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Performance Analysis of New Hybrid Beams

Performance Analysis of New Hybrid Beams

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1.1 Introduction

Worldwide developments in civil engineering construction have created the need for new construction materials to support the structural progress, particularly in the case of severe construction requirements like large span girders or high-rise buildings. One of the objectives for the construction industry is the design of light-weight flexural elements with high capacity and high resistance to corrosion. The outstanding engineering properties of the new materials, mainly their high strength-to-weight ratios and high resistance to corrosion, can change much in the construction and rehabilitation of infrastructure. The demonstrated superior characteristics of new materials such as Fibre Reinforced Polymers (FRP), and Ultra High Performance Concrete (UHPC), have shown that these materials will play an important role in future civil engineering projects.

However, the use of the innovative materials in structures requires the establishment of appropriate design and detailing guidelines before the broad acceptance of the application of these materials. In addition, the design methodology should ensure economic use of the materials. The application of advanced composite materials in civil engineering construction indicates that important technical and implementation issues need to be resolved prior to broad acceptance of these materials by the civil engineering design and construction community.

1.2 Research objectives

The work described in this dissertation is concerned with understanding of the behaviour of an innovative hybrid beam composed of a thin UHPC slab in the compression zone, and Steel Reinforced Polymers (SRP) or Carbon Fibre Reinforced Polymer (CFRP) sheets to resist the tension, separated and connected by a Glass Fibre Reinforced Polymer (GFRP) hollow box beam. All these materials have superior strength and durability compared to conventional construction materials, so they can be utilized in much smaller dimensions than that in traditional sections. The hybrid beam studied in this research differs from beams studied previously as reported in the literature by the absence of any conventional material from its components. The unidirectional SRP and CFRP sheets were bonded to the GFRP box section using a moisture-tolerant epoxy adhesive.

Moisture-insensitive epoxy adhesive was applied onto the top flange of the GFRP section before casting the wet UHPC slab to ensure full bond between the two materials. Composite action between the GFRP section and the top UHPC slab was enhanced by GFRP shear stud connectors. The presence of UHPC on the top side of the beam supports the top GFRP flange to avoid the buckling of this flange under high compressive stresses. In addition to the structural contribution of the GFRP box beam, the hollow box is also responsible for carrying the shear stresses in the beam and during construction, and acts as a permanent stay-in-place form for the wet concrete. The high deflections that would occur if the GFRP box section was used alone are significantly reduced by the addition of the high stiffness materials. Ease of construction was also included in the aims for the system being developed.

The behaviour of short and long hybrid beams was investigated experimentally and analytically in this research project to determine the feasibility and effectiveness of such structural sections as the main flexural members in bridge construction. Seven long beams were tested to study the flexural behaviour of the hybrid section: four beams had one layer of externally bonded SRP sheet on the tension side with spans between 5100 mm and 6000 mm while the other three beams had CFRP sheets with span lengths of 6000 mm. Simplified and finite element structural analysis of the hybrid beams has been implemented. The results of the analyses were compared to the experimental tests, and good agreement between the analytical and experimental work was achieved. Adding UHPC and CFRP/SRP sheets increased the GFRP section capacity significantly. Finite Element modelling for the hybrid beams was implemented to confirm the results of the compatibility analysis and the experimental tests. The behaviour of the different hybrid beams was compared to determine the different parameters that affect the behaviour of this new assemblage. Three short beams were tested quasi-statically to investigate the shear behaviour of the hybrid section. Two of the short beams had a single sheet of externally bonded SRP on the tension side while the other beam had two sheets of CFRP. Span lengths ranged from 500 mm to 800 mm.

1.3 Research significance

The innovative hybrid beam studied herein includes only high performance materials. The combination of the UHPC with SRP or CFRP and GFRP materials in the investigated hybrid beam is expected to result in low-weight, low-maintenance, high performance elements. It is expected that applying these materials effectively will lead to faster construction and less traffic interruption for bridges because of their light weight. In addition, reduced necessary maintenance is also expected due to the high durability of the materials, and good aesthetics should occur because of the small overall dimensions.

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1.4 Thesis layout

This thesis is composed of seven chapters. In this chapter an overview of the work, the objectives and significance of the research have been presented. In the next chapter the literature on research on hybrid sections and previous studies for the behaviour of the new construction materials is reviewed. The properties of the materials used and the experiments done to verify their mechanical properties and suitability for the project are described in chapter three. The preparation and assembly of the hybrid sections is also described in that chapter. A simple hybrid section was constructed during phase one of the research project. The simple section was easy to assemble. The aim was to investigate the general behaviour of a section composed of the innovative materials examined. In chapters four and five, the experimental and analytical work done on the hybrid beams of phase one are described respectively. The results from phase one were used to design two more sections to examine different behaviours for the hybrid beams. The experiments and results of the beams tested in phase two are illustrated in chapter six. Finally, the important conclusions and recommendations drawn from the results of this research are listed in chapter seven, together with suggestions for future research. 

hybrid beams



Several research studies have been conducted on the new materials (UHPC and FRP) to study their short- and long-term behaviours. Although most of the new materials exhibited acceptable mechanical properties for many projects, the relatively high initial price of the new materials limited its wide application. The main application for composite materials in structural construction nowadays is for the repair and rehabilitation of already existing structures, and many such applications have been presented by (Karbhari and Zhao 2000). This type of application relies on using small composite pieces which have high strength in order to raise the capacity of structures already constructed from traditional materials. A review of some of the important findings on new materials is presented in the following sections.

Hybrid FRP beams

Based on the high strength of FRP materials, high-capacity slim structural sections can be constructed from only FRP composites. However, serviceability limit states can control the design of the sections in many cases and restrain the full utilization of the FRP strength because of the relatively low modulus of elasticity of the FRP composites. The relatively low modulus of elasticity of the FRP composites is attributed mainly to the presence of the plastic resins in the FRP matrix.

The concept of a hybrid FRP beam was first proposed and investigated by Deskovic et al. (1995b) in order to reduce the expected deflection of GFRP beams and decrease the possibility of buckling of the FRP upper flange under compression. Deskovic et al. (1995b) studied the behaviour of three simple beams constructed from a GFRP hollow box section with a layer of conventional concrete on top and a CFRP sheet on the bottom. The presence of the CFRP improves the creep and fatigue behaviour of the beams in addition to carrying some of the tensile stresses. The authors performed quasistatic experimental tests on beams with a span of 3 m. The beams were tested under three-point loading till failure. The authors performed non-linear finite element modelling for the beams, and modelled only a quarter of the beams by making use of the symmetry.

