Hybrid FRP-Concrete Structural Member: Research and Development in Europe and Asia

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1 CICE The 5th International Conference on FRP Composites in Civil Engineering September 27-29, 2010 Beijing, China Hybrid FRP-Concrete Structural Member: Research and Development in Europe and Asia Donna Chen & Raafat El-Hacha (relhacha@ucalgary.ca) Department of Civil Engineering, University of Calgary, Calgary, Alberta, Canada ABSTRACT: Approximately 40% of Canadian bridges currently in service were constructed forty to fifty years ago. Many existing structures suffer from structural integrity problems, such as corrosion, and require significant repairs. In some cases, complete replacement of the structure is necessary. Fibre reinforced polymers (FRPs) have shown great potential as a structural material due to its high strength, resistance to corrosion, light weight and ease of construction. Hybrid FRP-concrete structural members have become of particular interest. This paper will investigate the different applications of hybrid FRP-concrete structural members and the experimental research studies performed on these members in Europe and Asia. Summaries of results from experimental tests and analytical investigations are provided with a focus on the effect of factors such as the type of shear bond used at the FRP-concrete interface and FRP fabrication methods on the ultimate load resistance and failure mode. 1 INTRODUCTION Reinforced concrete is the most commonly used building material today; however, despite its widespread use, frequent problems occur. Over the duration of a typical reinforced concrete structure s lifespan, maintenance, repairs and sometimes rehabilitation would be required due to corrosion of the steel reinforcement bars within the concrete. In Canada, the municipal infrastructure deficit, as calculated in 2007, is approximately $124 billion dollars, with $44.5 billion dollars directly related to the areas of transportation and transit (Mirza, 2007). In addition, it is estimated that $115 billion dollars will be required to accommodate the changing needs of new and growing communities through the construction of new infrastructure or the expansion of existing ones (Mirza, 2007). Research conducted within the past 20 years investigating the performance of Fibre Reinforced Polymers (FRPs) in Civil Engineering applications has shown that FRP materials have great potential for use in structures due to its high strength, resistance to corrosion, light weight and ease of construction. More recently, studies have been performed to investigate the replacement of reinforced concrete members with hybrid FRP-concrete members in structures, leading to the construction of several structures using the new hybrid system. This paper will present experimental details of current developments in the research of hybrid FRP-concrete structural members. A brief overall summary of the findings will also be provided at the end. 2 CURRENT RESEARCH AND DEVELOPMENT 2.1 FRP-Concrete Composite Deck - Korea Institute of Construction Technology, Korea Kim et al. (2005) tested a unique modular hybrid FRP-concrete deck system. To address failure in earlier models at the interface between adjacent modules, individual modules were fabricated by pultrusion, with male-female socket-shapes present at the extremities. Shear connecting plates perpendicular to the upper face of the module were produced monolithically with the FRP module in lieu of bonding using epoxy at a later stage to prevent failure at the bond interface. Though it was suspected that coarse sand coating alone would provide sufficient bond strength, the development of bond between the concrete and the FRP was achieved through the use of both the shear connecting plates as well as the coarse sand coating. Steel reinforcement bars, provided in the concrete deck, were supported either on top of the shear connection plates or through prefabricated holes through the plates. Figure 1 shows the section details.

