Advancements in Characterization and Potential Structural Seismic Performance of Alkali-Activated Concrete Incorporating Crumb Rubber: A State-of-the-Art Review

Abstract

The production of ordinary Portland cement has had a significant environmental impact, leading to increased interest in sustainable alternatives. This comprehensive review thus explores the performance and applications of rubberized alkali-activated concrete (RuAAC), an innovative material combining alkali-activated concrete with crumb rubber (CR) from waste tires as a coarse/fine aggregate replacement. The study examined current research on the components, physical and mechanical properties, and seismic performance of RuAAC structures. Key findings revealed that CR addition enhances dynamic characteristics while reducing compressive strength by up to 63% at 50% CR replacement, though ductility improvements partially offset this reduction. Novel CR pretreatment methods, such as eggshell catalyzation, can enhance seismic resilience potential. While studies on the structural seismic performance of RuAAC are limited, relevant research on rubberized conventional concrete indicated several potential benefits, highlighting a critical gap in the current body of knowledge. Research on the behavior of RuAAC in full-scale structural elements and under seismic loading conditions remains notably lacking. By examining existing research and identifying crucial research gaps, this review provides a foundation for future investigations into the structural behavior and seismic response of RuAAC, potentially paving the way for its practical implementation in earthquake-resistant and sustainable construction.

1. Introduction

The construction industry has a significant role in global environmental issues, with the production of ordinary Portland cement (OPC) alone contributing to approximately 8% of the world’s carbon dioxide (CO₂) emissions [1]. In response to this concern, researchers have been actively exploring sustainable alternatives to conventional concrete, such as alkali-activated concrete (AAC) [2,3]. AAC, which includes geopolymer concrete (GPC) as a prominent subset, has emerged as a promising solution, utilizing industrial by-products as the primary binder and exhibiting comparable, if not superior, mechanical properties, durability, and resistance under harsh environments to conventional concrete [4]. The successful application of AAC in new and existing structures has been demonstrated, in addition to its potential for other applications, including fire-resistant coatings, thermal insulation, and hazardous waste immobilization [5,6]. By developing and implementing AAC as a sustainable alternative to OPC-based concrete, the construction industry can significantly reduce its carbon footprint while maintaining or even improving the performance and durability of built structures [7].

In addition to the environmental impact of cement production, the growing volume of vehicles on highways and streets in developed and industrialized countries has led to a significant rise in end-of-life tires [8]. Discarded tire numbers are expected to rise to approximately five billion by the end of 2030 due to increasing demand for vehicles worldwide, posing a substantial environmental challenge [9,10]. Over half of these tires are disposed of in landfills, causing environmental and health risks such as fires and chemical leakage into groundwater [11]. To address this issue, researchers have explored incorporating recycled materials, such as waste tire rubber and fibers, as partial replacements for aggregates and reinforcements in concrete, respectively [12]. Crumb rubber (CR) is an effective waste tire rubber aggregate that can be used in concrete [13]. Rubberized AAC (RuAAC) reduces the consumption of raw materials and provides a sustainable solution for waste time management. RuAAC is an innovative and eco-friendly construction material that combines the sustainability benefits of AAC with the enhanced properties provided by the incorporation of CR [14]. By partially replacing natural aggregates in AAC with CR, RuAAC offers a promising solution to the growing environmental concerns associated with waste tire disposal and the depletion of natural resources associated with the production of natural aggregates [15]. This approach is consistent with circular economy principles, where waste materials are repurposed and reintegrated into new products, thereby minimizing their environmental impact and promoting resource conservation [16].

Several previous studies were conducted to investigate the fresh and hardened properties, durability, and potential applications of RuAAC. Fresh properties, including workability and setting time, are essential for successfully implementing RuAAC in construction projects [17,18]. Hardened properties, including compressive strength, flexural strength, and elastic modulus, are critical for assessing the structural performance and serviceability of RuAAC [18,19]. Durability aspects, such as resistance to chemical attack, freeze–thaw cycles, and fire, have also been explored to ensure the long-term performance of RuAAC in various environmental conditions [20,21].

RuAAC exhibits improved impact resistance, energy absorption, and durability compared to rubberized traditional concrete, making it suitable for various applications [22].

Table 1. Comparative overview of key performance characteristics between RuAAC and traditional concrete.

Property	Traditional Concrete	RuAAC	Reference
Environmental Impact	High CO2 emissions	Reduced emissions from replacing cement with waste materials + waste tire utilization	[29]
Energy Absorption	Limited	Significantly enhanced	[13]
Thermal Insulation	Low	Enhanced	[30]
Compressive, Flexural, and Tensile Strength	High	Moderate (trade-off)	[30]
Ductility	Brittle	Enhanced flexibility	[31]
Seismic Performance	Standard	Excellent energy dissipation	[32]

summarizes the key performance differences between RuAAC and traditional concrete. Its enhanced ductility and toughness, attributed to the elastic nature of CR particles, enable high energy dissipation and bridging of cracks, making RuAAC ideal for structures subjected to dynamic loads, such as those in earthquake-prone regions, high-rise buildings, blast-resistant structures, seismic dampers, roads, and airport runways [23]. Additionally, the CR’s low thermal conductivity results in better insulation, allowing RuAAC to be used in energy-efficient buildings [24]. These enhanced properties of RuAAC surpass those of traditional AAC, highlighting its potential uses in many applications. However, incorporating CR in AAC may also result in shortcomings in the mechanical properties, such as reduced compressive strength, flexural strength, and elastic modulus, critical parameters for structural applications [25]. Yet, strategies have been explored to mitigate this drawback, including optimizing CR content, selecting appropriate CR particle sizes, and employing CR pretreatment methods to enhance the compatibility and adhesion between CR and the alkali binder [26]. Additionally, incorporating fibers into AAC enhanced its tensile and flexural strengths, with 1% recycled steel fibers by volume significantly increasing the flexural strength and energy absorption capacity of AAC [27]. Using various types of fibers, such as steel or polypropylene fibers, in AAC enhanced the compressive and tensile strengths, workability, and flexural strength [28]. These findings emphasize the potential of AAC and its combination with CR and fibers to support sustainability in the construction industry.

As the construction industry faces the dual challenges of sustainability and performance, developing and implementing innovative materials such as RuAAC becomes increasingly important. By leveraging the combined benefits of AAC and CR, RuAAC offers a promising option for reducing environmental drawbacks while enhancing the mechanical properties and energy dissipation of concrete structures.

Table 2. Overview of key literature reviews on RuAAC and rubberized concrete development and applications.

Title	Year	Research Objective	Study Description
Scope of reusing waste shredded tires in concrete and cementitious composite materials: A review [11]	2021	To assess the feasibility of adding shredded tire CR waste to concrete and evaluate its environmental impact	Assessed the feasibility of incorporating shredded tire CR in concrete through a critical review of rheological, static/dynamic, mechanical, and durability properties.
Preparation and properties of rubberized geopolymer concrete: A review [12]	2021	To explore the integration of CR into AAC and evaluate its potential benefits	Examined compatibility between CR particles and alkali-activation matrix, highlighting limited studies on alternative aluminosilicate precursors as sustainable FA replacements.
One part alkali activated materials: A state-of-the-art review [33]	2022	To evaluate advancements in one-part AAMs incorporating CR aggregates	Highlighted the need for more comprehensive studies on the structural-level properties of one-part alkali-activated materials (AAMs) when incorporating CR and emphasized the lack of performance-based design standards for one-part AAMs.
Advancements in Characterization and Potential Structural Seismic Performance of Alkali-Activated Concrete Incorporating Crumb Rubber: A State-of-the-Art Review (Current Review)	2025	To examine RuAAC from material science to structural applications with a focus on seismic performance	This study examines RuAAC from material science to structural applications, with an emphasis on seismic performance. It explores recent research, identifies knowledge gaps, and offers recommendations for future studies.

presents an overview of significant literature reviews examining the development and applications of RuAAC and rubberized concrete from 2021, 2022, and 2025 (current review). These reviews show the evolution of research from initial feasibility studies of incorporating CR into concrete and AAC to more comprehensive analyses of structural applications. The earliest review in 2021 focused on fundamental aspects of CR incorporation, while subsequent studies expanded into examining material compatibility and one-part AAM systems. It is shown that the reviews lack in addressing the structural seismic performance. The lessons learned from recent strong seismic events, such as the Kahramanmaraş earthquake that devastated Türkiye and Syria on 6 February 2023, reminded the engineering community of the pressing need to address disaster resilience for a more sustainable built environment in earthquake-prone regions. Thus, the conducted literature review in this study aims to explore the development, properties, and potential applications of RuAAC. By examining the integration of AAC, recycled CR, and fibers, this review explores the advancements in materials that contribute to sustainable construction practices without compromising structural performance. It also discusses the challenges associated with the use of these materials, providing insights into mitigation techniques. The present literature review also highlights the applications of RuAAC, with a specific focus on the potential performance of earthquake-resistant structural members using rubberized concrete materials to be adopted in future research using RuAAC. This study also intends to update the database of RuAAC based on recent research studies for further experimental and numerical investigation. In addition to reporting the research findings, the gaps and limitations of the previous studies, specifically those pertaining to structural engineering applications, are addressed.

2. Methods and Statistics of Literature Review

This comprehensive review was conducted using a systematic approach to ensure a comprehensive examination of the current state of knowledge regarding RuAAC, its properties, and its potential for structural and seismic applications. The review began with a wide collection of relevant documents from Scopus academic databases. The search query used involved the material, testing, failure modes, response, and/or structural member/structure, as follows:

TITLE-ABS-KEY ((“geopolymer concrete” OR “geopolymer*” OR “geopolymer cement” OR “alkali-activated concrete” OR “alkali-activated cement” OR “alkali-activated*” OR “alkali activated*”) AND (“crumb*” OR “rubber*” OR “rubberized*” OR “rubberized concrete”) AND (“compressive*” OR “tensile*” OR “seismic*” OR “dynamic” OR “splitting tensile” OR “punching*” OR “beam*” OR “column*” OR “slab” OR “structure*”)).

This comprehensive literature review follows the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) framework guidelines for systematic review articles [34]. The initial search resulted in 250 documents published between 2009 and 2024, representing a 15-year period when research on this topic first emerged in the scientific literature. To focus on high-quality scientific research, the search was limited to peer-reviewed original research journal articles written in English, with the subject area restricted to engineering. These exclusion criteria yielded a collection of 108 documents, as shown in Figure 1. Following the collection, a screening process was conducted to ensure the relevance of the included studies. Publications were excluded based on criteria such as irrelevance to RuAAC, lack of focus on structural or seismic applications (studies focusing solely on advanced composite material or retrofitting), or inadequate scientific rigor. This screening process resulted in the exclusion of 25 documents, leaving a final selection of 83 documents for in-depth review and analysis.

