The mechanics and durability of rubber fiber hydraulic concrete on laboratory

This study delves into the mechanical and durability properties of rubber fiber hydraulic concrete (RFHC) using an orthogonal testing method. The optimal mix proportion for RFHC is determined through orthogonal range and variance analysis results. The influence of polypropylene fiber content, rubber particle content, and rubber particle size on the compressive strength, flexural strength, split tensile strength, brittleness, and crack resistance of rubber fiber concrete is thoroughly examined. Durability tests are conducted based on the optimal mix ratio, and a comparative analysis of the durability between RFHC and conventional concrete is carried out. The findings indicate that the mechanical properties of the concrete exhibit a significant decline with increasing rubber content and decreasing rubber particle size. The variation in rubber content notably affects the mechanical properties of hydraulic concrete. The frost resistance and impermeability of RFHC are considerably higher than those of conventional concrete, while the erosion resistance of RFHC surpasses that of the conventional benchmark concrete. This research offers valuable benchmarks for the theoretical design and practical engineering application of RFHC, contributing to the advancement of sustainable construction materials.

feature introduces a myriad of advantages including enhanced resistance to impact, erosion, earthquakes, and thermal variations.][14][15] The use of rubber aggregates in concrete also presents an opportunity to decrease reliance on natural sand resources, which are currently under severe depletion in several regions globally.Thus, substituting a fraction of natural sand with rubber aggregate not only addresses the environmental concerns resulting from over-exploitation of sand resources but also offers a practical solution for recycling waste rubber.][18] Hydraulic concrete, a cement-based material, has widespread applications in hydropower projects such as water retention, power generation, and sand discharge applications. 19However, the challenge lies in mitigating the temperature rise due to concrete hydration heat and enhancing the material's resistance to cracking during construction. 20,21][29] Despite extensive research on conventional rubber concrete, a research gap exists concerning the mix design, mechanical properties, and durability of rubber fiber hydraulic concrete (RFHC). 3,4As a multicomponent composite material, the RFHC's performance is significantly influenced by its composition and dosage.The inclusion of an appropriate quantity of rubber aggregate can potentially enhance the durability of hydraulic concrete, although the underlying mechanism is complex.
Recognizing the urgency for deeper exploration into the mix proportion design, mechanical properties, and durability of RFHC, this article aims to provide a comprehensive review of the current knowledge on RFC.We focus on its unique properties, applications, challenges, and potential research directions in this rapidly growing field.This study provides a comprehensive experimental investigation into the mechanics and durability of RFHC.The research offers new insights by determining the optimal mix design through orthogonal tests, evaluating the effects of various factors on mechanical properties, and comparatively analyzing the durability performance of RFHC versus conventional concrete.This study contributes to the existing body of knowledge in the following ways: • It provides useful benchmarks for the theoretical design and engineering application of RFHC through model tests.
• It offers a thorough analysis of the effects of polypropylene fiber content, rubber particle content, and rubber particle size on the mechanical properties of RFC.
• It presents novel insights into the durability of RFHC in comparison to conventional concrete, including aspects such as frost resistance, impermeability, and erosion resistance.
The remaining sections of this article elucidate the research methodology and findings.Initial tests were conducted using the orthogonal test method to evaluate basic mechanical performance attributes of RFHC such as compressive strength, split tensile strength, and flexural strength.The influential factors on the mechanical properties of RFHC were rigorously analyzed to determine the optimal mix ratio.Following this, specimens were prepared for durability tests (frost resistance, impermeability, erosion resistance) based on the ideal mix proportions, along with the corresponding traditional concrete mix proportion.The study concludes with a comparative evaluation of the durability of RFHC and conventional concrete.

MATERIALS AND METHODS
RFHC is a multifaceted composite material whose performance is significantly influenced by its composition and dosage.
Appropriately incorporating rubber aggregates has the potential to enhance the durability of hydraulic concrete, albeit through a complex mechanism.To delve into the intricacies of RFHC's mix proportion design, mechanical properties, and durability, this study utilizes the orthogonal test method.This approach facilitates the examination of the various factors impacting the mechanical properties of RFHC, enabling the determination of an optimal mix ratio.Following this, specimens are prepared for durability testing and comparison with conventional concrete.The findings from these tests offer indispensable insights for the theoretical design and practical engineering application of RFHC.

