Testing of recycled fine aggregate concrete’s mechanical properties using recycled fine aggregate concrete and research on technology for highway construction

Yali Liu

doi:10.1515/nleng-2025-0109

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Testing of recycled fine aggregate concrete’s mechanical properties using recycled fine aggregate concrete and research on technology for highway construction

Yali Liu

Veröffentlicht/Copyright: 18. April 2025

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Abstract

The rapid growth of transportation engineering has led to significant environmental challenges due to the accumulation of building waste and tailings. A promising solution is to utilize these materials to develop high-performance tailings recycled concrete (RFAC) for highway construction, promoting a green circular economy. This study advances existing research by exploring innovative mix design strategies and performance optimization techniques to enhance RFAC’s mechanical properties and industrial applicability. Specifically, this research investigates the effects of various iron tailings admixtures on the mechanical behavior and deformation characteristics of RFAC before and after carbonation, aiming to establish an optimal composition for transportation infrastructure. The materials used include iron tailings, recycled coarse aggregate, natural river sand, and 42.5-grade ordinary silicate cement. Unlike conventional approaches, this study emphasizes the influence of particle morphology and surface characteristics of laboratory-produced recycled fine aggregate, revealing its angular shape, rough texture, and larger specific surface area, which significantly impact fluidity, bulk density, and crushing value. Experimental results demonstrate that fiber type and tailings admixture ratio play a crucial role in improving RFAC’s slump, compressive strength, split tensile strength, and failure modes. Compared to previous studies, this research provides a more refined analysis of mix proportioning and introduces tailings-based performance enhancements, offering new insights for large-scale applications in transportation engineering. By optimizing tailings incorporation and aggregate proportions, this study establishes a systematic approach to improving RFAC’s mechanical performance, durability, and construction adaptability, laying the foundation for sustainable infrastructure development.

Keywords: recycled fine aggregate; high-performance concrete; mechanical properties

1 Introduction

The rapid development of modern civilization has ushered in a new phase in transportation engineering construction, with concrete playing a crucial role in infrastructure projects such as highways, rail transit, and tunnels. However, the simultaneous growth of the construction industry and urbanization has led to an increasing accumulation of building and tailing waste, posing significant environmental challenges [1]. Addressing this issue while promoting sustainable development has become a key concern in both academic and industrial sectors.

Recycled fine aggregate concrete has gained considerable attention due to its potential to reduce carbon emissions, conserve natural resources, and facilitate the reuse of construction waste [2]. While recycled aggregates retain some properties of natural aggregates, they also exhibit drawbacks such as rough surfaces, high water absorption, and reduced strength [3]. These characteristics significantly impact the workability and durability of concrete, necessitating further research and optimization.

Existing studies have explored the mechanical properties of iron tailings concrete, particularly in road engineering applications [4]. Previous research has examined its permeability, shrinkage, slump, and compressive strength [5]. However, most studies have focused on conventional tailings concrete, limiting its environmental and social benefits [6,7]. The adoption of high-performance recycled tailings concrete is hindered by variations in aggregate properties, weak interfacial transition zones, and challenges in cost control and large-scale implementation.

To address these limitations, researchers have explored performance enhancement strategies such as admixture incorporation, surface treatment technologies, and mix ratio optimization [8]. However, these approaches often lack standardized optimization criteria, and surface treatment remains costly for widespread adoption [9,10]. There is still a need for a systematic proportioning design and performance optimization strategy tailored to different application scenarios.

This study aims to develop high-performance tailings recycled concrete by partially replacing fine aggregate with iron tailings and incorporating recycled aggregates [11]. Through systematic testing of its mechanical properties, durability, and environmental adaptability, the research evaluates its feasibility in transportation engineering applications. The findings not only contribute to the resource utilization of construction waste but also provide theoretical support for the engineering application of tailings-based recycled concrete.

The main contributions of this study are as follows:

Proposing an optimized mix design incorporating iron tailings and recycled aggregates to enhance concrete performance.
Systematically evaluating the mechanical properties, durability, and environmental adaptability of the developed concrete.
Providing theoretical and practical insights for the application of high-performance tailings recycled concrete in transportation infrastructure.

