Use of CFRP for Strengthening Steel Structures & Bridges

Transcription

1 Use of CFRP for Strengthening Steel Structures & Bridges S. Rizkalla 1 and M. Dawood 2 1 Distinguished Professor of Civil Engineering and Construction, 2 Ph.D. Candidate, Constructed Facilities Laboratory, Department of Civil, Construction and Environmental Engineering, North Carolina State University, 1 sami_rizkalla@ncsu.edu, 2 mmdawood@ncsu.edu Abstract This paper presents the development of a CFRP system for strengthening and repair of steel bridges and structures. The research program included development of the strengthening system, investigation of the behavior of strengthened bridges and special structures, and detailing requirements. Based on the research comprehensive guidelines including structural detailing are provided. This paper demonstrates that high modulus CFRP materials can be effectively used for strengthening steel structures and bridges. Keywords: steel bridges, steel towers, fatigue, overloading, bond, environmental durability, design guidelines Introduction A number of researchers have demonstrated that carbon fiber reinforced polymer (CFRP) materials can be used to strengthen steel flexural members 1,2. Most of the early research studies focused on the use of conventional CFRP materials with a modulus of elasticity typically less than that of steel. Thus, although the materials could effectively enhance the strength of the structure, large amounts of strengthening materials were required to improve the serviceability. This paper summarizes various aspects related to the development of a high modulus (HM) CFRP strengthening system for steel bridges and structures. This paper describes the various phases of the research program, which has been on-going for the last six years, including system development, member behavior, and detailing of a proposed HM CFRP system for strengthening steel bridges and structures. Design guidelines are also proposed. This paper demonstrates that the proposed CFRP system can be effectively used to enhance the serviceability and ultimate strength of steel bridges and structures. Phase I: System Development The following sections describe the main stages of the system development including the basic characteristics of the CFRP materials, selection of the resin and adhesive for bonding the material to the steel surface, recommendations for the surface preparation and installation techniques. CFRP materials The proposed strengthening system consists of HM carbon fiber materials The carbon fiber materials included dry fiber tow sheets and precured CFRP plates. The fiber sheets are suitable for applications with complex geometrical configurations such as curved girders or monopole towers. These are typically impregnated with an epoxy resin in-situ which also acts to bond the fibers to the structure. The CFRP plates are suitable for applications requiring a higher level of strengthening. The relatively high modulus of the CFRP materials makes them well suited to enhance the serviceability of steel structures. The typical material properties of the carbon fibers and the CFRP plate are presented in Table 1.

2 Table 1: HM carbon fiber material properties Carbon Fiber 3 CFRP Plate * Elastic modulus, E 640 GPa 418,000 MPa Ultimate strength, f u 2600 MPa 1540 MPa Tensile rupture strain, ε u * measured according to ASTM D3039 Adhesive and resin selection The first phase of the system development focused on the selection of appropriate adhesives to bond the HM carbon fiber materials to steel surfaces. This included selection of a saturating resin to bond the dry fiber tow sheets and selection of an appropriate structural adhesive for bonding the pultruded CFRP plates 4. Ten different saturating resins were compared through a series of double-lap shear coupon tests to select a suitable resin for bonding carbon fiber tow sheets to steel surfaces. The poor performance of some of the resins was due to debonding of the CFRP from the steel surface. A second group of resins failed due to pull-out of the fibers from the resin which indicated possible incomplete wetting of the fibers. For the best performing resins, failure of the coupons was predominantly by rupture of the carbon fibers indicating complete utilization of the HM CFRP materials. An average shear stress of 12 MPa was measured for these resins prior to rupture of the fibers. Small-scale flexural tests were conducted to select an effective adhesive for bonding CFRP plates to steel beams. These tests were selected to represent typical bond stress distribution within the adhesive layer for beam applications. The beams were strengthened by bonding one or two layers of CFRP plates to the bottom of the tension flange. Various plate lengths were considered to determine the minimum bond length required to develop the full tension strength of the CFRP materials. The beams were loaded to failure in the four point bending configuration. A total of six different structural adhesives were evaluated using this test configuration. The SP Systems Spabond 345 two part epoxy adhesive was selected for the proposed HM CFRP strengthening system. Surface preparation Surface preparation of the base metal is fundamental in ensuring adequate bond and involves cleaning, followed by the removal of weak layers and then re-cleaning. The steel surface should first be completely cleaned of oils or other contaminants. A suitable solvent such as acetone can be used to remove these contaminants. Weak layers, such as mill scale, paint and corrosion products should be removed by grit blasting procedures using angular grit. Grit blasting should be completed until a white metal surface with a rough texture is achieved. After grit blasting, any surface dust should be removed by brushing, vacuuming or blowing with a clean uncontaminated air supply. As a guideline the adhesive or primer layer should be applied to the cleaned steel surface within 24 hours after cleaning. Surface preparation should not be conducted in conditions of high humidity or precipitation. Surface preparation of the CFRP materials is minimal since many CFRP plates come with a protected, pre-roughened surface. The surface of smooth CFRP plates should be lightly sanded. Any dust on the surface of the plate should be removed with an appropriate solvent such as methanol. Typically carbon fiber sheets do not require any surface preparation.

