Experimental Investigation into the Flexural Behaviour of Basalt FRP Reinforced Concrete Beams

This study involved conducting four-point bending tests on three types of rebar-reinforced concrete beam: those reinforced with steel bars, those reinforced using basalt fiber-reinforced polymer rods, and hybrid reinforced concrete beams that used both Basalt fiber-reinforced plastic and reinforcing steel. Various reinforcement ratios and configurations were used to analyze mid-span deflection, flexural capacity, fracture distribution, and Basalt FRP bars strain. The results suggest that using a hybrid reinforcement system that combines Basalt fiber-reinforced plastic and reinforcing steel can improve the transverse rupture strength of rebar-reinforced concrete beam, reduce deflection and cracking, and use the high-strength characteristics of Basalt FRP bars.


Introduction
The robustness of concrete structures incorporating steel bars has become a major concern as they are near the end of their life cycle [1][2].Corrosion of steel bars in a corrosive environment can reduce the safety and durability of the structure, and researchers are dedicated to finding solutions to this problem [3,4].Reinforced concrete beams are the primary resistance components in traditional structures.However, after concrete cracks, the atmospheric environment can cause steel bars to corrode, posing a serious threat to the structure's life cycle [5].To mitigate corrosion damage to steel bars of structures, one effective approach is to replace traditional steel bars with FRP composite materials bars as stressed steel bars [6,7].FRP bars have good corrosion resistance and can significantly increase the load-bearing capacity [8,9], but they have a high elastic modulus and do not have the significant yield point characteristics of steel bars, which can result in relatively poor overall force performance in concrete structures when the structure is damaged.
To address these issues, some researchers have proposed a new form of hybrid reinforcement with fiber-reinforced plastic and reinforcing steel [10][11][12][13].This hybrid reinforcement form can retain the partial ductility of traditional reinforced concrete structures while fully utilizing the high-strength characteristics of FRP bars, significantly improving the bearing capacity of concrete structures [14,15].However, compared with reinforced concrete structures, concrete structures reinforced using basalt fiber-reinforced polymer rods tend to exhibit larger deflection deformations and wider cracks during normal use conditions [16,17].To evaluate the effectiveness of BFRP bars and concrete beams working in tandem, this study proposes a method that replaces steel bars with BFRP bars of equivalent strength and section, and the performance under stress of BFRP rebar-strengthened concrete beams are compared and analyzed.

Materials
The physical and mechanical parameters of the BFRP and 12 mm diameter HRB400 grade bars are demonstrated in table 1.To produce the concrete for the experiment, P.O 32.5 ordinary silicate cement was selected, along with river sand as the fine aggregate and crushed stones less than 20mm in size as the coarse aggregate.The compressive strength measurement was performed in compliance with the Chinese code GB/T 50081-2002.

Test Specimens
To investigate the bending performance of concrete beams reinforced with both BFRP bars alone and combined BFRP and steel bars, 18 concrete beams were fabricated, consisting of three different types.Table 2 illustrates the details of these beams.

Test Setup
A external strain gauge was mounted on the bottom longitudinal bar, at the midpoint of the beam and where the load is applied, to gauge the deformation of the steel or BFRP bars.Two strain gauges were inserted at the upper and lower sides of the beam, and three strain gauges were installed at the center of the lateral surface.surface of the beam at a spacing of 50 mm.Data was automatically collected using the uT7121Y static strain test equipment, and the graded monotonic displacement loading test was performed in accordance with Standard for Test Method of Concrete Structures (GB50152-2012).
Preloading was done before the official start, and 0.2 mm was loaded until the concrete fractured at each stage.Following cracking, 0.8 mm was gradedly loaded.The beam underwent graduated loading at 0.2 mm till failure after it reached the maximum bending moment.Figure 2 depicts the beams' loading mechanism.

Moment-deflection Curves
The bending moment-deflection curves of each beam are demonstrated in the figure 3, based on the deflection and corresponding moment data.Three distinct deformation stages were observed within the  Each beam exhibited significant stiffness since the concrete at its base had not yet reached the precracking load.The moment-deflection curve followed a linear trend, and all beams displayed linear deformation characteristics.As the postcracking deflection increased, the slope of the momentdeflection curve decreased.Among the three beam types, the reinforced concrete beam 2S12 had the highest stiffness, where beam S12-B12 was stiffer than beam S12-B10.The stiffness of rebar-reinforced concrete beam with BFRP bars was the lowest, with beam 2B12 having the highest stiffness, followed by beam 2B10, and beams B8-B10 exhibiting the lowest stiffness.As the deflection increased subsequent to the steel bars reaching their yield point, the momentdeflection curve of beam 2S12 remained nearly horizontal during the stage following the yielding of the steel bars.The moment-deflection curves of beams S12-B10 and S12-B12 displaced a second inflection point, where the predominant load was borne by the BFRP bars, with an approximately linear increase following buckling of reinforcement.Beams S12-B10 and S12-B12 exhibited a longer period of plastic deformation after the yielding of the steel bars, lasting until the beams were damaged, in contrast to beam 2S12.
The mid-span deflection and flexural bearing capacity of each beam are presented in table 3. The experimental findings suggested that strengthened with basalt fiber-reinforced polymer rods have the highest mid-span deflection when compared to those Strengthened with steel reinforcement, while the flexural capacity was similar.As the proportion of BFRP reinforcement to steel reinforcement increased, The central deflection of the concrete beam reinforced with BFRP and a combination of rebars was reduced while the flexural bearing capacity increased (S12-B10 and S12-B12).Figure 4 illustrates the mid-span deflection and flexural capacity with equal strength substitution (beam 2B10 and beam 2S12, beam S12-B10 and beam 2S12) and equal section substitution (beam 2B12 and beam 2S12, beam S12-B12 and beam 2S12) of the reinforcement.

