A Comprehensive Review on the use of Recycled Materials in Self-Compacting Concrete

A Comprehensive Review on the use of Recycled Materials in Self-Compacting Concrete

Yaqoob Saif

University of Sfax, National School of Engineers of Sfax, LGC, Laboratory of Civil Engineering

Sfax, Tunisia

AbstractWith the rising inclination of the construction industry towards sustainable materials and practices to minimize environmental impacts and ensure resource efficiency, Here is what is happening. Self-Compacting Concrete (SCC) is a very fluid and no segregating concrete, and it is best due to its workability and reduction of labour. This literature review paper presents the current state of knowledge on the use of recycled materials such as Recycled Concrete Aggregate (RCA), Crumb Rubber (CR), and Fly Ash (FA) as SCC materials in pursuit of sustainability with the required performance. This paper discusses the effects of the above materials on fresh, mechanical and durability properties of SCC, highlights main advantages and disadvantages of their applications and provides the direction for the future research. Findings indicate that SCC mix design and treatment methods reveal that sustainable and recycled materials can efficiently be used in the production of SCC without performance degradation, thus paving the way for the development of SCC along green lines, i.e., towards more sustainable concrete technologies.

Keywords Self-Compacting Concrete (SCC), Recycled Concrete Aggregate (RCA, Crumb Rubber (CR), Fly Ash (FA), Sustainable Construction, Recycled Materials, Durability, Green Concrete.

1. INTRODUCTION

   The construction sector is expanding worldwide and is followed by a high demand for concrete, contributing to the over-exploitation of natural resources, adding to the construction & demolition waste. However, this poses major environmental issues and for that reason, so much research and engineering activity has been focused on finding replacement materials to achieve sustainable production of concrete. One such solution which has been developed to overcome both environmental and on-site performance challenges is Self-Compacting Concrete (SCC) which a high- performance concrete that has the ability to flow and fill the required application area (without the use of mechanical vibration) even where heavily reinforced [1].

   SCC has multiple benefits over customary concrete such as improved workability, decreased labor cost, superior surface finish, and reduced noise on sites. Nonetheless, SCC typically necessitates higher cement contents and larger quantities of fine materials, elevating both the environmental footprint and economic expense associated with the mix. In order to tackle these matters, including recycled materials in SCC materialization has emerged as a promising approach that increases sustainability enabled by performance preservation or improvement[2].

   Various recycled materials used in self-compacting concrete (SCC) have been studied in recent works, including Recycled Concrete Aggregate (RCA), waste tire-based Crumb Rubber (CR), and Fly Ash (FA), a by-product resulting from the combustion of coals in the industry. Recycling concrete as RCA and using it as a natural aggregate replacement has been explored as a potential solution for this issue and a possible recycling outlet for construction waste. One of the advantages of adding crumb rubber is that its elastic properties may contribute to the impact resistance and ductility of SCC, but it causes lower compressive strength at higher levels. But fly ash is commonly used as a supplementary cementitious material to enhance SCC properties in terms of workability, long-term behavior of concrete, and minimizing CO2 emission due to the less use of cement[3][6].

   This review paper strives to highlight the recent research on developing sustainable SCC with the influence of RCA, CR, and FA. It outlines the impact of these materials on the fresh and hardened properties of SCC, analyzes the primary advantages and disadvantages of their usage and pinpoints a research gap and future perspectives on optimizing mix design strategies in sustainable construction.

   A. Methodology of the Literature Review

   The literature search was carried out based on a systematic method in order to include studies that were relevant, recent, and with high research quality. Peer-reviewed journal publications (between 2003 and 2024) were obtained from databases in Scopus, ScienceDirect, and Web of Science. Employing terms as: self-compacting concrete, recycled concrete aggregate, crumb rubber concrete, fly ash in SCC, and sustainable concrete materials.

   Only studies published in English and focused on SCC with recycled materials were considered. Papers were selected based on their experimental depth, clarity in reporting mix compositions, and relevance to either fresh, mechanical, or durability properties. Review articles and case studies were also considered for broader context

2. EFFECTS OF RECYCLED MATERIALS ON THE PROPERTIES OF SELF-COMPACTING CONCRETE

   The integration of recycled materials into Self-Compacting Concrete (SCC) requires a careful evaluation of how these additions influence both fresh and hardened properties. This section presents a review of recent findings on how three widely studied recycled materialsRecycled Concrete Aggregate (RCA), Crumb Rubber (CR), and Fly Ash (FA) affect the performance of SCC.

