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Effects of CFRP sheets on the flexural behavior of high-strength concrete beam

  • Rawya A. Abduljabbar , Sura F. Alkhafaji EMAIL logo , Hayder S. Abdulaali , Ali Abdulqader and Shagea Alqawzai
Published/Copyright: July 1, 2024
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Open Engineering
From the journal Open Engineering Volume 14 Issue 1

Abstract

The aim of this study is to evaluate numerically the effects of carbon fiber-reinforced polymer (CFRP) sheets strengthening on the flexural performance of high-strength concrete (HSC) beam using ABAQUS 3D finite element (FE) modeling software. The developed FE models were verified against the experimental results found in literature. The FE models can accurately estimate the performance of CFRP-strengthened high-strength reinforced concrete (RC) beams. Subsequent parametric analysis was performed to assess the performance of CFRP-strengthened concrete beams considering various parameters including compressive strength of concrete, CFRP width, thickness, length, number of CFRP layers, and CFRP strengthening schemes. Based on the results of FE analysis. It was demonstrated that using HSC significantly enhances the performance of CFRP-strengthened RC beams. It was also confirmed that width, thickness, and layer number of CFRP sheets improve the flexural behavior of CFRP-strengthened HSC beams by increasing the ultimate loads and strain-hardening behavior of the specimens. The strengthening schemes contribute to delaying or inhabiting the debonding especially when the CFRP sheets are added along the bottom of the beams. It was demonstrated that using CFRP sheets U-wrapping contributes to the prevention or delay of debonding and increases the capability of resisting the stress imposed on the concrete. Therefore, installing the CFRP sheets at the bottom face of beam below the tensile reinforcement enhances the performance of CFRP-strengthened HSC beams.

Keywords: carbon fiber-reinforced polymer; flexural performance; finite element modeling; high-strength concrete beam; strengthening schemes

1 Introduction

Over the past decades, many existing reinforced concrete (RC) structures all over the globe are in a severe deterioration state because of aging, lack of maintenance, steel reinforcement corrosion, errors in design, poor detailing, construction defects, harsh environmental conditions, and overloading [1]. Therefore, these structures have to be replaced or strengthened to improve the deteriorated capacity and/or retrofitted in some structural elements. Mostly, strengthening is the most economically and environmentally effective technique as compared with destruction and reconstruction. Structures that are made of low-strength concrete (LSC) are inherently unsafe [2,3]. These structures require to be retrofitted or strengthened using suitable techniques and materials in order to bring the strength, ductility and rigidity of the element and/or system to desired level.

Several techniques have been employed to strengthen the RC structures. Some of these techniques include wrapping the existing structural members with RC jacketing or with steel plates, increasing the load-bearing capacity of the member cross-section by increasing the size and dimension of cross section and changing the stress state. The load-bearing capacity of the cross-section can be also increased using the steel reinforcement or composite polymers such as fiber-reinforced polymers (FRPs) [4]. Recently, FRPs have gained wide acceptance and proven to be effective in rehabilitating and strengthening of structural members such as beams and slabs in flexural and shear to increase the life span of the structure. FRP sheets and plates are fabricated through embedding the high-strength fibers in a polymer matrix. Based on their materials, they are classified into Aramid-FRP (AFRP), Basalt-FRP (BFRP), Carbon-FRP (CFRP), and Glass-FRP (GFRP) [5]. They can be utilized as internal reinforcement for new RC structures or additional reinforcement to repair or strengthen the existing RC structures [6,7] and as an external confining reinforcement RC axial member [8,9,10]. Strengthening the RC structures using externally bounded FRP (EB-FRP) elements has become a worldwide practice and accepted by recent design standards [11,12]. EB-FRP sheets are the most commonly preferred technique for strengthening and retrofitting of RC structures in recent years due to the increased strength to width ratio, ease of handling, higher resistance to corrosion and less requirement for equipment installation, making them appropriate for concrete structures [13,14,15,16].

