Ductility improvement of GFRP-RC beams using precast confined concrete block in compression zone

Abstract

Fiber-reinforced polymers (FRPs) have received considerable research attention because of their high strength, corrosion resistance, and low weight. However, owing to the lack of ductility in this material and the quasi-brittle behavior of concrete, FRP-reinforced concrete (FRP-RC) beams, even with flexural failure, do not fail in a ductile manner. Because the limited deformation capacity of FRP-RC beams depends on the ductility of their compression zones, the present study proposes using a precast confined concrete block (PCCB) in the compression zone to improve the ductility of the beams. A control beam and four beams with different PCCBs were cast and tested under four-point bending conditions. The control beam failed due to shear, and the PCCBs exhibited different confinements and perforations. The goal was to find an appropriate PCCB for use in the compression zone of the beams, which not only improved the ductility but also changed the failure mode of the beams from shear to flexural. Among the employed blocks, a ductile PCCB with low equivalent compressive strength increased the ductility ratio of the beam to twice that of the control beam. The beam failed in pure flexure with considerable deformation capacity and without significant stiffness reduction.

Graphical abstract

Keywords

ductility / four-point bending test / glass fiber-reinforced polymer / precast confined concrete block

1 Introduction

Despite the merits of employing steel bars in reinforced concrete (RC) structures, using these elements in humid environments such as coastal zones has always been challenging because of the potential for corrosion. The occurrence of corrosion not only leads to a significant reduction in the resistance of the structure, but also the lack of access to the damaged bars for repair or replacement, necessitating use of alternative solutions. Several researchers have focused on identifying appropriate methods for solving this problem [1–4].

Replacing steel bars with fiber-reinforced polymer (FRP) bars is an attractive method because of their high strength, low weight, and corrosion resistance. Different aspects of FRP bars in concrete beams, such as their bonding to concrete, have been studied [5–7]. Furthermore, experimental, analytical, and numerical studies were conducted on the behavior of RC beams with various types of FRP bars. Al-Sunna et al. [8] investigated the response of FRP-RC beams by performing load–deflection experiments on flexural members reinforced by glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP), considering different reinforcement ratios. They determined that shear- and bond-induced deformations can play a major role in FRP-RC elements with moderate to high levels of reinforcement. Zhang et al. [9] conducted experimental and numerical studies on six concrete beams reinforced with varying ratios of basalt fiber-reinforced polymers (BFRPs) and one control beam (CB) reinforced with steel bars. Based on the test and numerical modeling results, they suggested a revised equation for flexural stiffness to determine the FRP-RC beam deflection. Carter and Genikomsou [10] presented a numerical model for predicting the response of BFRP concrete beams using nonlinear finite element analysis. The behavior of RC beams with GFRP bars was examined by performing full-scale tests. A comparison was made between the load and displacement results of the beams and those of typical concrete beams reinforced with steel bars [11]. Habeeb and Ashour [12] investigated the flexural performance of GFRP-RC beams with various levels of reinforcement. They reported that the load−deflection results of the test beams agreed well with the relationships of ACI 440.1R [13]. Considering the reinforcement percentage and depth-to-height ratio, Barris et al. [14] studied the behavior of GFRP-RC beams. A comparison between their results and those obtained from the codes indicated that although the codes provided reasonable relationships for determining the service load, they underestimated the ultimate load. Kara et al. [15] proposed an analytical model that utilized a stiffness matrix to forecast the performance of FRP-RC beams. The results of their model were in good agreement with the results of tests on continuous beams. The experiments revealed a linear response of the GFRP-RC beams compared to the ductile behavior of the beams reinforced with steel bars.

Owing to the linear behavior of FRP in tension and the brittle response of concrete under compression, the load−deflection behavior of FRP-RC beams experiences a lack of ductility [16–18]. Although the use of FRP bars solves the problem of corrosion, the challenge of the limited ductility of RC beams reinforced with FRP remains unsolved. To overcome this problem, some reputable codes of practice, such as ACI-440-1R [13], emphasize that FRP-RC beams must be designed to be over-reinforced; thus, with concrete crushing in compression, the deformability of concrete can be used to provide some degree of ductility in these beams [19]. The use of complementary methods to solve the problem of poor ductility in concrete beams reinforced with FRP bars has been considered by researchers.

One idea is the application of FRP bars together with steel bars (steel/FRP hybrid reinforcement system) [20–22]. Sun et al. [23,24] performed experimental studies based on this idea. In an experimental study [23], the load−displacement diagram of beams with the aforementioned idea was divided into three major phases. The formation of a tensile crack in the concrete, the yielding of the steel bars, and the crushing of the concrete during compression were the ends of these three stages. In Ref. [25], the flexural behavior of hybrid concrete beams (reinforced with GFRP and steel bars) was experimentally and numerically studied. Although the flexural behavior of hybrid RC beams using GFRP-steel bars could be predicted using a finite element model, the contribution of steel in this method would keep the corrosion problem unsolved.

