Shear and flexural performance of reinforced concrete beams with recycled concrete aggregates

Abbreviations

CS

compressive strength

CR

compression reinforcement

FS

flexural strength

ITZ

interfacial transition zone

RCA

recycled concrete aggregate

RCB

reinforced concrete beam

TR

tensile reinforcement

SS

stirrup spacing

STS

splitting tensile strength

1 Introduction

In today’s world, the concrete that is taken from age-old highways and other structures is frequently regarded as useless and eliminated as garbage from deconstruction. Recycled concrete aggregate (RCA) is produced by collecting and breaking old concrete. The implementation of RCA in contemporary building projects is still considered a relatively recent method. The use of recycled concrete in construction materials is primarily driven by the aim of making the building process more sustainable and environmentally friendly. According to Oikonomou [1], significant environmental concerns in construction include the extraction of 50% of raw resources from nature, the consumption of 40% of total energy, and the generation of 50% of total waste.

Throughout the years, numerous investigators have undertaken studies on the impact of utilizing RCA in the manufacturing of concrete, as documented in the previous literature. Tabsh and Abdelfatah [2] examined the factors impacting the compressive and tensile strengths of concrete that was mixed with four different kinds of coarse aggregates: natural aggregates (NAs), recycled aggregates sourced from landfill, crushed concrete with a 30 MPa strength, and crushed concrete with a 50 MPa strength, all derived from either old concrete with a known or unknown strength. According to the research, recycled coarse aggregate lost more strength and hardness than NA, but it was still tolerable. Recycled concrete typically has 10–25% less strength than regular concrete created with natural coarse aggregate. In another study, investigating the mechanical properties of different mixes of recycled and NAs in concrete, Pacheco et al. [3] analysed the impact of recycled aggregates on concrete’s mechanical properties and variability, and they set standards using standard estimates for NA concrete’s variability. In contrast to complete recycled aggregate incorporation, the results indicated that intermediate degrees of recycled aggregate concrete integration enhanced variability. Regardless of whether or not recycled materials were used, high-strength concrete mixes exhibited a greater degree of variability than other mixes. The results of a different study by Medina et al. [4] were examined by analysing different aspects of a study on the partial use of low-quality 25 and 50% recycled aggregate derived from construction and demolition waste in place of natural coarse aggregate when making concrete in the 30 MPa strength class. Higher replacement rates of recycled aggregate and concentrations of floating particles decreased the physical and mechanical characteristics of hardened concrete, including strength, density, absorbency, and water absorption, while adding up to 50% recycled aggregate did not negatively impact the workability of fresh concrete. The findings of the research carried out by Bai et al. [5] provided a concise summary of the ways in which the recycled aggregate replacement rate influences the mechanical characteristics of concrete. To achieve this, the quantitative correlations between the composition of old bonded mortar and the efficiency of recycled aggregate were examined. Based on the results, concrete’s tensile and flexural strengths (FS) were less affected by the addition of RA than its compressive strength (CS). Cabral et al. [6] examined the heterogeneity in the physical properties of building and demolition particles, which obstructs the use of recycled aggregates in concrete production. The research found that altering the water–cement ratio and the amount of NAs substituted with recycled aggregates resulted in reduced CS in concrete constructed with recycled aggregates, with the exception of recycled fine aggregate from brick ceramic, which enhanced strength.

