair and rehabilitation or replacement. In United States, steel bridges comprise more than 34% of the total number of bridges that are in serious need for upgrading. Many of these bridges needed to be repaired due to permanent damage to critically stressed locations of the tension members. These damages were caused by direct physical impact, corrosion, growth of small cracks under fatigue loading, and gradual loss of the cross section. The use of Fiber Reinforced Polymer composite (FRP) as a strengthening material has received a great attention in last decade due to its superior characteristics. Prestressing techniques increase the benefits obtained from using FRP and enhance the serviceability behaviour of strengthened steel-concrete composite beam. This paper reviews the analytical investigations, experimental studies, demonstration projects, and field applications of steel structures strengthened using non-prestressed and prestressed FRP techniques. KEYWORDS Steel concrete composite girder, FRP, non-prestressed, prestressed, flexural strengthening, externally bonded, anchorage, bond, fatigue. INTRODUCTION Strengthening using externally bonded FRP sheets and plates was introduced in the last few decades and proved to be successful in many applications of strengthening reinforced concrete and masonry structures. In strengthening systems, the material is generally expected to have higher or similar stiffness compared to the base material of the deficient structures. This is the case in masonry and reinforced concrete structures but not in steel structures. By introducing new types of high modulus FRP materials, the system shows good potential for use in strengthening steel structures because it has many advantages compared to conventional steel plate strengthening. Hollaway and Cadei (2002) in a state-of-the-art article on Progress in the Technique of Upgrading Metallic Structures with Advanced Polymer Composites described the basic principles of upgrading steel structures with advanced composite materials as well reviewed the problems associated with plate bonding onto steel structures and how to overcome these problems in addition to a review of the practical applications of this technique to steel structures including some case studies. A comprehensive state-of-the-art review of research work on FRP strengthening steel structures was conducted by Shaat et al (2004) on retrofitting steel structures using FRP including repair of naturally corroded or artificially notched beams, strengthening of intact beams, strengthening of steel-concrete composite girders, retrofitting of thin-walled tubular sections. The authors discussed important topics related to the fatigue behaviour, bond and force transfer mechanisms between steel and FRP. The durability of retrofitted systems particularly the issue of galvanic corrosion, and the available field applications were also presented. In general, the authors summarized the research findings such that FRP sheets and strips are not only effective in restoring the lost capacity of a damaged steel section but are also quite effective in strengthening of steel sections to resist higher loads, extend their fatigue life and reduce crack propagation, if adequate bond is provided and galvanic corrosion is prevented. The authors identified a number of potential future research areas that need to be investigated. A recent state-of-the-art paper on FRP strengthening steel structures by Zhao and Zhang (2007) provided a review of the current research areas that have developed rapidly on the bond between steel and FRP, strengthening of steel hollow section members, and fatigue crack propagation in the FRP steel system. The authors identified the future research topics needed such as the bond slip relationship, the stability of CFRP strengthened steel members, and fatigue crack propagation modeling. 951

2 This paper complements the available state-of-the-art review papers by focusing on prestressed FRP strengthening of steel-concrete composite girders. To set the background for the paper, strengthening steelconcrete composite girders using non-prestressed FRP is also summarized. NON-PRESTRRESSED FRPs FOR STRENGTHEING STEEL-CONCRETE COMPOSITE GIRDERS Mertz and Gillespie (1996) tested small scale specimens W8 10 (W200 15) rolled beams mm long, strengthened with 6 mm thick CFRP plates bonded to the tension flanges. The beams were tested in a four-point loading. The test results showed that there was approximately 20 % increase in the flexural stiffness and more than 50 % increase in strength before the test of these beams was stopped due to the buckling of the top cover plate. Also, full scale tests were carried out on two girders 610 mm deep, 6400 mm long I-beams taken from a demolished bridge that had a severely corroded bottom flange (approximately 80 % loss of