   1. Fresh Properties

      SCC’s fresh state behavior is crucial, as it must exhibit high flowability, passing ability, and resistance to segregation. When RCA is used as a partial replacement for natural aggregates, a reduction in flowability is often reported. This is attributed to RCAs high porosity and irregular surface texture, which increase water demand and reduce flow [7]. Measures such as increased superplasticizer dosage and adjusted mix proportions are typically required.

      Crumb rubber, derived from waste tires, tends to decrease workability as well, due to its low specific gravity and hydrophobicity. Alharbi et al. [8] observed that the addition of CR in SCC mixtures reduced slump flow and increased T50 time, though the extent depended on particle size and replacement ratio.

      Conversely, fly ash enhances the fresh properties of SCC. Its spherical particles and pozzolanic characteristics contribute to a lubricating effect in the mix. Bui et al. [9] reported that SCC mixes incorporating 25%40% fly ash demonstrated improved flowability and lower segregation, making FA a favorable addition in SCC formulations.

   2. Mechanical Properties

      The mechanical behavior of SCC with RCA shows a slight reduction in compressive strength compared to conventional mixes. This is mainly due to the weak interfacial transition zones and micro-cracks present in the RCA particles. However, Alqarni et al. [10] found that SCC containing up to 30% RCA maintained acceptable compressive and tensile strength values for structural applications.

      Crumb rubber has a more pronounced effect on mechanical performance. The inclusion of CR typically reduces compressive strength, especially at high replacement levels. In their study, Guneyisi et al. [11] reported a strength reduction of up to 35% when 20% CR was used as fine aggregate. Despite this, the concrete exhibited improved toughness and energy absorption, which is advantageous for impact-prone structures. Fly ash, in contrast, improves long-term mechanical performance. Singh and Munjal [12] showed that SCC incorporating 30% FA had slightly lower early-age strength but exceeded the strenth of control mixes at 90 days due to continued pozzolanic activity and microstructure densification.

   3. Durability Properties

      The durability of SCC is strongly influenced by the type of recycled material used. RCA generally increases permeability and drying shrinkage, raising concerns about freezethaw resistance and chloride ingress. Nevertheless, when properly treated or combined with mineral admixtures, its durability performance can be improved [10].

      Crumb rubber-modified SCC demonstrates enhanced performance under dynamic and thermal loading but suffers from increased water absorption and reduced abrasion resistance [11]. Surface treatment of rubber particles has been suggested as a potential remedy.

      Fly ash significantly contributes to the durability of SCC. According to Zhao et al. [13], fly ash reduces chloride ion permeability, enhances resistance to sulfate attack, and improves long-term durability by refining pore structure.

3. COMPARATIVE ANALYSIS OF SCC WITH RECYCLED MATERIALS

   To better understand the influence of various recycled materials on SCC, a comparative summary of key properties reported in selected studies is provided in Table 1. This table highlights variations in workability, compressive strength, and durability based on the type and percentage of recycled material used.

   Table I: Comparative Summary of SCC with RCA, CR, and FA

   Study (Author, Year)	Recycled Material	Replacement Level	Slump Flow (mm)	Compressive Strength (MPa)	Key Observations
   Kou & Poon, 2009	RCA	30%	670	38	Acceptable strength with adjusted mix
   Alharbi et al., 2022	CR	15%	600	28	Reduced flow, lower strength, better ductility
   Singh & Munjal, 2018	FA	30%(cement)	710	45	Improved long-term strength and flow
   Guneyisi et al., 2004	CR	20%	580	25	Significant strength drop at high CR content
   Bui et al., 2010	FA	25%	720	44	Improved workability and reduced segregation
   Alqarni et al., 2021	RCA	30%	660	36	Good performance at 30% RCA replacement
   Zhao et al., 2020	FA	35%	730	47	Enhanced durability and reduced permeability
   Ganesan et al., 2018	CR	10%	610	30	Low strength but improved flexibility
   Federle et al., 2022	CR	10%(treated)	640	34	Surface treatment improved bonding
   Li et al., 2020	FA	40%	750	46	Excellent flow and strength with 40% FA

4. BENEFITS AND DISADVANTAGES OF RECYCLING MATERIAL IN SCC

   Self-Compacting Concrete (SCC) is a modern approach to concrete design and construction that incorporates performance requirements and aims to minimize environmental impact.