Research has also been conducted on reinforced concrete beams (RCBs). Knaack and Kurama [7] examined the flexural and shear properties of RCBs that substitute coarse NAs with RCAs. The study presented experimental findings of 12 twin pairs of normal-strength concrete beams, comparing measured data with analysis models and standard coding techniques. The study observed no significant differences in nonlinear behaviour and rupture between RCA and NA beams in flexure-critical or shear-critical scenarios. Seara-Paz et al. [8] examined the flexural behaviour of recycled concrete under increasing loads until failure. According to the study, which used eight RCBs with recycled coarse aggregates, two different water–cement ratios of 0.5 and 0.65, and four different replacement percentages of 0, 20, 50, and 100%, the cracking moment decreased as the replacement percentage increased because recycled concretes had lower tensile strength. RCBs showed larger curvatures than conventional concrete due to higher stresses in both the concrete and the steel reinforcement. Kang et al. [9] performed flexural tests on 28 high-strength and normal-strength RCBs to assess the potential use of RCA in concrete structures, analysing the similarities in fracture patterns between NA and RCA specimens. The study’s findings indicated that a replacement rate of RCA up to 30% did not markedly alter the flexural behaviour. Arezoumandi et al. [10] examined the FS of full-scale RCBs made of normal concrete and 100% RCA. The results showed that RCA beams’ ultimate FS was comparable to normal concrete beams, though their deflection was approximately 13% greater. Additional research was conducted by Arezoumandi et al. [11] to investigate the shear strength of full-scale beams constructed from conventional concrete and 100% RCA. For this reason, the experimental program included 12 beams in all, six of which represented each kind of concrete. The investigation revealed that 100% RCA beams had shear strength roughly 12% inferior to that of standard concrete beams. Sato et al. [12] examined the flexural behaviour of recycled concrete blocks by analysing several factors, such as the water-to-binder ratio, curing conditions, and tension reinforcement ratio. According to the findings, structural concrete may employ recycled aggregates with mechanical qualities comparable to or better than those of the recycled aggregates used in this study. If required, deflection must be managed by considering the physical characteristics of the recycled aggregate concrete.

