Experimental Study on Flexural Performance of Precast Prestressed Concrete Beams with Fiber Reinforcement

Abstract

Fiber-reinforced concrete (FRC) has good toughness and a gentle stress–strain softening section, which can improve the inherent defects of concrete material such as high brittleness, easy cracking, and poor fracture toughness. In this paper, carbon fiber, aramid fiber and mixed fiber are introduced to enhance the performance of precast prestressed concrete beams (PPCB). The effects of different fiber types and adding rate on mechanical properties of FRC were studied via axial compression test and four-point bending test. Based on the flexural performance test of precast FRC beams, the failure form and the improvement degree of flexural ability of the beams were analyzed. Moreover, the load–deflection curve and the quantified ductility index obtained by the test were discussed, and the law of the improvement effect of fiber type on flexural property was revealed. The results show that the optimal addition rate of fiber is 0.6%. In addition, the addition of fiber significantly increased the cracking load and ultimate bearing capacity of the test beam, whereby the average increase in cracking load and ultimate bearing capacity was 40% and 20%, respectively. At the same time, the ductility of the beam is obviously enhanced by the action of fibers, among which the hybrid fiber has the best effect. Specifically, the ductility coefficient analysis verifies that aramid fiber plays an important role in improving the ductility of the components.

1. Introduction

In recent years, China’s transportation industry has achieved historic milestones. By the end of 2022, the total length of the comprehensive transportation network had exceeded 6 million kilometers, ranking first globally in both high-speed rail and expressway mileage [1]. Bridges play a vital role as key nodes in transportation infrastructure, and the safety of their structures is crucial for the normal functioning of cities and the coordinated development of regional economies. According to incomplete statistics, the total number of road and railway bridges in China exceeds 1 million, thus establishing China’s status as a bridge construction country, of which prestressed concrete bridges account for a considerable proportion [2,3].

With the advancement and widespread adoption of prefabricated construction technology, the prefabricated construction technique for bridge structures has seen rapid development in China [4]. In this innovative approach, raw materials such as steel and concrete are transported to prefabrication plants, where components forming the main structure are produced on assembly lines. Furthermore, the manufactured components undergo maintenance and storage in the prefabrication area until they are transported to the construction site as per the production plan and lifted into position to complete the construction of the main structure. Overall, the construction process resembles assembling blocks, enabling the factory prefabrication and standardization of bridge construction [5]. Compared to traditional cast-in-place methods, prefabricated construction offers advantages such as faster construction speed, reduced on-site personnel, and higher project quality. The construction process no longer requires rebar binding and wet concrete operations, resulting in minimal generation of construction waste, significantly reduced construction noise and dust pollution, and substantial time savings [6,7]. Additionally, the substantial reduction in the number of construction workers, construction waste, and carbon emissions contributes directly to the goals of achieving carbon peak and carbon neutrality [8]. Therefore, in the context of China’s strong advocacy and development of green and environmentally friendly prefabricated construction technology, the prefabrication and industrialization of prestressed concrete bridges have experienced rapid growth [9].

Concrete materials possess inherent drawbacks such as high brittleness, susceptibility to cracking, low tensile strength, and poor fracture toughness. Previous studies indicate that more than half of the prestressed concrete bridges face problems such as concrete surface cracking and reinforcement corrosion due to increased traffic and ongoing harmful environmental impacts. When significant cracks manifest in prestressed concrete structures, the concrete cover in some bridges undergoes detachment, thereby exposing and corroding the reinforcements, which severely compromises the safety of the bridge structures. Consequently, the issue of cracking defects in prestressed concrete bridges remains a focal point of engineering research.

