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Article

Investigation of Fresh, Mechanical, and Durability Properties of Rubberized Fibre-Reinforced Concrete Containing Macro-Synthetic Fibres and Tyre Waste Rubber

by
Nusrat Jahan Mim
1,
Mizan Ahmed
2,
Xihong Zhang
2,*,
Faiz Shaikh
1,
Ahmed Hamoda
3,
Vipulkumar Ishvarbhai Patel
4 and
Aref A. Abadel
5
1
School of Civil and Mechanical Engineering, Curtin University, Kent Street, Bentley, WA 6102, Australia
2
Centre for Infrastructure Monitoring and Protection, School of Civil and Mechanical Engineering, Curtin University, Kent Street, Bentley, WA 6102, Australia
3
Civil Engineering Department, Faculty of Engineering, Kafrelsheikh University, Kafrelsheikh 6860404, Egypt
4
School of Computing, Engineering and Mathematical Sciences, La Trobe University, Bendigo, VIC 3552, Australia
5
Department of Civil Engineering, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(15), 2778; https://doi.org/10.3390/buildings15152778
Submission received: 15 July 2025 / Revised: 2 August 2025 / Accepted: 5 August 2025 / Published: 6 August 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

The growing disposal of used tyres and plastic waste in landfills poses a significant environmental challenge. This study investigates the potential of utilizing used tyre rubber and macro-synthetic fibres (MSFs) made from recycled plastics in fibre-reinforced rubberized concrete (RuFRC). Various percentages of tyre rubber shreds were used to replace coarse aggregates, calculated as 10%, 20%, and 30% of the volume of fine aggregates; fibre dosages (0%, 0.25%, 0.5%, 0.75%, and 1% by volume) were incorporated into the mix, and a series of physical, mechanical, and durability properties were evaluated. The results show that, as the fibre and rubber content increased, the slump of RuFRC decreased, with the lowest value obtained for concrete with 1% fibre and 30% rubber. The density of RuFRC decreases as the rubber percentage increases due to air voids and increased porosity caused by the rubber. The strength properties of RuFRC were found to decline with the increase in the rubber content, with mixes containing 30% rubber exhibiting reductions of about 60% in compressive strength, 27% in tensile strength, and 13% in flexural strength compared to the control specimen. Durability testing revealed that an increased rubber content led to higher water absorption, water penetration, and chloride ion permeability, with 30% rubber showing the highest values. However, lower rubber content (10%) and higher fibre dosages improved the durability characteristics, with water absorption reduced by up to 5% and shrinkage strains lowered by about 7%, indicating better compaction and bonding. These results indicate that RuFRC with moderate rubber and higher fibre content offers a promising balance between sustainability and performance.

