Flexural Performance of Encased Pultruded GFRP I-Beam with High Strength Concrete under Static Loading

There is an interesting potential for the use of GFRP-pultruded profiles in hybrid GFRP-concrete structural elements, either for new constructions or for the rehabilitation of existing structures. This paper provides experimental and numerical investigations on the flexural performance of reinforced concrete (RC) specimens composite with encased pultruded GFRP I-sections. Five simply supported composite beams were tested in this experimental program to investigate the static flexural behavior of encased GFRP beams with high-strength concrete. Besides, the effect of using shear studs to improve the composite interaction between the GFRP beam and concrete as well as the effect of web stiffeners of GFRP were explored. Encasing the GFRP beam with concrete enhanced the peak load by 58.3%. Using shear connectors, web stiffeners, and both improved the peak loads by 100.6%, 97.3%, and 130.8%, respectively. The GFRP beams improved ductility by 21.6% relative to the reference one without the GFRP beam. Moreover, the shear connectors, web stiffeners, and both improved ductility by 185.5%, 119.8%, and 128.4%, respectively, relative to the encased reference beam. Furthermore, a non-linear Finite Element (FE) model was developed and validated by the experimental results to conduct a parametric study to investigate the effect of the concrete compressive strength and tensile strength of the GFRP beam. The developed FE model provided good agreement with the experimental results regarding deformations and damaged patterns.


Introduction
Fiber Reinforced Polymer (FRP) materials are taking place in several applications in civil construction such as bridges and buildings, which are new or degraded structures [1]. There are different types of FRP composites, which include Carbon FRP (CFRP), Glass FRP (GFRP), and Basalt FRP (BFRP). The CFRP has more excellent mechanical properties and fatigue/creep/corrosion resistance. However, it is expensive, which limits some engineering applications [2]. The mechanical properties of GFRP and BFRP are good with the desirable material cost. However, their long-term properties under the concrete alkaline environment are relatively poor due to some possible chemical degradation reactions [3]. The advantages of using GFRP pultruded beams are high strength and stiffness, lightweight, free formability, high durability even under offensive environments, low thermal conductivity, and corrosion resistance [1,4]. Moreover, encasing these beams in concrete could improve their long-term durability. On the other hand, the application of high-strength concrete has increased in construction, with the rapid progress of concrete technology [5][6][7][8][9]. However, the material behavior of high-strength concrete is different from normal strength concrete. For instance, the modulus of elasticity, tensile, and shear strength of concrete don't increase in direct proportion to the compressive strength [10]. The greater concern is the higher brittleness of high-strength concrete compared to that of normal strength concrete. Therefore, the advantages of encased pultruded GFRP section with concrete are significantly to improve the composite interaction between the GFRP beam and concrete as well as the effect of web stiffeners were explored. In addition, a non-linear FE model was developed and validated with the experimental results to conduct a parametric study to investigate the effect of the concrete compressive strength and tensile strength of the GFRP beam.

Experimental Program
Five simply supported composite beams were tested to investigate the static flexural behavior of encased GFRP pultruded I-beams with high-strength concrete. The test matrix for the conducted experimental program is listed in Table 1. In the adopted nomenclature, the symbol Ref refers to the reference beam, which was an RC beam and tested for comparison purposes. The symbol EG refers to the encased GFRP composite beam. The subsequent symbols S and W indicate using shear connectors and web stiffener, respectively.

Specimen Encoding GFRP I-Section Shear Connectors Web Stiffeners
Ref

Details of the Tested Specimens
All beams were simply supported with overall and effective lengths of 3000 mm and 2750 mm, respectively. Rectangular cross-sections with 200 mm in width and 300 mm in height were adopted. The details of the tested beams are shown in Figure 1. The tested beams were reinforced in the tension zone by 2Ø16 mm and in the compression zone by 2Ø10 mm. The transverse reinforcement consisted of closed stirrups of 10 mm diameter at an equal spacing of 125 mm. These steel reinforcements were designed according to ACI 318-19. For the encased beams (EG, EGS, EGW, and EGSW), an identical GFRP-section of 150 mm depth (d G ), 100 mm flange width (b f ), and flange and web thickness of 10 mm (t), as shown in Figure 2a. The encased GFRP section was positioned at the center of the concrete cross-section of each tested beam. To increase the composite action between the encased GFRP section and concrete, steel shear connectors with a diameter of 12 mm and a height of 60 mm were used. The shear connectors were fabricated at the top flange of the GFRP section using hexagonal nuts with a diameter of 18 mm, as shown in Figure 2b. These connectors were arranged in two rows at a longitudinal spacing of 375 mm, as illustrated in Figure 2c,d. Rectangular prisms with dimensions of 110 mm × 25 mm × 10 mm were prepared and attached to the GFRP I-beams on both sides, as web stiffeners, at a longitudinal spacing of 160 mm to strengthen the web against undesired premature failures, as shown in Figure 3.