Another idea for improving the ductility of RC beams with FRP is the use of fibers in the concrete texture of the beams. The flexural responses of concrete beams with FRP-RC hybrid reinforcements were experimentally investigated [26]. It was found that the crack width in these beams under a service load did not change compared to that in the FRP/plain concrete beams. However, the strain in the top fibers of the compression zone increased to 0.004. According to the deformation approach, the addition of PFs increased the ductility of the beams by up to 30%. In some studies, researchers leveraged the advantages of engineered cementitious composites (ECCs) have been exploited to improve the ductility of FRP-RC beams [27]. Experimental and numerical analyses were performed on the flexural behavior of concrete beams reinforced with basalt bars in both the conventional and ECC states [28]. The utilization of BFRP bars in an ECC matrix is considerably more efficient than that of bars in concrete. The beams with an ECC matrix failed in a highly ductile manner owing to the high strain capacity of the ECC during compression. In a previous study [29], the flexural responses of concrete beams reinforced with BFRP, GFRP, aramid fiber-reinforced polymer (AFRP), CFRP, and steel bars were evaluated by adding polypropylene, steel, and glass fibers. It was shown that the steel bar/steel fiber-RC (SFRC) and GFRP bar/SFRC beams had the highest and lowest improvements in ductility, respectively.

In the aforementioned methods, although the contribution of steel improved the ductility of the beams to some extent, the risk of steel corrosion was not eliminated because of the presence of steel fibers or bars. Nonmetallic fibers did not significantly affect the ductility of the FRP-RC beams.

In another line of research, the idea of utilizing the concept of Compressive Yielding (CY) has been proposed to enhance the ductility of FRP-RC beams [30]. Thus, a ductile cementitious composite or steel element is placed in the compressive region of the beam for ductility enhancement. When using a steel piece in the compression zone, yielding could result in nonlinear behavior of the beams, preventing their brittleness and sudden failure [31]. In addition to the test results, analytical studies revealed that using the idea of CY could lead to the plastic behavior of the beam, increasing its ductility [32]. Using the concept of the CY block, some researchers [33–36] found that any solution to improve the ductility of the CY block should not increase its strength; otherwise, the bars are likely to undergo brittle failure. Therefore, they suggested using slurry-infiltrated fiber concrete (SIFCON), in which the fibers improve ductility, while the holes control the strength of the block. Although the SIFCON blocks offered better corrosion resistance than beams thoroughly cast with SFRC, they were not free from the risk of corrosion. Thus, it can be concluded that developing a steel-free mechanism and maintain the merits of previous studies is imperative.

The present study proposes a corrosion-free technique using precast confined concrete blocks (PCCBs) in the compression zone of GFRP-RC beams. This technique has never been reported in the literature, and its main advantage is the elimination of steel in any form. Using this technique, it is expected that FRP-RC beams with adequate ductility and without corrosion will be cast and utilized. This study aims to improve the ductility of GFRP-RC beams by taking advantage of a ductile PCCB unit. Using PCCB in the compression zone of GFRP-RC beams enhances the capacity of the compressive strain, enhancing the ductility of the beams. CFRP sheets confine the PCCBs, whereas perforations cause the formation of weak planes in the blocks. Different configurations of confinement for PCCB were selected in discrete types, confinement with the middle gap, and spiral based on previous research [37–39]. Four PCCB-equipped beams with various arrangements of holes and confinements were subjected to four-point bending tests, in addition to a CB specimen without a PCCB. The results and ductilities of these beams are also presented.

2 Experimental program

2.1 Test specimens

Five GFRP-RC beams 180 mm wide, 270 mm high, and 2000 mm long were subjected to four-point bending tests in both conventional and PCCB-equipped forms. For each beam, three GFRP bars with a diameter of 16 mm were used to determine the tensile strength. Transverse steel bars with a diameter of 8 mm were also positioned as closed stirrups with a spacing of 50 mm to provide shear resistance, whereas no stirrups were utilized in the pure flexural zone. As shown in Fig.1, three strain gauges were installed in the PCCB-equipped beams: one at the center of the middle GFRP (position one), one on the CFRP strip in the PCCB (position two), and one on the PCCB (position three). In the CB, only two strain gauges were installed: one at the center of the middle GFRP bar (position one) and the other on the top fiber of the compression zone of the concrete (position two). Two bars with a diameter of 8 mm bent at an angle of 170° were used to sufficiently brace the PCCB in the concrete beam. The details of the specimens are shown in Fig.1. Two 12 mm bars were used only in the shear span in the compression region to support the shear stirrups. The geometric properties of the studied beams are listed in Tab.1. In this table, n, w, t, and s denote the number, width, thickness, and spacing of the CFRP strips utilized in the blocks, respectively, and types A−D represent the different confinement arrangements used for the PCCBs.

