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Article

Flexural and Shear Strengthening of High-Strength Concrete Beams Using near Surface Basalt Fiber Bars

by
Ahmed Ashteyat
1,2,
Ala’ Taleb Obaidat
3,
Ahmad Al-Khreisat
4 and
Mu’tasime Abdel-Jaber
1,*
1
Civil Engineering Department, The University of Jordan, Amman 11942, Jordan
2
Department of Civil Engineering, College of Engineering in Al-Kharj, Prince Sattam Bin Abdulaziz University, Al-Kharj 11942, Saudi Arabia
3
Civil Engineering Department, Philadelphia University, Amman 19392, Jordan
4
Civil Engineering Department, Cairo University, Cairo 12613, Egypt
*
Author to whom correspondence should be addressed.
Infrastructures 2025, 10(1), 1; https://doi.org/10.3390/infrastructures10010001
Submission received: 3 November 2024 / Revised: 10 December 2024 / Accepted: 17 December 2024 / Published: 24 December 2024
Browse Figures
Review Reports Versions Notes

Abstract

Strengthening of reinforced concrete (RC) structures has become a primary challenge in civil engineering. Different materials and procedures have been used in order to repair or strengthen RC structures. In this research, the NSM-Basalt Bar (NSM-BFRP) technique was used to strengthen high-strength reinforced concrete beams in flexure and shear. Twelve beams were designed, constructed, and tested under four-point loads. Six of them were designed to have insufficient longitudinal steel reinforcement to make sure that the failure would be a flexural failure in the control beam. Whereas, the other six specimens were designed to have insufficient transverse steel reinforcement to make sure that the failure will be a shear failure in the control beam. All RC beams were strengthened using NSM-BFRP with different configurations except control specimens. The load deflection curve, the cracking pattern and the failure mode were evaluated. The experimental results reveal that NSM-BFRP bars significantly enhance the ultimate load capacity of high-strength concrete beams, with flexural capacity improvements of up to 33.33% and shear capacity enhancements of up to 63.5%. However, the use of BFRP bars also led to a shift in failure modes from flexural to shear, particularly in specimens with increased flexural reinforcement. The findings suggest that while NSM-BFRP bars are highly effective in strengthening concrete beams, careful consideration of the reinforcement configuration is necessary to avoid premature shear failure and ensure balanced structural performance.