(A) (B)
F I G U R E 1 Rubber particles and polypropylene fibers: (A) 20 mesh rubber particles; (B) 12 mm single bundle polypropylene fibers.

Testing of raw materials
The raw materials used in RFHC include cement, fly ash, coarse aggregate, fine aggregate, rubber particles, polypropylene fiber, and admixtures.Conch Cement Co., Ltd provided the P⋅O 42.5 conventional Portland cement that exhibits chemical composition and performance indicators in Tables 1 and 2, respectively.Calcareous crushed stone with a continuous gradation of 5-25 mm and an apparent density of 2720 kg/m 3 serves as coarse aggregate, whereas, river sand with a fineness modulus of 2.55, and an apparent density of 2650 kg/m 3 serves as fine aggregate.The fly ash used is grade II with material properties shown in Table 3.Three types of rubber particles with 10 mesh (1140 kg/m 3 ), 20 mesh (1120 kg/m 3 ), and 40 mesh (1130 kg/m 3 ) were used, among which the appearance of 20 mesh particles is shown in Figure 1A.Polypropylene fiber, as detailed by the specific performance indicators in Table 4, is utilized.This involves the use of a single bundle of staple fiber with a length of 12 mm.Polycarboxylic acid superplasticizer with a water reduction rate of 20% is the admixture used in this study.

Testing scheme and mix ratio
In order to determine the material component in each cubic meter of RFHC, we followed the C40 strength grade conventional concrete mix ratio (JGJ55-2011) where a portion of the fine aggregates are replaced with rubber aggregates of equal volume.The multi-index mix design of RFHC required us to investigate the effects of the rubber content, polypropylene fiber content, and rubber particle size on the mechanical properties of concrete at various levels.To achieve this, we used the L9 (3 3 ) orthogonal testing scheme, where we selected three levels for each influencing factor to optimize the mix proportion and determine the best level combination.Table 5 depicts the levels of each factor, where polypropylene fiber content, rubber content, and rubber particle size are represented by (A1, A2, and A3), (B1, B2, and B3), and (C1, C2, and C3), respectively.The mix ratio for the C40 conventional concrete prepared with a slump of 100-120 mm, a sand ratio of 42%, and a water-cement ratio of 0.4, along with the mix ratio for the nine groups of RFHC obtained using the L9(3 3 ) orthogonal testing scheme, are presented in Table 6.

Testing of concrete properties and methodology
The testing of concrete properties encompasses two key aspects, namely mechanical performance and durability.In order to streamline the testing process, the orthogonal testing method was initially employed to conduct essential mechanical performance tests on RFHC.This involved the analysis of the main factors affecting RFHC's mechanical properties, and the determination of the optimal mix ratio.Subsequently, based on the optimal mix ratio and the C40 conventional concrete mix ratio, concrete specimens were produced for durability testing to discern the differences between RFHC and conventional concrete in this regard.In this process, mechanical performance testing of concrete involved evaluating its compressive, splitting tensile, and flexural strength.The mix proportion of each of the groups was developed based on the regulations provided by GB/T 50081-2016 (Standardization for Testing Methods of Mechanical Properties of Conventional Concrete) and CECS F I G U R E 2 Specimens for concrete performance testing.13-2009 (Standardization for Testing Methods of Fiber Concrete).A total of six testing blocks were made, comprising three blocks for both C40 conventional concrete and RFHC.Compressive strength and splitting tensile strength tests were conducted on cube specimens measuring 150 mm × 150 mm × 150 mm, while flexural strength was assessed using rectangular column blocks measuring 100 mm × 100 mm × 400 mm.
Durability testing of concrete was carried out based on the aforementioned optimal mix ratio of RFHC and C40 conventional concrete.This followed the requirements put forth by DL/T 5150-2017 (Testing Regulations for Hydraulic Concrete) and GB/T50082-2009 (Long-term Performance and Durability Testing of Conventional Concrete).Concrete frost resistance, impermeability and erosion resistance were the key factors assessed during durability testing.The frost resistance testing was evaluated on rectangular column specimens measuring 100 mm × 100 mm × 400 mm.Impermeability testing was done on truncated cone specimens of size 175 mm × 185 mm × 150 mm, while erosion resistance performance testing was undertaken on short cylindrical specimens measuring 300 mm × 100 mm.
To create the testing blocks, forced mixers were used to mix the concrete.After being put into the mold, the concrete was vibrated and compacted on a vibrating table, left to stand indoors for 24 h and then marked with a specific number before being put in the standard curing room to cure for 28 days.Once aged, concrete's basic mechanics and durability were tested in line with the standard testing protocols.Figure 2 illustrates the various specimens used in the testing process.

