Mechanical Experiments on Concrete with Hybrid Fiber Reinforcement for Structural Rehabilitation

Reinforced concrete is used in the construction of bridges, buildings, retaining walls, roads, and other engineered structures. Due to seismic activities, a lot of structures develop seismic cracks. The rehabilitation of such structures is necessary for public safety. The overall aim of this research study was to produce a high-performance hybrid fiber-reinforced concrete (HPHFRC) with enhanced properties as compared to plain high-performance concrete and high-performance fiber-reinforced concrete (HPFRC) for the rehabilitation of bridges and buildings. Kevlar fibers (KF) and glass fibers (GF) with lengths of 35 mm and 25 mm, respectively, were added and hybridized to 1.5% by mass of cement to create hybrid fiber-reinforced concrete mixes. Eight mixes were cast in total. The compressive strength (f′c), flexural strength (fr), splitting tensile strength (fs), and other mechanical properties, i.e., energy absorption and toughness index values, were enhanced in HPHFRC as compared to CM and HPFRC. It was found that the concrete hybridized with 0.75% KF and 0.75% GF (HF-G 0.75 K 0.75) had the most enhanced overall mechanical properties, illustrating its potential to be utilized in the rehabilitation of bridges and structures.


Introduction
Concrete is the most frequently used material in structures around the globe. Concrete deteriorates over time, while seismic activities can also reduce the life of a structure by damaging it. Except for a few conventional materials, other advanced materials are not yet well developed or universally implemented and their repair capabilities and long-term maintenance requirements have not been investigated [1]. Structures built in the 1960s and 1970s, which are now 50-60 years old, require rehabilitation or else they will pose a hazard to public safety. Among the various methods of concrete rehabilitation, the use of fiber-reinforced concrete (FRC) for rehabilitation and retrofitting has become very popular in the last decade, and significant studies have been performed on FRC [2][3][4]. As is clear from its name, FRC contains randomly distributed fibers in the concrete mix in all dimensions, which improves the tensile properties, help in resisting cracks, and make the concrete more ductile [5]. Several types of fiber, including natural and synthetic fibers, have been added to concrete, which in turn enhanced the performance [6]. To achieve the required properties, which cannot be achieved with the use of one fiber type in FRC, Table 1. Some of the available studies on hybrid fiber-reinforced concrete.

Limitations of Fiber Contribution Major Conclusions
Chen et al., 2020 [15] PF (0.03-0.09%), SF (0.5-1%) by volume UHPC with 0.03% PF and 0.5% steel fibers showed best results at temperatures of 300 • C, 400 • C, and 500 • C in compression and flexural strength but splitting tensile strength reduced. PF fibers burn and help reduce internal water pressure.

Research Motivation and Significance
The research motivation is to provide an enhanced material for the rehabilitation of structures, which is emerging as a large industry around the globe, as concrete requires repair after exposure to harsh environmental effects, wear and tear, and seismic activities. If rehabilitation is not properly performed, the structure at risk may fail, which can result in many causalities. Cracks that develop in structures due to seismic activity may cause moisture to reach the reinforcement and cause corrosion [35]. Corrosion of reinforcements is the leading cause of structural damage and premature degradation of RCC structures [36]. The research significance of high-performance concrete with hybrid fiber reinforcement using Kevlar and glass fibers for the rehabilitation of damaged bridges and structures is yet to be investigated. The motive of this research is to incorporate the synergetic effects of hybrid fibers into high-performance concrete to obtain a better rehabilitation material in order to improve infrastructure and increase public safety by eliminating the hazards posed by damaged structures. The innovative aspect of this research study is the creation of a high-performance hybrid fiber-reinforced concrete (HPHFRC) with enhanced properties as compared to plain high-performance concrete and high-performance fiber-reinforced concrete (HPFRC) for the rehabilitation of bridges and structures.