RC flexural members transmit the applied loads from floor to columns, which is then transmitted to the foundation. Therefore, RC beam is considered as one of the most essential elements in the RC frames. However, RC beams are subjected to deterioration due to numerous factors such as excessive load application, corrosion of reinforcement, differential settlement, and poor construction and design. Hence, the strengthening of RC beams is required to enhance their flexural load bearing capacity to meet the serviceability requirements [17]. The flexural strength of RC beams can be greatly improved by binding the FRP sheets/plates to the tension zone of the beam. A great effort of research has been done to investigate the performance of RC beams strengthened by various types of FRP sheets and plates. Many studies have investigated experimentally the flexural behavior of FRP-strengthened beams. For examples, Dong et al. [18] performed an experimental work on the RC beam strengthened with external flexural and flexural-shear FRP sheets composed of CFRP and GFRP. The influence of various parameters, such as layers’ numbers of CFRP sheets, pre-cracked width, thickness, concrete cover, ratio of shear and flexural reinforcement, depth of cross section, and concrete strength. It was found that the flexural–shear strengthening configuration is highly effective than the flexural counterparts in increasing the ultimate strength stiffness and hardening response of the RC beam. Chakrabortty and Khennane [19] conducted a study to investigate the behavior of a novel configuration of a hybrid FRP-concrete beam, consisting of CFRP laminate, GFRP pultruded profile, and a concrete block all wrapped up using filament winding. It was found that using this technique can improve the stiffness and load carrying capacity of beams through the imposed confinement applied to the concrete. Hawileh et al. [20] also conducted an experiment to evaluate the performance of RC beams strengthened in flexure by means of various combinations of EB-hybrid GFRP/CFRP sheets. It was observed that as compared to the control beam, the load carrying capacity of the strengthened beams increased relying on the hybrid sheets combination. Uriayer and Alam [21] performed an experimental study to assess the effectiveness of steel-CFRP composite as stirrups in concrete beam to carry the shearing force. Various parameters including the shape of stirrups, number of CFRP layers used in each stirrup, and number of stirrups used in shear spans were investigated. It was indicated that the steel-CFRP stirrups is an effective solution of premature failure of FRP stirrups at the bends. Sumathi [22] conducted a study to investigate experimentally the flexural behavior of FRP externally bounded RCC beams. It was indicated that using FRP sheets can improve the strength, stiffness, and ductility of the beam specimens. Salama et al. [23] conducted a research on the performance RC beams strengthened by externally side-bonded (ESB) CFRP sheets to assess the effects of various schemes of bottom-bonded (BB) and side-bonded CFRP sheets. It was found that the flexural strength of the strengthened beams with similar amount of reinforcement was increased, as compared to the control specimens. Kar and Biswal [24] carried out a research to explore the feasibility of using BFRP sheets for shear strengthening of RC beams. It was found that using BFRP sheets can enhance the shear and flexural strength of RC beams. More recently, Hosen et al. [25] conducted a study to assess the flexural performance of RC beams strengthened with ESB CFRP fabrics. And several parameters including the ratio of steel reinforcement and width of CFRP. It was found that increasing the ratio of steel reinforcement and CFRP width improved the first crack and ultimate loads of the strengthened specimens, in comparison to the control counterparts. Lattif and Hamdy [26] performed an experimental study to investigate the flexural strengthening of prestressed self-compacting RC beams using CFRP sheets and laminate composites. It was indicated that using CFRP to strengthen the RC beams can significantly enhance the load capacity of the beams. It was also observed that strengthening of beams using CFRP laminates can increase the load capacity and reduce the ductility as compared to the beams strengthened with CFRP sheets. Tiwary et al. [27] also performed a research to assess the damaged RC beams retrofitted in flexure with CFRP sheets. The influence of the CFRP sheet scheme were explored. It was shown that flexural strength as well as the stiffness of RC beams strengthened with CFRP sheets is functionally effective. It was also observed that the effectiveness of flexural retrofitting method appears to vary depending on the configurations of CFRP sheets. Most recently, Sabzi et al. [28] conducted a study to experimentally and analytically evaluate the influence of tensile reinforcement ratio and arrangement on the performance of FRP strengthened RC beams. It was found that increasing the number of tensile reinforcing bars and reducing the diameter of bars increase the axial load capacity of beam specimens made of normal and high-strength concrete (HSC).

In parallel to the experimental investigations, some studies have also been conducted to simulate numerically the performance of RC beams to assess the influence of FRP sheets on the flexural strength of beams. For instances, Godat et al. [29] also performed a numerical study to assess the flexural behavior of RC beams strengthened with CFRP using ABAQUS 3D finite element (FE) modeling. It was found that the developed FE models can accurately estimate the flexural performance of specimens. The effects of numerous parameters including width, length, thickness, elastic modulus, and strengthening schemes on the performance of CFRP-strengthened RC beams were assessed. It was demonstrated that the flexural strength of specimens increased with the increase in the width and elastic modulus of CFRP sheets. In addition, the flexural strength and stiffness of specimens increased with the increase in the thickness and length of CFRP sheets up to certain limits, where additional increase in the thickness and lengths of CFRP did not show any further increase in the flexural capacity of beams. It was also indicated that beams strengthened by bonding the CFRP sheets to the bottom face of beam below the longitudinal steel reinforcement is very effective in enhancing the flexural capacity of RC beams. Phan and Nguyen [30] numerically evaluated the effectiveness of RC beams flexural strengthening using three types of FRP materials including GFRP, AFRP, and CFRP using ANSYS software. The numerical simulation was validated with the design code results. It was observed that using FRP is more effective in terms of increasing the strength and reducing the cost, as compared to the traditional technique such as increasing the cross-section of the beam.

Based on the literature above, it can be found that the flexural strength of FRP-strengthened RC beams mainly depends on various parameters, such as CFRP thickness, width, length, number of CFRP layers, and strengthening schemes, implying that there is no sufficient research to cover most of these parameters because the research on the impacts of different CFRP parameters on the flexural strength of HSC beam is quite limited. Therefore, there is a need to conduct FE modeling using ABAQUS, which is a well-known and user-friendly analysis software that is capable of solving a wide-range of linear and nonlinear problems. It is an economic approach that can be used to simulate the effects of various parameters on the flexural behavior of HSC beam. As contribution to fill the need of studying the effects of several parameters on the performance of HSC beams strengthened by CFRP, numerical model is developed in this study using ABAQUS software. The accuracy of the developed model is validated by the experimental results proposed by Hosen et al. [25]. The validated model is utilized to evaluate the effects of concrete strength, width, thickness, lengths, number of layers. and strengthening schemes of CFRP on the performance of HSC beams.