the tension flange). These beams were strengthened by adhesively bonding 6 mm CFRP plates to the bottom flange. The test results showed a significant increase in the ultimate strength and flexural stiffness of the repaired beams. Sen et al., (2001) tested six specimens 6100 mm long W8 24 (W200 36) A36 wide flange steel beam acting compositely with a 114 mm thick by 710 mm wide reinforced concrete slab. Three of the beams had yield strength of 310 MPa, while the remaining had yield strength of 370 MPa. The beams were first loaded to cause yielding of the tension flanges to simulate severe service case. Then, three beams were strengthened using 2 mm CFRP plate, while the remaining three beams were strengthened using 5 mm thick CFRP plate. Two out of the three beams strengthened using 2 mm CFRP plate were tested without end anchorage, while the third beam was tested using a bolted clamp assembly anchorage consisting of a pair of bolts at 89 mm from the ends of the plates damaged beams. For the beams strengthened with 2 mm CFRP plate, the test was stopped because the load cell reached its capacity. The behaviour of the beam having anchorage did not differ from that with epoxy adhesive only, because no debonding was observed in both cases. For beams strengthened with 5 mm CFRP plate, one beam was tested without end anchorage which had a premature sudden failure by the delaminating of the CFRP plate. This beam was then re-tested to determine the ultimate capacity of the steel section that was used to finetune the finite element model used to determine the flexural behaviour of all specimens. The rest of the beams strengthened using 5 mm CFRP plate, had a bolted clamp assembly consisting of two lines of three 19 mm diameter bolts at each end. The failure mode was by the CFRP rupture along the bolts. The results showed a significant increase in the ultimate capacity of steel composite bridge sections containing 310 MPa wide flange shape. The increase in ultimate capacity was 21 % and 52 % for the 2 mm and 5 mm CFRP plates, respectively. Corresponding increase in ultimate load capacity for the bridge sections containing 370 MPa wide flange shapes were 9 % and 32 % for the 2 mm and5 mm CFRP plates respectively. The results indicated modest increases in the elastic stiffness, particularly when the thinner 2 mm CFRP plates were used. For all beams, the results indicated that failure was ductile because of the noticeable deflection before failure. Tavakkolizadeh and Saadatmanesh (2003) studied the effect of varying the number of layers of CFRP sheets to strengthen a steel-concrete composite beam. Three beams were fabricated using W A36 steel beams and a concrete slab of 75 mm thick and 910 mm wide. The beam had a span of 4780 mm and four point bending tests were performed with a 500 mm constant moment region. A wooden block was placed to fit tightly between the steel flanges at the location of the point load to prevent web crippling. A unidirectional CFRP sheet of width 76 mm and thickness of 1.27 mm was bonded to the bottom flange of the steel beam using one, three, and five layers for the three strengthened beams, respectively. The sheet had a tensile strength of 2137 MPa and a tensile modulus of 144 GPa, which were obtained from the coupon testing. A pair of 3950 mm long CFRP sheets was placed side to side for the beam strengthened using one layer. For the beam strengthened using three layers, three pairs of carbon sheets were cut to 3950 mm, 3650 mm, and 3350 mm long and placed side by side with 150 mm staggers. For the third beam strengthened using five layers, the sheets were cut to the lengths of 3950 mm, 3800 mm, 3650 mm, 3500 mm and 3350 mm. The strengthening technique used increased the ultimate capacity by 44 %, 51 %, and 76 % for one, three, and five layer retrofitting systems. The effect of strengthening on the stress level at the elastic range and the elastic stiffness was insignificant. A simple analytical model was used in the analysis of the behaviour of the beam and then compared to the AASHTO ultimate strength design method. The analytical model had good correlation with the experimental results. Shaat et al., (2004) plotted the test results of the two previous studies to emphasize the effect of the CFRP reinforcement ratio on the increase in the flexural strength for beams with different yield strengths. The strengthening ratio was considered as the ratio between the total area of the CFRP and the steel section. The figures showed an increase in the flexural strength by increasing the reinforcement ratio. Moreover, the flexural strength increase was higher for steel with lower yield strength. The effectiveness of the CFRP strengthening has reduced for thicker CFRP laminates as the failure was governed by debonding rather than by rupture. APFIS