Methodology
For the rehabilitation of concrete bridges and structures, Kevlar and glass fibers were hybridized in high-performance concrete in different percentages to evaluate their behavior and mechanical properties, keeping in mind the previous literature on hybrid fiber-reinforced concretes and rehabilitation. The mix designs and fiber percentages were chosen on the basis of the most suitable results in the trial testing. The materials were physically investigated before the mixes were prepared. ACI 211 guidelines were used to prepare the mix designs. The flow chart of this research study is shown in Figure 1. In the first step, the existing literature was evaluated to assess the need to rehabilitate damaged structures using HPHFRC. In the next steps, the concrete mixes were designed, and tests were performed according to ASTM standards. Finally, the best concrete mixes were recommended based on data collection and analysis for the purpose of rehabilitating concrete bridges. is the leading cause of structural damage and premature degradation of RCC structures [36]. The research significance of high-performance concrete with hybrid fiber reinforcement using Kevlar and glass fibers for the rehabilitation of damaged bridges and structures is yet to be investigated. The motive of this research is to incorporate the synergetic effects of hybrid fibers into high-performance concrete to obtain a better rehabilitation material in order to improve infrastructure and increase public safety by eliminating the hazards posed by damaged structures. The innovative aspect of this research study is the creation of a high-performance hybrid fiber-reinforced concrete (HPHFRC) with enhanced properties as compared to plain high-performance concrete and high-performance fiber-reinforced concrete (HPFRC) for the rehabilitation of bridges and structures.

Methodology
For the rehabilitation of concrete bridges and structures, Kevlar and glass fibers were hybridized in high-performance concrete in different percentages to evaluate their behavior and mechanical properties, keeping in mind the previous literature on hybrid fiber-reinforced concretes and rehabilitation. The mix designs and fiber percentages were chosen on the basis of the most suitable results in the trial testing. The materials were physically investigated before the mixes were prepared. ACI 211 guidelines were used to prepare the mix designs. The flow chart of this research study is shown in Figure 1. In the first step, the existing literature was evaluated to assess the need to rehabilitate damaged structures using HPHFRC. In the next steps, the concrete mixes were designed, and tests were performed according to ASTM standards. Finally, the best concrete mixes were recommended based on data collection and analysis for the purpose of rehabilitating concrete bridges.

High-Performance Concrete
The ingredients were batched by weight at a mix design ratio of 1:1.2:1.8 (cement/sand/aggregate) with 8% silica fumes (SF) and 0.6% high-performance waterreducing agent by mass of cement, mixed at a W/C + SF ratio of 0.31 for the high-

High-Performance Concrete
The ingredients were batched by weight at a mix design ratio of 1:1.2:1.8 (cement/sand/ aggregate) with 8% silica fumes (SF) and 0.6% high-performance water-reducing agent by mass of cement, mixed at a W/C + SF ratio of 0.31 for the high-performance concrete named the control mix (CM). The mix design ratio was selected after trial testing. The mix proportion for the control mix is shown in Table 2. Two types of aggregates were used, namely granite and Margala crush (10 and 5 mm), as shown in Figure 2. of aggregates were used, namely granite and Margala crush (10 and 5 mm), as shown in Figure 2.

Fibers
Kevlar and glass fibers were used in this study. The type of Kevlar used was Kevlar 29 and type of glass used was E-Glass fibers, as shown in Figure 3. The physical and mechanical properties of both fibers are shown in Table 3. The fibers used were bundled, which upon mixing in wet concrete dispersed homogeneously.

High-Performance Hybrid Fiber-Reinforced Concrete (HPHFRC)
In addition to the control mix (CM) of high-performance concrete, high-performance fiber-reinforced concrete (HPFRC) and high-performance hybrid fiber-reinforced concrete

Fibers
Kevlar and glass fibers were used in this study. The type of Kevlar used was Kevlar 29 and type of glass used was E-Glass fibers, as shown in Figure 3. The physical and mechanical properties of both fibers are shown in Table 3. The fibers used were bundled, which upon mixing in wet concrete dispersed homogeneously.    Table 4. Fiber combinations by percentage weight of cement.