Experiment methodology
To conduct the necessary mechanical performance testing of RFHC, the orthogonal testing method was utilized.A series of C40 conventional concrete and RFHC testing blocks were manufactured for the aforementioned mechanical performance tests.Each group consisted of three testing blocks, and each group's average was considered the testing result for each mechanical property.The testing procedure for evaluating the various mechanical properties of concrete is depicted in Figure 3A-C.

Testing results and analysis
Table 7 displays the compressive, flexural, and split tensile strengths of each group of specimens at 28 days, along with the flexure-compression and tension-compression ratios, as seen in Table 7. RFHC's 28d compressive, flexural, and split tensile strengths exhibited a decrease compared to conventional concrete.However, the tension-compression and  flexure-compression ratios both increased.Additionally, its toughness and deformation increased while enhancing its crack resistance.

Orthogonal range and variance analysis
To assess the data from Table 7, range and variance analysis were performed, and the results are shown in Table 8.The range analysis indicated a substantial impact on concrete strength by rubber content (B), which was followed by rubber particle size (C), with polypropylene fiber content (A) having the least impact among the three factors.The order of the influencing factors is B > C > A.
As for the compression-compression and the tension-compression ratios, the order of the influencing factors is A > B > C. The optimal ratios obtained for 28d compressive, flexural, and tensile strength, as well as the flexure-compression and tension-compression ratios, are A1B1C1, A2B1C1, A1B1C1, A2B3C3, and A2B3C3, respectively.It is crucial to note that the optimal ratios for different testing factors vary for different performance indicators, and each factor's primary and secondary influence order is also different.Thus, the analysis of variance is necessary to investigate each factor's primary and secondary influence order on different performance indicators and its overall significance.8, it is evident that the influence of factor B on 28d compressive strength is highly significant, while the remaining two factors have little impact.As for 28d flexural strength, factor B's influence is highly significant, followed by significant influence from factor C, whereas factor A has no significant impact.For 28 d split tensile strength, factor B's influence is highly significant, whereas factor C's impact is significant, and factor A has no significant effect.As a result, the primary influencing factor regarding concrete strength after 28 days is factor B, and factor C is the second most influential factor, while factor A has no significant influence.In terms of the compression ratio, both factors A and B have a high degree of significance, while none of the factors are substantial for the tension-compression ratio.
Based on the test data above, the comprehensive analysis results of range and variance demonstrate that fiber content, A, has a significant effect on the compression ratio, with optimal results obtained when using A2.For the 28d compressive, flexural, and tensile strengths of RFHC, the influence of rubber content, B, is highly significant, with B1 yielding the best results.The optimum rubber content is B1.As for rubber particle size, C, it significantly influences 28d flexural and tensile strengths, though insignificant for other indicators.Thus, the optimal rubber particle size is C1.Collectively, A2B1C1 is the most effective proportion for RFHC, as confirmed by the comprehensive analysis results from Table 8.
We can observe the analysis curves of various factors' effect on the fundamental mechanical properties of RFHC, per Table 8, in Figure 4A-C.Figure 4  (C) split tensile strength.