The rest of this article are structured as follows: Section 2 presents the experimental design and methodology. Section 3 discusses the mechanical properties and durability analysis. Section 4 evaluates the environmental adaptability of the concrete. Finally, Section 5 summarizes the key findings and suggests future research directions.

2 Setting the proportions and test material

The experimental design is well structured, fully considering the particle size distribution of tailings and their impact on concrete performance. The natural coarse aggregate used in the test was continuously graded artificial gravel with a particle size range of 5–20 mm, while the fine aggregate was natural river sand sourced from the Ba River. The cement used was 42.5-grade ordinary silicate cement, and the foundation concrete’s strength grade was C30. The recycled aggregate, produced by Xi’an Environmental Protection Science and Technology Company Limited, was a widely available material that had been in use for 30 years. It underwent continuous grading, screening, cleaning, drying, and sieving before being bagged for testing.

The iron tailings used in the experiment were obtained from Yogou tailings depot, Shaanxi Shangluo Baoming Mining Co. Prior to testing, the gradation of the recycled coarse aggregate and tailings was measured and compared with standard specifications. As shown in Figure 1, the particle gradation meets the required standards [12]. Additionally, Tables 1 and 2 summarize the physical properties of the primary materials before testing.

Figure 1

Aggregate particle grading: (a) limestone-A, (b) granite-A, (c) limestone-B, and (d) granite-B.

Table 1

Cement’s primary performance indicators

Water consumption for standard consistency (%)	Set time (min)	Final setting time (min)	Fineness (45)	Stability	Flexural strength (MPa)		Compressive strength (MPa)
30	170	285	3.0	Qualified	4 days	29 days	4 days	29 days
30	170	285	3.0	Qualified	5.5	6.9	20.5	43.6

Table 2

Key materials’ performance indicators

Performance index	Apparent density (kg/m³)	Bulk density (kg/m³)	Fragmentation value index (%)	Water absorption rate (%)	Mud content (%)	Moisture content (%)	Organic content	Alkali-aggregate reaction	Error range (%)
Natural coarse aggregates (NCA)	2,940	1,740	10.5	1.42	0.75	0.82	Qualified	Qualified	±2.5
Recycled coarse aggregate (RCA)	2,520	1,460	15.8	7.02	1.98	3.25	Qualified	Qualified	±3.0
Sand	2,760	1,830	12.5	2.26	1.4	4.2	Qualified	Qualified	±2.0
Iron tailings	2,740	1,826	18.6	8.85	2.8	1.52	Qualified	Qualified	±3.0

While the experimental setup effectively considers the impact of tailings particle distribution, the selection criteria for the tailings replacement rate require further elaboration [13]. To enhance scientific rigor, additional discussion on the rationale behind the chosen replacement percentages, along with supporting experimental data or references, would strengthen the validity of the study’s conclusions.

On the basis of this, high-performance tailings recycled concrete’s mix ratio design was completed. After establishing a 30% recycled aggregate substitution rate based on the pertinent literature, the mix ratios were created in accordance with the specifications [14], accounting for varying tailings mixing quantities. Table 3 displays the concrete mixing ratios under different working conditions following trial mixing and adjustment. For comparison and analysis, a water/cement ratio of 0.45 and a sand rate of 0.55 were chosen for each mixing ratio.

Table 3

Designing the mixing ratio for inventive concrete under various operating circumstances

NO	Cement	NCA	RCA	Sand	Iron ore tailings	Tailings replacement rate (%)	Error range (%)
NAC	530	1,062	0	570	0	0	±2.0
RAC-1	530	732	310	560	0	2	±2.5
RAC-2	530	731	315	510	60	8	±3.0
RAC-3	530	745	318	452	116	10	±3.0
RAC-4	530	752	320	450	158	20	±3.5
RAC-5	530	755	320	402	174	25	±3.5

