FLEXURAL BEHAVIOUR OF HYBRID FRP-UHPC GIRDERS UNDER STATIC LOADING

Transcription

1 Proceedings of 8 th International Conference on Short and Medium Span Bridge Niagara Falls, Canada 21 FLEXURAL BEHAVIOUR OF HYBRID FRP-UHPC GIRDERS UNDER STATIC LOADING Donna S. M. Chen Department of Civil Engineering, University of Calgary, Calgary, Alberta, Canada Raafat El-Hacha Department of Civil Engineering, University of Calgary, Calgary, Alberta, Canada ABSTRACT The behaviour of composite beams fabricated from Fibre Reinforced Polymers (FRPs) and Ultra-High Performance Concrete (UHPC) under static flexural loading was investigated. The specimens tested included one control beam consisting of a Glass FRP (GFRP) hollow box section as well as two hybrid specimens, composed from a layer of UHPC cast on the top flange of the GFRP hollow box section and either Carbon FRP (CFRP) or Steel FRP (SFRP) sheets bonded on the bottom flange of the GFRP box. In the hybrid beams, GFRP shear studs were used along with epoxy adhesive at the interface between the top flange of the GFRP hollow box section and the UHPC to provide shear and tensile resistance and ensure composite action between the two distinct materials; however, experimental testing showed that debonding failure in the hybrid beams occurred prior to the load expected for flexural failure. This paper will explore the core design methodologies as well as investigate and analyze the performance of these hybrid FRP-UHPC beams under flexural loading. 1. INTRODUCTION Currently, the most used material in Civil Engineering for the construction of structures is reinforced concrete. As a form of conventional civil engineering construction, there are numerous existing and well-established standards, specifications and procedures that are available due to intensive research, which ensure high structural quality and safety. However, over the years, it has been found that reinforced concrete structures do exhibit long-term problems related primarily to corrosion in the presence of moisture, though additional issues such as sulphate attack, alkaliaggregate reaction and freeze-thaw also do exist. These problems result in elevated costs of maintenance and repair that has the potential of matching the scale of the initial cost of construction. In recent years, newer methods of construction using high performance structural materials, including Fibre Reinforced Polymers (FRPs), have been investigated. The advantageous properties of FRPs include high strengthto-weight ratio, high strength, resistance to corrosion and ease of installation relative to other structural materials. Tests on hybrid FRP-concrete structural members, many for the purpose of bridge applications, have been performed at institutions internationally with promising results. When used in the fabrication of a hybrid structural member, the FRP section often has the added advantage of acting as a stay-in-place formwork for the concrete. The short and long-term performance of hybrid FRP-concrete structural members was investigated by Deskovic et al. (1995a, 199b), with emphasis on maximizing the advantageous properties of each material used. Fam and Rizkalla (2) studied the behaviour of hybrid concrete-filled tubes fabricated from filament wound GFRP, pultruded GFRP and steel, with comparisons of their performance as a hollow tube. Kim et al. (26) performed experimental tests on modular hybrid FRP-concrete bridge deck systems, using shear connecting plates fabricated monolithically with the FRP module as well as coarse sand coating to generate sufficient bond between the concrete and the FRP. At the University of California, Cheng and Karbhari (25) studied the performance of a composite FRP-concrete deck panel system that utilizes a series of shear ribs perpendicular to a series of vertical stiffeners bonded on top of a FRP deck panel fabricated from a combination of carbon and E-glass FRPs. An FRP bridge deck system could weigh 71-1