Material Properties
The concrete mix proportion of the used high-strength concrete is listed in Table 2. The maximum aggregate size was 12 mm. The compressive strength and modulus of elasticity of 53.8 MPa and 31,000 MPa, respectively, were obtained from testing concrete cylinders with 150 × 300 mm dimensions according to ASTM C39-39M [24] and ASTM C469-469M [25], respectively. Tension tests were carried out on steel rebars of 16 and 10 mm diameters. Three specimens for each diameter were tested in the Consulting Engineering Bureau, College of Engineering, the University of Baghdad according to ASTM A615/A615M-18 [26]. The obtained yield stresses were 520 and 408 MP, respectively. Whereas the ultimate stresses were 687 and 466 MPa, respectively.

Material Properties
The concrete mix proportion of the used high-strength concrete is listed in Table 2. The maximum aggregate size was 12 mm. The compressive strength and modulus of elasticity of 53.8 MPa and 31,000 MPa, respectively, were obtained from testing concrete cylinders with 150 × 300 mm dimensions according to ASTM C39-39M [24] and ASTM C469-469M [25], respectively. Tension tests were carried out on steel rebars of 16 and 10 mm diameters. Three specimens for each diameter were tested in the Consulting Engineering Bureau, College of Engineering, the University of Baghdad according to ASTM A615/A615M-18 [26]. The obtained yield stresses were 520 and 408 MP, respectively. Whereas the ultimate stresses were 687 and 466 MPa, respectively.

Material Properties
The concrete mix proportion of the used high-strength concrete is listed in Table 2. The maximum aggregate size was 12 mm. The compressive strength and modulus of elasticity of 53.8 MPa and 31,000 MPa, respectively, were obtained from testing concrete cylinders with 150 × 300 mm dimensions according to ASTM C39-39M [24] and ASTM C469-469M [25], respectively. Tension tests were carried out on steel rebars of 16 and 10 mm diameters. Three specimens for each diameter were tested in the Consulting Engineering Bureau, College of Engineering, the University of Baghdad according to ASTM A615/A615M-18 [26]. The obtained yield stresses were 520 and 408 MP, respectively. Whereas the ultimate stresses were 687 and 466 MPa, respectively. The GFRP I-beams used in this study were made by an international manufacturer of FRP products (Dura composites, United King). They were made of isophthalic polyester resins reinforced with E-glass fibers. The mechanical and geometric properties of the pultruded GFRP I-section are listed in Table 3. These properties were obtained from standard tests according to ASTM D695-15 [27] and ISO 527-4:2021 [28] for the compressive and tensile properties, respectively.

Experimental Setup and Instrumentations
The specimens were tested as simply supported beams under the effect of a concentrated load at the mid-spans. A hydraulic jack, with 490 kN capacity, was used to apply the load. The applied load and corresponding deflection were recorded using a load cell and linear variable differential transforms (LVDT), respectively, as shown in Figure 4a. Electrical strain gauges (ε 1 to ε 6 ) were used to measure the strains in the steel rebars, concrete, and GFRP I-section at the mid-spans, as illustrated in Figure 4b. The GFRP I-beams used in this study were made by an international manufacturer of FRP products (Dura composites, United King). They were made of isophthalic polyester resins reinforced with E-glass fibers. The mechanical and geometric properties of the pultruded GFRP I-section are listed in Table 3. These properties were obtained from standard tests according to ASTM D695-15 [27] and ISO 527-4:2021 [28] for the compressive and tensile properties, respectively.

Experimental Setup and Instrumentations
The specimens were tested as simply supported beams under the effect of a concentrated load at the mid-spans. A hydraulic jack, with 490 kN capacity, was used to apply the load. The applied load and corresponding deflection were recorded using a load cell and linear variable differential transforms (LVDT), respectively, as shown in Figure 4a. Electrical strain gauges (ε1 to ε6) were used to measure the strains in the steel rebars, concrete, and GFRP I-section at the mid-spans, as illustrated in Figure 4b.