2.1. Bibliometric Analysis Overview

The data of the bibliometric analysis in

Table 3. Main information obtained from the extracted publications (2009–2024).

Main information about the data	Sources (Journals)	36
	Number of documents	108
	Average citations per document	25
Documents’ contents	Keywords	838
	Author’s keywords	382
Authors’ information	Number of authors	374
	Co-Authors per document	4.47
Authors’ collaboration	International co-authorships %	38
	Single-authored documents	2

offer a concise overview of the literature on RuAAC from 2009 to 2024. With the selected 108 documents from 36 journals, with an average of 25 citations per document, the data demonstrate significant scholarly impact. The high number of keywords (1220 total) reflects the topic’s wide scope. With 374 authors, an average of 4.47 co-authors per document, and 38% international collaborations, the topic reflects strong collaborative research.

2.2. Production and Impact Analysis

Figure 2 illustrates the annual distribution and cumulative production of publications related to RuAAC from 2009 to 2024. The data reveal a clear development in research interest and outcomes in this topic. Initially, research activity was marginal, with only one publication in 2009, followed by a six-year gap until 2016, when another single publication appeared. This scarce start suggests that RuAAC was an unexplored topic with limited focus. However, the year 2018 marked the start of sustained annual publications, with five articles produced. A four-publication output was noted in 2019, indicating a growing interest in the field. A significant acceleration began in 2020 with 8 publications, followed by a further increase to 10 articles in 2021. The most dramatic spike occurs in 2022, 2023, and 2024, with an unprecedented 22, 27, and 30 publications, respectively. This sudden increase in publications reflects concentrated research into RuAAC’s potential, driven by sustainability funding, technological breakthroughs, and novel applications. The timing coincides with escalating rubber waste from tire production, waste that creates environmental hazards through land contamination and toxic burning emissions [35]. RuAAC addresses this crisis by incorporating industrial by-products like fly ash, slag, and rubber into construction materials, embodying circular economy principles [14].

The data for 2024 also show that a high level of research interest is being maintained. The total number of publications illustrates the field’s exponential growth, particularly from 2021 onwards, reaching 108 publications by 2024. This rapid recognition of RuAAC in recent years highlights this increasingly important research topic. The trend suggests that the field is continuously gaining interest, with more researchers contributing to its development. Given the strong research attention, continued growth is anticipated in this research area.

The global research landscape for RuAAC from 2009 to 2024 shows a diverse distribution across continents. Figure 3 reflects the growing importance of RuAAC in future construction practices worldwide. The concentration of publications in Asia is complemented by significant contributions from Western countries like the United States and United Kingdom, as well as notable activity in the Middle East, including the United Arab Emirates (UAE). A few publications have been produced in the UAE [36,37,38]. While modest compared to top producers, the Middle East’s contribution to this research area represents a potential starting point for future expansion, aligning with regions’ interests in innovative and environmentally friendly building practices. The global spread of research involving countries across different continents highlights the widespread interest in RuAAC and suggests opportunities for international collaboration.

2.3. Review Framework

This review follows a systematic approach to evaluate RuAAC’s potential for structural and seismic applications, as illustrated in Figure 4. The presented framework begins from material composition and continues until the testing phase, ensuring a comprehensive assessment of this innovative material. The examination begins with RuAAC components, precursors, activator solutions, rubber aggregates, fibers, and mixing methods, as these components determine material behavior. The review then advances to material testing, examining both physical properties (workability, setting time, absorption) and mechanical characteristics. These properties form the basis for understanding RuAAC’s structural applications potential. The structural dynamic performance section, while crucial for seismic applications, remains limited due to the scarcity of research in this area. Only three main aspects have been investigated: RuAAC-filled steel sections, cyclic stress–strain behavior, and finite element modeling. This limitation highlights a significant research gap, particularly in full-scale structural testing and seismic response evaluation. The presented framework concludes by examining potential structural applications and identifying research gaps, providing direction for future investigations. This organization ensures thorough coverage while emphasizing areas needing further study for practical implementation in seismic-resistant construction.

3. RuAAC Components, Mixing, and Curing

RuAAC is a novel material incorporating CR particles as a partial replacement for natural aggregates in an alkali-activated binder matrix. The main components of RuAAC include aluminosilicate precursors (binders), alkaline activator solutions, admixtures, and CR particles. It may also include different types of fibers as additives. The properties and performance of RuAAC are influenced by the characteristics and proportions of these constituents, as well as the mixing procedure, CR pretreatment methods, and curing conditions employed.

3.1. Precursors

The most used aluminosilicate precursors in RuAAC are fly ash (FA) [39,40,41] and ground granulated blast furnace slag (GGBFS) [13,42,43,44,45], which are industrial by-products rich in silica and alumina.

Table 4. Precursor types used in AAC and their percentages.

Precursor Type	Percentage Range	Primary Role	References
Fly Ash (Class F)	40–100%	Primary binder	[31,39,48,49,61]
GGBFS	10–100%	Primary or secondary binder	[44,50,51,52,53]
Metakaolin	Up to 100%	Primary precursor	[18,47,62,63]
Rice Husk Ash	15–30%	Supplementary precursor	[31,58,64]
Palm Oil Fly Ash	10–30%	Partial precursor replacement	[36,59]
Silica Fume	-	Supplementary precursor	[20,42,65,66]
Calcined Clay	-	Supplementary precursor	[60]
Wood Ash	10–30%	Partial precursor replacement	[25,31,67]

shows the studied precursor types and their percentages used. Metakaolin, a calcined clay, has also been investigated as a precursor [18,46,47]. These materials react with alkaline activators to form an alkali-activated binder that replaces OPC in AAC [19]. Previous studies also utilized various precursors for producing alkali-activated binders, including rice husk ash, palm oil FA, and others, as shown in Table 3. The main precursors and their replacement percentages are FA (Class F), used as the primary binder in many studies, with percentages ranging from 40% to 100% of the total binder content [31,48,49]. In some studies, GGBFS is used as a secondary or primary binder, with percentages ranging from 10% to 100% of the total binder content [44,50,51,52,53]. Many studies have used a combination of FA and GGBFS as precursors to achieve a balance between early strength development, setting time, workability, and long-term performance [54,55,56]. The FA/GGBFS ratio is a key parameter affecting the properties of AAC [54,57]. Various alternative materials, including metakaolin, rice husk ash, palm oil FA, silica fume, calcined clay, wood ash, and oil palm ash, were utilized as precursors in alkali-activated binders at different percentages to optimize mechanical and durability properties [58,59,60].

3.2. Activator Solutions and Chemical Admixtures

Alkaline activator solutions are essential components in the alkali-activation matrix, as they provide the necessary alkaline environment activation for the reactions of aluminosilicate precursors [48]. In the production of alkali-activated materials, the alkaline activators are typically prepared 24 h before mixing. These solutions facilitate the extraction of silica and alumina compounds from the aluminosilicate source materials [68]. The most used activators in RuAAC are NaOH and Na₂SiO₃ solutions [40,69,70]. NaOH is typically available as pellets, granules, or flakes, appearing white and dissolving slowly in water [33,71]. The NaOH molarity ranged from 8 M to 16 M, with 8 M at the lower end [68,69] and 16 M at the higher end [63]. Some studies also used 10 M [48,72], 12 M [50,73], and 14 M [18,57] NaOH solutions.

Na₂SiO₃ can be produced in a highly viscous liquid or solid form [71]. The ratio of Na₂SiO₃ to NaOH (SS/SH) is a crucial parameter that influences AAC’s mechanical properties and microstructure [56,68]. The lowest ratio reported was 0.25 [40], while the highest ratio was 3 [74]. Many studies used a ratio of 2.5 [44,50,52,64]. The alkali activator solution concentration and the components’ ratios significantly affected the properties of the resulting alkali-activated materials [48]. The choice of activator and its proportions varied depending on the desired characteristics and the precursors used in the mixtures. Other activators mentioned include KOH and K₂SiO₃ [75], anhydrous sodium metasilicate [17,65,76,77], calcium hydroxide (Ca(OH)₂) [43], and calcium silicate gel [60], as shown in

Table 5. Activator solution types used in RuAAC.

Activator Solutions	References
Sodium hydroxide (NaOH)	[18,48,50,57,63,68,69,72,73]
Sodium silicate (Na2SiO3)	[40,56,68,71,74]
Potassium hydroxide (KOH)	[75]
Potassium silicate (K2SiO3)	[75]
Anhydrous sodium metasilicate	[17,65,76,77]
Calcium hydroxide (Ca(OH)2)	[43]
Calcium silicate gel	[60]

.

The incorporation of chemical admixtures in RuAAC has been reported in several studies, even with varying dosages and purposes. Superplasticizers, such as polycarboxylate-based and naphthalene-sulfonate-based admixtures, have been used to improve the workability and flowability of fresh RuAAC mixes. For instance, in a study by Abdelmonim and Bompa [51], a polycarboxylate-based superplasticizer was added at 0.5% of the binder mass to enhance the mix’s workability. Retarders, such as barium chloride (BaCl₂), have been employed to control the setting time of RuAAC, which can be rapid due to the high reactivity of the alkaline activators. Zhang et al. [78] reported the use of BaCl₂ at 1% of the total mass of precursors to balance the setting time.

Moreover, alkali activators and chemical admixtures varied widely in different studies, highlighting the importance of their type and dosage to achieve the desired properties of AAC-containing waste CR. Chemical admixtures were incorporated in small amounts into alkali-activated concrete mixes to enhance various properties in fresh and hardened states, including workability, setting time, and strength, as shown in

Table 6. Admixtures and additives types used in RuAAC.

Admixtures and Additives	Primary Role	References
Superplasticizers	Additive	[21,39,50,54,64,68,69,75,79,80]
Barium chloride (BaCl2)	Retarder	[17,57,78,81]
Styrene-butadiene rubber latex	Polymer additive	[82]
Polyvinyl alcohol	Polymer additive	[54]
Ethylene-vinyl acetate	Polymer additive	[83]
Borax	Admixture	[65,76,77,84,85]
NaHCO3, NaCl, C12H22O11	Chemical additives	[83]
Additional water	Additive	[47,63,74,79,86,87]

. Specific admixtures were chosen based on particular construction needs [54,75]. Because optimizing the activator type, concentration, and ratio is essential for achieving desired RuAAC properties, future research should focus on systematically evaluating the effects of various activator combinations and concentrations on RuAAC performance, particularly in the context of incorporating waste crumb rubber.

3.3. Rubber Aggregates and Pretreatment of CR

CR is a material obtained by shredding, cutting, and grinding end-of-life tires, resulting in various sizes and shapes, including CR, rubber chips, and rubber fibers [64]. The specific gravity of rubbers is lower than that of natural aggregates, ranging from 0.04 for expanded polystyrene beads [88] to 1.3 for CR [72]. The sizes of CR particles can vary widely, ranging from fine aggregates of less than 1 mm [66] to coarse particles of up to 20 mm [85].