3 Installation techniques Prior to installation, all the CFRP materials should be cut to the required length. When the environmental conditions allow, the surface of the base metal and CFRP materials should be prepared as described above. The adhesive should then be thoroughly mixed, applied to the surface of the CFRP plate and clamped within the pot life of the epoxy. The adhesive may be applied to the CFRP surface only, but for highly irregular surfaces it is recommended to apply the epoxy to the CFRP and the steel surface to minimize the formation of air voids within the adhesive layer. The thickness of the bond line can be controlled by the use of a plastic trowel with a V shaped notch or be mixing a small amount of glass beads into the adhesive. In the current study, adhesive thicknesses ranging from 0.1 mm to 1.2 mm were successfully used. To prevent sagging of the CFRP plates a temporary clamping system should be installed until the adhesive has cured sufficiently, typically within 24 hours. Phase II: Member Behavior The following sections describe the behavior of typical strengthened steel bridges. The behavior of large-scale steel bridges is presented. The behavior of strengthened bridges under overloading and fatigue loading conditions is also examined. The behavior of strengthened special structures, such as steel monopole towers, is also discussed. Large-scale steel bridges Three large-scale steel-concrete composite beams were tested to investigate the effectiveness of different configurations of CFRP plates to increase the strength and stiffness of typical steelbridges 4. Both intermediate (IM) and high modulus CFRP plates were considered. The possibility of prestressing the HM CFRP strips prior to installation was also investigated. The typical test beams consisted of a W310x45 steel beam in composite action with an 840x100 mm reinforced concrete deck slab. The beams were strengthened with different configurations of CFRP materials and loaded monotonically to failure. The increase of the flexural strength and elastic stiffness is given in the Table 2 for the three strengthened beams. The different strengthening systems increased the elastic stiffness and the ultimate capacity of the strengthened beams by up to 36% and 45% respectively. The results indicate that the use of the prestressed strips helped to improve the efficiency of the strengthening system by reducing the amount of strengthening required to obtain a comparable increase of the elastic stiffness. Table 2: Strength and Stiffness increase for large-scale steel bridge beams Strengthening system Strength increase Stiffness increase Bonded IM CFRP 16% 10% Bonded HM CFRP 45% 36% Prestressed HM CFRP - 31% Overloading behavior Three small-scale steel-concrete composite beams were tested to study the behavior of the strengthening system under overloading conditions 5. The beams, shown in Figure 1(a) were tested in four-point bending with a span of 3050 mm and a 610 mm constant moment region. Two of the tested beams were strengthened with different reinforcement ratios of HM CFRP materials. Several loading and unloading cycles were conducted to simulate severe overloading

4 of the beams. A third unstrengthened beam was tested as a control beam. The typical loaddeflection behavior of the strengthened beams, shown in Figure 1(b) was essentially linear up to rupture of the CFRP with a slight non-linearity occurring after yielding of the steel beam. The behavior of the beams after rupture of the CFRP was similar to that of an unstrengthened beam. Failure occurred due to crushing of the concrete. In addition to increasing the ultimate capacity and elastic stiffness of the beams, installation of the high modulus CFRP materials also helped to increase the yield load. The presence of the CFRP materials also reduced the residual deflection of the strengthened beams after unloading compared to the unstrengthened beam. This suggests that due to an overloading event an unstrengthened beam would likely exhibit a significant residual deflection which may necessitate replacement of the member while a similar strengthened beam could remain in excellent serviceable condition. (a) Total Applied Load (kn) (b) Steel Yield CFRP Rupture Strengthened beam Concrete Crushing Unstrengthened beam 50 Figure 1: Typical overloading (a) test setup and (b) load-deflection behavior Net Mid-span Deflection (mm) Fatigue behavior Three additional small-scale beams were tested under fatigue loading conditions 5. Two of the beams were strengthened using the same reinforcement ratio of CFRP materials however, using different bonding techniques to investigate the effect of the bond on the fatigue performance of the system. The third beam remained unstrengthened to serve as a control beam for the fatigue study. All three of the test beams were subjected to three million fatigue loading cycles at a frequency of 3 Hz. The minimum applied load used for the cyclic loading for all three beams was selected to be equivalent to 30 percent of the calculated yield load of the unstrengthened beams to simulate the effect of the sustained dead-load for a typical bridge structure. For the unstrengthened beam, the maximum load in the loading cycle was selected to be equivalent to 60 percent of the calculated yield load to simulate the combined effect of dead-load and liveload. The maximum load for the strengthened beams simulated an increase of 20 percent of the allowable live-load level in comparison to the unstrengthened beam. The maximum applied load in a fatigue cycle was thus equal to 60 percent of the calculated increased yield load of the strengthened beams. Both strengthened beams sustained the three million load cycles at the simulated increased live load level without showing any indications of degradation or failure.