Fig.2 illustrates the fabrication stages of the GFRP-RC beams equipped with a PCCB. First, a cage of bars, including longitudinal and transverse GFRP bars, was prepared (Fig.2(a)). A strain gauge was mounted on the middle bar (Fig.2(b)). After placing the reinforcement cage in the mold, the top bars were cut to provide the space required to locate the PCCB (Fig.2(c)). Next, the PCCBs were placed and fixed at the middle of the beam, and concrete casting of the beam was performed (Fig.2(d)). Finally, after 28-d curing of the specimens in water at ambient temperature, the drilling process in the PCCBs was performed, as shown in Fig.2(e). The confinement and perforation configurations of the PCCBs used in this study are shown in Fig.3. Each PCCB was 300, 180, and 100 mm long, wide, and high, respectively. Several studies have shown that confinement is an efficient method for improving the ductility of concrete under compression. To select the type of confinement, the most efficient methods for increasing the ductility of concrete under compression were considered: discrete confinement [37], confinement with a middle gap [38], and spiral confinement [39].

All confinements were provided by one layer of CFRP sheet with a thickness of 0.167 mm. Confinement A was full-length with a middle gap of 50 mm, Confinement B included seven partial strips with a width of 30 mm, Confinement C was partial with unequal spacing, and s1 and s2 were equal to 25 and 50 mm, respectively. Confinement D was a spiral with 30 mm wide strips at a spacing of 15 mm. As shown in Fig.3, two steel shear stirrups were used to provide an appropriate connection between the block and the beam.

In addition to the confinement, perforations were applied to three specimens (GB-A, GB-B, and GB-D). The perforations decreased the equivalent compressive strength of the blocks. The location and diameter of the perforations were selected such that weak planes were formed in the block. These planes were expected to cause local damage under compression. In Tab.1, the geometrical properties of the blocks from the confinement and perforation perspectives are presented.

Fig.4 shows the step-by-step casting procedure for PCCBs. First, block molds with dimensions of 100 mm × 180 mm × 300 mm were fabricated, and the steel braces were fixed at the proper position (Fig.4(a)). The width of the block (180 mm) was selected to be equal to that of the beam. The block height (100 mm) was selected to be in pure compression when the beam carried the loads. The concrete was placed after mold lubrication (Fig.4(b)). Next, the specimens were demolded and cured for 28 d in water at ambient temperature (20 °C) (Fig.4(c)). Finally, after sandblasting, rounding of the corners, and cleaning of the surfaces of the blocks, wrapping with CFRP sheets was performed (Fig.4(d) and Fig.4(e)). As shown in Fig.1, the PCCBs were located in the middle of the beam at the top of the cross-section.

2.2 Materials

The properties of the GFRP bars, including the diameter, ultimate tensile strength, elastic modulus, and ultimate strain, provided by the manufacturer, are summarized in Tab.2. The yield stress, tensile strength, and ultimate strain of the transverse steel bars were 340 MPa, 520 MPa, and 0.19, respectively.

CFRP (C300) sheets were used to confine the PCCBs. The thickness, elastic modulus, ultimate strength, and ultimate strain of the CFRP sheets were 0.167 mm, 235 GPa, 4950 MPa, and 0.0292 mm, respectively. All of the aforementioned characteristics of the CFRP, GFRP, and steel bars were provided by their manufacturers.

According to ASTM C39, the compressive strength of the concrete in the CB and the main body of the PCCB-equipped beams was evaluated as 45 MPa. One cubic meter of concrete consisted of 459 kg/m³ of Ordinary Portland Cement (OPC), 1020 kg/m³ gravel, 680 kg/m³ sand, and 230 kg/m³ water. The water-to-cement ratio of concrete was 0.5. In addition, 0.8% of OPC of the superplasticizer was added to increase the concrete workability.

To ensure the failure of the PCCBs, their equivalent compressive strength was considered to be less than that of the concrete in the main body of the beam. Therefore, the water-to-cement ratio for PCCBs was regarded as unity. All the PCCBs were cast from the same concrete. To determine the compressive behavior of the PCCB specimens, a uniaxial compressive test was performed as a displacement control at 0.1 mm/min. These specimens were identical to the PCCBs utilized in the beams in terms of concrete type, geometrical properties, confinement, and perforation arrangements.

Fig.5 illustrates the stress-strain diagram of the three PCCBs (except for PCCB-type C) and one control specimen. The results for PCCB type C were lost because of unexpected human error. The control specimen was a concrete block that was neither confined nor perforated. The dimensions of this block were identical to those of the PCCBs.

