The effects of high-content rubber and steel fibers on the workability and mechanical properties of concrete

Abstract

This study aims to explore the effects of high-content rubber sand and steel fibers on the workability and mechanical properties of concrete, providing theoretical and data support for the resource utilization of waste tires and the engineering application of rubber sand concrete. Rubber particles replaced 60% of fine aggregates (by mass), and the volume fractions of steel fibers were set at 1.5%, 2.0%, 2.5%, and 3.0%. Ordinary concrete, rubber sand concrete, and rubber sand concrete with steel fibers were prepared, and slump, water absorption, compressive strength, and cyclic compressive performance experiments were conducted, along with microstructure analysis. The results show that the introduction of rubber slightly increased the slump of the concrete, but after adding steel fibers, the slump gradually decreased, with the most significant reduction when the steel fiber content reached 3.0%. Water absorption results indicate that rubber significantly increased the water absorption of concrete, and the increase in steel fiber content further exacerbated this trend. Compressive strength tests showed that the addition of rubber sand resulted in a 41.4% decrease in compressive strength, but an appropriate amount of steel fibers (2.0%) could significantly restore the compressive strength, with a recovery rate of up to 95.1%. However, when the fiber content exceeded 2.0%, strength decreased due to uneven fiber distribution and the aggregation effect. Cyclic compression performance studies showed that although the introduction of rubber enhanced the energy dissipation capacity of the concrete, it significantly reduced the load-bearing capacity, while the addition of steel fibers significantly improved the hysteretic energy dissipation and the damage resistance under cyclic loading, the sample with 2.5% steel fiber content exhibited the best performance. The distribution of rubber sand in the matrix weakened the interfacial bonding strength and increased the porosity, while the steel fibers significantly improved the concrete’s stiffness and damage resistance by bridging cracks and dispersing stress. When the steel fiber content was too high (2.5% and 3.0%), the aggregation effect of fibers weakened the uniformity of the matrix, leading to a performance decline. The results of this study provide a reference for the practical application of rubber concrete in pavement engineering.

Keywords

concrete steel fibers rubber workability mechanical properties

Introduction

According to statistics from the International Rubber Study Group, the annual global production of tires has exceeded 1.7 billion units. However, the average lifespan of a tire is only 3 to 5 years, making the disposal of waste tires an urgent issue to address.¹ Currently, for waste tires that have no recycling value, they are typically disposed of by landfilling or incineration. It is important to note that the chemical components in tires are difficult to degrade in soil, which can cause severe contamination of underground water.^2,3 Incinerating tires releases large amounts of harmful gases (such as sulfur dioxide), which severely pollutes the air and is harmful to human health.² Therefore, exploring environmentally friendly disposal methods for waste tires has become one of the key focuses in academia.^4,5 Fortunately, researchers have found that recycling waste tires into rubber particles for use as raw materials in concrete production holds significant value for environmental protection and resource recycling. The ways in which rubber particles are used in concrete are shown in Table 1.^6–12

Table 1.

Usage of rubber particles.

Report source	Rubber size	Replaced material	Maximum replacement ratio (%)
Gerges et al.⁶	≤1 mm	Fine aggregate	20
Liu et al.⁷	2 mm	Fine aggregate	15
Liu et al.⁸	0.178 mm, 1.11 mm, and 2 mm	Fine aggregate	20
Aslani⁹	0.5–5 mm	Fine aggregate	100
	5–25 mm	Coarse aggregate	100
	0.5–5 mm和5–25 mm	Coarse and fine aggregates	60
Fernández-Ruiz et al.¹⁰	63 μm–0.6 mm	Cement	10
Gheni et al.¹¹	200目	Cement	25
Jokar et al.¹²	9 mm	Coarse aggregate	15