3 Making and examining the morphology of recycled fine aggregates

3.1 Getting recycled fine aggregate ready

A 150 × 250 jaw crusher made by Henan Zhongke Engineering Technology Co. was used to crush the broken concrete blocks twice after the laboratory waste concrete beams were manually crushed with a hammer. The crushed material was then manually sieved, and the recycled fine aggregate selected for this test had particle sizes ranging from 0.15 to 2.40 mm. To enhance the reproducibility of the experiment, further details on the processing of tailings and recycled fine aggregates are provided. The tailings underwent drying, sieving, and impurity removal to ensure consistency in particle characteristics. Figure 2 presents the specific particle size distribution curves of the recycled fine aggregate, tailings, and other comparison materials. Additionally, standard sand and natural river sand were sieved simultaneously to achieve a gradation curve comparable to the recycled fine aggregate. To systematically analyze the impact of particle characteristics on mortar performance, various fine aggregates were prepared, as illustrated in Figure 3.

Figure 2

Recycled fine aggregate grading curve.

Figure 3

Test made use of three distinct fine aggregates. (a) Recycled fine aggregate, (b) same grade standard sand, and (c) same grade natural river sand.

3.2 Analysis of bluntness and observation of various fine aggregate morphologies

To analyze the differences in particle morphology between recycled fine aggregates and other types of fine aggregates, this study utilized the Occhio Scan700 particle size and shape analyzer. This analyzer operates based on advanced image scanning technology, processing the captured images through Callisto3D graphic analysis software to obtain detailed morphological parameters.

3.2.1 Particle shape and size distribution analysis

To ensure a comprehensive comparison, three different types of fine aggregates were selected for analysis. The Occhio Scan700 provided high-resolution two-dimensional shape parameters, enabling the evaluation of key morphological characteristics, such as:

Sphericity: Reflecting the roundness and angularity of particles, which directly impacts workability and packing density in concrete.
Aspect Ratio: Indicating the elongation of particles, which influences the mechanical interlocking and overall strength of concrete.
Surface Roughness: Affecting the adhesion between the aggregate and cement paste, playing a crucial role in interfacial bonding.

3.2.2 Comparison of morphological differences

The two-dimensional morphological characteristics of various fine aggregates are illustrated in Figure 4, showcasing the contrast between recycled fine aggregates, natural river sand, and standard sand. Preliminary analysis reveals that:

Recycled fine aggregates exhibit more angular and irregular shapes, with a rougher surface due to residual mortar, which can enhance interfacial bonding but may reduce fluidity.
Natural river sand is generally more rounded and smooth, leading to better workability but potentially weaker interfacial adhesion.
Standard sand maintains a uniform particle shape and size, often serving as a reference material for controlled experimental conditions.

Figure 4

Shape of the particles in various fine aggregates. (a) Recycled fine aggregate, (b) same grade standard sand, and (c) same grade natural river sand.

3.2.3 Impact on concrete performance

The particle shape and size distribution significantly influence the mechanical properties and durability of concrete. The angular and rough texture of recycled fine aggregates may enhance mechanical interlocking, potentially improving compressive and flexural strength, but may also lead to higher water demand and reduced workability. These findings highlight the need for further optimization, such as surface treatment of recycled fine aggregates, to achieve a balance between workability and strength.

By incorporating detailed shape analysis and discussing its implications on concrete performance, this section enhances the scientific rigor and practical significance of the study.

According to the morphological examination, the recycled fine aggregate is more angular, has more long and narrow particles, and is generally uneven when compared to standard sand and natural river sand. The software was used to determine the obtuseness of the particles in order to quantitatively analyze their shape. This involved drawing an internal tangent circle at each protruding part of the projected contour of the particles, dividing its diameter by the maximum diameter of the internal tangent circle of the particles, and taking the average value, which indicates the sharpness of the particles’ surface; the smaller the obtuseness, the sharper the particles’ surface is, and the circle’s obtuseness is 100%. The calculation method is shown in the following equation:

(1) B = ∑ i = 1 N d i N D n × 100 % ,

where B is the obtuseness, d i is the diameter of the ith protrusion, D n is the maximum diameter of the particles, and N is the number of protrusions on the projected contour of the particles.