In the production of RuAAC, recycled waste tire rubber has been used to partially replace natural aggregates, promoting sustainability and enhancing specific properties such as energy absorption and impact resistance [54,64]. Rubber in various forms, including CR, tire rubber, and rubber fibers, has been incorporated into AAC mixes as a partial replacement for aggregates, with replacement percentages ranging from 0.5% to 100% depending on the type of rubber and its application [66,67]. The rubber particle sizes examined in various studies were 0–4 mm, 0.25–4 mm, 1–5 mm, 2.36–4.75 mm, and 5–10 mm [75,89,90,91]. The addition of CR to AAC to produce RuAAC has shown various effects on the material’s properties. Increasing CR content reduced physical and mechanical properties, while improving ductility and energy absorption [48,72,73,92]. The most effective approach for optimizing RuAAC performance involved a combination of moderate CR content (10–20%) and a mix of fine and coarse CR particles. This approach maximizes the benefits of CR inclusion, such as enhanced sustainability and material properties, while minimizing negative impacts on mechanical performance.

While some studies focused on incorporating untreated CR directly into alkali-activated mixes, others have explored the effects of various CR pretreatment techniques on RuAAC’s physical and mechanical properties [80,93,94]. These pretreatments aimed to improve the interfacial bonding between the CR particles and the alkali-activated matrix, thereby enhancing the mechanical and durability properties of the RuAAC mixes [93]. The pretreatment techniques studied are listed in

Table 7. Pretreatment techniques of CR examined in previous studies.

Pretreatment Techniques	References
Water washing	[93,95]
NaOH immersion	[45,56,78,80,93,94,95]
Cement slurry coating	[93,95]
Ultra-fine slag (UFS) paste coating	[93,95]
Thermal pretreatment	[93]
Oxidation and sulphonation	[93]
Sulphuric acid treatment	[93]
Eggshell catalyzation	[75,93]
Geopolymer paste coating	[93,96]

.

One of the standard pretreatment methods for CR particles is soaking them in NaOH solution or water. The duration of this treatment ranges from 30 min to 24 h in several studies, followed by washing with tap water and drying to achieve a saturated surface dry condition [45,80]. NaOH treatment has been found to roughen the surface of CR particles and remove the zinc stearate layer, enhancing the interfacial bonding between CR and AAC paste [95]. In NaOH immersion, CR particles are typically soaked in a 5–10% concentration NaOH solution for a specified duration [45]. The benefits of water washing involve removing surface impurities of CR particles and enhancing their surface roughness, leading to improved bonding with the alkali-activated matrix [93,95].

Cement slurry coating is a pretreatment method that entails coating CR particles with a thin layer of cement slurry. This technique aims to improve the consistency between the CR and alkali-activated matrix, thereby enhancing their interfacial bonding [95]. Ultra-fine slag (UFS) paste coating is similar to cement slurry coating. In this method, CR particles are coated with a layer of UFS paste, which improves compatibility and bonding between the CR and the alkali-activated matrix [93].

Thermal pretreatment subjects the CR particles to elevated temperatures. This process alters their surface properties and increases their surface energy, leading to better adhesion with the alkali-activated matrix [93]. Moreover, oxidation and sulphonation are chemical treatments that modify the surface chemistry of CR particles. These methods introduce functional groups that promote better interaction with the alkali-activated matrix [93]. Additionally, sulphuric acid treatment involves treating the CR particles with sulphuric acid. This process modifies their surface properties and enhances their bonding with the alkali-activated matrix [93].

Eggshell catalyzation uses eggshell powder to treat the CR particles. The eggshell powder acts as a catalyst to improve the interfacial bonding between the CR and the alkali-activated matrix [75,93]. Finally, geopolymer paste coating is a technique that involves coating the CR particles with a layer of fresh geopolymer paste using a planetary ball mill, followed by air curing and oven curing. This coating improves the compatibility and bonding between the CR and the alkali-activated matrix [93,96].

Studies have shown that pretreated CR particles generally lead to improved mechanical properties (e.g., compressive strength, flexural strength, and toughness) and durability of alkali-activated mixes compared to those containing untreated CR [56,93,94,96]. The enhanced interfacial bonding between the pretreated CR and the alkali-activated matrix reduces the stress concentration at the interface, minimizing the likelihood of crack initiation and propagation.

Recent innovations in pretreatment techniques demonstrate even greater potential for performance optimization. Mohana and Bharathi [93] developed a novel eggshell catalyzation method that achieved remarkable 95.7% strength recovery compared to untreated CR, significantly outperforming conventional methods. Their comparative study revealed that while NaOH treatment achieved 23.67% strength recovery and thermal treatment reached 45.16%, the eggshell catalyzation method delivered superior results by making the rubber surface extremely hydrophilic (water contact angle reduced from 134° to 10°). Additionally, the method provided exceptional impact resistance (2.67 times higher than control specimens) while reducing embodied CO₂ emissions by 1.6 kg CO₂/kg. Saloni et al. [95] reported that NaOH treatment achieved the highest compressive strength among pretreatment methods, with specimens reaching 57.6 MPa at 10% CR replacement compared to 53.2 MPa for untreated CR, representing an 8.3% improvement. UFS treatment showed comparable results with 58.2 MPa, while cement paste treatment yielded 55.7 MPa and water treatment achieved 54.5 MPa. In terms of durability, NaOH-treated specimens exhibited superior acid resistance with only 7–18% strength loss under HCl and H₂SO₄ exposure compared to 9–24% loss for untreated CR. Water absorption also improved significantly with pretreatment, reducing from 9.4% (untreated) to 7.4% (NaOH-treated) at 10% CR replacement. These quantitative comparisons confirm that the eggshell catalyzation method and NaOH treatment provide optimal mechanical enhancement while UFS treatment may offer the best overall balance of strength and durability characteristics.

Based on these pretreatment techniques, several methods present distinct scalability profiles for industrial implementation. Laboratory-scale methods such as eggshell catalyzation, oxidation and sulphonation, and sulphuric acid treatment face significant scalability challenges due to specialized chemical handling requirements, complex processing steps, and safety considerations, limiting their use primarily to research applications. In contrast, industrially viable techniques include water washing, NaOH immersion, and cement slurry coating, which can be readily integrated into existing rubber processing facilities with minimal infrastructure modifications. The selection of pretreatment methods should therefore balance performance enhancement with practical constraints of processing complexity, chemical availability, and economic feasibility for successful industrial implementation of RuAAC technology.

3.4. Fibers

Several studies investigated the effects of various fiber types on the properties of RuAAC, as shown in

Table 8. Fiber types, ratios, and specifications investigated in previous studies to improve RuAAC properties.

Type of Fiber	Volume (%)	Length/Diameter	References
Hook-end steel fiber	1	30 mm × 0.75 mm	[13,99]
Hooked-end steel fibers	0.25, 0.5	-	[100]
New hooked-end steel fiber	0.5, 1.0, 1.5	25 mm × 0.5 mm	[70]
Polyvinyl alcohol fibers	0.5, 1.0, 1.5	-	[49,54]
Polypropylene fibers	0, 0.5, 1, 1.5, 2	24 mm × 0.3 mm	[31]
Polypropylene fibers	0.25, 0.5	~32.06 μm	[100]
Recycled steel fiber from tires	0.5, 1.0, 1.5	-	[14,57,101]
Micro steel fibers	0.25, 0.5	~237.8 μm	[100]
Polyethylene fibers	1.75	-	[43]
Waste tire textile fibers	0–0.4	-	[98]
Waste tire steel fibers	0–0.4	-	[98]
Straight steel fibers	2	13 mm × 0.2 mm	[57]
Glass fibers	0.15, 0.30, 0.45	-	[80]
Steel fibers	0.5, 1.0, 1.5	-	[80]
Recycled steel fibers	2	9.92 mm × 0.3 mm	[57]

[43,70,97,98]. Steel, polypropylene, polyvinyl alcohol, and glass fibers were used in these studies. The fiber content ranged from 0.15% to 2% by volume, with lengths varying from micro-scale (32.06 μm) to macro-scale (30 mm). The incorporation of fibers aimed to enhance the mechanical properties and durability of alkali-activated mixes [13]. Combining CR with other materials, such as steel fibers, can help mitigate the adverse effects of CR on the mechanical properties [67]. For instance, steel fibers, known for their high tensile strength, improved RuAAC crack resistance and flexural strength, which are particularly important in applications where load-bearing capacity and resistance to mechanical stress are critical [14]. Incorporating steel fibers at a content range of 0.5% to 1.5% by volume appears to be optimal for mitigating the adverse effects of crumb rubber on mechanical properties while enhancing the ductility and load-bearing capacity of RuAAC. Fiber selection should be prioritized while considering cost-effective options for practical implementation. Recycled steel fibers from tires offer an economical solution, aligning with circular economy principles while providing mechanical enhancement at lower costs than commercial alternatives. This approach maximizes both economic viability and environmental benefits in RuAAC applications. This combination shows promise for applications requiring high structural performance and resistance to mechanical stress.

3.5. Mixing and Curing Approaches

Various approaches were employed to produce RuAAC mixes. The typical mixing procedure involved dry mixing of the solid ingredients, adding the alkaline activator solution, and further mixing until a homogeneous mixture was obtained [101]. Several studies prepared the alkaline activator solution 24 h before mixing [17,43,92,102]. The dry ingredients, including the aluminosilicate precursors, coarse and fine aggregates, and CR, were typically mixed for 2 to 5 min before the addition of the activator solution [18,58,103]. If needed, extra water was added along with the activator solution and mixed for 2 to 5 min [67,96]. In some studies, the binder and alkaline activator were first mixed for 3 to 5 min, followed by adding natural aggregates and then CR, with a total mixing time of around 10 min before casting and compaction [80,101,104]. The fresh RuAAC mixes were poured into prepared molds, often in two or three layers, and compacted on a vibrating table to achieve dense samples [17,92]. After casting, the specimens were typically covered to prevent moisture loss and allowed to set at room temperature for a period ranging from 1 to 24 h before demolding [72,92]. Curing regimes varied among the studies, with some employing ambient curing [17,59] and others using heat curing at temperatures between 60 °C and 90 °C for 24 to 48 h [68,72,80].

The wide variation in mixing and curing approaches adopted in previous studies presents significant challenges for consistent industrial adoption of RuAAC. The observed differences in mixing sequences and durations (2–10 min) and the practice of preparing alkaline activator solutions 24 h in advance may complicate standardized production protocols and just-in-time manufacturing schedules. More critically, the choice between ambient curing and heat curing at 60–90 °C creates a trade-off between production costs and performance consistency, where heat curing accelerates strength development but requires additional energy infrastructure, while ambient curing, though more sustainable, may result in inconsistent strength development due to seasonal temperature variations and extended demolding times [105,106].