1. Introduction

Reinforced concrete (RC) is extensively utilized in the construction industry due to its exceptional structural strength, durability, and versatility. However, over time, factors such as environmental exposure, heavy loading, insufficient maintenance, or inherent design flaws can lead to the degradation of these structures. Consequently, effective strengthening and repair techniques are essential to maintain the structural integrity of RC elements and extend their service life. One promising approach is the use of near-surface mounted (NSM) reinforcement, which has gained attention as a reliable method for enhancing the performance of various RC components. This technique involves embedding high-strength fiber-reinforced polymer (FRP) bars or strips into the near-surface layer of concrete, thereby increasing the element’s load-carrying capacity and overall strength [1,2,3,4,5,6,7].
Fiber-reinforced polymer (FRP) materials are widely recognized as one of the most effective retrofit solutions for enhancing the performance of various structural components. There are several types of FRP bars available on the market, including those made from carbon, glass, aramid, and basalt fibers, commonly referred to as CFRP, GFRP, AFRP, and BFRP, respectively. In terms of improving the load-carrying capacity of retrofitted beams, particularly through near-surface mounted (NSM) strengthening, extensive research has demonstrated the efficacy of CFRP and GFRP bars. Most studies have focused on these two types of FRP due to their established performance characteristics. The NSM technique involves embedding FRP rods into pre-cut grooves in the concrete cover of the element to be strengthened [8,9,10,11,12,13,14].
In a study by Al-Zu’bi et al. [15], the shear behavior of reinforced concrete (RC) beams strengthened by near-surface mounted (NSM) strip was investigated. Different strengthening configurations were employed by varying the length and inclination angle of the CFRP laminates. Results indicated that NSM-CFRP strengthening increased the load-carrying capacity, ductility, stiffness, and toughness from 8% to 41%, 9% to 78%, 24% to 159%, and 22% to 254%, respectively. Results also confirmed that as the CFRP laminate length decreases, the efficacy of the strengthening process increases, where the load-carrying capacity, ductility, stiffness, and toughness improved from 8% to 19%, 10% to 21%, 8% to 68%, and 26% to 119%, respectively. Similarly, Qaisi et al. [16] examined the effects of NSM carbon fiber-reinforced polymer (CFRP) strips and ropes on the flexural performance of RC beams. The study compared two groups of beams: one set reinforced with CFRP ropes and the other with CFRP strips. The use of NSM-CFRP ropes was found to significantly increase the load-carrying capacity of the beams, with improvements ranging from 18% to 51% compared to the control group. Furthermore, Al-Khreisat et al. [17] conducted an experimental study to assess the effectiveness of NSM-CFRP ropes in strengthening RC deep beams exposed to high temperatures. The results indicated that the ultimate load capacity of the strengthened beams improved by 19% to 46%, while the repaired beams exhibited enhancements ranging from 40.8% to 64.6%. These studies highlight the potential of NSM-FRP reinforcement, particularly CFRP-based solutions, for enhancing the structural performance of reinforced concrete beams under various loading and environmental conditions.
To investigate the bond stress–slip relationship and better understand the bond behavior of the near-surface mounted (NSM) basalt fiber reinforced polymer (BFRP) bar technique in reinforced concrete members, Gopinath et al. [18] examined the pull-out behavior of BFRP bars. Key factors such as bonded length, bar diameter, and groove size were evaluated. The study revealed that increasing the groove size did not significantly enhance the failure load. Based on the results, the authors recommended a groove size-to-diameter ratio for BFRP bars between 1.5 and 2.0 for practical applications. Further research on RC beams reinforced with NSM BFRP reinforcement demonstrated the technique’s effectiveness, with no signs of debonding or bond failure in any of the reinforced beams. It was observed that increasing the percentage of NSM reinforcement improved both the flexural capacity and the effective pre-yield stiffness of the beams. However, deflection and energy ductility were significantly reduced as the NSM reinforcement ratio increased. The study concluded that to avoid unintentional sudden compression failure, the total percentage of steel reinforcement in a beam section should remain within the limits specified by design codes.
In another study, Saribiyik et al. [19] presented experimental and analytical research on the shear strengthening of reinforced concrete (RC) beams using BFRP composites. Their investigation focused on the impact of different wrapping techniques and the use of BFRP composites for enhancing the shear capacity of RC beams. The study compared experimental results with existing design methods to evaluate suitable calculation techniques for these strengthening approaches. The RC beams tested were designed with low-strength concrete, minimal steel reinforcement, and rectangular cross-sections, providing adequate flexural moment capacity but insufficient shear capacity. The beams were subjected to shear reinforcement using BFRP composites in various configurations, including strip, side-bonding, and full wrapping. A four-point flexural test was conducted under monotonic loading to examine the effects on shear strength, deflections, shear crack opening, and shear deformations. Experimental results showed that the shear capacity of RC beams reinforced with BFRP composites increased by 43% to 100%, with fully wrapped sheets and strips producing the most significant improvements. The study also identified an appropriate predictive model for the shear strengthening configurations.
Kamonna and Abd Al-Sada [20] investigated the flexural behavior of one-way reinforced concrete (RC) slabs reinforced with near-surface mounted (NSM) basalt fiber reinforced polymer (BFRP) bars. The study involved testing ten RC slabs under four-point loading until failure. Key parameters examined included the steel reinforcement ratio (0.48% and 0.95%), the NSM-BFRP reinforcement ratio (0.17%, 0.25%, and 0.35%), and two types of epoxy adhesives (Sikadur-30 and NSM-Gel). The results demonstrated that NSM-BFRP bars significantly enhanced the flexural performance of the slabs, especially those with lower steel reinforcement ratios. Ultimate loads increased by 51–123% for slabs with 0.48% steel reinforcement and by 21–44% for slabs with 0.95% steel reinforcement, compared to control slabs. Correspondingly, ductility indices improved by 10–53% and 29–79%, respectively. The type of epoxy adhesive had little effect on the flexural behavior. Depending on the reinforcement ratio, failure typically occurred due to concrete crushing or steel yielding, followed by rupture of the NSM-BFRP bars. Using ACI 440.2R formulations, predictions of ultimate loads, yield, and cracking for the strengthened slabs were in good agreement with the experimental data, though the predictions were conservative.
The experimental program presented by Diab and Sayed [21] focused on the shear strengthening of reinforced concrete T-beams using NSM-BFRP bars. A novel non-mechanical anchorage method was assessed, which involved using handcrafted closed or U-shaped hybrid BFRP stirrups, combining BFRP bars and sheets, to reinforce T-beams. The effects of these anchorage techniques were analyzed, revealing that the shear capacity of beams strengthened with NSM-BFRP bars without anchorage increased from 8% to 46%. In contrast, beams strengthened with the proposed anchorage system exhibited a more significant increase in the shear capacity, ranging from 39.6% to 81.6%. The maximum strains in the BFRP bars varied from 27% to 59% of their ultimate strains, depending on the NSM spacing and the presence of anchorage. The study confirmed that the anchorage system successfully prevented premature debonding of the NSM-BFRP bars.
In a separate study, the potential of NSM-BFRP bars for repairing heat-damaged reinforced concrete (RC) components was explored [22]. The flexural performance of rectangular RC beams subjected to heat exposure (650 °C for three hours) was examined both theoretically and experimentally. Twelve RC beams were designed, with seven repaired using NSM-BFRP bars and three repaired using NSM-carbon fiber-reinforced polymer (CFRP) ropes. The study focused on failure modes, ultimate capacity, and recovery of initial flexural capacity. Recovery percentages ranged from 88.2% to 127%, with the performance of the beams repaired with either NSM-BFRP bars or NSM-CFRP ropes being comparable. Experimental results were in good agreement with theoretical predictions and ACI 440.2R-08 guidelines.
Research on the flexural and shear strengthening of high-strength concrete (HSC) is relatively limited, despite the material’s superior load-bearing capacity [23,24,25,26,27]. Hussein and Amleh [28] investigated the creation of composite components by combining normal-strength concrete (NSC) or high-strength concrete (HSC) with ultra-high-performance fiber-reinforced concrete (UHPFRC). Their experimental program evaluated the flexural and shear capacities of UHPFRC-NSC/HSC prisms and beams without stirrups. In these composite beams, the UHPFRC layer was subjected to tension, while the NSC/HSC layer was under compression. The beams failed in shear at 1.6–2.0 times the force of standard NSC/HSC beams, with minimal performance differences between high-strength and normal-strength concrete. The strong bond between the layers eliminated the need for shear connections. The experimental data were used to validate an analytical model for predicting the ultimate shear capacity of the composite beams.
Li and Aoude [29] emphasized the advantages of ultra-high-performance concrete (UHPC) over traditional concrete, highlighting its superior toughness, durability, tensile resistance, and compressive strength. Despite its potential for retrofitting older buildings, there has been limited research on its application in reinforcing high-strength concrete (HSC) members with compressive strengths exceeding 80 MPa. Their study investigated the effectiveness of UHPC in improving the shear and flexural performance of shear-deficient HSC beams. Two stirrup-free, shear-deficient HSC beams, with steel ratios of 1.6% and 2.4%, were retrofitted with a thin UHPC jacket and evaluated under four-point bending. The results showed that the UHPC jacket significantly improved the flexural performance of the beams, offering 40–125% greater ductility, 20–35% higher flexural strength, and 80–85% more stiffness compared to control beams. As the steel ratio increased, the failure mechanism shifted from bar fracture to concrete crushing. Finite element modeling was used to explore additional retrofit options and the shear-span-to-depth ratio.
Dawood and Nabbat [30] investigated the flexural and shear behavior of high-strength, non-prismatic RC beams reinforced with carbon fiber-reinforced polymer (CFRP), both with and without openings for pipes and ducts. The study tested twelve beams under two-point loads as simply supported spans. Openings, which are often required for utilities, were found to significantly reduce the flexural and shear capacity of the beams due to load concentration. CFRP reinforcement increased the ultimate load by 15% for shear and 16% for flexural behavior. In beams with openings, CFRP reinforcement improved the ultimate load by 23–35% for flexural behavior and by 16–25% for shear behavior, mitigating the negative effects of openings on the structural performance.
Carbon Fiber Reinforced Polymer (CFRP) composites are widely utilized as an effective solution for enhancing the structural integrity of civil engineering elements. In recent decades, extensive research has focused on the performance of reinforced concrete beams strengthened with CFRP using the near-surface mounted (NSM) technique. However, there is limited research on the use of basalt fiber reinforced polymer (BFRP) bars for the flexural and shear strengthening of high-strength concrete using the NSM approach. Therefore, this study aims to investigate the effectiveness of basalt fiber reinforced polymer (BFRP) bars for strengthening high-strength concrete beams in terms of both flexural and using the near-surface mounted technique with different configurations. In this study, twelve high-strength beams were cast and strengthened in shear and flexure using NSM-BFRP.