2 Figure 1 - Section detail of modular hybrid FRP-concrete deck system (Kim et al., 2005) Disregarding the small degree of slip that took place between adjacent modules, it was assumed for analysis that a linear strain distribution was present through the deck section. The section was designed to initially act as a perfect composite section until failure of the concrete under fatigue; after concrete failure, the section was expected to act solely as a FRP section. At failure of the FRP section, the safety factor was designed to exceed 2, thus preventing brittle failure. Shear strength tests were performed to determine the effect of the shear connecting plates and the coarse sand coating. In the case where both systems were used, the failure shear strength in the transverse and longitudinal directions was 4.47 and 4.44 times the strength when only the coarse sand coating was used. Static tests were performed on a 2.5m span hybrid deck section. Punching failure occurred in the concrete. After punching failure, the load resistance of the deck reduces incrementally as the deflection of the deck section increases. Final failure of the section occurred at the connection points between FRP modules. Fatigue tests performed showed that the integrity and load resistance of the deck system could be maintained up to 2 million cycles. Static load tests were performed every 50,000 cycles in order to ascertain the degree of damage sustained due to fatigue loading. 2.2 Hybrid FRP-Concrete-Steel Double-Skin Tubular Columns - The Hong Kong Polytechnic University, Hong Kong Yu et al., (2004) tested a new hybrid FRP-concretesteel double-skin tubular column, composed of an interior steel tube, exterior FRP tube and concrete in the space between the two tubes, for use in seismic applications. The steel tube acted as longitudinal reinforcement; the FRP tube, prepared using the wet lay-up process with its fibres oriented in the circumferential direction, confined the concrete. Six specimens, with varying FRP tube thicknesses, were tested under concentric axial loading. For all specimens, failure occurred due to FRP rupture, after which the load resistance of the column dropped dramatically. Evaluation of the performance of the hybrid structural member and the effect of FRP confinement compared the ultimate resistance achieved through experimental testing with the expected individual axial capacities of the concrete and steel tube assuming no interaction occurred between the two components. It was found that application of one layer of FRP did not increase the axial resistance of the member. Two layers and three layers of applied FRP tube resulted in 27% and 48% increases in load capacities, respectively. The behaviour of the hybrid member approximated elastic-near plastic behaviour with higher ductility achieved when compared with unconfined concrete columns. Axial strains measured at failure for one-, two-, and threeply FRP tubes were 5.53, 7.69, and 8.96 times that of the unconfined concrete members. Flexural testing was performed on six specimens, with varying concrete strength, steel tube thickness and FRP tube thickness. Due to the limited amount of headroom clearance available in the loading frame used, testing was only performed to an approximate mid-span deflection of 150mm and not to failure. This is shown in more detail in Figure 2. Figure 2 - Failure of hybrid FRP-concrete-steel column (Yu et al., 2007) For pure bending applications, it was advised that additional strengthening and modifications to the hybrid member be provided. The thickness of the FRP tube used, based on the number of plies applied, did not greatly affect the load capacity of the member; however, the thickness of the inner steel tube did exert a significant influence. All the specimens showed a high degree of ductility. Due to the fact that none of the specimens were loaded to failure, it was concluded that greater ductility could be expected from the hybrid members. 2.3 Laboratory and Field Performance of Cellular Fiber-Reinforced Polymer Composite Bridge Section Swiss Federal Institute of Technology and Other Institutions Zhou et al., (2005) researched the development of composite bridge deck systems made from off-the shelf pultruded FRP sections. FRP structural square tubes were placed side-by-side spanning in the transverse direction of the bridge and linked together using adhesives and steel rods. FRP plates were placed on both the top and bottom of the assembled