1. Introduction

Due to the surge in the need for infrastructure, the production of concrete has significantly increased over the years. It is estimated that, by 2050, 20 billion cubic meters of concrete will be produced globally [1]. This is a rise of 43% compared to the production reported in 2020 [1]. Mainly due to its excellent strength, workability, durability, fire, and thermal resistance, concrete is considered the most-suited construction material for any infrastructure project. However, such a vast demand for concrete also increases the extraction of natural resources, such as natural sands and aggregates, required for concrete production. It is reported that over 50 billion tons of natural aggregates are extracted annually, and this increases by 4.7% per year [2]. Such excessive mining leads to the risk of depletion of these precious natural resources and affects our ecosystem [3]. At the same time, an enormous quantity of waste tyres is generated globally, approximately 1000 million tyres reach the end of their service life every year [4], which is comparable in volume to a substantial fraction of the aggregates used in concrete production. This presents an opportunity to partially substitute natural aggregates with rubber from recycled tyres, thereby addressing both the issue of aggregate depletion and the environmental challenges associated with tyre disposal [5,6,7]. Landfilling such a polymer material made of a large composition of chemicals is a great ecological concern as these chemicals dissolve and contaminate groundwater. Contaminating groundwater not only causes harm to human beings, but it also has a catastrophic impact on wildlife.
Roadside barriers are made of reinforced concrete (RC) and are often heavily reinforced with conventional reinforcement bars to protect the occupants of a vehicle in a collision. However, likewise with many RC structures, roadside barriers throughout the world suffer from corrosion of reinforcement bars, which significantly affects their structural performance. As a sustainable impact-resistant material, rubberized fibre-reinforced concrete (RuFRC), incorporating rubber and macro-synthetic fibres, has been proposed to reduce the required quantity of reinforcement bars and enhance the overall durability of barriers [8,9,10,11]. The inclusion of rubber increases the electrical resistivity of concrete, thereby reducing the likelihood of chloride-induced corrosion of reinforcement [12,13]. Furthermore, the incorporation of fibres helps to control crack propagation and limit permeability, which further restricts the ingress of aggressive agents such as chlorides and moisture [14]. Combined, these properties reduce corrosion risk and extend the service life of roadside barriers. In addition, in barriers, steel reinforcements take the impact load during a collision, thus requiring significant reinforcement. However, the application of RuFRC reduces the need for significant reinforcement as the rubber will absorb impact from the collision, thus providing economic benefits.
There has been a significant amount of research carried out to examine the mechanical and durability of rubberized concrete (RuC). It is well understood that the workability and the mechanical strength of RuC are compromised with the increase in rubber [5,15]. Nonetheless, Bu et al. [16] conducted a comprehensive review highlighting that RuC has a higher electrical resistivity and energy dissipation capacity than normal concrete, thereby enhancing both durability and impact performance. Li et al. [17] reported that partially replacing aggregates with rubber can reduce concrete density by up to 28%, improve freeze–thaw and acid resistance, and deliver substantial impact resilience when the rubber content is kept under 20–30%. Eltayeb et al. [18] and Xiong et al. [19] demonstrated how rubber particles reduce stiffness degradation and enhance strain-rate sensitivity under dynamic loading. However, the existing studies show that crumb rubber, which replaces fine aggregates, exhibited lower energy absorption capacity than shredded rubber due to the effects of particle size [20,21]. While durability studies showed that rubber particles increase the porosity in concrete, thus the shrinkage in rubberized concrete is higher compared to normal concrete [22], the synergy between rubber and fibres in RuFRC helps balance mechanical performance, impact resistance, and durability, making it a promising material for sustainable roadside barrier applications [14,23].
Macro-synthetic fibres (MSFs) are widely used in concrete to improve its mechanical properties. MSFs can be made of virgin or recycled plastics, such as polypropylene (PP), which is mostly used in packaging, stationery, and automotive components. MSFs made of PP offer a lower manufacturing cost and has higher tensile strength and elastic modulus compared to many other synthetic fibres [24,25,26]. MSFs resist initiating cracks and reduce the crack width in concrete, thus improving the strength and durability of concrete [27]. By effectively controlling cracks, fibre-reinforced concrete (FRC) reduces plastic and dry shrinkage of concrete [27]. The post-peak performance of concrete is also improved by adding MSFs to concrete. The research shows that MSFs also improve the toughness and flexural resistance of concrete significantly [28,29,30]. Moreover, recycled PP fibres contribute to sustainability goals by diverting plastic waste from landfills while maintaining effective mechanical performance [31]. Due to their favourable combination of cost-effectiveness, mechanical properties, and environmental benefits, PP-MSFs are particularly suited for applications subjected to high strain rates, such as impact and blast loading. Dopko et al. [32] studied the effects of different dosages of fibres, namely PP, polyvinyl alcohol (PVA), and alkali-resistant glass (ARG), on the flexural performance of FRC. It was found that concrete with ARG fibres exhibited the best performance in terms of toughness and retaining residual strength, followed by PP and PVA fibres. Furthermore, concrete with ARG fibres had the best workability and toughness followed by those with PP and PVA fibres. While ARG fibres demonstrated superior mechanical properties, PP fibres offer a more cost-effective and sustainable alternative, especially when sourced from recycled materials, which aligns with environmentally conscious construction practice.
In this context, fibre-reinforced rubberized concrete (RuFRC) made of shredded rubber sourced from recycled tyres and macro-synthetic fibres made of recycled plastic offers the advantages of both rubberized concrete and fibre-reinforced concrete. RuFRC can be used in applications where structural members are subjected to a high strain rate (e.g., impact load, blast load), such as in roadside barriers and blast-resistant panels, in which both fibres and rubbers will absorb significant energy resulting from dynamic loading. Furthermore, rubberized concrete made of recycled materials also offers a great range of environmental benefits. However, limited research has been conducted to systematically investigate the synergistic effects of varying rubber replacement levels and fibre dosages on the fresh, mechanical, and durability properties of RuFRC. Previous studies have often employed limited or non-systematic combinations of rubber percentages and fibre dosages, hindering the comprehensive understanding of their interactive effects on key performance characteristics. In particular, a detailed parametric study examining how incremental changes in fibre dosage influence the behaviour of rubberized concrete at different rubber replacement levels is still lacking. This study fills this research gap by investigating the performance of RuFRC, exploring the effects of fibre dosages ranging from 0%, 0.25%, 0.5%, 0.75%, and 1%, while rubber shreds replaced 10%, 20%, and 30% of the coarse aggregate. The fresh and mechanical properties of RuFRC, including slump, density, compressive strength, splitting tensile strength, and flexural strength, are examined. The durability of RuFRC is examined by analysing water absorption, water penetration, chloride ion permeability, and drying shrinkage.

2. Test Program

2.1. Details of Materials

Typical class 1 Portland cement complying with ASTM C150/C150M [33] was used. The chemical composition and physical properties of class 1 Portland cement are presented in Table 1. Two types of sand, namely quarry sand and red river sand, were used as coarse sand to achieve a better coarseness than fine sand and used for mixing in equal proportions (see Figure 1a). The fine sand used in this study was markedly available washed sand (see Figure 1b), with a fineness modulus of 2.6, a specific gravity of 2.63, crystalline silica >98%, minerals <2%, approximately 7.4 pH, and relative density between 2.0 and 3.0. The maximum nominal size of coarse aggregates was 10 mm. According to ASTM C33 [34] standards, the specific gravity of the aggregate was 2.69, with a water absorption rate of 0.47%. The typical rubber shreds used are shown in Figure 1c, extracted from recycled tyres and replaced coarse aggregates by 10%, 20%, and 30%. The rubber primarily consists of polymeric compounds (approximately 60–65% by weight), carbon black (typically 20–30%), along with various additives and fillers, including organic and inorganic materials, and possessed a specific gravity below 1. The tyre rubber shreds used in this study were flat and elongated in shape. The surface texture was irregular and rough due to the shredding process. Macro-synthetic fibres (MSF) called ‘eMesh’ fibres, illustrated in Figure 1d, used in this study were made of 100% recycled materials, with a specific gravity of 0.92 and a moisture content of 0.0%. The length of the fibres was 47 mm, with a minimum tensile strength of 348 MPa. Superplasticizers were used to improve the flowability of RuFRC.