However, fiber materials with high strength and toughness properties can effectively address the limitations of concrete materials [10,11]. The combination of concrete and fibers facilitates the maximization of their respective advantages, resulting in the development of fiber-reinforced concrete (FRC) with enhanced toughness and ductility [12,13]. In recent years, carbon fiber, aramid fiber, polyethylene fiber, and basalt fiber are the four fiber types that have developed most rapidly, and they have become the main raw materials for improving concrete performance, among which carbon fiber and aramid fiber are the most widely used [14,15]. Considerable progress has been made in the research of FRC, including material mix design, mechanical properties, and load-bearing characteristics. Ayub et al. [16] investigated the mechanical properties of short-cut basalt FRC with a fiber volume fraction of 3%, and they observed a significant improvement in the flexural performance of concrete after adding basalt fibers. Lee et al. [17] found that although both steel fiber and glass fiber have a certain degree of improvement in flexural toughness, the effect of steel fiber is better. Specifically, after the bending strength reaches its peak, there is still about 30% of the bearing capacity, and this gradually decreases until it is completely destroyed. Nie et al. [18] observed that aramid fiber presented better properties than carbon fiber to improve the mechanical performance of cement mortars. Avanaki et al. [19,20] carried out a study on the seismic performance of FRC tunnel segments, and they discussed the influence of different steel FRC composite materials on the bending response of segment joints under earthquake via numerical and experimental methods. Halvaei et al. [21] introduced carbon textiles and chopped carbon fibers as reinforcing materials of engineered cementitious composite (ECC). Wille et al. [22] proposed a new tensile device and carried out uniaxial tensile properties tests of ultra-high performance fiber-reinforced concrete (UHP-FRC) considering different volume fractions. Deng et al. [23] conducted four-point bending tests on 16 groups of ductile FRC specimens, finding that the flexural strength and bending toughness of the specimens gradually increased with an increase in fiber dosage. Wang et al. [24] examined the influence of carbon fiber length and stirrup ratio on the flexural capacity of carbon FRC beams. The results showed that carbon fiber effectively improved the brittle failure of the concrete beams, and as the carbon fiber length increased, the deflection at failure of the beams also increased, while the stirrup ratio had minimal impact on the deflection and strain of carbon FRC beams. Li [25] performed flexural performance tests on six PVA-FRC beams and conducted a detailed analysis of the effects of fiber volume fraction on the cracking load, ultimate load, and flexural stiffness of the concrete beams. Considering the excellent intrinsic strength, micro-fibrillated surface characteristics, and interfacial properties of aramid fibers, the research by Qiao et al. [26] indicated that aramid fibers exhibit superior crack resistance compared to polypropylene fibers. Meanwhile, aramid fibers have a positive impact on reinforcement and toughening under low water–cement ratio conditions. The improvement in flexural strength in high-fiber-content ECC using aramid fibers is significantly better than that achieved by PVA fibers, albeit with a slightly smaller deflection. Additionally, the employment of multiple types of hybrid fibers is currently a prominent research focus in FRC. When two or more types of fibers are added, the mechanical strength of concrete is further enhanced compared to single fiber reinforcement. Krūmiņš et al. [27] conducted four-point bending tests on hybrid FRC and observed an overall improvement in the performance of the concrete beams.

The present study investigates the influence of different types and proportions of carbon fiber, aramid fiber, and hybrid fibers on the basic mechanical properties of concrete specimens. The conducted tests include axial compressive tests and three-point bending tests. By studying the effect of fiber type and dosage on the properties of FRC, the flexure properties of precast prestressed concrete beam (PPCB) are further studied on the basis of selecting the best dosage ratio. The load-bearing process, failure modes, and crack propagation patterns of PPCB are systematically analyzed. The research findings provide important references for enhancing the performance and structural safety and durability of existing bridge structures.

2. Mechanical Properties of FRC

2.1. Material Properties

Carbon fiber (CF) has high strength and light weight, and is a good reinforcement material, which is widely used in anti-seepage [28] and anti-cracking projects [29]. Similarly, aramid fiber (AF) is an excellent toughening material with a high specific strength, large specific modulus, and high temperature resistance [30]. Therefore, CF and AF are selected as the main research objects in this paper for improving the toughness and crack resistance of concrete. As shown in Figure 1, CF and AF are produced by Nanjing Mankat Technology Co., Ltd. of Nanjing, China and Zhejiang Xuantai New Materials Co., Ltd. of Tongxiang, China, respectively. The short-cut fiber length is 1.2 mm, and the mechanical properties of the two short-cut fiber materials are shown in

Table 1. Mechanical properties of fiber materials.

Fiber Type	Density (kg/m3)	Cut Length (mm)	Diameter (μm)	Tensile Strength (MPa)	Tensile Modulus (GPa)	Elongation (%)
Carbon fiber (CF)	1750	12	8	3530	240	1.5
Aramid fiber (AF)	1440	12	12	3150	80	3.6

.