It should be noted that Ren et al.¹³ reported that rubber particles are relatively light and can be more evenly distributed in concrete when used as fine aggregates, thus reducing the likelihood of segregation. Additionally, rubber particles do not possess pozzolanic activity, so their use as a replacement for cement is generally not recognized.^14,15 Therefore, rubber particles derived from waste tires are typically used as fine aggregates in concrete production. Recently, several reports have discussed the performance of rubber sand concrete (concrete with rubber particles as fine aggregates).^16–19 These reports consistently indicate that the slump and density of fresh rubber sand concrete mixtures are lower than those of ordinary concrete, and as the rubber content increases, both the slump and density decrease further. Liu et al.⁷ reported the cracking performance of rubber sand concrete, and the results showed that when the rubber content was 15%, the bending strength was 1.26 times the compressive strength, indicating better cracking resistance compared to ordinary concrete. Furthermore, Liu et al.⁷ also pointed out that rubber sand concrete exhibits excellent fatigue resistance. Recent reports have reached similar conclusions.^20,21 Liu et al.²⁰ suggested that rubber particles act as bridges for internal cracks in concrete, slowing the rate of crack propagation, thus providing excellent fatigue resistance. It is also noteworthy that rubber particles can enhance the freeze-thaw resistance of concrete.^21,22

However, the application of rubber sand concrete is not without limitations, with the prominent issue being that the replacement of natural river sand with rubber particles significantly reduces the strength of the concrete.^14,15,22 For example, Jalal et al.¹⁴ reported that when the rubber sand replacement ratio was 10%, the compressive strength of concrete decreased by 30%. Recently, some reports have discussed methods to enhance the strength of rubber sand concrete, mainly through (1) modifying rubber particles using sodium hydroxide solution^23,24; (2) adding active²⁵ mineral admixtures^26,27; (3) incorporating fibers.^28–30 It should be noted that modified rubber particles primarily improve the bond strength with the cement matrix and do not significantly enhance compressive strength.²⁴ Additionally, the inclusion of active admixtures can improve the density of rubber sand concrete but does not significantly enhance its toughness.²⁷ In contrast, adding fibers to enhance the strength of rubber sand concrete is the most direct and effective method. It should be pointed out that previous reports consistently agree that steel fibers, with their high tensile strength and high elastic modulus, are particularly suitable for enhancing the strength of concrete structures.^31,32

Although previous reports have discussed the performance of steel fiber rubber sand concrete, the rubber sand content has never exceeded 40%.^30–32 Furthermore, previous reports typically set the steel fiber volume fraction at around 1%.^31,32 In this study, the rubber sand content was set to 60%, and the steel fiber volume fractions were set at 1.5%, 2.0%, 2.5%, and 3.0%. Slump, water absorption, compressive strength, and cyclic compression tests were conducted, and the microstructure of steel fiber rubber sand concrete was studied using SEM. This study aims to assess the effects of large amounts of steel fibers and rubber sand on the mechanical properties and failure modes of concrete. The results provide fundamental data for the wider application of rubber sand concrete and support breakthroughs in the use of rubber sand concrete for pavement construction.

Experiment

Materials

The cement used was ordinary Portland cement, with a 28-day compressive strength of 42.5 MPa, and chemical composition of cement is shown in Table 2. The coarse aggregate was natural crushed stone with a particle size of 5 mm to 15 mm. The fine aggregate was natural river sand with a particle size of 0.3 mm to 0.6 mm, and the fineness modulus of sand is 2.8. The screening curve of aggregate is shown in Figure 1. The steel fibers were corrugated, and their basic parameters are shown in Table 3. The rubber particles had a particle size of 20 mesh (approximately 0.85 mm). The appearance of steel fiber and rubber is shown in Figure 2.

Table 2.

Chemical composition of cement (unit: %).

Type	SiO₂	Fe₂O₃	CaO	Al₂O₃	K₂O	MgO	Na₂O	SO₃	TiO₂
Cement	20.6	3.86	63.14	5.59	1.33	1.65	0.137	3.09	0.289

Figure 1.

Sieve analysis curve: (a) Coarse aggregate and (b) fine aggregate.

Table 3.

Basic parameters of steel fibers.

Length/mm	Diameter/mm	Length-to-diameter ratio	Density/(g/cm³)	Tensile strength/MPa
45	0.5	90	7.8	≥600

Figure 2.

Appearance of raw material: (a) Steel fiber and (b) rubber.

Mix proportions

In this study, rubber was used to replace 60% of the fine aggregates (by mass), and the volume fractions of steel fibers were set at 1.5%, 2.0%, 2.5%, and 3.0%. The mix proportions in this study were designed based on the Chinese national standard “Specification for Mix Proportion Design of Ordinary Concrete” (JGJ 55-2011). The mix proportions and abbreviations of the samples are shown in Table 4.

Table 4.

Mix proportions and abbreviations of samples (unit: kg/m³).