In addition to the studies mentioned above, numerous investigations have explored flexural behaviour [13–18]. Regarding bearing strength, flexibility, maximum load and deformation, and cracking, Evangelista and De Brito [13] compared the outcomes of flexural tests conducted on RCBs that used recycled fine aggregates rather than NAs with those of a reference beam made with regular concrete. For this aim, two beams measuring 2.00 m in total length and 12 × 20 cm² in cross-section were assessed for every concrete mix. The findings indicate that the incorporation of recycled aggregates in structural parts does not substantially impact their flexural performance, but the load-bearing capacity is somewhat inferior to that of conventional concrete. Peng et al. [14] examined how RCA replacement ratio affected flexural performance and durability characteristics by exposing three beams with varying RCA replacement ratios – 0, 50, and 100% – to chloride attack and long-term loads. More chloride ion infiltration, worsened steel corrosion and cracking damage, and reduced flexural capacity are all consequences of an elevated RCA replacement ratio, according to the research. For the purpose of predicting the ultimate FS of recycled RCBs, Momeni et al. [15] developed an intelligent model and created a database from the literature that contained experimental tests conducted on concrete beams. Additionally, two experiments were carried out in the laboratory to improve the database that was described earlier. Based on the results of the flexural test, the FS of the RRC beam was 10% less than that of a normal beam composed of NAs. Another investigation on the response of recycled aggregate concrete to prolonged loading was carried out by Seara-Paz et al. [16]. Eight RCBs with four different replacement ratios (0, 20, 50, and 100%) and water-to-cement ratios of 0.50 and 0.65 were constructed for this investigation. According to these findings, recycled aggregate concrete exhibits more long-term deformations in terms of strain and deflection than conventional concrete. By using experimental data, Ji et al. [17] investigated the flexural performance of 15 aggregate concrete beams with replacement percentages of carbonated recycled concrete ranging from 0 to 30, 50, 70, and 100%. According to the results, beams with different proportions of recycled coarse aggregates but the same levels of carbonation showed only slight changes in ultimate FS and deflection, as well as a little decrease in the cracking moment. Ye et al. [18] conducted a study to evaluate the flexural behaviour of corroded reinforced recycled aggregate concrete beams compared to that of corroded reinforced NA concrete beams. The researchers subsequently investigated the potential of recycled aggregate concrete beams in corrosive environments. Four different amounts of RCA in the total mass of coarse material in concrete mixes were used as controls: 0, 33, 66, and 100. Two different concrete strengths, C30 and C60, were also used. The comparative findings demonstrate that, in contrast to the behaviour of NA concrete beams, the flexural behaviour of corroded reinforced recycled aggregate concrete beams with a suitable amount of recycled aggregate concrete is good. RCA replacement ratio has been expressed in terms of mass or volume, with variations across studies. Bai and Sun [19], Choi et al. [20], Knaack and Kurama [7], Seara-Paz et al. [8], and Alnahhal and Aljidda [21] characterized it as a volumetric proportion, while Sato et al. [12] and Ignjatović et al. [22] used the mass proportion. The RCA replacement ratio has also been studied in the context of shear behaviour [23–29]. To test the shear strength of 20 recycled aggregate concrete beams, Choi et al. [23] used various span-to-depth ratios (a/d = 1.50, 2.50, 3.25), longitudinal reinforcement ratios (s = 0.53, 0.83, 1.61%), and recycled aggregate replacement ratios (0, 30, 50, 100%). According to the test findings, as compared to NA concrete, the shear strength of concrete may decrease by as much as 30% at a 100% replacement ratio. Fathifazl et al. [24] performed another study. This study presented the findings of an examination into the shear capability of RCBs that lack stirrups. The beams were constructed with RCA–concrete, which consists of concrete utilizing RCAs, and the concrete mixes were formulated using an innovative technology. Arezoumandi et al. [25] conducted an experimental investigation to examine the mechanical characteristics and shear strength of full-scale beams, including three distinct longitudinal reinforcement ratios for 18 beams constructed using RCA, utilizing two RCA combinations with different RCA replacement ratios: one that replaced 50% of the pure aggregate with RCA and the other that replaced 100%. The findings of this investigation indicate that the RCA 100% replacement exhibits, on average, an 11% reduction in shear strength compared to the RCA 50% replacement and CC beams; yet, these beams demonstrated comparable shear resistance. Katkhuda and Shatarat [26] conducted an empirical and analytical investigation of the shear behaviour of ten full-scale RCBs composed of native, recycled, and treated recycled aggregates. Every beam was manufactured without stirrups with 50 or 100% recyclable aggregate. The performance of shear-critical beams was analysed by the reporting of load–deflection curves, ultimate load values, and crack propagation during static testing. A comparison was made between the experimental shear capacities of the beams and the theoretical values from several international codes and fracture mechanics approaches. When compared to natural and untreated recycled aggregate, the testing findings demonstrated that utilizing treated recycled aggregate increased the shear capacity of the beams by a small amount. Rahal and Alrefaei [27] reported the findings of an experimental study that examined how the shear strength of longitudinally RCBs was affected by the use of coarse RCAs. Natural coarse aggregates were replaced with coarse RCAs in the following percentages: 0, 10, 20, 35, 50, 75, and 100%. Partial replacement had an identical impact on all aggregate grades. According to the experimental findings and a comparison with the analytical models, the negative effects of adding RCAs to the concrete are not entirely considered by the square root of the CS factor, which is used to determine the shearing strength. Pradhan et al. [28] revealed the findings of an experimental study analysing the effect of coarse RCAs on the shear strength of longitudinally reinforced concrete beams (RCBs). In the following percentages, coarse RCAs were used in lieu of natural coarse aggregates: 0, 10, 20, 35, 50, 75, and 100%. The partial replacement had an equivalent effect on all aggregate classifications. The experimental data and comparison with analytical models indicate that the adverse impacts of including RCA into the concrete are not fully accounted for by the square root of the CS component, which is used to ascertain shearing strength. No general limitations currently exist on the use of coarse RCA in concrete mixes. Recommendations from studies [30,31] suggest that the maximum amount of coarse RCA that may be used as a substitute for coarse normal aggregate is 30%. However, this limit can increase to 50% or even 100% provided the mix design, batching methodology, and moisture state of the RCA are properly managed [32].

As mentioned above, the usage of RCA has been increasing and is encouraged as it is one way to preserve the environment and has many potential applications. For this reason, it is essential to have a comprehensive understanding of RCA properties to ensure the effectiveness of its use. Furthermore, the literature’s greatest deficiency is the inadequacy of determining the RCA utilization percentage, as evidenced by the previously mentioned studies. The purpose of this study is to provide information on the characteristics of RCA as well as its influence on the flexural and shear behaviour of RCBs. In addition, determining the optimal percentage of RCA usage, which remains unclear in the literature, is one of the unique aspects of this study.