2.2. Experimental Design

Compressive strength and flexural strength are important mechanical properties of cement-based materials. FRC has good toughness under compression and bending, and the influence of different fiber content on the mechanical properties of FRC is obvious. In order to obtain the best performance of FRC, it is necessary to carry out a basic mechanical study on the FRC test block to determine the optimal fiber content. The basic mechanical properties of FRC include axial compressive strength and flexural strength. During the test, CF represents the carbon fiber test block, AF refers to the aramid fiber test block, and CAF is the carbon and aramid 1:1 mixed fiber test block. The basic mechanical properties tests of FRC were carried out in the laboratory of Beijing University of Technology. The relevant test procedures were carried out according to the provisions of the “Standard for test methods of concrete physical and mechanical properties” (GB/T 50081-2019) [31].

A total of 77 groups (3 in each group) of prismatic blocks were set up in the experiment. Among them, 29 groups of 150 × 150 × 300 mm test blocks were used for compressive test, and 29 groups of 150 × 150 × 300 mm specimens were used for elastic modulus test. In addition, the bending test used 19 groups of 100 × 100 × 400 mm blocks, equaling a total of 57 blocks.

2.3. Axial Compressive Strength

There are three test blocks in each group in the test, and the test block whose compressive strength is closest to the average value and whose failure phenomenon is representative is selected for analysis, as shown in Figure 2.

As can be seen from Figure 2, the failure process of FRC is obviously different from that of ordinary concrete. X-type cracks appear in the section of ordinary concrete prismatic test block when it is compressed, and concrete collapse and sudden reduction in strength occur when it reaches peak load, which is a typical brittle failure. During the compression process of the fiber concrete test block, due to the cracking resistance and toughening effect of the fiber, the compressive toughness increases significantly. When the load reaches about 40% of the ultimate load, vertical micro-cracks appear in the middle of the specimen. With the increase in the load, the cracks extend to both ends, and new vertical cracks appear around the specimen. Failure of the test block was declared, with no spalling of concrete. The failure modes observed from this test are similar to those investigated by Huang et al. [10], which clearly indicated the action of the FRC.

The compressive strength results of the test block are shown in

Table 2. Axial compressive strength f_ck of concrete test block (unit: MPa).

Fiber Type	Fiber Content (%)
	0.00	0.15	0.30	0.45	0.60	0.75	0.90
CF	40.4	39.2	39.3	40.5	41.9	38.8	37.8
AF	40.4	38.7	39.5	40.9	44.3	39.4	36.9
CAF	40.4	39.7	39.4	40.6	42.4	41.2	40.5

. Combined with the analysis of the failure phenomenon of the test block, when the fiber content is small, the compressive strength is slightly improved, and the brittleness property is greatly improved. When the fiber content is increased to 0.6%, the compressive strength is the largest improvement, and the ductility property is improved compared with that of the 0.3% doped block. However, when the content is increased to 0.9%, the compressive strength of the test block is decreased. The reason for this result is that excessive fiber content leads to clumping, forming weak points and reducing ductility. Therefore, when the fiber content is 0.6%, the FRC performance is better played.

2.4. Flexural Performance

2.4.1. Flexural Strength

For the FRC bending performance, 100 × 100 × 400 mm beams are used, the calculated span is 300 mm, and the supports are located 50 mm from each end, and the test device and loading results are shown in Figure 3. During the loading process of the FRC test block, many micro-cracks are produced across the pure bend section, with the width of one main crack increasing; then, the load gradually decreases, and failure is declared, and although the deflection of the test block is large, the crack does not penetrate it, as shown in Figure 3c; and the test block can still maintain good integrity. As a control group, the failure of the ordinary concrete test block occurred very suddenly, accompanied by a “bang”, the loading point on one side of the test block suddenly broke into two segments, and the load curve displayed by the system dropped sharply.

According to the “Standard for Test Methods for Physical and Mechanical Properties of Concrete” [31], the flexural strength of FRC test blocks under different fiber types and fiber content is calculated, as shown in

Table 3. Flexural strength f_p of concrete test block (unit: MPa).