3 structure after which all the components were bonded using the vacuum bagging process. Four phases of the bridge deck system were developed, using different combinations of properties such as material quality, dimensions, adhesive type and level of environmental exposure. The completed deck section was then supported by three steel wide flange beams, spaced evenly apart about the center, leaving the section edges unsupported to create the worse load case scenario. During laboratory testing, loading patches were placed at designated locations so that loads designed to simulate the truck axle loads could be applied in different combinations and orientations to obtain the greatest load effect. Field testing of the bridge deck was put into operation at an environmentally exposed weigh station, which is expected to under approximately 0.5 million loading cycles at month. It was discovered that the cutout areas intended for insertion of the steel rods attracted higher concentrations of stress. Where the holes were placed in the web, loss of bearing resistance resulted, leading to shear cracks when more than many rods are located in close proximity to each other. To minimize cost and maintain the structural integrity of the section, it was suggested that only three rods, the minimum amount required to prevent movement of the FRP square tubes during curing, be provided. Additionally, when tested under the same applied load, larger deflection and strains were detected from the unsupported edges as compared to the centre of the deck. In real application, it was advised to avoid leaving the edges of the cellular FRP bridge deck section without support unless absolutely necessary; where unavoidable, supplementary transverse support along the edges must be provided. Through both laboratory and field testing, the cellular FRP bridge deck system has proven to be resilient to the effects of fatigue loading and did not display any signs of lowered strength or stiffness after 7 million cycles. As a result of the experimental data, Zhou et al., (2005) believed that the use of the FRP deck design for practical applications in highway bridges could a feasible possibility. 2.4 Innovative Design of FRP Combined with Concrete: Short and Long Term Behaviour Deskovic and Triantafillou (1995a, 1995b) designed and investigated the short and long term performance of composite flexural structural members to optimize the combined the use of concrete, GFRP and CFRP based on their individual material properties. GFRP pultruded shapes generally perform well as a structural beam but do not possess high compressive strengths or stiffness and are vulnerable to sudden failure. For the hybrid section tested, concrete was cast above the GFRP box section, which also acted as a stay-in-place formwork. CFRP sheet, with a failure strain smaller than that of the GFRP, was applied at the base of the beam to provide additional tensile reinforcement. The CFRP was designed to be the first element of the composite section to fail, allowing for the beam to fail in a pseudo-ductile manner by giving advanced warning of complete section failure. A cross-sectional view of the hybrid member is shown in Figure 3. Figure 3 - Configuration of hybrid beam using concrete, GFRP box section and CFRP sheets (Deskovic andtriantafillou, 1995a) Three hybrid FRP-concrete beams were tested for short term flexural behaviour. The first specimen failed due to debonding of the concrete and GFRP; as a result the remaining two specimens were fabricated with steel bolts to act as shear connectors, which then both failed in the intended pseudoductile manner. It was found that experimentation and finite element analysis provided very similar results. Investigation into long term behaviour, such as creep and shrinkage, of the hybrid design was conducted by testing four specimens. Two beams were first tested under one-day creep with the application of constant load. After the initial full day of sustained loading, the beams were then tested under alternating loading at a rate of 4.2Hz. The loads which were applied to the first two beams were different during both stages of testing. The latter two specimens were used to tested environmental effects, where one was tested indoors under constant humidity and temperature while the other was placed outdoors and subjected to weathering. At the time when the paper was published, six months of data was available for examination. From the creep and fatigue test results, it was found that under lower loads, deflection was governed primarily by the fatigue loading rather than the initial sustained load whereas higher applied loads reflect a greater importance on the initial load on the final deflection and response of the hybrid member. This phenomenon was attributed to the loss of stiffness over time as a result of sustained and cyclic loading, which becomes more apparent as the level of load applied increases. In addition, it was concluded that temperature variations between -5 C to 35 C resulted in

4 negligible effects on the behaviour of the hybrid members. Two analytical models were used for comparison, based on the laminate theory as well as semiempirical. Overall, the semi-empirical proved to be the more accurate of the two methods although at lower loads, the predictions made through the laminate theory were more exact. Both methods showed satisfactory correlation with actual experimental da- ta collected. 