2. Experimental Program

The flexural and shear behaviors of reinforced concrete beams were investigated through experiments using different NSM BFRP configurations. This was performed by testing ten RC beam specimens that were strengthened by BFRP bars under four-point loads.

2.1. Material

2.1.1. Concrete

For casting each beam specimen, high-strength concrete (HSC) with a compressive strength of 60 MPa was obtained from a commercial source. Table 1 and Figure 1, respectively, show the mix design and aggregate grading. The mixture included only one superplasticizer. The cast beams were wrapped in burlap and sprayed with water daily for 28 days. Three cylinders were tested after 28 days of curing and the compressive strength was 55.2 MPa.

2.1.2. Steel Reinforcement

In the first phase of this experimental program, six specimens were designed to primarily fail in flexure, in accordance with the ACI 318-19 guidelines. To achieve this, the shear capacity of the beams was initially set to twice that of the flexural capacity. The beams were reinforced with deformed steel bars of 12 mm and 10 mm diameter, Grade 60, to provide bending resistance, and 8 mm smooth stirrups to enhance shear strength. In the second phase of the program, six beams were designed to ensure shear failure.

2.1.3. BFRP and Adhesives

In this study, the BFRP bars was used to strengthen the tested specimens longitudinally and transversally. The properties of Basalt FRP-Reinforcement Bars are listed in Table 2. SikaDur 330 was used as an adhesive for bonding the BFRP bar to the concrete. Table 3 shows the details of the properties of the adhesives, which has been provided from the manufacturer.

2.2. Beam Specimens

Twelve rectangular beams were designed in accordance with ACI 318-19 (2019) and divided into two groups. The first group is governed by flexural failure and the second group is governed by shear failure.
Group 1 (Flexural Failure): Concrete compressive strength (f’c) was 70 MPa. Tensile reinforcement consisted of two Φ12 bars (As = 226 mm2). Shear reinforcement comprising Φ8 bars with a spacing of 100 mm (Av = 100.6 mm2). Yield strength of steel (fy) was 420 MPa. Beam dimensions were 150 mm width (b) by 250 mm height (h) with a length of 1400 mm.
Group 2 (Shear Failure): Similar properties of group one; however, only three stirrups were used. Figure 2 shows the beam design profile and cross-section.
The shear span-to-depth ratio (a/d) plays a crucial role in determining the failure mechanism of reinforced concrete beams, distinguishing between shear and flexural failures. In this study, two different a/d ratios were adopted to ensure the desired failure mode. For beams designed to fail in shear, an a/d ratio of 2 (shear span = 400 mm) was used. In contrast, for beams intended to fail in flexure, an a/d ratio of 3 (shear span = 600 mm) was employed, which encourages a flexural failure mechanism.
Table 4 shows the test matrix and sample details used in this study. As mentioned before, the twelve specimens were divided in two groups based in governed failure. The first group was governed by flexure failure had six specimens, whereas the second group was governed by shear had six specimens. Five configurations of BFRP were employed in the first and second groups to study the flexural and shear behavior of strengthened RC beams using CFRP rods as listed in Table 4.
Each group had six specimens, the first specimen is the control (control and no BFRP rods were used). Two BFRP rods at the bottom surface (B-2 L) strengthened the second specimen. Three BFRP rods at the bottom (B-3 L) strengthened the third specimen. The fourth specimen was strengthened by one rod in each side with a length of 40 cm (ST-40). The fifth specimen was strengthened by one rod on each side with a length of 120 cm (ST-120). The sixth specimen was strengthened by a rod in a trapezoidal shape on both sides (TR) (see Table 4 and Figure 3). The second group had six specimens as well; the first specimen is the control (control and no BFRP rods were used). The vertical bars were spaced at 7.5 cm strengthened the second specimen transversally (V-7.5 cm). The vertical bars spaced at 15 cm strengthened the third specimen transversally (V-15 cm). The inclined bars oriented at 30° strengthened the fourth specimen transversally (Inc-300). Inclined bars oriented at 45° strengthened the fifth specimen transversally (Inc-450). The full circulation bars spaced at 15 cm strengthened the sixth specimen transversally (U-15 cm) (see Table 4 and Figure 4).

