State-of-the-art review on the influence of crumb rubber on the strength, durability, and morphological properties of concrete

Abbreviations

ACI

American concrete institute

ACR

accelerated corrosion resistance

ASTM

American society for testing and materials

CO₂

carbon dioxide

CR

crumb rubber

CRMC

crumb rubber-modified concrete

ECC

engineered cementitious composite

ELT

end-of-life tyre

FTIR

Fourier transform infrared spectroscopy

IS

Indian standard

ITZ

interfacial transition zone

LCA

life cycle assessment

MIP

mercury intrusion porosimetry

MPa

megapascal

OPC

ordinary portland cement

RCM

rapid chloride migration

RCPT

rapid chloride penetration test

SCM

supplementary cementitious material

SD

standard deviation

SEM

scanning electron microscopy

TGA

thermogravimetric analysis

UV

ultraviolet treatment

W/C

water-to-cement ratio

XRD

X-ray diffraction

1 Introduction

Concrete, a vital material in modern infrastructure, supports a wide range of construction projects globally, from residential buildings to large-scale civil structures. However, concrete production is resource-intensive, requiring substantial amounts of cement, water, and aggregates, which contribute to environmental degradation and greenhouse gas emissions [1,2]. Cement manufacturing alone accounts for approximately 7–8% of global CO₂ emissions [3,4]. With urbanization increasing, the construction sector faces the dual challenge of meeting demand while reducing ecological impact [5].

Effective waste management is crucial for sustainable development, particularly regarding discarded tyres, which represent a growing environmental challenge. Incorporating crumb rubber (CR) into concrete not only addresses waste disposal issues but also enhances the material’s performance by improving durability and mechanical properties. CR integration reduces reliance on traditional aggregates, promoting resource efficiency and advancing the principles of a circular economy, aligning with global sustainability goals [6].

The rapid growth of the automotive industry has resulted in a significant increase in the production of passenger vehicles and, consequently, end-of-life tyres (ELTs) [7,8,9,10]. According to the International Organization of Motor Vehicle Manufacturers, the global car count exceeded 81 million by 2022 [11], leading to a surge in discarded tyres. Approximately one billion ELTs is generated annually worldwide [12], with another report estimating around 4 million tyres reaching scrap standards every day [13]. This substantial waste stream requires immediate and sustainable solutions to mitigate its environmental and economic impact. Figure 1 illustrates typical tyre stockpiles, which continue to grow globally due to limited recycling infrastructure and poor disposal practices.

Figure 1

Stockpiling scrap tyres in the environment [14].

In India, around 1.5 million tons of ELTs is generated annually, but only 450,000 tons is recycled [15]. Similarly, the United States has accumulated 3.8 million metric tons of used tyres [16]. Improper disposal, such as landfilling or stockpiling, causes serious environmental concerns, particularly in developing and developed countries alike [17,18,19,20]. Tyres are non-biodegradable, requiring hundreds of years to decompose, during which they release secondary pollutants, such as dioxins and furans, known as “black pollution” [21]. The environmental challenges posed by tyres are amplified due to their composition, which typically includes carbon black, synthetic rubber, natural rubber, fillers, steel, and accelerators [8].

Traditional disposal methods such as landfilling and incineration have proven inadequate. Landfills consume significant space, and tyres, when stockpiled, are prone to fires that release hazardous pollutants [22,23]. Incineration, while recovering energy, results in greenhouse gas emissions and further environmental harm [10]. Addressing this issue necessitates the adoption of sustainable management strategies for ELTs.

Several approaches have been proposed for handling ELTs, such as re-treading, reusing, landfilling, energy recovery, and recycling [24,25]. Recycling tyres to extract valuable raw materials offers a promising alternative to conventional disposal methods. Advanced recycling methods, including pyrolysis, thermolysis, gasification, and mechanical shredding, enable the recovery of resources such as carbon black, oil, steel fibers, and granulate [26,27]. Figure 2 illustrates the management strategies for ELTs [28].

Figure 2

Management strategies for waste tyres at the end of useful life [28].