The varied curing regimes, ranging from ambient to heat curing, offer flexibility in production while potentially affecting the material’s strength development and microstructure. As research in this field progresses, refining these procedures for specific RuAAC formulations and applications will be essential for maximizing the material’s potential in sustainable construction practices.

4. Physical and Mechanical Characteristics of RuAAC

This section reviews the RuAAC’s physical properties and mechanical characteristics, which allows for a comprehensive understanding of material behavior.

Table 9. Summary of the influencing factors and their effects on the physical properties of RuAAC.

Property	Influencing Factor	Specific Effects on RuAAC	References
Workability and Flowability	CR content	- Generally decreases with increasing CR content - Up to 20% CR replacement may maintain sufficient workability	[18,39,41,44,45,46,51,54,57,61,64,67,74,90,92,99,101,107]
	CR particle size	- Finer CR particles reduce workability more than coarser particles due to higher specific surface area and increased water absorption
	CR surface treatment	- Pretreatment, such as NaOH-treated CR, helps mitigate adverse effects on workability
	Fiber content	- Steel fibers can reduce workability by up to 33% - Combined use of CR and fibers may improve workability compared to fibers alone
	Binder content	- Increasing binder content generally improves workability
	Activator concentration	- Higher activator concentrations tend to reduce workability
	Water-to-binder ratio	- Increasing water-to-binder ratio generally improves workability
Setting Time	CR content	- Initial and final setting times increase with CR content - 10% CR can increase initial setting time by ~37% and final setting time by ~27%	[25,41]
Water Absorption and Porosity	CR content	- Generally increases with increasing CR content	[31,46,47,56,61,64,74,103]
	CR particle size	- Finer CR particles lead to higher water absorption and porosity
	Curing time and temperature	- Longer curing times and higher temperatures decrease water absorption and porosity

and

Table 10. Summary of the influencing factors and their effects on the mechanical properties of RuAAC.

Property	Influencing Factor	Specific Effects on RuAAC	References
Compressive Strength	CR particle size	- Larger CR particles lead to greater strength reduction - 4 mm particles can reduce strength by ~39%, while 0.25 mm particles reduce it by ~11%	[14,18,39,42,44,52,54,68,73,80,88,92,98,102,108,109,110,111]
	CR surface treatment	- NaOH and UFS can improve strength - Eggshell-treated CR showed 95.7% strength recovery compared to untreated CR
	NaOH molarity	- Increasing NaOH molarity from 10 M to 14 M can enhance strength
	Fiber content	- Steel fibers can help counteract strength reduction from CR
	CR content	- Generally decreases with increasing CR content - 30% CR can reduce strength by ~25–63% depending on mix design
Flexural Strength	CR particle size	- Finer CR (0.27 mm, 0.83 mm) particles perform better than coarser ones (1.7 mm, 4 mm) - Finer particles can maintain up to 99% of control mix strength	[13,19,31,39,41,46,50,51,58,61,65,67,70,71,99,112]
	CR surface treatment	- NaOH treatment for 24 h can increase flexural strength by 41%
	Fiber content	- 1% steel fibers + 0.3% glass fibers can increase flexural strength by ~17% in 15% CR mixes
	CR content	- Generally decreases with increasing CR content - 60% CR can reduce flexural strength by up to 46.3% - Some studies report improved strength with up to 10% CR
Splitting Tensile Strength	Fiber content	- 1% steel fibers can significantly improve tensile strength	[19,21,31,36,42,46,49,53,56,65,67,70,71,78,84,90,99,107,108,109,113]
	CR content	- Generally decreases with increasing CR content - 50% CR can reduce splitting tensile strength by up to 52% - Some studies report increased modulus with 5% CR (up to 52.2% increase)
Modulus of Elasticity	CR surface treatment	- NaOH and USF can increase modulus	[14,18,21,36,64,65,76,78,85,90,92,95,96]
	Fiber content	- Steel fibers can compensate for modulus reduction due to CR
	GGBFS content	- Higher GGBFS content increases modulus of elasticity
	CR content	- 60% CR can reduce modulus by up to 79%

provide a comprehensive overview of the key influencing factors and their effects on the RuAAC’s physical and mechanical properties. This table summarizes the findings in this section from numerous studies, illustrating how parameters such as CR content, CR particle size, surface treatment, fiber incorporation, and mix design components impact physical and mechanical properties. A more detailed review of the physical and mechanical characteristics of RuAAC is presented in subsequent sections.

4.1. Physical Properties

The physical properties of RuAAC mortar, including workability, flowability, setting time, water absorption, and porosity, are crucial factors that determine its performance and suitability for various applications. They influence aspects such as processing ease, strength development, and durability.

4.1.1. Workability and Flowability

Several studies investigated the effects of incorporating CR and fibers on the workability and flowability of RuAAC [41,51,61,90]. These properties were typically assessed using standard tests such as slump flow [99], flow table [39], or flow diameter measurements [44]. Multiple studies found that adding CR decreased the workability and flowability of alkali-activated mixtures, as shown in Figure 5. The data scatter reflects variations in precursor types, curing conditions, CR treatment methods, and activator compositions for the same CR content across these different studies. This reduction was attributed to CR particles’ rough surface texture and hydrophobic nature, which increased the friction between the CR and the AAC paste [18,39,46,61,64,74,92,107]. The extent of the workability reduction depended on several factors such as the CR content, particle size, and surface treatment. Finer CR particles tended to reduce workability and flowability more than coarser particles due to their higher specific surface area and increased water absorption [74]. Using treated CR particles, such as NaOH-treated CR, helped to mitigate the adverse effects on workability [45]. The incorporation of steel fibers also influenced the workability and flowability of alkali-activated mixtures. Zhong et al. [101] reported that recycled tire steel fibers reduced the workability by up to 33% compared to the control mix. However, the combined use of CR and steel fibers [101] or polypropylene fibers [67] was found to have an interactive effect, improving the workability compared to using fibers alone.

Previous studies suggested different values for the optimal proportions of CR and steel fibers for better workability and flowability depending on the specific mixture design and target properties. Pradhan et al. [64] indicated that geopolymer mixes with up to 20% CR replacement maintained sufficient workability. Similarly, Hamidi et al. [90] reported that RuAAC with up to 20% CR aggregates retained adequate flowability. For steel fibers, Zhong et al. [101] concluded that combining 5% CR and 1% recycled tire steel fibers provided the best balance between workability and mechanical properties. Other factors affecting the workability and flowability of alkali-activated mixtures included the binder content, activator concentration, and water-to-binder ratio. Increasing the binder content [107] or water-to-binder ratio [74] generally improved the workability, while higher activator concentrations tended to reduce it [46]. Incorporating CR and steel fibers in alkali-activated mixtures can significantly impact their workability and flowability.

To conclude, CR addition generally decreased workability due to its rough texture and hydrophobic nature, with finer particles having a more pronounced effect. Surface treatments like NaOH can mitigate these adverse effects. While steel fibers can reduce workability by up to 33%, combining CR and fibers can improve workability compared to fibers alone. Studies suggest an optimal CR content of around 20% for maintaining adequate workability. Other factors influencing workability include the binder content, activator concentration, and water-to-binder ratio.

4.1.2. Setting Time

The incorporation of CR in AAC can significantly affect its setting time [25]. Chindaprasirt and Ridtirud [41] observed that AAC initial and final setting times experienced a notable increase when CR content was increased (Figure 6). For example, at a replacement level of 10% CR, the initial setting time increased from 60 to 87 min, while the final setting time extended from 105 to 135 min. This prolongation of setting times can be attributed to the water present within the pores of the CR aggregates, which slightly increases the overall water content of the mixture. This prolongation of setting times due to the addition of CR has important implications for construction scheduling and workability. It potentially allows for longer placement and finishing windows but also delays formwork removal and subsequent construction activities.

The incorporation of rubber particles in alkali-activated materials influences their setting characteristics. According to Zhao et al. [17], one-part rubberized geopolymer mixtures exhibited 30% and 50% longer initial and final setting times compared to their two-part counterparts, respectively. This delay was attributed to the additional time required for solid activators to dissolve. The inclusion of a retarder (BaCl₂) further extended these setting times. In terms of absolute values, while the two-part geopolymer showed an initial setting time of 35 min, both one-part rubberized formulations demonstrated extended setting times, reaching 105 min [17]. El-Yamany et al. [82] studied alkali-activated slag systems, noting that rubber incorporation also increased setting times, with higher dosages leading to further delays.

In summary, the incorporation of rubber and the activator type significantly influenced the setting characteristics of alkali-activated concrete. While extended setting times may present challenges for construction scheduling, they offer advantages in terms of workability and placement of windows. Understanding these effects is crucial for optimizing mixture designs and ensuring proper construction practices in RuAAC’s future applications.

4.1.3. Water Absorption and Porosity

Several studies investigated the effect of incorporating CR and fibers on AAC water absorption and porosity [31,56,61,103]. Water absorption and porosity generally increased with increasing CR content due to the formation of more voids and microcracks in the alkali-activated matrix [46,64]. Figure 7 shows the water absorption changes with increasing CR content for various AAC precursor binders. However, adding steel fibers helped to reduce water absorption and porosity by densifying the microstructure [49]. The size of CR particles also influenced water absorption and porosity. Finer CR particles led to higher water absorption and porosity than coarser particles [74]. This is attributed to the higher surface area and water absorption of finer CR. Longer curing times and higher curing temperatures decreased water absorption and porosity in RuAAC mortars [47]. This is likely due to the more complete alkali reaction and denser microstructure formed under these conditions. Reis Ferreira et al. [47] statistically optimized the mix proportions for minimizing water absorption and porosity in RuAAC mortars. The optimal mix contained 1% replacement of sand with fine CR and was cured for approximately six days, resulting in 10.48% water absorption and an 18.58% void index. The best proportions of CR and steel fibers for minimizing water absorption and porosity may vary depending on the mix design and other factors. However, previous studies suggested that low percentages of fine CR (e.g., 1% sand replacement) and moderate dosages of steel fibers or latex (e.g., 1–2%) can provide a good balance of properties.

Generally, CR increased water absorption and porosity, with finer particles having a more pronounced effect. However, these impacts can be mitigated through strategic mix design, including the addition of steel fibers, optimizing CR content and particle size, and employing longer curing times at higher temperatures. Balancing these factors is crucial for developing RuAAC mixtures with acceptable water absorption and porosity characteristics for specific applications.

4.2. Mechanical Properties

Adding CR to AAC consistently decreases mechanical strength, regardless of the specific AAC mix design. While the magnitude of this reduction can vary depending on factors such as the alkali-activated binder composition, rubber particle characteristics, and curing conditions, the overall trend remains consistent. Different alkali-activated mix formulations may experience varying degrees of strength loss, but the general pattern of reduced mechanical properties with increased rubber content holds across different AAC compositions.