Specimens and Testing
For every concrete mix, 9 standard cylinders (100 × 200 mm) were cast for every test, giving a total of 72 cylinders. Three of them were used for the compression test, three for

High-Performance Hybrid Fiber-Reinforced Concrete (HPHFRC)
In addition to the control mix (CM) of high-performance concrete, high-performance fiber-reinforced concrete (HPFRC) and high-performance hybrid fiber-reinforced concrete (HPHFRC) mixes were also cast using the same mix design. Kevlar fibers (KF) and glass fibers (GF) with lengths of 35 and 25 mm, respectively, were added and hybridized at 1.5% by mass of cement to create HPFRC and HPHFRC (GF: KF of 1.5%: 0%, 1.25%: 0.25%, 1%: 0.5%, 0.75%: 0.75%, 0.5%: 1%, 0.25%: 1.25%, 0%: 1.5%). The KF length was selected to counter macro-cracks and the GF length was selected to counter micro-cracks. Eight mixes were cast in total, including one CM, two HPFRC (M-G 1.5 and M-K 1.5 ), and five HPHFRC mixes (HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , and HF-G 0.25 K 1.25 ). The "K" in the mix names represents Kevlar, "G" represents glass, and "HF" represents hybrid fibers. The subscript numbers in the mix names represent the percentages of fibers added by mass of cement, e.g., HF-G 0.5 K 1 is a hybrid fiber mix with 0.5% glass fibers by mass of cement and 1% Kevlar fibers by mass of cement. The fiber combinations of all mixes are given in Table 4. Table 4. Fiber combinations by percentage weight of cement.

Specimens and Testing
For every concrete mix, 9 standard cylinders (100 × 200 mm) were cast for every test, giving a total of 72 cylinders. Three of them were used for the compression test, three for the splitting tensile test, and the remaining three for the density and water absorption test. For each concrete mix, three standard beams (100 mm × 100 mm × 450 mm) were also cast to study the flexural behavior. The tests conducted in this study along with the ASTM codes are given in Table 5. Compressive strength (f c ), elastic modulus (E c ), Compression energy absorbed pre-peak (CEA pre ), Compression energy absorbed post-peak (CEA post ), total compression energy absorbed (TCE), compression toughness index (C-TI) Splitting tensile strength ASTM C496 [40] Splitting tensile strength (f s ), splitting tensile energy absorbed pre-peak (SEA pre ), splitting tensile energy absorbed post-peak (SEA post ), total splitting tensile energy absorbed (TSE), splitting tensile toughness index (S-TI) Flexural strength ASTM C1609 [41] Flexural strength (f r ), flexural energy absorbed pre-peak (FEA pre ), flexural energy absorbed post-peak (FEA post ), total flexural energy absorbed (TFE), flexural toughness index (F-TI)

Slump Behavior
Slump values of the control mix along with high-performance hybrid fiber mixes and other fiber mixes are shown in Table 6. Significant reductions in slump behavior were observed after the addition of fibers to the mixes. The slump behavior of M-K 1.5 was reduced by 32% as compared to M-G 1.5 . From M-G 1.5 to M-K 1.5 , the slump behavior was reduced due to the water absorption ability of the Kevlar fibers, while glass fibers on the other hand do not absorb water. The control mix was the most workable among the mixes. Special care was given to all fiber-reinforced and hybrid fiber-reinforced mixes when filling the molds.

Density and Water Absorption
The density values of the concrete mixes are shown in Table 7. The density levels of M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , and HF-G 0.75 K 0.75 increased by 7 kg/m 3 , 5.5 kg/m 3 , 2.5 kg/m 3 , and 1 kg/m 3 , respectively, as compared to CM. The increases in density were due to the addition of glass fibers, which have a greater density as compared to Kevlar fibers. The density levels of HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 decreased by 1.5 kg/m 3 , 4 kg/m 3 , and 5.5 kg/m 3 , respectively, as compared to CM. Decreases in density were observed as the percentage of Kevlar fibers increased in the hybrid mixes. This was due to the lower density of Kevlar fibers as compared to glass fibers. The water absorption levels of concrete mixes are shown in Table 7. It is noted that with increases in Kevlar fibers in the hybrid fiber mixes, the water absorption also increased. Kevlar fibers absorb more water than glass fibers. Furthermore, the density levels of Kevlar and glass fiber also significantly differ along with their other physical and mechanical properties.