CR, obtained by processing ELTs, has gained significant attention in civil engineering applications due to its environmental and structural benefits. Mechanical grinding and cryogenic processing are two primary methods for producing CR. Mechanical grinding uses cracker mills to reduce rubber into particles ranging from 5 to 0.5 mm, while cryogenic processing employs liquid nitrogen to freeze tyres, making them brittle and allowing finer particles (4.75–0.075 mm) to be obtained [28,29]. The resulting CR retains elastic and resilient properties, making it suitable for use in construction materials.

Globally, the use of CR in civil engineering reflects a pioneering approach to resource recovery and environmental conservation [30,31]. This innovation significantly reduces waste in landfills while conserving natural resources [32,33]. Additionally, more than 30% of reusable materials such as plastics and scrap tyres are still disposed of in landfills due to poor waste segregation practices, highlighting the need for improved recycling mechanisms [34,35].

The incorporation of tyre-derived materials in construction enhances shock absorption, sound insulation, and durability, particularly in pavements, concrete mixes, and geotechnical applications [19,36]. However, challenges persist, primarily due to the weak adhesion between rubber particles and cement paste, which can reduce mechanical performance [37,38]. Surface treatments using NaOH, silane coupling agents, and other chemical processes have shown promise in improving rubber–cement bonding and mitigating this issue [39,40].

While numerous studies have evaluated individual aspects of CR-modified concrete (CRMC), including its strength, durability, or microstructure, the existing literature remains fragmented, with inconsistent conclusions and limited synthesis across performance domains. There is a growing need to consolidate current research to provide a unified understanding of how CR influences the behavior of concrete as a whole. This review addresses that gap by systematically analyzing the effects of CR incorporation on fresh properties, mechanical strength, durability performance, microstructural and morphological characteristics. It also explores advancements in surface treatment, hybrid systems, and sustainable design strategies.

2 Applications of scrap tyres

The tyre life cycle typically includes five stages: extraction, production, consumption, tyre collection, and scrap tyre management. After collecting ELTs, the next steps involve recovery and landfilling. Landfilling tyres poses significant ecological risks due to hazardous and soluble components that can reduce biodiversity [41]. However, tyre landfilling has been decreasing globally as alternative recovery methods gain popularity. These methods include “energy recovery,” where discarded tyres are used as a substitute for fossil fuels, matching the calorific value of high-quality coal, and “chemical methods” such as thermolysis, pyrolysis, gasification, and granulate recovery. The tyre-shredding procedure is depicted in Figure 3.

Figure 3

Shredding of waste tyre [32,42].

Granulate recovery entails the process of breaking down and reducing tyres into little fragments utilizing large-scale machines. These components can be employed in diverse civil engineering endeavors, including the construction of concrete pavement (specifically pervious concrete), paving blocks, rubberized asphalt pavements, roofing materials, shock-absorbing carpets for playgrounds and sports stadiums, sub-grade fill in highways and embankments, and other geotechnical applications [19,42,43].

The recycling of waste tyre rubber can be accomplished by many methods, as depicted in Figure 4. Mechanical grinding is a process that utilizes shredders, rolling mills, and granulators to break down discarded tyres into rubber particles at room temperature. During the initial shredding phase, the size of these particles varies between 100 and 460 mm in length and 100 and 230 mm in width. During the secondary stage, the particle size is further reduced to around 13 mm using high-speed rotary mills, extruders, and granulators.

Figure 4

Waste tyre shredding: (a) discarded tyre being shredded, (b) large-size tyre shreds, (c) chipped tyre, (d) CR, and (e) fine ground rubber [28,29].

CR is manufactured using two methods: (a) by employing cracker mills at room temperature and (b) through cryogenic processing utilizing liquid nitrogen at temperatures lower than −80°C. The cracker mill operation decreases the size of the rubber particles from 5 to 0.5 mm by running the rubber through grooved steel rollers. Cryogenic processing involves freezing the tyre using liquid nitrogen and then crushing it into small particles that range in size from 4.75 to 0.075 mm.

Utilizing discarded tyres in civil engineering applications offers a practical approach to managing them sustainably and addressing both environmental and economic issues [27,44,45,46]. The incorporation of waste rubber particles into concrete gives two significant advantages: it aids in the reduction of excessive utilization of natural resources and offers a sustainable approach for recycling discarded tyres [47].