2 Experimental program description

The experimental investigations utilized to validate the developed FE models are presented by Hosen et al. [25]. Descriptions of experimental beams are summarized in Table 1, while the complete details of the test program are available in the corresponding references. Only four specimens were chosen to validate the developed FE models because these four specimens contain the basic parameters, such as number of CFRP layers, CFRP width, and strengthening schemes.

Table 1

Description of the experimental specimens

Beam designation FE designation Total length (mm) Strengthening method
Number of layers FRP width (mm) U-Wrap (mm)
CB-A CB 3,300 — — —
SE-2A SE-1 3,300 2 42 —
SE-2A SE-2 3,300 2 84 —
SE-2A SE-3 3,300 2 125 100

Hosen et al. [25] carried out an experimental investigation to evaluate the flexural strengthening of RC beams externally-side bonded reinforced (E-SBR) using CFRP sheets. The beam specimens were 150 mm in width, 250 mm in depth and 3,300 mm in length with effective span length of 3,000 mm, as illustrated in Figure 1a and b. Two reinforcing deformed steel bars with 10 mm diameter were used to reinforce the beam at the top and bottom of the beams. Round mild steel bars of 6 mm diameter were used as stirrups at a spacing of 75 mm. One beam was fabricated as reference or control specimen. Two beams were reinforced by E-SBR method utilizing double layers of CFRP fabrics with widths of 42 and 84 mm, respectively. The details of control and strengthened beams are illustrated in Figure 1a. Another beam was strengthened by E-SBR method utilizing two layers of CFRP fabrics with width of 125 mm as well as U-wrap end anchorage using CFRP fabrics with 100 mm width, as shown in Figure 1b.

Figure 1 Details of experimental specimens. (a) Control beam and strength end beam. (b) U-wrap end anchorage.
Figure 1

Details of experimental specimens. (a) Control beam and strength end beam. (b) U-wrap end anchorage.

The 28-day compressive strength with average strength of 63.15 MPa and high-strength steel reinforcing bars with yield stress of 520 MPa were used. The round mild steel of grade 300 was used as stirrups. The elastic modulus for both steel reinforcement was 200 GPa. The density, ultimate strength, elastic modulus, elongation at break, and of the CFRP sheets were 1.79 g/m3, 1.5%, 3.9 GPa, 230 GPa, and 0.17 mm, respectively.

3 FE modeling

In this research, numerical simulation of FRP flexural-strengthened HSC beam is performed using ABAQUS FE package (2016). Descriptions of element types, meshing, material models, boundary conditions, and load application are presented in Sections 3.1–3.3.

3.1 Element types and mesh

To simulate accurately the actual behavior of CFRP-strengthened HSC beam using ABAQUS, it is very essential to choose appropriate element types for each element of the beam. The CFRP-strengthened HSC beam is composed of several parts including concrete, steel reinforcing bars, stirrups, steel plates, and FRP sheets. In this study, the concrete was modeled with the 3D eight-node brick element with reduced integration (C3D8R), while the steel reinforcement bars and stirrups were represented by the 3D two-node linear truss element (T3D2). The CFRP sheets were modeled by the four-node shell element (S4R) [31], while the brick element (C3D8R) was also selected to model the supports and loading steel plates. Mesh sensitivity study was conducted to select the mesh size and it was found that global mesh size of 25 mm is appropriate to mesh the elements of FRP strengthened HSC beams. The schematic view of typical meshing for control RC beam is shown in Figure 2.

Figure 2 Typical meshing of the studied control specimen.
Figure 2

Typical meshing of the studied control specimen.

3.2 Materials constitutive models

3.2.1 Concrete

Numerous concrete models including Drucker–Prager (DP) model, concrete damage plasticity (CDP) model, smeared crack model and brittle crack model are provided in ABAQUS software.

The CDP model has been widely employed to model the complex response of concrete beams strengthened with CFRP sheets and it was reported that it has the ability to provide the best performance [29,32]. Therefore, the model is used to represent the nonlinear response of HSC in this study. The model proposed by Desayi and Krishnan [33], in which curve had been utilized to simulate the behavior of RC beams externally-bonded CFRP flexural-strengthened [29,32], was employed to simulate the uniaxial compressive stress–strain response of the concrete, as expressed by the following relations:

(1) f c = E c ε c 1 + ( R − R E ) ε c ε o − ( 2 R − 1 ) ε c ε o 2 + R ε c ε o 3 ,

(2) E c = 4 , 700 f c ′ ,

(3) R = R E ( R σ − 1 ) ( R ε − 1 ) 2 − 1 R ε ,

R σ = R ε = 4 ,

(4) R E = E c E o ,

(5) E o = f c ′ / ε o ,

where f c and ε c are the compressive stress of concrete and its corresponding strain, while f c ′ is the peak strength of concrete, ε 0 is the strain corresponding to f c ′ , and E c is the concrete elastic modulus.

In this model the compressive response of concrete is initially characterized by a linear elastic branch up to a stress equal to 50% of maximum compressive strength of the concrete. This linear elastic branch is represented by the concrete elastic modulus and Poisson ratio. The second branch of the curve follows nonlinear behavior represented by a stress hardening branch followed by strain-softening beyond the peak compressive strength of the concrete up to a strain ( ε c ) of 0.003 [32].

The tensile strength ( f t ) , according to the model of Willam [34], is expressed as follows:

(6) f t = 0.62 ( f c ′ ) 0.5 .