2.3. Installation of NSM-BFRP Materials

In order to ensure that the grooves were cut with exactness and meticulousness, guiding lines were marked at the predetermined points on the concrete surface before the cutting operation commenced, as visually depicted in Figure 5a. The groove itself was produced with great care utilizing an electric saw, resulting in a channel that was precisely 15 mm in width and 20 mm in depth. To eliminate any particles of dust and thereby facilitate a perfect bond between the concrete foundation and the epoxy resin, a device designed to expel air was employed to thoroughly clean the groove, as illustrated in Figure 5b. After that, the BFRP bars were installed in the grooves followed by packing with epoxy as shown in Figure 5c.

2.4. Testing Procedure

Twelve beam specimens were subjected to two-point loading in a simply supported configuration. Each specimen was supported at its extremities by steel rollers positioned 100 mm from the respective end supports. Symmetrically applied concentrated loads were introduced at a spacing of 400 mm along the beam span as shown in Figure 6. A hydraulic jack with a maximum load capacity of 750 kN was used in testing the beams. Mid-span deflection was monitored using linear variable displacement transducers (LVDTs) fixed underneath the beam specimen as shown in Figure 7. Two LVDTs were placed at the center of the beam bottom, which their reading was taken to calculate the displacement at the center of the beam. The other two LVDTs were placed beside the supports. The load was measured using a load cell and the average displacement of two LVDT’s were calculated. Load–displacement data were acquired electronically.

3. Experimental Results

3.1. Group One

Group one had six specimens and governed by flexural failure. The specimens in this group were strengthened with different configurations of BFRP and subjected to an experimental regimen.

3.1.1. Failure Mode and Crack Pattern for Group One

The experimental investigation of the flexural strengthening of high-strength concrete beams using basalt fiber reinforced polymer (BFRP) bars reveals significant perceptions that could influence the future design and reinforcement strategies in structural engineering. Starting with the control specimen, the first crack was observed at a load of 113 kN, and the beam yielded at 135 kN and ultimately failed at the same load with a displacement of 21.1 mm. The control specimen behaved typically to the unreinforced or conventional RC beam. It can be seen that flexural shear failure is the dominant mode of failure due to the lack of sufficient reinforcement to withstand higher loads or prevent excessive deformation.
In contrast, the BFRP-reinforced specimens (B-2L and B-3L) exhibited first cracks at considerably lower loads of 40 kN and 50 kN, respectively. This early cracking could be attributed to the difference in the mechanical properties between BFRP and the concrete matrix, such as stiffness and bond strength. However, despite the lower initial cracking loads, both BFRP-reinforced specimens demonstrated significant improvements in their load-bearing capacities during the subsequent stages of loading. The B-2L specimen yielded at 143 kN and ultimately failed at 159 kN, while the B-3L specimen showed even better performance, yielding at 149 kN and failing at 180 kN. These results represent enhancements in ultimate load capacity by 17.7% and 33.33%, respectively, compared to the control specimen.
However, despite the enhancement of the load-bearing capacity, the ductility of these specimens (B-2L and B-3L) decreased, as evidenced by the reduced ultimate displacements of 16.5 mm for B-2L and 15 mm for B-3L. The reduction in displacement indicates that while BFRP bars contribute significantly to the flexural strength of the beams, they may also make the structure more brittle, reducing its ability to deform under load without failing. The mode of failure for both BFRP-reinforced beams (B-2L and B-3L) changed from flexural to shear failure, which is a critical observation. This change suggests that the increased flexural capacity provided by BFRP bars can induce higher shear stresses, leading to shear failure if the beams are not adequately reinforced for shear. This result illustrates the importance of ensuring that the shear reinforcement is appropriately designed when utilizing BFRP bars because the advantages of flexural strengthening using BFRP bars might be decreased by early shear failure.
On the other hand, specimens ST-40 and ST-120, which were tested to evaluate the influence of different BFRP reinforcement configurations, exhibited further insights. The ST-120 specimen showed a yield load of 145 kN and an ultimate load of 170 kN, reflecting a 25.9% enhancement in ultimate load capacity as of the control specimen. However, similar to the BFRP-reinforced specimens B-2L and B-3L, ST-120 also failed in shear, indicating that the increase in flexural reinforcement in the form of BFRP bars leads to a change in failure mode if not carefully managed. This highlights a critical challenge in structural design, where increasing one aspect of the performance, such as flexural strength, can inadvertently create vulnerabilities in another, such as shear capacity. The ST-40 specimen, on the other hand, yielded at 110 kN and failed at 128 kN, showing a 5% decrease in ultimate load compared to the control. This suggests that not all reinforcement configurations are beneficial, and some may even reduce the structural capacity under certain conditions.
The TR specimen presented another interesting case, with a yield load of 117 kN and an ultimate load of 127 kN, slightly lower than the control specimen. This beam also failed in flexure, indicating that the reinforcement technique employed did not significantly alter the beam’s performance compared to the control. The results suggest that this particular technique may not be as effective as BFRP reinforcement in enhancing the flexural capacity of high-strength concrete beams, possibly due to differences in material properties or the configuration of the reinforcement or the location of BFRP bars. Table 5 shows the test results of group one, which include the first crack load, yield load, yield displacement, ultimate load, ultimate displacement, and failure modes. The crack patterns and modes of failure of group one are shown in Figure 8.