3 Al-Saidy et al., (2004) investigated the behaviour of steel-concrete composite beams damaged intentionally at their tension flange to simulate corrosion and then repaired. Strengthening of these beams was accomplished by adhesively bonding CFRP plates to their tension area side. A total of six steel-concrete composite beams were tested in this study. The steel beams were W8 15 (W200 22) grade A572 structural steel acting compositely with a 76 mm thick by 812 mm wide reinforced concrete slab. The experimental program consisted of two undamaged (control) girders and four girders that were damaged by removing part of their bottom flange (i.e. a percentage of the bottom flange area) to simulate field corrosion, and then repaired by adding the CFRP plate to restore the composite beams to their original strength. One of these damaged beams was done by removing 50 % of its bottom flange and no repair was installed. In the repaired beams, the damaged percentages were 50 % and 75 %. These were repaired by adding CFRP plates to the web or by adding CFRP plates to the web and to the bottom flange. One damaged beam with 75 % of its bottom flange removed was repaired using CFRP plates bonded to the web and at the bottom of the tension flange. Also, in this study, longitudinal strains as well as shear stresses along the CFRP plate of all tested beams were measured by utilizing extensive instrumentation along CFRP plate. The experimental results showed that the damage to the bottom flange affected both the flexural stiffness and the strength of the beam. The results indicated that the elastic flexural stiffness of damaged beams can be restored (up to 50 %) with the use of CFRP plates for the system investigated. In addition, the ultimate strength of the original beam (without damage) was fully restored. A theoretical nonlinear analysis of the repaired members was conducted to provide a tool for design and analysis. The analytical procedure presented in the study was based on dividing the section into layers and segments throughout the section to determine the moment curvature and load deflection relationships. The results of the experimental work indicated that there is a good agreement with the predicted results of the analytical method. Sayed-Ahmed (2004) investigated strengthening the web of the steel I-beam against web buckling. In-thin walled I-beams, the risk of web buckling is high, and for strengthening the tension flange, premature failure can occur due to web buckling. A nonlinear finite element analysis was conducted to predict the performance of strengthening the web of composite I-beams consisting of 229 mm by 20 mm flanges, and varying the ratio of the web height to the web thickness h w /t w for 49.8, 108, 148, and 296. Strengthening was performed through bonding 100 mm width by 1.4 mm thickness CFRP strips to the mid-height of the web from both sides. The web buckling load capacity increased by 22 %, 33 %, and 61 % for h w /t w = 108, 148, and 296, respectively. The flexural capacity increased also by 2 %, 3 %, and 9 %, respectively. Patnaik and Bauer (2004) used two different groups of build-up steel I-beams in their investigation. The first group consisted of three beams all designed to fail in flexure, with the web consisted of a mm plate and a mm plate for the flanges. The beams had a total length of 3550 mm. Two out of the three beams were strengthened using CFRP plates attached to the tension flange to cover the whole width. As the plates used had only 51 mm width, they were assembled first to the required width before being bonded. The CFRP plate had a tensile strength and tensile modulus of 2790 MPa and GPa, respectively. The beam left in this group was used as control beam. For the second group which also consisted of three beams designed to fail in shear, the web plate was mm, and the flanges were mm. The beams were strengthened by completely covering each side of the web with one layer of CFRP plate with the full length of the beam. Four point bending tests were performed with a span of 3150 mm, bearing stiffeners at loading and support points to prevent local buckling. For the first group, the flexural strength increase was 14.1 % and 14.5 %. For the second group, the shear strength increase was 26 % for one specimen and was negligible in the other one as a result of the failure at the butt joint. Schnerch et al., (2005) strengthened 6400 mm long steel-concrete composite girders consisting of W12 30 (W310 45) A36 I-steel beams with a 840 mm by 100 mm concrete slab. Strengthening was performed using Intermediate Modulus CFRP plates with elasticity modulus of 440 GPa and High Modulus CFRP plates with elasticity modulus of 660 GPa. The reinforcing using CFRP plates was calculated as a ratio of the steel beam area. The reinforcing ratios were 4.5 % for intermediate strengthened beam and 7.6 % for the