   Nonetheless, these benefits are accompanied by several significant technical challenges that need to be overcome to allow successful and reliable use of SCC in both structural and

   non-structural applications.

   1. Advantages

      The use of recycled materials in SCC has one of the biggest benefits being a decreased environmental impact. The use of recycled concrete aggregate (RCA) diverts construction and demolition materials from landfills and reuses them for structural application, thus creating a circular economy for the construction industry [14]. Furthermore, this means fewer quarries need to be dug for natural aggregates, thus helping to conserve non-renewable resources.

      CR is derived from waste tires, and its employment in SCC is advantageous, because it solves a global waste problem while enhancing specific performance obstructions such as impact resistance, noise reduction, and flexibility. These characteristics make rubberized SCC suitable for nonstructural applications, including pavements, barriers, and sports facility foundations [15].

      Fly ash (FA) enhances sustainability and performance. Fly ash is a pozzolanic material that diminishes the demand for cement, which in turn reduces the CO emissions generated from cement manufacturing. It further improves the durability and long-term strength of SCC by improving the microstructure and decreasing permeability [16].

      Additionally, materials that are recycled often lead to cost savings. While adding RCA or CR to the mix may involve an additional cost due to the need for prior processing and quality control, reductions in raw material consumption and transportation can offset other costs in larger projects[17].

   2. Challenges

   There are however a number of challenges preventing the widespread use of recycled materials in SCC, despite the sustainability benefits.

   Mix design gets difficult due to water absorption of RCA and quality instability, hence moisture corrections to be done cautiously. Additionally, the leftover mortar retained on recycled aggregates could impair the interfacial transition zone (ITZ) and cause strength loss and porosity growth [18].

   Using waste crumb rubber in concrete has its its issues such as compatibility, as bond and dispersion in the cement matrix. It is a hydrophobic surface with a smooth texture that leads to poor interfacial bonding, adversely affecting the compressive strength and workability. [19]), and studies are still being carried out to enhance these characteristics through pre- treatment methods (such as chemical coating or microwave activation).

   One of the major difficulties with fly ash is the delay in developing strength at early ages, particularly in cold weather. Fly ash, on the other hand, improves long-term performance, but its slower rate of reaction can lead to a less favorable effect on construction schedules that require quick set times, unless used in combination with accelerators or performance-blended with other supplementary materials [20].

   In addition, [the] uncertainty for engineers and contractors, as a result of regulatory constraints, lack of standardization, and varying quality of recycled materials, is detrimental to adoption. In many areas, due to an absence of well-established guidelines and performance-based specifications, the incorporation of recycled materials in SCC still displays a conservative approach [21].

5. FUTURE PERSPECTIVES

   With world-unifying agendas focusing more towards sustainable construction and climate resilience, the recycling in the Self-Compacting Concrete (SCC) sector using appropriate recycled materials could see even sharper growth. But, in order for this integration to have been fully recognized and well-landed in both developed and developing countries, several key research and practical gaps need to bridge. This section delineates the future perspectives and research directions that will steer the next level of development in this arena.

   1. Optimization of Mix Design through AI and Modeling

      The issue of potential variability between recycled and natural components leads to major challenge in SCC with recycled components to both achieve adequate performance an to design mixes that address the variability of the recycled materials. Further investigation shall be carried out mainly on SCC mix design incorporating different combinations of recycled concrete aggregate (RCA), crushed rock (CR), and fly ash (FA) utilising artificial intelligence (AI), machine learning (ML), and statistical models for predictions of fresh and hardened properties. These tools can assist in rapid optimization of proportions, lower the cost of experimental work, and supply reliable performance prediction for practical development [22].

   2. Advanced Treatment of Recycled Components

      New surface treatment techniques, particularly for RCA and crumb rubber, are anticipated to be key for optimizing their performance in SCC. Varying surface modification techniques, such as chemical coatings [23], nano-silica treatments, plasma treatments, and other surface preparation methods are being increasingly studied and have shown promising results in improving the bonding behavior while also reducing the porosity of recycled materials [24]. These solutions will permit greater levels of substitution without impacting the performance of concrete.