Sample	Water	Cement	Fine aggregate (natural river sand)	Rubber particles	Coarse aggregate (natural crushed stone)	Steel fibers
OC	180	360	720	0	1080	0
RC	180	360	288	432	1080	0
RSC1	180	360	288	432	1080	117
RSC2	180	360	288	432	1080	156
RSC3	180	360	288	432	1080	195
RSC4	180	360	288	432	1080	234

Note: OC represents ordinary concrete. RC refers to concrete with 60% rubber replacement for fine aggregate. RSC1–RSC4 indicate rubberized steel fiber-reinforced concrete mixtures with 60% rubber replacement and steel fiber volume fractions of 1.5%, 2.0%, 2.5%, and 3.0%, respectively.

Experimental procedure

(1) Slump test: The slump test was conducted according to the Chinese standard GB/T 50080-2016, which specifies the procedure and requirements for measuring the workability of concrete. First, mix the fresh concrete mixture thoroughly. Place the slump cone on a clean, wet steel plate and fill the cone in three layers, tamping each layer 25 times. Then, slowly raise the slump cone and allow the fresh mixture to settle freely. Measure the height difference between the top surface of the mixture and the top of the cone, which is the slump value.

(2) Water absorption: The water absorption test was conducted according to the Chinese standard GB/T 50080-2016; after curing the concrete samples for 28 days, dry them to a constant weight at a temperature of 105°C. Then, immerse the samples in water for 24 h, remove them, wipe off the surface water, and weigh the samples. The water absorption rate is calculated using the formula shown in equation (1).

W = \frac{M_{1} - M_{2}}{M_{2}} \times 100 %

(1)

where W is the water absorption rate, M₁ is the mass of the sample after water absorption, and M₂ is the mass of the sample after drying.

(3) Compressive strength test: The compressive strength of concrete was determined in accordance with GB/T 50081-2019, conducted using displacement loading, with a loading rate of 2 mm/min.

(4) Cyclic compression test: The cyclic compression test was carried out following ASTM C1609. The initial load was applied up to 5 kN, followed by incremental loading with a 5 kN stress interval. Each unloading was to 0.5 kN. Both loading and unloading processes were conducted using stress control mode, with a control rate of 500 N/s.

Results and discussion

Slump

The slump experiment of the mixture is shown in Figure 3 and test results are shown in Figure 4. It can be observed that the slump of OC is 182 mm, indicating that the mix design in this study is reasonable and the concrete exhibits good workability and flowability. The slump of RC is 191 mm, which is a 4.95% increase compared to OC. This result is consistent with the findings of Ramdani et al.²⁶ and Steyn et al.³³ This is mainly because the surface of the rubber sand particles is smoother, reducing the frictional resistance between the aggregates and thus increasing the slump of the concrete.²⁶ Additionally, the density of rubber particles is lower than that of natural river sand, which reduces the overall weight of the concrete and contributes to the increased slump.³⁴

Figure 3.

Slump experiment of the mixture: (a) OC, (b) RC, and (c) RSC3.

Figure 4.

Slump.

From Figure 4, it can also be seen that when the steel fiber content is 1.5%, the slump decreases to 175 mm, a reduction of 16 mm compared to the rubber sand concrete without fibers (191 mm). This indicates that the steel fibers at this level have a minor impact on the workability of the concrete but have started to influence it. When the fiber content increases to 2.5% and 3.0%, the slump further decreases to 120 mm and 87 mm, respectively, with a more significant reduction. This shows that at these higher fiber contents, a fiber network structure is formed within the concrete, hindering the movement of the aggregates and paste, significantly reducing the workability of the concrete. It is worth noting that as the steel fiber content increases, the reduction in slump becomes more pronounced. These findings are consistent with previous reports.^35–37

Water absorption

The results of the water absorption calculation are shown in Figure 5. From Figure 5, it can be observed that compared to OC, the water absorption rate of RC increased by 41.46%. This is inconsistent with the findings of Benazzouk et al.,³⁸ whose report shows that the water absorption of samples containing 50% rubber particles decreased by five times, but it is consistent with the report of Steyn et al.³³ This may be due to differences in the rubber used. In this study, the rubber particles have smooth surfaces, which leads to weaker adhesion with the cement paste. This could result in the formation of micropores at the interface, increasing the pathways for water penetration inside the concrete.

Figure 5.

Water absorption.