high modulus beam. Each girder was loaded up to 60 % of the specified yield stresses and then unloaded before strengthening. In order to decrease the peeling stresses at the location of discontinuity, the plates were wrapped around the flange using 330 mm CFRP sheets at the splice locations and the ends. Also the plate s ends were detailed using a 20 degree reverse bevel. The ultimate flexural capacity was reported to be increased by 16 % and 45 %, respectively, for each strengthening type specimen. Meanwhile, the flexural stiffness increased only by 10 % and 36 %. This was due to the high elasticity modulus of the CFRP plates. Lenwari et al., (2005) strengthened seven W beams using three different lengths of CFRP plates. Steel plates of 200 mm wide by 12.2 mm thick were welded at the upper flange of the steel beam in order to ensure no compressive yielding. A four point loading scheme was used to test the beams having a span of 1800 mm. The APFIS

4 CFRP plates were bonded to the bottom flange with lengths of 500 mm for two beams, 650 mm for two beams, and 1200 mm for three beams. The failure mode was by debonding for the beams with CFRP plate with length 5/18 and 1/3 of the beam span. For the beams strengthened with CFRP plate with length 2/3 of the beam span, the failure was by rupture of the CFRP plate. The authors re-used the two beams tested with 500 mm CFRP plate by bonding 650 mm and 1200 mm CFRP plate. The effect of the pre-existing yielding before bonding the CFRP plate was found to have no significance for the results as found by comparing the non-damaged specimen with the already yielded specimen. For beams with adequate development length (1200 mm) such that the plate extends to the sufficiently low stress region, the increase in the strength was 61 % and 75 %, respectively. Colombi and Poggi (2006) used CFRP plates to strengthen traditional hot rolled HEA 140 shaped beams. Four identical steel beams were strengthened with one or two layers of CFRP plates using two different epoxy adhesives. The beams were 2500 mm long and the layers of the CFRP plates were made by a pair of parallel plates each of 2000 mm in length. For the specimen reinforced using 2 layers, two pairs of CFRP plates of length 2000 mm and 1800 mm, respectively, were bonded to the tension bottom flange. The beams were tested under three point loading. Two pairs of lateral bracing were provided to the remaining specimens to prevent the torsional buckling that took place while testing the first beam. This investigation focused on the evaluation of the local stresses at the laminate ends and the effect of using different adhesives and number of layers on both the stiffness and load carrying capacity. In this work, an analytical model using ABAQUS was developed to predict the behaviour of the strengthened beams. Standard two nodes beam elements were used to model the steel beam while standard eight nodes plane stress elements were used to model both the adhesive and the CFRP plates. The finite element mesh was refined at the ends of the laminate to capture the stress concentration at this zone. For the elastic stiffness of the strengthened beams, the effect was negligible for the beams reinforced using one layer of CFRP plates, and a 13.8 % increase was recorded for beams strengthened using two layers. The yield load increased by % and % for beams strengthened using one layer and two layers, respectively. The use of two different epoxy adhesives had no significance on the beam behaviour. The proposed model showed good agreement with the experimental results. Stratford and Cadei (2006) investigated the elastic analysis of adhesion stresses for the design of a cast iron beam strengthened using bonded CFRP plate. The beam span was 6000 mm strengthened with 4000 mm long CFRP plate having 11 mm thick and 356 mm width and an elastic modulus of 360 GPa. The bond stress analysis used assumed that the load transferred between the plate and the beam was a combination of shear stresses, which was parallel to the joint, and peeling stresses, which were normal to the joint. The first case of loading in this study was by applying an additional 40 kn/m compared to the beam without strengthening; the zones of influence of the shear stresses and the peeling stresses were found to be 200 mm and 75 mm, respectively from the support. Hence, the maximum stresses were reduced by extending the end of plate to be at 500 mm from the support instead of 1000 mm, which causes a 45 % reduction at the peak shear stresses. The second loading case had a uniform temperature increase of 30 ºC; the stresses caused by T= 30 ºC were three times larger than those from the applied load in the first loading case. The third loading case was by applying a clamping force of 10 kn at the plate ends. The clamping force resulted in only peeling stresses