Abstract

Self-compacting concrete (SCC) has revolutionized modern construction due to its superior workability and structural performance. Using fine and coarse recycled Aggregates has proven to be very effective in enhancing the qualities of the SCC in both fresh and hardened states. It also contributes as a solution to the trending issues of sustainability. This review explores findings from studies on self-compacting concrete's fresh and hardened properties (SCC) incorporating RCCA and RFCA. The review covers fluidity, viscosity, filling ability, and segregation resistance in the fresh state, compressive, tensile, and flexural strengths, and shrinkage in the hardened state. Results indicate that while RCA can detrimentally affect certain properties, strategic use, and optimized mix designs can mitigate these impacts, making RCA a viable component for eco-friendly SCC.

Keywords

Introduction

       
           
       
Figure 1: Aggregates production (tonnes) per capita by EU countries [3]

Using recycled aggregate in concrete offers a promising solution to the issue of construction and demolition waste. Currently, most recycled aggregate is utilized in lower-end applications. However, in some developed economies, it is also used in structural concrete due to its certified quality, which is validated by the Conformité Européenne (CE) Certificate [4].

As defined in [5] Self Compacting/Consolidating Concrete (SCC)is a mix that expels entrapped air without vibration and travels around obstacles, such as reinforcement to fill space within the formwork. Self-compacting concrete has been described as “the most revolutionary development in concrete construction for several decades” [6]. One of SCC's primary advantages is its ease of pouring into highly reinforced structures' formwork without any vibration. Some benefits of this feature include reducing construction time, optimizing design freedom, and improving the quality of the final product and working environment [7]. Chemical admixtures such as superplasticizers and viscosity modifiers are commonly used to achieve the aforementioned properties. The dosage of these chemical admixtures is a crucial factor influencing the fresh-state properties of SCC. Numerous research studies have demonstrated that chemical admixtures play an essential role in attaining desirable fresh-state properties in SCC.

As articulated by [8], The study of self-compacting concrete (SCC) with Recycled Aggregate (RA) is an approach contributing to the needs of the clients in the construction industry as well as the technological activities of the industry. The ability to satisfy the needs of clients and the relationship performance-quality-cost are the main distinctive factors of healthy competition in the construction sector. One major factor contributing to the RA's loss of quality is the existence of adhered mortar. As a result, RA are less mechanically strong, more water soluble, and have a lower density than NA. When it comes to the SCC's performance, these characteristics are the ones that have the biggest impact [9], [10] also made a similar comment by concluding that the quality of the source concrete affects the qualities of RA, mostly because of the type and amount of mortar that is applied to its surface. The water/cement (w/c) ratio utilized in the source concrete determines the mortar's quality, which is further influenced by the concrete's strength and grinding technique. [11] created four varieties of SCC with varying replacement percentages of CRA in place of coarse NA (CNA) (0%, 20%, 40%, and 60%) and they discovered that, due to the presence of old adhered mortar in the CRA and the higher deformability of the cement paste, the lower stiffness of CRA compared with CNA explained the performance degradation in terms of mechanical behavior (compressive and splitting tensile strengths and modulus of elasticity). Regarding durability, it was noted that SCC made with RA can be regarded as hermetic in terms of oxygen permeability.

As a result of several studies e.g. [8],[12],[13],[14].  in the area of self-compacting concrete SCC, the following properties are identified as the properties of fresh SCC;

1. Fluidity and flow speed in the absence of obstructions.
2. Viscosity,
3. filling ability
4. ability to pass through small openings, spaces,
5. Sieve segregation test,
6. Passing ability through confined spaces,
7. Self-compaction

On the other hand, the hardened properties of SCC include;

1. Compressive strength,
2. Splitting tensile strength,
3. Flexural tensile strength, and

Shrinkage.

This review paper aims to comprehensively evaluate the fresh and hardened properties of self-compacting concrete (SCC) incorporated with recycled concrete aggregates (coarse and fine). This includes a detailed examination of the workability, viscosity, stability, compressive strength, flexural and tensile strengths, durability, and microstructural characteristics. By synthesizing existing research, the paper aims to identify the benefits, challenges, and potential improvements in the use of RCA in SCC, providing insights for future research and practical applications in sustainable construction.

