Interactive Influence of Recycled Concrete Aggregate and Recycled Steel Fibers on the Fresh and Hardened Performance of Eco-Efficient Fiber-Reinforced Self-Compacting Concrete

Abstract

This study investigates the synergistic influence of recycled concrete aggregate (RCA) and recycled steel fibers (RSF) on the fresh and hardened performance of eco-efficient fiber-reinforced self-compacting concrete (SCC). Twelve C30/37.5 mixtures were produced using demolition waste as coarse RCA at replacement levels of 25, 50, 75, and 100% by mass, combined with RSF recovered from scrap tires at volume fractions of 0.25, 0.50, and 0.75%. Fresh properties were assessed in accordance with EFNARC guidelines using slump-flow (T500), V-funnel, L-box, and J-ring tests, while hardened performance was evaluated through compressive, splitting tensile, and flexural strengths at 28 days, together with density and ultrasonic pulse velocity (UPV). Increasing RCA and RSF contents reduced workability, reflected in lower slump-flow diameters and higher T500 and V-funnel times, although most mixtures maintained satisfactory self-compacting behaviour. Compressive strength decreased with RCA content and, to a lesser extent, with higher RSF, with a maximum reduction of about 39% at 100% RCA relative to the control mix, yet values remained structurally acceptable. In contrast, RSF markedly enhanced tensile and flexural responses: at 25% RCA, 0.75% RSF increased splitting tensile and flexural strengths by approximately 41% and 29%, respectively, compared with the corresponding fiber-free mix. RCA reduced density and UPV by about 10–14%, but these reductions were partially mitigated by RSF addition. Overall, the results demonstrate that SCC with moderate RCA (25–50%) and RSF (0.50–0.75%) can achieve a favourable balance between rheological performance and enhanced tensile and flexural behaviour, offering a viable composite solution for sustainable structural applications.

1. Introduction

The construction sector is increasingly oriented toward circular-economy principles, with particular emphasis on diverting construction and demolition (C&D) waste from landfills and reducing the extraction of virgin raw materials. Within this context, self-compacting concrete (SCC) has become an enabling technology for sustainable and resilient construction because it can flow under its own weight, fill heavily reinforced formwork, and consolidate without external vibration. SCC is commonly characterized by its filling ability, passing ability, and resistance to segregation, which are verified using standardized tests such as slump-flow, V-funnel, and L-box, as summarized in the EFNARC guidelines and related SCC specifications [1,2,3,4,5,6,7]. The elimination of vibration can reduce on-site energy consumption and noise and may improve the reproducibility of in situ compaction, thereby supporting the wider adoption of low-carbon binders and recycled constituents in high-quality concrete.

In recent years, an expanding body of work has investigated the feasibility of producing SCC with recycled concrete aggregate (RCA), motivated by the need to valorize C&D waste. Nevertheless, RCA typically contains adhered mortar and microcracks, exhibits higher porosity and water absorption, and presents more irregular shape and texture than natural aggregate [8,9,10,11,12]. These characteristics can increase water and superplasticizer demand, elevate yield stress and plastic viscosity, and reduce workability retention—effects that become more pronounced at high replacement ratios and when both coarse and fine RCA are used [13,14,15,16,17,18,19,20,21,22]. At the hardened state, the presence of residual mortar and a weaker interfacial transition zone may reduce compressive strength and stiffness and increase permeability-related durability indicators if mixture proportioning is not adjusted [23,24,25,26,27,28]. Recent MDPI studies, however, demonstrate that SCC with high RCA contents—including full replacement of coarse natural aggregate—can satisfy SCC fresh-property requirements and achieve competitive mechanical performance when accompanied by judicious binder and admixture optimization and, in some cases, fiber reinforcement [29,30,31,32,33].

Accordingly, current research on RCA-based SCC has shifted from simple replacement studies to performance-oriented proportioning and conditioning strategies. Reported approaches include pre-saturation or controlled moisture conditioning of RCA (or tailored water addition to compensate for early absorption), the use of limestone filler or other fine powders to enhance packing density and viscosity, and the incorporation of supplementary cementitious materials (SCMs) such as ground granulated blast-furnace slag to refine pore structure and improve workability retention [26,27,30]. Systematic analyses and meta-analyses indicate that mixture robustness depends on the coupled selection of water-to-binder ratio, paste volume, type and dosage of polycarboxylate-based superplasticizer, and aggregate gradation, rather than on RCA replacement alone [28,30,31]. Particle-packing-based SCC proportioning methods, such as that proposed by Nan Su and co-workers, remain widely used as a baseline and have been adapted for recycled aggregates by adjusting the packing factor and paste content [16,29,30]. These developments suggest that the sustainable design of RCA-based SCC should be framed as a multi-parameter optimization problem balancing rheology, strength, and durability.