The compression test setup and failure modes of the PCCBs are shown in Fig.6. The equivalent compressive strength of the blocks was defined as the maximum load-carrying capacity divided by the loaded area. The values for the control PCCB and PCCB types A, B, and D were 15.3, 18.8, 14.6, and 5 MPa, respectively.

As expected, the control specimens did not exhibit substantial ductility. In PCCB type A, the equivalent compressive strength and ductility were higher than those of the control blocks. As this specimen was heavily confined, only a limited number of perforations were made in the middle, as shown in Fig.3. The failure of this specimen began with concrete crushing in the middle gap and reached the ultimate limit state with the failure of the CFRP sheets (Fig.6). As shown in Fig.5, the confinement and perforation of the PCCB type B did not affect the equivalent compressive strength of the specimen. However, the ductility of the specimens was improved. In this specimen, concrete crushing in the perforated area and failure of the CFRP sheets were also observed.

Tab.1 and Fig.3 show 15 perforations with a 16 mm diameter and eight perforations with a 12 mm diameter (N15D16−N8D12) for PCCB type D, so it had a high level of perforations and light confinement. This specimen exhibited an extremely low equivalent compressive strength and acceptable ductility. The crushing of the concrete initiated the failure of this specimen in a heavily perforated area. Subsequently, the concrete crushing mode was observed in the middle with the failure of the middle sheets and the elimination of confinement in this part.

2.3 Test setup and procedures

Fig.7 illustrates the four-point bending test setup. The loading was imposed through a 500-kN capacity load cell. The two supports were located 1800 mm apart, and the distance between them was 600 mm. Three linear variable differential transformers (LVDTs) (LVDT1 and LVDT3 under the loading points and LVDT2 at the midspan) were used along the bending span to record the displacements. Furthermore, the strain results for the top fibers of the concrete in the PCCB, CFRP sheet, and GFRP bar were collected using a data logger at every loading increment. Loading was applied to the beams using a hydraulic jack, and a rigid steel element was used to perform a four-point bending test. The load was applied in increments of 5 kN. At the end of each load increment, the load was maintained constant to map the crack patterns. The tests were performed on a rigid floor to prevent secondary displacement caused by support settlement.

3 Results and discussion

This section describes the results of the four-point bending tests on the PCCB-equipped GFRP-RC beams. The results include the load−deflection of the beams and the load against the strain of the top fiber in the PCCB (SG3 in Fig.1), strain of the CFRP sheet (SG2 in Fig.1), and strain of the GFRP bar (SG1 in Fig.1 and Fig.2).

3.1 Load−deflection characteristics

The parameters describing the load−deflection behavior of the beams, including cracking load (P_c), maximum load (P_max), cracking displacement (Δ_c), corresponding displacement of the maximum load (Δ₀), initial stiffness (k), and ductility (µ), are presented in Tab.3. The load and displacement related to the change in the slope of the load−displacement curve are regarded as the cracking load and displacement, respectively. A shear mode was considered for the failure of the CB so that the effect of the PCCBs on the failure mode change could be observed.

Fig.8 depicts the load−displacement records of all the beams. In the CB, as the load increased, some flexural−shear cracks occurred, and the beam stiffness reduced. Because the beam was not strong enough in shear, the beam underwent a shear failure, as expected. This beam did not have significant ductility, and the flexural−shear cracks and contribution of the steel stirrups were responsible for this ductility. For GB-C, consisting of a PCCB with unequal spacing and no holes in the compression zone, no remarkable change was observed in the flexural strength. This beam experienced a pure shear failure mode similar to that of the CB. In GB-A, the use of a confined and perforated PCCB with a strength of 18.8 MPa, which was weaker than PCCB C, resulted in ductility improvement to some extent before failure. Beam experienced a flexural−shear failure. In GB-B, using a PCCB with a strength of 14.6 MPa, which was less than PCCBs types A and C, resulted in some plastic deformations in the compressive PCCB, providing a level of ductility and leading to a flexural−shear failure. Eventually, in GB-D, which contained a ductile and heavily perforated PCCB with 5 MPa strength, some sources of ductility were clearly observed in the load−displacement curve of the beam. Apparently, the ductile behavior of the block in the middle third of the beam caused a substantial rotational capacity, which influenced the load−displacement response of the beam. Notably, this beam failed in pure flexure with crushing of the PCCB. The low equivalent compressive strength of the PCCB caused the flexural capacity of the beam to be less than its shear strength; therefore, the beam failed in flexure. The contribution of the PCCB can change the failure mode of the beam from brittle (shear) to completely ductile (flexure). Because the beam did not experience any shear cracks, the entire ductility exhibited by the beam can be attributed to the flexural response of the beam, which was mostly due to the formation of numerous cracks in PCCB D.