in the adhesive without reducing the shear stresses though it is more likely to increase the shear strength. Shaat and Fam (2006a) investigated the behaviour of artificially damaged steel-concrete composite beam strengthened using two types of CFRP sheets. Four beams were tested under four point bending. The beam consists of W hot rolled steel section with 465 mm width and 75mm thick concrete flange. The tension flange of three beams was notched at mid-span to simulated the fatigue cracks or section loss. One beam was strengthened usnig Type 1 UHM-CFRP sheets having a tensile strength and modulus of elasticity of 510 MPa and 230 GPa, respectively, while two beams were strengthened with Type 2 CFRP sheets having a tensile strength and modulus of elasticity of 1130 MPa and 107 GPa, respectively. Using 14 layers of Type 1 UHM- CFRP sheets recovered all the loss in strength and stiffness and even increased the strength and stiffness by 10 % and 25 %, respectively over the undamaged control beam, and the failure was by rupture of the high modulus CFRP sheets. However, using 5 layers of Type 2 CFRP sheets recovered only 87 % and 85 % of the original strength and stiffness for one beam and 50 % and 91 % in the other beam due to the premature debonding failure mode which could be due to the low modulus of elasticity of the Type 2 CFRP sheets. The low recovery level of original strength and stiffness in the last beam could also be due to the long time interval between the sand blasting and application of the CFRP sheets. Another test was conducted by the authors, in which 8 layers of Type 1 UHM-CFRP sheets were used to repair a damaged steel-concrete (Shaat and Fam 2006 b). Tests results showed that with 8 layers, although the original load carrying capacity was not recovered, the stiffness was recovered and exceeded by 12%. It was shown that the 8 layers were not sufficient to compensate for the loss of the entire steel flange as well to sustain the specimen until first yielding of the steel cross section, and failed by rupture of CFRP sheets at a load level of 54% less than the maximum load attained in the control beam. APFIS

5 Ragab and El-Hacha (2006a, and 2006b) and El-Hacha and Ragab (2006) tested six steel-concrete composite beams having a 6000 mm span. The girder s steel cross section was a standard W200x19.3 hot rolled I-beam with a concrete flange of 56 mm thick by 435 mm wide. One beam was used as unstrengthened control and the remaining five beams were strengthened using different strengthening materials including: Steel Reinforced Polymer (SRP) sheets, Intermediate Modulus Carbon Fiber Reinforced Polymer (IM CFRP) pultruded plate, unidirectional pre-impregnated Carbon Fiber Reinforced Polymer (CFRP) sheets, High Modulus Carbon Fiber Reinforced Polymer (HM CFRP) pultruded plate, and Ultra High Modulus Carbon Fiber Reinforced Polymer (UHM CFRP) pultruded plate. The FRP reinforcement ratio was 8.1 %, 3.87 %, 21.4 %, 5.64 %, and 6.45 %, for the SRP sheet, IM CFRP plate, CFRP sheet, HM CFRP plate, and UHM CFRP plate, respectively based on properties reported by the manufacturer. The FRP reinforcement ratio was calculated as the ratio between the applied FRP area and the area of the steel section. The increase in the yield load for the strengthened beams with respect to the unstrengthened control beam was 22.5 %, 29.0 %, 21.3 %, 52.4 %, and 70.8 %, respectively while the increase in the ultimate load for the strengthened beams with respect to the unstrengthened control beam was 31.0 %, 37.4 %, 24.3 %, 48.7 %, and 42.9 %, for the SRP sheet, IM CFRP plate, CFRP sheet, HM CFRP plate, and UHM CFRP plate, respectively. The experimental results were compared to the numerical results obtained from modeling the beams using the finite element software ANSYS (Ragab 2007). The flexural behaviour had good correlation with the experimental results. Ragab and El-Hacha (2007) conducted another experimental investigation on strengthening 2400 mm long steelconcrete composite girders for horizontal shear. Six girders were tested. Two beams were used as unstrengthened control and two beams were strengthened in flexure only using SRP sheets for one beam and IM CFRP plate for the other beam. Because of the relatively small span of the beams, stud shear failures were observed close to supports; therefore the last two beams were strengthened for shear and flexure. Strengthening system for horizontal shear was applied by adding additional CFRP sheets bonded to the lower face of the upper flange of the steel section and the lower face of the concrete slab. This new strengthening system for horizontal shear proved efficient to overcome catastrophic stud failure. The failure in case of beams strengthened for horizontal shear plus flexure was initiated by concrete crushing. Consecutively, debonding of CFRP sheets