From Figure 5, it can also be seen that as the steel fiber content increased from 1.5% to 3.0%, the water absorption rate of the concrete increased from 6.0% to 7.2%, showing a gradual increase with a nonlinear growth trend. The reasons for this are as follows: (1) After the addition of steel fibers, a complex network structure was formed between the aggregates and paste inside the concrete. At lower fiber content (1.5%), the fibers were more evenly distributed, and their effect on the density of the concrete was minimal, causing only a slight increase in the water absorption rate to 6.0%³⁹; (2) as the steel fiber content increased, its distribution within the concrete may have become uneven, leading to local aggregation. This would create more microcracks and pores at the interface between the steel fibers and the cement paste, providing more channels for water penetration, thus significantly increasing the water absorption rate^40,41; (3) when the steel fiber content reached 3.0%, the water absorption rate increased to 7.2%, indicating a significant decrease in the density of the concrete. The interlaced structure of the fibers severely affected the encapsulation and flowability of the paste, ultimately leading to a significant increase in the water absorption rate.⁴²

Compressive strength

Figure 6 shows the results of the compressive strength test. The damage pattern of the sample is shown in Figure 7. The results indicate that when 60% of the natural river sand in the concrete is replaced with rubber, the compressive strength decreases from 30.4 MPa for ordinary concrete to 17.8 MPa, resulting in a strength loss of 41.4%. Similar trends have been reported in previous studies.^6,22,43 This is mainly due to the low stiffness and low strength properties of rubber, which reduce the rigid skeleton effect within the concrete.⁴⁴ Additionally, Aslani⁹ noted that the smooth surface of rubber and the poor bonding performance in the interfacial transition zone with the cement paste can easily lead to the formation of microcracks or pores, thus lowering the concrete’s load-bearing capacity.

Figure 6.

Compressive strength.

Figure 7.

Sample failure pattern. (a) OC, (b) RC, (c) RSC1, (d) RSC2, (e) RSC3, and (f) RSC4.

From Figure 6, it can also be seen that as the steel fiber content increases, the compressive strength gradually recovers. For example, at a fiber content of 1.5%, the compressive strength increases to 24.6 MPa, recovering 69.1% of the original strength. When the steel fiber content reaches 2.0%, the compressive strength further increases to 28.9 MPa, recovering 95.1% of the original strength. This is because at this stage, the fibers form a well-distributed network, significantly hindering crack initiation and propagation, making crack growth require more energy. It should be noted that when the steel fiber content increases to 2.5% and 3.0%, the compressive strength decreases to 26.3 MPa and 22.5 MPa, respectively, and the recovery rate drops to 86.5% and 74.0%. This indicates that at high fiber contents, uneven distribution and fiber aggregation effects may reduce the density of the concrete, thus affecting the recovery of compressive strength.

Stress–strain curve

Figure 8 shows the stress–strain curves obtained from the compressive strength test. It can be seen that the stress–strain curve of OC has a steep ascending section, demonstrating high stiffness and load-bearing capacity, with the peak stress being the highest. After failure, the stress decreases rapidly, showing a clear brittle failure characteristic. The stress–strain curve of RC has a more gradual ascending section, with a significantly reduced peak stress. The post-failure stress drop is slow, indicating that the introduction of rubber sand reduces the load-bearing capacity and stiffness of the concrete but improves its plastic deformation ability, making the failure process more gradual.

Figure 8.

Stress–strain curve obtained from the compressive strength test.

Observing the stress–strain curves of RSC1, RSC2, RSC3, and RSC4, it can be seen that the ascending section of the curve becomes steeper, the peak stress increases to varying degrees, and the post-peak curve shows a slower decline. This indicates that the addition of steel fibers effectively enhances the load-bearing capacity of the concrete and significantly improves its ductility. It should be noted that the stress–strain curve of RSC2 shows the highest peak stress, and the curve after failure drops more slowly, demonstrating the best strength and ductility. This suggests that the steel fibers in this sample are more evenly distributed, effectively playing the roles of crack bridging and stress dispersion.

The peak strain can be derived from the stress–strain curve, and the results are shown in Figure 9. It can be observed that the peak strain value of OC is the smallest, indicating that ordinary concrete has limited deformation ability when reaching peak stress, exhibiting typical brittle failure characteristics with poor ductility. The peak strain of RC, although only slightly higher than that of OC, shows that the introduction of rubber slightly improves the deformation capacity of the concrete. This is due to the good elasticity and deformation ability of rubber, which plays a cushioning role during compression and improves the plastic failure characteristics of the concrete.⁴⁵

Figure 9.