Fiber Type	Fiber Content (%)
	0.00	0.15	0.30	0.45	0.60	0.75	0.90
CF	5.82	5.80	5.62	5.92	7.26	6.49	7.00
AF	5.82	5.72	5.94	5.89	6.18	5.85	5.94
CAF	5.82	5.81	6.32	6.64	7.00	6.52	6.25

. It can be seen from the table that when the fiber addition amount is small, the bending strength of the test block is basically unchanged, and when the addition rate is greater than 0.45%, the bending strength of the test block is greatly improved, and the increase is about 20%. The trend is similar to that of previous studies [10]. Among them, carbon fiber has the greatest effect of improving the flexural strength, followed by hybrid fiber (CAF).

The curves of load and deflection with loading time are shown in Figure 4. It can be seen from the curve that carbon fiber still shows a certain brittleness when the content is low; that is, the falling section of the curve is relatively steep. When the block load is reduced to half of the ultimate load, the deflection value of the AF block is the largest, followed by CAF, which shows that aramid fiber plays an important role in improving the toughness of concrete.

2.4.2. Flexural Toughness (FT)

Due to the large human error in the judgment of initial crack deflection, Banthia et al. [32] proposed a toughness analysis method bounded by peak load, which divided the load–deflection curve area into two parts, pre-peak (E_pre) and post-peak (E_post,m), and considered the energy consumption of fiber concrete during deformation with specific deflection (L/m) as the variable. The proposed resilience indicator PCS_m is defined as follows:

$$PC{S}_{\mathrm{m}}=\frac{E_{post,m} L}{(\frac{L}{m}-\delta )b{h}^2}$$

(1)

where E_post,m is the area surrounded by the load–deflection curve after peak loading; L is the span of the beam (take 300 mm); δ is the deflection value corresponding to the peak load; b and h are the width and height of the beam section, respectively; and L/m is defined as the deflection corresponding to the load falling to 20% of the peak load.

According to Equation (1), the flexural toughness index PCS_m of the FRC test block is calculated as shown in

Table 4. Flexural toughness PCS_m of concrete test block.

Fiber Type	Fiber Content (%)
	0.00	0.15	0.30	0.45	0.60	0.75	0.90
CF	1.07	0.86	0.73	1.15	2.86	2.44	2.56
AF	1.07	1.59	2.53	2.39	2.73	2.53	3.59
CAF	1.07	1.68	1.32	2.63	3.37	2.04	2.95

. On the whole, the flexural toughness of FRC test block is significantly improved compared with that of the ordinary concrete test block, the improvement effect of aramid fiber on the toughness of concrete test block is more obvious, and the maximum increase range is 235%. The degree of improvement in mixed fibers was secondary, reflecting the positive effect of mixing the two fibers.

3. Experimental Program of FPCB

3.1. Element Design

According to the results of the mechanical properties of the FRC test block, 0.6% was selected as the optimal fiber addition rate, and the bending performance of the fiber concrete test beam was tested. All test members are simple support beams, rectangular sections are 200 mm × 180 mm, the total length of the beam is 4 m, and the calculated span diameter is 3.7 m. The upper, middle, and lower ends of the beam are equipped with 2φ6 non-prestressed steel bars, all test beams are equipped with stirrups of No. 12 iron wire φ2.8@100, and the stirrups within 500 mm at each end of the beam are encrypted to φ2.8@50. The ordinary steel bar used in the component is HPB300, and the prestressed steel bar adopts φ7 prestressed steel wire, and its tensile stress is 0.73 times the standard value of tensile strength and the maximum tensile value does not exceed 1.05 times, and the material parameters of the steel bar are shown in

Table 5. Mechanical properties of rebar materials.

Rebar Type	Diameter (mm)	Yield Strength (MPa)	Ultimate Strength (MPa)	Elongation (%)
Regular rebar	6	312	407	25
Prestressing tendon	7	1410	1603	4.2

. The detailed reinforcement details of the PPCB are shown in Figure 5, and the production process of the components is shown in Figure 6.