The fracture energy ( G f ) associated with the area under the softening zone of curve is calculated using the model expressed as follows:

(7) G f = ( 0.0469 d 2 − 0.5 d + 26 ) ( f c ′ ) 0.7 ,

where d is the maximum aggregate size.

3.2.2 Reinforcing steel and FRP

The steel reinforcements were modeled by an elastic-plastic constitutive relationship with linear strain-hardening. The elastic modulus, yield strength, and Poisson’s ratio of high-strength steel bars were 200 GPa, 520 MPa, and 0.3, respectively. The round mild steel of grade 300 was used as stirrups. The modulus of elasticity for both steel reinforcement was 200 GPa. The supports and loading plates are also represented with elastic material properties of 200 GPa for the elastic modulus with the Poisson’s ratio of 0.3. LAMINA material property was used to simulate the elastic behavior of CFRP sheets up to the brittle failure mode at the peak tensile stress [35]. The property of CFRP sheets were similar to the properties reported by Hosen et al. [25]. The elastic properties of LAMINA material type used in ABAQUS FE modeling are listed in Table 2.

Table 2

Elastic properties of CFRP sheets

Property E1 (MPa) E2 (MPa) NU12 G12 (MPa) G13 (MPa) G23 (MPa)
Value 230,000 23,000 0.30 5,405 5,405 5,405

3.3 Interaction, boundary conditions, and load application

In this study, bond between the CFRP and concrete was assumed to be perfect, which was achieved by tying the FRP sheets to the adjacent concrete face by using tie constraints options in ABAQUS interaction module. The steel reinforcement bars and stirrups were embedded into the concrete. In order to simulate the actual experimental boundary conditions, the load was applied to the middle of loading plates as line load using the four-point bending test. For all the beams, all three translation-degrees of freedom at both supports were restrained in x, y, and z directions, while the rotational degrees of freedom at end were unrestrained in order to simulate the fixed and pinned support conditions. The load was applied using displacement control method with total number of loading steps of 100.

4 Validation of FE models

To validate the developed FE model’s accuracy, the results of the four beams explained in the previous sections were modeled and analyzed in ABAQUS software. The results of the established FE models were compared against the test data reported by Hosen et al. [25]. The comparison between the test data and the FE modeling are made by considering the load–deflection curves ( P – ∆ ) and the ultimate load ( P u ) is shown in Figure 3. Table 3 summarizes the comparison between the FE predicted ultimate load ( P u , FEM ) and experimental ultimate load ( P u , EXP ) , respectively. The ratios of FE predicted and experimental ultimate loads ( P u , FEM / P u , EXP ) are also presented in Table 3. As shown in Figure 3, the FEM generated P – ∆ curves are in good agreement with the experimental test except the initial stiffness. The reasons were mainly that there were differences in the mechanical behavior of materials between the tested specimens and steel as well as concrete coupons [36]. Besides, the difference in the initial stiffness was possibly due to the slight gap between the tested specimens and axial compression testing machine before the loading, which is not the concern in FE modeling. It is also seen from Table 3 that the ratio of P u , FEM / P u , EXP for specimens CBA, SEA1, SEA2, and SEA3 are 1.05, 1, 0.93, and 0.88, respectively. The mean and corresponding standard deviation (SD) of P u , FEM / P u , EXP were 0.96 and 0.060, respectively. This confirms that the FE models can be used to estimate the performance of HSC RC beams reinforced by different schemes with side-bonded CFRP sheets. Therefore, the developed FE model can be effectively utilized to conduct subsequent parametric analysis in order to evaluate the influences of various parameters including concrete strength, thickness, width, and number of CFRP layers and CFRP strengthening configurations on the response of CFRP-strengthened HSC beams.

Figure 3 Comparison of load-deflection curve at mid-span between the experimental and FE model results. (a) Control specimens. (b) Beams strengthened with two layers of 42 mm width. (c) Beams strengthened with two layers of 84 mm width. (d) Beams strengthened with two layers of 125 mm width and 100 mm wide U-anchor.
Figure 3

Comparison of load-deflection curve at mid-span between the experimental and FE model results. (a) Control specimens. (b) Beams strengthened with two layers of 42 mm width. (c) Beams strengthened with two layers of 84 mm width. (d) Beams strengthened with two layers of 125 mm width and 100 mm wide U-anchor.

Table 3

Results of comparison between the test and FE models

Designation P u , EXP (kN) P u , FEM (kN) P u , FEM / P u , EXP
SBA 37.30 39.10 1.05
SEA1 73.31 73.01 1.00
SEA2 86.18 80.44 0.93
SEA3 101.93 89.92 0.88
Average 0.96
SD 0.06

5 Parametric study

The parametric study is carried out to evaluate the influence of various parameters including concrete strength, width, thickness, length, number of CFRP layers and strengthening configurations on the performance of HSC beams in terms of load-deflection curve, ultimate load, and ductility. Several approaches have been proposed to define the ductility index of composite members [37,38]. However, the ductility expression proposed by Obaydullah et al. [39] is adopted to define the ductility ( μ ) , where it is defined as the ratio of the deformation at ultimate load to the deformation at the yielding of steel in the ascending stage and expressed as follows:

(8) μ = ∆ u ∆ y ,

where ∆ u and ∆ y are the deflections at ultimate load and tension steel yield, respectively.