2.2. Mix Design and Sample Preparation

This study carried out tests on RuFRC with various fibre dosages and rubber percentages to examine the effects of various rubber percentages for each fibre dose. A total of 13 distinct categories of mix designs were generated by substituting fibre dosages and rubber percentages to evaluate the fresh, mechanical, and durability properties of the concrete. The test program included a control sample (C-0-0) without fibre and rubber. The fibre dosages of the specimens were varied between 0%, 0.25%, 0.5%, 0.75%, and 1% by volume. For each fibre dose, rubber shreds were varied. For all concrete mixtures in this study, a consistent water-to-binder ratio of 0.40 was preserved. In the naming of the specimens, ‘RuC’ stands for the fibre-reinforced rubberized concrete, followed by the fibre dose and rubber percentage. The proportions of all mixtures are detailed in Table 2.
The mixing of concrete followed the procedure specified in AS1012.2 [36]. All aggregates were prepared in saturated dry conditions obtained by soaking for 24 h, followed by air drying for an additional 24 h prior to testing. Their SSD condition was checked following ASTM C127 [37] for coarse aggregates and ASTM C128 [38] for fine aggregates by measuring water absorption and surface moisture. This was to ensure the maintenance of the water/cement ratio developed for the mix design. A water/cement ratio of 0.4 was used for all mixes. In the mixing process, aggregates, fibre, and cement were dry mixed for 2 min before water and superplasticizers were added. The concrete was then mixed for an additional 2 min before conducting slump tests. Slump measurements were taken for each mix according to AS 1012.3.1:2014 [39] within the subsequent 3 min. The samples for mechanical and durability testing were then cast using the prepared moulds. The samples were vibrated using a vibrator table for proper compaction. The samples were tested after 28 days of curing.

2.3. Test Setups

Figure 2 presents the program of the experimental studies. For determining the flowability of RuFRC, the slump test was conducted in accordance with AS 1012.3.1:2014 [39], where AS 1141.4 [40] was followed to determine the density of all the produced mixes.
The compressive strength of the concrete specimens was measured from testing cylinders of 100 × 200 mm, according to AS 1012.9:2014 [41]. The splitting tensile test was performed using 150 mm × 300 mm cylindrical samples subjected to a constant rate of 1.5 MPa per minute until failure, in compliance with AS 1012.10-2000 [42]. The flexural strength of concrete was measured according to AS 1012.11-2000 [43] by testing rectangular samples measured as 400 mm × 150 mm × 150 mm in size with a four-point bending loading. The mechanical testing setup of the RuFRC samples is shown in Figure 3. The results were averaged from testing three samples for each type of test.
To study the durability of RuFRC, the RCPT test was conducted using 50 mm × 100 mm samples cut from 100 mm × 200 mm cylinders as per ASTM C1202 [45], as shown in Figure 4b. The air temperature during testing was kept between 20 °C and 25 °C. The depth of water penetration was measured as per BS EN 12390-8 [46] standards using 10 mm × 200 mm samples tested after 28 days of curing. A water pressure of 500 ± 50 kPa was applied for 72 ± 2 h. After testing, the specimens were wiped, split perpendicularly to the exposed face, and the waterfront depth was marked and measured to the nearest 1 mm. Additionally, the drying shrinkage test was conducted following ASTM C157 [47], using 75 mm × 75 mm × 285 mm samples, spanning from day 1 to day 28. For the water absorption test, samples with a size of 50 mm × 100 mm, as specified in AS 1012.21 [48], were utilized. Figure 4 visually illustrates the setup for all testing procedures.

3. Physical Properties of RuFRC

3.1. Slump

The recorded slump values of RuFRC with various fibre dosages and rubber percentages are presented in Figure 5, along with the control specimen. It can be observed that, with the increase in the fibres and rubbers, the slump of concrete decreased. Nonetheless, almost all the RuFRC batches exhibited reasonable slump making it ideal for construction. However, as expected, the slump value decreased as the rubber percentages increased. The highest slump was found at 140 mm for the control sample and 125 mm for RuFRC with 0.25% fibre and 10% rubber, whereas the lowest one was measured at 62 mm for RuFRC with 1% fibre and 30% rubber, which is around 55.71% lower than the control mix. This reduction is attributed not only to the mechanical friction between the rubber shreds and fibres, which restricts the mobility of the coarse aggregates [27], but also to the hydrophobic surface of rubber particles that hinders proper wetting and lubrication by the cement paste, thereby increasing internal resistance [49]. Rheological studies conducted by Long et al. [49] have similarly reported that the presence of hydrophobic or rough-surfaced rubber particles increases yield stress and plastic viscosity, reducing workability. Increasing either fibre dosages or rubber percentage decreases the slump. The higher dosages of fibres were found to hinder compaction and cause uneven dispersion, which eventually reduces workability as well as leads to voids and lower strength [50]. Although with the increase in fibre dosages and rubber percentages the slumps reduce, a proper vibration method can be used for proper placement and uniformity as suggested by Gettu et al. [51].

3.2. Density

Figure 6 illustrates the density of RuFRC with varying fibre dosages and rubber percentages. The density is influenced by the rubber content. As the rubber percentage increases from 10% to 30%, the density decreases regardless of the fibre content. This reduction is attributed to the higher air content introduced by the rubber, which lowers the overall density of the concrete [52]. These findings align with studies by Di Mundo et al. [53] and Eisa et al. [52], who observed that adding rubber increases the overall porosity and air voids in cement composites.

