Experimental Study on the Shear Behavior of HTRCS-Reinforced Concrete Beams

Abstract

High-toughness resin concrete steel mesh (HTRCS) composites, as a novel reinforcement material, are extensively employed, yet there has been a lack of comprehensive quantitative studies on them, and our knowledge about them predominantly relies on experimental investigations. To delve into the shear performance of reinforced concrete beams fortified with HTRCS, this research executed four-point bending static load experiments on a benchmark of two standard beams and six HTRCS-reinforced beams. The results demonstrate that the shear bearing capacity of the reinforced concrete beams was notably enhanced with HTRCS, ranging from approximately 10% to 65%. Further examination revealed that the stiffness of the specimens is significantly influenced by the HTRCS thickness, shear–span ratio, and concrete strength, with the shear–span ratio exerting the most notable impact on stiffness. This analysis furnishes a solid theoretical foundation for the utilization of HTRCS.

1. Introduction

Reinforced concrete structures (RC structures) are extensively utilized in global infrastructure development due to their excellent fire resistance, durability, moldability, and the ease of obtaining materials. Nonetheless, the long-term exposure of RC structures to both natural and operational environments can gradually compromise their functionality, resulting in a cumulative deterioration. This gradual deterioration undermines the safety, functionality, and longevity of these structures, thereby shortening their service life and making them unsuitable for routine use. In such scenarios, demolition and reconstruction would inevitably entail substantial economic losses, underlining the importance of regular inspections, repairs, and reinforcement measures for these structures [1,2,3,4]. Presently, the primary strategies for reinforcing RC structures encompass the increasing section method, pasting technique, structural force system modification, energy release approach, and external prestress reinforcement method [5,6]. Wang [7] reinforced concrete beams by increasing the cross-section and prestressing, finding that the force and deformation properties of the beam were significantly improved, the bearing capacity was increased, and hat the deflection of the beam could be effectively controlled after reinforcing. By using the self-developed CFRP version of the anchorage and tensioning device, Deng [8] used prestressed CFRP plates to reinforce the side and bottom faces of eight reinforced concrete flexural members. Through the bending performance test, Deng [8] investigated the effects of the reinforcement of the side and bottom faces of the prestressed CFRP plates on the bearing capacity of the concrete beams, the emergence and development of deformation and cracking during the service stage, and the stress distribution and extension of the reinforced beams during the service stage of the CFRP plates. Despite their prevalent usage, these methods are not flawless, with drawbacks including sluggish construction speeds, inadequate bonding capabilities, and prestress loss. Consequently, there is an urgent need to delve into innovative reinforcement techniques for RC structures, aimed at addressing these challenges and enhancing their overall performance and longevity.

High-toughness resin concrete steel mesh (HTRCS), a sophisticated composite material [9], incorporates super-high-strength and high-toughness resin concrete as its foundation, interwoven with robust steel wire mesh. In recent years, HTRCS has garnered immense attention owing to its unparalleled attributes, encompassing remarkable strength, unparalleled toughness, robust adhesion, swift hardening, exceptional fluidity, and resilience against acid and alkali corrosion. The concept and application of HTRCS composites were initially proposed and implemented by Yan [10]. Utilizing orthogonal testing methodologies, the optimal blend proportions for the resin concrete matrix were identified by solely adjusting the aggregate content, taking into account compressive strength, the modulus of elasticity, fluidity, market pricing, apparent density, and air content. Xu [11] advocated for the incorporation of steel fiber concrete materials within main girders and deck pavement layers, aiming to enhance the bridge deck’s resilience against girder-end deformations and mitigate stress concentration. Nevertheless, the exploration of HTRCS composites’ mechanical properties remains a relatively understudied field.