Each specimen was designated using symbols that includes all the variables of parametric study. For example, SE is used to indicate E-SBR, in which the numbers 1, 2, and 3 represent the strengths of concrete of 25, 40, and 70, respectively. The letter “W” represents the width of CFRP sheets, where the numbers 1, 2, 3, and 4 is used to define the CFRP width of 125, 100, 75, and 50 mm, respectively. The letter “T” describes the thickness of CFRP sheets and the numbers 1, 2, 3, and 4 designate the CFRP layer thickness of 0.5, 0.75, 1, and 1.25 mm, respectively. The form “1L2” indicate the CFRP layer number and length. Three additional abbreviations, BE, UE, and FE, were used to indicate the CFRP strengthening schemes of HSC beams of bottom-SBR, U-wrapping E-SBR, and full wrapping at the shear span between the loading plates E-SBR, respectively.

Table 4 summarizes the details of CFRP-reinforced beams used for parametric study. The effects of these parameters on the performance of CFRP-reinforced HSC beams are graphically shown in Figures 5–10, as presented in Sections 5.1–5.4.

Table 4

Details of FRP-reinforced HSC beams for parametric study

Designation Compressive strength (MPa) Techniques of strengthening Layers no. Length (mm) Width (mm) Thickness (mm)
BC1 25 — — — — —
BC2 40 — — — — —
BC1 70 — — — — —
SE1-W1-t1 25 Sides 1 2,900 125 0.5
SE2-W1-t1 40 Sides 1 2,900 125 0.5
SE3-W1-t1 70 Sides 1 2,900 125 0.5
SE3-W2-t1 70 Sides 1 2,900 100 0.5
SE3-W3-t1 70 Sides 1 2,900 75 0.5
SE3-W4-t1 70 Sides 1 2,900 50 0.5
SE3-W1-t2 70 Sides 1 2,900 125 0.75
SE3-W1-t3 70 Sides 1 2,900 125 1
SE3-W1-t4 70 Sides 1 2,900 125 1.25
SE3-W1-t1-1L2 70 Sides 1 2,500 125 0.5
SE3-W1-t1-1L3 70 Sides 1 2,000 125 0.5
SE3-W1-t1-1L4 70 Sides 2 1,500 125 0.5
SE3-W1-t1-2L4 70 Sides 2 1,500 125 0.5
SE3-W1-t1-3L4 70 Sides 2 1,500 125 0.5
BE3-W1-t2 70 Bottom 1 2,900 125 0.75
UE3-W1-t1 70 U wrapped 1 2,900 150* 0.5
50**
FE3- W1-t2 70 Full mid-wrapped 1 700 125 0.75

*150: U-wrapped FRP across the width; **50: U-wrapped FRP across the depth.

5.1 Effects of concrete compressive strength

To evaluate the effects of concrete compressive strength on the performance of CFRP-strengthened concrete beams, three types of concrete including low-strength concrete (LSC), normal-strength concrete (NSC), and HSC were employed with corresponding compressive strength of 25, 40, and 70 MPa, respectively. The concrete type was classified based on the provision of EC2 [40], where the value of compressive strength for NSC is less than 50 MPa and the value of compressive strength for HSC is between 50 and 90, while the value of compressive strength for UHSC is greater than 90 MPa [41].

Figure 4 shows the effects of different concrete strength on the load-deflection curve of externally bounded CFRP-strengthened concrete beams. It is seen from Figure 4a that the concrete compressive strength has a significant influence on the performance of control RC beams, where the stiffness and peak strength of load–displacement curve is enhanced with the increase in the compressive strength of concrete. Similarly, it is also observed from Figure 4b that increasing the strength of concrete results in increasing the stiffness and peak strength of load–displacement for CFRP-strengthened HSC beams. Figure 5 shows the effects of concrete strength on the ultimate load and ductility of control and CFRP-strengthened HSC beams. As compared with specimen BC1, the ultimate loads of control specimens increase by increasing the strength of concrete with increment percentages of 19.31 and 41.57% for specimens BC2 and BC3, respectively. As compared with specimen SE1-W1-t1, the ultimate flexural loads of specimens SE2-W1-t1 and SE3-W1-t1 increase with the increase in the strength of concrete with increment percentages of 19.54 and 41.57%, respectively, as shown in Figure 5a. It can be noticed from Figure 5b that increasing the compressive strength of concrete could lead to improving the ductility of control specimens without CFRP strengthening, while increasing the concrete compressive strength can results in a slight reduction in the ductility of CFRP-strengthened beams, which mainly attributed to the increase in the CFRP strengthening and modes of debonding failure [42]. Overall, increasing the compressive strength of concrete leads to the improvement of flexural load carrying capacity and ductility of RC beams as well as the load capacity of CFRP-strengthened RC beams with slight reduction in the ductility of CFRP-strengthened RC beams.

Figure 4 Effects of compressive strength on the load–deflection curves of CFRP-strengthened RC beams. (a) Load–deflection curve for control beams. (b) Load–deflection curve for CFRP-strengthened HSC beams.
Figure 4

Effects of compressive strength on the load–deflection curves of CFRP-strengthened RC beams. (a) Load–deflection curve for control beams. (b) Load–deflection curve for CFRP-strengthened HSC beams.