Fig.9 shows the load−displacement curves of GB-D and GB-B for all the three LVDTs. As expected, owing to the symmetry of the specimen and loading, the results for LVDT1 and LVDT3, which were located under the two loading points, were similar. This similarity shows that the beam maintained its symmetry at the end of the loading, and in this regard, the test was performed precisely. In GB-D, at an approximate load of 100 kN, a remarkable change was observed in the slope of the load−displacement diagram. In fact, at this stage of loading, the stiffness of the PCCB type D decreased (Fig.5). This can be attributed to the propagation of cracks in the blocks. When the load exceeded 150 kN, the load−displacement curve of the specimen decreased owing to the failure of the CFRP sheets in the block. In the case of GB-B, the cause of the shear failure effect, the results of the load−deflection in LVDT1 and LVDT3 did not match exactly.

The placement of the PCCB in the compression region of a beam significantly influences its behavior. Depending on the strength and ductility of the PCCB, the addition of this block changed its load−displacement response, ductility, load-carrying capacity, and failure mode. A comparison of the stiffnesses of the studied beams revealed that adding PCCBs, even those with low strength, did not result in a remarkable change in their stiffness. Thus, the main advantage of using PCCBs in the beams is improvement in ductility without a considerable decrease in stiffness.

3.2 Strain distribution

Fig.10 shows the relationship between the applied load and the strain of the concrete at the extreme compression fiber and the strain of the GFRP bar. Given the numerous cracks in the PCCB in GB-D, the strain gauge attached to the top fiber of this block could not thoroughly record the strains (to the end of loading), and it was separated from the beam at approximately 120 kN.

Fig.10(b) shows the strain of the GFRP bars against an applied load. All the measured strains in the reinforcement are less than the ultimate tensile strain of the GFRP. The maximum strain recorded by the gauges is 0.0086, which is significantly lower than the minimum GFRP ultimate tensile strain of 0.0283 (Tab.2). This implies that none of the GFRP bars failed under tension, which agrees with the test observations. The beam deficiency in shear does not allow the GFRP bars to experience their tensile capacity.

3.3 Ductility

The ductility of a beam represents its capacity to sustain plastic deformation without a decrease in its bearing capacity. According to this definition, ductility can be calculated using the deformation or energy dissipation of a beam. For concrete beams reinforced with steel bars, ductility is defined as the ratio of the beam displacement in the ultimate limit state to the displacement corresponding to the yield of the steel bars. However, FRP-RC beams have no definite yielding points. Therefore, this definition of ductility cannot be applied to these beams. Several techniques have been recommended for calculating the ductility factors of GFRP-RC beams. According to previous research [31], the ductility factor in a load−displacement curve with a descending part after the maximum load is equal to the ratio of the displacement corresponding to 0.8F_max to the displacement corresponding to the intersection of a horizontal line passing through the maximum load and the line connecting the origin to 0.75F_max. Vijay and GangaRao [40] suggested another definition of the deformability factor: the ratio of the area under the load−deflection curve up to the ultimate load to that up to a displacement of span/180. The present study determined ductility using an energy method [41,42]. Accordingly, a bilinear curve was fitted to the load−displacement curve of each specimen up to the strength limit, such that the area under the bilinear curve was equal to the area under the load−displacement curve. Thus, as shown in Fig.11, the areas confined above and below the curve were equal after bilinearization. Finally, the ductility factor (µ) is calculated using the ultimate deflection (Δ_u) and the yield deflection (Δ_y), as shown in Eq. (1).

(1)\mu =\frac{{\mathrm{\Delta }}_{\mathrm{u}}}{{\mathrm{\Delta }}_{\mathrm{y}}}.

Fig.12 shows the bilinearization results of the load−displacement curves for different specimens and a column chart comparing the ductility ratios. The ductility factors obtained from the studied beams are listed in Tab.3 and shown in Fig.12(f). Considering that the CB would fail under shear stress, the block was expected to improve the ductility factor and change the failure mode to ductile. A comparison of the ductility factors indicates that the use of a PCCB with an inappropriate strength causes a negligible change in the ductility factor. In these cases, the block is not activated, and the beams experience a shear (GB-C) or flexural−shear (GB-A and GB-C) failure mode. The ductility of the block affected the deformation capacity of the beams. However, this is not promising because the blocks are not the main components of failure.

A designed block with appropriate strength and ductility can improve ductility and alter the failure mode to a desirable one. In this study, the greatest effect of the PCCBs was observed in GB-D, with a ductility factor of 4.84, which was approximately twice that of the CB. The beam failed during flexure, and the block was the main source of its deformation capacity. This outcome clearly indicates that using the plastic capacity of the compression zone is an efficient method for increasing the ductility of GFRP-RC beams. Notably, a load-carrying capacity loss in GB-D was observed, which is attributable to the fact that the failure mode of the CB had to be altered to a ductile one. To force the beams to fail in pure flexure, the moment resistance of the beams (in the middle) must be decreased to less than their shear strength. In this case, a decrease in the load-carrying capacity of the beams is inevitable.