Extensive research has conclusively proven the exceptional suitability of high-toughness resin concrete steel mesh (HTRCS) composites for reinforcing reinforced concrete elements. These composites boast rapid reinforcement capabilities, simplicity in construction, and stable operational performance. The normal serviceability and load-bearing properties of concrete structures after consolidation with HTRCS are improved to a certain extent, and their service life and durability are also increased. Liu [12] conducted an exhaustive study investigating the normal service performance of prestressed hollow slab beams reinforced with HTRCS. Utilizing full-scale model static load testing coupled with theoretical analysis, the study yielded invaluable insights into the practical utilization of HTRCS in structural reinforcement. Pu et al. [13,14] and Yang et al. [15] further delved into experimental research on short columns reinforced with HTRCS. Their research affirmed that reinforcing the flange of the specimens significantly curbed the progression of cracks. Notably, the crack load capacity of specimens exhibiting substantial bias was markedly enhanced, emphasizing the efficacy of HTRCS in bolstering structural integrity. Bian [16] performed bending reinforcement tests on model beams. The results indicated that the strengthened beams exhibited thin and closely spaced cracks during crack development. This finding suggests that HTRCS reinforcement fosters a more controlled and evenly distributed crack pattern, indicative of enhanced ductility and resilience in the reinforced structures. In conclusion, the research underscores the immense potential of HTRCS as a superior reinforcement material for concrete structures, offering a seamless blend of rapid application, user-friendliness, and enhanced performance, particularly in crack control and load-bearing capacity. While the current research findings have firmly established the reliability of reinforced concrete members strengthened with HTRCS composites, there remains a paucity of comprehensive quantitative systematic research, with a predominant focus on experimental aspects.

Therefore, the present study conducted a comprehensive four-point bend test on two conventional reinforced concrete beams and six high-temperature-resistant ceramic fiber-reinforced concrete beams, varying in shear–span ratio and thickness. The objective was to meticulously evaluate the impact of various variables on their failure modes, flexural strength, and stress characteristics.

2. Experimental Overview

2.1. Test Piece Design

In order to study the shear performance of reinforced concrete beams strengthened with HTRCS, eight specimens were designed in the experiment. The primary variables under investigation were the HTRCS thickness, shear–span ratio, and concrete strength grade. RC-J-1 and RC-J-7 are ordinary reinforced concrete beams without HTRCS reinforcement, while the other six types of specimens are all shear-reinforced with HTRCS. To assess the influence of the varying reinforcement layer thicknesses on the shear performance of HTRCS-reinforced concrete beams, the thicknesses for RC-J-2, RC-J-3, and RC-J-4 were meticulously set at 10 mm, 15 mm, and 20 mm, respectively. Furthermore, to prevent the specimens from undergoing diagonal tensile or compression failure, the shear–span ratios for RC-J-5 and RC-J-6 were strategically designed to be 2.36 and 2.8, respectively. The concrete design strength grade for specimens RC-J-7 and RC-J-8 is C50. The specific design parameters of the specimens are detailed in

Table 1. Datils of tested beams.

Specimens	Shear–Span Ratio	Thickness/cm	Reinforcement Material	Concrete Strength
RC-J-1	2.78	/	/	C30
RC-J-2	2.78	1	HTRCS	C30
RC-J-3	2.78	1.5	HTRCS	C30
RC-J-4	2.78	2	HTRCS	C30
RC-J-5	2.36	1	HTRCS	C30
RC-J-6	2.08	1	HTRCS	C30
RC-J-7	2.78	/	/	C50
RC-J-8	2.78	1	HTRCS	C50

.

The concrete beams are all uniformly designed with a standard length of 2200 mm and a rectangular cross-section measuring 200 mm by 400 mm. The spans for test specimens RC-J-1 to RC-J-4 and RC-J-7 to RC-J-8 have been calculated to be 2000 mm, whereas the spans for RC-J-5 and RC-J-6 are 1700 mm and 1500 mm, respectively. To induce a shear failure mode, meticulous reinforcement strategies were implemented to ensure that the beams’ flexural strength far surpasses their shear capacity. The lower tensile and upper compression zones of the specimens were fortified with an innovative combination of three 25 mm and two 20 mm diameter HRB400 hot-rolled ribbed steel bars which were made in China, yielding a longitudinal tensile reinforcement ratio of 2.4%. The stirrups, crafted from an HPB300 reinforcement with an 8 mm diameter, are strategically spaced at 200 mm intervals, achieving a stirrup reinforcement ratio of 0.502%, and featuring a protective layer thickness of 20 mm. The section size and reinforcement details of the test specimens are illustrated in Figure 1.