2 Materials and methods

RCAs were used in this study to replace NAs. The RCAs were obtained from previously tested concrete specimens that had failed in earlier experiments. The original concrete was designed for a target CS of 20 MPa, and the average measured CS of the reference cubes was 23.83 MPa. As shown in Figure 1, the RCAs were produced by breaking down the failed cube and beam specimens into smaller pieces using a hammer.

Figure 1

Utilized RCA view in samples.

To ensure a valid comparison between the two aggregate types, efforts were made to match the particle size distributions of the NAs and the RCAs. Prior to their incorporation into the concrete mixes, several sieve analyses were conducted to adjust the gradation of the RCAs to be similar to that of the NAs. The final gradation results for the RCAs used in this study are presented in Figure 2. As shown in Figure 2, the particle size distribution curves of the RCAs and NAs are very similar, indicating that the aggregate gradations were successfully aligned. Although minor differences exist, particularly in the 1–10 mm range, where the RCA exhibits slightly fewer fine particles, the distributions are considered sufficiently close for laboratory-scale comparison. Both aggregate types achieved full (100%) sieve passage, and permeability increased with particle size, further supporting their comparability.

Figure 2

Sieve analysis for RCA and NA.

In this investigation, RCAs were used at replacement ratios of 10, 20, and 40%, along with reference specimens that contained no RCA. The mechanical properties of the specimens are presented in Figure 3. For the CS tests, 150 mm × 150 mm × 150 mm cube specimens were used [33]. Splitting tensile strength (STS) tests were conducted on 100 mm × 200 mm cylindrical specimens [34], and 100 mm × 100 mm × 400 mm beams were utilized for FS tests [35]. The average CS of the reference sample was 23.83 MPa. With 10, 20, and 40% RCAs, average CS was decreased to 22.88, 21.25, and 18.94 MPa, respectively. According to these results, the average CSs of the samples containing 10, 20, and 40% RCA decreased by 3.99, 10.99, and 20.52%, respectively, compared to the reference specimen.

Figure 3

Mechanical test results.

On the other hand, the average STS of reference was 1.87 MPa. With 10, 20, and 40% RCAs, average STS was decreased to 1.80, 1.57, and 1.33 MPa. According to these results, the average STS decreased by 3.74, 16.04, and 28.88% when 10, 20, and 40% RCA were used, respectively. Furthermore, the FS of the reference beam was 8.82 MPa. With 10, 20, and 40% RCAs, FS was decreased to 8.53, 7.69, and 6.58 MPa. In other words, when compared with the reference specimen, 3.29% FS loss occurred at 10% RCA content, 12.81% at 20% RCA content, and 25.40% at 40% RCA content.

As illustrated in Figure 3, the CS of concrete tends to decrease as the RCA content increases, which is consistent with previously reported findings [36]. This reduction is primarily attributed to the presence of two distinct interfacial transition zones (ITZs) in RCA concrete: one between the old aggregate and its adhered mortar, and another between the adhered mortar and the new cement paste [31,37–39]. In conventional concrete using NAs, a single ITZ forms between the aggregate and the surrounding mortar. However, in RCA concrete, the dual ITZ structure tends to be weaker and more porous, resulting in diminished mechanical performance [31,39–41]. Additionally, concrete mixtures incorporating RCA typically require more mixing water to achieve similar workability, which further reduces the overall strength. The residual aged mortar adhering to RCA particles also has lower density and strength, contributing to the reduction in CS [42]. A similar declining trend is observed in FS values, in agreement with findings in the literature [43]. In particular, studies have shown that FS decreases when saturated RCA is incorporated into the concrete mix [44].

A total of 24 RCBs were fabricated for the experimental program. The detailed properties of these specimens are presented in Table 1. The beams were categorized into two groups based on their intended failure modes: the first 12 beams were designed to exhibit ductile flexural behaviour (bending beams), while the remaining 12 beams were deliberately designed to be shear-deficient to promote shear failure (shear beams). To achieve the intended failure modes, variations were made in both the longitudinal reinforcement and stirrup spacing. All beams were constructed with compressive reinforcement of 2Ø6, while the tensile reinforcement consisted of 2Ø8, 2Ø10, or 2Ø12 bars, depending on the specimen. In the beam design, Ø6 stirrups were used with spacing values of 100, 160, 200, and 270 mm. Figure 4 illustrates the geometric details of the beam specimens, including the reinforcement layout and the four-point bending test setup used in the experimental program.