Figure 5 Effects of compressive strength on ultimate load and ductility of control and CFRP-strengthened RC beams. (a) Ultimate strength. (b) Ductility.
Figure 5

Effects of compressive strength on ultimate load and ductility of control and CFRP-strengthened RC beams. (a) Ultimate strength. (b) Ductility.

5.2 Effects of FRP width and thickness

In order to assess the effects of CFRP width and thickness on the performance of FRP-strengthened HSC beams, four types of beams, ESB with CFRP sheets with various widths of 125, 100, 75, and 50 mm, and another four types of beams, ESB with CFRP sheets with various thicknesses of 0.5, 0.75, 1, and 1.25 mm, were used. Figure 6 illustrates the effects of CFRP width and thickness on the performance of CFRP-strengthened HSC beams.

Figure 6 Effects of CFRP width on the load–deflection curves of CFRP-strengthened RC beams.
Figure 6

Effects of CFRP width on the load–deflection curves of CFRP-strengthened RC beams.

It is seen from Figure 6 that increasing the width of CFRP results in a slight improvement in the load–deflection response of specimens. The FE predicted load–deflection curves seem to slightly increase with the increase in the CFRP width due to the larger zone to transfer the stress between the CFRP sheets and concrete resulting in the delay of debonding and increasing the load carrying capacity [29]. It is deduced from Figure 7a that increasing the width of CFRP leads to negligible influence on the stiffness of load–deflection curve of specimens with pronounced increase in the ultimate loads of specimens, which decrease from 89.34 to 80.76 kN with the increase in the CFRP widths from 50 to 125 mm with percentage increase of 9.6%. Similar observation was reported by Teng et al. [43], where it was indicated that the bond stresses from the FRP laminate to the concrete increases when low width of FRP is used. However, Godat et al. [29] conversely observed that using larger CFRP width leads to proportional increase in the load capacity. Özakça et al. [44] also stated that using larger CFRP width results in improving the flexural capacity of CFRP beam due to the fact that larger width of CFRP contributes to the failure concentration at the FRP-concrete interface. It is also observed from Figure 7b that increasing the width of CFRP results in a slight decrease in the ductility of specimens, which confirms the findings of Teng et al. [43]. This is because increasing the width of CFRP accelerates the debonding failure, hence adversely affects the ductility of CFRP beam [44].

Figure 7 Effects of CFRP width on the ultimate load and ductility of CFRP-strengthened HSC beams. (a) Ultimate load. (b) Ductility.
Figure 7

Effects of CFRP width on the ultimate load and ductility of CFRP-strengthened HSC beams. (a) Ultimate load. (b) Ductility.

It confirms that the ultimate load of HSC beam is improved with the increase in the width of CFRP sheets, since the reduction in CFRP sheets width results in the decrease in moment arm in the tensile stress of CFRP and hence, reducing the efficiency of ESB sheet method. However, increasing the width of CFRP sheets seems to induce a slight decrease in the ductility of specimens.

Figure 8 shows the effects of CFRP thickness on the performance of CFRP-strengthened HSC beams. It is seen from Figure 8 that increasing the thickness of CFRP results in a considerable enhancement in the load–deflection response of beams specimens, especially the peak strength without noticeable improvement in the initial stiffness of load–deflection curve. The FE predicted load–deflection curves seem to significantly improve with the usage of thicker CFRP sheets. It is also noticed from Figure 9a that using larger thickness of CFRP sheets improves the ultimate loads of HSC beams increase from 89.34 to 111.44 kN with pronounced increment of 8.90, 15.20, and 19.8% for HSC beams reinforced by CFRP sheets with a thickness of 0.75, 1, and 1.25 mm, respectively, as compared with specimens strengthened with CFRP sheets of 0.50 mm. However, it was indicated by Godat et al. [29] and Pathak and Zhang [45] that the beam capacity enhances with the increase in the FRP thickness up to a certain limit and reduces beyond that due to the accelerations of the FRP debonding by increasing the FRP. Conversely, Esfahani et al. [46] concluded that the thickness of FRP sheet shows no significant impact on the ultimate capacity of FRP strengthened beams. It is also seen from Figure 9b that the FRP thickness does not induce a significant effect on the ductility of CFRP beam specimens, which confirms the findings of Shin and Lee [47].

Figure 8 Effects of FRP thickness on the load–deflection curves for CFRP-strengthened beams.
Figure 8

Effects of FRP thickness on the load–deflection curves for CFRP-strengthened beams.

Figure 9 Effects of FRP thickness on the ultimate load and ductility of FRP-strengthened HSC beams. (a) Ultimate load. (b) Ductility.
Figure 9

Effects of FRP thickness on the ultimate load and ductility of FRP-strengthened HSC beams. (a) Ultimate load. (b) Ductility.

5.3 Effect of FRP length and number of layers

To assess the impacts of FRP length and layers number on the behavior of CFRP-strengthened HSC beams, four beams were externally strengthened by CFRP sheets with different lengths of 2,900, 2,500, 2,000, and 1,500 mm and three beams were externally strengthened by single, double, and triple CFRP layers with length of 1,500 mm. Figure 10 depicts the influence of FRP sheets length on the load–deflection behavior of HSC beams. It is evident from Figure 10 that increasing the lengths of CFRP sheets has no influence on the load–deflection curves and peak load of HSC beams. However, this is different than the study of Godat et al. [29], who indicated that the ultimate load capacity improvement with the increase in the FRP length with a significant improvement in ultimate load capacity is obtained for two-thirds of FRP length of the beam span. It can be observed from Figure 11a and b that increasing the length of the CFRP sheet does not add a pronounced contribution to the ultimate load and ductility of specimens, which may be attributed to the crack propagation related to the change in CFRP length according to Li et al. [48].