The detailed manufacturing process of the test beam, illustrated in Figure 2, comprises the following pivotal steps:

(1)

Assemble the reinforcing cage, construct the wooden formwork, and subsequently pour reinforced concrete beams.

(2)

Subject the concrete beams to a standard curing process for a duration of 28 days. The reinforcement layer of the test specimen employs HTRCS, comprising resin, curing agent, and aggregate.

The specific implementation steps for the HTRCS reinforcement are outlined below:

(1)

Thoroughly clean the joint surface of the concrete beam to eliminate dust and debris, and follow it with a wetting treatment to ensure proper adhesion.

(2)

Carefully pour the prepared resin concrete into the reinforcement bond of the reinforced concrete beam, facilitating the formation of the HTRCS layer.

(3)

After allowing for 24 h of natural curing, meticulously release the formwork and finalize the reinforcement work, ensuring a seamless integration of the HTRCS layer.

2.2. Mechanical Properties of Materials

The test specimens are designed with concrete strength grades of C30 and C50, with their mix proportions listed in

Table 2. Mixture ratio of concrete.

Design Strength	Material Consumption kg/m3
	Cement	Sand	Stone	Water	Water Reducing Agent	Mineral Powder	Fly Ash	Composite Mineral Admixture
C30	243	897	1025	159	159	24	-	72
C50	365	809	1055	157	11.7	26	39	-

. The cement specified is PO42.5 ordinary Portland cement. The coarse aggregate comprises crushed stone with a uniform particle size range of 5–10 mm, while the fine aggregate is sourced from typical river sand. The fly ash utilized is of the primary variety, and the water-reducing agent is a polycarboxylic acid-based high-efficiency type. Following a standard curing duration of twenty-eight days, the average compressive strengths of the concrete cubes were recorded at 42 MPa for the C30 grade and 63 MPa for the C50 grade. The elastic modulus of C30 and C50 concrete were 2.8 × 10⁴ MPa and 3.4 × 10⁴ MPa, respectively. The strength and elastic modulus of the concrete are in accordance with the specification [17].

The high-performance tensile reinforcement composite system (HTRCS) incorporates a meticulous blend of resin, curing agent, and aggregate in a mass ratio of 2:1:16. The compressive strength of HTRCS, determined through rigorous material property testing following a standard curing period of 7 days, is comprehensively presented in

Table 3. Material properties of HTRCS.

Compressive Strength (MPa)	Tensile Strength (MPa)	Flexural Strength (MPa)	Axial Tensile Strength (MPa)	Elastic Modulus (MPa)
90.97	10.83	27.20	17.91	1.9 × 104

. The longitudinal reinforcement and stirrup have been measured to exhibit yield strengths of 435 MPa and 418 MPa, respectively. As stipulated by “Metallic Materials—Tensile Testing—Part 1” (GB/T228.1-2010) [18], the reinforcement bars used in this study, with diameters not exceeding 50 mm, conform to the minimum yield strength requirement of 335 MPa, thereby validating their suitability.

2.3. Test Method and Measuring Point Arrangement

During this experiment, a three-point bending static loading method was utilized, with a hinged support placed at the top of the span of the test beam to transmit the vertical concentrated load. The vertical external load was imparted utilizing an electro-hydraulic servo pressure testing machine, henceforth known as ‘the press’, while a load cell positioned beneath it meticulously documented any numerical fluctuations in the load throughout the loading procedure. Before the formal loading process, a pre-pressure of approximately 5 kN should be applied to ensure full contact between the loading point, support, and the surface of the test piece for all test devices, including the pre-pressure pressing press, force sensor, hinge support, and test beam. The test piece underwent loading in two distinct stages: initially, up to 10 kN prior to the onset of cracking, and subsequently, up to 20 kN post-cracking, with each stage sustained for a duration of 3 min. Adjustments to the shear–span ratio of the specimen were achieved by meticulously repositioning the support beneath the specimen. Specifically, the shear–span ‘a’ measures 800 mm for test piece RC-J-5, 750 mm for RC-J-6, and a uniform 1000 mm for the six remaining test beams. The test loading device is depicted in Figure 3.