Load-Deflection Curves
The load versus mid-span deflection curves for the tested beams are shown in Figure  5. The relationships show four distinguished stages: (a) initial concrete cracking, (b) tensile reinforcement yielding, (c) concrete crushing and fracture of the GFRP beams, and (d) rupture of the tensile reinforcement. All curves started with the same abrupt slop in the first stage until the loading level of 15 kN. Then, the curves started to deviate from each other. As the applied load increased, the slops of the curves of specimens with shear connectors and web stiffeners became steeper towards the ultimate points. The peak loads of the tested beams EG, EGS, EGW, and EGSW were 58.3%, 100.6%, 97.3%, and 130.8% higher than the reference beam Ref, respectively, as listed in Table 4. After reaching the ultimate points, the applied loads were dropped due to the fracture of the GFRP beams. The bond-slip was responsible for the reduction in the slope beyond the ultimate load. Compared to the encased GFRP beams with shear connectors or web

Load-Deflection Curves
The load versus mid-span deflection curves for the tested beams are shown in Figure 5. The relationships show four distinguished stages: (a) initial concrete cracking, (b) tensile reinforcement yielding, (c) concrete crushing and fracture of the GFRP beams, and (d) rupture of the tensile reinforcement. All curves started with the same abrupt slop in the first stage until the loading level of 15 kN. Then, the curves started to deviate from each other. As the applied load increased, the slops of the curves of specimens with shear connectors and web stiffeners became steeper towards the ultimate points. The peak loads of the tested beams EG, EGS, EGW, and EGSW were 58.3%, 100.6%, 97.3%, and 130.8% higher than the reference beam Ref, respectively, as listed in Table 4. After reaching the ultimate points, the applied loads were dropped due to the fracture of the GFRP beams. The bond-slip was responsible for the reduction in the slope beyond the ultimate load. Compared to the encased GFRP beams with shear connectors or web stiffeners or both (EGS, EGW, and EGSW beams), the EG beam solely depended on the friction between the GFRP beam and concrete while the shear connectors and web stiffeners provided better anchorage and bond. Therefore, the shear connectors and web stiffeners were provided to treat the bond-slip problem, and it was proven in this research. The ultimate load and corresponding central displacement are shown in Table 4. The central displacements were proportional to the ultimate load. More ductility was obtained when using the shear connectors in specimens EGS and EGSW.
Materials 2022, 14, x FOR PEER REVIEW 7 of 20 stiffeners or both (EGS, EGW, and EGSW beams), the EG beam solely depended on the friction between the GFRP beam and concrete while the shear connectors and web stiffeners provided better anchorage and bond. Therefore, the shear connectors and web stiffeners were provided to treat the bond-slip problem, and it was proven in this research. The ultimate load and corresponding central displacement are shown in Table. 4. The central displacements were proportional to the ultimate load. More ductility was obtained when using the shear connectors in specimens EGS and EGSW.

Crack Patterns and Failure Modes
The initial crack load for each specimen is listed in Table 4. Cracks were initiated when the tensile stress at the bottom fiber of the tested beam was larger than the tensile strength of concrete. Then, the stresses started to be transferred to the steel rebars. The initial crack loads were very close to each other because the encased GFRP beam didn't contribute to the first stage of loading. As the applied load increased, new cracks were formed and appeared. The crack patterns and failure modes are shown in Figure 6. The reference beam, Ref, experienced yielding of steel reinforcement followed by crushing of concrete in the compression zone and more cracks were developed along with the depth of the concrete cross-section. Finally, a rupture in the bottom steel rebars occurred. This rupture caused the cracks to penetrate the specimen's cross-section and led to cutting the beam into two separate parts, as shown in Figure 6a. For the encased beams EG, EGS, EGW, and EGSW, the major cracks were developed at the mid-spans and propagated towards the compression zones. After reaching the ultimate loads, major cracks were observed joining each other and shear cracks were created. The final failure modes were concrete crushing followed by cover spalling and buckling of the top steel rebars. At the same time, ruptures in the GFRP beams and the bottom steel rebars occurred. As shown in Figure 6c-e, the shear connectors and web stiffeners increased the tested beams' rigidity, which caused more flexural cracks formation in these beams. Moreover, the