started near the supports. For the CFRP sheets bonded to concrete, delamination occurred with a thin layer of concrete attached to the CFRP sheets; this proves the efficiency of the epoxy used in bonding concrete and CFRP sheets. PRESTRESSED FRPs FOR STRENGTHENING STEEL-CONCRETE COMPOSITE GIRDERS There are few researchers concerned about strengthening steel concrete composite beams using prestressed FRP. Miyamoto et al., (1994) developed an analytical model for prestressed steel-concrete composite beams with FRP cables. Using this model, a parametric study was conducted to emphasize the effect of the most important parameters influencing the performance of the prestressed girders. The parameters studied depended on the purpose of usage whether it is an existing structure that needs strengthening or a new one. For existing structures, the parameters were cable arrangement in which the most important is the cable eccentricity, cable crosssectional area, material properties of the cables, prestressing force, and slab thickness increase. For new structures, more parameters were considered such as the steel cross-section, slab thickness, and material properties of the cross-section. The objective of the study was to improve yield load, ultimate load, and flexural rigidity. In the analytical modeling, four points loading and trapezoidal FRP cable profile with zero eccentricity at zero bending at the girder support was chosen. Providing eccentricity outside the steel section at the mid-span showed the best results in increasing yield load, flexural rigidity and ultimate load. If clear height below the bridge is limited, the eccentricity may not be allowed to increase outside the steel section. Yield load of the prestressed composite girder increased linearly with the FRP cable area provided and the ratio E t /E s, where E t is the elastic modulus of the FRP cable, and E s is the elastic modulus of the steel section. For all values of E t /E s, and with small values of FRP cable area, there was a negligible increase in the ultimate load due to rupture of FRP cable before failure of the section. On the other hand, there was a noticeable increase in ultimate load at higher FRP cable areas, and also, by ranging E t /E s from 0.35 to 1.3, the improvement was about 1.14 times the ultimate load for the same area of FRP cable. That means that the effect of increasing E t /E s on the ultimate load was not as pronounced as for yield load. As for flexural rigidity, increasing E t /E s from 0.35 to 1.3 boosted the increase in flexural rigidity from 1.02 to 1.2 for the same cross-sectional area. Schnerch (2005b) investigated the serviceability behaviour of three scaled steel-concrete composite beams that are typical of bridge structures strengthened with high modulus CFRP strip. Different parameters were studied, the first parameter was the modulus of elasticity of the CFRP strip, the second parameter was the cross sectional area of the longitudinal CFRP strip used, and the third parameter was if the CFRP strip r was adhesively bonded without prestressing or if it was prestressed first. The process of prestressing was done by bonding the CFRP strips to a special steel tabs and left for cure for seven days. After preparation of steel surface of steel beam APFIS

6 bottom flange, the adhesive was applied to the CFRP strips and additional adhesive was applied directly to the steel surface in contact with the CFRP strips. Prior to bonding process, holes were made in the bottom steel flange to bolt the steel tab at the fixed end and to bolt the fixture used to apply the jacking force on the other end. The jacking force was applied by means of tighten threaded rods that was positioned in the same level of the CFRP strips to avoid twisting of the tabs. The force was monitored by strain gauge placed on CFRP strips at mid-span. After reaching the required strain which was 0.06 percent, the CFRP strips was bonded to the steel beam using the same procedure used in the non-prestressed CFRP strips. The beams were subjected to three loading cyclic. The first cycle was up to 60 percent the yield stress of steel beam before strengthening, the second cycle was up to 60 percent the yield stress of steel beam after strengthening and the third cycle was up to ultimate load. The author concluded that prestressing CFRP strips makes an economical use of material and increases the stiffness of the beam under service load while maintaining the original ductility of the section. Wipf et al., (2003) documented the techniques used for strengthening two three span steel girder bridges in Guthrie County, Iowa, on state highway IA 141 approximately 1.6 miles west of Bayard, Iowa using CFRP rods. The bridges were strengthened in an effort to improve their live load carrying capacity. The first bridge was strengthened by installing