Analysis of compressive strength
Figure 4A shows that altering the rubber content of fiber-rubber concrete significantly impacts its compressive strength.The concrete cubes' compressive strength exhibits a notable downward trend with an increase in rubber content.This is due to the fact that rubber particles are low-strength organic, elastic, hydrophobic materials with low compatibility with the cement matrix.As a result, a weak bonding surface forms between the two materials, decreasing the effective bearing area inside the concrete and reducing concrete strength.This phenomenon is known as the "soft point effect" of rubber particles, which affects the strength of the concrete.The impact of different rubber particle sizes on the compressive strength of concrete is also significant.Figure 4A demonstrates that the compressive strength of the concrete mixed with rubber particles of particle size level 1 (10 mesh) is considerably higher than that of particle sizes of level 2 (20 mesh) and level 3 (40 mesh) when the rubber volume is equal.Consequently, adding coarse rubber particles can diminish the degree of weakening of the compressive strength of concrete.However, the specific surface area of fine-grained rubber particles is relatively large.Therefore, the more cement slurry consumed to wrap the rubber particles, the larger the weak bonding surface formed between the rubber particles and the cement matrix, which is more unfavorable to the concrete's compressive strength.
The most significant effect of polypropylene fibers on concrete strength is its weak interface effect and crack resistance effect.The higher fineness of polypropylene fibers allows for a more extensive contact interface with the concrete's cement substrate.Also, the hydrophobicity of polypropylene fiber increases the substrate water-binder ratio at the interface.Thus, the interface's concrete strength decreases, resulting in a weak interface effect due to the polypropylene fibers.This effect is known as the anti-cracking effect, which means that the polypropylene fibers evenly distributed within the concrete entangle with coarse aggregates and interweave to form a three-dimensional network structure, acting as a "reinforcing rib."When the concrete specimen deforms under stress, the local stress concentration can be reduced, thereby inhibiting the generation and development of internal cracks in the specimen and improving concrete strength.
Figure 4A indicates that the change in concrete compressive strength is not a simple linear relationship with an increase in polypropylene fiber content.Generally, the compressive strength initially declines and then rebounds.When the polypropylene fiber content increased from level 1 (0.6 kg/m 3 ) to level 2 (0.9 kg/m 3 ), the compressive strength decreased, indicating that the weak interface effect of the polypropylene fibers was more evident at this stage.When the fiber content continued to increase to level 3 (1.5 kg/m 3 ), the compressive strength of the concrete increased again, indicating that polypropylene fiber's anti-cracking effect played a leading role at this stage.

Analysis of flexural strength
Figure 4B shows that an increase in the amount of rubber has an apparent adverse effect on concrete's flexural strength, caused by the "soft point effect" of the rubber particles.The "soft point effect" becomes more evident as the rubbers increase.In addition, as the rubber particle size decreases, the flexural strength of concrete decreases substantially.When the rubber particle size changes from level 2 (20 mesh) to level 3 (40 mesh), the extent of the strength reduction is more significant.
According to Figure 4B, the flexural strength of concrete initially increases and gradually decreases with an increase in polypropylene fiber content.The crack resistance effect of polypropylene fibers is dominant within a specific content range, strengthening the toughness and delaying the development of cracks.The flexural strength of concrete improves with an increase in fiber content.However, weaker interfaces will appear in the concrete matrix as the polypropylene fiber content increases, and the weaker interface effect of polypropylene fibers will become more obvious.The crack resistance effect subsequently appears.When the content exceeds the critical value, the weak interface effect of the polypropylene fibers plays a leading role, leading to a decrease in the concrete matrix strength.Consequently, the flexural strength of the concrete gradually decreases with an increase in polypropylene fiber content.