LITERATURE REVIEW

Several papers were consulted in this study and they are evaluated according to their methodologies, findings, and other information that are relevant to our areas of interest in the following paragraphs.

 In an experiment made by Nalanth et al., 2014 To evaluate the Fresh and Hardened Properties of Steel Fibre fiber-reinforced self-compacting Concrete Using Recycled Aggregates as a Replacement Material, the conclusion was made that Recycled aggregates exhibit higher water absorption and lower specific gravity compared to conventional aggregates. Test results demonstrate that at a 30% replacement with recycled aggregates, self-consolidating concrete (SCC) meets the required compressive strength after 28 days. However, experimental investigations reveal that the 28-day flexural strength of SCC decreases across all replacement ratios of recycled aggregates. Overall, the compressive, flexural, and split tensile strengths of SCC decline as the proportion of recycled aggregates increases. [15]

In a study by Mo et al. (2021), crushed recycled concrete aggregate (RCA) of sizes ranging from 5 to 10 mm was incorporated into self-compacting concrete (SCC) at different weight ratios of 0%, 25%, 50%, 75%, and 100% to check its suitability as a possible replacement for natural aggregate (NA). The results show that RCA has effects on properties such as mechanical strengths, water absorption, and permeable voids in SCC. But despite these effects on SCC, they are less pronounced compared to their effects on normal vibrated Concrete. Also, despite the RCA-mortar interface exhibiting lower micro-hardness, the thickness of the interfacial transition zone (ITZ) was comparable to that of the natural aggregate mortar interface. These findings indicate that SCC is an appropriate matrix for incorporating RCA, and based on the properties examined, a maximum RCA replacement level of 25% is recommended.[16]

Revathi et al. (2013) in a study to explore the flow and strength characteristics of self-compacting concrete (SCC) using recycled concrete aggregate (RCA) as coarse aggregates. Five mixture series were prepared with varying proportions of recycled coarse concrete aggregates RCCA at 0%, 25%, 50%, 75%, and 100?sed on Nan Su’s mix proportioning method. The fresh concrete properties were assessed through slump flow, J-ring, and V-funnel tests, while compressive and tensile strengths were also evaluated. The results demonstrated that SCC could be successfully developed and improved with the incorporation of recycled aggregates.[17]

Poongodi et al., 2021 in the study “Durability Properties of Self-compacting Concrete Made with Recycled Aggregate,” specifically in pavement applications developed mixtures that were subjected to tests of permeability, water absorption, and chloride penetration. Three replacement levels (20%, 40%, and 60%) of recycled concrete aggregate (RCA) were prepared and evaluated alongside a control mixture containing 0% RCA. An M40 grade ternary blended concrete was formulated using Ordinary Portland Cement (OPC), fly ash, and silica fume. The fresh properties of the concrete were assessed using slump flow and V-funnel flow tests. The findings revealed that mixtures with up to 40% RCA demonstrated greater resistance to water absorption and chloride ingress compared to normal-weight concrete.[18]

Ait Mohamed Amer et al., 2021 investigate the impact of using recycled concrete aggregates (RCA) on the mechanical properties, rheological properties, and total shrinkage of concrete. The analyzed mechanical properties encompass compressive strength, tensile strength, and modulus of elasticity. Also, rheological properties such as yield stress, workability, and plastic viscosity were evaluated. In the experiments, coarse natural aggregates (CNA) were partially replaced with recycled concrete aggregate (RCA), superplasticizer was added to maintain consistent workability. After the results were evaluated, recycled aggregates showed a positive influence on mechanical strengths, the effect is associated with the water-to-cement (w/c) ratio, superplasticizer dosage, and substitution percentage. Concrete mixtures containing RCA showed higher values for rheological properties and greater total shrinkage, both of which depend on the water-to-cement ratio.[19]