In parallel, the incorporation of steel fibers into SCC has been shown to enhance tensile strength, fracture energy, and impact/fatigue resistance through crack bridging and pull-out mechanisms; however, fibers can impair flowability and passing ability if not supported by adequate paste cohesiveness [13,14,17,34]. Recycled steel fibers (RSF), including recycled tire steel fibers, offer an attractive circular-economy alternative to industrial fibers. Recent MDPI contributions confirm the technical viability of recycled tire steel fibers as reinforcement, reporting improved post-cracking response and fracture-related indices in concrete and demonstrating that lightweight SCC can accommodate recycled tire steel fibers at moderate volume fractions without deleterious effects on flowability when mixture design is optimized [32,35]. Moreover, the interaction between recycled steel fibers and the surrounding matrix—including bond behavior in aggressive environments—has begun to receive targeted attention; for example, recycled tire steel-fiber-reinforced SCC with BFRP reinforcement has been evaluated after seawater exposure, highlighting the need to consider durability alongside rheology and strength [33]. Comprehensive reviews further emphasize that fiber geometry, surface condition, and dosage govern both the fresh-state rheology and the resulting mechanical benefits, underscoring the importance of SCC-specific optimization rather than direct transfer of conventional fiber-reinforced concrete mixture proportions [34].

Despite these advances, comparatively few studies have systematically addressed SCC mixtures that simultaneously incorporate RCA and RSF across broad ranges of aggregate replacement and fiber contents while maintaining compliance with SCC rheological criteria. The combined use of RCA and RSF is non-trivial because RCA can increase variability, water demand, and paste requirements, whereas fibers can reduce passing ability and increase the risk of blocking or heterogeneity if dispersion is inadequate [8,28,34].

Accordingly, the novelty of the present work lies in a systematic and EFNARC-compliant assessment of the coupled (synergistic) effects of demolition-derived coarse RCA and tire-derived RSF on both fresh and hardened performance of SCC. Specifically, this study contributes: (i) a parametric experimental matrix spanning 0–100% coarse RCA replacement and 0.25–0.75% RSF volume fractions in a target C30/37.5 SCC framework; (ii) comprehensive verification of filling and passing ability using slump-flow (T500), V-funnel, L-box, and J-ring tests, enabling direct benchmarking against EFNARC limits; (iii) coupled evaluation of mechanical performance (compressive, splitting tensile, and flexural strengths) together with density and ultrasonic pulse velocity (UPV) as quality-related indicators; and (iv) identification of a practical mixture domain that preserves self-compacting behavior while enhancing tensile and flexural response through recycled fiber bridging, thereby supporting performance-based selection of eco-efficient SCC mixtures [8,25,29,32].

On this basis, SCC mixtures incorporating varying RCA replacement ratios and RSF dosages were designed using established SCC proportioning principles and evaluated in a consistent experimental program. The overarching objective is to identify RCA–RSF combinations that maximize workability and stability while providing enhanced mechanical performance and improved sustainability, thereby contributing data needed for practical and performance-based design of environmentally responsible SCC.

2. Experimental Program

To accomplish the research objectives, twelve concrete mixes were prepared using demolition waste (crushed concrete) as coarse aggregate. Recycled materials were incorporated at varying percentages (25%, 50%, 75%, and 100%) to replace the coarse aggregate. Additionally, steel fibers obtained from recycled scrap tires were added at ratios of 0.25%, 0.5%, and 0.75% by volume of concrete to enhance the properties of self-compacting concrete (SCC). The optimal fiber content for the fresh state of SCC was examined. A cube specimen measuring 10 × 10 × 10 cm was tested to assess the compressive strength and density of the mixes at 7 and 28 days. Cylindrical specimens with a diameter of 10 cm and a length of 20 cm were used to evaluate the splitting tensile strength, while prisms measuring 10 × 10 × 50 cm were employed to determine the flexural strength of the mixes.

2.1. Materials

The materials used in this study are Portland Cement (type I) produced by MASS cement plant (42.5 R—B.S) 12/96 in Kurdistan-Iraq. Natural rounded river aggregate was used as coarse aggregate with a maximum size of 12 mm, recycled coarse aggregate (RCA) was obtained from demolition of concrete cubes tested from labs by projects inside Erbil city, natural river sand was used as fine aggregate with a maximum size of 4.75 mm, recycled steel fiber (RSF) recovered from scrap tires, high water reduction admixture (HWRA) as super plasticizer (SP) was used to provide high workability that essentially required for SCC, and Silica fume is also used by replacement of 2.85% of cement; the properties of the used materials are listed in the

Table 1. Chemical composition and physical properties of Cement.

Chemical Composition	OPC Type I (42.5 R)
CaO	65.10%
SiO2	19.10%
Al2O3	4.20%
Fe2O3	2.70%
SO3	2.90%
MgO	1.40%
Na2O	0.65%
K2O	0.98
Physical Properties	OPC Type I (42.5 R)
Specific Gravity (g/m3)	3.16
Specific surface area (m2/kg)	325.2
Loss on ignition	3.9%

,

Table 2. Characterization of the Physical Properties of Natural Aggregates.