3.2.4
Splitting tensile strength analysis Figure 4C reveals that the effects of rubber content and rubber particle size on the split tensile strength of concrete are similar to their impact on flexural and compressive strength.The split tensile strength of concrete shows a substantial downward trend with an increase in rubber content and decrease in rubber particle size, particularly since the rubber content change's impact on the split tensile strength of concrete is significant.The splitting strength of concrete is more sensitive to cracks, and the "soft point effect" of rubber particles is more evident in the concrete splitting strength.Figure 4C indicates that the polypropylene fiber content's effect on the split tensile strength of concrete is not notable.Although the splitting strength of concrete slightly decreases with an increase in fiber content, the overall change is not significant.Moreover, according to the variance analysis in Table 8, rubber content is essential for the split tensile strength of concrete, rubber particle size is significant, and the polypropylene fiber content is not significant.The split tensile strength of the material is primarily influenced by rubber content, a factor intrinsically related to stress conditions and failure patterns observed during the concrete split tensile test.In a compressive examination, the substantial contact area of the cubic specimen restricts the material's failure.This internal failure ensues in a progressive manner, where the fiber content markedly mitigates its development.In the flexural test, as deflection steadily increases, the internal failure of the prismatic specimen also unfolds gradually.Hence, the fiber content significantly impacts the flexural strength.Contrastingly, in a split tensile test, the contact area of the cubic specimen is minimal.Absent of surface contact constraints or deflection deformations, the specimen succumbs to brittle failure.The internal failure does not follow a pronounced gradual progression.Consequently, such failure characteristics stipulate that the split tensile strength is substantially affected by the predetermined rubber content, whereas the set fiber content exhibits limited influence on the specimen's split tensile strength.

Analysis of brittleness and crack resistance
Table 7 shows that the tension-compression ratio and flexure-compression ratio of each group of RFHC are generally more significant than those of conventional concrete.This demonstrates that polypropylene fibers and rubber particles can improve concrete's brittleness and increase its toughness and crack resistance performance.The failure of the conventional concrete and RFHC specimens under the ultimate load is compared in Figure 5A,B.The figure shows that cracks first appear in the upper and lower corners of the specimen under compression testing as load increases and gradually develop towards the middle, forming a vertical crack in the middle of the specimen.Upon reaching the ultimate load, the concrete on the specimen's surface begins to bulge and peel off, with the four corners of the concrete peeling off most severely, displaying typical brittle failure.
During the entire RFHC loading process, small cracks only appear on the surface when the ultimate load is reached.Slight damage appears on the end face at ultimate failure.The remaining parts remain intact, with no apparent vertical penetration cracks.Compared with the conventional concrete failure form, the RFHC exhibits good deformation performance.The deformation performance improves with an increase in rubber particles and polypropylene fiber parameters.When rubber particles and polypropylene fibers reach good gradation, the two materials can mutually promote and improve concrete's brittleness and deformation properties.During flexural testing, as the load increases, the conventional concrete specimen begins to crack at the bottom of the specimen and extends upward in a straight line.When the ultimate load is reached, the crack penetrates instantly with violent and crisp sounds, instantly becoming zero.The entire process displays typical brittle failure, with the fracture surface relatively neat.As the load increases, cracks at the bottom of the RFHC specimens extend nonlinearly, with smaller crack widths and shorter extension lengths.As a result, the sound is softer and lower when the specimens break, with its cross-section zigzagging, as shown in Figure 5C.

DURABILITY TESTING
To provide an in-depth study of the differences and influencing factors of the durability of recycled fiber-reinforced high-performance concrete (RFHC) and conventional concrete, specimens were made to test the concrete durability according to the optimal mix ratio of RFHC A2B1C1 and the mix ratio of the above C40 conventional concrete.Testing mainly included freezing resistance testing, impermeability testing, and abrasion resistance testing.

4.1
Freezing resistance testing

Testing method
The frost resistance testing of concrete followed the quick freeze method outlined in the "Testing Standardization for Hydraulic Concrete" (DL/T 5150-2017).The evaluation indexes of frost resistance were the relative dynamic elastic modulus and mass loss rate.The relative dynamic modulus of elasticity was measured using the nondestructive testing method, which is highly sensitive to the internal damage of concrete structures under the action of freeze-thaw cycles, making it an essential parameter for analyzing the freeze-thaw resistance of concrete.The mass loss rate reflects the external cracking and spalling of the concrete under the action of freeze-thaw cycles, making it an auxiliary parameter for evaluating frost resistance.The specimen's dynamic elastic modulus and quality were tested 25 times during the freezing and thawing cycle, following the requirements of the quick freeze method.The formula used is as follows: where f n and m n are the natural frequency and mass of the specimen after n freeze-thaw cycles, respectively, and f 0 and m 0 are the natural frequency and mass of the specimen before freezing and thawing, respectively.The evaluation standard of the testing results is as follows: when the relative dynamic elastic modulus drops to 60% or the mass loss rate reaches 5%, it can be considered that the specimen has been destroyed and the test is terminated, or the test is terminated after freezing and thawing to a predetermined number of cycles.The corresponding number of freeze-thaw cycles is used as the concrete's frost resistance level (indicated by F).