2 about 2% of a similar RC deck (Keller 25). The behaviour of pultruded GFRP shapes in hybrid FRP-concrete member for slabs and girders were tested experimentally and in conjunction with analytical investigations by Honickman (28). Most recently, Lee and Hong (29) experimented with different types of connection details between adjacent modular composite GFRP deck sections. In this paper, research concerning the construction and testing of a newly constructed hybrid bridge girder system composed of different types of high performance materials including FRPs and Ultra-High Performance Concrete (UHPC) will be discussed. The used UHPC incorporates small steel fibres within the mixture instead of any passive internal steel reinforcement bars. Combined with other FRP materials, such as Carbon FRP (CFRP), Glass FRP (GFRP) and Steel FRP (SFRP), the proposed hybrid FRP-UHPC structural member, under two different configurations, was investigated under static flexural loading. Currently, research in hybrid bridge girders has not yet included the use of both UHPC and SFRP materials. Experimental findings from this research will allow for a better understanding of the behaviour and interactions between the high performance materials used in the hybrid girder. With continuing research, particularly in regards to the behaviour under fatigue loading, which will be addressed in the next stage of the project, this may lead towards the development of full-scale bridge deck systems composed of numerous adjacent hybrid bridge girders with one UHPC layer cast overtop. 2. DESIGN OF NEW HYBRID SYSTEM The use of high performance materials, such as FRPs and UHPC, in the design of the proposed hybrid cross-section is intended to allow for higher strength to be achieved while reducing section weight and overall size. The crosssection of the hybrid structural member consists of a thin layer of UHPC supported on the top flange of a GFRP hollow box section. Along the base of the GFRP box section, sheets of tensile reinforcement, made from either CFRP or SFRP, are applied. In order to provide sufficient bond along the interface between the GFRP and UHPC, moisture insensitive high modulus epoxy adhesive is applied to withstand the shear forces along the interface, with supplementary GFRP shear studs acting as the secondary shear resistance system. Under loading, the physical behaviour of FRP is linear elastic until failure. In a structural member, this type of failure can be catastrophic. In order to create a degree of safety with visible warning signs in the structure prior to collapse, a pseudo-ductile type of failure is desirable. Tensile reinforcement sheets with ultimate strain limits less than that of the GFRP hollow box section, which would also not simultaneously result in compressive failure of the UHPC, assuming perfect bond behaviour throughout, were used. The anticipated failure mechanism would consist of the tensile reinforcement sheets on the bottom of the GFRP section fracturing, followed by crushing of the UHPC prior to failure of the GFRP hollow box section and complete section failure. 3. MATERIALS According to the manufacturer, the UHPC has a modulus of elasticity that can range between 5 and 6 GPa with an ultimate compressive strength between 15 to 18 MPa at 28 days and a compressive strength of 3 MPa after 24 hours of casting. If it is heat-treated, it has the potential of reaching greater strengths that surpasses 2 MPa (Lafarge 27). The density of the UHPC is 25 kg/m 3 (Lafarge 28). Concrete cylinders were tested at 28 days as well as prior to testing of beam specimens under static loading, in accordance with ASTM standard C39. Experimental results from these tests, using equations and guidelines from ASTM standard C469, CSA A Cl , ACI and CSA S6-, show that the average compressive strength of the UHPC was 138 MPa with a standard deviation of ±18.4 MPa with a modulus of elasticity of 56 GPa with a standard deviation of ±8179 MPa. The GFRP hollow box section is a pultruded shape with internal glass strand rovings for longitudinal strength and either continuous strand glass mats or stitched reinforcements for transverse resistance. From manufacturer specifications, the modulus of elasticity is 17.2 GPa with a maximum tensile strength of 27 MPa (Strongwell 29a). GFRP shear rods were also used as shear connectors, with a shear resistance under single shear of 16 kn (Strongwell 29b). Experimental material property values obtained from tension coupon tests, performed according to ASTM D339, showed that the modulus of elasticity is 26.4 GPa with a maximum tensile strength of 321 MPa. 71-2