Crack Patterns and Failure Modes
The initial crack load for each specimen is listed in Table 4. Cracks were initiated when the tensile stress at the bottom fiber of the tested beam was larger than the tensile strength of concrete. Then, the stresses started to be transferred to the steel rebars. The initial crack loads were very close to each other because the encased GFRP beam didn't contribute to the first stage of loading. As the applied load increased, new cracks were formed and appeared. The crack patterns and failure modes are shown in Figure 6. The reference beam, Ref, experienced yielding of steel reinforcement followed by crushing of concrete in the compression zone and more cracks were developed along with the depth of the concrete cross-section. Finally, a rupture in the bottom steel rebars occurred. This rupture caused the cracks to penetrate the specimen's cross-section and led to cutting the beam into two separate parts, as shown in Figure 6a. For the encased beams EG, EGS, EGW, and EGSW, the major cracks were developed at the mid-spans and propagated towards the compression zones. After reaching the ultimate loads, major cracks were observed joining each other and shear cracks were created. The final failure modes were concrete crushing followed by cover spalling and buckling of the top steel rebars. At the same time, ruptures in the GFRP beams and the bottom steel rebars occurred. As shown in Figure 6c-e, the shear connectors and web stiffeners increased the tested beams' rigidity, which caused more flexural cracks formation in these beams. Moreover, the crushed concrete zones

Load-Strain Relationships
The strain records are illustrated in Figures 7-9. For the reference beam Ref, the maximum measured compressive strain in concrete was 0.0015 when yielding in the tensile reinforcement occurred. Whereas these values were 0.0023, 0.0032, 0.0025, and 0.0028 for the encased beams EG, EGS, EGW, and EGSW, respectively. The GFRP beam enhanced the encased beams' capacities and led to increases in the measured ultimate strains in concrete (see Figure 9). Based on these records, the tensile reinforcements in all beams firstly reached yielding at different loading levels. The yielding loads for the encased beams EG, EGS, EGW, and EGSW were 99.1 kN, 101.1 kN, 120.5 kN, and 155.3 kN with 23.7%, 26.2%, 50.1%, and 93.7% increases relative to the reference beam, respectively. Table 5 lists the maximum strain measurements in concrete and steel

Load-Strain Relationships
The strain records are illustrated in Figures 7-9. For the reference beam Ref, the maximum measured compressive strain in concrete was 0.0015 when yielding in the tensile reinforcement occurred. Whereas these values were 0.0023, 0.0032, 0.0025, and 0.0028 for the encased beams EG, EGS, EGW, and EGSW, respectively. The GFRP beam enhanced the encased beams' capacities and led to increases in the measured ultimate strains in concrete (see Figure 9). Based on these records, the tensile reinforcements in all beams firstly reached yielding at different loading levels. The yielding loads for the encased beams EG, EGS, EGW, and EGSW were 99.1 kN, 101.1 kN, 120.5 kN, and 155.3 kN with 23.7%, 26.2%, 50.1%, and 93.7% increases relative to the reference beam, respectively. Table 5 lists the maximum strain measurements in concrete and steel reinforcement. The measured strains in the tensile and compressive steel reinforcement for the encased beams were higher than those of the reference beam. The EGW beam experienced the highest tensile strain of 0.0145. The existence of studs and web stiffeners in beam EGSW improved the beam rigidity and released the tensile strains in the steel reinforcement. Therefore, this beam showed much improvement in the flexural behavior relative to the other encased beams. The strains in the GFRP beams increased almost linearly until failure. From these values of strain measurements in each part of the GFRP beams (top flange, bottom flange, and web), the GFRP beams contributed to providing more strength to the encased beams. The contribution of the GFRP beams increased by adding the studs and web stiffeners because the bottom flanges of these beams exhibited additional strains due to increasing the composite interaction with concrete.
Materials 2022, 14, x FOR PEER REVIEW 9 of 20 reinforcement. The measured strains in the tensile and compressive steel reinforcement for the encased beams were higher than those of the reference beam. The EGW beam experienced the highest tensile strain of 0.0145. The existence of studs and web stiffeners in beam EGSW improved the beam rigidity and released the tensile strains in the steel reinforcement. Therefore, this beam showed much improvement in the flexural behavior relative to the other encased beams. The strains in the GFRP beams increased almost linearly until failure. From these values of strain measurements in each part of the GFRP beams (top flange, bottom flange, and web), the GFRP beams contributed to providing more strength to the encased beams. The contribution of the GFRP beams increased by adding the studs and web stiffeners because the bottom flanges of these beams exhibited additional strains due to increasing the composite interaction with concrete. reinforcement. The measured strains in the tensile and compressive steel reinforcement for the encased beams were higher than those of the reference beam. The EGW beam experienced the highest tensile strain of 0.0145. The existence of studs and web stiffeners in beam EGSW improved the beam rigidity and released the tensile strains in the steel reinforcement. Therefore, this beam showed much improvement in the flexural behavior relative to the other encased beams. The strains in the GFRP beams increased almost linearly until failure. From these values of strain measurements in each part of the GFRP beams (top flange, bottom flange, and web), the GFRP beams contributed to providing more strength to the encased beams. The contribution of the GFRP beams increased by adding the studs and web stiffeners because the bottom flanges of these beams exhibited additional strains due to increasing the composite interaction with concrete.