2.5 Innovative Externally Bonded FRP-Concrete Hybrid Flexural Members Ibaraki University, Japan and Tongji University, China Wu et al., (2005) experimented with a new hybrid FRP-concrete beam design. he cross-section consisted of a concrete beam, prepared with rounded edges on all four corners, reinforced longitudinally with CFRP sheets at the bottom and then encased in the hoop direction by GFRP/CFRP sheets for the purpose of providing supplementary anchorage to the CFRP sheets as well as concrete confinement. Six specimens were tested, where Type A beams were fabricated with no steel reinforcement bars and Type B beams included four steel bars; neither categories included the use of steel stirrups. The effect of CFRP reinforcement ratio and the number and type of FRP layers used in the hoop direction were tested. It was found through experimentation that the stiffness of the hybrid beam in the stage after initial cracking and prior to debonding of the FRP sheets is proportionally related to the CFRP reinforcement ratio. By increasing the CFRP reinforcement ratio, the failure mode of the hybrid beam transitions from a tensile to compressive failure. Without the presence of steel reinforcement bars in the Type A specimens, a sudden and catastrophic style of failure occurred. Due to these findings, for safety reasons, steel reinforcement bars equal to the minimum reinforcement ratio should be provided to control the development and growth of cracks. Overall, of the six specimens tested, the one that showed the greatest flexural strength contained the minimum required steel reinforcement ratio with equal application of both GFRP and CFRP sheets in the hoop direction. When compared to a reinforced concrete with similar dimensions and a reinforcement ratio of 2.5%, the specimen mentioned above outperformed by 60% in regards to flexural strength. Comparison between analytical models, which characterized the concrete either as confined or unconfined, showed that confinement as a result of FRP sheets applied in the hoop direction does occur. Whereas the unconfined model predicted compression failure and a significantly lower flexural strength that that exhibited through experimentation, the confined concrete model correctly predicted a balanced failure mode with concrete crushing and CFRP rupturing almost simultaneously. 2.6 Composite GFRP Com posite Decks School of the Civil and Environmental Engineering, Kookmin University, Korea Lee and Hong (2009) developed modular composite GFRP deck cross-sections with different connections details. In the first stage, decks using tongueand-groove connections were tested. The deck profile consists of three adjacent trapezoidal cells, fabricated by pultrusion, with 8800 Tex E-glass roving in the longitudinal direction and stitched fabric woven at 90 and 45 degrees to the primary fibres. The resin used was unsaturated polyester. The deck de- sign with its connection detail is shown in Figure 4. Figure 4 GFRP composite deck with tongue-and-groove connection (Lee and Hong, 2009) Through testing, it was discovered that the above design problems during application. Firstly, due to the fact that the modules are required to be assembled by horizontal sliding adjacent deck sections together, shear studs cannot be inserted until after all of the decks are placed in position. This causes a high potential for mismatch and misalignment between the pre-drilled shear stud holes, making it difficult for assembly. Additionally, the design requires for the shear studs to be welded on-site to either steel girders or the steel plates, which are attached to concrete girders. The limited workspaces as well as the size of the shear stud holes make welding quite difficult to performing. As a result, a new type of composite GFRP deck was created with snap-fit connections. The crosssection is shown in Figure 5. Figure 5 GFRP composite deck with snap-fit connection (Lee and Hong, 2009) In this design, adjacent deck modules are connected vertically, which greatly increases workability while allowing for shear studs to be inserted with better accuracy since vertical assemblage reduces

5 the amount of horizontal discrepancy between the pre-drilled holes for shear studs. A diagram showing the module assembly is given in Figure 6. Figure 6 Vertical assembly of GFRP composite deck with snap-fit connection (Lee and Hong, 2009) With this connection design, it can also be used in curved bridge construction, which was not an option for the previous tongue-and-groove connection design. Additionally, the modules can be constructed with adhesives at the connection interface, allowing for ease in dissembling and reuse of the GFRP composite deck section if required, as in the case of temporary bridge constructions. The application of both the tongue-and-groove connection and snap-fit connection GFRP composite bridge deck sections have been seen in more than 20 traffic, pedestrian and walkway expansions bridges located in Korea. In the future, more bridges using this type of technology are planned for use in Tai- wan. 2.7 Use of Uni-Axial Pseudo-Ductile Hybrid FRP Sheet to Strengthen Existing Reinforced Concrete Flexural Members HanKyong National University, Korea Choi et al., (2009) investigated methods of strengthening existing reinforced concrete structures using hybrid FRP sheets using varying volumetric ratios between E-glass fibres (GF) and high-strength carbon fibres (CF). Using previous research performed by Manders et al. (1981) relating the optimum ratio between GF and CF to allow for a pseudo-ductile failure where the CF are designed to fail first, leaving the remaining GF to carry the applied load, the GF:CF ratio used to manufacture the hybrid FRP sheets in this research was The load-elongation behaviour of this type of hybrid FRP sheet is shown in Figure 7. Figure 7 Load-elongation