3 The CFRP sheet used is a uni-directional carbon fabric with a modulus of elasticity of 23 GPa and an ultimate tensile strength of 379 MPa. For the gross laminate, the thickness is 1. mm, the modulus of elasticity is 95.8 GPa and the ultimate tensile strength is 986 MPa (Fyfe 29a). The SFRP (Hardwire TM Tape) is made of unidirectional brass-coated ultra-high strength twisted steel wires forming cords which are assembled into a sheet that can be impregnated using various resin systems. The sheet s density consists of 2 aligned steel cords per 25.4 mm (7.87cord/cm) with each cord consisting of three.35 mm diameter straight wires wrapped by two wires at a high twist angle. During the manufacturing process, the cords are assembled side-by-side with the help of thin silicon mesh on one side to make the required width of the sheet. The individual cord has a diameter of.89mm with a breaking load of 1539 N, modulus of elasticity of 16 GPa, and strain at failure of 2.1%. The composite SFRP sheet has a net cross section of.38 mm 2 /mm and a thickness of 1.23 mm. According to the manufacturer, the ultimate tensile strength and effective modulus of elasticity for the 1.23 mm thick composite SFRP sheet are 985 MPa and 66.1 GPa, respectively (Hardwire 21). Tension test results showed that the average tensile strength and modulus of elasticity were 941 MPa and 6.1 GPa, respectively. Three types of epoxy adhesives were used. Epoxy Type A (Sikadur 33) is a moisture tolerant epoxy with a tensile strength of 3 MPa, tensile elongation limit of 1.5% and modulus of elasticity of 3.8 GPa (Sika 27). This epoxy adhesive was used to bond the SFRP sheets onto the bottom flange of the GFRP box. Epoxy Type B (Sikadur 32 Hi-Mod) is a moisture insensitive epoxy that has a tensile strength of 48 MPa, tensile elongation limit of 1.9% and modulus of elasticity of 3726 MPa (Sika 28). This epoxy adhesive was used at the interface between the wet UHPC and the top flange of the GFRP box. The bond strength is 11.7MPa (Sika 28). Epoxy Type C (Tyfo S) is primarily used for wet lay-up of CFRP sheets; the tensile strength is 72.4 MPa, with a tensile elongation of 5.1% and modulus of elasticity of 318 MPa (Fyfe 29b). 4. DESCRIPTION OF SPECIMENS 4.1. Beam Dimensions The GFRP hollow box beam is mm wide, mm height, with 11.1 mm thick walls all around. In the hybrid specimens, the layer of UHPC on top is 53mm in height with variable width for CFRP and SFRP reinforced beams. The thickness of the CFRP and SFRP sheets are 1. mm and 1.23 mm, respectively. The design crosssection of the control GFRP beam as well as typical hybrid FRP-UHPC beam is shown in Figure 1. Figure 1. Cross-section of beams Fabrication of Specimens The total lengths of the three specimens varied between 3 mm to 31 mm. To achieve identical span lengths for all, 25.4 mm thick steel plates, with a width of 1 mm and a length of 2 mm, were epoxy bonded at the base of the beams, after bonding of the tensile reinforcement sheets, so that the final center-to-center spacing between the plates was 29 mm uniformly for all specimens. It is important to note that this method of construction is only 71-3

4 applicable to the construction of new bridge structures; for existing structures, cut-off of the tensile sheets in conjunction with adequate anchorage prior to the beam supports would be required. For the two hybrid specimens, the top and bottom surfaces of the GFRP hollow box sections were prepared by sanding to ensure good bonding between the GFRP and CFRP/SFRP sheets using epoxy adhesive. Holes for the GFRP shear studs were then drilled into the top flange of the hollow box section. GFRP shear studs were placed at transverse spacing of 75 mm, centered about the beam, with a longitudinal spacing of 1 mm. The shear studs were 55 mm long, with 12 mm anchored into the drilled holes in the GFRP hollow box section and 43 mm embedded inside the UHPC layer. To ensure the GFRP shear studs are anchored properly, epoxy Type A was applied to one end of the studs prior to being inserted into the holes. Figure 2 shows the preparation of the specimens prior to pouring of concrete. Figure 2. Formwork installation with GFRP studs in place before casting. Prior to bonding of the tensile reinforcement sheets, the surface was clean from any dust with acetone. Then, a layer of epoxy was applied to both the surface of the GFRP bottom flange as well as one side of the CFRP or SFRP sheet. The sheet was then placed in position, with the epoxy applied side placed face down on the GFRP bottom flange. An additional layer of epoxy was then applied onto the exposed side of the sheet until