The cumulative and fractional curves of the obtuseness distribution were produced for the three distinct fine aggregates, as well as the equivalent volume percentage of various obtuseness particles in the overall volume, as seen in Figure 5. As illustrated in Figure 5(b), the peak bluntness corresponding to the standard sand and natural river sand is approximately 75 and 90%, respectively, whereas the minimum bluntness corresponding to the peak of recycled fine aggregate is 60%, or nearly 20%, under the same grading conditions. According to the cumulative scaling curves, the natural river sand has less passivation, indicating that there are comparatively few spherical particles in it, whereas the majority of the standard sand is characterized by higher passivation particles, which are less angular and closer to spheres. There are more angular particles with comparatively sharp boundaries in recycled fine aggregate.

Figure 5

Bluntness equivalent volume distribution curves for various fine aggregates: (a) cumulative curves and (b) dividing curves.

3.3 Evaluation and description of the morphological characteristics of various fine aggregates

Digital image processing was utilized to quantitatively describe the morphological characteristics of fine aggregates in order to further study the morphological parameters of the three distinct fine aggregates and to examine the distribution of various aggregates in various particle size intervals [15]. This test’s primary morphological parameters are aspect ratio, sphericity, and firmness. The following are the methods used to calculate the three parameters: Aspect ratio is a parameter that describes the morphology of the particles and assesses the size and shape of a single particle; the closer the aspect ratio is to 100%, the closer the length and width of the particles are; the smaller the aspect ratio, the longer and narrower the shape of the particles is. The following equation illustrates the calculation process:

(2) A sp = X F min / X F max ,

where X F max is the maximum Frett diameter, X F min is the minimum Frett diameter, and Asp is the aspect ratio. To a certain extent, firmness can reflect the size of the specific surface area of the particles. Firmness is a measure of the overall concavity of the particles, which is used to reflect whether the particle surface is concave or convex as well as the degree of concavity and convexity. The closer the firmness is to 100%, the closer the convex area is to the actual area of the particles and the less the concave portion of the particle contour, which can be calculated as shown in the following equation:

(3) S = A / A c ,

where A is the equivalent plane area of the particle edge, A c is the area of the convex envelope of the particle border, and S is the solidity. The degree to which particles resemble a sphere is known as their sphericity, and it is used to depict their shape in three dimensions. Three-axis equivalency has the highest sphericity, while flaky and cylindrical particles have the lowest sphericity. The following equation provides the calculation:

(4) Q = V p / V cs .

Wald’s sphericity is denoted by Q, the particles’ actual volume by V p , and their smallest exterior sphere volume by V cs . Using information from the Occhio Scan700 particle size analyzer and Callisto3D graphical analysis software, the aspect ratio, hardness, and sphericity of the three types of fine aggregates in various size intervals were computed, as seen in Figure 6.

Figure 6

Comparison of several fine aggregates’ morphological characteristics: (a) average aspect ratio, (b) average solidity, and (c) average roundness.

According to the average aspect ratios of three distinct fine aggregates, the recycled fine aggregates exhibit lower aspect ratios in the particle size range above 0.3 mm. Specifically, standard sand has an average aspect ratio of 75%, while recycled fine aggregate, which has the highest proportion within the gradation range, demonstrates an aspect ratio of only 67% for particles in the 1.15–2.35 mm range. This indicates that, compared to ordinary sand and natural river sand, the recycled fine aggregate’s smaller particles possess a relatively low aspect ratio. Moreover, particles larger than 0.3 mm in the recycled fine aggregate show a significantly reduced aspect ratio. This difference suggests that the recycled fine aggregate has a more angular and irregular shape, which could influence the workability and strength characteristics of concrete.

The recycled fine aggregates exhibit less firmness in the particle size range above 0.3 mm, which is consistent with the aspect ratio curves’ trend for the average firmness data. This suggests that throughout the crushing process, the mortar that was adhered to the recycled fine aggregate’s surface was left in place, giving the surface a certain amount of roughness that lessens its firmness. By looking at the sphericity comparison of several fine aggregates in figure, the sphericity of the same gradation of standard sand reaches 83%, that of natural river sand is approximately 80%, and that of recycled aggregate is approximately 73% in the range of 1.15–2.35 mm, which is the largest proportion of gradation. This suggests that the recycled aggregate is less spherical after crushing than the natural river sand because of the mortar that is attached to its surface, while the sphericity of standard sand and natural river sand is nearly equal and spherical particles are more prevalent in natural river sand.