Physical Properties	Coarse Aggregate	Fine Aggregate
Specific Gravity (g/m3)	2.64	2.68
Water Absorption %	0.41	0.24
Fineness Modulus	6.4	2.75

,

Table 3. Physical Characterisation of Recycled Coarse Aggregates.

Physical Properties	Recycled Coarse Aggregate
Specific Gravity (g/m3)	2.52
Water Absorption %	4.3

,

Table 4. Physical properties of Recycled Steel fiber.

Physical Properties	Recycled Steel Fiber
Diameter(mm)	0.25–0.3
Length(mm)	20–35
Specific Gravity (g/m3)	7.85
Tensile Strength (Mpa)	1250

,

Table 5. Chemical compositions and physical properties of Superplasticizer.

Chemical Compositions and Physical Properties	Superplasticizer
Form	Liquid
Color	Light Yellow
Odor	Slight/Faint
Boiling Point (C)	>100
Freezing point	−4
Relative Density	1.05–1.08
Water Solubility	Soluble

and

Table 6. Chemical compositions and physical properties of Silica Fume.

Chemical Composition	Silica Fume
CaO	1.50%
SiO2	95.10%
Al2O3	1.20%
Fe2O3	1%
SO3	0.12%
MgO	0.9%
Na2O	0.24%
K2O	0.78
Specific Gravity (g/m3)	2.21
Specific surface area (m2/kg)	2.0
Loss on ignition	1.5%

below:

2.2. Mix Proportions

The mix design followed EFNARC guidelines for self-compacting concrete (SCC), targeting a C30/37.5 grade. A control mix was initially prepared based on the trial mix design, while four additional mixes included partial replacement of coarse aggregate with recycled concrete aggregate at replacement levels of 25%, 50%, 75%, and 100%, respectively. The materials were first mixed in their dry state. Subsequently, 70% of the total water was added and thoroughly mixed. The remaining 30% of water, combined with a superplasticizer, was then incorporated into the mix. The recycled steel fibers were added separately to SCC mixtures at volume fractions 0.25%, 0.5%, and 0.75% of concrete. The flow properties were evaluated using slump flow, J-Ring, V-funnel, and L-box tests. The hardened properties of the concrete, including compressive strength, split tensile strength, and flexural strength, were assessed at 28 days. quantities and proportion details of all mixes are shown in

Table 7. Concrete mix proportioning for recycled SCC grade C30/37.5 design mix (Kg/m³).

Mix Code		Cement kg/m3	Fine Aggregate (Sand) kg/m3	Coarse Aggregate (Gravel) kg/m3	Recycled Coarse Aggregate kg/m3	Recycled Steel Fiber kg/m3	Silica Fume kg/m3	Water Liter/m3	Admixture (SP) Liter/m3
	CM	350	925	850	0	0	10	180	12
R25	M1	350	925	638	212	19	10	180	12
R50	M2	350	925	425	425	19	10	180	12
R75	M3	350	925	212	638	19	10	180	12
R100	M4	350	925	0	850	19	10	180	12
R25	M5	350	925	638	212	39	10	180	12
R50	M6	350	925	425	425	39	10	180	12
R75	M7	350	925	212	638	39	10	180	12
R100	M8	350	925	0	850	39	10	180	12
R25	M9	350	925	638	212	59	10	180	12
R50	M10	350	925	425	425	59	10	180	12
R75	M11	350	925	212	638	59	10	180	12
R100	M12	350	925	0	850	59	10	180	12

below:

2.3. Casting and Testing Methods

A drum mixer was employed for all batching operations. Initially, the fine and coarse aggregates were dry-mixed for approximately 3 min, during which water was added in accordance with their measured absorption capacity to achieve a surface-saturated condition. Silica fume and cement were then introduced, and the mixture was blended for a further 5 min. Subsequently, the mixing water together with 90% of the total superplasticizer dosage was added and mixing continued for an additional 5 min. The recycled steel fibers were then gradually dispersed manually into the rotating mixer, followed by the remaining 10% of the water/superplasticizer solution. Mixing was maintained for a further 3–5 min until the mixture exhibited the characteristic flowability and homogeneity associated with self-compacting concrete (SCC).

2.3.1. Rheological Properties

The fresh properties of the self-compacting concrete were evaluated in accordance with the guidelines and performance limits specified in EFNARC [1], as no dedicated standard for fiber-reinforced SCC is currently available. Consequently, the same EFNARC provisions were adopted for mixtures containing fibers. In this study, slump-flow diameter, T₅₀₀ slump-flow time, V-funnel flow time, J-ring, L-box ratio, and segregation resistance tests were performed to characterize the filling ability, passing ability, and stability of the SCC mixtures.