full saturation was achieved. Epoxy Type A was used for bonding the SFRP sheets and epoxy Type C was used for the CFRP sheets. Figure 3 shows the bonding of the CFRP sheet onto the GFRP. Figure 3. Bonding CFRP and SFRP sheets on the GFRP box. On the top surface of the GFRP hollow box beam within the formwork, wax was thinly spread onto the plywood to close any gaps that may have been present and to facilitate the formwork removal. Epoxy Type B was then applied along the entire length of the beam. To cast the concrete layer, the UHPC was mixed by an experienced technician from the manufacturer. The concrete mixture was slowly poured into the formwork, commencing from one end to the other. Pouring was performed behind the concrete flow line so that there is only one flow interface, preventing the steel fibres in the formwork to collide and disrupt the required random orientation of steel fibres within the UHPC. Figure 4 shows the pouring of UHPC into the formwork on top of the GFRP hollow box section. 71-4

5 Figure 4. Wax applied onto plywood, and casting the UHPC. To prevent bearing failure at the supports, concrete end blocks were used. In all of the specimens, at the two ends, wooden boards were inserted and held in place by silicon into the core of the GFRP box at a distance of 18 mm from the end as well as on the outside of the box. Larger holes, with a diameter of 5mm, were then drilled on the top web 1 mm from the end of the beam. When the UHPC was poured into the formwork for the compressive flange on top of the specimens, the concrete also flowed directly into the space limited internally by the wooden boards. The end support concrete block before and after concrete casting is shown in Figure 5. Figure 5. Construction of end block before and after casting of concrete. 5. TEST SET-UP AND INSTRUMENTATIONS All of the specimens were tested under four-point loading with span length of 29 mm between the center of supports. The specimens were tested under displacement control mode at a constant loading rate of 1mm/min. Strain gauges were placed at the bottom of the beam directly at midspan, the two point loads as well as halfway between the point load and the support at the two ends. At midspan, strain gauges were also placed on the side of the GFRP hollow box section at quarter depths; as well, strains gauges were also placed at the top of the beam. Linear Strain Conversion Transducers (LSCTs) similar to Linear Variable Displacement Transducer (LVDTs) were used to measure vertical deflection at midspan and the location of the point loads; one LSCT was also placed at midspan to measure lateral deflection. The load and instrumentation set-up is shown in Figure EXPERIMENTAL RESULTS 6.1. GFRP Hollow Box Section Beam: G-S The G-S beam was tested alone without UHPC cast on top or tensile reinforcement sheets bonded on the bottom of the GFRP box. The beam behaved in a linear elastic fashion, up until failure at a load of kn. Failure occurred due to crushing and subsequent collapse of the top flange at the center of the beam between the locations of the point loads. The failure strain at midspan at the top and bottom of the section was 5223 microstrain in compression and 699 microstrain in tension. The midspan deflection at failure was 48 mm. The location of the neutral axis at failure was 127 mm above the bottom of the beam. The final failure of beam G-S is shown in Figure 7. The load-midspan deflection behaviour of beam G-S is shown in Figure 8 in comparison with beams C-S and S-S. 71-5

6 Load (kn) Load (kn) The load-midspan strain behaviour of beam G-S is shown in Figure 9 in comparison with beams C-S and S-S. The strain profile on the bottom flange of beam G-S along its span, taken at different load levels, as well as the strain distribution through the depth of the section at midspan is shown in Figure 1. Figure 6. Test set-up and instrumentations. Failure Figure 7. Failure of G-S beam C-S C-S C-S S-S S-S G-S G-S S-S G-S Midspan Deflection (mm) Figure 8. Load-midspan deflection of all beams Compression Strain (με) Tension Figure 9. Load-midspan strain behaviour of all beams. 71-6