Compressive Properties
The 28 days compressive strength results for the concrete specimens are shown in Table 8. Increases in f c were noted for M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 as compared to CM of 9.7%, 10.1%, 10.6%, 13.4%, 9.5%, 8.6%, and 9.4%, respectively. Significant increases in the compressive strength values of HPHFRC mixes were observed as compared to the CM and HPFRC mixes. This was due to the hybridization effect of the fibers. The failure modes under compressive loading for all specimens are shown in Figure 4. 5.5 kg/m 3 , respectively, as compared to CM. Decreases in density were observed as the percentage of Kevlar fibers increased in the hybrid mixes. This was due to the lower density of Kevlar fibers as compared to glass fibers. The water absorption levels of concrete mixes are shown in Table 7. It is noted that with increases in Kevlar fibers in the hybrid fiber mixes, the water absorption also increased. Kevlar fibers absorb more water than glass fibers. Furthermore, the density levels of Kevlar and glass fiber also significantly differ along with their other physical and mechanical properties.

Compressive Properties
The 28 days compressive strength results for the concrete specimens are shown in Table 8.    The elastic modulus (Ec) values of all concrete mixes are given in Table 8. As compared to CM, the elastic modulus values of M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 increased by 12.2%, 14.2%, 15.5%, 17.0%, 19.0%, 21.0%, and 22.6%, respectively. The elastic modulus of M-K 1.5 increased by 9.2% as compared to M-G 1.5 . The elastic modulus values were calculated from the stress-strain curves shown in Figure 5.

Flexural Properties
The flexural strength (f r ) values of the beam specimens tested on the 28th day of curing are shown in Table 9. The flexural strength values of M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 and M-K 1.5 increased by 50.8%, 56.7%, 60.3%, 61.1%, 61.9%, 58.7%, and 57.9%, respectively, as compared to that of CM. The highest flexural strength was observed for the HF-G 0.5 K 1 specimen, which was the result of the hybridization of the fibers. The effect of loading under flexural loading is shown in Figure 7. The load deflection curves of concrete specimens under flexural loading are shown in Figure 8. The energy absorbed before cracking, i.e., the flexural energy absorbed pre-peak (FEApre), is the area underneath the load-deflection curve from the beginning to the peak load. The energy absorbed after cracking, i.e., the flexural energy absorption post-peak (FEApost), is the area underneath the load-deflection curve from peak load to failure load. The summation of FEA pre and FEA post is regarded as the total flexural energy absorption (TCE). The toughness index during flexural loading (F-TI) is ratio of the total flexural energy absorption to flexural energy absorbed pre-peak (TFE/FEA pre ). All described parameters under compression loading are shown in Table 9. Bridging effects under flexural loading are shown in Figure 7. Increases of 123.7%, 125.3%, 131.7%, 134.9%, 132.1%, 145.4% and 162.3% were observed for FEA pre values of M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 , respectively. The highest FEA pre was observed for M-K 1.5 . The FEA post for CM was observed to be zero because it failed after the first crack and could not absorb energy after the maximum load. Therefore, the increases in the FEA post values of HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 specimens equaled 4.6%, 6.5%, 9.0%, 13.4%, 12.7%, and 8.3%, respectively, as compared to M-G 1.5 , which was increased by 100% as compared to CM.