3.4 Crack propagations

The ductility of the beams was primarily due to their cracking and deformation capacities. The initiation and propagation of cracks in all elements of the studied beams were of crucial importance in interpreting the obtained results. Fig.13 shows the crack patterns and failure modes of the CB and GB-D in their ultimate limit states. A close view of the other PCCBs is shown in Fig.14. These figures show that cracking occurred in all PCCBs. The cracking mechanism can provide a new source of ductility compared to the CB. Owing to the specific mechanical properties of the blocks in GB-A and GB-C from the perspectives of strength and ductility points of view, the blocks could not change the failure mode of the beams. Although the blocks improved the ductility of the beams, the flexural capacity of the beams was higher than their shear strength, and flexural failures could not occur. In fact, the full deformation capacity of the PCCBs was not utilized in these beams. In GB-D, a PCCB with low strength and high ductility was employed. The results are presented in Fig.13, fewer cracks were observed in the main body, and most of the plastic deformations of the beam were concentrated in the PCCB. In fact, all energies dissipated by the rupture of CFRP, cracking, and crushing of concrete contributed to the behavior of the beam and caused not only a change in the failure mode, but also exhibited promising ductility.

4 Analytical study

Fig.15(a) shows a side view of a GFRP-RC beam equipped with a PCCB. Based on Bernoulli’s principle, the distribution of the strain over the height of the beams is considered to be linear. Fig.15(b) and Fig.15(c) depict the distributions of the strain, stress, and internal forces over the sections of these beams in both states, with and without the PCCB, respectively. To calculate the moment resistance of the over-reinforced FRP-RC sections (Fig.15(b)), it is well established that the strain at the farthest compressive fiber of concrete is assumed to be its ultimate value (ε_cu = 0.0035). In sections with PCCB (Fig.15(c)), there are two modes of failure: one is governed by block collapse and the other is due to the compressive failure of normal concrete. In a logical analytical approach, failure with the maximum moment resistance can be regarded as the governing failure mode. In GB-D, owing to the high strain capacity of the block (approximately 2%), the failure of the section was governed by normal concrete. In fact, the maximum moment resistance of this section happens when the farthest compressive fiber of normal concrete reaches its ultimate strain (ε_h = ε_cu = 0.0035). Based on these assumptions, there is a known value related to the top fiber of the normal concrete in the linear strain diagram. A neutral-axis depth was required to determine the entire strain diagram. This value can be easily determined using the equilibrium equation for the horizontal forces acting on a section. Notably, the stress−strain diagrams of all effective components can be utilized to convert the strain to stress variations along the height of the beam. As illustrated in Fig.15(c), for the GFRP-RC beam equipped with the PCCB, the stress in the compressive zone exhibited two different distributions for the PCCB and normal concrete. The total compressive force includes two forces: the compressive force exerted by the PCCB and that exerted by normal concrete. After the neutral axis was calculated, the moment resistance of the section was determined by summing the tensile and compressive forces multiplied by the arms at a single point.

Because only GB-D failed in pure flexure, the behavior of this beam could be predicted using the above method. According to the equations and calculations presented in Electronic Supplementary Materials, the moment resistance and load-carrying capacity of the beam were 45 kN·m and 150.2 kN, respectively. In our experimental study, GB-D failed under a 158.33 kN load. It can be observed that the model prediction of the load-carrying capacity is consistent with the test results.

5 Conclusions

This study examines the use of a ductile block (PCCB) in the compression zone of a GFRP-RC beam. The goal was to utilize the deformation capacity of PCCBs to enhance the ductility of the beams. The CB failed in shear with 45 MPa concrete compressive strength, and the PCCBs exhibited different confinement and perforation configurations. In addition to the CB, four RC beams equipped with PCCBs were cast and tested under four-point bending conditions. A uniaxial compressive test was performed to determine the compressive behavior of the PCCBs. The highest and lowest equivalent compressive strength were 19 and 5MPa, respectively. It was found that PCCBs with inappropriate strengths caused negligible changes in the ductility of the beams. The block is not fully activated in these cases, and the beams experience a shear or flexural−shear failure mode. Although the block’s ductility affected the beams' deformation capacity, this improvement was negligible because the blocks were not the main components of the failure. A concisely designed block with appropriate strength and ductility can considerably improve ductility and alter the failure mode to a desirable one. In this study, the most ductile block with the lowest strength was the most effective block for the behavior of the beams. This beam exhibited a ductility factor of 4.84, approximately twice more than that of the CB. The beam failed during flexure, and the block was the main source of its deformation capacity.

An analytical study based on Bernoulli’s principle was conducted to predict the moment resistances of beams. In the sections with a ductile block at the top, owing to their high strain capacity, the failure of the sections was governed by normal concrete. The neutral-axis depth was determined based on the satisfaction of the equilibrium equation. Because only one beam failed in pure flexure, the behavior of this beam could be predicted using this method. The difference between the load-carrying capacity predicted by the model and the test results was only 5%.