Good agreement was found between the experimental and the finite element results. The upper concrete layer was only bonded to the lower GFRP box section by epoxy adhesive, and this first beam failed by debonding of the concrete from the GFRP box section. In addition to the epoxy adhesive, all subsequent beams had two post-tensioned steel bolts every 190 mm joining the concrete to the box section. These other two beams failed first by debonding of the CFRP layer then by crushing of the concrete.

The researchers then examined the long–term behaviour of four hybrid beams, Deskovic et al. (1995a). The time-dependent behaviours like shrinkage, creep and fatigue were studied by satisfying the compatibility and equilibrium of the section after redistribution of the strains and stresses. Two beams were subjected to a constant load for one day by hydraulic jack, and then the same two beams were subjected to fatigue cyclic loading.

Heavy concrete blocks were hung on another two beams for several years. The results showed that the time-dependent behaviour of the polymer matrix of the GFRP box section material is the only property needed to determine the structural creep response. The fatigue response was found to be dependent on the load levels, and a reduction of the section stiffness was noted under both creep and fatigue loading. The GFRP box section studied by Deskovic et al. (1995b) were prepare by filament-winding technique which differs from the pultruded GFRP box studied in this thesis. In addition, the concrete type used by Deskovic et al. (1995b) was regular concrete with an average compressive strength of 45.5 MPa, while type of concrete used in this research project was an UHPC.

The UHPC has more than three times higher compressive strength and a completely different stress-strain relationship than the ordinary concrete, as will be presented in the next chapter. Consequently, it can be expected than the capacity of the hybrid beam presented in this thesis will have higher strength, and should be more durable because of the presence of the UHPC.

Nordin and Täljsten (2004) studied the behaviour of a hybrid beam composed of a GFRP I-beam with a block of conventional concrete on top and a CFRP sheet on the bottom (Figure 1). The concrete was bonded to the GFRP I-section with adhesive epoxy in one beam while steel shear studs were used in the other. The 3 m beams were tested under four point loading configurations. The authors reported the flange-web connection as a potential point of weakness. Instability of the section occurred at the support regions in the first (control) beam, so wood stiffeners were added to subsequent beams. The behaviour of all the beams was linear, and the authors suggested simple elastic beam calculations for predicting the beam behaviour. The control beam had no concrete or CFRP sheet, and started to fail by buckling of the upper flange followed by instability at one of its supports at a load of 133 kN. The beam with steel shear studs failed at 275 kN (about double the load of the control beam) by crushing of the top of the concrete under one of the loading points. The last beam had the concrete cast in a separate form and was bonded to the GFRP I-beam after 28 days. Failure of this beam started by a crack in the concrete just above the bonding line, followed by tensile cracks in the lower part of the concrete before the complete failure of the beam at a load 292 kN.

Garden (2004) reported on the different economic industrial application of composite materials with concrete in construction. The author presented the results of experiments conducted on an 18.0 m full-scale hybrid FRP-concrete beam. The beam was comprised of GFRP 350X460 mm box sections with a 150×500 mm concrete slab cast on top . Four CFRP layers were embedded in the GFRP lower flange, and vertical timber rods were added every 500 mm as web stiffeners to prevent web buckling. The 18 m GFRP-CFRP beam weighed 150kg without the concrete. The beam was tested under four point loading, and failed by debonding of the concrete near the sides of the beam.

Separate adhesive failure between the concrete and composite occurred along the beam length. The onset of the failure was difficult to determine because of the sudden collapse of the beam. The records from the strain gauges at the beam mid-span confirmed that the plane sections remained plane throughout the test.

Figure 1: Cross-section of the composite beam (Garden 2004)

The behaviour of hybrid sections of FRPs and conventional materials have been studied widely in recent years in order to reduce the initial construction costs. Aref et al. (2005) and Alnahhal and Aref (2008) performed experimental and finite element analysis on a hybrid FRP-concrete structural system. The authors derived simplified design equations for their system based on the results of their analysis and experiments. The equations are thus only applicable to the specific hybrid bridge studied. The simply supported one-lane bridge has an 18.3 m single span, and a cross-section of three trapezoidal GFRP box beams with a concrete layer under the top flange (Figure 2.4). The dimensions shown are millimetres. Solid and shell elements were used in the three dimensional ABAQUS model.

In a different theme, Fam and Skutezky (2006) studied experimentally the behaviour of seven GFRP tubes connected to a high-early strength concrete slab using GFRP dowels. Five of the tubes were filled with concrete and two tubes were left hollow. One of the beams had CFRP laminate bonded to the bottom flange to increase the flexural capacity. The beam with the CFRP sheet was 31% stronger in flexure than the one without this layer. However, the authors found that the increase in the beam flexural stiffness was insignificant due to the very small CFRP reinforcement area. The composite beam failed by tensile fracture of the CFRP laminate, followed by an immediate fracture of the GFRP flange due to the sudden decrease in the section capacity. The progressive failure observed by Deskovic et al. (1995b) did not occur in this case. While concrete filled beams had much higher stiffness, the authors reported a small increase in the flexural strength of the concrete-filled beams when compared to hollow beams. In addition, three small shear samples were tested to assess the efficiency of the GFRP dowels as shear connectors in concrete-filled and hollow tubes. The dowels in the concrete filled tubes worked much better due to the good fixation of the dowel ends, which expose the stud to shear stresses only.

Keller et al. (2007) tested eight hybrid bridge decks as simple beams with spans of 3m. The section of the hybrid beam had FRP at the lower side and UHPC at the top and lightweight concrete in-between. The authors reported that the failure was brittle and the capacity almost doubled when the FRP was bonded to the wet light-weight concrete by epoxy adhesive. The unbounded beams relied only on mechanical interlocking between the FRP and light weight concrete, and exhibited a very ductile behaviour. There are also  many online dissertation services which can help you in writing a dissertation at the cheap  prices.

Correia et al. (2007) tested the behaviour of a hybrid beam composed of a GFRP I-girder and a reinforced concrete layer on top of it. The top concrete was connected to the GFRP girder by steel shear studs. Two simply-supported beams were tested with spans of 4 mind 1.8 m to assess the flexural and shear behaviour of the proposed section. The results were compared to those of a GFRP I-girder alone. The long beam failed suddenly by crushing of the concrete associated by longitudinal interlaminar failure of the web of the I-girder, while the GFRP I-girder failed by local buckling when tested without concretion top of it. The flexural stiffness and strength of the hybrid beams were increased by about 350% and 300% respectively, over the values for the GFRP I-girder alone. The short beam also failed due to shear without warning, with the shear failure occurring at the top of the web of the GFRP I-girder where the top concrete and the top flange separated from the hybrid section. In a different study, the authors investigated the option of bonding the reinforced concrete slab to the GFRP I-girder using only an epoxy adhesive (Correia et al. 2009a). The beams constructed with epoxy adhesive failed by debonding between the upper concrete and the lower GFRP I-girder. One of first beams failed by web compressive failure at the support, so web stiffeners were added at the supports of future beams to avoid this premature failure mechanism. The authors then tested two hybrid multi-span beams with the upper concrete bonded to the lower I-girder by epoxy adhesive only (Correia et al. 2009b). One of the two new beams was simply supported with double cantilevers, with the load being applied at the ends of the cantilevers. This beam was subjected mainly to negative moment and failed by lateraltorsional buckling of the GFRP I-girder (Figure 2.6) at a load corresponding to the ultimate compressive capacity of the GFRP materials. Good bond was achieved between13the concrete and the GFRP I-girder, in the sense that there was no deboning before failure – probably because of the low failure load of this beam. The second beam was continuous with two 2.8 m spans. Each span was loaded by one concentrated load. The beam failure was initiated by deboning between the concrete and the GFRP I-girder followed by local buckling of the top GFRP flange, then concrete crushing.