Five dial gauges are strategically positioned—two at the beam’s end supports and three (at L/4, L/2, and L/3) along the midspan—to comprehensively capture vertical deflection data. The strain of the stirrup, longitudinal tensile reinforcement, and the surface of the test piece’s reinforcement is to be measured using embedded resistance strain gauges. The strain gauges mounted on the reinforcement surface of the test piece must be aligned precisely along the direct line connecting the loading point to the bearing position. The load cell should be situated between the press above the loading point and the hinge bearing. The load and strain data will be automatically collected by the IMP data acquisition system. Figure 4 clearly illustrates the layout of test points for each specimen, while the observation of crack initiation and their subsequent length extension during the test loading must be conducted meticulously by hand.

3. Analysis of Test Phenomena

3.1. Failure Modes

Figure 5 illustrates the damage states of each specimen after the loading test. The failure mode exhibited by the concrete beam reinforced with HTRCS mirrored that of a conventional RC beam. All the beams failed in the shear mode after the test. At a load of 75 kN, vertical flexural cracks emerged at the span of the RC-J-1 beam in the case of the conventional RC beam. As the loading intensified, supplementary inclined cracks surfaced, gradually expanding the crack region to encompass both lateral sides of the beam. By the time the load reached 260 kN, the crack region had vertically expanded to a height of 200 mm. With further loading, shear-induced inclined cracks began to form, tracing a path from the loading point to the support. As the load increases, a proliferation of fine cracks emerged, accompanied by a marked increase in the crack width. Ultimately, the concrete in the compression zone underwent crushing, thus concluding the loading test.

As indicated in Figure 5b–h, the beams fortified with HTRCS underwent a cracking progression reminiscent of traditional RC beams, culminating in shear failure, with the exception of the RC-J-4 specimen post-testing. Considering the RC-J-2 specimen as a representative case, upon the application of a load reaching 160 kN, flexural cracks emerged prominently at the beam’s midspan. As the load further increased to 200 kN, the flexural cracks at the midspan extended upwards and also extended to both sides of the midspan. Upon the application of a 260 kN load, a discernible flexural crack materialized between the loading point and the support. With the continued loading, the flexural crack began to develop as inclined cracks. As the loading progressed, the inclined crack extended to the loading point, and the crack width progressively increased. At a load of 360 kN, the beam’s shear capacity underwent a marked decline, ultimately resulting in failure.

The failure mode of the RC-J-4 specimen is shown in Figure 5d, and it is characterized as a bending failure. Initially, at a load of 230 kN, the first minor crack emerged at the bottom of the test beam. As the load increased to 260 kN, vertical bending cracks manifested within the span of the beam. Upon reaching a load of 270 kN, an inclined crack, which extended towards the loading point with a substantial angle, became visible on the opposite side of the beam. With the continued application of load, the vertical crack in the mid-span of the beam extended to the top of the beam, accompanied by an interfacial delamination failure of the resin material layer at the bottom of the beam. As the loading persisted, the widths of the primary vertical bending crack and the main oblique crack expanded further. At the point when the load was applied up to 560 kN, the width of the main vertical bending crack notably increased, and the deflection of the beam became significantly pronounced, culminating in the specimen’s failure.

3.2. Crack Analysis

As Figure 6 vividly illustrates, all specimens underwent shear failure, with the exception of the RC-J-4 specimen, which uniquely demonstrated a bending failure mode. The initial crack for all specimens emerged at the bottom of the mid-span of the test beam. Subsequent to the initial crack, numerous minute fractures emerged in proximity to the mid-span of the test beam. Upon reaching a critical load threshold, a diagonal fracture progressively materialized, traversing the path connecting the loading point to the bearing. A notable distinction lies between the high-temperature-resistant ceramic steel (HTRCS)-reinforced concrete beam and its conventional counterpart, as evidenced by the significant decrease in surface cracks on the reinforced beam. This underscores the effectiveness of the reinforcement layer in curbing crack propagation. As the reinforcement layer’s thickness escalated, the number of cracks on the test beam dwindled, resulting in a concentration of fractures within the span. Upon comparing the RC-J-5 and RC-J-6 specimens with the RC-J-2 specimen, it becomes evident that there is no marked variation in the surface crack count. This finding underscores the limited influence of the shear–span ratio on the total number of cracks present on the test beam. By augmenting the concrete strength of the test beam, as exemplified in Figure 6h, the crack distribution across its surface expanded in scope, while the total number of cracks remained largely unchanged.