4.2.1. Compressive Strength

Compressive strength is one of concrete’s most critical mechanical properties, directly related to its load-bearing capacity and overall structural performance. However, the addition of rubber particles significantly affects the compressive strength of AAC, which is a crucial mechanical property for structural applications. Several factors influence the compressive strength, including the content and size of rubber, the ratio and concentration of activators, and the pretreatment of CR.

(i) Impact of CR Content:

One of the primary factors affecting the compressive strength of RuAAC is the rubber content. Several studies have investigated the influence of incorporating CR on AAC’s compressive strength [14,80,98,109]. The consensus is that increasing the rubber content reduces compressive strength, even to varying degrees, depending on factors such as the type of geopolymer binder, rubber particle size, and curing conditions. Figure 8 illustrates the impact of CR content replacement on compressive strength across multiple studies. The data scatter reflects variations in precursor types, curing conditions, CR treatment methods, and activator compositions for the same CR content across these different studies. A clear trend emerges: as the percentage of CR increases, compressive strength generally decreases. The initial rate of this reduction varies significantly among previous studies, revealing two distinct patterns of strength reduction. Some studies, such as Sarkaz et al. [73], demonstrate a sharp initial drop in compressive strength. The data of the latter study show a dramatic decline from about 70 MPa to 30 MPa with just a 10% rubber replacement. However, other studies like Alsaif et al. [18] and Ipek et al. [39] indicated a more gradual initial strength reduction. These studies show a less severe reduction in strength over the same 0–10% replacement range, indicating that their concrete mixtures may be more tolerant to initial rubber inclusion. However, the rate and extent of this strength reduction vary among the previous studies, likely due to differences in mix designs or materials used.

Numerous specific studies across various types of RuAAC mixtures further support this general trend of decreasing compressive strength with increasing CR content. Saloni et al. [92] found that incorporating 30% CR as a replacement for fine aggregates in alkali-activated low-calcium FA concrete resulted in a 25% reduction in 28-day compressive strength compared to the control mix. Similarly, Alsaif et al. [18] reported a 63.2% decrease in compressive strength when 50% of fine aggregates were replaced with CR in metakaolin-based geopolymer mortars. A study by Elzeadani et al. [65] reported that 15%, 30%, 45%, and 60% CR replacement reduced the 28-day cylinder strength by 47.7%, 74.4%, 81.7%, and 86.4%, respectively, compared to the reference mix.

(ii) Impact of CR Particle Size:

Moreover, increasing the size of rubber particles leads to a more pronounced reduction in compressive strength. Kuang et al. [114] investigated the high-temperature performance of rubberized geopolymer mortar prepared with different rubber particle sizes (4 mm, 1.7 mm, 0.83 mm, and 0.25 mm). The compressive strength decreased with increasing rubber particle size. At ambient temperature, the compressive strength decreased by approximately 39%, 24%, 23%, and 11% for 4 mm, 1.7 mm, 0.83 mm, and 0.25 mm rubber particles, respectively, at 5% replacement of river sand mass [114]. Kuang et al. [74] also investigated the influence of CR particle size (4 mm, 1.7 mm, 0.83 mm, and 0.25 mm) on the compressive strength of slag-based geopolymer mortar exposed to high temperatures. The results indicated that larger rubber particle sizes resulted in lower compressive strengths due to decreased gap-filling ability and less compact internal structure [74]. Figure 9 shows the reduction in compressive strength with increasing the CR particle size, though the magnitude and rate of this decrease vary. Sarkaz et al.’s [73] study demonstrates a notable strength reduction, starting at around 43 MPa for ≤1 mm particles and dropping to about 29 MPa for 4 mm particles. Kuang et al.’s studies show remarkably similar trends, beginning with much higher compressive strengths (approximately 80 MPa) for the smallest particles, followed by a sharp initial decrease to about 70 MPa at 0.4 mm, and then a more gradual decline to around 50 MPa for 4 mm particles. Notably, the rate of strength reduction is not linear across all studies, being steeper for smaller particle sizes and leveling off for larger ones. These findings suggest that while incorporating CR reduces concrete compressive strength, the extent of this reduction is influenced by particle size, with smaller particles having less impact than their larger counterparts.

(iii) Mitigation of CR Adverse Effects:

Several mechanisms contribute to the reduction in compressive strength when CR is incorporated into AAC, including the following [29,65,78,108]:

• Rubber particles’ lower stiffness and higher deformability compared to natural aggregates decrease the composite’s overall stiffness;
• Weak interfacial bonding between rubber particles and the geopolymer matrix leads to stress concentrations and early crack initiation;
• The hydrophobic nature of rubber particles can entrap air and increase the porosity of the geopolymer matrix, reducing its density and strength;
• Replacing stiffer natural aggregates with softer rubber particles alters the stress distribution within the matrix, resulting in a less efficient load transfer mechanism.

The surface treatment of rubber particles has been explored to mitigate the adverse effects on compressive strength [94]. Pretreatment techniques aim to improve the interfacial bonding between the rubber particles and the geopolymer matrix, which is crucial for the composite’s mechanical properties (Table 7). Mohana et al. [93] developed a method for rubber surface modification using eggshell catalyzation. The eggshell-treated CR geopolymer mortar exhibited a 95.7% strength recovery compared to the untreated CR mortar, attributed to the formation of secondary calcium silicate hydrate (CSH) gels and improved interfacial bonding [93]. Saloni et al. [95] investigated five different pretreatment methods applied to CR before incorporating it into alkali-activated slag mortars at replacement levels of 10%, 20%, and 30% of fine aggregates by volume. The pretreatment methods included no treatment, water treatment, NaOH treatment, cement paste treatment, and ultrafine slag treatment. The results showed that the NaOH and UFS treatments were the most effective in improving the compressive strength of rubberized AAS mortars. The mixes containing UFS-treated CR showed the lowest reduction in strength, with a 7% reduction at 10% CR after 28 days. Conversely, the mixes without pretreatment experienced the most significant reduction, with a 35% decrease at 30% CR after 28 days. The study attributed the strength improvement to the increased surface roughness and reactivity of CR particles after NaOH and UFS treatments, leading to better bonding with the matrix.

Although the compressive strength generally decreased with increasing CR content and particle size in AAC, strategies such as using finer rubber particles, surface treatments, or additives can mitigate the strength reduction to a certain extent [78]. For example, adding steel fibers or optimizing the geopolymer binder composition can help counteract the adverse effects of rubber on compressive strength [115].

4.2.2. Flexural Strength

(i) Impact of CR Content:

Flexural strength is a critical mechanical property of concrete, reflecting the material’s ability to resist bending stress. Like compressive strength, RuAAC’s flexural strength declined as the percentage of CR aggregate replacement increased [71]. Figure 10 illustrates the impact of CR replacement on flexural strength across multiple studies. The data scatter reflects variations in precursor types, curing conditions, CR treatment methods, and activator compositions for the same CR content across these different studies. The general trend shows a decrease in flexural strength as the percentage of CR increases, with most studies exhibiting a steeper decline within the 0–20% replacement range. The steepness of the flexural strength decline varies considerably between studies. Some studies, such as Alsaif et al. [18] and Sagir et al. [42], show a plateau effect at higher replacement percentages, while others indicate a continued reduction in flexural strength. Sagir et al.’s [42] study stands out with the highest initial strength and most dramatic strength reduction.

Several studies have investigated the factors influencing the flexural strength of RuAAC, particularly when incorporating waste materials such as CR [13,19,31,50,58]. In a study by Elzeadani et al. [65], the flexural strength of RuAAC decreased with increasing CR content. Compared to the control mix, the 28-day flexural strength values decreased by 3.4%, 12.8%, 33%, and 46.3% for mixes containing 15%, 30%, 45%, and 60% CR, respectively. The reduction in flexural strength was attributed to the lower stiffness and strength of rubber particles compared to natural aggregates and the weak interfacial bonding between rubber and the geopolymer matrix. Similarly, Obeidy et al. [46] reported a reduction in flexural strength with increasing CR content in AAC. The 28-day flexural strength values of mixes with 10%, 20%, and 25% coarse aggregate replacement by CR were 3.75, 3.29, and 3.34 MPa, representing reductions of 6.25, 12.75, and 16.5%, respectively, compared to the control mix. The authors attributed the strength loss to the weak adhesion between the rubber particles and the geopolymer matrix, as well as the high Poisson’s ratio of rubber, which could cause premature bond failure under flexural loading. Sagir et al. [42] also reported a reduction in flexural strength with increasing waste rubber content in slag-based geopolymer mortars. Compared to the control mix, the 28-day flexural strength decreased by 6.4–27.39% for mixes containing 5–15% waste rubber. However, the flexural strengths of all mixes remained above 4.5 MPa, indicating their potential suitability for various applications. Interestingly, Arunkumar et al. [67] found that adding CR increased the flexural strength of FA-slag-based RuAAC. Compared to the control mix, the 28-day flexural strength increased by 1.85% and 3.19% for mixes with 0.5% and 1% rubber replacement, respectively. The study attributed this improvement to the enhanced ductility and crack-bridging effect provided by the rubber particles. However, further increasing the rubber content to 1.5% and 2% reduced flexural strength.

(ii) Impact of CR Particle Size:

The size of CR particles also influences the flexural strength of RuAAC [74]. In a study by Kuang et al. [114], mixes with finer CR (0.27 mm and 0.83 mm) exhibited higher flexural strengths than those with coarser rubber (1.7 mm and 4 mm). The 28-day flexural strengths of mixes with 0.27 mm and 0.83 mm rubber were 99% and 93% of the control mix, respectively, while mixes with 1.7 mm and 4 mm rubber had flexural strengths of 87% and 75% of the control mix. The better performance of finer rubber particles was attributed to their improved bonding with the geopolymer matrix and the ability to control crack propagation. In addition to the particle size, the surface texture and morphology of the rubber particles can also influence the flexural strength. Deng et al. [109] reported that rubber particles with rough surfaces and irregular shapes led to better mechanical interlocking and improved bonding with the geopolymer matrix, resulting in higher flexural strength than smooth and rounded particles.

(iii) Mitigation of CR Adverse Effects:

Several strategies have been proposed to mitigate the negative impact of CR on flexural strength. One approach combines CR and fibers, such as steel or polypropylene fibers. Gill et al. [80] demonstrated that adding 1% steel fibers and 0.3% glass fibers to AAC with 15% CR increased flexural strength by around 17% compared to the mix with 15% CR and no fibers. The fibers help to bridge cracks and provide post-cracking resistance, thereby compensating for the weakening effect of CR. Pretreatment methods aim to improve the interfacial bonding between the rubber particles and the geopolymer matrix, which can enhance mechanical properties, including flexural strength [90]. One common pretreatment method is soaking the CR particles in NaOH solution [80,94]. In a study by Ameri et al. [45], the flexural strength of alkali-activated mortar incorporating NaOH-treated CR was evaluated. The results showed that NaOH treatment for 1 h increased the 28-day flexural strength by 28% compared to untreated rubber at 15% replacement. Extending the treatment time to 24 h improved flexural strength, resulting in a 41% increase. The enhanced flexural performance was attributed to the improved interfacial bonding between the treated rubber particles and the geopolymer matrix. Pretreatment and surface treatment methods have been shown to enhance the flexural strength of RuAAC by improving the interfacial bonding between the rubber particles and the geopolymer matrix [92].