Flexural Properties
The flexural strength (ƒᵣ) values of the beam specimens tested on the 28th day of curing are shown in Table 9 Figure  7. The load deflection curves of concrete specimens under flexural loading are shown in Figure 8. The energy absorbed before cracking, i.e., the flexural energy absorbed pre-peak (FEApre), is the area underneath the load-deflection curve from the beginning to the peak load. The energy absorbed after cracking, i.e., the flexural energy absorption post-peak (FEApost), is the area underneath the load-deflection curve from peak load to failure load. The summation of FEApre and FEApost is regarded as the total flexural energy absorption (TCE). The toughness index during flexural loading (F-TI) is ratio of the total flexural energy absorption to flexural energy absorbed pre-peak (TFE/FEApre). All described parameters under compression loading are shown in Table 9

Splitting Tensile Properties
The splitting tensile strength (f s ) values of cylindrical specimens are shown in Table 10. The increases in f s for M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 as compared to CM were observed to be 26.5%, 27.5%, 29.3%, 27.8%, 28.5%, 27.3%, and 25.9%, respectively. The highest f s was observed for the HF-G 1 K 0.5 specimen, which was a hybrid fiber mix. The failure modes under splitting tensile loading for all specimens are shown in Figure 10. specimens are shown in Figure 10, while the load-time curves of the specimens can be seen in Figure 11. The toughness index during splitting tensile loading (S-TI) is the ratio of the total splitting tensile energy absorption to splitting tensile energy absorption prepeak (TSE/SEApre). The SEApre, SEApost, TSE, and S-TI values for the concrete specimens are given in Table 10.  Figure 12.  The splitting tensile energy absorption pre-peak (SEA pre ) is the area underneath the load-time curve from the beginning to the peak load. The splitting tensile energy absorption post-peak (SEA post ) is the area underneath the load-time curve from the peak load to the failure load. The summation of the SEA pre and SEA post values is regarded as the total splitting tensile energy absorption (TSE). The splitting tensile failure modes of the specimens are shown in Figure 10, while the load-time curves of the specimens can be seen in Figure 11. The toughness index during splitting tensile loading (S-TI) is the ratio of the total splitting tensile energy absorption to splitting tensile energy absorption prepeak (TSE/SEA pre ). The SEA pre , SEA post , TSE, and S-TI values for the concrete specimens are given in Table 10. The increases in SEA pre for M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 as compared to CM were observed to be 72.3%, 86.4%, 97.4%, 84.1%, 97.4%, 99.6%, and 101.9%, respectively. The highest SEA pre was noted for M-K 1.5 . The decreases in SEA post for HF-G 1.25 K 0.25 and HF-G 1 K 0.5 equaled 1.5% and 1.8%, respectively, as compared to M-G 1.5 . For SEA post , the increases for HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 equaled 6.4%, 6.5%, 12.9%, and 19.0%, respectively. The highest value was observed for M-K 1.5 . The increases in TSE for M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 as compared to CM equaled 144.9%, 157.9%, 168.7%, 161.3%, 174.7%, 181.6%, and 188.4%, respectively. The increases in S-TI for M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 , HF-G 0.25 K 1.25 , and M-K 1.5 as compared to CM equaled 42.2%, 38.4%, 36.1%, 42.0%, 39.2%, 41.1%, and 42.8%, respectively. The highest S-TI was noted for M-K 1.5 . A percentage comparison of the concrete specimens under splitting tensile loading is given in Figure 12.

Discussion
The optimization of concrete mixes is shown in Table 11. The mix with the best results was HF-G0.75K0.75. The HF-G0.75K0.75 mix is recommended for rehabilitation purposes. The Ec, f'c, TCE, C-TI, fr, TFE, F-TI, fs, TSE, and S-TI values of recommended mix (HF-G0.75K0.75) increased by 14.6%, 11.8%, 35.0%, 15.9%, 37.9%, 81.6%, 56.9%, 21.7%, 61.7%, and 29.6%, respectively, as compared to CM. The results for HF-G0.75K0.75 were the best, as shown in Table 11. The top three mixes with the most enhanced properties were hybrid fiber mixes (HF-G1K0.5, HF-G0.75K0.75, and HF-G0.5K1). These mixes can be used for rehabilitation work. Based on the above results, hybrid fiber concrete can be used in rehabilitation because of its enhanced mechanical properties in comparison to fiber-reinforced concrete. A comparison of the properties of HFRC with FRC is given in Figure 13. Table 11. Comparison of minimum, maximum, and recommended mixes.