4. Analysis of Test Results

4.1. Load–Displacement Curve

The comprehensive test results for each specimen are systematically presented in

Table 4. Feature points of all specimens.

Groups	Specimens	Dcar/mm	Fcar/kN	Dmax/mm	Fmax/kN
G1: Thickness	RC-J-1	0.54	75	8.45	365
	RC-J-2	1.14	160	8.33	401
	RC-J-3	1.22	160	20.435	540
	RC-J-4	1.72	230	15.12	560
G2: Shear–span ratio	RC-J-1	0.54	75	8.45	365
	RC-J-2	1.14	160	8.33	401
	RC-J-5	1.09	200	8.72	600
	RC-J-6	0.43	120	2.91	420
G3: Concrete strength	RC-J-2	1.14	160	8.33	401
	RC-J-7	0.53	70	8.35	440
	RC-J-8	4.49	340	8.38	480

for clear analysis. Upon analyzing the data, it becomes evident that the specimens reinforced with high-temperature-resistant ceramic steel (HTRCS), specifically RC-J-2, RC-J-6, and RC-J-8, have witnessed a notable enhancement in cracking load, ranging from a substantial 60% to an impressive 200%, when compared to RC-J-1 and RC-J-7. Additionally, there has been a marked improvement in the shear bearing capacity, ranging from approximately 10–65%, emphasizing the profound efficacy of HTRCS in reinforcing the shear strength of reinforced concrete (RC) beams. Moreover, within the cohort of specimens strengthened with HTRCS, RC-J-4 exhibits a significant increase in cracking load, approximately 43%, in comparison to RC-J-2 and RC-J-3.

Correspondingly, the shear bearing capacity has also augmented, with notable improvements of roughly 4% and 3.7%, respectively. This correlation highlights a positive correlation between the thickness of the HTRCS layer and the enhanced shear bearing capacity, indicating its pivotal role in reinforcing the beams. Upon scrutinizing the RC-J-5 specimen, it is observed to display a cracking load that is approximately 25% higher than RC-J-2, accompanied by a substantial increase in the bending bearing capacity of roughly 50%. Nonetheless, it is noteworthy that the shear bearing capacity of RC-J-6 is found to be lower than RC-J-5, which can be primarily attributed to the premature delamination between the HTRCS layer and the underlying concrete substrate, occurring prior to the vertical grid fracture within the HTRCS material.

Finally, the RC-J-8 specimen stands out with a remarkable increase in the cracking load of approximately 112% when compared to RC-J-2, and its shear bearing capacity has been significantly bolstered by approximately 20%. These significant findings underscore the pivotal role of concrete strength in determining the shear bearing capacity of reinforced concrete (RC) beams reinforced with high-temperature-resistant ceramic steel (HTRCS). The utilization of high-strength concrete is notably effective in mitigating the propagation of cracks.

Figure 7 depicts the intricate interplay between the load applied and the resulting displacement in the beam body, meticulously recorded at each pivotal stage of the test beams’ performance. It becomes strikingly clear from this visualization that all test beams traverse a progression of phases: an initial elastic loading phase, a working phase marked by the emergence of cracks, and ultimately, a failure loading phase under the relentless accrual of load. During the elastic phase, the load–displacement relationship exhibits a pristine linearity, signifying a steadfast response to the applied forces. Notably, the stiffness of the reinforced concrete beams, fortified with high-temperature-resistant ceramic steel (HTRCS), remains largely consistent across varying influencing factors, closely mirroring the inherent stiffness of traditional plain reinforced concrete (RC) beams. As the first crack stealthily materializes on the surface of the test beam, it heralds a pivotal transition. The compression zone’s height dwindles, and the once-straight load–displacement line gracefully bends into a curve, marking the entrance into the elastic–plastic realm the working phase characterized by the presence of cracks. This phase persists until the test beam reaches its yield load, the threshold where further stress begins to elicit visible changes in material behavior. As the load relentlessly mounts, the tensile reinforcement within the test beam yields, leading to a pronounced decline in the slope of the load–displacement curve. This development signifies a marked decrease in the beam’s stiffness, indicative of a more compliant structure under stress. Consequently, the deflection of the test beam escalates rapidly, accompanied by the irreversible deterioration of the concrete matrix.