2.3.2. Mechanical Properties

Hardened properties were determined at 28 days using calibrated testing equipment. Compressive strength was measured on 100 mm cubes using a hydraulic compression testing machine (Model: UTEST EN 12390-4), under a controlled loading rate consistent with ASTM C39 (stress rate: 0.25 ± 0.05 MPa/s). Splitting tensile strength was tested on 100 mm × 200 mm cylinders using the same testing frame equipped with a splitting tensile fixture (Model: UTEST EN 12390-4) in accordance with ASTM C496; the load was applied continuously without shock to achieve a splitting tensile stress rate within the ASTM-specified range (0.7–1.4 MPa/min). Flexural strength was obtained from four-point bending tests on 100 mm × 100 mm × 500 mm prisms using a flexural testing frame and third-point loading arrangement; the clear span was 400 mm and the rate of increase in extreme-fiber stress was maintained within the ASTM C78 limits (0.86–1.21 MPa/min). For all mechanical tests, load and time histories were recorded digitally.

2.3.3. Density and Ultrasonic Pulse Velocity Testing

In this study, the physical properties of self-compacting concrete (SCC) incorporating recycled concrete aggregate (RCA) and recycled steel fibers (RSF) were evaluated using density and ultrasonic pulse velocity (UPV) measurements. The density of hardened concrete was determined in accordance with standard procedures by calculating the mass-to-volume ratio of specimens cured for 28 days, while UPV testing was performed in accordance with ASTM C597 to assess the internal homogeneity and overall quality of the SCC mixtures

3. Experimental Results and Discussion

3.1. Fresh Properties of Recycled Self-Compacting Concrete

All fresh-state tests were conducted on three independent batches per mixture, and the results are reported as mean values with the corresponding standard deviations (

Table 8. Rheological Performance of All Concrete Mixes.

Mix Code	RCA Content (%)	RSF Content (%)	Slump Flow (mm)	Slump T500 (s)	V-Funnel (s)	L-Box (H2/H1)	J-Ring (mm)
Control Mix	0%	0%	780 ± 12	2.0 ± 0.1	7.5 ± 0.3	0.98 ± 0.01	700 ± 15
	25%	0%	700 ± 15	2.5 ± 0.2	8.0 ± 0.3	0.95 ± 0.02	680 ± 14
M1		0.25%	680 ± 18	3.0 ± 0.2	8.5 ± 0.4	0.93 ± 0.02	660 ± 16
M5		0.5%	660 ± 20	3.5 ± 0.3	9.0 ± 0.4	0.90 ± 0.03	640 ± 18
M9		0.75%	640 ± 22	4.0 ± 0.3	9.5 ± 0.5	0.88 ± 0.03	620 ± 20
	50%	0%	680 ± 17	3.0 ± 0.2	9.0 ± 0.4	0.92 ± 0.02	660 ± 16
M2		0.25%	660 ± 19	3.5 ± 0.3	9.5 ± 0.5	0.90 ± 0.03	640 ± 18
M6		0.5%	640 ± 21	4.0 ± 0.3	10.0 ± 0.5	0.87 ± 0.03	620 ± 19
M10		0.75%	620 ± 23	4.5 ± 0.4	10.5 ± 0.6	0.85 ± 0.04	600 ± 21
	75%	0%	650 ± 18	3.5 ± 0.3	9.5 ± 0.4	0.90 ± 0.03	630 ± 17
M3		0.25%	630 ± 20	4.0 ± 0.3	10.0 ± 0.5	0.88 ± 0.03	610 ± 18
M7		0.5%	610 ± 22	4.5 ± 0.4	10.5 ± 0.6	0.85 ± 0.04	590 ± 20
M11		0.75%	780 ± 12	2.0 ± 0.1	7.5 ± 0.3	0.98 ± 0.01	700 ± 15
	100%	0%	700 ± 15	2.5 ± 0.2	8.0 ± 0.3	0.95 ± 0.02	680 ± 14
M4		0.25%	680 ± 18	3.0 ± 0.2	8.5 ± 0.4	0.93 ± 0.02	660 ± 16
M8		0.5%	660 ± 20	3.5 ± 0.3	9.0 ± 0.4	0.90 ± 0.03	640 ± 18
M12		0.75%	640 ± 22	4.0 ± 0.3	9.5 ± 0.5	0.88 ± 0.03	620 ± 20
EFNARK Limits			650–800	2–6	8–12	0.8–1.0	650–800

Note: Values represent the mean of three independent tests ± standard deviation (SD). All results fall within EFNARC [1] acceptance limits for self-compacting concrete.