4. Mechanical Properties of RuFRC

4.1. Compressive Strength

Figure 7 illustrates the compressive strength of and strength variations in different mixes with varying fibre dosages and rubber contents. It can be seen that, as the amount of rubber aggregate increases, the compressive strength of the concrete consistently decreases across all fibre volume fractions. The control specimen exhibited the highest compressive strength of 60.5 MPa, while the most significant reductions were observed at a 30% rubber content, where strength variations ranged from 60% to 62%. This reduction is attributed to the lower stiffness, weaker interfacial bonding, and higher porosity introduced by rubber particles, which negatively affect load transfer within the concrete matrix [54,55,56].
However, the presence of fibres slightly helps to mitigate this reduction by enhancing crack bridging and increasing the toughness of the matrix, especially at lower rubber replacement levels (e.g., RuC-1-10 with 40.50 MPa). At higher rubber contents (30%), the advantages of the fibres become less effective. The percentage decrease in compressive strength for each fibre dose group, due to the increase in the rubber content, was remarkably consistent across different fibre dosages. For example, at a 0.25% fibre dose, the compressive strength reduced by 44%, 46%, and 62% when the rubber amount increased from 0% to 10%, 20%, and 30%, respectively. At 1% fibre dose, the reductions were 33%, 48%, and 61%. The slight variations in compressive strength can be attributed to the poor quality of the fibres, and the weak interfacial transition zone between the fibre and cement matrix as well as the air voids and the cement matrix dilution, as discussed in [57,58].

4.2. Tensile Strength

Figure 8 depicts the tensile strength and variations in strength across different mixes with varying fibre dosages and rubber contents. It can be seen that the RuFRC has a higher tensile strength than C-0-0. Specifically, the RuFRC mixes with 10% and 20% rubber content displayed nearly identical tensile strength values. For example, at a 0.5% fibre dose, increasing rubber content to 10% and 20% resulted in tensile strengths of 3.34 MPa and 3.46 MPa, respectively, which are 11.06% and 14.95% higher than the control mix. Similarly, at a 0.75% fibre dose, the tensile strength increased by 20.35% and 17.34%, while at a 1% fibre dose, the increments were 28.94% and 27.19%, respectively. Although rubber particles themselves have lower stiffness and strength than natural aggregates, their inclusion in moderate amounts (10–20%) in the presence of fibres can contribute positively due to the combined effect of fibres and rubber at the interfacial transition zone (ITZ). The softer rubber particles act as stress absorbers, redistributing stress around crack tips and delaying crack initiation, while the macro-synthetic fibres bridge developing cracks and transfer tensile forces across them. Together, this fibre–rubber synergy enhances the concrete’s ability to resist crack propagation, thereby improving tensile strength despite the inherent weakness of rubber [50,59].
However, with a higher rubber content (30%), the tensile strength significantly decreased. The increased rubber content introduces more air voids and weakens the bond between the rubber particles and the cement matrix, further diminishing the tensile strength [55,59,60]. The lowest tensile strength, 2.73 MPa, was observed for mixes with 0.25% fibre dose and 30% rubber content, which is 9.49% lower than the control sample. The lower fibre dosages and the presence of higher rubber content lead to poor compaction, creating weak zones within the mix. The lower fibre dosages were ineffective in strengthening these weak zones by bridging cracks, thereby lowering the tensile strength [54,55,61]. However, for mixes with fibre dosages higher than 0.25%, the tensile strength was higher than the control mix. For example, the improvement in the tensile strength for specimens with 30% rubber was estimated as 6% and 8% when the fibre dosages were 0.75% and 1%, respectively. This shows that higher fibre dosages effectively bridge cracks and improve the tensile strength of concrete for mixes with a higher rubber content.

4.3. Flexural Strength

Figure 9 depicts the flexural strength and variations in strength across different mixes with varying fibre dosages and rubber aggregates. The data shows that, except for the mix with 30% rubber content, most RuFRC mixes exhibited flexural strengths comparable to or slightly higher than the control specimen. As the fibre dose increased, a general trend of slight improvement in flexural strength was observed, particularly for mixes containing 10% and 20% rubber. For instance, the mix with 0.25% fibre dose and 10% rubber showed a 3.93% increase in flexural strength, while the mixes with 20% and 30% rubber demonstrated reductions of 7.48% and 13.18%, respectively. The trend continued with higher fibre dosages, where the 0.50% fibre dose and 20% rubber content exhibited a 4.37% improvement in flexural strength, whereas the 30% rubber content mix saw a notable reduction of 12.55%. This improvement at 0.50% fibre dosage with 20% rubber can be attributed to the combined effect of fibre–rubber interaction and effective fibre spacing. According to Naaman [62], when fibres are adequately distributed at appropriate dosages, the spacing between them is small enough to efficiently bridge cracks as they initiate, thereby improving load transfer and toughness. In the case of RuFRC with 10–20% rubber, the softer rubber particles help redistribute stresses at the interfacial transition zone (ITZ), while the macro-synthetic fibres bridge developing cracks [14]. This synergy enhances the crack-bridging efficiency and delays crack propagation, leading to better flexural performance. However, as the rubber content increases beyond 20%, excessive void formation and reduced matrix cohesion likely outweigh the reinforcing benefits of fibres, resulting in a drop in strength.
At a 0.75% fibre dose, the mix with 10% rubber had the highest increase in flexural strength (10.33%), whereas the mixes with 20% and 30% rubber content showed only slight reductions. At the highest fibre dose of 1%, the mix with 10% rubber showed a 2.03% increase in flexural strength, but the mix with 30% rubber experienced an 8.30% decrease. These results suggest that a lower rubber content enhances flexural strength, likely due to the fibres’ role in improving crack resistance and matrix toughness. However, as the rubber content increases, the mix’s compaction and bonding properties deteriorate, leading to a reduction in flexural strength. This is particularly evident in mixes with 30% rubber, where poor compaction, higher porosity, and weakened bonding between rubber particles and the cement matrix significantly degrade the flexural strength [52,54].