Notably, the present study is only a preliminary step toward proving the efficiency of the proposed idea. The following steps of the investigation are to use PCCBs in FRP-RC beams with a flexural mode of failure, find various methods of block construction to provide higher ductility, determine the precise properties of PCCBs with respect to the beam properties, and provide analytical relationships to estimate the PCCB properties. However, these investigations require additional experiments.

References

[1] Salama A S D, Hawileh R A, Abdalla J A. Performance of externally strengthened RC beams with side-bonded CFRP sheets. Composite Structures, 2019, 212: 281–290

[2] Naser M Z, Hawileh R A, Abdalla J A. Fiber-reinforced polymer composites in strengthening reinforced concrete structures: A critical review. Engineering Structures, 2019, 198: 109542

[3] Abuodeh R O, Abdalla J A, Hawileh R A. Flexural strengthening of RC beams using aluminum alloy plates with mechanically-fastened anchorage systems: An experimental investigation. Engineering Structures, 2021, 234: 111969

[4] Hawileh R A, Mhanna H H, Al Rashed A, Abdalla J A, Naser M Z. Flexural behavior of RC beams externally bonded with polyethylene terephthalate (PET) fiber reinforced polymer (FRP) laminates. Engineering Structures, 2022, 256: 114036

[5] Ehsani M R, Saadatmanesh H, Tao S. Design recommendations for bond of GFRP rebars to concrete. Journal of Structural Engineering, 1996, 122(3): 247–254

[6] Tastani S P, Pantazopoulou S J. Bond of GFRP bars in concrete: Experimental study and analytical interpretation. Journal of Composites for Construction, 2006, 10(5): 381–391

[7] Antonietta Aiello M, Leone M, Pecce M. Bond performances of FRP rebars-reinforced concrete. Journal of Materials in Civil Engineering, 2007, 19(3): 205–213

[8] Al-Sunna R, Pilakoutas K, Hajirasouliha I, Guadagnini M. Deflection behaviour of FRP reinforced concrete beams and slabs: An experimental investigation. Composites. Part B, Engineering, 2012, 43(5): 2125–2134

[9] Zhang L, Sun Y, Xiong W. Experimental study on the flexural deflections of concrete beam reinforced with Basalt FRP bars. Materials and Structures, 2015, 48(10): 3279–3293

[10] Carter J, Genikomsou A S. Investigation on modeling parameters of concrete beams reinforced with basalt FRP bars. Frontiers of Structural and Civil Engineering, 2019, 13(6): 1520–1530

[11] Benmokrane B, Zhang B, Laoubi K, Tighiouart B, Lord I. Mechanical and bond properties of new generation of carbon fibre reinforced polymer reinforcing bars for concrete structures. Canadian Journal of Civil Engineering, 2002, 29(2): 338–343

[12] Habeeb M, Ashour A F. Flexural behavior of continuous GFRP reinforced concrete beams. Journal of Composites for Construction, 2008, 12(2): 115–124

[13] AlkhrdajiTFyfeE RKorffJSchupackMBakisC EGentryT RShieldC K. Guide for the design and construction of structural concrete reinforced with FRP bars. In: Proceedings of the American Concrete Institute (ACI) committe 440. Detroit: American Concrete Institute (ACI), 2006

[14] Barris C, Torres L, Turon A, Baena M, Catalan A. An experimental study of the flexural behaviour of GFRP RC beams and comparison with prediction models. Composite Structures, 2009, 91(3): 286–295

[15] Kara I F, Ashour A F, Dundar C. Deflection of concrete structures reinforced with FRP bars. Composites. Part B, Engineering, 2013, 44(1): 375–384

[16] Adam M A, Said M, Mahmoud A A, Shanour A S. Analytical and experimental flexural behavior of concrete beams reinforced with glass fiber reinforced polymers bars. Construction & Building Materials, 2015, 84: 354–366

[17] Mousa S, Mohamed H M, Benmokrane B, Nanni A. Flexural behavior of long-span square reinforced concrete members with uniformly distributed fiber-reinforced polymer bars. ACI Structural Journal, 2020, 117(4): 209–222

[18] Deitz D, Harik I, Gesund H. Physical properties of glass fiber reinforced polymer rebars in compression. Journal of Composites for Construction, 2003, 7(4): 363–366

[19] ACICommittee. Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer (FRP) Bars (ACI 440.1R-15). Farmington Hills, MI: American Concrete Institute, 2015

[20] Zhu P, Xu J, Qu W. Fatigue shear performance of concrete beams reinforced with hybrid (glass-fiber-reinforced polymer + steel) rebars and stirrups. Frontiers of Structural and Civil Engineering, 2021, 15(3): 576–594