Analysis of testing results
Figure 6A,B present the outcomes of concrete rapid freeze-thaw cycle testing and dynamic elastic modulus testing, respectively.Based on laboratory testing results, the relationship curves between freeze-thaw cycles are illustrated in Figure 7A,B.
Figure 7A shows that both the RFHC and conventional concrete experience a decrease in relative dynamic elastic modulus as the number of freeze-thaw cycles increases.In particular, the decline is insignificant during the initial phase of freeze-thaw cycling but rises significantly in the later stages of freeze-thaw action.When the number of cycles is below 100, the decline is relatively gentle, and the relative dynamic elastic modulus can be sustained above 90%.However, when the number of cycles exceeds 125, the relative dynamic modulus of elasticity decreases drastically.Notably, the decline in conventional concrete is more severe, with an obvious inflection point in the curve change.When subject to 175 freeze-thaw cycles, the relative dynamic elastic modulus of the conventional concrete decrease to 58.2%, leading to freeze-thaw failure due to significant surface erosion, exposure of aggregates, and local cracks.On the other hand, Figure 7B shows that, at the beginning of the freeze-thaw cycle, the mass-loss rate data of the two specimens (RFHC and conventional) is relatively scattered and without regularity.This is mainly due to the freezing and thawing cycling, which causes the surface of the specimen to crack, thus reducing mass.Simultaneously, the microcracks created on the surface of the specimen open up channels, allowing moisture to permeate, leading to an increase in quality.Consequently, the quality loss rate of the specimen fluctuates positively and negatively due to the combined effects of the two aspects.The figure demonstrates that when the number of freeze-thaw cycles exceeds 50, surface damage gradually escalates, and the water saturation state essentially occurs, causing the mass loss rate to increase slowly.After 100 freeze-thaw cycles, the degradation effect intensifies, forming an inflection point in the curve change.Afterward, the mass loss rate increases rapidly, and the surface peeling of the specimen intensifies until freeze-thaw damage occurs.During testing, the surface spalling phenomenon of conventional concrete is more severe than that of RFHC, with a faster rate of quality loss.
The change law is consistent with the change law of the relative dynamic elastic modulus, except that the inflection point of the mass loss rate precedes the relative dynamic elastic modulus.This is because the mass loss rate mainly reflects freeze-thaw damage on the concrete surface, while the relative dynamic elastic modulus reflects freeze-thaw effects and damage inside the concrete.Moreover, the failure patterns of RFHC and conventional concrete under different freeze-thaw cycles are depicted in Figure 8.
During testing, the relative dynamic elastic modulus, mass-loss rate, and appearance of the conventional concrete changed conspicuously as the number of freeze-thaw cycles increased.In contrast, the changing trend of RFHC was relatively gentle, with minor appearance changes.Therefore, RFHC exhibited superior frost resistance to conventional concrete due to the appropriate mixing of rubber and fiber, which increased the energy dissipation of concrete during freeze-thaw damage, limited the expansion and spread of microcracks, and buffered internal stresses to improve the freezing resistance performance of concrete.

Testing method
The penetration height method is outlined in the "Testing Standardization for Hydraulic Concrete" (DL/T 5150-2017).This method determines the impermeability of concrete, enables the calculation of relative permeability coefficients between various concretes, and facilitates comparison of their impermeabilities.To initiate testing, the water pressure of the impermeability tester is added to a predetermined set pressure value, typically 1.0 ± 0.2 MPa, and 1.0 MPa for this test.After the pressure has stabilized for 24 h, the pressure is reduced, and the concrete specimen is removed.The specimen is then split along the longitudinal section and divided into ten equal parts, from which the average water penetration height is measured and calculated.The relative permeability coefficient K r is subsequently computed using formula (3), derived from the average of the measured values of six specimens in each group.The formula used is as follows: where K r is the relative permeability coefficient, mm/h;  is the absorption rate of concrete, 0.03; D m is the average seepage height, mm; t is the constant pressure time, h; and H is the water pressure, expressed as the water column height, mm.