4 Results

The slump findings for various concretes are displayed in Figure 7, which suggests that fiber significantly affects RFAC’s flowability. All forms of fiber-reinforced RFAC have a droop that is less than that of RFAC without fibers and that considerably lessens as the fiber volume fraction rises. Steel fibers have a bigger impact on RFAC flowability than flexible fibers do. Steel fibers create a skeleton in the combination because of their length and high stiffness, which prevents aggregate slip and lowers slump. The larger surface area of polypropylene fiber (PPF) and basalt fiber (BF), on the other hand, requires more mortar coverage, further reducing RFAC slump. It is worth noting that the slump of R100 is the same as that of normal concrete (200 mm). This is because, when RFAC is cast, the additional water that saturates the fine recycled aggregate (FRA) is added together with the effective water engaged in hydration; yet, the portion of the water does not penetrate the interior of the FRA. The slump of R100 was not decreased because some of the FRA pores were filled with cement particles during the concrete mixing process, increasing the actual water-to-cement ratio and counteracting the friction effect caused by the FRA’s sharpness [16].

Figure 7

Slump of different kinds of concrete.

Using the slump of R100 as a reference, the normalized droop of each type of RFAC was analyzed to visually evaluate the effect of fiber type on RFAC flowability. Figure 8 displays the normalized slump vs fiber volume fraction for each fiber-reinforced RFAC, with the closed lines representing different fiber volume percentages from the exterior to the interior (0 to the greatest value), and the normalized droop is the point where the lines cross the coordinate axes. According to the findings, the end-hooked steel fiber (ESF)-reinforced RFAC slump reduced most noticeably as the volume fraction increased, and the fibers’ detrimental impact on RFAC flow was rated as follows: ESF > MSF > PPF > BF. Due to their higher modulus of elasticity, steel fibers had a bigger effect than flexible fibers. Additionally, the majority of the steel fibers formed a skeleton structure after mixing, which further decreased the RFAC flow.

$Figure 8 Each fiber-reinforced RFAC with a different fiber volume fraction has a normalized slump.$

Figure 8

Each fiber-reinforced RFAC with a different fiber volume fraction has a normalized slump.

4.1 Split tensile characteristics, steel fiber distribution, and damage mode

With a single crack running across the cross-section and the specimen shattering into two pieces, the R100 specimen exhibits typical brittle damage (Figure 9(a)). The R100, R100ESF1.0, R100MSF1.0, R100PPF0.132, and R100BF0.10 specimens’ damage under split tensile stress is depicted in Figure 9. The damage pattern of 1.0% ESF- and MSF-reinforced RFAC is shown in Figure 9(b) and (c). The specimen shows repeated cracking and the damage changes from brittle to ductile, mostly as a result of end-hook resistance and bridging loads between the steel fibers and the matrix.

Figure 9

Final failure pictures of different types of specimens: (a) R100, (b) R100ESF1.0, (c) R100MSF1.0, (d) R100PPF0.132, and (e) R100BF0.10.

Although the surface of the R100PPF0.132 and R100BF0.10 specimens had several little fractures, the damage came from the main crack penetration (Figure 9(d), (e)). Because there was not enough bond stress, the majority of the flexible fibers snapped at the fissures. R100PPF0.132 did not break entirely because some of the PPF fibers were still in the elastic stage at the cracking, which is caused by the low modulus of elasticity and high elongation of the flexible fibers (Figure 9(d)).

4.2 Steel fiber distribution in RFAC

The distribution of steel fibers in concrete has been shown to have a significant impact on the material’s mechanical qualities. The 1.0% steel fiber-reinforced 150 mm × 150 mm × 150 mm cubic specimens cut with the size of 40 mm × 40 mm × 40 mm in this test were subjected to X-ray computed tomography scanning in order to examine the dispersion of steel fibers in RFAC. The distribution of steel fibers in the R100ESF1.0 and R100MSF1.0 specimens is displayed in Figure 10. The steel fibers are dispersed at random angles throughout the RFAC, and while micro steel fiber (MSF) is smaller than ESF, its distribution is clearly denser.