Figure 2: Lateral torsional buckling for double cantilever hybrid beam (Correia etal. 2009b)

Alnahhal et al. (2008) studied experimentally and analytically the behaviour of a hybrid FRP-concrete bridge deck on steel girders (Figure 2). The deck was composed of GFRP trapezoidal cells with a thin layer of concrete in the top zone. The GFRP offered environmental protection for the concrete, while the concrete supported and enhanced the stiffness of the GFRP laminates. GFRP shear studs were installed in staggered positions and concrete was cast at the shear key locations. The results showed that the structural performance of the section exceeded specifications of the American Association of State Highway and Transportation Officials (AASHTO). The authors noticed that the hybrid deck was interacting with the steel girders in a partially composite action under service loading conditions.

Several authors investigated different hybrid sections of FRP to overcome the low stiffness of FRP materials and make use of its light weight. Bank et al. (2009) studied the possibility and application of hybrid FRP sections to act as a stay-in-place form for the concrete slab. Two hybrid sections were presented, one with timber planks and FRP, and the other had FRP with mechanically fastened cement boards or cast-in-place concrete.

Mufti (2003) presented the possibilities of using multi-cell FRP sections filled with concrete to act as bridge deck, and showed several Canadian projects where FRP was15applied successfully. Wu et al. (2006) tested a reinforced concrete beam with CFRP sheets bonded to the lower side of the beam, and the beam then wrapped with GFRP sheets to aid in load transfer to the FRP reinforcement through shear. The concept was also used mainly to raise the capacity of a traditional section by the FRP sheets (Sayed-Ahmed et al. 2009).

Figure 3

2.3 Bond strength of FRP laminates

Good bonding is essential for successful composite action in hybrid sections. Therefore, special emphasis was placed in the current project on the bond between the GFRP and the wet UHPC, and between the SRP sheet and the GFRP section. Many researchers have16studied experimentally and/or analytically the bond between FRP and other materials. Forexample, Oehlers et al. (2003) recommended different structural detailing approaches tobe considered in order to avoid deboning mechanisms. Shao et al. (2005) investigated the wet-bonding between dry FRP laminates and wet cast-in-place concrete through several pullout tests. The authors compared the results from wet-bond samples with the more conventional dry-bond – that is between FRP and matured dry concrete. The behaviours and results of the two bonding cases were very close, and the authors suggested that appropriate bond can be achieved by casting wet concrete directly onto composite FRPs through an epoxy adhesive.

Camli and Binici (2007) tested experimentally the strength of CFRP sheets bonded toconcrete prisms and hollow clay tiles using a double shear push-out test setup. The results of the tests revealed that embedded anchors acting as shear connectors doubled the bonding strength. The bond strength was more affected by the anchorage length and width than by the concrete compressive strength. When the results of experiments on specimens with and without plaster were compared, the presence of a plaster finish was observed to reduce the bond strength by two-thirds. Finally, a slight improvement was noticed in the bond strength when the ends of the CFRP laminates were spread up to 30°.The bond strength of concrete to GFRP plates was determined through double shear and bending experiments, and from the results, Xiao et al. (2004) recommended that the effective bond length for a direct shear test is 100 mm while for flexural bond of the FRP plate in the negative moment zone to be about 300 mm.

Bakay et al. (2009) studied experimentally the effect of concrete compressive strength and the area of steel shear reinforcement on the deboning of CFRP laminates from the bottom of flexural reinforced concrete beams. The authors also investigated the strain distributions and the compatibility between the strains in the CFRP laminates and the concrete. The authors then proposed design procedures for strengthening flexural members with FRP laminates.

On the theoretical side, Yuan et al. (2004) derived a closed-form expression for calculating the interfacial shear stress distribution and load-displacement response at low and high loads. Jianguo et al. (2005) studied an analytical model for defining non-linear bond stress-slip models of FRP-concrete interfaces. Pullout tests were also conducted using different FRP materials and stiffness’s, and with different adhesives. The results from the analytical model showed good agreement with the experimental data In predicting the non-linear shear behaviour at the FRP sheet-concrete interfaces.

Closed-form solutions for determining the elastic shear and peel stresses in the bonding area between a strengthening plate and a beam were investigated by Stratford and Cadei(2006). The methods included the loading and temperature effects in addition to plate pre-strains, and can be used in designing a strengthening system for beams. The procedure allows for a variable plate area along the beam length, and can determine the sensitivity of an adhesive joint to bond defects.

Ferracuti et al. (2007) developed a procedure for deriving the non-linear mode II interface law for FRP-concrete bonding based on previous experimental results. The formula included the non-linear contribution of the epoxy adhesive and the concrete cover at high shear stresses considering pull-pull delaminating tests. The authors applied a numerical simulation for a bond-slip model, and obtained good agreement with the experimental results for the FRP strains, shear stresses and slips in the bonded zone. You may also be  interested in getting help from online dissertation writing services if you find any difficulty in writing a dissertation on hybrid beams. 


Sayed-Ahmed et al. (2009) reviewed the different modes of deboning between FRP laminates and reinforced concrete flexural members, and presented the currently accepted models for predicting the bond strength between concrete and FRP laminates.

2.4 Mechanical and fatigue behaviour of high performance materials

Mirmiran et al. (1999) tested five concrete-filled FRP tubes under axial and transverse loads to develop a moment-thrust interaction diagram for the hybrid column. Relative slippage between the FRP tube and the filling concrete was prevented by using internal shear connector ribs. The filled tubes were as strong as a conventional reinforced concrete column with a 6% reinforcement ratio. The failure of the filled tubes was ductile, and Euler-Bernoulli beam theory was applicable with the fully developed composite action between the concrete and the FRP tube.