Observations and Fracture Distribution
To track the development of cracks in the beams during the loading test, a red marker was used, and the failure patterns of each beam are illustrated in figure 5.During the initial phase of applying the load, the crack development pattern of each beam was similar, with vertical cracks appearing in the middle.As a reference, the cracks in beam 2S12 were thin and short, whereas those inBRFP concrete beam B8-B10, beam 2B10, and beam 2B12 were wider and longer, extending up to one-half of the depth of the beam.With the increased of deflection, diagonal fissures emerged within the bending-shear zone of the beams, and continued to extend to the loading point.
The failure pattern of beam 2S12 was in the form of crushed concrete in the compressed zone after the yielding of the steel bars, with a crack width of 0.9 mm.Nevertheless, the failure patterns of beam B8-B10, beam 2B10, and beam 2B12 were in the form of crushed concrete in the compressed zone, and the BFRP bars had not yet reached the ultimate tensile stress, with the crack widths of 3.3 mm, 3.9 mm, and 4.6 mm, respectively.Comparatively, beam S12-B10 and beam S12-B12 were damaged in the form of crushed concrete in the compressed zone, subsequent to the steel bars reaching their elastic limit, while the BFRP bars did not reach the ultimate tensile stress.The measured crack widths were 3.5 mm and 3.7 mm, respectively, as shown in table 3.
As depicted in figure 6, when the reinforcement was substituted with equal strength (beam 2B10 and beam 2S12, beam S12-B10 and beam 2S12), the crack widths of beam S12-B10 and beam 2B10 were approximately four times that of beam 2S12.When steel bars were substituted with equal sections (beam 2B12 and beam 2S12, beam S12-B12 and beam 2S12), the crack widths of beam S12-B12 and beam 2B12 were approximately four and five times that of beam 2S12, respectively.Based on the experimental data, It is clear that a higher ratio of BFRP reinforcement relative to steel reinforcement led to wider cracks in both concrete beams reinforced with BFRP bars and Rebar, and those with a combination of BFRP bars and steel Rebar.Therefore, it can be inferred that concrete beams that are coreinforced with a combination of BFRP and steel bars.are more effective at inhibiting the development of concrete cracks than concrete beams exclusively strengthened with BFRP bars of equal strength or equal section of the reinforcement.

Conclusions
The principal findings of this study can be summarized as follows: (1) The bending moment-deflection curve of the material characteristics of steel and BFRP reinforcement bars.exhibits a three-stage curve, with beam cracking and a bending moment peak as the turning point.The bending moment-deflection curves of BFRP bar and steel bar hybrid Rebar-reinforced concrete beam exhibit a four-stage curve, with beam cracking, steel bar yielding, and a bending moment peak as the turning point.
(2) Increasing the proportion of BFRP bars to steel bars results in wider cracks in concrete beams reinforced with BFRP bars and the hybrid-Rebar-reinforced concrete beam with BFRP and steel bars.However, the flexural capacity increases while the mid-span deflection decreases.
(3) Concrete beams reinforced with a hybrid combination of Basalt Fiber Reinforced Polymer (BFRP) and steel rebars exhibit enhanced crack containment compared to beams solely reinforced with BFRP.This hybrid reinforcement strategy results in an augmented flexural capacity, diminished mid-span deflection, and optimizes the exploitation of BFRP's superior tensile properties.

Notes:
The symbol "2S12" refers to two steel bars having a diameter of 12 mm, while "B8-B10" denotes one BFRP bars having a diameter of 8 mm and another BFRP bars having a diameter of 10 mm.Similarly, "S12-B10" indicates one steel bars having a diameter of 12 mm and one BFRP bars having a diameter of 10 mm.As for the variables, "As" represents the sectional area of steel bars, "Af" refers to the sectional area of BFRP bars, and "Af / As" represents the area ratio of BFRP bars to steel bars.To compare the performance of beams with various kinds of reinforcement, bars with equal strength and equal cross-section (diameter) were substituted and analyzed.The three types of beams considered were: a. Reinforced concrete girder with steel bars, such as beam 2S12, which had two steel bars having a diameter of 12 mm at the bottom; b.Concrete beams reinforced with BFRP bars, such as beam B8-B10, which had one steel bars having a diameter of 8 mm and one BFRP bars having a diameter of 10 mm c.Concrete beams hybrid-reinforced with BFRP and steel bars, as beam S12-B10, with one steel bars having a diameter of 12 mm and one BFRP bars having a diameter of 10 mm.Furthermore, beam 2B12, featuring two 12 mm diameter BFRP bars, was designed following the equal section substitution of beam 2S12, while beam S12-B10 and beam S12-B12 were designed based on the equal section substitution of beam 2S12.The measurements and reinforcement details of each beam are depicted in figure1.

Figure 1 .
Figure 1.Schematic diagram of the size and reinforcements of beams.(mm)

Notes:
CC1 means concrete crushed in a compressed area after the steel bars have yielded; CC2 means concrete crushed in a compressed area but the BFRP bars have not reached the ultimate tensile stress; SY-CC means concrete crushed in a compressed area after the steel bars have yielded and the BFRP bars have not reached the ultimate tensile stress.

Figure 5 .
Figure 5.Typical failure modes of the beams.

Figure 6 .
Figure 6.Comparison of the crack widths of the beams.

Table 1 .
The material characteristics of steel and BFRP reinforcement bars.

Table 2 .
Main parameters of the beams.

Table 3 .
The experimental findings of the beams.