). As expected, a degree of experimental scatter was observed in the fresh-state rheological measurements of SCC. This variability can be attributed to the high sensitivity of SCC to minor fluctuations in moisture condition and admixture effectiveness, as well as to the intrinsic heterogeneity of recycled concrete aggregate (RCA) and the challenges associated with achieving uniform dispersion and orientation of recycled steel fibers (RSF) within the fresh matrix.

At higher RSF dosages, additional sources of variability may arise from fiber–fiber interactions, occasional local clustering, and the associated increase in interparticle friction. These mechanisms can influence flowability (filling ability) as reflected by slump-flow diameter and T500, and viscosity-related performance as indicated by V-funnel time. Similarly, RCA surface roughness and elevated water absorption may contribute to small batch-to-batch differences that affect both filling ability and passing ability. Nevertheless, despite this natural scatter, the results exhibit consistent and reproducible trends across the full RCA–RSF matrix. In particular, the progressive reduction in slump-flow and passing ability (L-box and J-ring), together with the increase in T₅₀₀ and V-funnel times, provides robust evidence of the systematic influence of RCA replacement and RSF addition on fresh-state behavior. Importantly, the measured values remain within the EFNARC [1] acceptance limits (Table 8), confirming that adequate filling ability, passing ability, and stability were maintained for self-compacting performance.

As summarized in Figure 1, the combined use of RCA and RSF produces a systematic shift in the fresh-property indicators toward the less-flowable end of the EFNARC window. The reduction in normalized slump-flow and J-ring flow, together with the increase in normalized T500 and V-funnel times, confirms the progressive rise in mixture viscosity and internal friction as RCA replacement increases and fibers are introduced. Nevertheless, the normalized values generally remain within the EFNARC acceptance band (0–1), demonstrating that the adopted SCC proportioning strategy maintained adequate filling ability, passing ability, and stability across the investigated RCA–RSF matrix. This figure therefore provides a compact, multi-criteria verification that EFNARC compliance is preserved while highlighting the expected trade-off between fresh workability and the sustainability/mechanical benefits of recycled constituents.

3.1.1. Slump Flow

As demonstrated in Figure 2 and Figure 3, slump-flow diameter decreased consistently with increasing recycled concrete aggregate (RCA) replacement and steel fiber content, reflecting the progressive increase in mix viscosity. For the fiber-free control mixtures, slump flow decreased from 700 mm at 25% RCA to 600 mm at 100% RCA, whereas the incorporation of steel fibers led to a further reduction, reaching 540 mm at 0.75% fiber content for the mix with 100% RCA. Conversely, the T₅₀₀ time (the time required for the slump flow to reach 500 mm) increased with both RCA level and fiber dosage, confirming the higher resistance to flow. The control mix exhibited an increase in T₅₀₀ from 2.5 s at 25% RCA to 4.0 s at 100% RCA, while the addition of steel fibers raised this value to 5.5 s at 0.75% fiber content for 100% RCA.

3.1.2. The V-Funnel Flow

The V-funnel flow time exhibited a trend consistent with the slump-flow and T₅₀₀ results, increasing with both RCA replacement level and steel fiber content, which reflects the progressive rise in mix viscosity and internal friction. As shown in Figure 4, the fiber-free mixtures recorded V-funnel times increasing from 8.0 s at 25% RCA to 10.0 s at 100% RCA. The incorporation of recycled steel fibers further accentuated this effect: at each RCA level, mixes containing 0.25–0.75% RSF showed longer flow times than their corresponding plain mixes, with the maximum value of 11.5 s obtained for the mixture with 0.75% RSF and 100% RCA. This behaviour indicates that the combined use of RCA and RSF reduces the rate of flow through the V-funnel, although all measured values remain within the acceptable range for self-compacting concrete.

3.1.3. The L-Box Blocking Ratio (H₂/H₁)

As illustrated in Figure 5, the L-box blocking ratio (H₂/H₁), which reflects the passing ability and overall flowability of SCC, decreased slightly with increasing recycled aggregate and steel fiber contents. For the fiber-free mixtures, the L-box ratio declined from about 0.95 at 25% RCA to 0.85 at 100% RCA, indicating a modest reduction in the capacity of the mix to pass through simulated reinforcement. The incorporation of recycled steel fibers further accentuated this trend: at each RCA level, mixes with 0.25–0.75% RSF exhibited lower H₂/H₁ values than their corresponding plain mixes, with the minimum ratio of approximately 0.78 recorded for the mixture containing 0.75% RSF and 100% RCA. These results, as shown in Figure 5, suggest a slight restriction in flow and passing ability at high RCA and fiber dosages, although most mixes remained close to the EFNARC recommended range for self-compacting concrete.