5 Special structures: steel monopole towers Three scaled steel monopoles were fabricated from A572 grade 65 steel with similar proportions to monopoles that are typically used as cellular phone towers. Three different strengthening configurations were examined including wet lay-up of high modulus carbon fiber sheets, adhesive bonding of CFRP plates that were pultruded using the same high modulus fibers and CFRP plates that were pultruded with an intermediate modulus fiber. Strengthening the monopole by wet lay-up of CFRP sheets reduced the net deflection of the monopole by 25 percent at the mid-length and by 17 percent at the tip compared to the unstrengthened monopole. Failure occurred due to rupture of the sheets on the tension side underneath the anchorage. Following rupture, redistribution of the stresses in the monopole resulted in local buckling of the monopole on the compression side. This buckling ruptured the longitudinal and transverse fibers surrounding the buckled region. Figure 2: Typical monopole tower test setup For the monopole strengthened with high modulus CFRP strips the net deflection of the monopole was reduced by 39 percent at mid-length and by 30 percent at the tip. The monopole, strengthened with the intermediate modulus CFRP strips had the greatest stiffness increase, with a reduction of the net mid-length and tip deflection by 53 percent and 39 percent respectively. Phase III: Detailing The following sections present an experimental program which was conducted to develop detailing recommendations for the strengthening system. The bond behavior of the CFRP plates and the environmental durability of the system are both considered. Bond behavior A detailed experimental program was conducted to study the bond behavior of the proposed strengthening system 6. The experimental program consisted of a total of eight double lap shear coupons and ten steel beams with a CFRP splice joint at the midspan location within the constant moment region. The different parameters studied included the shape of the plate end, the presence of additional mechanical anchorage and the total length of the splice plate. All of the tested coupons and beams failed by sudden debonding of the splice plate prior to rupture of the CFRP. The findings indicate that the shape of the plate end had the most significant effect on the joint capacity. While a square plate end is currently the most commonly used configuration, the experimental results indicate that implementation of a reverse tapered joint detail can approximately double the strength of the spliced connection. The beam tests indicated that mechanical anchorage near the plate end did not increase the joint strength. Small-scale tests were conducted to determine the characteristics of the adhesive and the bond interface. Both the shear strength and the pure tension strength of the adhesive and the bond interface were considered. Pull-off tests were conducted to determine the pure tension strength of the bond interface. The average measured bond strength was 18 MPa which is five times higher than the typical bond strength of CFRP to concrete. The measured tension strengthen of the adhesive was 38 MPa. The pure shear strength of the adhesive and of the bond interface was

6 measured using a specially designed torsion test device. The average measured shear strength of the adhesive and the bonded interface were 48 MPa and 23 MPa respectively. A finite element analysis is currently in progress to evaluate the distribution of bond stresses in the adhesive. The analysis indicates that significant shear and normal stress concentrations are induced in the adhesive near the plate end. The analysis further indicates that implementation of the reverse tapered plate end detail, shown in Figure 2, can reduce the magnitude of the shear stress concentration by approximately 20% and can practically eliminate the peeling stress concentration at that location. Steel tension flange Main CFRP plate Note: Not to scale End of CFRP splice plate Figure 4: Reverse tapered plate end detail Center of splice Typical plate end Environmental durability A comprehensive experimental program is ongoing to study the environmental durability of the proposed strengthening system. The experimental program includes a total of 52 CFRP-to-steel double lap shear coupons. Two methods to enhance the durability of the system were considered. A glass fiber insulating layer was included between the steel and the carbon fiber for some of the test specimens to prevent galvanic corrosion. For some specimens the steel was pretreated with a silane primer. Additional specimens implemented both methods of protection while the final series of specimens did not include any additional protection. The effect of sustained load was also considered. The degradation of the bond was accelerated by exposing the specimens to wet/dry cycles in a 100 o F, 5% salt water solution. To simulate the performance of the system under typical environmental conditions, another series of specimens was subjected to typical outdoor environmental conditions. The test setup is shown in Figure 3(a). To date a total of 34 coupons have been tested. These include 14 unconditioned control coupons, 10 coupons which were subjected to accelerated corrosion for one month and 10 coupons exposed for four months. The remaining tests are currently in progress. The average measured strength and the range of measured strengths for all of the tested coupons are given in Figure 3(b). For the specimens that did not include any form of environmental protection, the average strength decreased by 45 percent after four months of severe environmental exposure. For the specimens that included an additional glass fiber layer the average strength decreased by 50 percent after four months. The average strength of the specimens which included a silane coupling agent remained essentially constant for the four month duration. For the coupons that included both glass and silane the average strength decreased by only 12 percent.