Sasanipour & Aslani, 2020 investigated the influence of incorporating fine and coarse recycled concrete aggregates (RCA) on the properties of self-compacting concrete (SCC) where RCA was substituted at levels of 25%, 50%, 75%, and 100% for natural aggregates in SCC mixes, with a constant cement content of 420 kg/m?3; and a water-cement ratio of 0.4. Mechanical properties, such as compressive strength, tensile strength, and ultrasonic pulse velocity, were assessed. Also, properties such as water absorption, volume of permeable voids, electrical resistivity, and resistance to chloride ion penetration. The fresh properties of self-compacting concrete (SCC) were examined using slump flow and J-ring tests. Three mix series were prepared: one replacing natural coarse aggregates NCA with recycled coarse concrete aggregates (RCCA), another substituting natural fine aggregates with recycled fine concrete aggregates (RFCA), and a third incorporating a combination of both recycled coarse and fine concrete aggregates. Results revealed a decrease in compressive strength across all mixes with increasing RCA content, while the effect on tensile strength was minimal. Durability properties of water absorption and volume of permeable voids, worsened as the proportion of RCA increased in the mixes. Notably, the resistance to chloride ion penetration decreased significantly with higher levels of fine RCA replacement. However, a 25% replacement of RCCA had negligible effects on electrical resistivity and chloride ion resistance. Strong correlations were observed between electrical resistivity and water absorption, as well as between total charge passed and permeable voids in mixes containing RCA. Despite the adverse effects on certain properties, the study suggests that limited substitution of RCCA may still be viable without compromising the overall durability of SCC.[14]

Another study is by Kapoor et al., 2020 on the Effect of recycled aggregates on fresh and hardened properties of self-compacting concrete. In this study, Natural Coarse Aggregates (NCA) and was substituted with Recycled Coarse Concrete Aggregates (RCCA) at varying percentages of 0%, 50%, and 100% respectively. On similar account, Natural Fine Aggregates (NFA) were replaced by Recycled Fine Concrete Aggregates (RFCA) at varying percentages of 0%, 25%, 50%, 75%, and 100% replacement levels. There was also partial replacement of Portland cement with fly ash by 30% in an attempt to promote sustainability. The fresh properties of various (SCC) mixtures were assessed through workability tests, including the Slump-flow test, T500 test, V-funnel test, and L-box test. The hardened properties were assessed through the Ultrasonic Pulse Velocity (UPV) test. Furthermore, the compression strength of all SCC mixtures was evaluated for their workability.

The study concluded that replacing natural coarse aggregates (NCA) and natural fine aggregates (NFA) with recycled concrete aggregates (RCA) and recycled fine aggregates (RFA), respectively, led to decreased workability due to the higher water absorption and larger surface area of RCA and RFA compared to NCA and NFA. To address these effects, chemical admixtures were added to the SCC mixtures in varying dosages to maintain their workability and stability. The ultrasonic pulse velocity (UPV) values of all SCC mixtures were within the excellent and good categories, indicating favorable hardened properties.[20]

A review by Akhtar & Sarmah, 2018 provides a comprehensive analysis of global construction and demolition waste (C&D) generation trends and examines recent efforts to enhance the properties of recycled aggregate concrete through supplementary materials. Drawing upon data from 40 countries across six continents, the review critically evaluates current C&D waste generation levels and governmental policies, while also discussing future goals and targets. The analysis reveals a significant increase in global C&D waste generation, particularly in developing countries like India and China, necessitating comprehensive management systems and government initiatives. Recycled aggregates from C&D waste often exhibit inferior quality, prompting researchers to explore the use of pozzolanic materials and recommending their incorporation at levels of 30% to 50% in concrete mixes to achieve strength parity with natural aggregate concrete. These findings underscore the urgent need for innovative strategies and collaborative efforts to address the growing challenges of C&D waste management and promote sustainable practices in the construction industry globally.[1]