The incorporation of CR in AAC generally decreases flexural strength, with reductions up to 46.3% at 60% CR content. However, some studies report slight increases (1.85–3.19%) at low CR contents (0.5–1%). Finer CR particles (0.27–0.83 mm) perform better than coarser ones (1.7–4 mm), maintaining up to 99% of control mix strength. Mitigation strategies include combining CR with steel or glass fibers, which can increase flexural strength by 17%, and NaOH pretreatment of CR, improving strength by up to 41%.

4.2.3. Splitting Tensile Strength

RuAAC’s splitting tensile strength also declined as the percentage of CR aggregate replacement increased [42,46,84,107]. Several studies have investigated the factors influencing the splitting tensile strength of RuAAC, particularly when incorporating waste materials such as CR [19,31,49,107,109]. In a study by Al-Fasih et al. [108], the splitting tensile strength of RuAAC decreased with increasing CR content. At 28 days, the splitting tensile strength decreased from 4.62 MPa for the control mix (0% CR) to 3.48 MPa, 2.88 MPa, 2.80 MPa, 2.65 MPa, and 2.22 MPa for mixes with 10%, 20%, 30%, 40%, and 50% CR replacement, respectively. This represents a reduction of up to 52% in splitting tensile strength at 50% CR content compared to the control mix. Similarly, Obeidy et al. [46] reported that the splitting tensile strength of RuAAC decreased with increasing CR content. The 28-day splitting tensile strength decreased by 10.6%, 15.2%, and 21.2% for mixes with 10%, 20%, and 30% CR replacement, respectively, compared to the control mix without rubber. However, the rate of decrease in splitting tensile strength was lower than that of compressive strength. Furthermore, Moghaddam et al. [113] explored the combined effect of CR and steel fibers on the tensile strength of FA-based AAC. The study found that adding 10% CR reduced tensile strength compared to control mixes without CR. However, including steel fibers (up to 1%) significantly improved tensile strength, with the highest value of 4.7 MPa observed in a mix with 20% OPC, 10% CR, and 1% steel fiber. This suggests that while CR alone may decrease tensile strength, its combined use with steel fibers can enhance mechanical properties. Similarly, Hamidi et al. [90] focused on producing RuAAC with varying CR aggregate content. The results showed that up to 10% CR replacement improved tensile strength, with the highest values of 1.92 MPa (7 days) and 2.35 MPa (28 days) for 10% CR. Beyond 10% CR, tensile strength decreased, suggesting an optimal CR content for enhancing tensile properties. Figure 11 shows the influence of rubber type and replacement with steel fiber incorporation on the splitting tensile strength [99]. The recycled tire rubber materials used were tire rubber chips as coarse rubber aggregate and fine CR as fine aggregates. In addition, 1% content of steel fibers was introduced. Adding rubber decreases the splitting tensile strength compared to the control mixture with no rubber or fiber. Higher replacement ratios of rubber (15% vs. 10%) lead to lower splitting tensile strength for both CR (Figure 11a) and tire rubber chips (Figure 11b). Incorporating 1% steel fibers consistently improves the splitting tensile strength for all rubber types and replacement ratios compared to their counterparts without steel fibers. This strengthening effect can be attributed to two key mechanisms: (1) the hydrophilic nature of steel fibers improving the fiber–matrix bond interface and (2) the high elastic modulus of SF facilitating better stress distribution and crack bridging throughout the matrix. These findings align with prior investigations demonstrating steel fiber’s capability to enhance the mechanical properties of alkali-activated concrete composites [99].

It should be noted that few studies have reported that increasing CR content in AAC positively affects tensile strength [36,109]. For instance, Yolcu et al. [53] investigated the effects of binder dosage and waste rubber fiber content on the mechanical properties of AAC. The results showed that increasing the waste rubber fiber content positively influenced the tensile strength, with 15% waste rubber fiber content resulting in the highest tensile strength improvement due to the bridging effect of the rubber fibers. Similarly, Zhang et al. [78] studied the replacement of river sand with CR at different ratios. The highest splitting tensile strength was observed with a 10% CR replacement, achieving 4.3 MPa, 37.2% higher than the control mix (0% CR). However, increasing the CR content beyond 10% decreased tensile strength, indicating an optimal CR replacement level for maximizing tensile strength. Luhar et al. [21] highlighted that AAC with CR exhibited higher tensile strength than OPC concrete, primarily due to better bonding between the geopolymer matrix and aggregates. The tensile strength increased with higher CR content, with the maximum strength of 5.49 MPa achieved in a mix with 30% CR after 365 days. This indicates a gradual improvement in tensile strength with increasing CR content. Incorporating CR in AAC enhances tensile strength to an optimal content level, beyond which the tensile strength may decrease. Additional reinforcing materials, such as steel or polypropylene fibers, can improve tensile properties further [67].

To conclude, the impact of CR on the splitting tensile strength of AAC is complex. While most studies report a decrease in strength with increasing CR content (up to 52% reduction at 50% CR), some find improvements at low CR levels (up to 10%). The optimal CR content varies between studies, with some reporting peak strength at 10% CR replacement. Incorporating steel fibers (1%) can significantly enhance tensile strength, even in CR-containing mixes. Interestingly, some research shows AAC with CR can outperform normal concrete in tensile strength, attributed to better matrix–aggregate bonding.

4.2.4. Modulus of Elasticity and Stress–Strain Relationship

Several studies investigated AAC’s modulus of elasticity and stress–strain behavior when incorporating CR and fibers. The modulus of elasticity is an important mechanical property that reflects concrete’s stiffness and deformation resistance. The stress–strain relationship provides insights into the material’s behavior under increasing stresses, including its elastic and plastic deformation, peak stress, and post-peak response. Incorporating CR as a partial replacement for fine aggregates generally reduced the modulus of elasticity of RuAAC [18,21,36,64,76,85]. For instance, Saloni et al. [92] reported that the modulus of elasticity decreased with increasing CR content, with 30% rubber reducing the modulus by around 13% compared to the control mix without rubber. Elzeadani et al. [65] reported that the modulus of elasticity of rubberized AAC decreased with increasing rubber content. The 28-day modulus reduced from 28.31 GPa for the control mix to 5.96 GPa for the mix with 60% rubber replacement, representing a 79% reduction. The latter study attributed this to the rubber’s lower elastic modulus and the matrix’s increased porosity. Additionally, 10–30% CR reduced the modulus by 25–58% [48], and the elastic modulus of RuAAC decreased with increasing CR content, as evidenced by the lower slopes of the stress–strain curves [96]. However, some studies reported that low percentages of CR could increase the elastic modulus. For example, 5% CR increased the elastic modulus by 52.2% to 19.5 GPa, and then it decreased at higher CR% values [78].

Some methods were reported to compensate for the strength loss, such as pretreatment of CR, which generally improved the modulus of elasticity, with NaOH-treated and UFS-treated mixes having the highest moduli among CR mixes [95]. Furthermore, Eren et al. [99] reported that adding steel fibers compensated for the strength loss and even enhanced the modulus of elasticity. However, Che et al. [14] found that recycled steel fibers did not significantly affect the elastic modulus but improved the ductility of RuAAC by delaying failure, as reflected in the more gradual descending branches of the stress–strain curves.

Factors affecting the stress–strain response and modulus of elasticity include GGBFS content, steel fiber addition, and CR replacement [65]. Higher GGBFS content increased the modulus of elasticity and reduced axial/lateral crushing strain, while 10% steel fibers reduced the modulus of elasticity and increased axial/lateral crushing strain. Increasing CR content led to a more ductile and less steep post-peak response, indicating a transition from brittle to quasi-brittle failure [65]. The ductile nature of CR aggregates and their ability to fill voids contributed to the improved softening behavior of RuAACs [90]. The stress–strain curves of RuAAC exhibited ascending and descending branches, with increasing recycled steel fibers and CR contents leading to more gradual descents after reaching peak stress [14].

Figure 12 shows the typical stress–strain curve of RuAAC compared to other materials, such as normal concrete, AAC, and rubberized concrete. The graph illustrates distinct behavioral patterns for each material type. Normal concrete and AAC exhibit higher initial stiffness, as evidenced by their steeper initial slopes. AAC reaches the highest peak stress at a slightly higher strain than normal concrete [116,117]. Both normal concrete and AAC show relatively brittle behavior post-peak, with AAC displaying a slightly steeper descent at high strain [118]. In contrast, rubberized concrete and RuAAC demonstrate significantly lower initial stiffness and peak stresses but much higher ductility [119]. Rubberized concrete achieves a slightly higher peak stress than RuAAC. The post-peak behavior of these rubberized variants is significantly more gradual, indicating superior energy absorption and deformation capacity. This comparison highlights the trade-off between strength and ductility: while normal concrete and AAC offer higher strength, rubberized alternatives provide enhanced deformability and energy absorption. RuAAC combines characteristics of both AAC and rubberized concrete, offering improved ductility compared to AAC but with reduced strength compared to all other variants.

Furthermore, a study by Arunkumar et al. [31] produced various hybrid fiber-reinforced GPC specimens with different polypropylene fiber and rubber fiber dosages. The ductility factor of these hybrid fiber-reinforced GPC specimens was investigated. The ultimate and yield deflection values were measured by fixing dial gauges at the center point while applying flexural load on prism specimens to calculate the ductility factor. As shown in Figure 13, the specimen with 1% rubber fiber and no polypropylene fiber showed a maximum ductility factor of 1.54. Utilizing rubber fiber increased both the ultimate and yield deflection. In comparison, the specimens with higher polypropylene fiber percentages had the least ductility, as the high polypropylene fiber content affected the stability of the mix. It is noteworthy that this study focused on one type of fiber only.

Conclusively, while incorporating CR in AAC generally reduces the modulus of elasticity, with up to 79% reduction at 60% CR replacement, some studies report increases at low CR contents. CR enhances ductility and energy absorption, transitioning failure modes from brittle to quasi-brittle. RuAAC exhibits lower initial stiffness and peak stress but higher ductility compared to conventional concrete and AAC. Fiber reinforcement can compensate for strength loss and further improve ductility. These characteristics highlight RuAAC’s potential for applications requiring high energy absorption and deformation capacity, despite its lower strength.

5. Seismic Performance of RuAAC and Rubberized Concrete

This section focuses on the seismic performance testing conducted on rubberized concrete, AAC, and the limited studies encompassing RuAAC to enhance the seismic performance of concrete structures. This review of the utilization of rubberized concrete and AAC in structural applications aims to examine the structural behavior of rubberized concrete and AAC elements, such as columns, beams, and slabs, under various loading conditions, including seismic loading. Additionally, the experimental setup and testing procedures for rubberized concrete and AAC structural members are reviewed, emphasizing different testing techniques under the effect of seismic and dynamic loadings to guide future RuAAC seismic performance assessment studies.