4.4. Statistical Relationship Among Strength Properties

While the experimental determination of concrete properties can be challenging due to large-scale requirements, analytical relationships offer a useful alternative for predicting properties to aid in finite element analysis and the design of new concrete mixes. In this study, an effort was made to develop empirical equations that can predict the splitting tensile strength based on compressive strength data, accounting for the effects of rubber and fibres in fibre-reinforced concrete. A commonly used simple 0.5 power law model was applied to predict the tensile strength of concrete in this study, which is expressed as follows:
f c t = 0.59 f c '
where f c t and f c ' are the tensile and compressive strength of concrete at 28 days, respectively. This equation was compared with those suggested by different codes, including ACI 318-19 [63], AS 3600-18 [64], and Choi et al. [65], as shown in Table 3.
Figure 10 illustrates the correlation between compressive strength and tensile strength using both experimental data and various code equations. The comparison indicates that the predicted values from AS 3600-18 [64] and the prediction equation closely aligns with the experimental results. In contrast, the equation by Choi et al. [65] tends to underestimate the tensile strength predictions while overestimating the compressive strength in the range from 30 to 45 MPa. Overall, a linear relationship is observed, showing that, as the compressive strength of concrete increases, its tensile strength also rises.
Based on the statistical analysis of the test data, the formula to predict the flexural strength of RuFRC is proposed as follows:
f r = 0.91 f c '
Figure 11 presents the relationship between compressive strength and flexural strength, comparing the experimental results with predictions from various code equations, including ACI 318-19 [63], AS 3600-18 [64], Xu et al. [66], and Perumal [67], as summarized in Table 4. The comparison reveals that ACI 318-19 [63], AS 3600-18 [64], and Xu et al. [66] tend to underestimate the flexural strength when compared to the experimental results. On the other hand, the equation by Perumal [67] initially underestimated the flexural strength up to a compressive strength of 30 MPa. For compressive strength between 30 MPa and 40 MPa, the predictions became closer to the experimental values but started to overestimate it beyond 42 MPa. On the other hand, the proposed model provides a better estimation than other models.

5. Failure Modes

Figure 12 illustrates the failure modes of RuFRC specimens subjected to axial compression. As seen in Figure 12a, the control specimen C-0-0 experienced shear-type failure, where cracks propagated diagonally, leading to a loss of concrete material from the core section. This type of brittle failure is generally observed for concrete with high strength, recorded as 60.5 MPa for the control specimen. Moreover, this type of failure typically occurs when the material’s shear strength is exceeded, causing the concrete to fracture along an inclined plane. On the contrary, RuFRC specimens exhibited a ductile mode of failure, as can be seen in Figure 12. Generally, for RuFRC specimens, the most common failure was small concrete pieces spalling off. For some specimens, some concrete parts were broken, mainly on the top part, exhibiting a compressive type of failure pattern. However, fibres were shown to hold them rather than spall off.
Figure 13 illustrates the failure mode of the specimens subjected to the tensile splitting test, where the specimens fractured into two distinct sections near the central line. This behaviour is typical in splitting tests, where tensile stresses generated during loading lead to a crack formation that propagates through the specimen along the plane of maximum tensile stress. The fracture along the central axis suggests that the material’s ability to resist tension was compromised, which could be due to the reduced interfacial bonding and increased porosity caused by the rubber aggregates in the mix.
When subjected to flexural loading, the RuFRC samples displayed rupture of fibres. This resulted in RuFRC specimens deforming more plastically before failure, displaying greater resistance to fracture due to the presence of fibres, as illustrated in Figure 14. The fibresu in the RuFRC mix play a key role in this behavior, as they enhance the material’s crack-bridging ability, allowing the specimens to sustain higher deformations before failure. Additionally, the presence of rubber aggregates might contribute to this improved ductility, as the rubber particles can absorb some of the stresses, leading to more gradual failure progression rather than brittle fracture. Thus, the combination of fibres and rubber aggregates in RuFRC enhances its toughness and ductility under flexural loading, distinguishing it from conventional concrete, which tends to fail more abruptly.

6. Durability of RuFRC

6.1. Water Absorption

Figure 15 illustrates the rate of water absorption for RuFRC specimens with varying fibre dosages and rubber content. An increase in water absorption as the rubber content increases can be seen, particularly at lower fibre dosages. For example, at a 0.25% fibre dose, the water absorption increases from 4.40% for 10% rubber to 4.99% for 30% rubber. Similar increments are observed for other fibre dosages, with the 30% rubber content consistently showing higher water absorption compared to the control mix (C-0-0), except for the 1% fibre dose where absorption returns to the initial level of 4.2%. The overall trend suggests that a higher rubber content contributes to increased water absorption, primarily due to the introduction of additional voids and the weak interfacial bonding between rubber particles and the cement paste [68,69]. Rubber particles create porous zones and microcracks at the interfacial transition zone (ITZ), which facilitate water ingress [53,70]. Additionally, the uneven mixing of higher amounts of rubber can create weak bonding sites in the concrete, allowing more water to be absorbed [17,71]. The effect of the fibre content on water absorption shows more mixed results. Generally, the water absorption rate slightly fluctuates with different fibre dosages. For example, at 0.25% fibre dose, the water absorption increases from 4.40 for the 10% rubber mix to 4.99 for the 30% rubber mix, but the increase is less pronounced when the fibre dose is higher. At 1% fibre dose, the absorption rates for 10%, 20%, and 30% rubber mixes stabilize at lower values (around 4.2 to 4.4). The presence of fibres generally contributes to slightly reduced water absorption in some cases, possibly due to improved concrete compaction and enhanced bonding [72], though the effect seems overshadowed by the rubber content’s influence on water uptake.