Scrap tyres, whether in their intact state or after being processed, have been widely utilized in civil engineering applications [48,49]. Scrap tyres in the industrial sector are subjected to pyrolysis, a thermal treatment technique, to recover valuable resources, including carbon black, oil, and scrap steel from the organic components of the tyres [50].

In civil engineering, scrap tyres are used to improve asphalt in road construction, act as sound barriers, provide insulation under gravel-surfaced roads, serve as lightweight aggregates in concrete, and perform well in the creation of low-noise pavements [51,52,53].

Intact scrap tyres and processed forms have gained considerable interest in geotechnical engineering for their practical applications [54,55]. These applications encompass various uses such as strengthening retaining walls and slopes, stabilizing slopes with both economic and technical advantages, rehabilitating tropical soil slopes, enhancing drainage performance, and reducing settlement and backfill pressure on retaining structures [23,56,57,58].

Recycled tyres have proven to be effective in a range of geotechnical applications. They have been effectively integrated as subgrade materials to hinder the upward movement of water by capillary action and improve drainage systems. Moreover, the use of lightweight backfills has been found to be beneficial in terms of reducing settlement, improving stability, and enhancing drainage properties for retaining structures [59,60].

3 Effect of CR on fresh concrete properties

The incorporation of CR into concrete significantly influences its fresh properties, primarily affecting workability, water demand, and air content. These changes are closely related to the particle size, replacement levels, and inherent properties of rubber, such as its hydrophobic nature and irregular surface texture. While rubberized concrete offers environmental and functional benefits, challenges associated with its fresh properties necessitate strategies for improvement, including the use of supplementary cementitious materials (SCMs) and chemical additives.

One of the most significant effects of CR on fresh concrete is the reduction in workability. The hydrophobic nature of rubber particles hinders their ability to absorb water, causing uneven dispersion within the cement matrix and disrupting the smooth flow of the mix. As the content of CR increases, the reduction in workability becomes more pronounced. Increasing the proportion of rubber in concrete results in a significant reduction in unit weight [61]. This reduction is attributed to the inherently low specific gravity of rubber compared to conventional aggregates [62]. This has been attributed to the creation of voids and capillaries by the rubber particles, which interfere with the packing density and consistency of the mix [63]. Furthermore, larger particle sizes exacerbate this issue due to their greater surface area and irregular shapes, which demand additional water to maintain flowability.

The increase in water absorption is another critical challenge observed in rubberized concrete. The porous nature of rubber particles leads to higher water demand, which in turn reduces workability. The incorporation of SCMs such as silica fume, fly ash, slag, and metakaolin has been found to mitigate these drawbacks [64]. These SCMs not only improve the rheological properties of the mix but also enhance the overall density of the concrete, thereby reducing the adverse impact of rubber particles on water absorption and fresh-state performance.

In addition to reduced workability, the presence of CR increases the air content in concrete. Rubber particles tend to act as nucleation sites for air voids, resulting in a higher volume of entrapped air within the matrix [65]. This increase in air content is attributed to the non-polar nature of rubber, which resists bonding with water and promotes air entrapment during mixing. While excessive air content can compromise the mechanical strength of hardened concrete, it may offer certain benefits, particularly in enhancing freeze–thaw resistance. The uniformly distributed air voids accommodate the expansion of water during freezing cycles, reducing internal stresses and mitigating freeze–thaw damage.

The size and distribution of rubber particles play a critical role in determining their impact on the fresh properties of concrete. Smaller particles are less disruptive to the mix and exhibit better dispersion, whereas larger particles increase the likelihood of segregation and inconsistencies. Optimizing particle size, in combination with chemical admixtures such as superplasticizers, has been shown to alleviate the reduction in workability and improve mix homogeneity [64].

While fresh concrete behavior reflects early handling characteristics, the mechanical performance of CRMC determines its suitability for structural applications. The next section examines how CR content influences compressive, tensile, and flexural strength, as well as energy absorption and elastic modulus.

4 Effect of CR on mechanical properties

The mechanical properties of CRMC are significantly influenced by the level of rubber replacement, particle size, and surface treatment of rubber aggregates. While CR generally reduces the compressive strength and stiffness of concrete due to weak bonding at the interfacial transition zone (ITZ), it substantially enhances toughness, ductility, and energy absorption. These changes make CRMC particularly suitable for applications requiring deformation capacity, crack resistance, and impact absorption.