7 Strain (με) Height from bottom of section (mm) 7 Midspan Service (26.5 kn) 25 Top flange of GFRP box at 6 kn at 9 kn Ultimate (124.5 kn) 2 15 GFRP box mid-depth Service (26.5 kn) at 6 kn at 9 kn Ultimate (124.5 kn) Bottom flange of GFRP box Distance from support (mm) Compression Midspan Strain (με) Tension Figure 1. Longitudinal strain profile and midspan strain distribution of GFRP hollow box beam (G-S) Hybrid CFRP-UHPC Beam: C-S The hybrid CFRP-UHPC beam (C-S) failed at a load equal to kn and a corresponding midspan deflection of 49.9 mm. The average midspan tensile strain of the CFRP sheet at ultimate failure was 665 microstrain. The corresponding average concrete compressive strain at the top of the beam was 16 microstrain. The hybrid beam behaved in a linear elastic manner in the early stages of loading; however, debonding between the top flange of the GFRP hollow box section and the UHPC, particularly in one half span of the beam, occurred at a peak load of kn and a corresponding midspan deflection of 28.5 mm, causing an abrupt reduction in load resistance down to kn and a corresponding midspan deflection of 29.6 mm. The average midspan tensile strain of the CFRP sheet as well as the concrete compressive strain at peak load was 5477 microstrain and 1922 microstrain, respectively. Hairline fractures were visible at the bond interface between the UHPC and the top flange of the GFRP box that increased in size as loading resumed. Outward buckling of the webs in the GFRP hollow box section occurred at ultimate failure. At one end of the beam that exhibited large signs of debonding, the gap between the GFRP top flange and the UHPC was clearly visible at failure, where the GFRP shear studs were seen to be pulled directly out of their holes. When examined from the side of the beam, it was observed that, of the GFRP shear studs pulled out of their holes in the GFRP top flange, the ones near the support remained relatively vertical while those closer to midspan were noticeably at an angle close to 45⁰ from vertical. Failure of the C-S beam can be seen in Figure 11. Figure 11. Failure of hybrid CFRP-UHPC beam (C-S) The load-midspan-deflection and load-strain curves are shown in Figures 8 and 9, respectively, with the longitudinal strain profile along the beam span and the strain distribution across the depth of the beam at midspan shown in Figure 12. The location of the neutral axes at ultimate failure was at a height of 117 mm from the bottom of the beam in the GFRP hollow box section and at 242mm in the UHPC layer. 71-7

8 Strain (με) Height from bottom of section (mm) Midspan Service (79.4 kn) at 125 kn at 175 kn Peak (222.2 kn) Ultimate (182.6 kn) Top of concrete Top flange of GFRP box GFRP box mid-depth Service (79.4 kn) at 125 kn at 175 kn Peak (222.2 kn) Ultimate (182.6 kn) Bottom flange of GFRP box Distance from support (mm) Compression Midspan Strain (με) Tension Figure 12. Longitudinal strain profile and midspan strain distribution of hybrid CFRP-UHPC beam (C-S) Hybrid SFRP-UHPC Beam: S-S The ultimate load was kn, resulting in an average tensile strain in the SFRP sheet of 6876 microstrain and average compressive strain in the UHPC of 782 microstrain. The maximum midspan deflection reached was 56.6 mm. Debonding between the GFRP hollow box section and the UHPC layer, resulting in visible fracture lines at the interface, commenced at a load of kn, causing loss of load resistance; considerable debonding was also experienced when the load reached 131 kn after specimen loading resumed. Larger strain levels for the top portion of the hybrid beam were achieved at a load of kn, with the concrete strain at the top reaching 192 microstrain. After this point, a small amount of additional load (about 8.5%) was carried by the beam until ultimate failure. The beam exhibited linear elastic behaviour until debonding occurred. Upon the application of additional load, the relationship between the applied load and midspan deflection began to change, with higher levels of deflection carried under the same increment of applied load. Failure of beam S-S is shown in Figure 13. Figure 13. Failure of hybrid SFRP-UHPC beam (S-S). At failure, tension cracks in the UHPC layer were clearly visible at the location of the point loads showing that the neutral axis of the beam was located within the UHPC. The load-midspan-deflection and load-strain curves for the S-S beam are shown in Figures 8 and 9, respectively in comparison with the other two beams. The longitudinal strain profile and midspan strain distribution curves are shown in Figure 14. At failure the location of the neutral axes was 115 mm above the bottom of the beam in the GFRP hollow box section and 252 mm in the UHPC layer. 71-8