behaviour of the hybrid GF/CF sheet (Choi et al., 2009) Five reinforced concrete beams were prepared for flexural testing, with one beam left unreinforced as a control, three beams reinforced with 1-, 2- and 3- plies of the hybrid sheet and the last beam reinforced with only CF tensile sheets. To avoid premature debonding failure during loading tests, special design considerations were put into a new anchorage me- chanism for the sheets. The loading setup for the beam is shown in Figure 8. Figure 8 Load setup and section details of reinforced concrete beams strengthening with the hybrid sheet (Choi et al., 2009) From tests, the results showed that applying one ply of the hybrid sheet showed the best outcome in regards to improved strength and ductility while taking into consideration economic feasibility. It was found that though both the structural behaviour of the beam strengthened with one ply of the hybrid sheet performed similarly to that strengthened with CF only up until failure, the latter loses strength abruptly past the peak, resulting in the beam to exhibit the same load-strain behaviour of the control beam. Additionally, the amount of carbon fibres used to achieve the same load resistance up to failure in the two strengthening configuration is approximately 5 times greater in the CF strengthened reinforced beam, significantly increasing the cost of construction in practical applications. When the number of hybrid sheets is increased, it results in higher peak load resistance, equal to 88.9kN and 96.5kN from 79.6kN for the reinforced concrete beam, for the 1- ply and 2-ply strengthened beams, respectively. It also results in less ductility. In the case of the reinforced concrete beams reinforced with three sheets of the hybrid sheet, collapse was caused due to anchorage failure and debonding despite the additional considerations made in the design of the special anchorage mechanism. 3 CONCLUSIONS In this literature review, it was shown that FRP material can fulfill the roles of a stay-in-place form-

6 work, non-corrosive reinforcement as well as concrete confinement in columns, simultaneously when used in hybrid structural members. When used in the hoop direction around columns, the confinement effect is achieved, resulting in an associated increase in axial load resistance; furthermore, the increase in ductility with every additional layer of FRP applied is incredibly substantial. In regards to flexural members, linear elastic failure due to the natural physical property of FRP material can be avoided in lieu of a pseudo-ductile failure mode by using hybrid FRP sheets manufactured from a combination of highstrength carbon fibres and E-glass fibres, with a recommended ratio of 1 to 8.8. It is also important that adequate bond strength is provided, particularly in flexural members, to prevent early failure due to debonding. Under long-term loading, there is a loss in stiffness in hybrid FRP-concrete structural members causing larger deflections to occur. This becomes more apparent as the magnitude of the applied load increases. Analytical models formed using the laminate theory as well as semi-empirically did show good correlation to experimental data, where the laminate theory provided more accurate results at lower loads. Wu, Z., Li, W., and Sakuma, N Innovative Externally Bonded FRP/Concrete Hybrid Flexural Members, Composite Structures, Vol. 72, No. 3, March 2006, pp Yu, T., Wong, Y.L., Teng, J.G., and Dong, S.L Structural Behaviour of Hybrid FRP-Concrete-Steel Double- Skin Tubular Columns. Proceeding from 2004 ANCER Annual Meeting,. Honolulu, Hawaii, July 28-30, 2004, (CD-Rom, 11p) Zhou, A., Coleman, J.T., Temeless, A.B., Lesko, J.J., and Cousins, T.E Laboratory and Field Performance of Cellular Fiber-Reinforced Polymer Composite Bridge Deck Systems. Journal of Composites for Construction, ASCE, September-October 2005, pp REFERENCES Choi, D., Ha, S., and Kim, K Use of Uni-Axial Pseudo- Ductile Hybrid FRP Sheet to Strengthen Existing Reinforced Concrete Flexural Members. Proceedings of the 2 nd International Conference of International Institute for FRP in Construction for Asia-Pacific Region. Seoul, Korea, December 9-11, 2009, pp Deskovic, N., Triantafillou, T. C., and Meier, U. 1995a. Innovative Design of FRP Combined with Concrete: Short- Term Behavior. Journal of Structural Engineering, 121(7), pp Deskovic, N., Meier, U., and Triantafillou, T. C. 1995b. Innovative Design of FRP Combined with Concrete: Long- Term Behavior. Journal of Structural Engineering, 121(7), pp Kim, B.-S., Cho, J.-R., Park, S. Y., and Cho, K Toward Hybrid Bridge Deck: An Innovative FRP-Concrete Composite Deck. Proceedings of International Joint Seminar of the KSCE and the JSCE: 2006 KSCE Annual Conference on Recent Progress of Concrete-Steel-FRP Hybrid Structures. Gwangju, Korea, October 13, 2006, pp Lee, S and Hong, K-J The Current and Future Applications of FRP Decks. Proceedings of the 2 nd International Conference of International Institute for FRP in Construction for Asia-Pacific Region. Seoul, Korea, December 9 11, 2009, p.p Mirza, S Danger Ahead: The Coming Collapse of Canada's Municipal Infrastructure. A report prepared for the Federation of Canadian Municipalities, Ottawa, Ontario, November 2007.