Figure 10 Effects of FRP length on the load deflection curve of FRP-strengthened HSC beams.
Figure 10

Effects of FRP length on the load deflection curve of FRP-strengthened HSC beams.

Figure 11 Effects of FRP length on the ultimate load and ductility of FRP-strengthened HSC beams. (a) Ultimate strength. (b) Ductility.
Figure 11

Effects of FRP length on the ultimate load and ductility of FRP-strengthened HSC beams. (a) Ultimate strength. (b) Ductility.

Figure 12 presents the effects of CFRP layer number on the load–deflection performance of CFRP-strengthened HSC beams. It is seen from Figure 12 that using more layers of CFRP sheets significantly improves the initial stiffness and peak of load–deflection curves for HSC beams. It can be also seen from Figure 13a and b that the ultimate load and ductility of CFRP-strengthened beams significantly improves when double and triple layers were used with a percentage increment in the ultimate load of 13.41 and 15.59%, compared to specimens strengthened with single layer of CFP sheet. These results are similar to that of Godat et al. [29], who confirmed that increasing the layer numbers lead to the increase in the stiffness of beams and hence, the ultimate load is improved.

Figure 12 Effects of CFRP layers number on the load-deflection of FRP strengthened HSC beams.
Figure 12

Effects of CFRP layers number on the load-deflection of FRP strengthened HSC beams.

Figure 13 Effects of CFRP layers number on the ultimate load and ductility of FRP-strengthened HSC beams. (a) Ultimate load. (b) Ductility.
Figure 13

Effects of CFRP layers number on the ultimate load and ductility of FRP-strengthened HSC beams. (a) Ultimate load. (b) Ductility.

5.4 Effects of CFRP strengthening schemes

To study the effects of CFRP schemes configurations, various scheme methods were implemented including bottom, sides, U-wrap, and full wrapping between the loading plates. Figure 14 shows the effects of different strengthening techniques on the load–deflection behavior of HSC beams. It can be found from Figure 14 that fixing the CFRP sheets at the bottom of HSC beam results in the greatest stiffness and enhancement in the peak load of load–deflection curves compared to the other strengthening schemes. This is mainly attributed to the capability of transferring additional stress when the CFRP sheets are placed at the bottom of beam face below the longitudinal reinforcing bars, thus the contribution of CFRP sheets are increased [29]. It can be noticed from Figure 15a that wrapping the beam fully by CFRP at the middle part of the beam between the support attained the lowest ultimate load of 65.56 kN, while attaching the CFRP sheets to the sides of the HSC beams leads to further improve in ultimate load of 89.34 kN. Strengthening the HSC beams using U-wrapped shows an ultimate load 157.49 kN, while attaching the CFRP sheet installed at the bottom of the beam shows the highest ultimate load of 170.72 kN. It also observed from Figure 15b that strengthening the specimen with U-wrapped shows the highest ductility, while the utilization of other strengthening schemes show less contribution to the improvement of ductility. It is confirmed that the strengthening schemes delays or prevents the debonding especially when the CFRP sheets are added along the bottom of the beams.

Figure 14 Effects of FRP strengthening scheme on the load-deflection curve of FRP-reinforced HSC beams.
Figure 14

Effects of FRP strengthening scheme on the load-deflection curve of FRP-reinforced HSC beams.

Figure 15 Effects of FRP strengthening scheme on the ultimate load and ductility of FRP-reinforced HSC beams. (a) Ultimate load. (b) Ductility.
Figure 15

Effects of FRP strengthening scheme on the ultimate load and ductility of FRP-reinforced HSC beams. (a) Ultimate load. (b) Ductility.

6 Conclusion

This study is conducted to investigate numerically the effects of CFRP sheets on the performance of HSC beams using ABAQUS software considering various parameters such as concrete strength, width, thickness, length, layer number and strengthening schemes of CFRP sheets. Based on the findings, the following conclusions were drawn:

  1. The compressive strength of concrete plays a vital role on the behavior of RC beam and CFRP-strengthened HSC beams. It is shown that increasing the concrete compressive strength from 25 to 70 MPa leads to an increase in the ultimate load of CFRP-strengthened beams by 41.57%, indicating that using HSC results in improving the ultimate load capacity and strain-hardening response of the flexural members. It can be also found that increasing the compressive strength of concrete could lead to improving the ductility of control specimens without CFRP strengthening, while increasing the concrete compressive strength can result in a slight reduction in the ductility of CFRP-strengthened beams.

  2. The width and thickness of CFRP sheets shows a substantial effect on the flexural performance of CFRP-strengthened HSC beams, where the load-deflection response and ultimate loads seem to increase with decreasing the width of CFRP sheets, while it increases with increasing the thickness of CFRP sheets. The ultimate loads of specimens seem to decrease from 89.34 to 80.76 kN with the increase in the CFRP widths from 50 to 125 mm. Similarly, using larger thickness of CFRP sheets improves the ultimate loads of HSC beams, where the ultimate loads increase by 8.90, 15.20, and 19.8% for HSC beams reinforced by CFRP sheet thickness of 0.75, 1, and 1.25 mm, respectively, as compared with the specimens strengthened with CFRP sheets of 0.50 mm. It is also observed that increasing the width of CFRP results in a slight decrease in the ductility of the specimens, while increasing the FRP thickness does not induce a significant effect on the ductility of CFRP beam specimens.