$Figure 10 With a 1.0% fiber volume fraction, the distribution of steel fiber in RFAC is (a) ESF and (b) MSF.$

Figure 10

With a 1.0% fiber volume fraction, the distribution of steel fiber in RFAC is (a) ESF and (b) MSF.

4.3 Load–deformation curve

Figure 11 shows the NC and R100 splitting tensile load–deformation curves. Although R100’s peak splitting tensile stress was 13.9% lower than NC’s, no appreciable improvement in toughness was observed, and all specimens displayed brittle damage with mild transverse distortion. Figure 12 displays the splitting tensile load–deflection curves of RFAC reinforced with different fibers. The results demonstrate that the peak splitting tensile load and ductility of RFAC increased as the fiber volume percentage increased. MSF is superior to ESF, and steel fibers toughen and reinforce RFAC more effectively than flexible fibers. MSF-reinforced RFAC exhibits superior splitting tensile capabilities over ESF-reinforced RFAC due to the MSF’s smaller diameter, low aspect ratio, higher MSF at the same volume fraction, and increased likelihood of cracks passing through the MSF. Additionally, Liang et al. [12] found that as the volume % of steel fibers increases, so do the peak splitting tensile load and ductility of steel-fiber-reinforced recycled concrete. The fraction rises, the peak load and critical deformation of steel fiber-reinforced recycled concrete in the shear load–deflection curve rise noticeably, and the load decreases after the peak load, all of which are consistent with the splitting and tensile behavior of steel fiber-reinforced RFAC in this study.

Figure 11

NC and R100 split tensile load–deflection curves.

Figure 12

Tensile load–deflection curves for several fiber-reinforced RFAC types are shown as follows: (a) ESF, (b) MSF, (c) PPF, and (d) BF.

The splitting tensile load–deformation curves of fiber-reinforced RFAC generally exhibit three stages: (1) the elastic stage, in which the load increases linearly with lateral displacement until it reaches the cracking load; (2) the elasto-plastic stage, in which the load increases nonlinearly with displacement until it reaches the peak, during which the fibers are pulled out and the cracks gradually expand; and (3) the softening stage, in which the load gradually decreases, the displacement increases, and the cracks expand until they are destroyed. A typical split tensile load–deformation curve is depicted in Figure 13, where the deformation associated with the peak load is easily visible. The cracking loads for each type of concrete were examined to examine the impact of fiber type on RFAC toughness. The findings indicated that the trend was in line with the ultimate compression load. This is because the damage buildup mechanism for split tensile and compressive stresses is comparable, and the specimens are in the elastic stage prior to cracking. Figure 14 further showed the linear relationship between ultimate compressive load and cracking load, and there was a strong correlation between the experimental data and the fitting results.

Figure 13

Fiber-reinforced RFAC typical load–deflection curves.

Figure 14

Connection between ultimate compression stresses and cracking loads.

4.4 Analysis of characteristic parameters

Figure 15, which is normalized to R100, shows the relationship between the fiber volume percentage and the normalized characteristic parameters of the split tensile load–deflection curves of the various types of fiber-reinforced RFACs. The closed line shows the fiber volume percentage gradually increasing from 0 from inside to outside. The results show that an increase in the fiber volume percentage significantly improves the splitting tensile properties of RFAC. Figure 15(a) and (b) illustrates how different fibers affect the normalized cracking load and peak splitting tensile load, respectively. The highest enhancement impact is seen in the MSF-enhanced RFAC. For an MSF volume fraction of 1.0%, the cracking load is 1.52 times greater than R100, and for a volume fraction of ESF of 1.0%, the peak tensile stress is 2.1 times greater than R100; with a 1.0% increase in volume fraction, the peak tensile load is higher than R100 (R100 by 32.3%); with a PPF volume fraction of 0.198%, the peak tensile load was increased by 15.5%. Fibers have been demonstrated to improve the splitting tensile characteristics of recycled concrete, provided that the PPF volume portion is appropriately managed.