Ductility
There are many definitions for ductility and ductility index of conventional RC structures with only steel rebars. The ratio of deflection at ultimate load to the deflection at yield load can define the ductility of these members. The FRP materials have a linear stress-strain relationship until failure, and the energy released for FRP beams is linear, which is different from that for steel reinforcement. Therefore, the ductility in this study is based on the energy theory [29]. The ductility (μE) was calculated based on Equation (1) depending on the load-deflection relationships for the tested beams.

Ductility
There are many definitions for ductility and ductility index of conventional RC structures with only steel rebars. The ratio of deflection at ultimate load to the deflection at yield load can define the ductility of these members. The FRP materials have a linear stress-strain relationship until failure, and the energy released for FRP beams is linear, which is different from that for steel reinforcement. Therefore, the ductility in this study is based on the energy theory [29]. The ductility (μE) was calculated based on Equation (1) depending on the load-deflection relationships for the tested beams.

Ductility
There are many definitions for ductility and ductility index of conventional RC structures with only steel rebars. The ratio of deflection at ultimate load to the deflection at yield load can define the ductility of these members. The FRP materials have a linear stress-strain relationship until failure, and the energy released for FRP beams is linear, which is different from that for steel reinforcement. Therefore, the ductility in this study is based on the energy theory [29]. The ductility (µ E ) was calculated based on Equation (1) depending on the load-deflection relationships for the tested beams.
where E T is the total energy calculated from the area under the load-deflection curve, E E is the stored elastic energy calculated from the load-deflection relationships as shown in Figure 10. The slope (S) was obtained from the following equation: where P 1 was the load at the end of the elastic stage, S 1 was the slop of the elastic stage, P 2 was the peak load at the end of the second line, and S 2 was the slope of the second line.
where ET is the total energy calculated from the area under the load-deflection curve, EE is the stored elastic energy calculated from the load-deflection relationships as shown in Figure 10. The slope (S) was obtained from the following equation: where 1 was the load at the end of the elastic stage, 1 was the slop of the elastic stage, 2 was the peak load at the end of the second line, and 2 was the slope of the second line. Figure 10. The ductility mode that used in this study [29].
The total energy, elastic energy, and ductility of the tested specimens are illustrated in Table 6. The ductility of beam Ref was 3.52, which was smaller than the encased beams. The GFRP beams improved the ductility by 21.6% relative to the reference one. Moreover, the shear connectors, web stiffeners, and both improved the ductility by 185.5%, 119.8%, and 128.4%, respectively, relative to the encased beam EG. Table 6. The ductility of the tested specimens.

Numerical Modeling
Finite Element (FE) models were developed, using the general-purpose FE code Abaqus [30], to simulate the three-dimensional (3D) modeling of encased pultruded GFRP beams under static loading.

FE Modelling of Encased Beam
Different element types were used to model the different components of the encased beams (concrete, steel reinforcement, pultruded GFRP beam, shear connectors, and web stiffeners). Continuum eight-node solid element with reduced integration (C3D8R) was used to simulate the nonlinear behavior of concrete, shear connectors, and steel plates. The interface between the bearing plates and concrete was modeled as a Figure 10. The ductility mode that used in this study [29].
The total energy, elastic energy, and ductility of the tested specimens are illustrated in Table 6. The ductility of beam Ref was 3.52, which was smaller than the encased beams. The GFRP beams improved the ductility by 21.6% relative to the reference one. Moreover, the shear connectors, web stiffeners, and both improved the ductility by 185.5%, 119.8%, and 128.4%, respectively, relative to the encased beam EG.

Numerical Modeling
Finite Element (FE) models were developed, using the general-purpose FE code Abaqus [30], to simulate the three-dimensional (3D) modeling of encased pultruded GFRP beams under static loading.