3.1.2. Flexural Load Displacement Response of the Strengthened Beam

In this group, six beam specimens were constructed of high strength of 60 MPa subjected to two loading points and designed to fail in flexure. Five beam specimens were strengthened with BFRP rods with different configurations. Figure 9 shows the flexural load displacement response for all specimens of the first group and shows the effect of BFRP configuration in flexural response.
Figure 9 and Table 5 show that the control specimen exhibited an ultimate load and ultimate displacement of 135 MPa and 22 mm, respectively. The beam specimen strengthened with two rods of BFRP at the bottom (B-2L), showed an increasing ultimate load and decreasing ultimate displacement of about 17.7% and 22%, respectively, compared to the control beam; moreover, resulted in a shear failure mode. Using three rods of BFRP at the bottom in strengthening the beam specimen (B-3L), showed an increasing ultimate load and ultimate displacement of about 33.33% and 29%, respectively, compared to the control specimen, which also resulted in shear failure mode. It can be concluded that as the number of rods increases or the spacing between rods decreases, the ultimate load increases and the ultimate displacement decreases. Regarding the specimens that were strengthened with the BFRP rods at both sides (ST-40, ST-120 and TR), the specimen ST-40 which was strengthened with straight one rod of BFRP at each side with a length of 40 cm exhibited an ultimate load and ultimate displacement of 128 MPa and 13 mm, respectively. The beam specimen (ST-40) decreased the ultimate load and ultimate displacement about 5% and 39%, respectively, as of the control specimen. However, using one straight rod of BFRP on each side with a length of 120 cm to strengthen the specimen (ST-120) exhibited an ultimate load and ultimate displacement of 170 MPa and 18 mm, respectively. The beam specimen (ST-120) increased the ultimate load and decreased the ultimate displacement by about 26% and 15%, respectively. It can be seen that in specimen ST-40 the length of the rod was not enough and did not coincide with the moment profile. This was the reason of decreasing the ultimate load and ultimate displacement. Finally, the specimen strengthened with the trapezoidal BFRP rod at each side, showed about 5% and 25%, reduction in ultimate load and ultimate displacement, respectively, compared with the control specimen.

3.2. Group Two

Group two had six specimens and governed by shear failure. The specimens in this group were strengthened with different configurations of BFRP and subjected to an experimental regimen.

3.2.1. Failure Mode and Crack Pattern for Groupe Two

The experimental results of group two provided critical insights into the behavior of high-strength concrete beams reinforced with basalt fiber reinforced polymer (BFRP) bars, particularly under conditions intended to induce shear failure. The control shear specimen exhibited a first crack at 57 kN, yielding at 110 kN, and ultimately failing at 170 kN with a displacement of 10.4 mm (Table 6), experiencing a typical shear failure (Figure 10). This control specimen serves as a baseline for evaluating the performance of the various reinforcement strategies employed in the other specimens.
The V-7.5 cm specimen, reinforced with vertical BFRP bars spaced at 7.5 cm, demonstrated an initial cracking load of 40 kN, a yield load of 120 kN, and an ultimate load of 224 kN. The enhancement in ultimate load capacity by 31.7% compared to the control is significant. This specimen ultimately failed in flexure with an ultimate displacement of 14.5 mm, indicating that the close spacing of the BFRP bars effectively enhanced the shear strength of the beam, shifting the failure mode from shear to flexural.
The V-15 cm specimen, reinforced with vertical BFRP bars with a wider spacing of 15 cm, exhibited a first crack at 35 kN, a yield load of 118 kN, and an ultimate load of 190 kN. This configuration resulted in an 11.7% increase in the ultimate load capacity as of the control specimen but led to a mixed shear-flexural failure mode with an ultimate displacement of 8 mm. The results suggest that while the BFRP reinforcement improved load capacity, the wider spacing may have reduced the effectiveness of the bars in providing uniform support, leading to a combination of failure modes.
The Inc-30° specimen, designed with inclined BFRP bars at 30 degrees, showed a significant improvement in performance. The first crack occurred at 67 kN, and the beam yielded at 170 kN, with an ultimate load of 278 kN—representing a substantial 63.5% enhancement in the load capacity compared with the control specimen. However, this specimen experienced complex failure modes, including shear failure, concrete cover spalling (CS), and de-bonding, with an ultimate displacement of 17.7 mm. These findings suggest that inclined reinforcement at 30 degrees effectively enhances both shear and flexural capacities, but the interaction between the different failure mechanisms needs to be carefully considered in the design.
The Inc-45° specimen, with BFRP bars inclined at 45 degrees, also performed well, with a first crack at 65 kN, a yield load of 180 kN, and an ultimate load of 211 kN, resulting in a 24.1% increase in ultimate load capacity as control specimen. The beam failed in flexure at a displacement of 15.3 mm, indicating that the 45-degree inclination provided significant flexural strength enhancement while preventing shear failure.
The U-15 cm specimen, reinforced with U-shaped BFRP bars spaced at 15 cm, exhibited a first crack at 50 kN, a yield load of 205 kN, and an ultimate load of 226 kN. The ultimate load capacity was enhanced by 32.9%, and the beam failed in shear at an ultimate displacement of 16 mm. This result suggests that the U-shaped reinforcement effectively increased the load capacity and improved ductility, but shear failure remained the dominant failure mode, indicating that additional measures might be necessary to prevent shear failure entirely.
Overall, the results from group two highlight the effectiveness of different BFRP reinforcement configurations in enhancing the structural performance of high-strength concrete beams. The inclined BFRP bars at 30 degrees (Inc-30°) provided the most significant increase in the ultimate load capacity, but they also introduced complex failure modes that require careful attention. The V-7.5 cm and U-15 cm specimens demonstrated substantial improvements in both the load capacity and ductility, though the potential for shear failure suggests that these configurations may benefit from additional shear reinforcement or adjustments in bar spacing. The mixed shear-flexural failure observed in the V-15 cm specimen underscores the importance of optimizing bar spacing to ensure uniform reinforcement and prevent premature failure. Table 6 shows the test result of group two, which includes the first crack load, yield load, yield displacement, ultimate load, ultimate displacement, and failure modes. The crack patterns and modes of failure are shown in Figure 10.