5.1. Seismic Performance of RuAAC

Seismic performance remains a critical consideration in structural engineering, particularly for innovative materials like RuAAC. Recent research has investigated their behavior under dynamic loading conditions through experimental studies on material properties, structural elements, and computational modeling. The studies demonstrate that while rubber content generally reduces strength and stiffness, it can enhance energy dissipation, ductility, and damping characteristics, which are vital properties for seismic resistance. This section examines key findings from experimental investigations into RuAAC-filled steel tubes, stress–strain behavior under various loading conditions, and finite element analyses of damping properties.

5.1.1. RuAAC-Filled Steel Sections

Elzeadani et al. [120] investigated the axial compressive behavior of concrete-filled steel tubes using RuAAC as an innovative infill material. The researchers conducted an experimental program testing 54 specimens with circular and square cross-sections, varying the rubber content (0%, 30%, and 60% replacement of natural aggregates) and using two different length-to-diameter/width ratios (2 and 4). The methodology involved preparing RuAAC mixes with GGBFS and FA and testing the specimens under axial compression while monitoring their load-shortening response. Increasing rubber content led to significant reductions in axial strength and stiffness: approximately 45.3% reduction at 30% rubber replacement and 57.3% at 60% replacement. Circular tubes outperformed square tubes, achieving an 18.3% higher peak confined stress and substantially higher ductility. These results agree with previous work conducted by Elzeadani et al. [121]. Energy ductility for circular specimens increased dramatically, with 30% CR and 60% CR specimens showing 2.5 and 8.6 times the ductility of non-rubberized specimens, respectively. Mechanical property analysis showed concrete compressive strength dropped from 71.8 MPa to 22.1 MPa (69.2% reduction) at 30% rubber replacement and to 8.7 MPa (87.9% reduction) at 60% replacement.

Furthermore, to examine the cyclic performance, Elghazouli et al. [122] comprehensively examined the seismic performance of square steel tubes infilled with RuAAC through severe cyclic loading tests and numerical simulations. The study subjected nine concrete-filled steel tube specimens to lateral cyclic displacements with varying crumb rubber replacement ratios (0%, 30%, and 60%) and axial load levels (0–20% of cross-section capacity). Critically, the cyclic test results demonstrated significant improvements in seismic response compared to hollow steel tubes. The RuAAC-infilled tubes exhibited enhanced hysteretic behavior, with the concrete infill delaying local buckling and providing more stable energy dissipation characteristics. However, increasing the rubber content progressively impacted seismic performance: displacement ductility reduced by 33.9%, while the normalized energy dissipation remained comparatively consistent across different rubber replacement ratios. The plastic hinge length decreased by 24.7% as the rubber content increased, and axial loading further modified the cyclic response. Notably, the study revealed that the confinement provided by the steel tube was crucial in mobilizing the seismic potential of RuAAC, with thicker tubes showing better ability to utilize the material’s plastic deformation capabilities. The researchers concluded that RuAAC-filled steel tubes offer promising seismic performance, particularly for members experiencing combined bending and axial loading, with the tube’s confinement playing a critical role in maintaining structural integrity during cyclic loading.

5.1.2. Monotonic and Cyclic Stress–Strain Behavior

Elzeadani et al. [76] conducted an experimental investigation into the monotonic and cyclic stress–strain behavior of RuAAC under varying strain rates and rubber contents. The study utilized cylindrical specimens composed of GGBFS and FA as precursors, with sodium metasilicate as the alkaline activator. Crumb rubber replaced 0%, 30%, and 60% of total aggregates by volume. As indicated in other studies, this study confirmed that increasing rubber content significantly reduced the compressive strength, elastic modulus, and crushing strain, with a 60% rubber content resulting in an average 87.7% decrease in compressive strength. Conversely, higher strain rates enhanced strength, stiffness, and ductility. Cyclic loading envelope curves closely matched monotonic responses, with rubberized mixes exhibiting stable post-peak cyclic degradation. Energy dissipation increased with strain rate but decreased with higher rubber content, suggesting a trade-off between these parameters. The unloading modulus decreased with increasing unloading strain and rubber content, while the plastic residual strain increased proportionally with unloading strain. The study developed analytical models to predict critical properties, including compressive strength as a function of rubber content and dynamic increase factors for strength, elastic modulus, and crushing strain based on strain rate. For example, the compressive strength (f_c) as a function of rubber content (ρ_vr) was given by Equation (1), as follows [76]:

$${\displaystyle \frac{f_c}{f_{c0}}}={\displaystyle \frac{1}{1+2{\left(\frac{3{\lambda \rho }_{vr}}{2}\right)}^{\frac{3}{2}}}}$$

(1)

where f_c0 is the reference mix strength, and λ is a factor related to aggregate replacement.

The research revealed that RuAAC exhibits distinctively lower stiffness compared to conventional concrete despite similar compressive strengths, attributed to the binding gels formed during activation. While the study successfully developed analytical expressions and constitutive models for mechanical properties under both loading conditions, several limitations were noted. The expressions are only applicable for GGBFS-based RuAAC with rubber contents up to 60% and strain rates between 2.08 × 10⁻⁵ and 2.08 × 10⁻² s⁻¹. The research did not address partial unloading/reloading cycles or the effects of different precursor combinations and activator contents. Future studies should investigate long-term properties like shrinkage and creep behavior, examine the unloading and reloading response for various material combinations, and explore the microstructural effects of binding gels on material stiffness.

5.1.3. Finite Element Modeling of Damping Characteristics

Chen et al. [32] conducted a detailed finite element analysis to evaluate the damping behavior of RuAAC cantilever beams using the ABAQUS software. The researchers modeled cantilever beams measuring 100 mm × 150 mm × 1200 mm with rubber contents varying from 0–15% by volume to assess their response under free vibration conditions. The results demonstrated that incorporating rubber particles significantly enhanced the energy dissipation properties of the AAC matrix. The specimens containing 10% rubber content exhibited optimal performance, achieving the highest overall total energy consumption of 18,415 J compared to 15,731 J, 16,759 J, and 16,352 J for the control mix without rubber, the 5% rubber content mix, and the 15% rubber content mix, respectively. The damping loss factors showed progressive improvement with increasing rubber content up to 15%, reaching values of 1.58, 1.75, 2.02, and 2.15 for rubber contents of 0%, 5%, 10%, and 15%, respectively, when subjected to damage displacement. Although 15% rubber content achieves higher peak damping loss factors, it shows pronounced performance degradation at severe damage levels; 10% rubber content maintains superior consistency under the same conditions, demonstrating better structural reliability. The 10% optimum recommendation is based on overall performance across multiple criteria and damage stages, not just peak damping loss factor values. The study found that excessive rubber particles reduced the energy dissipation resilience of the structure in the failure stage.

The hysteresis curves in Figure 14 illustrate the load-displacement behavior of plain AAC with 0% rubber addition (Figure 14a) and RuAAC with 10% rubber content (Figure 14b). The hysteresis curves obtained from the analysis revealed distinct behavioral differences between plain AAC and RuAAC with 10% rubber content. While the plain AAC demonstrated higher load-carrying capacity, the RuAAC specimens exhibited more pronounced hysteresis loops, indicating superior energy dissipation capabilities. The larger enclosed area of the hysteresis curves for RuAAC confirmed enhanced ductility and damping capacity, properties particularly beneficial for structures subject to cyclic loading or seismic events. The research concluded that a 10% rubber content represented an optimal balance, providing enhanced damping performance while maintaining structural integrity. This finding offers valuable guidance for practical applications where improved vibration resistance is desired without excessive compromising of mechanical properties.

While these findings demonstrate the potential of RuAAC for enhancing structural damping performance, several limitations and areas for future research should be noted. The study focused solely on one beam geometry and a limited range of rubber contents, suggesting the need for investigation of different structural configurations and rubber replacement levels. Additionally, the long-term durability and performance of RuAAC under sustained cyclic loading conditions remains to be fully characterized. Further research examining the effects of rubber particle size distribution, surface treatment methods, and the influence of different alkali activators could provide valuable insights for optimizing the damping properties of these innovative composites.

5.2. Seismic Performance of Rubberized Concrete

Table 11. Overview of seismic performance experiments conducted on rubberized concrete structures.