Manzi et al., 2017 study examines the creep and shrinkage behavior of self-compacting concrete incorporating coarse and fine recycled concrete aggregates, with a maximum replacement rate of 40% of the total aggregate volume. It investigates the physical properties and porosity of the concrete and analyzes their correlation with mechanical properties. The experimental investigation yielded several key findings: Firstly, it was demonstrated that self-compacting concrete (SCC) can be successfully produced with coarse and fine recycled concrete aggregates at levels of up to 40% in the mix design. Secondly, the mechanical properties of SCC containing high-quality recycled concrete aggregates can match or surpass those of the reference mix composed entirely of natural aggregates, attributed to the development of a denser microstructure. Thirdly, while both shrinkage and creep behaviors are influenced by the presence of recycled aggregates, the impact on creep is more pronounced, albeit with limited variations compared to traditional concrete. The mix that shows the best creep behavior is the one labeled CR100 and that is due to its highest compressive strength and lowest pore size distributions. Lastly, Porosity investigations revealed that the microstructure of the mixes is influenced by the content and type of recycled aggregates, resulting in variations in pore size distributions. These variations subsequently affect the mechanical properties of the concrete in both the short and long term.[12]

Señas et al. (2016) investigated the impact of recycled aggregates on the properties of self-consolidating concrete (SCC). In this study, 50% of the Natural Coarse Aggregate (NCA) was replaced with Recycled Coarse Concrete Aggregate (RCCA) sourced from Pentagonia gravel. Also 20% Natural Fine Aggregate (NFA) was replaced with crushed concrete powder to serve as Recycled Fine Concrete Aggregate (RFCA). An assessment of the fresh properties was carried out. Physical and mechanical properties were also assessed in the hardened state. Petrographic analysis was conducted to examine the concrete's composition, focusing on the interfacial transition zone and the contribution of the powders to the mortar microtexture. The findings exhibited variability based on the type of admixture and aggregate used. however, the study demonstrated the feasibility of incorporating these crushed aggregates to produce high-quality self-consolidating concrete.[21]

Kou & Poon, 2009 evaluated the fresh and hardened properties of self-compacting concrete (SCC) using recycled concrete aggregate (RCA) as both coarse and fine aggregates where they prepared three series of mixtures; Series I contains 100% coarse RCA with levels of fine RCA replacing river sand at 0%, 25%, 50%, 75%, and 100% with a water-to-binder ratio of 0.53, Series II with the same fine RCA levels but a water-to-binder of 0.44, and finally Series III containing 100% recycled concrete aggregates (RCA) both coarse and fine at water-to-binder ratios of 0.44, 0.40, and 0.35. The cement content was kept constant across all mixtures. Tests were conducted to assess the fresh, hardened, and durability properties of the SCC mixtures. The results indicated that Self Compacting Concrete (SCC) made with river sand and crushed fine recycled aggregates exhibited only slight differences in properties.[22]

Carro-López et al., 2015 studied the rheology of self-compacting concrete with fine recycled concrete aggregates. The study involved analyzing the effect of incorporating fine recycled aggregates on the rheology of self-compacting concrete over time using empirical tests such as J-Ring, slump-flow, L-Box, etc. Results show that Self-compacting concrete characteristics were lost at 90 minutes with 50% and 100% recycled sand, while compressive strength significantly decreased in these mixes. The mix with 20% replacement had a decrease of less than 10%.[23]

Nischay T G. et al., 2015 also Studied the Properties of Self Compacting Concrete using Recycled Aggregate in Fresh and Hardened State and produced recycled aggregates from hardened concrete waste, conducting material characterization, testing different replacement percentages in M-20 self-compacting concrete mixes, and fresh and hardened properties were systematically evaluated. The properties of recycled aggregates met the standards for natural aggregates, and 50% replacement in the M-20 concrete mix showed satisfactory flow and compressive strength values.[24]

Olafusi et al., 2015 on evaluation of Fresh and Hardened Properties of Self-Compacting Concrete. The methodology involved comparing rheological properties and compressive strengths of SCC and conventional cement concrete, examining flowability and segregation resistance, varying dosage of superplasticizer and VMA, conducting compressive strength tests at different ages, and studying the effect of water-cement ratio on plastic properties. The results show that Self-compacting concrete (SCC) gains strength more slowly compared to conventional cement concrete, exhibiting lower strength at 28 days but potentially higher strength beyond 90 days. The rate of strength gain in SCC is slower than that of conventional concrete up to 28 days; however, SCC demonstrates greater compressive strength at 90 days. A well-designed SCC mix achieves 85% to 95% of the compressive strength of conventional concrete at 28 days, with the potential for even greater strength at 90 days and beyond.[25]