Parameters
Mix

Discussion
The optimization of concrete mixes is shown in Table 11. The mix with the best results was HF-G0.75K0.75. The HF-G0.75K0.75 mix is recommended for rehabilitation purposes. The Ec, f'c, TCE, C-TI, fr, TFE, F-TI, fs, TSE, and S-TI values of recommended mix (HF-G0.75K0.75) increased by 14.6%, 11.8%, 35.0%, 15.9%, 37.9%, 81.6%, 56.9%, 21.7%, 61.7%, and 29.6%, respectively, as compared to CM. The results for HF-G0.75K0.75 were the best, as shown in Table 11. The top three mixes with the most enhanced properties were hybrid fiber mixes (HF-G1K0.5, HF-G0.75K0.75, and HF-G0.5K1). These mixes can be used for rehabilitation work. Based on the above results, hybrid fiber concrete can be used in rehabilitation because of its enhanced mechanical properties in comparison to fiber-reinforced concrete. A comparison of the properties of HFRC with FRC is given in Figure 13.

Discussion
The optimization of concrete mixes is shown in Table 11. The mix with the best results was HF-G 0.75 K 0.75 . The HF-G 0.75 K 0.75 mix is recommended for rehabilitation purposes. The E c , f c , TCE, C-TI, f r , TFE, F-TI, f s , TSE, and S-TI values of recommended mix (HF-G 0.75 K 0.75 ) increased by 14.6%, 11.8%, 35.0%, 15.9%, 37.9%, 81.6%, 56.9%, 21.7%, 61.7%, and 29.6%, respectively, as compared to CM. The results for HF-G 0.75 K 0.75 were the best, as shown in Table 11. The top three mixes with the most enhanced properties were hybrid fiber mixes (HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , and HF-G 0.5 K 1 ). These mixes can be used for rehabilitation work. Based on the above results, hybrid fiber concrete can be used in rehabilitation because of its enhanced mechanical properties in comparison to fiber-reinforced concrete. A comparison of the properties of HFRC with FRC is given in Figure 13. Table 11. Comparison of minimum, maximum, and recommended mixes.

Empirical Equations for Predicting Toughness Index
The toughness index shows the energy absorption of a material post-fracture, which is given primary importance when choosing a material for rehabilitation. The empirical equations (Equations (1)-(3)) for predicting toughness indexes (C-TI, F-TI, and S-TI) were developed for fiber percentages by using experimental data from compressive, flexural, and splitting tensile strength tests. The R 2 values ranged from 0.66 to 0.84 and the maximum variance observed between experimental and numerical values was 12%, as shown in Figure 14.
For the compression toughness index: For the splitting tensile toughness index:

Empirical Equations for Predicting Toughness Index
The toughness index shows the energy absorption of a material post-fracture, which is given primary importance when choosing a material for rehabilitation. The empirical equations (Equations (1)-(3)) for predicting toughness indexes (C-TI, F-TI, and S-TI) were developed for fiber percentages by using experimental data from compressive, flexural, and splitting tensile strength tests. The R 2 values ranged from 0.66 to 0.84 and the maximum variance observed between experimental and numerical values was 12%, as shown in Figure 14.
For the compression toughness index: For the flexural toughness index: For the splitting tensile toughness index:

Conclusions
Significant enhancements in the overall properties of high-performance hybrid fiberreinforced concrete (HPHFRC) were observed as compared to high-performance fiberreinforced concrete (HPFRC) and the control mix (CM), proving the usefulness of the fiber hybridization method in the concrete industry. In the rehabilitation of concrete bridges and structures, these enhancements can be put to use.
The following are some of the conclusions extracted from this study: 1. The workability of concrete was significantly reduced with the addition of fibers. As the percentage of Kevlar fibers increased in the concrete, the workability decreased due to the water absorption of Kevlar fibers; 2. The maximum density was observed for mixes with dominant glass fiber percentages (M-G1.5, HF-G1.25K0.25, HF-G1K0.5, HF-G0.75K0.75), because glass fibers are denser than Kevlar fiber. A correlation with the minimum water absorption was also observed in glass-fiber-dominant mixes. This was due to the water absorption properties of the Kevlar fibers; 3. The overall best properties were observed for the hybrid mix (HF-G0.75K0.75) with 0.75% of glass fibers and 0.75% of Kevlar fibers. Among all mixes, the most dominant properties were for the HF-G0.75K0.75 mix due to the equal quantities of fibers exhibiting the best synergetic effect; 4. The elastic modulus of HF-G0.75K0.75 increased by 17% as compared to CM, which was very near to the maximum value observed. The compressive strength, total compressive energy, and toughness index during compression for HF-G0.75K0.75 were increased by 13.4%, 58.6%, and 18.8% as compared to CM, which were the maximum values observed out of all mixes; 5. The flexural strength, total flexural energy absorbed, and toughness index during flexural loading for HF-G0.75K0.75 increased by 61.1%, 444.7%, and 131.9% as compared to CM; 6. As compared to CM, increases of 127.8%, 261.3%, and 142% were observed in terms of the splitting tensile strength, total splitting tensile energy absorbed, and splitting

Conclusions
Significant enhancements in the overall properties of high-performance hybrid fiberreinforced concrete (HPHFRC) were observed as compared to high-performance fiberreinforced concrete (HPFRC) and the control mix (CM), proving the usefulness of the fiber hybridization method in the concrete industry. In the rehabilitation of concrete bridges and structures, these enhancements can be put to use.
The following are some of the conclusions extracted from this study: 1.
The workability of concrete was significantly reduced with the addition of fibers. As the percentage of Kevlar fibers increased in the concrete, the workability decreased due to the water absorption of Kevlar fibers; 2.
The maximum density was observed for mixes with dominant glass fiber percentages (M-G 1.5 , HF-G 1.25 K 0.25 , HF-G 1 K 0.5 , HF-G 0.75 K 0.75 ), because glass fibers are denser than Kevlar fiber. A correlation with the minimum water absorption was also observed in glass-fiber-dominant mixes. This was due to the water absorption properties of the Kevlar fibers; 3.
The overall best properties were observed for the hybrid mix (HF-G 0.75 K 0.75 ) with 0.75% of glass fibers and 0.75% of Kevlar fibers. Among all mixes, the most dominant properties were for the HF-G 0.75 K 0.75 mix due to the equal quantities of fibers exhibiting the best synergetic effect; 4.
The elastic modulus of HF-G 0.75 K 0.75 increased by 17% as compared to CM, which was very near to the maximum value observed. The compressive strength, total compressive energy, and toughness index during compression for HF-G 0.75 K 0.75 were increased by 13.4%, 58.6%, and 18.8% as compared to CM, which were the maximum values observed out of all mixes;
As compared to CM, increases of 127.8%, 261.3%, and 142% were observed in terms of the splitting tensile strength, total splitting tensile energy absorbed, and splitting tensile toughness index of HF-G 0.75 K 0.75 ; 7.
The high-performance hybrid fiber-reinforced concrete illustrated the best performance as compared to the CM and HPFRC mixes. The top three mixes that showed the best properties were hybrid fiber-reinforced concrete mixes (HF-G 1 K 0.5 , HF-G 0.75 K 0.75 , HF-G 0.5 K 1 ); 8.
Based on these conclusions, the hybrid fiber mixes demonstrated more enhanced overall properties as compared to FRCs. These improved mechanical properties of the HFRC can be utilized in the rehabilitation of bridges and structures.

Recommendations
A long-term durability study of HPHFRC for rehabilitation needs to be performed. The chemical resistance should be evaluated before the practical implementation of the HFRC.
Fibers of different length should be used to check their hybridization effects.
To improve the mix's workability, different super plasticizers need to be checked to identify the best results.
A cost analysis is recommended before any commercial application. Informed Consent Statement: Not applicable.

Data Availability Statement:
The data can be made available from the corresponding author upon reasonable request.