FE Modelling of Encased Beam
Different element types were used to model the different components of the encased beams (concrete, steel reinforcement, pultruded GFRP beam, shear connectors, and web stiffeners). Continuum eight-node solid element with reduced integration (C3D8R) was used to simulate the nonlinear behavior of concrete, shear connectors, and steel plates. The interface between the bearing plates and concrete was modeled as a surface-to-surface interaction. The pultruded GFRP beam and web stiffeners were modeled by using the four-node doubly curve shell element with reduced integration (S4R). The longitudinal and transverse reinforcements were modeled using the 2-node linear 3D truss element (T3D2). Several nonlinear analyses with different element sizes were performed to select the best mesh size to obtain accurate results at a reasonable solving time. The FE mesh showing the different components of the encased beams is illustrated in Figure 11. solving time. The FE mesh showing the different components of the encased beams is illustrated in Figure 11.
The experimental boundary conditions were adopted in the FE analysis as simply supported beams. The first support was constrained in the Y-and Z-directions, representing the hinge support. At the same time, the second support was constrained only in the Y-direction, which expressed the roller support. The whole model was constrained in the X-direction. The full bond technique was assumed to simulate the connection between the concrete and steel rebars. However, the bond between the GFRP beam surface and the surrounded concrete was simulated using surface-to-surface contact pairs. The contact property was represented by the tangential behavior with a penalty friction formulation. The tangential shear stress was adopted from the push-out test as 0.422 MPa [31] and the friction coefficient was used equally at 0.55 according to the test of Hadi and Yuan [16]. However, the full bond between the shear studs and concrete was assumed. The displacement-controlled strategy was employed to load the analyzed beams by defining the vertical displacement value of the reference point.

Material Modeling
The compressive behavior of concrete was defined using the concrete damagedplasticity (CDP) model [32]. In this model, the material principal failure mechanisms were compressive crushing, tensile cracking, and loss of elastic stiffness. The value of the The experimental boundary conditions were adopted in the FE analysis as simply supported beams. The first support was constrained in the Y-and Z-directions, representing the hinge support. At the same time, the second support was constrained only in the Y-direction, which expressed the roller support. The whole model was constrained in the X-direction. The full bond technique was assumed to simulate the connection between the concrete and steel rebars. However, the bond between the GFRP beam surface and the surrounded concrete was simulated using surface-to-surface contact pairs. The contact property was represented by the tangential behavior with a penalty friction formulation. The tangential shear stress was adopted from the push-out test as 0.422 MPa [31] and the friction coefficient was used equally at 0.55 according to the test of Hadi and Yuan [16]. However, the full bond between the shear studs and concrete was assumed. The displacement-controlled strategy was employed to load the analyzed beams by defining the vertical displacement value of the reference point.

Material Modeling
The compressive behavior of concrete was defined using the concrete damagedplasticity (CDP) model [32]. In this model, the material principal failure mechanisms were compressive crushing, tensile cracking, and loss of elastic stiffness. The value of the dilation angle ψ was 36 • , the value of the plastic flow potential eccentricity (e) was 0.1, and the ratio of the initial equibiaxial compressive yield stress to initial uniaxial compressive yield stress (σ bo /σ co ) was 1.16. Moreover, the coefficient (K c ) was 2/3 and the viscosity parameter was 0.001. The elastic properties of concrete were specified according to the ACI 363R-92 [5] and ACI 318-19 [33] guidelines. The used uniaxial compressive stress-strain and damage compression-crushing strain relationships of concrete are shown in Figure 12. The concrete behavior under uniaxial tension was represented by the tension softening mechanism and tension stiffening due to the tensile resistance of concrete surrounding the tensile reinforcement, which was forced by bond stresses to extend simultaneously with reinforcement [34,35]. Figure 13 presents the adopted post-failure tensile stress and tensile damage with cracking strains curves. dilation angle was 36°, the value of the plastic flow potential eccentricity (e) was 0.1, and the ratio of the initial equibiaxial compressive yield stress to initial uniaxial compressive yield stress (σbo/σco) was 1.16. Moreover, the coefficient (Kc) was 2/3 and the viscosity parameter was 0.001. The elastic properties of concrete were specified according to the ACI 363R-92 [5] and ACI 318-19 [33] guidelines. The used uniaxial compressive stress-strain and damage compression-crushing strain relationships of concrete are shown in Figure 12. The concrete behavior under uniaxial tension was represented by the tension softening mechanism and tension stiffening due to the tensile resistance of concrete surrounding the tensile reinforcement, which was forced by bond stresses to extend simultaneously with reinforcement [34,35]. Figure 13 presents the adopted post-failure tensile stress and tensile damage with cracking strains curves. The steel reinforcement was simulated as an elastic-perfectly plastic material. The modulus of elasticity and yielding stress were used to define the stress-strain curve. The degradation in the GFRP I-beam was modeled according to Hashin's criteria [36]. According to the damage evolution law, the material stiffness of the GFRP I-beam degraded after satisfying the damage initiation criteria. The Hashin damage initiation criterion is used in Abaqus, along with a progressive damage variable based on stress state and fracture energy Gf as listed in Tables 3 and 7, respectively. dilation angle was 36°, the value of the plastic flow potential eccentricity (e) was 0.1, and the ratio of the initial equibiaxial compressive yield stress to initial uniaxial compressive yield stress (σbo/σco) was 1.16. Moreover, the coefficient (Kc) was 2/3 and the viscosity parameter was 0.001. The elastic properties of concrete were specified according to the ACI 363R-92 [5] and ACI 318-19 [33] guidelines. The used uniaxial compressive stress-strain and damage compression-crushing strain relationships of concrete are shown in Figure 12. The concrete behavior under uniaxial tension was represented by the tension softening mechanism and tension stiffening due to the tensile resistance of concrete surrounding the tensile reinforcement, which was forced by bond stresses to extend simultaneously with reinforcement [34,35]. Figure 13 presents the adopted post-failure tensile stress and tensile damage with cracking strains curves. The steel reinforcement was simulated as an elastic-perfectly plastic material. The modulus of elasticity and yielding stress were used to define the stress-strain curve. The degradation in the GFRP I-beam was modeled according to Hashin's criteria [36]. According to the damage evolution law, the material stiffness of the GFRP I-beam degraded after satisfying the damage initiation criteria. The Hashin damage initiation criterion is used in Abaqus, along with a progressive damage variable based on stress state and fracture energy Gf as listed in Tables 3 and 7, respectively. The steel reinforcement was simulated as an elastic-perfectly plastic material. The modulus of elasticity and yielding stress were used to define the stress-strain curve. The degradation in the GFRP I-beam was modeled according to Hashin's criteria [36]. According to the damage evolution law, the material stiffness of the GFRP I-beam degraded after satisfying the damage initiation criteria. The Hashin damage initiation criterion is used in Abaqus, along with a progressive damage variable based on stress state and fracture energy G f as listed in Tables 3 and 7, respectively.