7 Submerged water heater (a) Test coupons during dry cycle Figure 3: Environmental durability phase (a) test setup (b) test results Measured Tension Strength (kn) Control 1 Month 4 Months no change 45% Adhesive Adhesive + Silane 50% Adhesive + Glass (b) 12% Adhesive + Glass + Silane Proposed Design Guidelines For a given beam, the CFRP strengthening system can be designed to achieve a specified increase of the allowable live load level. The design is based on a moment-curvature analysis and accounts for the non-linear behavior of the concrete and the steel materials. The analysis procedure and a worked example are presented in detail elsewhere 7. The proposed design methodology is based on three criteria shown in Figure 4 relative to the moment-curvature relationship of a typical strengthened bridge beam. The combined effect of the dead load, M D, and the increased live load, M L, should not exceed 60% of the increased yield load of the strengthened member, M y,s to ensure elastic behavior under service loading conditions. To satisfy the strength limit state, the total factored load, with the appropriate dead and live load factors, α D and α L respectively, should not exceed the ultimate capacity of the strengthened beam, M U,S, with an appropriate strength reduction factor, φ. To maintain the safety of the structure in case of an unexpected loss of the strengthening system, the total effect of the applied dead load, M D, and the increased live load, M L, should not exceed the nominal capacity of the unstrengthened beam, M n,us. The remaining limit states which are applicable to unstrengthened steel-concrete composite beams should also be checked at the increased load level. Strengthened Nominal Strengthened Capacity, M n,s Ultimate Strengthened Capacity, M U,S = φm n,s M D + M L 0.6 M YS α D M D + α L M L M U,S = φm n,s M D + M L M n,us Curvature M Y,US M Y,S Nominal Unstrengthened Unstrengthened Capacity, M n,us Factored Moment Live Load, M L (α D M D + α L M L ) Dead Load, M D Moment Figure 4: Proposed design guidelines

8 Conclusions This paper presents the development of a high modulus CFRP system for strengthening steel bridges and structures. The experimental and analytical results demonstrate that the proposed system can be effectively used to enhance the serviceability and ultimate strength of steel beams. The modulus of the CFRP, the level of prestressing, the bond detailing and the degree of environmental protection applied can all be varied to suit the needs of the specific application. The experimental results were confirmed by analytical and finite element techniques. Flexural design guidelines are presented which ensure the safety and serviceability of the strengthened member. This paper demonstrates that carbon fiber materials can be effectively used for strengthening of steel bridges and structures. Acknowledgements The authors would like to acknowledge the support of the National Science Foundation (NSF) Industry/University Cooperative Research Center (I/UCRC) on Repair of Buildings and Bridges with Composites (RB 2 C). The generous support and contributions of Mitsubishi Chemical FP America Inc. and Fyfe Co. LLC are also greatly appreciated. References 1. Sen R, Libby L, Mullins G., 2001, Strengthening steel bridge sections using CFRP laminates, Comp: Part B, 39, Tavakkolizadeh M, Saadatmanesh H., 2003, Strengthening of steel-concrete composite girders using carbon fiber reinforced polymer sheets, Jour Struc Eng, 129(1), Mitsubishi Chemical FP America Inc., 2004, Dialead: High performance coal tar pitch based carbon fiber. 4. Schnerch, D. Strengthening of steel structures with high modulus carbon fiber reinforced polymer (CFRP) Materials, 2005, Ph.D. dissertation, North Carolina State University. 5. Dawood M, Rizkalla S, Sumner E., 2007, Fatigue and overloading behavior of steel-concrete composite flexural members strengthened with high modulus CFRP materials, J. Comp. Const., 11(6), Rizkalla, S., Dawood, M. and Schnerch, D., in press, Development of a carbon fiber reinforced polymer system for strengthening steel structures. Comp.: Part A, (2007), doi: /j.compositesa Schnerch D, Dawood M, Rizkalla S, Sumner E., 2007, Proposed design guidelines for strengthening of steel bridges with FRP materials, Const. Build. Mat. 21,