Grdic et al., 2010 studied the potential of using coarse recycled aggregate from crushed concrete in the production of self-compacting concrete. Three types of concrete mixtures were prepared with coarse aggregate replacement levels of 0%, 50%, and 100% by recycled aggregate. Consistency was maintained across all mixtures during the mixing process. The results show that the properties of these concrete exhibit only minor differences, demonstrating that coarse recycled aggregate can be effectively used to produce self-compacting concrete.[26]

The study by Barroqueiro et al. (2020) investigated the feasibility of producing high-performance self-compacting concrete (SCC) with a reduced environmental impact by using fine and coarse recycled aggregates from the precast industry as substitutes for natural aggregates. The goal was to ensure the resulting SCC maintained suitable durability for industrial applications. Six different SCC mixes were prepared with varying ratios of fine to coarse recycled aggregates (FRA/CRA): 0/0, 25/25, 50/50, 100/100, 0/100, and 100/0 percent. To assess the primary transport and degradation processes affecting the concrete, the following tests were conducted; water absorption by immersion and capillarity, oxygen permeability, chloride ion migration, electrical resistivity, and carbonation depth. The results of the findings indicated that, despite some adverse effects of using recycled aggregates, it is feasible to produce high-performance SCC with satisfactory durability.[27]

Güneyisi et al., 2016 Rheological and fresh properties of self-compacting concretes containing coarse and fine recycled concrete aggregates. The methodology involved preparing SCC mixtures with varying levels of natural aggregate replacements for RCCA and RFCA, replacing 20% of cement content with fly ash, using a water-to-binder ratio of 0.32, and adopting Herschel-Bulkley and modified Bingham models to describe the rheological behavior. The study concluded that the Herschel-Bulkley and modified Bingham models effectively represent the rheological behavior of SCC containing recycled concrete aggregates (RCA). Also, the concrete property of self-compactibility was significantly improved by the different replacement levels of coarse recycled concrete aggregates (CRCA) and fine recycled concrete aggregates (FRCA) used in the SCC mixtures.[28]

Sagoe-Crentsil et al., 2001 evaluate the performance of concrete made with commercially produced coarse recycled concrete aggregate. To assess the fresh and hardened qualities of the concrete, performance tests utilizing naturally occurring fine sand and commercially generated coarse recycled concrete aggregate were conducted. The study aimed to compare the characteristics of recycled aggregate concrete with those of natural aggregate concrete, highlighting their similarities. Test results showed that the differences between RFCA and NFA concrete are smaller than previously reported for laboratory-crushed recycled aggregates. For mixtures without blast furnace slag and with similar proportions and workability, no significant differences were found in compressive and tensile strengths. Water absorption rates and carbonation levels were also comparable for both concrete types. However, recycled aggregate concrete exhibited approximately 12% higher abrasion loss and 25% higher drying shrinkage after one year. The splitting tensile strength to compressive strength ratio matched established values for equivalent grade concretes made with natural aggregates. Using blast furnace slag cement can further enhance durability. The superior properties of recycled concrete aggregate, compared to laboratory-crushed aggregates, are attributed to better aggregate grading and quality resulting from plant crushing operations.[29]

Silva et al., 2016, Properties of self-compacting concrete on fresh and hardened with residue of masonry and recycled concrete. The study evaluated the impact of recycled aggregates on the properties of self-compacting concrete (SCC). Five SCC mixes were tested, with natural coarse aggregate (NCA) replaced by recycled concrete aggregate (RCA) at varying levels from 0% to 100% by volume. The cement content was kept constant at 480 kg/m?3;, comprising Portland cement and 20% by weight of recycled materials (RM). An assessment of the Rheological properties of the fresh SCC using slump and flow tests, V-funnel, and L-box tests was carried out demonstrating suitable workability, segregation resistance, filling capacity, and passing ability. Although incorporating recycled coarse aggregate reduced mechanical properties due to poor adhesion between old mortar and aggregate, the results remained within acceptable limits for use in various construction elements.[30]