Validations of the FE Results
The conventional load-deformation curves for the tested specimens were used to validate the FE results, as shown in Figures 14 and 15. Initially, the FE deformations exhibited linear elastic behaviors with a higher stiffness than the experimental records. This difference in behavior could be attributed to the used constitutive models for materials and the full bond assumed between the concrete and steel rebars in the FE analysis. As the applied load gradually increased, cracks were formed and the non-linear deformation behaviors were obtained in good agreement with the experimental results.

Validations of the FE Results
The conventional load-deformation curves for the tested specimens were used to validate the FE results, as shown in Figures 14 and 15. Initially, the FE deformations exhibited linear elastic behaviors with a higher stiffness than the experimental records. This difference in behavior could be attributed to the used constitutive models for materials and the full bond assumed between the concrete and steel rebars in the FE analysis. As the applied load gradually increased, cracks were formed and the non-linear deformation behaviors were obtained in good agreement with the experimental results. Table 8 summarizes the experimental and FE results regarding the maximum deflections and loads. The comparisons show that the difference between the maximum applied loads reached about 4.25% for the specimen EGW.
The FE crack patterns for concrete were represented in Abaqus by defining the post-cracking damage properties for the CDP model by visualizing the tensile damage at the integration point DAMAGET. Figure 15 shows a closed agreement between the FE and experimental crack patterns, indicating that the proposed FE model effectively predicted the failure behavior of the tested specimens. Based on these comparisons, the proposed FE model can simulate the tested specimens with and without the GFRP beam. Therefore, the proposed model was used to evaluate a comparative parametric study.  The FE crack patterns for concrete were represented in Abaqus by defining the postcracking damage properties for the CDP model by visualizing the tensile damage at the integration point DAMAGET. Figure 15 shows a closed agreement between the FE and experimental crack patterns, indicating that the proposed FE model effectively predicted the failure behavior of the tested specimens. Based on these comparisons, the proposed FE model can simulate the tested specimens with and without the GFRP beam. Therefore, the proposed model was used to evaluate a comparative parametric study.

Parametric Study
The influences of the compressive strength of concrete and tensile strength of the GFRP beam on the flexural behavior of encased GFRP I-beam were investigated using the verified FE model.