Ma and Schneider (2002) presented the behaviour of a modified UHPC where the cementer silica fume was partially replaced by fine quartz powder. A self-compacting UHPC with compressive strength of 155 MPa was produced without heat treatment. A very good fire resistance of Ductal® UHPC was measured by Behloul et al. (2002) for different structural shapes (shells, girders, columns).The high performance and good characteristics of the reactive powder UHPC supported its application in highway bridges. Vernet (2003) performed a micro structural and durability assessment of the UHPC and found a high resistance to aggressive media. The author attributed this good durability to three factors: the high stability of the hydrates, the very low connectivity of the pores at the nanometre level (which prevent the aggressive elements from diffusing in the matrix), and the self-sealing mechanism by the residual clinker hydration (where the reaction of the residual clinker can rapidly heal some micro-cracks). One of the reasons for the good mechanical and durability characteristics of the UHPC is its high density, which even suggests UHPC as a candidate for storage of nuclear wastes (Matte et al.1998).

In order to take a step towards applicable design guides for Ductal® reactive powder UHPC, Chanvillard and Rigaud (2003) studied the tensile properties of the material according to the French recommendations. The authors investigated the first crack strength and the post-crack resistance. Resplendino and Petitjean (2003) highlighted the main features of French recommendations for applying the UHPC. The durability of the UHPC was also confirmed by Graybeal and Hartmann (2003b) with their UHPC samples showing good resistance to rapid chloride penetration, freeze-thaw and scaling. Experimental results by Graybeal and Hartmann (2003a) indicated that flexural fatigue of the untracked concrete matrix is unlikely to be a design controlling factor, but fatigue of the fibres may occur after cracking. The results of Stiel et al. (2004) showed that the flexural strength of horizontally-cast CARDIFRC UHPC beams is five times higher than for vertically-cast beams because the alignment of the fibres was normal to the casting direction. The authors also pointed that the fibre orientation was affected by the size of the cast specimen and the mix workability.

Acker and Behloul (2004) also discussed the potential of the Ductal® UHPC in structural elements and presented results indicating that this material has very little creep and shrinkage compared to normal concrete. The authors indicate that the creep coefficient for thermally treated UHPC is about a quarter of that of UHPC not thermally treated, and the pressurising losses can be substantially reduced. The Ductal® also did not exhibit any drying shrinkage because of the very low water to cement ratio. The superior mechanical20characteristics of UHPC have encouraged many researchers to explore applications indifferent structural elements. Graybeal et al. (2004) presented a summary of the investigation sponsored by the United States Department of Transportation in order to usethe UHPC in highway bridges. The Ductal® UHPC also has excellent durability and verygood resistance to chemical attack and ageing. Pimienta and Chanvillard (2005)demonstrated the excellent durability of the UHPC through a series of experiments oncracked UHPC specimens. Some of the specimens had steel fibres while others had organic fibres. The aggressive environments for the specimens were hot water, sodium chloride solution and wet-dry cycles.

Zhou et al. (2005) studied the performance of cellular FRP bridge deck systems by studying the deck stiffness, strength and failure mode. The authors used two methods of deck contact loading: a steel patch according to the AASHTO Bridge Design Specifications and a simulated tire patch constructed from a real truck tire. The authors noticed that the simulated tire load developed greater global deflections under static loading, and the failure mode was localized and dominated by transverse bending failure of the composites, rather than the punching shear observed for the steel patch load. The authors also tested the field performance of the bridge deck under service conditions, and found no significant loss of deck capacity after more than one year. Dissertation Writing UK is the leading name in writing dissertations on hybrid beams

The material properties of the UHPC were extensively studied under the supervision of the Federal Highway Administration (Graybeal 2006a) in addition to the structural behaviour of the UHPC girders (Graybeal 2006b).

In another study, Graybeal (2007) examined the compressive behaviour of both treated and steam-untreated UHPC. The author analyzed the stress-strain relationships and21presented equations for predicting the strength gain with time and the modulus of elasticity as a function of the compressive strength. Farhat et al. (2007) studied the performance of CARDIFRC UHPC, and got excellent fatigue and shrinkage results because of the even distribution of the fibres throughout the material. The authors also retrofitted a damaged conventional concrete beam with a thin bottom layer of the UHPC. The retrofitted beam was subjected to thermal cycles and no deterioration was recorded for the bond or the strength of the UHPC layer.

Shaheen and Shrive (2007) tested Ductal® reactive powder UHPC prisms (40x40x160mm) under bending fatigue. The rate of crack propagation created by the cyclic fatigue loading was monitored, and the cracks were studied with a high magnification microscope. The results showed that the material has excellent resistance to low- and high-strain cyclic fatigue load even in the presence of a crack, and showed that the Ductal® UHPC is tougher than other concretes. Farhat et al. (2007) reported as well that the distribution of the fibres within the concrete increases the fatigue endurance limits of the UHPC significantly.

Static and fatigue tests were performed by Parsekian et al. (2008) on a UHPC girder. The Ductal® UHPC used had a compressive strength of over 200 MPa. A girder with only steel fibres and no reinforcement bars was tested under fatigue loads. The girder showed high fatigue resistance with no degradation in stiffness being observed.

Ahmad et al. (2008b) tested eight FRP cylindrical beams filled with concrete under dynamic fatigue loads. The authors reported enhanced fatigue life for samples with higher reinforcement ratios and for samples with end restraints that limited the slippage of the concrete core. The authors concluded that the fatigue behaviour of the22beams was governed by the characteristics of the FRP tube, and recommended a maximum load level of 25% of the static capacity to be the fatigue design limits.

Ahmad et al. (2008a) then studied the behaviour of short and deep concrete-filled FRP tubes. None of the beams tested failed by shear. The concrete core started to slip relative to the FRP tube after the first flexural crack. The deep beams had higher bending capacities due to the arching and strut action.

Helmi et al. (2008) tested GFRP coupons under static and dynamic loads. The coupons were cut from hollow GFRP tubes, and were subjected to either tension or tension compression loads. The authors found that the fatigue life was reduced when the load23included a compression part, and also with decreasing the load frequency. The authors recommended that the maximum stress to be limited to 0.25 of the ultimate strength of the FRP in order to achieve at least one million cycles.

Gautam and Matsumoto (2009) studied the shear deformation of CFRP box beams with some of the beams filled with concrete. The authors concluded that the contribution of shear deformation in the beam deflection was up to 60%, and attributed this high percentage to the low shear modulus of the orthotropic CFRP material. The authors also did not find any significant effect of the filling concrete on the shear load, and found that almost all the shear load was transferred through the CFRP beam. The beam stiffness was doubled in the beams where the filling concrete was bonded to the CFRP beam compared to the case of no bond.