Testing results and analysis
Figure 9A-C depict the concrete impermeability testing outcomes, along with the apparent water seepage of the split surface of the specimen.The results indicate a water seepage height of 39 mm for the conventional concrete specimen, with the calculated relative permeability coefficient being 9.32 × 10 −6 mm/h.In comparison, the RFHC specimen yielded an average water penetration height of 24 mm, and the relative permeability coefficient was calculated to be 3.53 × 10 −6 mm/h.These figures demonstrate that RFHC boasts significantly higher impermeability than its conventional concrete counterpart.Moreover, rubber particles and fibers incorporated into concrete not only hinder the initiation of cracks and delay the propagation of cracks under external forces but also improve the internal pore structure of the concrete and obstruct the continuity and penetration of capillary channels to advance the concrete's impermeability.

Testing method
Concrete abrasion resistance testing utilizes the underwater steel ball approach detailed in the "Testing Standardization for Hydraulic Concrete" (DL/T 5150-2017).This method measures the relative resistance of the concrete surface to abrasion caused by high-speed underwater flow, thereby assessing the concrete surface's relative abrasion resistance performance.In accordance with this testing method, the specimen must be immersed in water for at least 48 h before testing.
During the test, the specimen is removed, and any surface water is wiped off; subsequently, it is weighed and placed into the anti-abrasive testing machine for a total of 72 h.It is washed clean upon removal, and its surface moisture is wiped dry before being weighed again.Using the abrasion resistance strength as the evaluation index, the concrete abrasion resistance performance is calculated using the formula provided.Its calculation formula is as follows:  where f a is the impact resistance, which is the time required for a worn unit mass per unit area, h/(kg m 2 ); T is the cumulative time of the test, h; A is the wear area of the specimen, m 2 ; and △M is the cumulative mass of the specimen that is lost after being ground during period T, kg.The average of the measured values of three specimens is taken as the testing result.

Testing results and analysis
Figure 10A-C display the results of the concrete scouring and abrasion resistance testing, along with the specimens following erosion.Table 9 presents the results of the concrete erosion resistance testing, indicating that RFHC showcases higher abrasion resistance than conventional concrete; however, the difference is not significant.Considering both the surface moisture content of the specimen and the weighing error during testing, differentiating between the two results is challenging.
The abrasion resistance performance of RFHC with an appropriate mix ratio surpasses that of conventional concrete.However, factors such as rubber and fiber parameters, concrete strength grade, and testing conditions will inevitably cause differences in the testing results.As depicted in Figure 10B,C, coarse aggregates become exposed on the surface of the two specimens after washing and abrading.The surface of the conventional concrete specimen is uneven, with the cement matrix between the coarse aggregates significantly abraded.On the other hand, the surface of the rubber concrete specimen is relatively flat, with rubber particles and fibers uniformly embedded in the cement matrix, rendering a specific abrasion resistance effect.The erosion failure of concrete results from the abrasion, impact, and cavitation of the concrete surface by the high-speed water flow containing steel balls.Since the strength of the cement matrix is lower than that of the aggregates, the destructive erosion force first causes the cement matrix parts to gradually abrade into pits, with the aggregates eventually protruding.Upon reaching a certain extent, the protruding aggregates are washed away, forming erosion damage.For RFHC, rubber particles and fibers' specific absorption effect on the impact energy of water flow contributes to its improved abrasion resistance performance.