The reduction in compressive strength is one of the most widely reported drawbacks of incorporating CR into concrete. This decline is primarily caused by the hydrophobic nature of rubber particles, which hinders proper bonding with the cement matrix and disrupts the hydration process. Stress concentration points develop around the rubber particles, leading to premature cracking and reduced load-bearing capacity. Reported compressive strength losses range from 20 to 50% when fine or coarse aggregates are replaced with CR at levels between 10 and 20% [38,66]. A replacement of 15% of natural aggregates with tyre rubber has shown strength reductions of up to 50% [67]. The extent of strength loss is influenced by both particle size and replacement level, with coarse rubber aggregates creating larger voids and discontinuities that weaken the load path.

Predictive equations have been developed relating CR dosage to compressive strength and elastic modulus [68]. While mechanical properties tend to decline with increasing CR, compressive strength reductions of 15–25% have been observed at 5–10% fine aggregate replacement using untreated CR [40]. However, treating rubber particles with sodium hydroxide (NaOH) improved the rubber–cement bond, resulting in a 10–12% recovery in strength. At lower replacement levels, such as 12.5%, the reductions are less significant, and compressive strengths exceeding 30 MPa have been achieved, which is acceptable for non-structural or moderate-strength applications [69].

A comprehensive synthesis of findings from over a dozen studies fitted quadratic regression curves to mechanical performance trends across CR dosages, confirming that compressive and flexural strength decline consistently beyond 20% CR content, highlighting the importance of limiting replacement levels to preserve structural performance [70].

In contrast to compressive strength, CR enhances the flexural strength, toughness, and ductility of concrete, particularly at moderate replacement levels. Rubber particles act as crack arrestors and contribute to energy dissipation during loading, improving resistance to flexural and impact loads. Flexural strength improvements of 15–20% have been reported with the inclusion of rubber fibers [71]. These fibers effectively bridge cracks, prevent propagation, and enhance post-cracking behavior. Surface treatment of rubber aggregates with NaOH has further improved flexural strength by 8–15%, highlighting the benefits of chemical modification [39].

In rubberized mortars, replacing sand with 30% CR has shown a 10–12% reduction in drying shrinkage while maintaining acceptable flexural strength, indicating suitability for applications requiring crack resistance and dimensional stability [72].

The modulus of elasticity of CRMC consistently decreases with increasing CR content due to the lower stiffness and higher elasticity of rubber compared to mineral aggregates. Reductions of 40–50% in elastic modulus have been observed with 25–50% CR replacement [73]. While this limits the applicability of CRMC in stiffness-critical structures, it enhances flexibility and deformability, making it suitable for pavements, seismic-resistant structures, and vibration-damping applications.

Energy absorption and impact resistance of CRMC are significantly improved, offering advantages for dynamic load applications. Rubber particles dissipate impact energy and reduce crack propagation due to their elasticity. Narrower crack widths and higher ductility have been reported at CR levels of 12.5–25% [73]. Hollow blocks with 6.5% CR content demonstrated enhanced energy absorption, sound insulation, and thermal conductivity [74]. Despite some loss in compressive strength, coarse aggregate replacement with CR significantly improves residual strength and toughness under flexural loading [75].

The weak bond between rubber and cement paste remains a concern. Various surface treatments have been explored to improve rubber–cement adhesion. NaOH treatment increases surface roughness and improves compressive and flexural strength by 10–15% [39]. Acid and alkali pretreatments have also been shown to preserve strength and increase ductility of rubber fibers [76]. Combined use of latex- and NaOH-treated rubber with silica fume and fly ash has led to 20–27% improvements in compressive strength and over 40% reduction in chloride ion penetration [77]. Ultraviolet (UV) treatment of rubber aggregates has improved deformability and crack resistance, further validating the potential of physical and chemical modifications [78].

These trends have been confirmed through a recent meta-review compiling data from over 20 studies, which reported average reductions of 50.3% in compressive strength, 24.5% in tensile strength, and 31.9% in flexural strength at high CR levels. The study also noted a ductility increase of over 85% and found that optimal mechanical performance typically occurred at CR contents below 10% and water–cement ratios ≤0.4 [79].