Study	Experiment Type	Scope	Composition	Main Findings
Khan et al. [113]	Shake table testing	- Compared normal concrete frame with rubberized concrete frame - Investigated the dynamic characteristics and seismic performance parameters	15% CR	- Prototype structure fundamental period: 7.27% higher for rubberized concrete than normal concrete. - Damping ratio: 18.5% higher for rubberized concrete. - Rubberized concrete frame showed 20.40% lower peak seismic response acceleration. - Rubberized concrete frame exhibited 150% higher energy dissipation.
Xue et al. [114]	Free vibration and shaking table testing	- Investigated the potential use of rubberized concrete as a structural material with enhanced energy dissipation capability	0%, 5%, 10%, 15%, 20% CR	- Average damping ratio: 62% increase for rubberized concrete. - 27% reduction in peak response acceleration for rubberized concrete. - Natural frequency 7.85 Hz for normal concrete columns and 5.65 Hz for rubberized concrete columns. - Rubberized concrete shows potential for improving seismic performance due to enhanced energy dissipation.
Moustafa et al. [115]	Shake table testing	- Compared rubberized concrete column with conventional reinforced concrete column - Focused on effects of seismic intensity and axial load ratio	20% CR	- Rubberized concrete column had 7% higher fundamental period initially - Rubberized column exhibited 15% lower accelerations after rebar fracture. - Rubberized column had lower residual drifts until rebar fracture - Rubberized column dissipated 16.5% more cumulative energy by 200% in design earthquake
Chao et al. [116]	Cyclic loading tests	- Compared conventional concrete, aeolian sand concrete, rubberized concrete, and rubberized aeolian sand concrete columns. - Focused on residual drift, load-bearing capacity, and ductility.	10% CR and 30% aeolian sand	- Rubberized aeolian sand concrete column had 82% lower normalized residual drift up to 2% drift. - Rubberized concrete column showed 4.7% decrease in yield load, 7.7% decrease in peak load, 6.9% decrease in breaking load, and 22% increase in displacement ductility coefficient. - Rubberized concrete column had 36.5% higher ductility coefficient compared to conventional concrete. - Columns with CR exhibited smaller crack widths and less concrete peeling.
Kalman Šipoš et al. [117]	Cyclic loading tests	- Evaluated the seismic performance of reinforced concrete columns and frames made with partially replaced aggregate and recycled rubber particles.	10% and 15% CR	For Columns: - Normal concrete max shear force 56 kN, while rubberized concrete specimens max force 51 kN at almost 3% drift. - Rubberized columns showed higher deformability, smaller plastic hinge area, and narrower cracks For Frames: - Rubberized frames had only 3% lower load capacity - Less damage and smaller plastic joint region in rubberized frames.
Youssf et al. [123]	Axial compression and cyclic loading	- Explore the possible use of rubberized concrete for structural columns	20% CR	- Rubberized concrete column had higher natural frequency (18.3 Hz vs. 12.1–15.3 Hz). - Rubberized concrete column had 3.8 times higher initial stiffness. - Rubberized concrete column had 49% lower viscous damping ratio.
Hassanli et al. [124]	Cyclic loading, eccentrically applied monotonic axial loading, free vibration tests (for beams) and finite element analysis	- Observe the effect of rubber on the behavior of rubberized concrete columns. - Numerically model rubberized concrete beam and column members using FE analysis.	0%, 6%, 12%, 18% CR	Beam Testing: - Compressive strain capacity increased with rubber content (up to 16.2% increase). - Deflection capacity increased (7.7% to 27.9%). - Damping ratio increased (up to 27.8% increase). - Hysteretic energy dissipation decreased (6–10% reduction). Column Testing: - Ultimate capacity reduced by about 12% (for 18% rubber content) - Similar crack patterns, but more cracks with smaller widths as rubber content increased.
Mohamed et al. [125]	Numerical analysis for cyclic loading, axial loading	- Assess the effect of CR on the cyclic behavior of reinforced concrete columns. - Validate a finite element model for predicting rubberized reinforced column behavior under cyclic loading.	0%, 10%, and 15% CR	- Lateral displacement increased 26.5% (10% CR), 34.5% (15% CR). - Displacement ductility improved 80.47% (10% CR), 125.58% (15% CR). - Damping ratio enhanced 33.67% (10% CR), 44.02% (15% CR). - Enhanced energy dissipation and reduced crack propagation. - Delayed concrete cover splitting.
Zhang et al. [126]	Cyclic loading test	- Investigate the impact of rubber aggregates on crack resistance and energy dissipation properties of rubberized concrete	- CR particles: 1–3 mm, 2–4 mm, 3–5 mm - CR content: 5–60% by volume	Enhanced damping properties: - Loss factor: 4.4–8.4% (higher than conventional concrete). - Linear increase with stress levels. Higher rubber content: - Significantly increased hysteresis loop area Energy dissipation increased with: - Higher rubber volume content - Larger loading force amplitude

summarizes the key findings from several research studies that investigated the seismic behavior of rubberized concrete in structural elements, particularly columns and frames. These studies employed varied testing methodologies, including shake table tests, cyclic loading experiments, and free vibration analyses, to evaluate rubberized concrete structures’ dynamic characteristics and seismic response compared to their conventional alternatives. While Table 11 presents seismic performance data from rubberized concrete studies to guide future RuAAC research directions, the reported benefits (including enhanced damping and ductility) may not directly translate to AAC due to the distinct chemical composition, hydration process, and interfacial bonding characteristics inherent to AAC systems compared to normal concrete.

The research findings reveal several notable trends in the seismic performance of rubberized concrete structures. One consistent observation across multiple studies is the increased damping capacity of rubberized concrete. For instance, Khan et al. [127] reported an 18.5% higher damping ratio in rubberized concrete frames. Xue et al. [128] also observed a substantial 62% increase in the average damping ratio for rubberized concrete columns. This enhanced damping property contributes to improved energy dissipation during seismic events, as evidenced by the 150% higher energy dissipation reported by Khan et al. [127] and the 16.5% increase in cumulative energy dissipation noted by Moustafa et al. [129].

Another significant finding is reduced peak seismic response accelerations in rubberized concrete structures. Khan et al. [127] observed a 20.40% lower peak seismic response acceleration in rubberized concrete frames, while Xue et al. [128] reported a 27% reduction. This decrease in acceleration response suggests that rubberized concrete structures may experience lower seismic forces during earthquakes, potentially leading to reduced structural damage.

Previous studies also indicate that rubberized concrete structures generally exhibit higher deformability and ductility. Chao et al. [130] found a 22% increase in displacement ductility coefficient for rubberized concrete columns and a 36.5% higher ductility coefficient for rubberized aeolian sand concrete columns compared to conventional concrete. This enhanced ductility is crucial for seismic performance, allowing structures to undergo larger deformations without failure.

However, it is important to note that the incorporation of rubber particles typically reduced concrete strength. This is reflected in the slight decreases in yielding, peak, and breaking loads observed by Chao et al. [130]. Despite this, the overall seismic performance benefits outweigh the strength reduction, as evidenced by the improved behavior under cyclic loading and the reduction in damage patterns reported by Youssf et al. [123]. The fundamental period of rubberized concrete structures tends to be longer than that of conventional concrete structures, as reported by Moustafa et al. [129] and Khan et al. [127]. This shift in the structure’s natural frequency could benefit specific seismic design scenarios, potentially moving the structure’s response away from the dominant frequencies of earthquake ground motions.

Interestingly, previous studies show that rubberized concrete structures often exhibit delayed onset of cracking and reduced crack widths. Moustafa et al. [129] observed delayed onset of cracking, rebar yielding, and fracture in rubberized columns. Chao et al. [130] and Turatsinze et al. [131] also reported that adding rubber aggregates from used tires to concrete reduces the tendency of concrete toward cracking and leads to smaller crack widths and less concrete peeling. This behavior improved the durability and reduced maintenance requirements for structures in seismic regions.

While the seismic performance of rubberized concrete has been extensively studied, research on the seismic behavior of RuAAC remains limited, particularly at the structural level. However, the significant seismic performance characteristics observed in rubberized concrete provide a valuable foundation for evaluating and potentially enhancing the seismic response of RuAAC structures.

The key findings from rubberized concrete studies, including increased damping capacity, improved energy dissipation, reduced peak seismic response accelerations, enhanced deformability and ductility, and improved cracking behavior, offer promising insights that could apply to RuAAC. These characteristics suggest that RuAAC might exhibit similar seismic performance benefits.

6. Conclusions

6.1. Summary and Key Findings

This comprehensive literature review explored the development, properties, and potential applications of RuAAC, an innovative and sustainable construction material that combines the benefits of AAC with the improved properties provided by the incorporation of CR. The review covers various aspects of RuAAC, including its components, physical and mechanical properties, and available limited studies on its seismic performance.

The key findings are as follows:

• Material Components: FA and GGBFS serve as the main aluminosilicate precursors with NaOH and Na₂SiO₃ solutions as activators. Interface bonding improves through pretreatment methods like NaOH immersion and cement slurry coating.
• Fresh Properties: CR content affects mixtures by reducing workability/flowability and extending setting times. Both initial and final setting times increase with higher CR content.
• Physical Properties: Higher CR content consistently leads to increased water absorption and porosity in the hardened material.
• Mechanical Properties:

  ⚬

  Most mechanical properties decline with CR addition;

  ⚬

  Compressive strength shows major reduction (up to 63.2% at 50% CR);

  ⚬

  Flexural strength decreases more gradually than compressive strength;

  ⚬

  Elastic modulus typically decreases except for small improvements at 5% CR;

  ⚬

  Splitting tensile strength varies, with possible gains of up to 10% CR content.

• Dynamic Performance: CR enhances dynamic behavior through improved energy dissipation and damping. Analysis shows 10% rubber content optimizes damping, while yielding better seismic performance through reduced peak accelerations and enhanced deformability, which suggests potential applications such as in bridge bearings or seismic isolation layers.

6.2. Future Recommendations and Research Gaps

Based on the comprehensive literature review of RuAAC herein and its potential applications in structural engineering, particularly earthquake-resistant structural members and systems, several research gaps have been identified. Research gaps are organized into three systematic categories: material characterization, structural performance, and standardization. These categories represent sequential knowledge requirements, where material understanding forms the foundation for structural applications, which in turn require standardized protocols for widespread implementation.

(i) Material characterization gaps: Understanding the fundamental material behavior remains incomplete, limiting optimization potential and performance predictability in RuAAC applications, particularly in the following research areas:

• Lack of studies on the effect of different types of fibers on RuAAC: While various rubber types have been studied in RuAAC, there is a notable absence of research investigating the incorporation of various types of fibers, such as steel, basalt, and carbon fibers, in RuAAC.
• Insufficient comparative studies of CR pretreatment effects on RuAAC: Comparative studies examining different rubber pretreatment methods and their influence on bond characteristics and mechanical and long-term performance remain scarce.

(ii) Structural performance gaps: Translating material properties into structural-level performance requires comprehensive testing across multiple scales and loading conditions as follows:

• Limited studies on the structural applications of RuAAC: While numerous studies have investigated the mechanical properties of RuAAC at the material level, there is a lack of research on its performance in structural-level elements such as beams, columns, slabs, and beam–column connections.
• Insufficient research on the seismic performance of RuAAC structural members: While extensive research highlighted the seismic behavior of rubberized concrete structures, the combination of these two materials (rubber + AAC) has not been adequately investigated in the context of seismic performance. Further studies are recommended to validate the energy dissipation of RuAAC structural members under cyclic loading.
• Limited experimental testing of large-scale RuAAC structural systems: The testing programs conducted previously for rubberized concrete structural systems, such as cyclic loading and shake table testing, have not been widely adopted for RuAAC. There is a pressing need for comprehensive experimental investigations to assess the structural behavior and seismic performance of RuAAC structural systems, particularly under actual earthquake loading.
• Impacts of construction technology and cost on RuAAC practical structural applications: Further studies are needed to verify the relative impacts of different construction technologies and related costs on RuAAC practical structural applications.

(iii) Standardization gaps: Consistent evaluation methods and design frameworks are essential for reliable implementation and quality assurance across the construction industry as follows:

• Lack of standardized experimental protocols: The experimental conditions across different studies reviewed in the present study could be a limitation when conducting comparative analysis. Several measures were taken in the present study to address this issue, including comparing studies that use similar mix quantities in terms of CR content, sizes, and curing conditions, and focusing on percentage changes and trends rather than absolute values for mechanical property evaluation. However, standardized experimental protocols across the research community should be developed and adopted to enhance data comparability.

Systematic resolution of these interconnected research gaps will provide valuable insights into the structural behavior and seismic performance of RuAAC and establish the knowledge foundation necessary for widespread RuAAC adoption in sustainable and resilient construction practices. The identified research gaps directly impact sustainable construction advancement through RuAAC implementation. Studying seismic performance for RuAAC, including large-scale testing, will enable its adoption in earthquake-prone regions, addressing the United Nations (UN) Sustainable Development Goals (SDGs) 9 and 11. Reducing CO₂ emissions from Portland cement production using AAC and tire waste disposal addresses SDGs 12 and 13. These research advances would contribute to circular economy principles by creating valuable applications for waste materials for more sustainable cities and communities.