3.2. Load Equipment and System

The schematic diagram of the test loading and the location of the measurement points are shown in Figure 7. The test adopts the three-point loading method, a total of 4 test beams, including 1 ordinary concrete prestressed beam for the control test, and 3 concrete beams mixed with different fibers. During the test, L represents the prototype scaled test beam, CL is the carbon fiber test beam, AL is the aramid fiber test beam, and CAL refers to the carbon and aramid 1:1 mixed fiber test beam. The loading test adopts a hierarchical loading system, and before the cracks of the test beam, 3 kN is loaded in each stage, and data acquisition and crack observation are started after each stage is loaded for 10 min. When loaded to 80% of the calculated value of cracking load, the loading of each stage is changed to 1 kN; after the cracking of the test beam, it is changed to 3 kN per stage for loading; and, when the load reaches 80% of the calculated value of the bending bearing capacity, the displacement control is used to obtain the accurate ultimate bearing capacity, maximum deflection, and maximum crack width.

4. Experimental Results and Discussion

4.1. Failure Pattern

For the prototype beam L, the first crack appeared near the loading point when the load was increased to 12 kN. As the load increased, new cracks continued to form, and when loaded to 24 kN, the cracks were essentially continuous. Among them, the first major crack developed at the fastest rate, and the deflection of the beam increased significantly. Finally, when the load reached 38.7 kN, the tension reinforcement yielded, the surface concrete spalled, and the test beam failed. The failure modes of the PPCB test beams are shown in Figure 8. The test process showed that the first crack appeared in the beams when the loads reached 17 kN, 16 kN, and 16.5 kN, respectively. As the loading continued, the number of cracks increased. Unlike the prototype beam, when loaded to 80% of the ultimate load, the crack quantity in the FRC-enhanced beams stabilized, and the crack height was smaller than that of the prototype beam L. Among them, the crack development in beam CL was the slowest, with the fewest cracks, only 15. The final failure mode was the crushing of the upper concrete and crack overloading. Beam AL had an increased range and number of cracks compared to beam CL, showing good ductility. Even after the crack width exceeded the limit, the beam deflection continued to increase until the reinforcement yielded, without concrete crushing. Beam CAL exhibited a positive hybrid effect, with a significant redistribution of stresses after cracking. The crack distribution was the widest, and the crack spacing and width were the smallest, and in the end, it produced 21 cracks.

Table 6. Flexural test results.

Beam Type	Cracking Load Fcr (kN)	Cracking Moment Mcr (kN·m)	Yield Load Fy (kN)	Yield Moment My (kN·m)	Ultimate Load Fu (kN)	Ultimate Moment Mu (kN·m)
L	12.0	7.4	30.9	19.09	38.8	23.9
CL	17.0	10.5	41.6	25.64	49.0	30.2
AL	16.1	9.9	35.3	21.78	43.5	26.8
CAL	16.5	10.2	38.3	23.61	44.9	27.7

presents the cracking load, yielding load, and ultimate load, as well as the converted section bending moments for the four test beams under static loading. From Table 6, it can be observed that the cracking load, yielding load, and ultimate load of FRC beams have all increased, with minimum increases of 30%, 14%, and 12%, respectively. This indicates that the use of fibers has greatly improved the drawbacks of concrete, such as brittleness and susceptibility to cracking. Carbon fiber has the largest influence on the cracking load and flexural performance of the test beams, followed by hybrid fibers.

4.2. Strain Results of Concrete

During the bending test, concrete strains along the height of the cross-section at the mid-span were measured for the four test beams. Figure 9 shows the development of concrete strains at different load levels before failure. It should be noted that due to the gradual increase in load during the test, the strains at the bottom of the beam exceeded the limit tensile value and caused damage to the strain gauges, resulting in missing data.

Figure 9a for beam L, it can be observed that the strain and load of the concrete at the mid-span cross-section are directly proportional. Under the first two load levels, the stress–strain curve of the concrete cross-section is linear, and the neutral axis of the test beam continuously shifts upward as the load increases, in accordance with the assumption of a plane section. In the initial loading stage, the neutral axis of prototype beam L is around 80 mm. As the load increases, the tension zone of the concrete develops cracks and gradually becomes inactive, causing the height of the compressed zone to decrease and the neutral axis to move upward. When the test beam is close to failure, the neutral axis reaches around 110 mm, with a significant upward shift. At the point of imminent failure, cracks develop rapidly, resulting in damage to the strain gauge at the bottom of the compressed zone, leading to missing strain values for the last two load levels.