Figure 4

Reinforcement assembly: (a) bending, (b) shear, and (c) beam loading at four-points.

3 Experimental results

Table 2 presents the key parameters obtained from the load–displacement curves of each tested specimen. The maximum load recorded during testing is denoted as P _max, and the corresponding displacement at this load was determined. The stiffness at a maximum load was calculated as the ratio of P _max to its associated displacement. To define the yield point, a load value equivalent to 85% of P _max (denoted as P _u ) was selected. The corresponding displacement at P _u was identified as δ _y . Yield stiffness was calculated by dividing P _u by δ _y . On the descending branch of the curve, the displacement corresponding again to 85% of P _max was identified as δ _u . Finally, the ductility ratio for each specimen was computed as the ratio δ _u /δ _y .

The energy dissipation capacity of each specimen was calculated as the area under the load–displacement curve. These results are summarized in Table 3. The elastic energy dissipation (also referred to as yield energy) was calculated as the area under the curve up to the point where the load reached P _u (85% of P _max) on the ascending branch. The total energy dissipation was determined as the area under the curve up to the displacement value δ _u , which corresponds to 85% of P _max on the descending branch. The plastic energy dissipation was obtained by subtracting the elastic energy from the total energy dissipation. Representative load–displacement curves for selected specimens are shown in Figure 5, while the corresponding crack patterns are shown in Figure 6.

Figure 5

Load–displacement graph.

Figure 6

Crack behaviour for (a) bending and (b) shear.

3.1 Load-carrying behaviour of recycled aggregate concrete beams under shear and flexure

The results presented in Figure 7 show how the maximum load (P _max) values varied with RCA content and reinforcement diameter. In flexural beams with Ø8 tensile reinforcement, increasing the RCA ratio from 0 to 40% led to a gradual reduction in P _max: 38.93, 38.41, 36.46, and 35.92 kN – corresponding to decreases of 1.34, 6.34, and 7.73 relative to the reference specimen. When Ø10 tensile reinforcement was used, the P _max values were 54.55, 54.40, 48.85, and 48.42 kN for RCA contents of 0, 10, 20, and 40%, respectively. This represents decreases of 0.27, 10.45, and 11.24%. Similarly, with Ø12 tensile reinforcement, the values were 63.64, 63.58, 62.73, and 58.03 kN – corresponding to reductions of 0.09, 1.43, and 8.82% with increasing RCA. These results confirm a general trend: the flexural load capacity of beams tends to decrease with increasing RCA content. This reduction appears more pronounced in specimens with smaller reinforcement diameters (e.g. Ø8), whereas beams with larger diameters (e.g. Ø12) show a relatively moderate decline, suggesting that increasing longitudinal reinforcement may partially compensate for the strength loss introduced by RCA.

Figure 7

Load-carrying values according to changing RCA contents.

An additional comparison was made to evaluate the effect of longitudinal reinforcement diameter on flexural capacity at different RCA contents. When the RCA ratio was held constant at 0%, increasing the reinforcement from Ø8 to Ø10 and Ø12 raised the load capacity from 38.93 to 54.55 and 63.64 kN, representing increases of 40.12 and 63.47%, respectively. At 10% RCA, the corresponding load capacities were 38.41, 54.40, and 63.58 kN, resulting in increases of 41.58 and 65.57%. For 20% RCA, values of 36.46, 48.85, and 62.73 kN were observed, showing increases of 33.98 and 72.05%. Finally, at 40% RCA, load capacities increased from 35.92 to 48.42  and 58.03 kN with Ø10 and Ø12 reinforcements, corresponding to increases of 34.80 and 61.55%, respectively. As expected, increasing the reinforcement diameter consistently improved the flexural load capacity of the beams across all RCA contents. While the absolute capacity gains remained significant, the relative rate of improvement did not show a strictly decreasing trend with increasing RCA content. Notably, the highest relative gain from Ø8 to Ø12 was observed at 20% RCA. These results highlight the effectiveness of using larger diameter reinforcement to partially compensate for the strength reductions associated with higher RCA content, emphasizing the importance of proper reinforcement design in sustainable concrete applications.