The analysis of Figure 7a indicates that the shear bearing capacity of reinforced concrete beams that have been reinforced with high-temperature-resistant ceramic steel (HTRCS) is significantly higher than that of beams without such reinforcement. Additionally, both the shear bearing capacity and deflection of the test beams increase as the thickness of the HTRCS reinforcement layer increases. This suggests that the thickness of the HTRCS layer is a critical factor in enhancing the structural performance of the beams. As depicted in Figure 7b, the shear capacity and the load at which cracking initiates in the specimens can be enhanced by reducing the ratio of reinforcement to the shear–span ratio of the test beams. The curve demonstrates that the thickness and shear–span ratio of the HTRCS reinforcement layer significantly contribute to the stiffness of the reinforced concrete beam prior to the yield point. This implies that optimizing these parameters can lead to a stiffer structure before the onset of yielding. Furthermore, Figure 7c reveals that increasing the strength of the concrete can improve the shear bearing capacity of beams that have been strengthened with HTRCS. However, before the yield of the reinforcement, the stiffness of the test beams with higher-strength concrete is relatively lower. This observation suggests that, in beams with higher concrete strength, HTRCS plays a more substantial role after the tensile reinforcement of the concrete beam has yielded. In essence, the contribution of HTRCS becomes more pronounced in high-strength concrete beams, particularly post-yield, highlighting the importance of the material selection and reinforcement strategy in structural design.

4.2. Strain Distribution of Concrete

An exhaustive analysis of a cluster of right-angle strain gauges, strategically positioned to traverse the diagonal fractures within the central shear–span of the concrete, situated on the compromised extremity of the test beam, was performed utilizing the precise formula tailored for such gauges, as clearly illustrated in Figure 8. As discernible from Figure 8, under uniform conditions, the primary shear strain exhibited by reinforced concrete beams reinforced with HTRCS surpasses that of their unreinforced counterparts, demonstrating the effectiveness of the strengthening method. This phenomenon stems from the fact that the strengthened test beams undergo lesser deformation compared to their unreinforced counterparts under identical loading conditions, underscoring the superior resilience of the HTRCS-enhanced beams. Furthermore, a notable observation is the slower rate of strain escalation in the reinforced concrete beams, in contrast to their unreinforced versions, hinting at a more pronounced brittle nature during the shear failure sequence. This underscores the need for careful consideration when assessing the structural integrity of such beams.

The principal strain within the reinforcement layer is determined employing the formula tailored for right-angle strain gauges, as depicted in Figure 9. An examination of Figure 9a divulges that augmenting the reinforcement layer’s thickness from 10 mm to 20 mm results in a decrease in principal strain under equivalent loads. Conversely, thinner reinforcement layers exhibit heightened strains during shear, increasing the vulnerability to cracking and facilitating the rapid propagation of cracks. As a result, components endowed with thinner reinforcement layers manifest more prominent brittleness traits, emphasizing the imperative for meticulous contemplation in the design of shear capacity. A comparative analysis of load–strain curves across varying shear–span ratios and concrete strengths suggests that both factors exert an influence on strain variation, albeit the effect remains relatively insignificant.

4.3. Longitudinal Bar Load–Strain Curve

For the purpose of analysis, we selectively examined the strain gauge data emanating from the longitudinal bars situated at the mid-span of the test beam. Figure 10 illustrates the relationship curves between the strain and load for the horizontal longitudinal bars of each test specimen under various parameters. Upon meticulous comparison of the curves across different test specimens, a discernible pattern emerged: the load–strain curve of the longitudinal bars naturally segregates into two distinct phases, demarcated by the emergence of cracks. The advent of cracks triggers a notable inflection point in the test specimen’s curve, signifying the formation of a flexural crack traversing the tensile longitudinal bar, which aligns seamlessly with the initial inflection point observed on the load–displacement curve, offering a cohesive narrative of the structural response.