The preceding studies demonstrate the superior durability of the new high performance materials. However, no-one has yet examined hybrid beams made of combinations of these materials. There have been studies where some high performance materials have been used in the combination studied here, but there is no study as yet where the complete beam has been fabricated from high performance materials. Reports on tests on hybrid beams frequently indicate a brittle manner of failure. Thus, different sections were designed here to study the possibility of controlling the brittle failure of the hybrid beams by changing the dimensions of the elements and consequently the failure mode. 


3.1 Introduction

Many different high performance materials were used in the research work described in this thesis. All the materials used in the research have superior strength and durability compared to conventional construction materials. As such, the materials can be utilized in much smaller dimensions than sections constructed from traditional materials. The first element in the hybrid section was the ductul.  UHPC which carried the compressive stresses. The UHPC was used in a small layer, about 50 mm deep, on top of the beam because of its high strength, and contributed about quarter of the total stiffness of the section. The second component was a pultruded GFRP box section, which contributed about sixty percent of the total stiffness of the section. The GFRP box section had many functions: limit the deflection of the beam, carry the shear stresses, carry most of the tensile forces by the lower flange, act as a permanent stay-in-place form for part of the concrete, and finally connect the top concrete layer with the lower sheet under tension. Unidirectional CFRP sheets or SRP strips were used to carry some of the tensile forces on the lower side of the GFRP box section. Several adhesive epoxy types were used to connect the components together in addition to 9.5 mm GFRP shear studs between the concrete and the box section. Experimental tests were performed on the different materials to confirm the manufacturer-provided mechanical properties. The stress-strain behaviours of the materials are presented in Figure 3.1. The stress-strain relationships were obtained from tests performed on specimens of the actual materials used, except for Sika CFRP fibres and laminate whose relationships were based on the manufacturer’s supplied data sheet (Sika 2007d). No tests were done on the Sika CFRP sheets because25the results of the tests done by the manufacturer were well reported. The UHPC was tested under compression loads, while the FRPs were tested in tension. The strains of the UHPC were plotted as positive values in Figure 3.1 to allow easy comparison with the other materials.

3.2 Properties of the Materials

The constituent materials formulated to make the UHPC include Portland cement(Lafarge Ductal BS 1000.1006 #41296642), silica fume, quartz flour, fine silica sand, a high range water reducing admixture based on modified polycarboxylates (CHRYSO®Fluid Premia 150) which also protects from freezing (Chryso 2010), water, and high26carbon steel or organic (poly-vinyl alcohol) fibres (Figure 3.2) (Lafarge 2007). Some of the available UHPC products on the market can reach compressive strengths more than200 MPa and flexural tensile strengths of up to 50 MPa (Lafarge 2007). In order to reach the maximum strength, the UHPC should be steamed at 90 oC and 95 percent relative humidity for 48 hours (Graybeal 2006a). This standard manufacturer-recommended curing treatment includes two hours of increasing steam and two hours of decreasing steam, and should be initiated within four hours after demolding. In this research project, none of the samples were treated thermally according to the standard treatment procedure because that would have affected the bonding efficiency of the adhesive epoxy based on the recommendation of the manufacturer of the epoxy ((Sika 2007b), (Sika 2007c)). The type of UHPC used had high-carbon-steel fibres and reached a compressive strength of170 MPa after 28 days, about 75% of the strength which could have been achieved with thermal treatment of the UHPC. The UHPC has a very low shrinkage ratio because of its very low water-to-cement ratio, and no post-treatment shrinkage (Graybeal 2006a). The distribution of the fibres within the concrete matrix also increased the fatigue endurance limits of the UHPC significantly as reported by Farhat et al. (2007). Shaheen and Shrive(2007) tested UHPC beams experimentally under cyclic fatigue loading: the results showed that the UHPC material has an excellent resistance to low and high cyclic fatigueload even in the presence of a crack. In addition the fatigue tests performed by Parsekianet al. (2008) on an UHPC bridge girder demonstrated high fatigue resistance with nodegradation observed in the girder stiffness, even after two millions loading cycles.

Compression tests were performed on cylinders (Figure 3.3) of the UHPC cast from eachof the batches used in the beams and on eight prisms (Figure 3.4) extracted from thebeams after they had been tested. Typically, the cylinders of the UHPC were 75 mm indiameter and 150 mm long, which is smaller than the conventional concrete cylinders. The smaller UHPC cylinders ensured that the cylinders could be tested to failure in compression without exceeding the capacity limits of the testing machine. The UHPC prisms were cut from the actual tested Beam 1 using a water-jet to a length of 100 mmand square cross-section of 50 mm side length. The compressive strengths of the UHPCprisms and cylinders were not the same because of the differences in the shapes of the specimens and the number of days between casting and testing the specimens. Tables 3.1and 3.2 summarize the results of the compressive tests on the UHPC specimens. The average compressive strength of the UHPC cylinders for Beams 1 and 2 after 28 days was 185.8 MPa with a standard deviation of 1.34 MPa. Four of the eight prisms extracted from the beams after testing represented the compressive strength of the beams in the longitudinal direction, while the other four prisms had the 100 mm side parallel to the lateral direction of the beams. 60 days after casting the UHPC, the average compressive strength and standard deviation for longitudinal prisms were 163.4 MPa and 4.16 MPa, respectively, and 155.3 MPa and 8.49 MPa for the transverse prisms. The P value of atwo-tailed Student’s t-test on the longitudinal and transverse prism strengths is 0.1385, so the small difference in strength cannot be considered statistically significant. The P value assumes that each set of samples has the same mean as the other set, and the observed difference is just a coincidence of random sampling. The 0.1385 P value can be interpreted that if both the longitudinal and transverse prisms have the same compressive29strength, 14% of the samples will have a larger difference than that observed in the experiments. In addition, the 95% confidence interval of the difference between both standard deviations ranges from -3.495 to 19.645, which means that there is 95% chance that the true difference between the longitudinal and transverse strengths will be included in this range. As a result of this information, the UHPC was taken to be a homogenous material in the finite element modeling of the beams.