9 Strain (με) Height from bottom of section (mm) Midspan Service (85.9 kn) at 125 kn at 175 kn Ultimate (187.4 kn) Top of concrete Top flange of GFRP box Service (85.9 kn) at 124 kn GFRP box mid-depth at 175 kn Ultimate (187.4 kn) Bottom flange of GFRP box Distance from support (mm) Compression Midspan Strain (με) Tension Figure 14. Longitudinal strain profile and midspan strain distribution of hybrid SFRP-UHPC beam (S-S). 7. INTERPRETATION OF RESULTS The GFRP hollow box section alone was tested in order to act as a control beam that did not incorporate the UHPC cast on the compression side and the SFRP or CFRP sheets bonded on the tension side. The behaviour of the specimen was as expected for GFRP material, exhibiting near-perfect linear elastic behaviour until failure. The strain at the top and bottom of the beam at midspan were approximately the same in magnitude, showing that the neutral axis remained at mid-depth during loading. This was shown in Figure 9 in the strain distribution through depth at midspan. However, the experimental failure strain encountered during loading was not the same as expected through reported manufacturer specifications. Based on the test results of 321 MPa and GPa for ultimate tensile strength and modulus of elasticity, respectively, the ultimate failure strain of the GFRP pultruded shape would be 12,159 microstrain. This value compared with experimental failure strain values at the extreme top and bottom fibres equal to 5223 and 699 microstrain respectively, indicate that the GFRP material only reached between 43% and 5% of its maximum potential failure strain. This suggests that the ultimate load reached in experimentation of kn did not reach the full capacity of the GFRP material and represented approximately 5% of the ultimate load of the beam, had ultimate strain in the material been reached. The reason that the full material strength of the GFRP was not reached in experimentation is due to the lack of lateral support of the compression flange of the GFRP hollow box beam G-S. This resulted in buckling and crushing of the top flange to occur prior to rupture of the bottom flange at ultimate strain values. For the specimen with UHPC cast on the top flange of the box, buckling of the top flange is prevented by the UHPC, avoiding this type of premature failure. The hybrid CFRP-UHPC beam showed good linear elastic behaviour until debonding failure between the GFRP hollow box section and the UHPC layer occurred at a peak load of 222 kn. After debonding, the linear behaviour of the hybrid beam began to deviate, showing slightly greater increases in strain and deflection for intervals of applied load. The strain incompatibility that occurred at the GFRP-UHPC interface that resulted as an effect of the debonding caused the UHPC and the GFRP hollow box section beam reinforced with the CFRP sheet at its base to act as two separate structure components, with the formation of two individual neutral axes. This can be seen in the C-S midspan strain profile shown in Figure 12, where a significant strain difference exists at the base of the UHPC layer. In the same figure, strains show that tension forces were exerted on the bottom half of the UHPC layer. The strain values reached by the UHPC and CFRP during testing, either at peak load (222 kn) or at ultimate failure (182kN), are significantly less than the ultimate strain of these two materials due to the premature debonding failure at the UHPC-GFRP interface. To achieve the desired design failure of CFRP sheet rupture, the strain at the base of the beam would need to reach 1, microstrain, which is 54% greater than the experimental value. Therefore, if debonding had not occurred, assuming that the beam continued to behave linear elastically, the expected load resistance of the hybrid CFRP-UHPC beam would have reached 342 kn. The hybrid SFRP-UHPC beam experienced debonding failure at loads of 124 kn and 133 kn, reaching ultimate failure at a load of 187 kn. After the first debonding failure, the beam continued to perform in the same linear elastic manner as exhibited prior to debonding; however, after the second debonding failure, the structural behaviour of the hybrid beam began to transition into nonlinear behaviour. In the testing of this beam, after debonding occurred, 71-9