   3. Performance-Based Standards and Codes

      The absence of consensus protocols for incorporating recycled resources into SCC is a major impediment to its adoption. Hence, the future efforts should be the development of international codes and performance-based standards that would provide for the flexibility in the material used while ensuring both safety and durability. Using these guidelines, engineers can design SCC mixes with recycled content for all structural applications. [25].

   4. Integration into Circular Economy and Life-Cycle Analysis (LCA)

      Future developments will also emphasize the holistic assessment of environmental impact through life-cycle assessment (LCA). This includes not only the carbon footprint reduction achieved by using recycled materials, but also water use, energy demand, and waste minimization. Coupling SCC technology with circular economy strategies will lead to more robust and sustainable construction practices [26].

   5. Broader Applications in Infrastructure and 3D Printing

      As the mechanical performance of recycled SCC improves, its applications will expand beyond non-structural use to bridges, tunnels, high-rise buildings, and 3D-printed structures. Several pilot studies have already demonstrated the feasibility of using optimized recycled SCC in automated construction, which offers tremendous potential for resource efficiency and innovation [27]

   6. Real-World Applications of SCC with Recycled Materials Although much of the research on recycled SCC remains within academic or laboratory settings, several real-world applications have emerged. For example, in Japan and parts of Europe, SCC with RCA has been used in precast tunnel linings and noise barrier walls, demonstrating satisfactory performance under service conditions. In the Middle East, projects using FA-based SCC in high-temperature climates have shown improved thermal stability and reduced cracking. Rubberized SCC has also been employed in sports courts, pedestrian pavements, and roadside curbs, where energy absorption and flexibility are prioritized over compressive strength.

   These case studies affirm the feasibility of integrating recycled materials into functional and durable concrete structures, especially for non-structural or lightly loaded applications.

6. ENVIRONMENTAL AND ECONOMIC ASSESSMENT

   The environmental benefits of using recycled materials in SCC are well-documented in terms of carbon footprint reduction, energy savings, and waste diversion. Life-cycle assessment (LCA) studies show that substituting 30% natural aggregates with RCA can reduce total embodied carbon by up to 20%, while using FA instead of cement can cut CO emissions by 3040%. Economically, these substitutions reduce raw material costs and minimize disposal fees, although costs may increase slightly due to processing and quality control of recycled materials.

   When viewed from a life-cycle perspective, SCC with recycled materials offers a more sustainable solution compared to conventional concrete, particularly when optimized for local material availability and environmental conditions.

7. RESEARCH GAPS AND FUTURE RECOMMENDATIONS

   Despite the promising results, several research gaps remain. Most studies focus on short-term performance; therefore, long-term durability and service life under real environmental exposures (e.g., freezethaw, corrosion, sulfate attack) remain underexplored. In addition, the combination of multiple recycled materials in a single SCC mix (e.g., RCA + CR + FA) has been sparsely investigated and could yield synergistic effects.

   Future research should also explore:

   • The use of recycled SCC in structural elements (beams, columns)
   • Behavior under seismic and dynamic loading
   • Performance in extreme climates
   • Standardization of testing protocols and mix design tools
   • Integration into 3D printing applications
8. CONCLUSION

The transition toward sustainable construction practices has underscored the need to incorporate recycled materials into modern concrete technologies such as Self-Compacting Concrete (SCC). This literature review has demonstrated that the use of Recycled Concrete Aggregate (RCA), Crumb Rubber (CR), and Fly Ash (FA) in SCC presents a viable pathway to reduce environmental burdens, enhance material circularity, and contribute to more eco-efficient infrastructure development.

Each of the reviewed materials offers distinct advantages: RCA promotes resource conservation and waste reduction, CR enhances ductility and shock resistance, and FA significantly improves workability and long-term durability while lowering cement consumption. However, the successful application of these materials depends on understanding and mitigating the associated challenges, such as strength reduction, increased porosity, and variability in material quality.

Advancements in material treatment, AI-based mix design optimization, and the development of international performance-based codes are expected to drive wider adoption of recycled SCC. Future research should continue to focus on long-term performance, standardization, and the integration of SCC into broader sustainability frameworks such as life-cycle assessment and circular economy models.

In conclusion, the use of recycled materials in SCC is not only technically feasible but also essential for the future of green construction. With continued innovation and policy support, recycled SCC can play a critical role in achieving the sustainability goals of the construction industry on a global scale.

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