6.2. Drying Shrinkage Test

The drying shrinkage test results of RuFRC specimens are presented in Figure 16. The results show that adding fibre to rubberized concrete significantly improves the drying shrinkage of concrete. Generally, adding rubber to concrete increases the drying shrinkage of the specimens due to the presence of voids and rubber being deformed easily, as discussed in the literature [73,74]. However, the inclusion of MSFs significantly improves the shrinkage behaviour of rubberized concrete. This is because the existing studies show fibres improve the drying shrinkage of concrete due to the reduction in porosity and improve the bonding to the cement paste [52,54,55,59,60]. When observing the test results (Figure 16b), it can be observed that the RuFRC with up to 0.75% fibre exhibited a reduced drying shrinkage compared to C-0-0. This indicates that, up to this dosage, fibres were effective in bridging microcracks, restraining shrinkage strain, and improving the matrix integrity. However, specimens with 1% fibre exhibited a higher shrinkage strain than other specimens and the variability observed at 1% fibre dosage is more pronounced. While RuC-1-10 showed relatively low shrinkage, RuC-1-20 and RuC-1-30 displayed significantly higher values. This inconsistency suggests that, at a higher fibre content, the interaction between the fibres and rubber becomes highly sensitive to fibre dispersion, fibre–matrix bonding, and the extent of void formation. The reason behind this could be due to the fact that the excessive fibre content (1% in this case), combined with rubber particles, likely caused poor bonding within the matrix, leading to weak interfacial zones and the formation of microvoids. As a result, the effectiveness of the fibres was reduced, and higher porosity developed in these samples. A similar result was reported by [75] for fibre-reinforced concrete where 1% fibre exhibited a very similar performance to the control specimen. This suggests a threshold effect, where beyond a certain fibre content, the benefits of crack bridging are offset by fibre agglomeration, poor dispersion, and increased porosity in the matrix.

6.3. Water Permeability Test

Figure 17 shows the rate of water penetration for RuFRC specimens with varying fibre dosages and rubber content. The data reveals a general increasing trend in water penetration as the rubber content increases, regardless of the fibre dose. For instance, the control had a water penetration rate of 2.20, whereas mixes with 30% rubber exhibited higher rates, ranging from 3.1 to 3.7. This increase can be attributed to the hydrophobic and elastic nature of rubber, which disrupts the uniformity of the concrete mix, creating voids and weak points that facilitate deeper water penetration [17,76]. Furthermore, the threads present in the tyre rubber may act as additional pathways for water, exacerbating penetration as the rubber content rises [70].
The effect of fibre content on water penetration exhibited variability depending on the rubber percentage. At lower rubber contents, higher fibre dosages tend to slightly reduce water penetration due to improved crack bridging and densification of the matrix [55,61]. For instance, at 10% rubber, the water penetration rate decreases from 2.95 (RuC-0.25-10) to 2.30 (RuC-0.75-10), demonstrating the positive influence of fibres in minimizing water ingress. However, at higher rubber contents (e.g., 30%), the influence of fibres becomes less pronounced as the rubber content dominates the behaviour, increasing water penetration regardless of the fibre dosage.

6.4. Rapid Chloride Penetration Test

The average charge passed during the RCPT test of RuFRC with varying fibre dosages and rubber content is presented in Figure 18. It can be seen that the rubber aggregate content significantly influences chloride ion permeability, with higher rubber proportions generally resulting in increased permeability. For example, mixes with 30% rubber (e.g., RuC-0.25-30 and RuC-0.50-30) exhibit RCPT values above 5000 C, indicating “high” permeability, according to ASTM classifications. This trend can be attributed to the hydrophobic and elastic nature of rubber, which creates microvoids and weakens the bonding in the concrete matrix, facilitating chloride ion ingress [17,70,77]. Conversely, at lower rubber contents (10%), the chloride ion permeability is significantly reduced, as seen in RuC-0.25-10 and RuC-0.75-10, due to a better balance between rubber-induced flexibility and matrix integrity.
The addition of fibres shows a mixed effect depending on the rubber content. At lower rubber percentages (e.g., 10%), increasing the fibre dose significantly reduces chloride ion permeability, likely due to the fibres enhancing the matrix’s crack resistance and densification [78,79], which limits pathways for chloride ions. However, in mixes with higher rubber content (e.g., 20% and 30%), the influence of fibres is less pronounced, as the rubber-induced voids and weak interfaces dominate the permeability behaviour [55]. For example, RuC-0.25-30 (5046 C) and RuC-0.50-30 (5246.5 C) show high permeability despite the inclusion of fibres, emphasizing that the effect of fibres is overshadowed when rubber disrupts the matrix integrity.
According to ASTM C1202 [45], the charge-passed categories indicate that the control specimen (C-0-0) and most mixes with higher rubber contents fall into the “high” chloride ion permeability range (>4000 C). However, mixes with 10% rubber and moderate to high fibre dosages (1786 C for the RuC-0.75-10 mix and 1975 C for the RuC-1-10 mix) achieve “moderate” (2000–4000 C) or even “low” (1000–2000 C) permeability, reflecting the positive impact of fibres and lower rubber dosages on chloride resistance. On the other hand, mixes with 20% or 30% rubber often exhibit “high” or “moderate” permeability, emphasizing the adverse effect of excessive rubber on the concrete’s resistance to chloride penetration. These results suggest that, while moderate rubber content and appropriate fibre dosage can enhance chloride resistance, excessive rubber undermines the concrete’s durability by increasing porosity and reducing compaction.