Figure 9b–d show that under the various load levels, the concrete strains of the FRC-enhanced beams along the cross-section height generally follow the assumption of a plane section. Before the concrete cracks, the position of the neutral axis is around 80 mm, and as the load continues to increase, the concrete gradually cracks, causing the neutral axis to move upward. Near the point of failure, the neutral axis is around 102 mm, slightly lower compared to beam L. Among the beams, the neutral axis of beam CL has the slowest upward movement, followed by beam CAL. Additionally, the inclusion of carbon fiber, aramid fiber, and hybrid fibers results in smaller concrete strains compared to ordinary concrete strains under the same load. This indicates that the fibers in the concrete act as micro-reinforcement, sharing the load with the internal reinforcement of the beam. It not only improves the ultimate load-carrying capacity of the test beams but also effectively delays the occurrence and development of cracks.

4.3. Strain Results of Rebar

In the rebar strain analysis, it is noted that the steel reinforcement in the bending members is the main load-bearing component. For the partially prestressed concrete beams with a low reinforcement ratio, once the mild steel reinforcement yields, it indicates that the beam is approaching failure. Therefore, it is necessary to analyze the load–rebar strain relationship and investigate the influence of different treatment methods on the flexural performance of partially prestressed concrete beams. Figure 10 shows the strain curves of mild steel reinforcement and prestressed steel reinforcement in PPCB.

From Figure 10a, it can be observed that the strain curves of mild steel reinforcement overlap and exhibit linear growth before concrete cracking, with a relatively slow increase in strain. This is because before cracking, the steel reinforcement and concrete jointly bear the sectional stress, with the concrete bearing the majority of the stress. After concrete cracking, the strain of mild steel reinforcement in Beam L increases rapidly due to the concrete no longer contributing to the load-carrying capacity, and the stress is solely transferred to the steel reinforcement. In contrast, the strain increment of steel reinforcement in beams CL, AL, and CAL is smaller than that of the prototype beam after concrete cracking. This is attributed to the inhibition of crack propagation by nearby fibers, partially offsetting the stress concentration. As the load continues to increase, fibers are pulled out or broken, resulting in significantly smaller slopes of the strain curves for beams CL, AL, and CAL compared to the pre-cracking phase, with similar trends among them. When the tensile reinforcement reaches yield, the steel reinforcement in beams AL and CAL yields later than that in beam L, while in beam CL, the cracks have already exceeded the permissible limit before steel reinforcement yielding due to its higher stiffness. Under the same load, beam CL exhibits the smallest strain in mild steel reinforcement, beam AL shows the largest strain, and beam CAL, with hybrid fibers, falls between the two, indicating a positive hybrid effect. The analysis suggests that fibers provide a certain tensile resistance in concrete, thus enhancing the flexural performance of concrete beams.

In this study, the pre-tensioning method was used for prestressing the steel reinforcement, which resulted in initial strain in the prestressed steel reinforcement before the experiment. Therefore, the strain increment was analyzed in this section. Figure 10b indicates that the prestressed steel reinforcement did not reach yield until the failure of the test beams. Before concrete cracking, the strain values of the prestressed steel reinforcement did not differ significantly and exhibited a linear relationship. After cracking, stress redistribution occurred, with the prestressed steel reinforcement bearing more load and thus experiencing increased strain. Under the same load, the prestressed steel reinforcement strain in beam L was greater than that in the FRC beam. This can be attributed to the bridging effect of fibers, which impedes crack propagation and facilitates energy transfer between fibers and concrete, resulting in a more balanced stress distribution between the mild steel reinforcement and prestressed reinforcement.

4.4. Load–Deflection Curve

The line displacement along the axis perpendicular to the cross-section, known as deflection, during bending deformation reflects the deformation capacity of the test beam under load. Figure 11 shows the load–deflection curve for PPCB.