3.2.2. Shear Load Displacement Response of the Strengthened Beam

In the second group, six beam specimens were constructed of high strength of 60 MPa subjected to two loading points and designed to fail in shear. Five beam specimens were strengthened with BFRP rods with different configurations. Figure 11 shows the shear load displacement response for all specimens in the second group and shows the effect of BFRP configuration in shear response.
Figure 11 and Table 6 show that the control specimen resulted in an ultimate load and ultimate displacement of 170 MPa and 18 mm, respectively. Specimens V-7.5 strengthened by vertical BFRP rods at 7.5 cm spacing showed an increase in the ultimate load and ultimate displacement of about 32% and 40%, respectively; moreover, resulted in a flexure failure mode. However, the specimens V-15 strengthened with BFRP rods with a spacing of 15 cm, exhibited an increase in ultimate load and decrease in ultimate displacement of about 12% and 23%, respectively, resulted in a shear-flexural failure mode. It can be concluded that as the spacing between rods increases the ultimate load decreases and the ultimate displacement increases. Regarding the specimens that were strengthened with inclined BFRP rods at both sides (Inc-30° and Inc-45°), specimen Inc-30°, which strengthened with inclined one rod of BFRP at each side, exhibited an ultimate load and ultimate displacement of 278 MPa and 18 mm, respectively. The beam specimen Inc-300 increased the ultimate load and ultimate displacement by about 64% and 70%, respectively. However, using one inclined rod of BFRP in each side at 45° degree gave an ultimate load and ultimate displacement of 211 MPa and 16 mm, respectively. Using BFRP rods at 45° degree increased the ultimate load and ultimate displacement by about 24% and 47%, respectively, as shown in beam specimen Inc-45°. Finally, the specimen strengthened with U shape of the BFRP rod with a spacing of 15 cm, showed about 33% and 254%, increasing in ultimate load and ultimate displacement, respectively, compared with the control specimen.

4. Effectiveness of NSM-Basalt Fiber Reinforced Polymer Bars in High-Strength Concrete Beams

In this study, an experimental work was conducted to investigate the effect of using near-surface mounted (NSM) basalt fiber reinforced polymer (BFRP) bars in strengthening high-strength RC beams on its flexural and shear performance.

4.1. Flexural Strength Enhancement

The use of BFRP bars in flexural strengthening, as observed in group one, resulted in significant improvements in load-bearing capacity. The BFRP-reinforced specimens (B-2L and B-3L) exhibited ultimate load enhancements of 17.7% and 33.33%, respectively, compared to the control specimen (Figure 12). This marked increase underscores the effectiveness of NSM-BFRP bars in elevating the flexural strength of high-strength concrete beams. However, the reduced ductility observed in these specimens, along with the shift from flexural to shear failure, suggests a critical trade-off; while BFRP bars enhance flexural capacity, they may induce higher shear stresses, leading to premature shear failure if shear reinforcement is not adequately provided.

4.2. Shear Strength and Failure Mode Shift

The results from group two show the effectiveness of NSM-BFRP bars, particularly in shear strengthening. The V-7.5 cm specimen, with closely spaced BFRP bars, demonstrated a 31.7% enhancement in ultimate load capacity, shifting the failure mode from shear to flexural. This shift highlights the ability of BFRP bars to not only reinforce but also modify the failure behavior of high-strength concrete beams, ensuring a more desirable failure mode under increased loads.
The inclined BFRP bars, especially those at a 30 degree angle (Inc-30°), provided the most substantial enhancement in load capacity (63.5%). However, the complex failure modes observed in this configuration, including shear failure, concrete cover spalling, and de-bonding, indicate that while the technique is highly effective in increasing load capacity, it also introduces new challenges that must be carefully addressed in design and application (Figure 13).