Peak strain.

From Figure 9, it can also be seen that the peak strain of the samples containing steel fibers is greater than that of RC, and the peak strain gradually increases with the increase in steel fiber content. This indicates that the introduction of steel fibers effectively suppresses the rapid expansion of cracks, delays the failure process of the concrete, and enhances its plastic deformation ability.

Elastic modulus

The results of the elastic modulus calculation are shown in Figure 10. The results show that the elastic modulus of OC is 17.596 GPa, reflecting the high stiffness of the concrete material and its good resistance to deformation. In contrast, the elastic modulus of RC decreased to 14.024 GPa, a reduction of 20.3%. This indicates that replacing natural river sand with rubber significantly weakened the stiffness of the concrete. This decrease is primarily due to the lower elastic modulus and poor stiffness of rubber, as well as its weak bonding performance at the interface with the cement paste, leading to a reduction in the overall stiffness of the concrete.⁴⁶

Figure 10.

Elastic modulus.

From Figure 10, it can also be observed that steel fibers significantly enhance the elastic modulus of concrete. For example, the elastic modulus of RSC1 is 18.916 GPa, which is a 34.9% increase compared to RC and even slightly higher than OC (a 7.5% increase). This is consistent with results from previous reports.⁴⁷ This result indicates that the introduction of steel fibers effectively improves crack suppression and significantly enhances the deformation resistance of the concrete. When the steel fiber content is 2.0%, the elastic modulus further increases to 26.851 GPa, achieving a recovery rate of 152.6%, which significantly exceeds the elastic modulus of OC. However, when the steel fiber content continues to increase to 2.5% (RSC3) and 3.0% (RSC4), the elastic modulus decreases to 22.540 GPa and 19.690 GPa, respectively. This indicates that at an appropriate fiber content, steel fibers can form a uniformly distributed reinforcement network within the concrete, bridging cracks and dispersing stress, effectively improving the stiffness and deformation resistance of the concrete.

Cyclic compressive performance

Load–strain curve

The load–strain curves obtained from the cyclic compressive test are shown in Figure 11. The results show that OC can withstand 8 cycles of loading, with the curve exhibiting high initial stiffness, but the hysteretic loop area is small. The unloading path is steep, and the rebound effect is poor, demonstrating typical brittle characteristics. The stiffness quickly degrades during cyclic loading, with the peak load reaching 40.954 kN, indicating that OC has high load-bearing capacity but poor ductility. Compared to OC, the RC curve is much flatter, with a relatively larger hysteretic loop area. However, the peak load decreases to 18.533 kN, which is only 45.25% of OC, and the stiffness degradation is more pronounced. The unloading path shows greater deformation and plastic characteristics, indicating that while rubber enhances the ductility and energy dissipation capacity of the concrete, it significantly weakens its load-bearing capacity and compressive strength.

Figure 11.

Cyclic compressive load–strain curves: (a) OC, (b) RC, (c) RSC1, (d) RSC2, (e) RSC3, and (f) RSC4.

The results in Figure 11 also show that RSC1 can withstand five cycles of loading, with the peak load recovering to 27.918 kN. The hysteretic loop area increases further, and the curve shows better energy dissipation performance and ductility. The peak load of RSC2 reaches 36.697 kN, allowing for seven cycles of loading, with a smoother unloading path, demonstrating excellent ductility and damage resistance. RSC3 (2.5% steel fiber) performs the best, with the peak load reaching 44.633 kN, exceeding that of OC. However, the performance of RSC4 declines, with the peak load at 30.842 kN, and the number of cycles it can withstand decreases to 6. The hysteretic loop area also decreases, and the unloading path is steeper, indicating a trend of increased stiffness degradation and reduced plastic energy dissipation capacity. Similar results were obtained by Su and Lin.⁴⁸ This may be due to the uneven distribution or aggregation of high steel fiber content within the concrete, limiting its enhancement effect.^49,50

Loading deformation modulus

Based on the experimental data obtained from cyclic compression, the loading deformation modulus was calculated, and the results are shown in Figure 12. It can be observed that the loading deformation modulus of all samples gradually increased with the increase in the number of cyclic compressions. The initial loading deformation modulus of OC was 1354.918 MPa, and it gradually increased to 8526.5823 MPa by the 9th loading. This indicates that ordinary concrete exhibits some strain hardening during cyclic loading, meaning that as the number of cycles increases, the microcracks within the material are compressed or closed, thereby improving its deformation resistance and stiffness.