3.1.4. J-Ring Assessment

Figure 6 presents the J-ring test results, which confirm the progressive influence of RCA and recycled steel fibers (RSF) on the passing ability of the SCC mixtures. For the fiber-free mixtures, the J-ring flow diameter decreased from approximately 680 mm at 25% RCA to about 580 mm at 100% RCA, indicating a gradual reduction in the ability of the concrete to pass through simulated reinforcement as RCA content increased. The inclusion of RSF further accentuated this trend; at 100% RCA, the J-ring flow decreased from about 580 mm without fibers to approximately 520 mm at 0.75% RSF, reflecting increased blocking effects associated with fiber interlocking and higher internal friction. Nevertheless, as shown in Figure 6, the mixtures retained satisfactory self-compacting performance, and the measured J-ring values remained within the EFNARC acceptance criteria for passing ability.

3.2. Mechanical Properties of the Recycled SCC Mixtures

The hardened mechanical behaviour of the recycled self-compacting concrete (SCC) mixtures was evaluated through compressive, splitting tensile, and flexural strength tests on standard specimens after 28 days of curing. As summarized in

Table 9. Mechanical properties of all SCC mixtures incorporating RCA and RSF.

Mix Code	RCA Content (%)	RSF Content (%)	Compressive Strength (MPa)	Splitting Tensile Strength (MPa)	Flexural Strength (MPa)
Control Mix	0%	0%	44.2	3.4	6.5
	25%	0%	41.4	3.2	6.2
M1		0.25%	42.1	3.7	6.9
M5		0.50%	40.2	4.1	7.4
M9		0.75%	38.0	4.5	8.0
	50%	0%	35.6	3.0	6.0
M2		0.25%	36.5	3.5	6.8
M6		0.50%	32.0	4.0	7.5
M10		0.75%	30.5	4.3	7.8
	75%	0%	30.0	2.8	5.8
M3		0.25%	29.5	3.3	6.5
M7		0.50%	27.0	3.7	7.0
M11		0.75%	25.5	4.0	7.3
	100%	0%	27.0	2.5	5.5
M4		0.25%	24.5	3.0	6.2
M8		0.50%	23.0	3.4	6.7
M12		0.75%	22.5	3.7	7.0

, increasing the recycled concrete aggregate (RCA) replacement level led to a systematic reduction in compressive strength, which can be attributed to the lower density, higher porosity, and weaker interfacial transition zones associated with RCA. In contrast, the incorporation of recycled steel fibers (RSF) produced a pronounced enhancement in the tensile-related properties: both splitting tensile and flexural strengths increased with fiber dosage owing to improved crack bridging and control. The mixtures containing the highest fiber volume fraction (0.75% RSF) exhibited the greatest gains in splitting tensile and flexural strengths, even at elevated RCA contents, indicating that RSF can partially compensate for the adverse influence of RCA on mechanical performance. Overall, these results show that, although compressive strength diminishes with increasing RCA content, the judicious use of steel fibers significantly improves the tensile and flexural response of recycled SCC, leading to a more ductile and crack-resistant composite material.

The mechanical properties of recycled aggregate and recycled steel fiber self-compacting concrete show distinct trends with varying RCA and fiber content.

3.2.1. Compressive Strength Development

Figure 7 demonstrates that compressive strength is primarily governed by RCA replacement level, exhibiting a clear downward trend as natural coarse aggregate is progressively substituted by RCA. This behaviour is consistent with the higher porosity and weaker interfacial transition zone associated with RCA, which reduce the effective load-carrying capacity of the hardened matrix. At a given RCA level, the influence of RSF on compressive strength is comparatively secondary: a marginal improvement may be observed at low fiber dosage, while higher fiber contents can lead to slight reductions. This response is attributed to the workability/compaction sensitivity of fiber-reinforced SCC—greater fiber volume increases internal friction and the risk of entrapped air or local heterogeneity, which can outweigh any microcrack-restraining benefit of fibers under predominantly compressive loading. Importantly, despite the systematic reductions at high RCA contents, the overall pattern in Figure 7 remains consistent and supports the broader conclusion that RCA governs compressive strength, whereas RSF contributes more significantly to tensile-related performance.

3.2.2. Splitting Tensile Strength Behaviour of SCC Incorporating RCA and RSF

The splitting tensile strength of the recycled SCC mixtures increased markedly with the addition of recycled steel fibers, as shown in Figure 8. For the fiber-free control mixes, tensile strength decreased with increasing RCA content, dropping from 3.2 MPa at 25% RCA to 2.5 MPa at 100% RCA, reflecting the weaker and more porous nature of the recycled aggregate. When steel fibers were incorporated, this negative effect was more than compensated: at 100% RCA, the splitting tensile strength improved from 2.5 MPa at 0% fiber to 3.7 MPa at 0.75% fiber. The highest tensile strength of 4.5 MPa was obtained for the mixture with 25% RCA and 0.75% fiber, demonstrating that increasing fiber volume fraction enhances crack-bridging capacity and significantly improves the tensile resistance of recycled SCC.