  3. Increasing the CFRP sheets length shows no influence on the load–deflection response, ultimate strength, and strain-hardening performance of CFRP-strengthened HSC beams. However, increasing the number of FRP layers significantly enhances the load–deflection response, ultimate load, and straining-hardening behavior of CFRP-strengthened HSC beams. Using double and triple CFRP layers to strengthen the beam specimens shows ultimate loads of 103.17 and 105.84 kN with percentage increases of 13.41 and 15.59%, as compared with using single layer of CFRP sheet.

  4. The strengthening schemes show a great contribution to delaying or inhabiting the debonding especially when the CFRP sheets are added along the bottom of the beams. As compared to the wrapping of the beam fully by CFRP at the middle part of the beam between the support, which attained the lowest ultimate load of 65.56 kN, attaching the CFRP sheets to the sides of the HSC beams results in an ultimate load of 89.34 kN with percentage increment of 26.6%. Besides, strengthening the HSC beams using U-wrapped shows a higher ultimate load of 157.49 kN, while attaching the CFRP sheet installed at the bottom of the beam shows the highest ultimate load of 170.72 kN with percentage increment of 58.4 and 61.6%, respectively, compared to attaching the CFRP at the middle part of the beam. It is also observed that the U-wrapped strengthening scheme shows the highest ductility, while the utilization of other strengthening schemes shows less contribution to the improvement of ductility.

Acknowledgement

The authors would like to acknowledge the assistance and encouragement from colleagues for giving us the support and sharing their knowledge and enriching research experience. The authors are grateful to the anonymous reviewers for their constructive comments and suggestions that enhanced the quality of this work.

  1. Funding information: Authors state no funding involved.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and consented to its submission to the journal, reviewed all the results and approved the final version of the manuscript. RAA: data acquisition, methodology, conceptualization, simulation, and writing – original draft. SFA: conceptualization, visualization, validation, and resources. HSA: guidance, critical revision, interpretation, resources, and validation. AA: guidance, resources, and validation. SA: critical revision, resources, simulation, visualization, and writing – reviewing and editing.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: The datasets generated and analysed during the current study are available in the Department of Construction and Projects, Mustansiriyah University. Data are available upon reasonable request and with the permission of the Department of Construction and Projects, Mustansiriyah University. Interested parties may contact sura.alkhafaji@uomustansiriyah.edu.iq (corresponding author) for inquiries regarding dataset access.

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Received: 2024-01-27
Revised: 2024-04-14
Accepted: 2024-05-16
Published Online: 2024-07-01

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  90. Studying the effect of using CFRP warping on strength of husk rice concrete columns
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  126. Thermal buckling analysis for hybrid and composite laminated plate by using new displacement function
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  147. Optimization of local modified metakaolin-based geopolymer concrete by Taguchi method
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  193. Geogrid reinforcement for improving bearing capacity and stability of square foundations
  194. Evaluation of the effluent concentrations of Karbala wastewater treatment plant using reliability analysis
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  199. Analyzing the impact of transfer learning on explanation accuracy in deep learning-based ECG recognition systems
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  202. The impact of using prestressed CFRP bars on the development of flexural strength
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  206. Special Issue: 51st KKBN - Part I
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https://doi.org/10.1515/eng-2024-0048
Keywords for this article
carbon fiber-reinforced polymer; flexural performance; finite element modeling; high-strength concrete beam; strengthening schemes
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BY 4.0

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  145. Assessment of agricultural drought in Iraq employing Landsat and MODIS imagery
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  150. Seismic performance evaluation of concrete buttress dram (Dynamic linear analysis)
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  158. Self-sensing behavior of hot asphalt mixture with steel fiber-based additive
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  161. Investigation of the hydraulic characteristics and homogeneity of the microstructure of the air voids in the sustainable rigid pavement
  162. Enhanced biogas production from municipal solid waste via digestion with cow manure: A case study
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  164. Preparation and investigation of cobalt nanoparticles by laser ablation: Structure, linear, and nonlinear optical properties
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  166. The effect of urban environment on large-scale path loss model’s main parameters for mmWave 5G mobile network in Iraq
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  170. In-depth analysis of critical factors affecting Iraqi construction projects performance
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  175. Evaluating the interaction for embedded H-steel section in normal concrete under monotonic and repeated loads
  176. Estimation of the settlement of pile head using ANN and multivariate linear regression based on the results of load transfer method
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  180. An experimental study of some mechanical properties and absorption for polymer-modified cement mortar modified with superplasticizer
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  183. Design and optimization of two-stage controller for three-phase multi-converter/multi-machine electric vehicle
  184. Evaluation of microstructure and mechanical properties of Al1050/Al2O3/Gr composite processed by forming operation ECAP
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  201. Improving multi-object detection and tracking with deep learning, DeepSORT, and frame cancellation techniques
  202. The impact of using prestressed CFRP bars on the development of flexural strength
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  205. Modification of the 5D Lorenz chaotic map with fuzzy numbers for video encryption in cloud computing
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  207. Evaluation of static bending caused damage of glass-fiber composite structure using terahertz inspection
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