The key mechanical trends observed across studies are synthesized in Table 1, including the effects of dosage, strength decline, and mitigation strategies.

Beyond strength and stiffness, long-term exposure to environmental stressors requires durable concrete systems. The following section explores how CR affects permeability, chemical resistance, freeze–thaw behavior, and fire performance.

5 Effect of CR on durability

The incorporation of CR in concrete significantly influences its durability properties, including resistance to environmental deterioration, water penetration, freeze–thaw cycles, chloride ingress, carbonation, and acid attack. While rubberized concrete often exhibits higher porosity and permeability, advancements in surface treatments and mix design have shown promising improvements in durability performance. These factors of CRMC is a subject of increasing interest for sustainable construction applications.

One of the key findings in durability studies is the improved resistance of rubberized concrete to chloride ion penetration and impact loading. Rubber aggregates are known to create a discontinuous pore structure that hinders the ingress of chloride ions, thereby enhancing durability in chloride-rich environments [62,80]. However, higher rubber content has been shown to increase water absorption and permeability due to the hydrophobic nature of rubber, which interferes with the hydration process and promotes void formation by trapping air and repelling water during mixing (Figure 5) [81]. This trade-off necessitates optimization of replacement levels to balance durability performance with mechanical properties.

Figure 5

Entrapped air bubble in water-submerged CR [82].

Rubberized concrete demonstrates notable improvements in impact resistance and damping capacity, which enhance performance under dynamic and seismic loading conditions [83,84]. It exhibits a higher damping ratio and significant reductions in seismic force transmission, making it suitable for vibration-damping applications. Enhanced flexural toughness and resistance to impact loads are also reported, as rubber particles dissipate energy and prevent rapid crack propagation [84].

The effect of CR on water absorption and carbonation depth has been studied extensively, with results indicating increased porosity at higher rubber contents, which contributes to increased water absorption and carbonation depth [67,85]. However, carbonation levels generally remain within acceptable limits for reinforced concrete. After 150 freeze–thaw cycles, concrete with 25 and 50% CR retained over 86% of its original compressive strength, while 75% CR mixes showed a decline to approximately 76.7% strength retention. This enhanced freeze–thaw durability was attributed to rubber’s ability to slow pore coarsening, reduce macrocrack propagation, and maintain internal matrix integrity during cycling [86]. Rubber ash concrete has also demonstrated excellent chloride resistance [85].

Rubber aggregates have been found to significantly enhance abrasion resistance and permeability, especially in wear-critical applications such as pavements [71]. Although CR generally reduces compressive strength due to its elastic nature, abrasion resistance improves proportionally with rubber content, owing to the material’s ability to dissipate energy under abrasive forces [87]. Despite the strength reduction, the findings support the suitability of CR for applications requiring durability under mechanical wear. However, mix optimization and surface treatments are necessary to balance strength and durability by improving the rubber–cement interface.

Durability performance under acidic conditions has yielded contrasting outcomes. Incorporating 5–20% waste tyre rubber powder as a fine aggregate replacement decreased compressive strength but improved shrinkage resistance and dimensional stability at lower replacement levels. Shrinkage increased by 35% at 5% CR replacement and by 95% at 20% [88]. Under sulfuric acid exposure, rapid formation of ettringite and gypsum was observed, leading to moderate strength losses; however, specimens maintained superior residual performance [89].

Chemical and physical modifications have been shown to improve durability. Acid and alkali pretreatment of rubber fibers enhanced their bond strength with the cement matrix, improving resistance to aggressive environments while preserving ductility [76]. NaOH pretreatment significantly enhanced resistance to water penetration, freeze–thaw cycles, and chloride ingress by strengthening the rubber–cement interface [90]. Aqua-thermal cleaning, a physical method for surface impurity removal, achieved compressive strengths close to control specimens when 10% CR was used, without chemically altering the rubber [91].