Figure 12.

Loading deformation modulus.

The loading deformation modulus of RC is significantly lower than that of OC, reflecting the negative impact of rubber’s low stiffness and the interface weakening effect on the overall stiffness of the concrete. From Figure 8, it can also be seen that the addition of steel fibers has a significant strengthening effect on the loading deformation modulus of rubber sand concrete. Among all the groups, RSC3 has the highest loading deformation modulus, increasing from an initial 2268.0365 MPa to 10,474.691 MPa, indicating that steel fibers significantly enhance the concrete’s resistance to deformation.

In summary, the data on the loading deformation modulus show that both ordinary concrete and rubber concrete exhibit a significant increase in stiffness during cyclic loading, but the introduction of rubber results in a lower stiffness level compared to ordinary concrete. Adding steel fibers can significantly improve the stiffness, with the highest loading deformation modulus observed when 2.5% steel fibers are incorporated.

Dissipated energy

The results of the dissipated energy calculation are shown in Figure 13. The results show that the dissipated energy of OC gradually increases throughout the entire cyclic process. The dissipated energy in the first cycle is 0.00,547 J/m³, and it reaches 0.04,077 J/m³ in the eighth cycle, showing a steady cumulative growth trend. This indicates that ordinary concrete gradually absorbs energy during cyclic loading, with limited plastic energy dissipation capacity. The energy loss is primarily controlled by brittle cracking, and the rate of increase in cumulative dissipated energy is slow.

Figure 13.

Dissipated energy.

In comparison to OC, RC exhibits greater dissipated energy under cyclic loading. For example, in the first cycle, the dissipated energy is 0.01,446 J/m³, approximately 2.6 times that of ordinary concrete, indicating that the introduction of rubber significantly improves the energy dissipation capacity of the concrete.

From Figure 13, it can also be observed that RSC1 demonstrates higher energy dissipation capacity. The dissipated energy in the first cycle is 0.00,435 J/m³, which is slightly lower than RC, but after five cycles, the total dissipated energy reaches 0.02,033 J/m³. This shows that the introduction of steel fibers suppressed crack propagation, improving the ductility and energy dissipation ability of the concrete. However, the enhancement effect was not fully realized with a 1.5% steel fiber content. The dissipated energy in the first cycle of RSC2 is 0.00,225 J/m³, which is slightly lower initially, but as the number of cycles increases, its cumulative dissipated energy reaches 0.03464 J/m³ in the seventh cycle. RSC3 shows good performance in terms of dissipated energy but with a slightly lower enhancement effect compared to RSC2. Although RSC4 exhibits higher dissipated energy, the number of cycles decreases to 6, and the dissipated energy reaches 0.04496 J/m³ in the sixth cycle, which is slightly lower than that of RSC2 and RSC3. This suggests that excessively high steel fiber content may cause fiber aggregation, limiting further improvement in plastic energy dissipation capacity and causing faster stiffness degradation.

In summary, OC has the lowest dissipated energy, exhibiting typical brittle characteristics. RC shows a significant increase in dissipated energy but with faster damage accumulation. Furthermore, reasonably controlling the steel fiber content is crucial for improving the energy dissipation capacity and cyclic loading performance of rubber concrete.

Microstructure

Figure 14 shows the microstructure of the samples from each group. It can be observed that OC displays a typical ordinary concrete matrix structure, primarily composed of the cement matrix, with unhydrated cement particles and air voids dispersed within. Cracks mainly propagate along the interface between the cement matrix and aggregates, indicating that the matrix and interfacial regions are the weak points of the material. Microcracks are densely distributed and are mostly through cracks, revealing the brittle characteristics of OC under loading.

Figure 14.

SEM microstructure. (a) OC, (b) RC, (c) RSC1, (d) RSC2, (e) RSC3, and (f) RSC4.

The microstructure of RC shows a distinct rubber particle distribution. Rubber particles are embedded within the cement matrix, with noticeable pores and gaps at the matrix interface. Cracks predominantly propagate along the interface between the rubber particles and the matrix, highlighting the weakening effect of the interfacial transition zone. In RSC1, long steel fibers are evenly distributed within the matrix and bond well with the matrix interface. The steel fibers play a bridging role during crack propagation, effectively preventing further crack development and dispersing localized stress concentration. Compared to RC, RSC1 shows significant enhancement in crack control and deformation resistance, but the 1.5% fiber content may not have reached an optimal distribution, and its enhancement effect is relatively limited.