7. Conclusions

This study investigates the effects of different combinations of fibre dosages and rubber content on the fresh properties, mechanical performance, and durability of fibre-reinforced concrete (RuFRC). The experimental results were thoroughly analysed, and the key findings are summarized as follows:
  • As the fibre and rubber content increased, the slump of RuFRC decreased, with the lowest value (55.71% reduction from the control) observed in the 1% fibre and 30% rubber mix. This reduction is mainly due to friction and uneven dispersion, though proper vibration can improve workability.
  • The density also decreases as the rubber percentage increases due to higher air voids and porosity introduced by rubber.
  • As the rubber content increased, the compressive strength of RuFRC demonstrated a decline, with the most significant reductions observed at 30% rubber content, where strength dropped by 60–62%. While the fibres slightly improve strength by enhancing crack bridging, their effectiveness diminishes at a higher rubber content (30%). The RuFRC mixes with 10% and 20% rubber content showed flexural and tensile strengths that were either comparable to or slightly higher than the control, with improvements seen as the fibre dosage increased. However, at 30% rubber content, the strength properties decreased compared to mixes with 10% and 20% rubber content due to factors such as poor compaction, higher porosity, and weakened bonding between the rubber particles and the cement matrix.
  • The proposed empirical formulae for the strength properties of RuFRC demonstrated accurate predictions when compared to different code provisions.
  • Rubber aggregate content significantly affects the durability of RuFRC mixes. An increase in rubber content (especially 30%) leads to a higher water absorption, penetration, and chloride ion permeability due to rubber’s hydrophobic nature, void creation, and weak bonding. In contrast, a lower rubber content (10%) and higher fibre dosages improve compaction, bonding, and crack bridging, reducing water absorption and permeability, though fibre effectiveness diminishes with a higher rubber content due to disrupted matrix integrity.

Recommendation and Limitation

This study was only concerned with the investigation of the physical, mechanical, and durability properties of RuFRC. Future studies should focus on the impact performance of RuFRC, such as investigating drop-weight tests, Charpy tests, or split-Hopkinson bar tests. Furthermore, due to the limited data, simplified formulae based on the power law were developed to predict the tensile and flexural strength. However, future studies should focus on developing a multivariate regression model that predicts strength properties as a function of fc′, rubber content, and fibre content. Also, detailed SEM investigations should be performed in future research. Based on the research findings, Table 5 proposes suggestions for the applications of RuFRC.

Author Contributions

Conceptualization, M.A.; Methodology, M.A.; Validation, M.A.; Formal analysis, M.A. and N.J.M.; Investigation, M.A. and N.J.M.; Data curation, M.A. and N.J.M.; Writing—original draft, M.A. and N.J.M.; Writing—review and editing, F.S., X.Z., V.I.P., A.H. and A.A.A.; Visualization, F.S., X.Z., V.I.P., A.H. and A.A.A.; Funding, M.A. and A.A.A.; Project administration, M.A. and X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the Ongoing Research Funding Program (ORF-2025-343), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Acknowledgments