In high-performance concrete, the effects of CR vary with particle size and replacement level. Replacing up to 10% of natural sand with rubber particles caused minor reductions in compressive and tensile strength and modulus of elasticity, while enhancing malleability, energy absorption, and vibration resistance. At 15% replacement, strength losses became significant (30–50%). Particle sizes of 0.6 mm or less helped mitigate these losses, and silica fume addition improved load distribution [92].

The use of powdered tyre rubber as partial cement replacement and chipped rubber as coarse aggregate replacement has also been investigated. At 5% cement replacement, compressive strength dropped by less than 5%, while 7.5–10% replacement caused 20–40% strength loss. Chipped rubber increased tensile strength but also water absorption and permeability, whereas powdered rubber reduced absorption due to its finer size [93].

Thermal behavior and fire resistance of rubberized engineered cementitious composites have also been evaluated. Incorporation of CR improved resistance to explosive spalling under elevated temperatures. The addition of nano-silica further enhanced strength, shrinkage resistance, and microstructural stability by densifying the matrix through increased calcium silicate hydrate (C–S–H) gel formation [94]. In a study involving CR concrete blended with waste stone residue, compressive strength losses of 50–60% were reported at 600°C, with structural collapse at 800°C. Nevertheless, the system showed reduced embodied carbon and cost, making it suitable for sustainable, non-load-bearing applications in fire-prone zones [95].

Thermal behavior studies revealed that rubberized concrete lost less weight than conventional concrete when heated to 200°C, and even at 600°C, weight loss remained under 10%, indicating high thermal stability [96]. Rubberized concrete also demonstrated over 400% improvement in frost resistance, 63.5% recovery of lost carbonation resistance, and achieved RCM-II classification for chloride resistance. These results were attributed to enhanced particle elasticity and improved bond characteristics that restricted pore connectivity and delayed crack propagation under thermal and environmental stress [97].

Table 2 summarizes the major durability effects associated with CR incorporation, highlighting dosage sensitivity and mitigation approaches.

Many of the durability issues observed in CR concrete originate at the microstructural level. Therefore, it is essential to understand the morphological changes induced by rubber inclusion and how they influence material behavior under service conditions.

6 Effect of CR on microstructural and morphological properties

The microstructure of CRMC governs the material’s mechanical integrity, durability, and functional performance. The inclusion of rubber aggregates alters the internal morphology by introducing weak interfaces, increasing heterogeneity, and affecting the continuity of the cementitious matrix.

Figure 6 illustrates the SEM image of untreated CR particles at 50× magnification. The image captures two adjacent rubber particles exhibiting contrasting surface textures: one with a smooth, angular surface likely originating from the tyre’s inner layer and the other with a rough, porous surface attributed to the outer layer [98]. The smooth-surfaced particle shows limited mechanical interlocking and reduced wettability, resulting in weak adhesion with the cement paste. Conversely, the porous particle, despite its roughness, contains surface cavities that hinder proper paste penetration, further impairing bond quality. These morphological features confirm earlier findings that attributed the weak CR–matrix bond to particle smoothness and irregular geometry [37].

Figure 6

Scanning electron microscopy (SEM) of untreated CR [98].

The presence of rubber particles increases matrix heterogeneity and porosity, ultimately reducing composite density [99,100]. This degradation in structural uniformity directly compromises compressive strength. SEM and mercury intrusion porosimetry (MIP) analyses reveal that rubber disrupts cement hydration zones, creating weak ITZs characterized by microvoids and incomplete hydration. Compared to conventional concrete, the ITZ in CR concrete is more porous and irregular. Improved particle distribution and bonding in alkali-activated rubberized systems have been shown to enhance matrix homogeneity and offer added thermal and acoustic insulation benefits [46]. These improvements are attributed to rubber’s lower density and elastic behavior, which introduce voids that impair wave transmission and reduce acoustic velocity and stiffness, i.e., the traits that simultaneously improve ductility and energy dissipation.

Further microstructural observations presented in Figure 7 confirm these disruptions. SEM images show CR aggregates embedded in discontinuous cement matrices surrounded by unhydrated paste, as reported in prior studies [101]. This irregular ITZ weakens stress transfer and facilitates crack initiation under loading. However, such a matrix is also more deformable and flexible, enabling improved energy absorption under impact and cyclic conditions.

Figure 7

Rubberized concrete microstructure SEM images [101].