[21] Wu C, He X, He L, Zhang J, Wang J. Combination form analysis and experimental study of mechanical properties on steel sheet glass fiber reinforced polymer composite bar. Frontiers of Structural and Civil Engineering, 2021, 15(4): 834–850

[22] Fakharifar M, Dalvand A, Sharbatdar M K, Chen G, Sneed L. Innovative hybrid reinforcement constituting conventional longitudinal steel and FRP stirrups for improved seismic strength and ductility of RC structures. Frontiers of Structural and Civil Engineering, 2016, 10(1): 44–62

[23] Sun Z, Tang Y, Luo Y, Wu G, He X. Mechanical properties of steel-FRP composite bars under tensile and compressive loading. International Journal of Polymer Science, 2017, 2017: 1–11

[24] Sun Z, Fu L, Feng D C, Vatuloka A R, Wei Y, Wu G. Experimental study on the flexural behavior of concrete beams reinforced with bundled hybrid steel/FRP bars. Engineering Structures, 2019, 197: 109443

[25] Xingyu G, Yiqing D, Jiwang J. Flexural behavior investigation of steel-GFRP hybrid-reinforced concrete beams based on experimental and numerical methods. Engineering Structures, 2020, 206: 110117

[26] Wang H, Belarbi A. Ductility characteristics of fiber-reinforced-concrete beams reinforced with FRP rebars. Construction & Building Materials, 2011, 25(5): 2391–2401

[27] Qiao Z, Pan Z, Xue W, Meng S. Experimental study on flexural behavior of ECC/RC composite beams with U-shaped ECC permanent formwork. Frontiers of Structural and Civil Engineering, 2019, 13(5): 1271–1287

[28] Cai J, Pan J, Zhou X. Flexural behavior of basalt FRP reinforced ECC and concrete beams. Construction & Building Materials, 2017, 142: 423–430

[29] Aydın E, Boru E, Aydın F. Effects of FRP bar type and fiber reinforced concrete on the flexural behavior of hybrid beams. Construction & Building Materials, 2021, 279: 122407

[30] Guo B, Lin X, Wu Y, Zhang L. Evaluation of flexural resistance of compression yielded concrete beams reinforced with fibre reinforced polymers. Engineering Structures, 2022, 250: 113416

[31] Wu Y F. New avenue of achieving ductility for reinforced concrete members. Journal of Structural Engineering, 2006, 132(9): 1502–1506

[32] ShenXLiXShiWChenY. Numerical study on flexural behaviour of FRP reinforced beams with compression yielding blocks. Case Studies in Construction Materials, 2022, 17(12): e01169

[33] Wu Y F, Zhou Y W, He X Q. Performance-based optimal design of compression-yielding FRP-reinforced concrete beams. Composite Structures, 2010, 93(1): 113–123

[34] LiuX CWuY FLeungA Y THouJ G. Mechanical behavior of mild steel compressive yielding blocks. In: Proceedings of the First Asia-Pacific Conference on FRP in Structures. Hong Kong: ePublications@scu, 2007, 12–14

[35] Wu Y F, Jiang J F, Liu K. Perforated SIFCON blocks—An extraordinarily ductile material ideal for use in compression yielding structural systems. Construction & Building Materials, 2010, 24(12): 2454–2465

[36] Zhou Y, Wu Y, Teng J, Leung A. Ductility analysis of compression-yielding FRP-reinforced composite beams. Cement and Concrete Composites, 2009, 31(9): 682–691

[37] Barros J A, Ferreira D R. Assessing the efficiency of CFRP discrete confinement systems for concrete cylinders. Journal of Composites for Construction, 2008, 12(2): 134–148

[38] Wang W, Sheikh M N, Al-Baali A Q, Hadi M N S. Compressive behaviour of partially FRP confined concrete: Experimental observations and assessment of the stress−strain models. Construction & Building Materials, 2018, 192(20): 785–797

[39] Ismail R, Rashid R S M, Chan W C, Jaafar M S, Hejazi F. Compressive behavior of concrete cylinder fully and partially confined by carbon fibre-reinforced polymer (CFRP). Construction & Building Materials, 2019, 201: 196–206

[40] Vijay P, GangaRao H V. Bending behavior and deformability of glass fiber-reinforced polymer reinforced concrete members. Structural Journal, 2001, 98(6): 834–842

[41] ZhouY WWuY FTengJ GLeungA Y T. Parametric space for the optimal design of compression-yielding FRP-reinforced concrete beams. Materials and Structures, 2010, 43(1−2): 81−97

[42] Wu Y F, Oehlers D J, Griffith M C. Rational definition of the flexural deformation capacity of RC column sections. Engineering Structures, 2004, 26(5): 641–650

RIGHTS & PERMISSIONS

Higher Education Press