3.2.2 Properties of GFRP box section

Based on the results of ASTM tests provided by the manufacturer (Strongwell 2007), theminimum ultimate mechanical properties for the hollow box GFRP coupon material areas follows: the longitudinal tensile and compressive strengths are 207 MPa; the transverse tensile and compressive stress capacities are 48.3 MPa and 103 MPa, respectively; thelongitudinal tensile and compressive modulus of elasticity is 17,200 MPa; and thetransverse tensile and compressive modulus is 5.52 GPa. The manual of the manufacturermentioned that these values are the minimum achieved values, and that the mechanicalproperties of the GFRP box material shall meet or exceed these values. The GFRPmaterial has a specific gravity of 1.7, and coefficient of linear thermal expansion of1.2×10-5 mm/mm/Co. Tension tests were performed on strips extracted from the GFRP box section in both the longitudinal and transverse directions using the water-jet cutter. The GFRP strips are shown in Figure 3.7, where the shorter strips are from the transverse direction, and the longer strips from the longitudinal direction of the box. The steel stubs through which the specimens were gripped, were 25 mm wide by 100 mm long, with thelength of the GFRP specimens between the steel stubs being 100 and 190 mm for the transverse and longitudinal specimens, respectively. This latter is the direction oforientation of the glass fibres. Two of the longitudinal strips were taken out from thelower flange of the box section after Sika CFRP sheets were bonded to it. The results ofthe GFRP with the bonded CFRP sheet can help in understanding the percentage of contribution of the lower CFRP sheet to the total tensile strength and stiffness of the lower GFRP flange. Each GFRP strip has a 15 mm width and has the same 11.11 mm thickness as the wall thickness of the hollow box section. Some of the samples were34instrumented with additional strain gauges in the transverse direction to confirm the 0.33Poisson’s ratio reported by the manufacturer.

GFRP strips taken from the box beam

Figure 4: GFRP strips taken from the box beam

The GFRP strips are shown in Figure 3.8 during a tension test and the associated failed specimens. The GFRP strips failed by tensile rupture of the specimens at locations veryclose to the steel grips. This failure location suggests possible premature failure because of the stress concentration due to the gripping of the specimens during the tension test. The GFRP strips were instrumented with longitudinal and transverse strain gauges. The stress-strain curves for the GFRP samples are shown in Figure 3.9, and Table 3.3 lists the failure loads for all the samples. The stress-strain curve of the GFRP strip which had a35bonded CFRP sheet shows some slight sudden drops with an associated small change in slope along its path. The changes can be due to the straightening of the CFRP sheet at some locations because the sheet was laid up by hand on the GFRP box and cannot be considered perfectly straight as the glass fibres in the pultruded box. The average modulus of elasticity for the GFRP material was 26 GPa and Poisson’s ratio of 0.33. Thethickness of the two CFRP sheets (0.762 mm combined) is much smaller than the GFRPsection (11.11 mm). The results of the GFRP strips with the bonded CFRP sheet were 15and 13 percent higher than for the GFRP alone in terms of the elastic modulus and tensilestrength, respectively.

3.2.3 Properties of GFRP shear studs

From the previous investigation by Deskovic et al. (1995b), it was determined that using adhesive epoxy only for bonding the concrete to the GFRP sections may not provide sufficient shear strength. Debonding failure was reported to occur at stresses much lower than expected, which suggested that premature Mode II fracture might initiate allocations of epoxy discontinuity in the bonding area. Mode II fracture failure is possible when the shear forces which cause the failure are in the same direction as the direction of propagation of the crack. The discontinuity spots act as flaws which initiate crack propagation at low stresses. All the beams studied in this research thesis had GFRP shear connectors in addition to the moisture-insensitive epoxy to guarantee full composite action between the UHPC and the GFRP box section. Five double shear tests (Figure3.11) were performed on the 9.5 mm diameter GFRP studs, and the ultimate single shearvalues were 10.965, 11.695, 8.495, 8.860, and 9.845 kN. The average ultimate singleshear capacity for each stud was 9.97 kN and a standard deviation of 1. 36 kN, and 10 kNis the value considered in the design and analysis of the hybrid beam (Appendix A)

3.2.4 Properties of the CFRP sheets used in Phase I

Based on the technical data sheets provided by the manufacturer, Sika Canada Inc., the ultimate tensile strength of the unidirectional CFRP fibres (Figure 3.12) is 3,450 MPa, the Modulus of Elasticity of the fibre sheet is 230 GPa and the elongation at failure is greater than 1.5%. From the results of ASTM D3039-00 (ASTM Standard 2002) tests done on0.381 mm thick unidirectional CFRP laminate coupons with a moisture-tolerant epoxy (SikaWrap® Hex 230C with Sikadur® 330 epoxy), the manufacturer reported that the average ultimate tensile strength is 894 MPa, the modulus of elasticity is 65.4 GPa, and the average elongation at failure is 1.33% (Sika 2007d).Figure 3.12: Sika CFRP sheet

3.3 Bond Tests

Eighteen UHPC samples were prepared and tested according to the Japan Society of CivilEngineers (JSCE) standard E 543-2000 (JSCE Concrete Engineering Series 2001). Thistest aims to assess the efficiency of bonding concrete to FRP. Each test specimen consistsof two concrete blocks each 100x100x250 mm. The two blocks of each test specimen are connected only by two similar FRP strips bonded to two opposite faces of the concrete46blocks by the same epoxy adhesive. The bonding area of each FRP strip to each concrete block has a width of 50 mm and length of 200 mm. The two concrete blocks are pulled away from each other during the test through two steel bars embedded in each concrete block. A comparison between the debonding failure loads of the different specimens can evaluate the efficiency of the bonding between the concrete and the FRP.

Twelve UHPC prisms were cast initially and used the moisture-insensitive epoxy, and after they were tested six of them were sanded and re-prepared with the moisture-tolerant epoxy. The outside dimensions of the UHPC for each sample were 100×100×500 mm. Each sample was divided into two equal parts which were separated completely by a 3mm thick wood sheet. Two threaded rods, 25.4 mm diameter, of mild steel were embedded in each sample, one in each part, aligned with the longitudinal axis of the sample so the sample could be gripped during the tension test. The tension force in each rod during the bond test was expected to be about 45% of the ultimate measured tensile capacity for the threaded rod (202 kN). Three nuts were inserted on each rod to increase the resistance of the rod against slip during the pull-out test (Figure 3.18). 


4.1 Introduction

Phase one included the experiments and analytical study done on the beams constructed with section 1. The dimensions of section 1 are shown in Figure 4.1, where the 127 mm lower tensile sheet is two sheets of Sika CFRP on one beam and one sheet of SRP on the other beam. After the long beams had been tested, short sections were cut from those beams to constitute some of the short beams. Long and short beams were tested to investigate the flexural and shear behaviour of the beam cross-sections, respectively. The spans of the beams ranged between 500 and 6000 mm. Seven tests were performed: five beams were tested under static loading with both long and short spans to determine the flexural behaviour and shear strengths; and one beam was tested under fatigue loading toassess the fatigue behaviour of the beam. One of the long beams was quasi-statically tested twice because of incomplete first test.

Figure 5: Cross-section for beams of phase one


4.3 Experimental Results

The first tested beam (Beam 1) had one layer of SRP sheet bonded to the lower flange to work as the tension reinforcement. The relatively thin bearing plate at the supports of Beam 1 (Figure 4.7) caused stress concentration and resulted in failure through web crippling at one of the supports (Figure 4.8) at a load level about 80% of the estimatedsection flexural capacity. Therefore, thicker bearing plates were used for Beams 2 and 3(Figure 4.9) to avoid the premature crippling failure.