Li et al., 2016, Experimental study on mechanical behaviors of concrete with large-size recycled coarse aggregate. The paper aims to design and prepare a new type of recycled concrete aggregate (RCA) using recycled concrete as large-sized coarse recycled aggregates and to evaluate its strength indexes and failure patterns experimentally. The study involved creating RCA with large-sized coarse recycled aggregates LRCA up to 80 mm in size, simplifying the crushing process and enhancing production efficiency. Various parameters, including the incorporation rate of the LRCA, the maximum size of LRCA, the original strength of the recycled concrete, and the strength of the corresponding natural concrete aggregate (NCA), were considered. Cube-compressive tests were conducted to measure strength and analyze failure patterns. The results indicated that the cube compressive strength of recycled concrete aggregate RCA with large recycled concrete aggregates LRCA closely matched that of natural concrete aggregate NCA, showing only a 14?crease when the maximum size of LRCA was 80 mm and the incorporation rate was 40%. No significant reduction in compressive strength was observed when the incorporation rate was below 30%. The ratio of axial compressive strength to cube compressive strength for RCA with 80 mm LRCA was 12% lower than that of NCA. The difference in splitting tensile strength between RAC and NAC was less than 10%. Additionally, the cracking surfaces in the crushed specimens frequently passed through the LRCA. This study demonstrates the feasibility of producing high-strength RCA using LRCA, with minimal reductions in compressive and tensile strengths compared to NAC, while also highlighting the efficiency benefits due to the simplified crushing process.[31]

ANALYSIS OF RESULTS

Fresh Properties:

1. Fluidity and Flow Speed:

From studies [17], [18], and [21] it is observed that incorporating recycled aggregates in SCC mixes results in decreased workability due to higher water absorption and rougher surfaces of recycled aggregates. Slump flow tests consistently showed reduced fluidity as the percentage of recycled aggregates increased.  [20] noted that despite these challenges, introducing chemical admixtures helped maintain acceptable workability and stability parameters across various replacement levels.

2. Viscosity:

 Viscosity is examined in studies [19] and [28]. Results from these studies indicate that SCC with recycled aggregates demonstrates increased yield stress and plastic viscosity, which can be managed with appropriate superplasticizer dosages.

 [28] found that the Herschel-Bulkley and modified Bingham models effectively described the rheological behavior of SCC containing recycled concrete aggregates (RCA), indicating a significant influence of replacement levels on viscosity.

3. Filling Ability and Passing Ability:

 Studies [15] and [17] evaluated the filling capacity and ability of SCC to pass through small openings using V-funnel and L-box tests. SCC mixtures with recycled aggregates generally showed adequate filling ability and passing capacity, though performance slightly diminished as the recycled aggregate content increased.

 [20] confirmed these findings, with SCC maintaining suitable workability and resistance to segregation, passing ability, and filling capacity up to moderate replacement levels

4. Segregation Resistance:

 [24] and [30] noted that SCC with up to 50% recycled aggregate substitution exhibited satisfactory segregation resistance, assessed through sieve segregation tests. Segregation resistance was maintained by adjusting mix proportions and incorporating appropriate admixtures.

 [18] found that mixtures containing up to 40% RCA maintained good resistance to segregation, comparable to conventional SCC.

5. Compaction:

The study in [14] revealed that proper compaction could be achieved in SCC mixes with recycled aggregates, provided the mix design is appropriately adjusted to account for the higher absorption and rough texture of recycled aggregates. [31] highlighted that SCC with large-size recycled coarse aggregates (LRCA) demonstrated compaction and strength properties similar to conventional concrete, emphasizing the importance of maintaining a consistent mix design.

Hardened Properties:

1. Compressive Strength:

[15], [17], and [26] consistently showed a reduction in compressive strength with increasing recycled aggregate content. However, SCC mixes with up to 30% recycled aggregates still met the required compressive strength after 28 days.

[22] and [31] also confirmed that although compressive strength decreases with higher recycled aggregate content, SCC with moderate levels of RCA can still achieve satisfactory strength.

2. Splitting Tensile Strength:

Studies [15], [17], and [29] reported similar trends in splitting tensile strength, which generally decreased with higher recycled aggregate content. However, the splitting tensile strength to compressive strength ratio remained within acceptable ranges for equivalent grade concretes.