Effect of the Concrete Compressive Strength
The influence of various concrete compressive strengths of 45 MPa, 53.8 MPa, and 65 MPa on the flexural performance of encased beams under static load was studied. The different compressive strengths were implemented in Abaqus through the stress-strain curves as well as the CDP model. The peak loads of the analyzed specimens are shown in Figure 16. Enhancements in the peak loads, service load, and maximum deflections of the analyzed specimens were obtained as the concrete compressive strength increased, as listed in Table 9. The percentages were estimated for the reference concrete compressive strength of 45 MPa. The peak loads increased with increasing the compressive strength of concrete. For the reference beam Ref, the peak load increased by 3.76% and 11.92% for the compressive strength of 53.8 MPa and 65 MPa, respectively, relative to the compressive strength of 45 MPa. The most enhancements in the peak loads were for the encased beam EG, which were 24.78% and 32.32% for compressive strengths of 53.8 MPa 65 MPa, respectively, concerning the compressive strength of 45 MPa. Moreover, the mid-span deflections at the peak loads were reduced as the concrete compressive strength increased.

Effect of the Tensile Strength of the GFRP Beam
To study the effect of tensile strength of GFRP I-beam on the flexural behavior of the encased beams, two tensile strengths of 258 MPa and 416.6 MPa [37] were investigated, besides the tensile strength of 347.5 MPa, which was obtained in this study according to (ASTM Designation: D 695-15) [27]. The peak loads and the corresponding mid-span deflections increased as the tensile strength of GFRP increased as listed in Table 9 and shown in Figure 17. In these comparisons, the reference tensile strength was 258 MPa. The increase in the peak loads ranged between 6% and 18% when the tensile strength was 347.5 MPa, while the improvements ranged between 14% and 27% for tensile strength of 416 MPa, as listed in Table 10.

Effect of the Tensile Strength of the GFRP Beam
To study the effect of tensile strength of GFRP I-beam on the flexural behavior of the encased beams, two tensile strengths of 258 MPa and 416.6 MPa [37] were investigated, besides the tensile strength of 347.5 MPa, which was obtained in this study according to (ASTM Designation: D 695-15) [27]. The peak loads and the corresponding mid-span deflections increased as the tensile strength of GFRP increased as listed in Table 9 and shown in Figure 17. In these comparisons, the reference tensile strength was 258 MPa. The increase in the peak loads ranged between 6% and 18% when the tensile strength was 347.5 MPa, while the improvements ranged between 14% and 27% for tensile strength of 416 MPa, as listed in Table 10.

Conclusions
This paper provides experimental and numerical investigations on the flexural performance of RC specimens composite with encased pultruded GFRP I-sections. Five simply supported composite beams were tested in this experimental program to investigate the static flexural behavior of encased GFRP pultruded I-beams with

Conclusions
This paper provides experimental and numerical investigations on the flexural performance of RC specimens composite with encased pultruded GFRP I-sections. Five simply supported composite beams were tested in this experimental program to investigate the static flexural behavior of encased GFRP pultruded I-beams with high-strength concrete. Besides, the effect of using shear studs to improve the composite interaction between the GFRP beam and concrete as well as the effect of web stiffeners of GFRP were explored. Moreover, a non-linear FE model was developed and validated by the experimental results to conduct a parametric study to investigate the effect of the concrete compressive strength and tensile strength of the GFRP beam. The following conclusions can be drawn based on the experimental and FE results.

1.
Encasing the GFRP beam with concrete enhanced the peak load by 58.3%. Using shear connectors, web stiffeners, and both improved the peak loads by 100.6%, 97.3%, and 130.8%, respectively, relative to the classical reinforced concrete. The shear connectors and web stiffeners increased the beams' rigidity. In addition, the GFRP beams improved the ductility by 21.6% relative to the reference one. Moreover, the shear connectors, web stiffeners, and both improved the ductility by 185.5%, 119.8%, and 128.4%, respectively, relative to the reference beam.

2.
The strains of the pultruded GFRP beams increased almost linearly until failure. The GFRP beams contributed to providing more strength to the encased beams. The contribution of the GFRP beams increased by adding the studs and web stiffeners because the bottom flanges of these beams exhibited additional strains due to increasing the composite interaction with concrete. 3.
The peak loads increased with increasing the compressive strength of concrete. For the reference beam without a GFRP beam, the peak load increased by 3.76% and 11.92% for the compressive strength of 53.8 MPa and 65 MPa, respectively. However, the most enhancements in the peak loads were for the encased beam, which were 24.78% and 32.32% for the compressive strengths of 53.8 MPa 65 MPa, respectively, with respect to the compressive strength of 45 MPa.

4.
The peak loads and the corresponding mid-span deflections increased as the tensile strength of GFRP increased. The increase in the peak loads ranged between 6% and 18% when the tensile strength was 347.5 MPa, while the improvements ranged between 14% and 27% for the tensile strength of 416 MPa.