The weak adhesion at the CR–cement interface remains a fundamental challenge in CRMC systems. Poor ITZ bonding contributes to microcracking and early strength loss [85,71]. However, controlled CR dispersion and polymer interleaving have been shown to mitigate these effects, improving ductility and post-crack behavior [71]. Replacing coarse aggregates with rubber particles increases permeability and vulnerability to carbonation and chloride ingress due to enhanced porosity, highlighting the need for optimized particle size and dosage control [67].

Surface treatment has emerged as an effective strategy to address these morphological deficiencies. UV treatment has been shown to improve CR deformability and promote chemical bonding with the cement matrix, thereby reducing void formation at the interface and achieving more homogeneous paste coverage [78]. Similarly, acid and alkali pretreatment techniques have demonstrated significant improvements in CR–cement adhesion, resulting in better microstructural integrity and improved strength retention [76].

Particle size optimization also plays a crucial role in improving matrix quality. Finer CR particles, when uniformly distributed, reduce void size and frequency, thereby enhancing compactness and cohesion [102]. These particles also contribute to higher strength and lower permeability. In a hybrid matrix incorporating steel fibers and porcelain waste, a synergistic structure was observed that improved strength, crack control, and toughness by reducing discontinuities and improving composite integrity [103].

The incorporation of SCMs further refines the microstructure by filling voids and contributing to secondary hydration. In mixes containing fly ash and waste rubber, the use of coupling agents and controlled vulcanization improved the bonding between rubber and hydration products, leading to reduced porosity and enhanced thermal stability [104]. SCMs such as fly ash and silica fume promote dense packing, thus counteracting the adverse effects of rubber on strength and permeability.

The distribution and bonding of CR particles influence not only mechanical and durability properties but also energy efficiency. Alkali activation of rubberized concrete improves rubber–matrix compatibility, thereby enhancing thermal and acoustic insulation performance [46]. These findings underscore the multifunctional potential of CRMC, where microstructural refinement directly contributes to performance enhancement and sustainability.

Table 3 summarizes the critical research gaps identified through this review and outlines focused directions for future investigations. These span structural-scale validation, microstructural quantification, lifecycle modeling, and design code evolution.

The findings discussed across fresh, mechanical, durability, and microstructural domains reveal consistent patterns and gaps. The concluding section synthesizes these insights and identifies strategic research directions for improving CR concrete technology.

7 Conclusions

This review critically synthesized state-of-the-art research on CRMC, evaluating its fresh behavior, mechanical performance, durability characteristics, and morphological evolution. Based on the integrated analysis of the peer-reviewed studies, the following conclusions are drawn:

1. Compressive strength consistently decreases with increasing CR content, primarily due to weak bonding, air entrapment, and elastic incompatibility at the ITZ. Strength losses typically range between 30 and 60% at replacement levels exceeding 15–20% by volume. However, mixes with ≤10–12.5% CR content, optimized for particle size and surface texture, can still achieve compressive strengths above 30 MPa.

2. Elastic modulus experiences greater reduction than compressive strength, often exceeding 40% at moderate CR content. This loss in stiffness is offset by enhanced deformability and ductility, making CR concrete particularly suitable for energy-dissipative applications such as pavements, impact zones, and seismic-resistant components.

3. Flexural strength shows comparatively lower sensitivity to CR addition, with typical reductions of 10–25%. Rubber particles act as crack arrestors and contribute to post-cracking toughness. At controlled dosage levels, CRMC demonstrates improved load redistribution and fracture energy absorption.

4. Durability behavior is highly dosage- and matrix-dependent. Untreated CR increases permeability, sorptivity, and water absorption due to entrapped voids. However, combinations with pozzolanic materials such as fly ash or silica fume refine the pore network, reducing capillary connectivity and enhancing resistance to chloride ingress, abrasion, and freeze–thaw cycles. Treated CR concretes show 30–50% better performance in durability indices compared to untreated counterparts.

5. Microstructural investigations reveal poor hydration and weak bonding around untreated rubber particles, with voids and detached zones dominating the ITZ. Surface-treated CR particles, especially those subjected to alkali activation or physical roughening, promote interfacial adhesion and reduce microcrack density. Particle size optimization (below 0.6 mm) further improves packing density and minimizes heterogeneity.