3.2.3. Flexural Strength Development of SCC Mixtures

The flexural strength of the recycled SCC mixtures exhibited a trend similar to that of the splitting tensile strength, showing clear improvements with increasing steel fiber content, as presented in Figure 9. For the fiber-free control mixes, flexural strength decreased with higher RCA replacement, from 6.2 MPa at 25% RCA to 5.5 MPa at 100% RCA, reflecting the weaker matrix associated with recycled aggregates. The incorporation of recycled steel fibers substantially enhanced flexural performance: at 25% RCA, the mix with 0.75% fiber achieved a maximum flexural strength of 8.0 MPa, while at 100% RCA the corresponding value reached 7.0 MPa. Overall, although compressive strength tends to decline with increasing RCA and fiber content, the significant gains in tensile and flexural strengths indicate improved ductility and a more favourable flexural response in the recycled SCC mixtures.

3.3. Density and Ultrasonic Pulse Velocity (UPV) of Recycled SCC Mixtures

Previous studies have consistently shown that replacing natural coarse aggregate with recycled concrete aggregate (RCA) leads to a systematic reduction in concrete density, primarily because RCA exhibits lower specific gravity and higher porosity than natural aggregates [20,21,22,23,24,25,26,27,28,29]. The same trend was observed in this study: as illustrated in Figure 10, the density decreased from 2355 kg/m³ for the control mix (0% RCA, 0% RSF) to 2115.8 kg/m³ for the mix with 100% RCA and no fibers. At each RCA level, the incorporation of recycled steel fibers (RSF) recovered from post-consumer waste tires produced a slight but consistent increase in density—for example, at 25% RCA the density rose from 2290.4 kg/m³ (0% RSF) to 2294.8 kg/m³ at 0.75% RSF, and at 100% RCA from 2115.8 kg/m³ (0% RSF) to 2129.6 kg/m³ at 0.75% RSF—indicating improved matrix compaction and the added mass of the fibers [33,34,35].

The ultrasonic pulse velocity (UPV) results showed a similar pattern. As presented in Figure 11, UPV decreased with increasing RCA content, from 4.3 km/s for the control mix to 3.7 km/s for the 100% RCA mix without fibers, reflecting a more heterogeneous microstructure and a weaker interfacial transition zone (ITZ), in agreement with previous findings [4,5]. The presence of RSF, however, led to incremental increases in UPV at all RCA levels; for instance, at 100% RCA, UPV improved from 3.7 km/s (0% RSF) to 3.85 km/s at 0.75% RSF, which can be attributed to the crack-bridging and densifying effects of the steel fibers that promote a more continuous matrix and reduce internal defects [3,9]. Overall, these observations confirm that RSF can effectively mitigate the adverse effects of RCA on both density and UPV, thereby contributing to the production of more durable and sustainable SCC in accordance with EFNARC [1]) performance requirements. The physical property results (density and UPV) for all mixtures are summarized in

Table 10. Density and ultrasonic pulse velocity (UPV) of recycled SCC mixtures with varying RCA replacement levels and RSF contents.

Mix Code	RCA Content (%)	RSF Content (%)	Density (Kg/m3)	Ultrasonic Pulse Velocity UPV (km/s)
Control Mix	0%	0%	2355	4.3
	25%	0%	2290.4	4.1
M1		0.25%	2284.5	4.2
M5		0.50%	2289.2	4.25
M9		0.75%	2294.8	4.3
	50%	0%	2230.5	4.0
M2		0.25%	2237.2	4.05
M6		0.50%	2242.7	4.1
M10		0.75%	2247.4	4.15
	75%	0%	2155.5	3.85
M3		0.25%	2166.9	3.9
M7		0.50%	2171.3	3.95
M11		0.75%	2176.8	4
	100%	0%	2115.8	3.7
M4		0.25%	2119.1	3.75
M8		0.50%	2124.5	3.8
M12		0.75%	2129.6	3.85

.

3.4. Correlation Between Ultrasonic Pulse Velocity and Mechanical Performance

Ultrasonic pulse velocity (UPV) is widely used as a non-destructive indicator of internal quality, as it reflects the continuity of the cementitious matrix and the presence of pores, microcracks, and weak interfacial zones. To address the reviewer’s suggestion, the measured UPV values were correlated with the 28-day compressive strength fc, splitting tensile strength fst, and flexural strength ff for all SCC mixtures (Figure 12).

Simple linear regressions (V in km/s) yield: fc = 31.00 V − 92.14 (R² = 0.68), f_st = 1.95 V − 4.31 (R² = 0.46), and f_f = 2.28 V − 2.38 (R² = 0.38). These relationships confirm that the reduction in UPV caused by increasing RCA replacement—primarily linked to higher porosity and a weaker interfacial transition zone—is accompanied by a corresponding degradation in load-bearing capacity. The addition of RSF slightly increases UPV, consistent with improved crack-bridging and stiffer transmission paths; however, its influence on compressive strength remains secondary compared with the dominant effect of RCA, while it more directly benefits tensile-related performance. Overall, UPV can therefore serve as a practical screening parameter for the quality of RCA–RSF self-compacting concrete, provided that the combined influence of recycled aggregate and fiber content is considered in interpretation.