[12] observed that the presence of recycled aggregates influenced tensile strength, but high-quality recycled aggregates could achieve comparable results to natural aggregates.

3. Flexural Tensile Strength:

 [15] and [23] found that flexural strength generally decreased with increased recycled aggregate content. However, SCC with up to 20% fine recycled aggregates showed a minimal reduction in flexural strength, indicating potential for certain structural applications.

 [30] confirmed that flexural tensile strength remained within acceptable limits for construction use, provided the mix proportions and quality of recycled aggregates are properly managed.

4. Shrinkage:

Studies [12], [19], and [29] consistently reported higher drying shrinkage in SCC with recycled aggregates. This was attributed to the greater porosity and water absorption characteristics of recycled aggregates.

Despite the increased shrinkage, [12] noted that proper mix design and quality control can mitigate these effects, ensuring the structural integrity and durability of SCC with recycled aggregates.

CONCLUSION

The integration of recycled concrete aggregates (RCA) into self-compacting concrete (SCC) presents a promising approach to enhancing sustainability in construction. The reviewed studies provide valuable insights into the effects of RCA on the fresh and hardened properties of SCC.

Fresh Properties: Studies consistently show that the use of RCA generally results in a decrease in fluidity and flow speed due to the higher water absorption capacity of RCA. This necessitates the use of superplasticizers to achieve comparable fluidity to SCC made with natural aggregates (NA). Increased viscosity is a common outcome when incorporating RCA, attributed to the irregular shape and rough texture of the aggregates, which can hinder the flowability of SCC. RCA tends to decrease the filling ability and passing ability of SCC due to the increased surface roughness and angularity of the aggregates, which impedes the smooth flow of the mix through narrow spaces and around obstructions. The use of RCA can lead to higher segregation rates, especially at higher replacement levels, due to the weaker bond between the recycled aggregate and the new cement paste. However, proper use of viscosity-modifying agents can improve segregation resistance. The self-compacting nature of SCC is generally maintained with RCA, though adjustments in mix design, such as increased binder content and use of admixtures, are often necessary to compensate for the lower quality of RCA compared to NA.

Hardened Properties: There is a consensus that compressive strength decreases as the percentage of RCA increases. This is primarily due to the inferior quality and higher porosity of RCA. However, mixtures with up to 25% RCA can still achieve satisfactory compressive strengths for many structural applications. Similar to compressive strength, splitting tensile strength is adversely affected by higher RCA content. The presence of adhered mortar in RCA contributes to a weaker interfacial transition zone, leading to reduced tensile strength. Flexural strength generally shows a reduction with increasing RCA content. This reduction is attributed to the same factors affecting compressive and tensile strengths, with the old adhered mortar on RCA playing a significant role. SCC incorporating RCA exhibits higher shrinkage compared to SCC with NA. This is due to the higher water absorption and greater porosity of RCA, which results in more significant moisture loss over time.

Durability and Microstructural Characteristics: The incorporation of RCA tends to reduce the durability of SCC, particularly in terms of water absorption, permeability, and resistance to chloride ion penetration. However, these effects can be mitigated to some extent by optimizing the mix design and using supplementary cementitious materials like fly ash and silica fume. The microstructure of SCC with RCA is generally more porous and heterogeneous compared to SCC with NA. This results in a weaker interfacial transition zone and contributes to the reduced mechanical properties observed in such mixes.

Recommendations

To balance performance and sustainability, a replacement level of up to 25% RCA is recommended for maintaining satisfactory mechanical properties and workability in SCC. Incorporating superplasticizers and viscosity-modifying agents is crucial to compensate for the adverse effects of RCA on the fresh properties of SCC. The use of materials like fly ash and silica fume can enhance the durability and mechanical properties of SCC containing RCA.

In conclusion, while the use of RCA in SCC poses certain challenges, appropriate mix design and the use of chemical admixtures can mitigate these effects, making it a viable option for sustainable construction practices. Future research should focus on long-term performance and the development of standardized guidelines for the use of RCA in SCC.

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