6. Thermal and acoustic insulation properties are significantly enhanced due to the low thermal conductivity and high damping capacity of rubber. Rubberized mixes show up to 30% lower thermal conductivity and improved sound absorption characteristics. Fire resistance, while limited by rubber decomposition at elevated temperatures, remains acceptable for non-structural applications up to 600°C at low CR dosages.

7. Hybrid mixes incorporating steel fibers, nano-silica, or geopolymeric binders exhibit synergistic behavior, achieving compressive strengths above 35 MPa at moderate CR levels, while simultaneously improving crack resistance, dimensional stability, and impact tolerance.

8. Environmental benefits are tangible, with CR integration reducing the demand for virgin aggregates, diverting tyre waste from landfills, and cutting embodied carbon by up to 25% in optimized SCM-supported mixes. However, lifecycle modeling is essential to fully capture emissions from pretreatment, transport, and service-life durability.

9. Hybrid mixes combining CR with SCMs and nano-modifiers such as silica fume, fly ash, nano-silica, or metakaolin demonstrate strong synergistic effects. These systems deliver enhanced bond strength, pore refinement, and matrix densification, enabling improved structural and durability performance beyond what is achievable by CR or SCMs alone.

As a synthesis of the findings presented across mechanical, durability, and microstructural domains, a performance-based classification framework for CRMC is proposed in Table 4. This classification links rubber dosage levels with compressive strength targets, ductility profiles, durability trends, treatment requirements, and practical structural applications. It is intended to serve as a reference tool for researchers and practitioners seeking to tailor CRMC mix designs based on performance objectives and exposure conditions.

8 Future Scope

To advance the scientific and practical deployment of CR concrete, the following future research directions are proposed:

1. Field-scale validation through long-term trials on structural elements such as slabs, beams, and walls under real environmental and loading conditions is necessary to assess durability and performance retention.

2. Development of performance-based mix proportioning frameworks linking CR characteristics, treatment methods, and supplementary binders to mechanical and durability outputs is essential for practical adoption.

3. Microstructural modeling using advanced techniques such as nanoindentation, MIP, and 3D X-ray tomography can reveal ITZ behavior, hydration patterns, and crack propagation mechanisms, enabling predictive modeling.

4. Systematic investigation of multifunctional composite systems, combining CR with industrial by-products, nanoscale modifiers, and recycled fibers, can enhance strength, toughness, shrinkage control, and resilience.

5. Lifecycle assessment (LCA) studies should be conducted comparing CR concretes to conventional and alternative sustainable concretes, accounting for pre-processing, emissions, durability-driven maintenance, and end-of-life recyclability.

6. Exploration of cost–benefit and techno-economic models for CR concrete production should be prioritized to support large-scale adoption. Parametric optimization of mix designs for different performance targets under budget constraints will accelerate industry uptake.

7. Integration into design codes and standards is vital. Standardization of classification schemes, testing methods, and durability benchmarks will enable wider acceptance by engineers, contractors, and policymakers.

From a techno-economic standpoint, CR incorporation in concrete offers cost offsets through aggregate replacement and landfill diversion, but it introduces additional costs for surface treatment and quality control. Untreated CR may reduce initial mix costs by 5–10%, particularly at low dosages (<10%), due to savings on natural aggregates. However, these gains diminish at higher dosages due to strength loss and the need for compensatory SCMs or additives. Surface treatment processes, such as alkali or acid washing, can add 10–20% to raw material processing costs, but they significantly improve bond strength and mechanical performance, reducing long-term maintenance and premature repair. When rubber is used in hybrid systems with industrial by-products (e.g., fly ash, silica fume), the overall lifecycle cost may improve due to enhanced durability and reduced environmental penalties. In terms of sustainability, optimized CRMC mixes can reduce embodied CO₂ by 15–25% and eliminate the need for natural sand in certain applications, supporting circular economy objectives. The economic feasibility of CRMC thus depends on dosage, treatment approach, and targeted performance class, with the best cost–benefit balance found in low- to medium-dosage mixes with hybrid SCM integration.

Table 5 outlines comparative landscape of existing concrete design standards and their limitations in addressing CRMC systems.