4. Conclusions

This study evaluated the fresh, mechanical, and physical performance of self-compacting concrete (SCC) incorporating recycled concrete aggregate (RCA) and recycled steel fibers (RSF). The results demonstrate that, although RCA and RSF significantly modify the rheology and strength of SCC, appropriately proportioned mixtures can still satisfy EFNARC requirements while delivering meaningful sustainability benefits through the reuse of construction and demolition waste and tire-derived steel fibers. The main findings are:

• Rheological effect of RCA: Increasing RCA content systematically reduced workability: slump-flow decreased and T₅₀₀, V-funnel, L-box, and J-ring values increased, due to the rougher texture, higher porosity, and greater water absorption of RCA.
• Rheological effect of RSF: RSF further decreased flowability by increasing mix viscosity and interparticle blocking, but all mixes remained within EFNARC [1] limits for filling and passing ability when admixture dosage was properly adjusted.
• Optimal fiber content for fresh performance: Very high fiber volume (0.75% RSF) noticeably impaired workability; an RSF range of about 0.25–0.50% offered the best compromise between self-compacting ability and strength enhancement.
• Compressive strength: Compressive strength declined with higher RCA replacement and, to a lesser extent, with increasing RSF, mainly because of higher matrix porosity and reduced compactability. Despite this reduction, all mixes achieved compressive strengths acceptable for the targeted C30/37.5 strength class.
• Splitting tensile and flexural strengths: RSF produced substantial gains in splitting tensile and flexural strengths across all RCA levels, with the highest values at 0.75% RSF, particularly for 25% RCA mixtures. The fibers improved crack-bridging and energy absorption, resulting in more ductile behaviour even when compressive strength was slightly reduced.
• Density and ultrasonic pulse velocity (UPV): Density and UPV decreased with increasing RCA content, reflecting the lighter, more porous nature of recycled aggregates and a more heterogeneous internal structure. RSF slightly increased both density and UPV at each RCA level, indicating improved matrix integrity and fewer internal defects.
• Selection criteria for the recommended mix window: To identify the recommended range of RCA and RSF contents, a two-stage screening approach was applied based on: (i) fresh-property compliance and (ii) mechanical-performance adequacy. For the fresh state, mixtures were required to satisfy the EFNARC [1] acceptance thresholds adopted in this study—slump flow, T₅₀₀, V-funnel time, L-box blocking ratio, and J-ring response (Table 8)—together with stable self-compacting behaviour (i.e., proper flow and filling without visible segregation). For the hardened state, mixtures were considered mechanically acceptable when the 28-day compressive strength met the intended structural-grade target (C30/37.5 design objective), while maintaining or improving the tensile-related performance (splitting tensile and flexural strengths) relative to the corresponding fiber-free mixtures at the same RCA level, thereby ensuring a balanced structural response rather than strength gain in only one metric.
• Key quantitative highlights: In the fresh state, increasing RCA and RSF contents produced the expected reduction in workability (lower slump-flow and higher T₅₀₀ and V-funnel times), while the majority of mixtures retained satisfactory self-compacting behaviour in accordance with EFNARC-based performance criteria. In the hardened state, compressive strength decreased with RCA replacement (and to a lesser extent with increasing RSF), with an RCA-induced maximum reduction of ≈39% at 100% RCA relative to the control mixture. In contrast, RSF markedly enhanced tensile-related performance: at 25% RCA, 0.75% RSF increased splitting tensile and flexural strengths by ≈41% and ≈29%, respectively, compared with the corresponding fiber-free mixture. RCA reduced density and ultrasonic pulse velocity (UPV) by approximately 10–14%, and these reductions were partially mitigated by RSF addition, indicating improved matrix continuity and crack-bridging effects.
• Perspectives and future work: The present findings provide a performance-based basis for designing eco-efficient SCC with recycled constituents; however, several avenues merit further investigation. Future work should (i) quantify durability under aggressive exposure conditions (e.g., chloride ingress, carbonation, sulfate attack, and freeze–thaw cycling) and evaluate transport properties; (ii) address time-dependent behaviour (shrinkage, creep, and cracking propensity) in RCA–RSF systems; (iii) employ microstructural techniques (e.g., SEM/EDS and X-ray micro-CT) to elucidate the RCA–paste and RSF–matrix interfacial mechanisms underlying the observed macroscopic trends; (iv) examine robustness and field implementation issues including workability retention, pumpability, and fiber dispersion control at larger batching scales; and (v) integrate life-cycle assessment and multi-objective optimization to identify mixture domains that simultaneously maximize structural performance and sustainability.