Beam 2 had two layers of Sika CFRP sheets bonded to the lower flange. The failure ofBeam 2 occurred by crushing of the UHPC at one of the loading points (Figure 4.10),90which was immediately followed by delamination between the concrete and the GFRPbox section. Once the top UHPC layer failed, the high compressive stresses which werecarried by the UHPC transferred to the GFRP box section. The GFRP box section couldnot sustain the high level of compressive forces, and the upper portion of the GFRP boxbuckled (Figure 4.11) and the beam failed immediately. The sudden failure occurred at the section where maximum shear and normal stresses developed. It is worth mentioning that the failure was located at the middle of the loading plate. The delamination of the UHPC after the failure of Beam 2 is shown in Figure 4.12, where a thin layer of Ductal®can be seen still bonded to the top flange of the GFRP box section. The nature of the delamination failure proves the excellent wet-bonding efficiency between the UHPC and the top-flange of the GFRP box section. The lower flange of the GFRP box section did not fail or rupture at the failure location because the beam failure was initiated at the top of the beam.


Chapter Five: Analytical Programme of Phase One

5.1 Introduction

An analytical study was performed on the beams tested in phase one in order to study the behaviour of the beams, and to give a better idea about the distribution of stresses in thedifferent section components. A good understanding of the behaviour of the hybrid section should help in designing a more efficient section with better utilization of its components as outlined in Appendix A.4. Strain compatibility analysis of the beam mid-span sections was studied under flexure (Appendix A.1). In addition, Finite element analysis was completed on two long beams using a commercial program.

5.2 Strain Compatibility Analysis

A section strain compatibility analysis method was used to predict the behaviour of thehybrid section (Appendix A.1). The main assumptions adopted in this analysis were:

  • The transverse plane section remains plane before and after the bending. The beam is considered to be sufficiently long to provide a span to depth ratio for Euler-Bernoulli beam theory to be applicable. The strain at any fibre in the section isdirectly proportional to the distance of the fibre from the neutral axis of the section. The failure of the section can be predicted from the maximum-strain failure criterion. This means that the material whose strain will reach its maximum failure strain will fail first and will govern the section capacity.
  • The materials used in the section can have either linear or non-linear stress-strain relationships. Examining the possibility of getting accurate results from considering all the materials to have linear behaviours is emphasized. If all the materials used122have linear elastic relationships and there is perfect bonding between them, then the whole hybrid system will have linear behaviour.
  • Full bond exists between all the section components. The GFRP shear studs is assumed to help reduce the shear stresses in the epoxy along the UHPC-GFRP interface and thereby to reduce the effect of the known non-linear behaviour of the epoxy at high shear stress levels (e.g. Jianguo et al. (2005) and Ferracuti et al.(2007)).
  • The beam section is narrow enough to consider that shear stresses are uniform across the width of the section.
  • The effect of the presence of the shear studs was neglected as the studs are embedded in the concrete and their volume is small relative to the concrete volume.
  • The effect of the holes which were drilled in the top flange of the GFRP box was neglected because they were filled with GFRP studs.
  • The thickness of the epoxy between the UHPC and the GFRP hollow box was very small and was neglected.
  • The length of the beam is short enough to ignore the effect of shrinkage of the UHPC.
  • The fillets at the corners of the GFRP section have an inner radius of about 1.6 mm and an outer radius of 12.7 mm which is small relative to the thickness of the box section (11.11 mm), and were ignored in the analysis.

5.3 Finite Element Modeling of the Beams

A three-dimensional linear finite element analysis was performed for both Beams 1 and 2using a commercial finite element program (ABAQUS 6.7). The finite element models were built to represent the actual beam geometry and materials (Figure 5.10), including the GFRP box fillet edges. The mechanical properties of the different materials were based on the results from the experiments described in chapter three. The modulus of elasticity of the UHPC was taken as 46 GPa, the GFRP box section was taken as 23.8GPa, the SRP sheet 66.2 GPa, and the Sika CFRP sheet 65.4 GPa. A linear analysis was performed for the beams because the experiments on the materials showed that the stress strain relationships for all materials were almost linear up to failure. 4-node linear quadrilateral shell elements were used to model both the SRP and CFRP sheets, while 8-node linear hexahedron brick elements were adopted in the modelling of the GFRP box section and the UHPC. The UHPC was considered homogeneous, and modelled as anelastic isotropic material. Hour glassing control was used in the analysis, and the running time of the three dimensional problem was decreased by using the reduced integrationtechnique. Hourglassing is a numerical problem caused by displacement modes where astatus of zero strain or stress is created at the integration points. The number of hour glassing modes generally increases as the number of integration points decreases.The loads and boundary conditions were distributed over a number of adjacent nodesalong the transverse direction. The elements had a uniform mesh size of about 10 mm. Part of the meshed model for the hybrid beam is shown in Figure 5.11. A mesh sensitivity test was performed to ensure that the model has an adequate number of elements and that the results are not mesh dependent. The results of smaller sizes for the elements did not131differ than the results presented here. The GFRP shear connectors were neglected in the model. Full bond was assumed along the entire interacting surfaces of the different materials because it was assumed that the GFRP studs lower the shear stresses in the epoxy to avoid the non-linear effect. 


5.1 Introduction

The research work presented in this thesis investigates the behaviour of an UHPC-FRP hybrid beam. Since all the used materials have almost linear stress-strain relationships, different failure scenarios were investigated in order to determine the safest failure mode with the least brittle failure. Some beams were designed to fail by crushing of the UHPC, while others should fail in the tension side. The failures at the tension side were due to either the tensile failure of the bonded CFRP sheets or the tensile failure of the lower flange of the GFRP box section. The early warning signs of failure were closely watched in each case before the complete collapse of the beam.

Both long and short beams were tested to study the flexural and shear behaviour of the hybrid beams. In addition, two beams were exposed to cyclical temperature and humidity environmental conditions to assess the endurance of the hybrid section. Finally, five beams were tested under fatigue loading to study the behaviour of the hybrid beam under cyclic loading.