A similar comparison was conducted to evaluate the effect of RCA content on shear capacity under varying stirrup spacings. For beams with 160 mm stirrup spacing, increasing the RCA ratio from 0 to 40% led to a decrease in P _max from 53.66 to 44.66 kN, corresponding to reductions of 0.30, 6.30, and 16.77% for 10, 20, and 40% RCA, respectively. With 200 mm spacing, the P _max values declined from 50.48 to 39.89 kN as RCA content increased, equivalent to losses of 11.49, 14.82, and 20.98%. At the widest spacing of 270 mm, the reductions were more severe: P _max dropped from 38.97 to 20.71 kN, representing declines of 7.16, 19.37, and 46.86% across increasing RCA ratios. These results clearly indicate that both higher RCA content and wider stirrup spacing significantly reduce the shear capacity of RCBs. Therefore, to maintain adequate shear strength in RCA-based beams, the RCA replacement ratio should be limited, and the stirrup spacing should be carefully optimized.

3.2 Rigidity

The flexural rigidity of an RCB refers to its resistance against bending deformation. In other words, higher flexural stiffness corresponds to lower curvature under the same bending moment. In structural engineering, concrete stiffness is often used to describe the material’s ability to resist displacement under loading. As presented in Figure 8, the stiffness of RCBs varies with stirrup spacing and RCA content. For specimens with Ø8 reinforcement, stiffness tended to decrease with increasing RCA content. However, when the reinforcement diameter was increased to Ø10 and Ø12, beams containing 20% RCA exhibited relatively higher stiffness values. Furthermore, an increase in stirrup spacing was also found to influence stiffness. Notably, the highest stiffness was observed in the specimen with 10% RCA and 270 mm stirrup spacing. These findings suggest that both reinforcement detailing and RCA content jointly affect the flexural rigidity of RCBs, and optimal combinations may provide improved performance despite the use of recycled materials.

Figure 8

Rigidity of specimens.

These results confirm that increasing the RCA content leads to a significant reduction in beam stiffness. Although the use of larger reinforcement diameters enhances stiffness, this improvement is not sufficient to fully compensate for the stiffness loss associated with RCA incorporation.

According to previous studies [45], the addition of coarse RCA into concrete mixtures leads to a reduction in the elastic modulus of concrete. The extent of this reduction depends on several factors, including the mechanical properties of RCA, its replacement ratio, and the overall mix design [46]. This behaviour can be attributed to the presence of residual adhered mortar in RCA, which is significantly weaker and more porous than NAs. Moreover, the RCA particles may also experience internal microcracking or other forms of structural degradation during the recycling process [47,48].

Based on these results, it is evident that the RCA content, longitudinal reinforcement diameter, and stirrup spacing play critical roles in determining the stiffness of RCBs. As the RCA ratio increases, a noticeable reduction in stiffness is observed. Conversely, increasing the reinforcement diameter contributes to higher stiffness values at maximum load. Although variations in stirrup spacing also affect stiffness, this influence may be more complex and context-dependent, especially in beams governed by shear behaviour.