CONCLUSIONS
This investigation employed an orthogonal testing approach to assess the mechanical behavior and durability of RFHC.
A meticulous analysis scrutinized the effects of polypropylene fiber and rubber particle content, as well as rubber particle size, on the mechanical attributes of concrete.The optimal mixture ratio for RFHC was deduced from the results of range and variance analyses.An exhaustive evaluation was conducted to comprehend the influences of these variables on the compressive, flexural, and split tensile strength, as well as the brittleness and crack resistance of RFHC.Upon identifying the ideal mixture proportions for both RFHC and conventional C40 concrete, a sequence of durability assessments was performed, encompassing tests for frost resistance, impermeability, and abrasion resistance in concrete.A nuanced comparison underscored the variances in durability between RFHC and conventional concrete types.The salient conclusions derived from this investigation are as follows: 1.The orthogonal testing methodology effectively optimizes the RFHC mixture proportion; among the parameters examined, rubber content and particle size exhibit the most substantial impact on the mechanical characteristics.2. RFHC displays a pronounced improvement in crack resistance compared to conventional concrete, substantiated by elevated tension-compression and flexure-compression ratios.
3. When optimized, RFHC distinctly outperforms conventional concrete in terms of freezing resistance and impermeability.4. The findings of this study contribute valuable knowledge about the mechanical attributes, optimal mix design, and comparative durability advantages of RFHC, thereby paving the way for the construction of advanced hydraulic concrete structures.
This study augments the durability performance of high-volume hydraulic concrete, enhancing its resistance to cracking, abrasion, frost, and permeability.In the context of thin-walled concrete structures, such as prefabricated channels, these improvements are complemented by an increase in the ductility of RFC.Future research directions and expectations include further investigations to optimize the mix proportion design of RFHC, delving deeper into its mechanics and durability.The study could be extended to explore the use of alternative fibers or admixtures in hydraulic concrete to enhance performance.Additionally, the environmental and economic benefits of employing RFHC in construction projects could be assessed, and the long-term performance and durability of RFHC in real-world applications could be examined to provide more data for its practical implementation.wear area of specimen ΔM cumulative lost mass after abrasion time T AUTHOR CONTRIBUTIONS Jingkui Zhang: Conceptualization (equal); data curation (equal); formal analysis (equal); investigation (equal); methodology (equal); validation (equal); writing -original draft (equal); writing -review and editing (equal).Changshun Liu: Data curation (equal); investigation (equal).Jugang Luo: Data curation (equal); investigation (equal).Juncai Xu: Conceptualization (equal); data curation (equal); investigation (equal); methodology (equal); software (equal); visualization (equal); writing -review and editing (equal).

NOMENCLATURE
Compressive strength testing of the block; (B) flexural strength testing of the block; (C) splitting tensile strength testing of the block.

F
provides a more intuitive representation of the influence law of different factors on every performance index.I G U R E 4 Influence of different factors on the basic mechanical properties of RFHC: (A) Compressive strength; (B) flexural strength;

F I G U R E 5
Failure of different concrete specimens under the ultimate load: (A) RFHC testing block; (B) conventional concrete testing block; (C) sections of the flexural testing of different concrete specimens.
U R E 6 (A) Concrete rapid freeze-thaw testing; (B) concrete dynamic elastic modulus testing.U R E 7 (A) Relationship between the relative dynamic elastic modulus and the number of freeze-thaw cycles; (B) relationship between the mass loss rate and the number of freeze-thaw cycles.

8
Failure mode of concrete under different numbers of freeze-thaw cycles: (A) The two types of specimens do not change substantially after 50 cycles; (B) the GC specimen began to peel off after 100 cycles, and the RC specimen appeared pitted; (C) the peeling of the GC specimens increased after 125 cycles, and the RC pockmarks increased; (D) the GC specimen failed and the RC was slightly eroded after 175 cycles.

9 F
Concrete impermeability testing and apparent water seepage on the split surface of the specimen: (A) Concrete impermeability testing; (B) RFHC testing block; (C) conventional concrete testing block.I G U R E 10 Concrete erosion resistance testing and concrete specimen after grinding: (A) Concrete erosion resistance testing; (B) RFHC testing block; (C) conventional concrete testing block.
TA B L E 1 Physical and chemical properties of fly ash.
TA B L E 3

E 5 Orthogonal testing scheme. Level Polypropylene fiber content /kg/m 3 (A) Rubber content /% (B) Rubber particle size/mesh (C)
Polypropylene fiber performance index.Mix ratio of conventional concrete and RFHC.
TA B L E 4
TA B L E 8-By examining Table Calculation results of concrete erosion and abrasion strength.
TA B L E 9