4.3. Ductility and Structural Integrity

In general, Table 5 and Table 6 show that using BFRP rods in strengthening of high-strength beams enhances ductility. The increase in ductility in a strengthened beam governed by flexure ranged from 101% to 208%; however, in the specimens that governed by shear, the increase in ductility ranged from 154% to 234%, compared with the control specimen. In group one governed by flexure, using a trapezoidal configuration in each side for strengthening exhibited the large enhancement in ductility of about 208% compared to the control specimen. On the other hand, in group two that was governed by shear, the U-shape configuration showed a significant increase in ductility of about 238% compared to the control specimen. The use of U-shaped BFRP bars (TR and U-15 cm) showcased an impressive (5% and 323%) enhancement in the ultimate load capacity, along with improved ductility of 208% and 234%, respectively. Despite the dominance of shear failure in this specimen, the increased ductility suggests that NSM-BFRP bars can contribute to not only higher load-bearing capacities but also to better overall structural resilience.

5. Conclusions

The experimental investigation on using NSM-basalt fiber reinforced polymer (BFRP) bars for strengthening high-strength concrete beams demonstrates significant improvements in both flexural and shear capacities. The research highlights the following key conclusions:
  • The application of NSM-BFRP bars resulted in notable increases in flexural capacity, with enhancements of up to 33.33% compared to unreinforced control specimens. However, the increased flexural capacity also led to a reduction in ductility, indicating a trade-off between strength and flexibility that must be managed in design.
  • The results showed that using BFRP reinforcement can significantly increase shear capacity, with a maximum enhancement of 63.5%. However, the introduction of BFRP bars also altered the failure modes, shifting them from shear to flexural or to complex modes involving concrete cover spalling and de-bonding. This shift underscores the need for careful reinforcement design to balance flexural and shear strengths.
  • The study found that different configurations of BFRP reinforcement produced varying results, with closely spaced and inclined bars generally providing better performance. However, certain configurations, such as wider bar spacing, led to mixed or less desirable failure modes, emphasizing the importance of optimizing reinforcement layout.
  • While NSM-BFRP bars effectively increased load capacity, they also reduced the ductility of the beams, particularly in flexural strengthening applications. This reduction in ductility suggests a need for additional measures, such as supplementary shear reinforcement, to maintain structural resilience.
In conclusion, NSM-BFRP bars offer a promising solution for enhancing the strength of high-strength concrete beams. However, the trade-offs between increased strength and reduced ductility, as well as the potential for altered failure modes, must be carefully addressed in the design and implementation of this reinforcement technique. Future research should focus on optimizing reinforcement configurations to maximize the benefits of NSM-BFRP bars while mitigating the associated risks.

Author Contributions

Conceptualization, A.A.; methodology, A.T.O. and A.A.; validation, A.T.O. and A.A.; formal analysis, A.A., A.A.-K. and A.T.O.; investigation, A.T.O.; resources, M.A.-J. and A.A.; data curation, A.T.O. and A.A.; writing original draft preparation, A.A.-K., A.A. and A.T.O.; writing review and editing, A.A., A.A.-K., M.A.-J. and A.T.O.; visualization, A.A. and A.T.O.; supervision, A.A.; project administration, A.T.O. and A.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The article includes all the research data.

Acknowledgments

This work was carried out during the sabbatical leave granted to the author Ahmed Ashteyat from the University of Jordan during the academic year 2024–2025. Finally, the authors appreciate the support of the university of Jordan and Philadelphia university.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

fc′Compressive strength of concrete (MPa).
EcModulus of elasticity of concrete (GPa).
dEffective depth of the beam (mm).
hTotal depth of the beam (mm).
a/dShear span-to-depth ratio.
AsArea of steel reinforcement (mm2)
PApplied load (kN).
NSM-BFRPNear-Surface-Mounted Basalt Fiber Reinforced Polymer bars.