non-prestressed CFRP plates to the soffit of the bottom flange and the second bridge was strengthen using post-tension CFRP rods. In both bridges the strengthening material was placed in the positive moment region of the beam. For the second bridge, the anchorage was bolted to the web of the beam using high strength bolt. The CFRP rods were placed in position between each pair of anchorage brackets on both sides of the beam web. The nominal prestressing force applied to all rods (four rods per side) was 53.3 kn. The bridge was instrumented to measure the strain at certain location. The authors concluded that, prestressing does not reduce live load deflections, it increases the live load carrying capacity of the bridge by generating strain opposite to those produced by dead load and, thus, it was determined that approximately 5 to 10 percent of live load moment was reduced by the prestressing moment which allows the bridge to carry additional traffic live load. The average loss in the prestressing force after two years was found to be 11.6 kn. RESEARCH IN PROGRESS Currently at the University of Calgary under the supervision of Dr. El-Hacha a comprehensive research study is ongoing to investigate analytically and experimentally the efficiency of using externally bonded prestressed various FRP materials to strengthen 6000 mm long steel-concrete composite girders. The girder s steel cross section was a standard W hot rolled I-beam with a reinforced concrete flange of 56 mm thick by 435 mm wide. Different parameters were investigated, including the type of FRP used and the induced prestressing force level. Both, Reinforced Polymer (SRP) sheets and Intermediate Modulus Carbon Fiber Reinforced Polymer (IM CFRP) pultruded plates were prestressed to 15%, 30% and 45% of their ultimate tensile strength. The complete flexural behaviour, the behaviour in the elastic and plastic zones, ductility, ultimate strength and modes of failure are also studied. An innovating, practical, mechanical system for anchoring and prestressing the FRP against the girder itself has been developed and design guidelines to predict the ultimate load of the strengthened composite steel girder after strengthening are being proposed. So far, four steel-concrete composite beams were tested. Three beams were strengthened with Steel Reinforced Polymer (SRP) sheets with prestressing force equal to 15%, 30% and 45 % of the ultimate tensile strength of the SRP sheets, and one beam was strengthened using CFRP plate with a prestressing force equal to 15% of the ultimate tensile strength of the CFRP plate. Preliminary test results showed that prestressing externally bonded SRP sheets enhanced the flexural behaviour of the strengthened beams especially in the elastic region, and improved their serviceability behaviour although the ultimate load carrying capacity was not affected compared to similar beam strengthened with non-prestressed SRP sheets tested by Ragab (2007). CONCLUSIONS The following conclusions can be drawn from the literature review presented in this paper on strengthening steel-concrete composite girders with non-prestressed and prestressed FRP: FRP proved to be effective in strengthening steel-concrete composite section due its light weight, easy handle, and corrosion resistance. It can be used to increase the stiffness, the serviceability behaviour and the flexural strength of the beam. Non-prestressed FRPs increase the ultimate loads and decrease deflections, although they can support only additional live loads applied to a structure, and are unable to carry the dead load. Prestressed FRPs can carry both a portion of the dead load and the additional live load carried by the structure, and improve the serviceability of the members. APFIS

7 By prestressing the FRP, the materials can be used more efficiently, the strengthened member becomes stronger, the yielding load is significantly higher than for members strengthened with non-prestressed, and the ultimate carrying capacity can be further increased. Prestressing FRP increases the stiffness under service loading while maintaining the ductility of the beam. In case of using high modulus FRP, prestressing reduces the overloading damage due to overloading condition. Strengthening using prestressed FRP laminates is very attractive, because it combines the advantage of using non-corrosive and light-weight materials with the high efficiency offered by external prestressing. REFERENCES Al-Saidy, A.H., Klaiber, F.W., and Wipf, T.J. (2004). Repair of steel composite beams with carbon fiberreinforced polymer plates, Journal of Composites for Construction, ASCE, 8(2), Colombi, P., and Poggi C. (2006). An experimental, analytical and numerical study of the static behaviour of steel beams reinforced by pultruded CFRP strips, Composites Part B: Engineering, Elsevier publisher, UK, 