Another inflection point is shown to be caused by the emergence of a vertical bending crack. Subsequent to the formation of bending cracks, with the exception of specimen RCJ-4, the maximum tensile strain of the longitudinal bars in the mid-span of the remaining reinforcement specimens surpassed the theoretical yield strain, yet they progressed linearly in terms of actual strain growth, devoid of a yield plateau. This suggests that the tensile longitudinal bars in each specimen failed to attain the yield point. Consequently, analyzing the maximum strain data from the concrete and longitudinal reinforcement in the mid-span compression zone, it is concluded that the ultimate failure mode for each specimen is uniformly identified as the shear failure of the beam. The strain distribution diagram of the comparative test specimen reveals that the stiffness of the load–strain curve for the longitudinal reinforcement in each test specimen remains virtually unchanged prior to crack formation. However, after the appearance of the crack, factors such as reinforcement layer thickness, shear–span ratio, and concrete strength exert varying degrees of influence on the slope of this curve. Notably, the shear–span ratio emerges as the most influential factor in determining the stiffness of the specimens.

4.4. Load–Strain Curve of Stirrup

Figure 11 vividly illustrates the intricate relationship between the load applied to the test specimens and the corresponding strain exhibited by the stirrups. Upon scrutinizing Figure 11, it becomes evident that the evolution of stirrup strain within each beam can be distinctly categorized into two phases, marked by the appearance of the primary inclined crack. During the initial phase, the stirrups exhibit a relatively stable strain level, remaining largely unchanged until the primary inclined crack emerges. This implies that, during this stage, the primary resistance against shear forces primarily stems from the interlocking effect of the concrete and the reinforcing bars running longitudinally. As the test progresses into the subsequent phase, the threshold load for the primary inclined crack formation in unreinforced specimens is approximately 150 kN and 200 kN, respectively. Conversely, the reinforced specimens demonstrate a significantly higher load capacity of approximately 300 kN, which coincides precisely with the inflection point on the load–displacement curve. This underscores the efficacy of HTRCS reinforcement in delaying the onset of the primary inclined crack, thereby enhancing the structural integrity of the specimens. Notably, once the concrete at the site of the primary inclined crack loses its structural contribution, the stirrups undergo a sudden and significant stress shift, yielding as the applied load gradually intensifies. This observation highlights the pivotal significance of stirrups in governing the post-crack response of beams, as well as the superior performance of HTRCS reinforcement in mitigating shear forces and retarding crack propagation.

Compared to the unreinforced specimens, the escalation of strain within the main diagonal crack is notably mitigated, demonstrating that the high-performance tensile reinforcement composite system (HTRCS) reinforcement layer and stirrups efficiently withstand the applied shear force within this critical area. Consequently, this phenomenon significantly curbs the widening of the main diagonal crack under equivalent loading conditions, thus postponing the attainment of the ultimate failure state in the reinforced specimen. Upon scrutinizing Figure 11a,b, it becomes evident that the shear–span ratio is the pivotal parameter influencing the strain experienced by the stirrups. This suggests that the shear contribution of the stirrups in the short shear–span is more significant than in the long shear–span specimens. In long shear–span specimens, cracking manifests at an earlier stage, and the resulting diagonal cracks assume greater widths, facilitating the unfettered expression of the shear reinforcement’s capabilities within the web.

5. Conclusions

In this paper, two reinforced concrete beams and six reinforced concrete beams strengthened with HTRCS were tested and studied. The main results of this study are summarized as follows:

(1)

The predominant failure mode observed in the majority of specimens is shear-compression failure. Compared with ordinary concrete beams, the number of cracks on the surface of reinforced concrete beams strengthened with HTRCS is obviously smaller, signifying that the reinforcement layer effectively mitigates the occurrence of cracks. The concrete strength of the test beams significantly impacts the distribution of surface cracks, with a notable influence on the overall crack pattern.

(2)

Reinforcing RC beams with HTRCS significantly increases the shear capacity. The cracking load of HTRCS-reinforced specimens was increased by about 60–200% compared with RC-J-1 and RC-J-7, and the shear capacity was increased by about 10–65%.

(3)

Concrete strength plays a pivotal role in determining the shear bearing capacity of RC beams reinforced with HTRCS. The utilization of high-strength concrete is notably effective in mitigating the propagation of cracks. The shear capacity of reinforced concrete beams strengthened with HTRCS increases with the increase in the thickness of the reinforced concrete layer and the strength of the concrete.