The authors acknowledge the support provided by Flexiroc Australia by supplying the shredded rubber and concrete mix design. The support from Enviromesh, formerly Fibercon, is also acknowledged for supplying the eMesh fibres used in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Materials used in RuFRC. (a) Coarse sand; (b) fine sand; (c) rubber [35]; (d) MSF; (e) sieve analysis.
Figure 1. Materials used in RuFRC. (a) Coarse sand; (b) fine sand; (c) rubber [35]; (d) MSF; (e) sieve analysis.
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Figure 2. Testing program.
Figure 2. Testing program.
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Figure 3. Mechanical test setup of RuFRC specimens [44]. (a) Compressive test; (b) tensile splitting test; (c) flexural test.
Figure 3. Mechanical test setup of RuFRC specimens [44]. (a) Compressive test; (b) tensile splitting test; (c) flexural test.
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Figure 4. Experimental setup. (a) Slump test; (b) RCPT; (c) depth of water penetration; (d) drying shrinkage.
Figure 4. Experimental setup. (a) Slump test; (b) RCPT; (c) depth of water penetration; (d) drying shrinkage.
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Figure 5. Slump of RuFRC mixes.
Figure 5. Slump of RuFRC mixes.
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Figure 6. Density of RuFRC mixes.
Figure 6. Density of RuFRC mixes.
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Figure 7. Compressive strength of and strength variation in RuFRC mixes. (a) Compressive strength; (b) strength variation.
Figure 7. Compressive strength of and strength variation in RuFRC mixes. (a) Compressive strength; (b) strength variation.
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Figure 8. Tensile strength of and strength variation in RuFRC mixes. (a) Tensile strength; (b) strength variation.
Figure 8. Tensile strength of and strength variation in RuFRC mixes. (a) Tensile strength; (b) strength variation.
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Figure 9. Flexural strength of and strength variation in RuFRC mixes. (a) Flexural strength; (b) strength variation.
Figure 9. Flexural strength of and strength variation in RuFRC mixes. (a) Flexural strength; (b) strength variation.
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Figure 10. Accuracy of the design models to predict the tensile strength of RuFRC.
Figure 10. Accuracy of the design models to predict the tensile strength of RuFRC.
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Figure 11. Accuracy of the design models to predict the flexural strength of RuFRC.
Figure 11. Accuracy of the design models to predict the flexural strength of RuFRC.
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Figure 12. Failure modes of samples during compression. (a) C-0-0; (b) RuC-0.25-10; (c) RuC-0.25-20; (d) RuC-0.25-30; (e) RuC-0.5-10; (f) RuC-0.5-20; (g) RuC-0.5-30; (h) RuC-0.75-10; (i) RuC-0.75-20; (j) RuC-0.75-30; (k) RuC-1-10; (l) RuC-1-20; (m) RuC-1-30.
Figure 12. Failure modes of samples during compression. (a) C-0-0; (b) RuC-0.25-10; (c) RuC-0.25-20; (d) RuC-0.25-30; (e) RuC-0.5-10; (f) RuC-0.5-20; (g) RuC-0.5-30; (h) RuC-0.75-10; (i) RuC-0.75-20; (j) RuC-0.75-30; (k) RuC-1-10; (l) RuC-1-20; (m) RuC-1-30.
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Figure 13. Failure modes of RuFRC during tension.
Figure 13. Failure modes of RuFRC during tension.
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Figure 14. Failure modes of RuFRC during flexure [44].
Figure 14. Failure modes of RuFRC during flexure [44].
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Figure 15. Rate of water absorption of RuFRC.
Figure 15. Rate of water absorption of RuFRC.
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Figure 16. Changes in the shrinkage strain of mixes with (a) 0.25% and 0.5% fibre and (b) 0.75% and 1% fibre over the 28-day period.
Figure 16. Changes in the shrinkage strain of mixes with (a) 0.25% and 0.5% fibre and (b) 0.75% and 1% fibre over the 28-day period.
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Figure 17. Water penetration of RuFRC.
Figure 17. Water penetration of RuFRC.
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Figure 18. Rapid chloride penetration test results of RuFRC.
Figure 18. Rapid chloride penetration test results of RuFRC.
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Table 1. Chemical composition and physical properties of the Portland cement used in this study [35].
Table 1. Chemical composition and physical properties of the Portland cement used in this study [35].
Chemical Composition
CaO 64
SiO2 20.2
Al2O3 5.3
Fe2O3 2.7
MgO 1.4
MnO -
K2O -
Na2O 0.6
P2O5 -
TiO2 -
Cl 0.01
SO3
Others 0.25
Loss on ignition (LOI) 2.3
Physical Properties
pHApproximately 12
Specific Gravity2.5 to 3.2
Melting Point>1200 °C
Bulk Density1000–1600 kg/m3
Particle size10–30% of particles are <7 µm
OdourOdourless
Table 2. Normalized weight of materials based on the weight of the cement for 1 m3 volume.
Table 2. Normalized weight of materials based on the weight of the cement for 1 m3 volume.
Specimen IDRubber
(%)		Fibre
(%)		CementFine
Aggregate		Coarse AggregateRubberMSF
C-0-00011.962.040.000
RuC-0.25-10100.2511.961.860.070.0051
RuC-0.25-202011.961.660.15
RuC-0.25-303011.961.500.22
RuC-0.50-10100.511.961.860.070.0102
RuC-0.50-202011.961.660.15
RuC-0.50-303011.961.500.22
RuC-0.75-10100.7511.961.860.070.0153
RuC-0.75-202011.961.660.15
RuC-0.75-303011.961.500.22
RuC-1-1010111.961.860.070.0204
RuC-1-202011.961.660.15
RuC-1-303011.961.500.22
Table 3. Design formulas for calculating tensile strength used to compare with test results reported in this study.
Table 3. Design formulas for calculating tensile strength used to compare with test results reported in this study.
ModelEquation
ACI 318-19 [63] f c t , A C I = 0.62 f c ′
AS 3600-18 [64] f c t , A S 3600 = 0.6 f c ′
Choi et al. [65] f c t , C h o i = 0.55 f c ′
Table 4. Design formulas for calculating flexural strength used to compare with test results reported in this study.
Table 4. Design formulas for calculating flexural strength used to compare with test results reported in this study.
ModelEquation
ACI 318-19 [63] f r , A C I = 0.62 f c ′
AS 3600-18 [64] f r , A S 3600 = 0.6 f c ′
Xu et al. [66] f r , X u = 0.39 ( f c ′ ) 0.59
Perumal [67] f r , P e r u m a l = 0.259 ( f c ′ ) 0.843
Table 5. Suggested applications of RuFRC.
Table 5. Suggested applications of RuFRC.
Region/SiteApplicationRecommendations
Roadside barrierStructural, absorbing impact from collisionsYes
Sound barriersSound-proofing barriersYes
Earthquake-prone zonesAs energy-dissipating materialYes
PavementsNon-structural, impact-absorbing layersYes
Precast elementsStructural, blocks, tilesYes
Structural columns and beamsAs the main load-bearing elementNo, due to the lower compressive strength
Fire-sensitive zonesAs the main load-bearing elementNo, due to the combustible nature of rubber materials
Cold regions with freeze–thaw cyclesStructural elementNo, due to the higher porosity in concrete, durability issues may exist
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MDPI and ACS Style

Mim, N.J.; Ahmed, M.; Zhang, X.; Shaikh, F.; Hamoda, A.; Patel, V.I.; Abadel, A.A. Investigation of Fresh, Mechanical, and Durability Properties of Rubberized Fibre-Reinforced Concrete Containing Macro-Synthetic Fibres and Tyre Waste Rubber. Buildings 2025, 15, 2778. https://doi.org/10.3390/buildings15152778

AMA Style

Mim NJ, Ahmed M, Zhang X, Shaikh F, Hamoda A, Patel VI, Abadel AA. Investigation of Fresh, Mechanical, and Durability Properties of Rubberized Fibre-Reinforced Concrete Containing Macro-Synthetic Fibres and Tyre Waste Rubber. Buildings. 2025; 15(15):2778. https://doi.org/10.3390/buildings15152778

Chicago/Turabian Style

Mim, Nusrat Jahan, Mizan Ahmed, Xihong Zhang, Faiz Shaikh, Ahmed Hamoda, Vipulkumar Ishvarbhai Patel, and Aref A. Abadel. 2025. "Investigation of Fresh, Mechanical, and Durability Properties of Rubberized Fibre-Reinforced Concrete Containing Macro-Synthetic Fibres and Tyre Waste Rubber" Buildings 15, no. 15: 2778. https://doi.org/10.3390/buildings15152778

APA Style

Mim, N. J., Ahmed, M., Zhang, X., Shaikh, F., Hamoda, A., Patel, V. I., & Abadel, A. A. (2025). Investigation of Fresh, Mechanical, and Durability Properties of Rubberized Fibre-Reinforced Concrete Containing Macro-Synthetic Fibres and Tyre Waste Rubber. Buildings, 15(15), 2778. https://doi.org/10.3390/buildings15152778

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