$Figure 15 Split tensile load–deflection curves of different fiber-reinforced RFACs with varying fiber volume fractions and their normalized characteristic parameters. (a) Normalized cracking load, (b) normalized peak splitting tensile load, (c) normalized deformation at peak load, (d) normalized peak toughness, and (e) normalized residual toughness.$

Figure 15

Split tensile load–deflection curves of different fiber-reinforced RFACs with varying fiber volume fractions and their normalized characteristic parameters. (a) Normalized cracking load, (b) normalized peak splitting tensile load, (c) normalized deformation at peak load, (d) normalized peak toughness, and (e) normalized residual toughness.

With different fiber volume fractions, Figure 15(c–e) illustrates how the kind of fiber affects the normalized deformation as a function of peak splitting tensile load, peak toughness, and residual toughness of RFAC. The RFAC’s peak splitting tensile load deformation, peak toughness, and residual toughness all increased in tandem with the fiber volume percentage. Fiber toughness and ductility were greatly enhanced by RFAC as compared to split tensile strength. The specimen with a 1% MSF volume fraction had deformation, peak toughness, and residual toughness that were 10.89, 20.54, and 33.7 times greater than R100, respectively [17]; the specimen with a 1% ESF volume fraction had deformation, peak toughness, and residual toughness that were 8.31, 10.31, and 32.61 times greater than R100. Because flexible fibers increase the ductility of RFAC, Figure 15(c) and (d) demonstrates that the improvement of residual toughness by flexible fibers is noticeably superior to the augmentation of peak toughness. The ductility and toughness of RFAC were enhanced with the increase in the volume fraction of PPF and BF fibers, even if the splitting tensile property enhancement of RFAC by flexible fibers was not as excellent as that of steel fibers. Overall, steel fibers outperformed flexible fibers in improving the split tensile properties of RFAC because of their high tensile strength and superior bonding qualities. The split tensile properties of RFAC were improved by several fiber types in the following order: MSF > ESF > BF > PPF. The PPF and BF fibers were primarily broken because of their lower tensile strength, whereas the steel fibers were taken out of the concrete matrix after the specimen fractured under split tensile loading. The tensile property damage patterns of several fiber types under split tensile loading are displayed in Figure 16, and scanning electron microscopy was used to observe the fracture of PPF and BF. The findings indicate that whereas PPF and BF broke, the steel fiber end hooks straightened.

Figure 16

Patterns of failure for various fiber types include (a) ESF, (b) MSF, (c) PPF, and (d) BF.

5 Conclusion

This study investigates the influence of varying iron tailings dosages on the properties of recycled fine aggregate concrete, assessing its feasibility for highway construction. The findings highlight that compared to natural sand, recycled fine aggregate exhibits higher angularity, roughness, and specific surface area, significantly affecting concrete performance. While its use as an aggregate is viable, optimizing the removal of adhered mortar is essential for improving property stability. The damage mode of recycled fine aggregate concrete transitions from brittle to ductile, with steel fibers enhancing crack resistance through bridging effects. However, the toughening effect remains limited due to premature fiber fracture before crack penetration.

Future research should focus on the long-term durability of concrete with varying iron tailings content, particularly in terms of resistance to freeze–thaw cycles, chloride penetration, and fatigue under traffic loads. Additionally, evaluating its applicability across different highway construction environments, including extreme climates and high-load conditions, will help establish comprehensive engineering guidelines and facilitate practical implementation.

Funding information: This study was supported by Research on the Integration of Blockchain and Transportation Logistics, Teaching Technology (2023) No. 359; Project Number: 24B580001.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2024-12-16

Revised: 2025-02-13

Accepted: 2025-02-25

Published Online: 2025-04-18

This work is licensed under the Creative Commons Attribution 4.0 International License.

Artikel in diesem Heft

https://doi.org/10.1515/nleng-2025-0109

Schlagwörter für diesen Artikel

recycled fine aggregate; high-performance concrete; mechanical properties

Creative Commons

BY 4.0