3.3 Ductility

Ductility refers to a material’s ability to undergo significant plastic deformation prior to failure. In structural engineering, ductility is essential for energy dissipation and preventing sudden collapse. Ductile beams can dissipate substantial energy through deformation while maintaining load-carrying capacity. As shown in Figure 9, ductility ratios for flexural specimens tended to decrease with increasing RCA content, although a minor increase was observed at 40% RCA content; the ductility ratio remained significantly lower than the reference. For Ø8 reinforcement, ductility values were 12.40, 11.98, 11.68, and 11.38 for RCA contents of 0, 10, 20, and 40%, respectively, corresponding to reductions of 3.39, 5.81, and 8.23%. For Ø10 reinforcement, ductility dropped more sharply: from 10.05 to 5.61 as RCA content increased to 40%, with intermediate values of 8.09 and 2.22. The respective decreases were 19.50, 77.91, and 44.18%. Beams with Ø12 reinforcement exhibited ductility ratios of 5.37, 3.65, 2.20, and 3.05 for increasing RCA levels, corresponding to reductions of 32.03, 59.03, and 43.20%. This trend indicates a consistent decline in ductility with increasing RCA. A general trend of reduced ductility with increasing RCA content was observed, particularly for beams with Ø8 reinforcement. This decline is likely due to the inherent limitations of RCA, such as adhered mortar and internal microcracks, which tend to cause interfacial failure within the aggregate rather than promoting crack deflection around it [49]. However, at higher RCA contents, some irregularities emerged in specimens with Ø10 and Ø12 bars, suggesting that the interaction between RCA characteristics and reinforcement diameter is not strictly linear and may involve more complex mechanisms.

Figure 9

Ductility of specimens.

For shear-dominated specimens, ductility behaviour was more variable. At 160 mm stirrup spacing, increasing RCA from 0 to 40% resulted in ductility values of 1.91, 1.55, 1.54, and 1.63, reflecting reductions of 18.85, 19.37, and 14.66%. However, at 200 mm spacing, values of 1.43, 1.84, 1.65, and 1.87 were recorded, showing mixed results with percentage increases of 28.67, 15.38, and 30.77%. At 270 mm spacing, ductility values were 1.54, 1.23, 1.63, and 2.07, with an initial drop (20.13%) followed by increases of 5.84 and 34.42%.

These results indicate that although ductility tends to decline consistently in flexural specimens with increasing RCA content, shear-critical beams display more irregular patterns. Interestingly, wider stirrup spacings seem to improve ductility in beams with high RCA content, potentially due to enhanced deformation capacity and energy dissipation mechanisms. Therefore, appropriate stirrup detailing plays a key role in compensating for ductility losses in RCA-based shear-dominated elements.

3.4 Energy dissipation

Due to their inherent deformability, RCBs exhibit the capacity to dissipate mechanical energy under loading. Energy dissipation, a key structural performance indicator – especially for seismic applications – is typically quantified as the area under the load–displacement curve. This area captures both the elastic and plastic energy dissipated before failure. According to Liu et al. [50], energy dissipation is commonly assessed up to the point where the applied load drops to 90% of its peak value.

As shown in Figure 10 and summarized in Table 3, the total energy dissipation generally decreased with increasing RCA content. For Ø8-reinforced specimens, the highest energy dissipation was observed at 20% RCA, whereas in Ø10 and Ø12 specimens, maximum values of 3.042 and 1.669 kJ were recorded at 0% RCA, respectively. While some fluctuations exist across RCA levels, the overall trend indicates that higher RCA content tends to impair the energy dissipation capacity.

Figure 10

Energy dissipation of RCBs.

A similar pattern was observed in shear-dominant specimens. For instance, at 160 mm stirrup spacing, the highest energy dissipation (0.436 kJ) occurred at 0% RCA, with progressive declines observed at higher RCA ratios. At 200 mm spacing, a slight peak (0.303 kJ) was found at 10% RCA – marking a modest 3.77% increase over the 0% RCA case – followed by a reduction. At the widest spacing of 270 mm, energy dissipation again peaked at 0% RCA (0.110 kJ).

These results confirm a generally negative correlation between RCA content and energy dissipation under both flexural and shear loading conditions. This degradation is primarily attributed to the inherent material deficiencies of RCA, such as the presence of adhered old mortar, internal microcracks, and higher porosity, which weaken the stiffness and fracture resistance of the composite [51,52]. As also noted by Chen et al. [53], the heterogeneous and inconsistent nature of RCA can introduce variability in energy dissipation behaviour, particularly at higher replacement levels.

In conclusion, increasing RCA content tends to diminish the energy dissipation capacity of both flexural and shear-critical RCBs. Although minor improvements may be observed in certain shear specimens with wider stirrup spacing, the seismic performance of RCA-based members is generally compromised. Therefore, the use of RCA in earthquake-resistant design should be carefully evaluated, with particular attention to stirrup configuration and reinforcement detailing.