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Figure 1. Sieve analysis of aggregate grading.
Figure 1. Sieve analysis of aggregate grading.
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Figure 2. Beam design profile and cross-section.
Figure 2. Beam design profile and cross-section.
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Figure 3. Flexural Strengthening Configurations.
Figure 3. Flexural Strengthening Configurations.
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Figure 4. Shear Strengthening Configurations.
Figure 4. Shear Strengthening Configurations.
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Figure 5. Process of creating grooves.
Figure 5. Process of creating grooves.
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Figure 6. Test setup.
Figure 6. Test setup.
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Figure 7. Placement of LVDTs.
Figure 7. Placement of LVDTs.
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Figure 8. Cracks Patterns and Modes of Failure for Group One.
Figure 8. Cracks Patterns and Modes of Failure for Group One.
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Figure 9. Load–Displacement Curve for Group One.
Figure 9. Load–Displacement Curve for Group One.
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Figure 10. Cracks Patterns and Modes of Failure for Group Two.
Figure 10. Cracks Patterns and Modes of Failure for Group Two.
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Figure 11. Load–Displacement Curve for Group Two.
Figure 11. Load–Displacement Curve for Group Two.
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Figure 12. Ultimate loads for group one.
Figure 12. Ultimate loads for group one.
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Figure 13. Ultimate loads for group two.
Figure 13. Ultimate loads for group two.
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Table 1. Mix design proportions.
Table 1. Mix design proportions.
Materials for ProportioningWeight (kg/m3)
Cement OPC450.0
Water171.3 (Total water)
Coarse Aggregate827.1
Fine Aggregate946.5
Admixture (Adcon PC550)8.0
Table 2. Properties of Basalt FRP-Reinforcement Bars.
Table 2. Properties of Basalt FRP-Reinforcement Bars.
Size,
mm		Cross-Sectional Area mm2Ultimate Tensile Strength, MPaElastic Modulus GPaUltimate Strain mm/mm
161801089580.020
Table 3. SikaDur 330 properties.
Table 3. SikaDur 330 properties.
Properties Standard
Modulus of Elasticity in Flexure~3800 N/mm2 (7 d, +23 °C)(EN 1465)
Tensile Strength~30 N/mm2 (7 d, +23 °C)(ISO 527)
Modulus of Elasticity in Tension~4500 N/mm2 (7 d, +23 °C)(ISO 527)
Elongation at Break~0.9% (7 d, +23 °C)(ISO 527)
Tensile Adhesion StrengthConcrete fracture (>4 N/mm2) on a sandblasted substrate(EN ISO 4624)
Coefficient of Thermal Expansion4.5 × 10−5 1/K (Temperature range −10 °C min./+40 °C max.)(EN 1770)
Glass Transition TemperatureCuring timeCuring temperatureTG(EN 12614)
30 d+30°+58 °C
Heat Deflection TemperatureCuring timeCuring temperatureHDT(ASTM D 648)
7 d+10 °C+36 °C
7 d+23 °C+47 °C
7 d+35 °C+53 °C
Resistance to continuous exposure up to +45 °C
Service Temperature−400 °C min./+45 °C max.
Table 4. Strengthening technique.
Table 4. Strengthening technique.
GroupIntended FailureSpecimen IDDetailsBFRP TypeStrengthening Layout Based on
OneFlexuralControlNo rodsBarsFigure 3A
B-2LTwo rods at the bottom surfaceBarsFigure 3B
B-3LThree rods at the bottom surfaceBarsFigure 3C
ST-40One rod in each side with a length of 40 cmBarsFigure 3D
ST-120One rod in each side with a length of 120 cmBarsFigure 3E
TRRod in trapezoidal shape in both sidesBarsFigure 3F
TwoShearControl shearNo rodsBarsFigure 4A
V-7.5 cmVertical bars spaced at 7.5 cmBarsFigure 4B
V-15 cmVertical bars spaced at 15 cmBarsFigure 4C
Inc-30°Inclined bars oriented at 30°BarsFigure 4D
Inc-45°Inclined bars oriented at 45°BarsFigure 4E
U-15 cmFull circulation bars spaced at 15 cmBarsFigure 5b
Table 5. Group One Results.
Table 5. Group One Results.
Group oneSpecimen IDFirst Crack
(kN)		Yield Load
(kN)		Yield Displacement
(mm)		Ultimate Load
(kN)		Ultimate Displacement
(mm)		%UL%UDDuctilityMode of Failure
Control451136.813522-0169Flexural shear
B-2L4014311.51592217.721.80184Shear failure
B-3L50149101802033.33−28.91182Shear failure
ST-40651101012830−5−38.38300Flexural failure
ST-12065145131702225.9−14.69220Shear failure
TR701178.512730−5−24.17375Flexural failure
%UL and %UD: Degree of Enhancement in Ultimate Load and Ultimate displacement.
Table 6. Group Two Results.
Table 6. Group Two Results.
Group TwoSpecimen IDFirst Crack
(kN)		Yield Load
(kN)		Yield Displacement
(mm)		Ultimate Load
(kN)		Yield Displacement
(mm)		Ultimate Displacement
(mm)		%UL%UD%DuctilityMode of Failure
Control shear571106.91701018-0180Shear failure
V-7.5 cm401207.522482531.739.42312Flexural failure
V-15 cm35118519072211.7−23.07315Shear- Flexural failure
Inc-30°671708.527892563.570.19278Shear failure, CS, De-bonding
Inc-45°651806.521182324.147.12288Flexural failure
U-15 cm502058.922673532.953.85500Shear failure
CS: Concrete Cover Spalling. %UL and %UD: Degree of Enhancement in Ultimate Load and Ultimate displacement.
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MDPI and ACS Style

Ashteyat, A.; Obaidat, A.T.; Al-Khreisat, A.; Abdel-Jaber, M. Flexural and Shear Strengthening of High-Strength Concrete Beams Using near Surface Basalt Fiber Bars. Infrastructures 2025, 10, 1. https://doi.org/10.3390/infrastructures10010001

AMA Style

Ashteyat A, Obaidat AT, Al-Khreisat A, Abdel-Jaber M. Flexural and Shear Strengthening of High-Strength Concrete Beams Using near Surface Basalt Fiber Bars. Infrastructures. 2025; 10(1):1. https://doi.org/10.3390/infrastructures10010001

Chicago/Turabian Style

Ashteyat, Ahmed, Ala’ Taleb Obaidat, Ahmad Al-Khreisat, and Mu’tasime Abdel-Jaber. 2025. "Flexural and Shear Strengthening of High-Strength Concrete Beams Using near Surface Basalt Fiber Bars" Infrastructures 10, no. 1: 1. https://doi.org/10.3390/infrastructures10010001

APA Style

Ashteyat, A., Obaidat, A. T., Al-Khreisat, A., & Abdel-Jaber, M. (2025). Flexural and Shear Strengthening of High-Strength Concrete Beams Using near Surface Basalt Fiber Bars. Infrastructures, 10(1), 1. https://doi.org/10.3390/infrastructures10010001

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