10 considerable deflection was sustained without significant increases to the load carried. Strain continuity was not maintained and after debonding, the beam ceased to perform as a composite member. Subsequently, this caused tension forces to be exerted at the bottom of the UHPC layer, resulting in tension cracks that were shown most prominently directly below the steel bearing plates of the two point loads. Similarly, to attain the SFRP rupture failure mode originally designed, where the ultimate tensile strain of the SFRP would be 15, microstrain, it would represent an increase of 124% in the load resistance, placing the expected load resistance at a value of 419 kn. Debonding along the interface between the UHPC and the top flange of the GFRP hollow box section occurred in both hybrid beams tested under static loading. This indicates that the bonding method used, which consisted of a combination of epoxy adhesive and GFRP shear studs, was not sufficient to withstand the shear and tension forces between the top flange of the GFRP hollow box section and the UHPC layer. Though debonding at the interface does provide an advance warning of ultimate failure, this mode of failure is not desirable due to the fact that the full potential of the high performance materials used in the hybrid structural member was not utilized. The design of shear resistance along the interface between the top flange of the GFRP and the UHPC layer used published material properties by the manufacturers, combining the shear resistance of both the epoxy and the GFRP shear studs for the total shear resistance, which surpassed the applied load required to reach the ultimate flexural resistance of the hybrid beams. It was noted that at failure, the GFRP shear studs appeared to be structurally intact, where tensile fracture and shearing did not occur. The cause of this premature failure in the hybrid beams may be due to insufficient bond strength and bond length between the GFRP shear stud and the vertical sides of drilled holes in the top flange of the GFRP hollow box section. Additional epoxy or higher strength epoxy may have been required to ensure proper bonding. In order to achieve failure loads that are better representations of the load carrying capabilities of the high performance materials used in the design of the hybrid beams, supplementary design changes and ameliorations to the bonding along the GFRP-UHPC interface are required. This may entail any combination of the following: increasing the amount of epoxy adhesive applied at the interface level, increased roughening of the GFRP contact surface to promote better bonding, decreasing the GFRP shear stud spacing, use of bonding agents with higher tensile and shear strengths, or post-tensioning the UHPC to the end block of the beam. In all three beams, G-S, C-S and S-S, the strain at the bottom of the beam recorded at ultimate failure was 699, 665 and 6876 microstrain, respectively, regardless of the mode of failure and the load level at which debonding, where applicable, occurred. These results may indicate that failure of the beam could be partially attributed to the strength of the GFRP hollow box section material. Compared with the experimental material property values for the GFRP, the expected maximum strain is equal to 12,159, indicating that the GFRP material properties did not attribute to the premature failure. A similar mode of failure was experienced by Honickman (28) where full ultimate strain was not attained due to debonding failure. Through parametric studies, it was speculated that a shear span-to-depth equal to or greater than 4.2 would be required to obtain flexural failure prior to debonding. 8. CONCLUSIONS Debonding of both hybrid beams at the interface between the top flange of the GFRP box section and the UHPC layer caused premature failure prior to reaching the ultimate flexural capacity. As a result, the desired failure mode by rupture of the tension reinforcement sheets occurring first followed by crushing of the UHPC was not achieved; however, the primary design objective for the hybrid beams to provide a pseudo-ductile failure with advanced visible warning signs prior to ultimate failure was obtained through debonding at the GFRP-UHPC interface. Testing of the hybrid beams have revealed that in addition to the UHPC layer acting to increase the resistance of the GFRP hollow box section beam, it also provides lateral support and prevents compressive flange