37(1), El-Hacha, R., and Ragab, N., (2006). Flexural strengthening of composite steel-concrete Girders using Advanced Composite Materials, Proceedings of the 3 rd International Conference on FRP composites in Civil Engineering (CICE 2006), December 13-15, Miami, Florida, US, Hollaway, L.C. and Cadei, J. (2002), Progress in the technique of upgrading metallic structures with advanced polymer composites, Progress in Structural Engineering and Materials, 4(2), Lenwari, A., Thepchartit., and Albrecht, P. (2005). Flexural response of steel beams strengthened with partiallength CFRP plates, Journal of Composites for Construction, ASCE, 9(4), July-August, Mertz, D., and Gillespie, J. (1996). Rehabilitation of steel bridge girders through the application of advanced composite material, NCHRP 93-ID11, Transportation Research Board, Washington, D.C., 20p. Miyamoto, A., Inoshita, Y., Mori, T., Yasuda, K. (1994). Mechanical behavior and design concept of prestressed composite girder with FRP tendons Fiber Reinforced Structures, Developments in Short and Medium Span Bridge Engineering, Patnaik, A.K., and Bauer, C.L. (2004). Strengthening of steel beams with carbon FRP laminates, Proceeding of the 4 th International conference Advanced Composite Materials in Bridges and Structures, Calgary, Alberta, Canada, July 20-13, 2004, (CD-Rom 8p). Ragab, N. (2007). Strengthening of steel-concrete composite girders using various advanced composite materials, MSc Dissertation, University of Calgary, Calgary, Canada. Ragab, N., and El-Hacha, R., (2006a). Flexural Strengthening of Steel-Concrete Composite Girders, Proceedings of the 1 st International Structural Specialty Conference (ISSC-1), CSCE Annual Conference, Calgary, Alberta, Canada, May 23-26, 2006, (CD Rom, Paper ST-096), 10p. Ragab, N., and El-Hacha, R., (2006b). Effectiveness of Various Repair Systems for Flexural Strengthening of Steel-Concrete Composite Girders, Proceedings of the 7 th International Conference on Short and Medium Span Bridges (SMSB-VII), August 23-25, 2006, Montreal, Quebec, Canada, (CD Rom, Paper RR-107), 11p. Ragab, N., and El-Hacha, R., (2007) FRP Shear Strengthening of Steel-Concrete Composite Girders, Proceedings of 8 th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures (FRPRCS-8), Patras, Greece, July 16-18, 2007 (CD-Rom 8p). Sayed-Ahmed, E.Y. (2004). Strengthening of thin walled steel I-section beams using CFRP strips, Proceeding of the 4 th International conference Advanced Composite Materials in Bridges and Structures, Calgary, Alberta, Canada, July 20-13, 2004, (CD-Rom 8p). Schnerch, D. (2005). Strengthening of steel structures with high modulus Carbon Fiber Reinforced Polymer (CFRP) materials, Ph.D. Dissertation, North Carolina State University, Raleigh, North Carolina, US, 281p. Schnerch, D.,Dawood, M., Sumner, E.A., and Rizkalla, S. (2005). Behaviour of Steel Concrete composite beams Strengthened with Unstressed and Prestressed High-Modulus CFRP Strips Proceedings of the Fourth Middle East Symposium on Structural Composites for Infrastructure Applications (MESC4), Alexandria, Egypt, May Sen, R., Liby, L.,and Mullins, G. (2001) Strengthening steel bridge sections using CFRP laminates, Composites Part B: Engineering, Elsevier publisher, UK, 32(4), Shaat, A., Schnerch, D., Fam, A., and Rizkalla, S. (2004), Retrofit of steel structures using Fiber-Reinforced Polymers (FRP): State-of-the-art. Transportation Research Board (TRB) Annual Meeting, Washington, CD-ROM ( ). Shaat, A. and Fam, A. (2006a) Effectiveness of Different Composite Materials for Repair of Steel Bridge Girders. Proceedings of the 3 rd International Conference on FRP composites in Civil Engineering (CICE 2006). December 13-15, 2006, Miami, Florida, USA, (CD-ROM, pp ). APFIS

8 Shaat, A. and Fam, A. (2006b) Rehabilitation of Damaged Steel-Concrete Composite Beams using High Modulus CFRP Sheets. Proceedings of the 7 th International Conference on Short and Medium Span Bridges (SMSB-VII). August 23-25, 2006, Montreal, Quebec, Canada. CD-ROM paper# RR p. Stratford, T., and Cadei, J. (2006). Elastic analysis of adhesion stresses for the design of a strengthening plate bonded to a beam, Construction and Building Materials, Elsevier publisher Ltd, UK, 20(1-2), Tavakkolizadeh, M., and Saadatmanesh, H. (2003). Strengthening of steel-concrete composite girders using carbon fiber reinforced polymers sheets, Journal of Structural Engineering, ASTM, 129(1), Wipf, T.J., Phares, B.M., Klaiber, F.W., and Si Lee Y., (2003). Evaluation of Post-tension Strengthened Steel Girder Bridge Using FRP Bars, CTRE Project 01-99, Center for Transportation Research and Education, Iowa State University, 2901 South Loop Drive, Suite 3100, Ames, IA Zhao, X.L. and Zhang, L. (2007). State of the Art Review on FRP Strengthened Steel Structures, Engineering Structures, 29(8), APFIS