From the curve, the turning points of the test beam’s cracking load, yield load, and ultimate load can be observed, indicating that the failure process of the test beam can be roughly divided into three stages: Stage I: Loading until concrete cracking. The load–deflection curve of the test beam shows linear growth, indicating that the test beam is operating in the full cross-section and exhibiting maximum stiffness, corresponding to the elastic working stage. Stage II: Working with cracks. After reaching the cracking load, the first crack appears in the pure bending zone, leading to a decrease in stiffness and a change in the load–deflection curve, with a turning point. Beyond the turning point, the load–deflection curve continues to increase until the reinforcement yields. Stage III: Reinforcement yielding until beam failure. After reinforcement yielding, the load-bearing capacity of the test beam increases slightly, crack propagation accelerates, and deflection significantly increases.

During the initial loading stage, when the load is relatively small, the test beam does not exhibit cracks, and the deflection change is not significant, following a linear increase. This indicates elastic deformation characteristics of the test beam. As the load increases, the test beam L is the first to crack. Under the same loading conditions, the deflection of FRC test beams is significantly smaller than that of the prototype beam. This is because the fibers exhibit excellent resistance to bending and crack prevention, and the fiber extraction and fracture process dissipate some energy, delaying the yielding of the reinforcement and improving the ultimate load-carrying capacity of the test beam. Under the same loading conditions, the deflection sequence from largest to smallest for FRC beams is as follows: AL, CAL, CL. This is due to carbon fibers significantly improving the brittleness of concrete and enhancing its stiffness, while aramid fibers improve the ductility of concrete. When combined, they fully exploit their respective advantages when incorporated into the concrete.

Ductility refers to the ability of a structure, member, or section to deform from initial yield to maximum bearing capacity without a significant reduction in bearing capacity. It reflects the deformation ability of the structure. The deformation ability of the structure with poor ductility will lead to sudden failure, so it is particularly important to analyze the ductility of test beams. The deflection ductility coefficient is used in this paper as μ₁ = ƒ_μ/ƒ_y, where ƒ_μ and ƒ_y represent the ultimate deflection and yield deflection of the test beam, respectively.

The ductility coefficient of test beams result is shown in

Table 7. The ductility coefficient of test beams.

Beam Type	fy/mm	fu/mm	μ1
L	37.1	59.80	1.61
CL	29.87	54.30	1.82
AL	27.61	57.34	2.08
CAL	27.52	56.05	2.04

. It can be seen from Table 7 that compared with the L beam, the ductility of the other fiber-reinforced beams except the CL-P beam is improved to varying degrees, and the elongation of the fiber concrete beams is larger. Compared with the effect of different fiber types, it can be seen that aramid fiber has a greater effect on beam ductility than carbon fiber, and the hybrid positive effect of the two fibers can be fully reflected by ductility values of the CAL beam.

For FRC beams, the ductility of AL beam is greatly improved by 29% due to the good toughness of aramid fibers. Meanwhile, the high strength of carbon fiber makes the ductility of CL beam minimally improved. Hybrid fibers combine the characteristics of the two fibers to achieve a complementary effect, and the ductility of CAL beam is improved by 26.7% compared with that of the prototype beams. It shows the positive effect of fiber mixing.

5. Conclusions

In order to study the flexural performance of fiber-reinforced prestressed reinforced concrete beams, the load–deflection curves, the strain between reinforcement and concrete, and the ductility properties of the beams under bending load were analyzed via test methods. The following conclusions were obtained:

(1)

The addition of fiber significantly increased the cracking load and ultimate bearing capacity of the test beam, with the cracking load increasing on average by 40% and ultimate bearing capacity by 20%. When the ordinary prestressed concrete test beam is damaged, the concrete in the compression zone will crush and fall off, while the FRC beam will maintain good integrity when it is damaged.

(2)

The deflection corresponding to the ultimate load of the fiber prestressed concrete beam is higher than that of the ordinary prestressed concrete beam, and according to the analysis of the ductility coefficient, the addition of fiber not only improves the bearing capacity of the test beam, but also improves the ductility of the beam body. Among the fiber concrete beams, beam AL has the best ductility, so it can be seen that aramid fiber plays an important role in improving the ductility of the component.

(3)

The short-chopped carbon fiber and aramid fiber as reinforcement materials can effectively improve the compressive strength and flexural performance of concrete, and they can be used to improve the technology of precast concrete bridge members, thereby increasing the service life of bridge structures. It should be noted that further tests and numerical analyses are still needed to verify the reliability of the overall mechanical properties of complex structures.