buckling at higher loads. Though debonding failure occurred, preventing the ultimate flexural resistance, as determined from material properties, from being attained, the failure load of both hybrid beams were higher than that reached by the beam composed of the GFRP hollow box section alone. This can be attributed to the integration of high performance materials such as UHPC and CFRP in the hybrid beam, allowing for greater resistance through composite action. With continuing research development, these hybrid beams may have potential for future use in bridge applications. The results from experimental testing under static load of the hybrid FRP-UHPC beams are part of a pilot test in an on-going experimental project. Further testing of these hybrid beams in the future will incorporate design features based on experimental findings presented in this paper, including, but not limited to, methods of augmenting the 71-1

11 shear resistance at the interface between the GFRP flange and the UHPC as well as alteration to beam dimensions to prevent the onset of debonding failure prior to flexural failure. 9. ACKNOWLEDGEMENT The authors would like to acknowledge Lafarge Canada, Sika Canada and Hardwire LLC for donating the materials, as well as the financial support received from the University of Calgary and from the Natural Sciences and Engineering Research Council of Canada. We are also grateful for the help and support received throughout this project from the technical staff members in the Department of Civil Engineering at the University of Calgary. 1. REFERENCES ACI Standard. 22. Building Code Requirements for Structural Concrete and Commentary. ACI American Concrete Institute, Farmington Hills, MI, USA. ASTM Standard. 21. Standard Test Method for Compressive Strength of Concrete Cylindrical Specimens. ASTM C39-1. ASTM International, West Conshohocken, PA, USA. ASTM Standard Standard Test Method for StaticModulus of Elasticity and Poisson's Ratio of Concrete in Compression. ASTM Standard C ASTM International, West Conshohocken, PA, USA. ASTM standard. 2. Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. ASTM standard D339-. ASTM International, West Conshohocken, PA, USA. Canadian Standards Association. 24. Design of Concrete Structures (CSA A23.3-4). Canadian Standards Association, Mississauga, ON, Canada. Cheng, L., and Karbhar, V. 25. Steel-Free Hybrid FRP Stiffened Panel-Concrete Deck System. Proceeding from ACI Special Publication SP at the 7th International Symposium on Fibre Reinforced Polymer Reinforcement on Concrete Members(FRPRCS-7. Kansas City, MO, USA. pp Deskovic, N., Meier, U., and Triantafillou, T. 25. Innovative Design of FRP Combined with Concrete: Long- Term Behavior. Journal of Structural Engineering, 121 (7): Deskovic, N., Triantafillou, T., and Meier, U Innovative Design of FRP Combined with Concrete: Short Term Behavior. Journal of Structural Engineering, 121 (7): Fyfe. 29a. Tyfo SCH-41 Composite Product Data Sheet. Fyfe Co. LLC. San Diego, CA, USA. Fyfe. 29b. Tyfo S Saturant Epoxy Product Sheet. Fyfe Co. LLC. San Diego, CA, USA. Hardwire X2 Cord Properties - Metric. Hardwire LLC. Pocomoke, MD, USA. Honickman, H. 28. Pultruded GFRP Sections as Stay-in-Place Structural Open Formwork for Concrete Slabs and Girders. M.Sc. Thesis. Queen's University, Department of Civil Engineering, Kingston, ON, Canada. Keller, T. 25. All-FRP and Hybrid-FRP Load-Bearing Structures Status and Future Prospects. 4 th Middle East Symposium on Structural Composites for Infrastructure Application (MESC-4), Alexandria, Egypt. pp Kim, B.-S., Cho, J.-R., Park, S. Y., and Cho, K. 26. Toward Hybrid Bridge Deck: An Innovative FRP-Concrete Composite Deck. Proceedings of International Joint Seminar of the KSCE and the JSCE: 26 KSCE Annual Conference on "Recent progress of concrete-steel-frp hybrid structures", Gwangju, Korea. pp Lafarge. 27. Fiche de caractéristiques techniques. Lafarge Canada. Calgary, AB, Canada. Lafarge. 28. Ductal Mechanical Performance. Lafarge Canada. Calgary, AB, Canada. Sika. 27. Sikadur 33 Product Data Sheet. Sika Canada Inc. Pointe-Claire, QC, Canada. Sika. 28. Sikadur 32 Hi-Mod Product Data Sheet. Sika Canada Inc. Pointe-Claire, QC, Canada Strongwell. 29a. EXTREN Fiberglass Structural Shapes and Plate. Strongwell Corporation. Bristol, VA, USA. Strongwell. 29b. FIBREBOLT Fiberglass Studs and Nuts. Strongwell Corporation, Bristol, VA, USA