4 Discussion

5 Conclusion

This study investigated the effects of RCA incorporation on the flexural and shear behaviour of RCBs, wherein RCA replaced natural coarse aggregate at proportions of 0, 10, 20, and 40 wt% by weight. The experimental program focused on evaluating the influence of RCA content on critical structural performance indicators, including load-bearing capacity, stiffness, ductility, and energy dissipation.

• Increasing the RCA content from 0 to 40% led to a reduction in the load-bearing capacity of the beams in both flexural and shear behaviour. In shear-dominant specimens with 270 mm stirrup spacing, the shear capacity decreased by 46.86% (38.97/20.71 kN). In flexural specimens with Ø10 longitudinal reinforcement, the maximum reduction in flexural capacity was recorded as 11.24% (54.55/48.42 kN).

• In flexural specimens, the highest stiffness value was recorded in the RCB specimen incorporating Ø12 longitudinal reinforcement and 0% RCA content, reaching 6.15 kN·mm⁻¹. Although the specimen with 20% RCA content also exhibited relatively high stiffness (4.91 kN·mm⁻¹), a general declining trend was observed with increasing RCA content. This observation underscores the importance of longitudinal reinforcement in mitigating the adverse effects of RCA, whose inherent characteristics – such as adhered old mortar and internal microcracks – reduce the bond strength and stiffness of the composite material.

• In shear-dominant specimens, although wider stirrup spacing typically leads to reduced transverse confinement and stiffness, the maximum stiffness value of 9.18 kN·mm⁻¹ was unexpectedly observed in the specimen with 10% RCA content and 270 mm stirrup spacing. This result deviates from the expected trend and may be attributed to localized crack patterns or complex interactions between RCA properties and stirrup configuration. Such irregularities highlight the need for further investigation into the combined influence of RCA content and shear reinforcement detailing.

• The ductility ratios of RCBs decreased significantly as the RCA content increased from 0 to 40%. This reduction is attributed to the weaker interfacial transition zone between the old mortar and the new cement paste in RCA, which promotes internal cracking within the aggregate particles rather than crack deflection around them. As a result, the energy dissipation through plastic deformation is reduced, leading to lower ductility.

• Although it is expected that the ductility decreases as the RCA content increases, it is observed that this trend is broken in some specimens, especially at 20 and 40%. This can be explained by the heterogeneous nature of RCA, crack propagation interacting with reinforcement diameter and variability in energy dissipation mechanisms. Such a non-linear trend of ductility behaviour reveals the complexity of the mechanical effects of RCA.

• An inverse relationship was observed between the RCA content and the energy dissipation capacity of RCBs. The use of higher RCA ratios reduced the beams’ ability to absorb and dissipate energy, mainly due to the presence of aged mortar and increased porosity in the recycled aggregates. These characteristics lead to lower stiffness and reduced post-peak deformation capacity, ultimately diminishing overall energy dissipation.

• The findings of this study contribute to a better understanding of the structural performance of reinforced concrete incorporating varying levels of RCA in both flexural and shear behaviour. The experimental data presented can serve as a valuable reference for engineers and designers aiming to develop sustainable concrete solutions for structural applications.

Despite the insights gained from this study, several aspects remain open for further investigation. Future research may focus on the following topics to address existing gaps and enhance the understanding of RCA-incorporated structural concrete.

• The structural and mechanical performance of RCA-based concrete elements under elevated temperature exposure should be further investigated, particularly focusing on thermal degradation, residual strength, and microstructural changes.

• The dynamic behaviour of RCA-incorporated structural components under seismic and blast loading conditions requires comprehensive investigation to assess their viability in critical infrastructure and safety-sensitive applications.

• The long-term durability performance of RCA-based structural elements, including freeze–thaw resistance, carbonation depth, chloride ion penetration, and alkali-silica reaction susceptibility, should be systematically investigated under realistic environmental conditions.

• The bond behaviour between recycled concrete and reinforcing steel should be experimentally evaluated, as RCA-induced porosity and weaker interfacial zones may affect capacity and crack development.

• Numerical modelling should be conducted to validate the experimental results and predict the structural behaviour of RCA-incorporated beams under various loading conditions.