The microstructure of RSC2 shows a denser and more uniform distribution of steel fibers, with a stronger bond at the interface. The cracks are clearly separated by the steel fibers, and the crack path becomes more complex and curved. The crack propagation process requires more energy, and the synergy between the fiber-matrix interface significantly improves the concrete’s stiffness and damage resistance. In the microstructure of RSC3, although the steel fibers are still uniformly distributed, there may be fiber aggregation in certain areas. The crack path is interfered with by the steel fibers and propagates more slowly, but localized aggregation could lead to stress concentration.

The microstructure of RSC4 shows a denser distribution of steel fibers, but there is a significant fiber aggregation phenomenon. In some areas, the spacing between fibers is too close, affecting the uniformity of the matrix. Cracks primarily form in the matrix around the fibers and lead to larger cracks in the aggregated regions.

Conclusion

This study systematically analyzed the effects of high-content rubber sand and steel fibers on the workability and mechanical properties of concrete through slump, water absorption, compressive strength, and cyclic compression tests. Based on the experimental results, the following conclusions were drawn:

(1) After replacing 60% of the fine aggregates with rubber sand, the slump of the concrete increased by 4.95%, significantly improving its flowability and workability. This is related to the smooth surface, light weight, and reduced friction between the rubber particles. However, the introduction of rubber sand significantly weakened the density of the matrix, leading to reduced bonding strength in the interfacial transition zone and increased microporosity, which resulted in a 41.4% reduction in compressive strength and a 41.46% increase in water absorption.

(2) When the steel fiber content is 2.0%, the compressive strength recovery rate of the concrete reached 95.1%, showing the best enhancement effect. Appropriate amounts of steel fibers improve the concrete’s stiffness and compressive strength by bridging cracks and improving the interfacial bond. However, when the steel fiber content exceeds 2.0%, the enhancement effect is reduced due to uneven fiber distribution and the aggregation effect.

(3) Although the introduction of rubber sand enhanced the concrete’s energy dissipation capacity, it significantly reduced its load-bearing capacity. After adding steel fibers, the hysteretic energy dissipation of the concrete increased significantly. The maximum dissipated energy occurred in the sample with 2.5% steel fiber content, indicating the significant role of steel fibers in enhancing the seismic and fatigue resistance of rubber sand concrete.

(4) The weak interfaces and pores formed by the rubber sand in the matrix are the primary reasons for the decline in performance. Steel fibers significantly enhanced the overall performance of the matrix by hindering crack propagation and improving the interfacial bond. However, when the fiber content is too high (2.5% and 3.0%), local fiber aggregation leads to stress concentration, reduces the uniformity of the matrix, and weakens the performance.

This study primarily investigated the effects of high-content rubber sand and steel fibers on the workability and mechanical properties of concrete, but certain limitations remain. Firstly, only a single water-to-cement ratio was considered, and the influence of different water-to-cement ratios on rubber sand steel fiber concrete was not explored. Additionally, while this study provided a detailed analysis of compressive strength and cyclic compression behavior, it did not evaluate the tensile strength, which is crucial for pavement applications. Moreover, due to experimental constraints, the long-term durability of the concrete, such as freeze–thaw resistance and sulfate attack under wet–dry cycles, was not assessed. Future studies should investigate the effects of different water-to-cement ratios and include tensile strength and durability tests to comprehensively evaluate the engineering applicability of rubber sand steel fiber concrete.

Footnotes

ORCID iD

Xiuling Chen

Author contributions

Xiuling Chen: Writing: review and editing; Genwang Ge: Funding acquisition and investigation; Yan Zhou: resources and methodology; Suiwei Pan: Writing: original draft; Anqi Ren: Supervision.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Anhui Provincial Department of Housing and Urban-Rural Development Science and Technology Program (2021-YF68); the Anhui Provincial Department of Housing and Urban-Rural Development Science and Technology Program (2021-YF69); the Key Scientific Research Project of Ma’anshan University (QS2022002); the Key Scientific Research Project of Anhui Provincial Universities (2022AH052716); and the Key Scientific Research Project of Ma’anshan University (QS2024003).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.*

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