7.2 Conclusions and Recommendations

The experimental testing of long and short hybrid beams has been performed together with analysis of the behaviour. The work undertaken helps to provide better understanding of the flexural and shear behaviours of the hybrid beam. The new knowledge can be used for future predictions of the behaviour of similar beams and for designing composite beams from the same materials after allowing for the appropriate safety factors. Based on the results of this study, the following conclusions can be drawn:

  • Adding UHPC to the compression side of GFRP box sections increased the capacity of the GFRP box sections significantly.
  • Composite action between the UHPC and the GFRP box section can be ensured using GFRP shear studs and moisture insensitive epoxy, and can be modelled by considering full contact bond between the interacting surfaces of the two materials.
  • None of the tested hybrid beams failed by debonding of lower tensile SRP/CFRPsheets. This can mainly be attributed to the presence of the steel bearing plate under the sheets, while normally in rehabilitation of existing girders the sheets are bonded in the clear span between the supports only.
  • The linear behaviours of GFRP, CFRP, SRP, and UHPC materials of the hybrid beams permit the use of linear modelling and analysis. Linear finite element analysis was found to be able to predict effectively the flexural behaviour of the similar hybrid FRP-UHPC beams up to failure.
  • A simple analysis technique and a linear finite element analysis were found to predict effectively the flexural behaviour of the hybrid FRP-UHPC beams. Linear analysis can be used in designing composite beams from the same materials after allowing for appropriate safety factors.
  • Adequate end blocks should be used at the supports to ensure the load path will notgo through the weak corners of the GFRP box section.
  • The proposed hybrid FRP-concrete beam assembly showed acceptable lateraltorsional stability, and none of the LSCT that were measuring the horizontal displacement of the beams recorded any significant side shift in any of the beams.
  • Drilling the top flange of the GFRP box section and installing the shear studs was ignored in the finite element modelling of the beams, and created no significant difference between the analytical and experimental results.
  • The stiffness contributions of the lower tensile SRP/CFRP sheets to the overall stiffness of the hybrid beam are very small, and the presence of the lower sheets can be neglected when determining the capacity of the beam unless the failure is in the tension side.
  • Wet bonding of UHPC to the GFRP box section using moisture insensitive epoxycan provide excellent bond, as a linear strain distribution was measured experimentally along the mid-span section and confirmed from the results of the finite element analysis. The bonding of wet UHPC to the GFRP box section can be used and satisfy the requirement of many projects.
  • When the strain gauges were bonded to solid and sanded UHPC surface, the straingauges remained fully functional and accurately recorded the compression strains up to loads close to the failure loads due to the fine nature of the UHPC constituents and the absence of coarse aggregate.
  • Both the recorded experimental values showed that the strain distribution across thedepth of the proposed hybrid FRP-concrete beams remained linear until failure of the beams. This confirmed the assumption that plane sections remain plane before and after bending.
  • The non-linearity of the properties of the materials had a minor effect in the calculation of strains and deflection of the hybrid beam. Consequently, linear186modelling of the hybrid beam should be sufficient to predict the behaviour of the hybrid beam.
  • The failure of the tested long hybrid beams was sudden and did not show any deformability when the UHPC governs the hybrid beam strength and the failure initiates at the top of the beam. This can be avoided by increasing the deformability through altering the tensile reinforcement ratio.
  • The fatigue resistance of the hybrid section is governed by the fatigue limits of the GFRP box section. The fatigue failure loads were much lower than the flexural and shear capacity of the hybrid beams, therefore the design capacity of the hybrid beams will be governed by the fatigue limits of the GFRP box section.
  • The beams that were subjected to many fatigue cycles at low loads failed under less number of cycles at higher loads, and the rate of loss of stiffness increased significantly at the higher loads.
  • When the cyclic loading starts, the deflections and strains of the hybrid beams were noticed to increase a little in the first few hundred cycles of fatigue loading, and then started to stabilize till the load increased. No physical signs of failure were noticed in the beams until failure of the GFRP box occurred at the end of the fatigue tests.
  • All the materials showed good resistance to the environmental cycles.
  • The epoxies used here were more effective in bonding CFRP plate than SRP sheet, the failures were observed either within the CFRP plate or the SRP-epoxy interface in the bond experiments.
  • The epoxies bonded to the CFRP plate better than to the UHPC, as there were nofailures along the CFRP-epoxy interface but there were some along the UHPC epoxy interface.
  • For good bond, the surface of hardened UHPC should be sandblasted before the epoxy is applied. If bonding is to be performed on dry UHPC, it is highly recommended that the surface be sand blasted before applying the epoxy.
  • Clamping one end of a sample in the pull-out test is adequate to ensure failure at the other end.
  • The strain values in the bonded laminate depend mainly on the modulus of elasticity of the laminate.
  • The strain along the length of the CFRP and SRP laminates drops significantly after100 mm for dry bonding, as also noted by (Xiao et al. 2004). This is not the case for wet bonding, where the strain is lost more gradually along the length of the specimen. The difference in strain distribution may be attributed to the fact that the epoxy penetrated the outer layer of the UHPC in wet bonding, so the effective thickness of the bonding layer is higher than with the dry bonding.
  • The SRP cords are assembled by a silicone mesh on one side of the laminates. The bond strength does not appear to be affected by the side of the SRP sheets used for bonding.
  • The measured values of the bond strength for these materials can be acceptable formany strengthening projects of FRP pultruded sections or UHPC elements, where the expected shear stress should be lower than that recorded (3.5 MPa for the materials used in this research) with an acceptable factor of safety.
  • The upper surface of UHPC should be covered with glass cloth sheet to obtain asmooth regular surface. This will be important for continuous beams if laminates are expected to be bonded to the upper surface.
  • The surface of the hardened UHPC should be sanded before bonding strain gauges to it in order to ensure good performance of the strain gauge up to the failure of the concrete. This will eliminate the chance of bonding the gauge over a possible hidden void or bubble location.
  • The moisture-insensitive epoxy used in this study was more effective in wet bonding CFRP than SRP. Wet bonding of CFRP reached 77% of the bond strength of sand blasted dry bonding, whereas with SRP, wet bonding reached only 57% of the sand blasted dry bond value. Notwithstanding the reduced strength, using epoxy to bond wet UHPC may suit the requirements of many future projects.
  • The strain values of wet bonding are higher than those of dry bonding (sand blastedor not) at the same load.

7.3 Suggestions for Future Work

Some future research work can be done in order to help in setting up the design guidelines of the hybrid beams:

  • Extensive fatigue and fracture experiments on the GFRP material of box section can help in establishing the fatigue strength limits.
  • Study the cases of bonding the lower CFRP sheets at separate locations instead ofbonding to the whole lower flange. This case should avoid the simultaneous failure of the CFRP sheet and the lower flange of the GFRP box section.
  • Study the behaviour of hybrid beams with the upper concrete bonded to the box section with wet epoxy only, without using shear connectors.
  • Studying the behaviour and stability of deeper hybrid beams. i.e. bonding two GFRP boxes on top of each others.
  • A detailed dynamic analysis is necessary to be conducted on a three dimensional bridge model designed from similar hybrid beams in order to satisfy all the serviceability limit states.


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