Experimental Study on Compressive Strength and Chloride Permeability Improvement of Recycled Aggregate Concrete Modified by Glazed Hollow Beads, Fly Ash, and Fiber Composites

Abstract

Recycled concrete aggregates (RCAs) typically exhibit higher chloride permeability and lower strength compared to natural aggregates, potentially accelerating steel corrosion and compromising the durability of reinforced concrete structures. While functional additives like fibers, fly ash (FA), and glazed hollow beads (GHBs) are known to improve concrete properties, the quantification of the synergistic effects of their hybridization in RAC and a systematic multicriteria-based performance assessment are still lacking. This study experimentally investigates the individual and combined effects of GHB, FA, BF, and PPF on the compressive strength and electric flux of RAC. Fourteen mixtures were designed with different RCA replacements (0, 30, 50, and 100%), FA contents (0, 10, 20, and 30%), GHB dosages (0, 15, and 30%), and PPF and BF hybridization (0, 0.1 and 0.2%). Compared to unmodified RAC with 50% RCA replacement, the addition of 30% GHB significantly decreased the electric flux by 34.1% but comprised the compressive strength by 9.4%, whereas FA provided a weaker electric flux reduction of 16.3% alongside a lower strength decrease of 6.0%. A multicriteria analysis revealed that the synergistic GHB-FA-BF-PPF hybridization achieved the best performance of all formulations, exhibiting a remarkable 40.7% reduction in electric flux and a slight 1.3% increase in compressive strength compared to the unmodified RAC specimen. These findings demonstrate that the practical use of RAC modified by GHB-FA-BF-PPF hybridization would be highly beneficial in terms of mechanical performance as well as chloride permeability.

1. Introduction

Global climate change intensifies environmental challenges, necessitating urgent measures to curb greenhouse gas emissions. China has committed to achieving “carbon peak” and “carbon neutrality” targets, vigorously promoting low-carbon economies and green development to foster sustainable human progress. Recycled concrete aggregates (RCAs) have been increasingly utilized in concrete production to conserve natural resources and efficiently repurpose construction and demolition waste accumulated in landfills [1]. However, previous studies indicate that higher replacement percentages of RCAs in recycled aggregate concrete (RAC) result in compromised durability and mechanical properties, primarily attributed to the adhered old mortar on RCAs [2,3,4]. Generally, RCAs exhibit greater inhomogeneity and porosity and lower density compared to natural aggregates (NAs), as 30–35% of their volume comprises old cement paste adhered to natural aggregates (65–70% volume). Another critical factor in RACs is the interfacial bond properties between the old paste and aggregates, where a weak transitional layer is observed. RAC differs fundamentally from normal concrete (NC) in that it contains a higher density of interfacial transition zones (ITZs) and critical defects, leading to the lower mechanical properties and durability performance of RACs [5,6,7].

When subjected to coastal and marine environments, chloride-induced corrosion represents a primary degradation mechanism for steel reinforcement in reinforced concrete (RC) structures. The accumulation of corrosion products at the steel–concrete interface progressively fills interfacial pores, compromising the bond strength between the reinforcement and the surrounding concrete [8]. Sustained corrosion leads to expansive pressure from rust products, generating internal microcracks that propagate to cause concrete cover spalling and a reduction in the steel cross-sectional area [9,10]. These processes synergistically diminish the structural load capacity and durability. From a microstructural perspective on RACs, increasing recycled aggregate replacement ratios elevates porosity and average pore size, resulting in reduced matrix compactness and inferior chloride penetration resistance [11,12,13]. Under corrosive conditions, RAC structures demonstrate accelerated reinforcement corrosion, earlier onset of cover cracking, and more severe crack propagation compared to conventional NC members [14,15]. Hence, RAC experiences exacerbated durability deterioration [16], significantly restricting its practical implementation. To expand RAC applications in RC structures, enhancing its chloride permeability is imperative, particularly through physical or chemical strengthening approaches.

From the perspective of practical engineering, a variety of supplementary functional materials (SFMs), including fly ash (FA), fiber, steel slag (SS), and glazed hollow beads (GHBs), are widely recommended for the mix design of RACs to improve their performance. Previous studies show that FA enhances the long-term strength of RACs, particularly at low replacement levels [17,18,19]. Regarding concrete durability, FA improves most key performance indicators [20,21]. These advantages motivate research on FA–ground granulated blast furnace slag (GGBS) binary blends to develop eco-friendly concrete with balanced mechanical and durability properties. It was demonstrated that FA-GGBS’s synergistic effects outperform single admixtures. Specifically, simultaneous incorporation of 15% FA and 15% GGBS in RAC with 50% RCAs achieved superior 90-day compressive strength, chloride penetration resistance, and frost resistance compared to conventional concrete, while incorporating FA slightly reduces the elastic modulus and strength of RAC. Brito and Kurda et al. [22] found that FA replacement levels lower than 40% yield 90-day compressive strength in FA-modified concrete equivalent or superior to 28-day reference concrete strength. Conversely, higher FA incorporation typically reduces the strength development of RACs.

Fiber reinforcement has been widely recognized to enhance the mechanical properties and durability of concrete by arresting crack formation and propagation through bridging mechanisms. Notably, single-type fibers exhibit crack-size-dependent efficacy influenced by the fiber geometry and stiffness [23]. To optimize crack-bridging performance, hybrid systems combining fibers of differing types and dimensions, e.g., the hybridization of basalt fiber (BF) and polypropylene fiber (PPF), have been developed. Fu et al. [24] demonstrated that PPF-BF hybridization increased the concrete energy absorption by up to 100% compared to plain concrete. Smarzewski [25] reported that hybrid basalt-polypropylene fibers in high-performance concrete (HPC) resulted in reduced compressive strength but significantly improved the tensile strength and fracture energy. Wang et al. [26] observed that an optimal hybridization (0.15% BF + 0.033% PPF) enhanced the flexural and splitting tensile strength of HPC by 22.8% and 48.6%, respectively, alongside a 14.1% increase in the compressive strength. Nevertheless, systematic studies on the chloride resistance of concrete with PPF-BF hybridization remain limited, with conflicting findings; e.g., Afroughsabet et al. [27] and Algin and Ozen [28] reported improved chloride penetration resistances with PPF/BF addition, whereas Sadrmomtazi et al. [29] observed reduced resistance due to fiber-induced microstructural changes. Recently, Ahmed and Lim [30] employed the fiber hybridization technique to enhance the performance of recycled concrete. It was demonstrated that the incorporation of single fibers, including BF and PPF, failed to surpass the durability performance improvements of the hybrid BF-PPF system.

The incorporation of glass hollow beads (GHBs) with distinct physical and mechanical properties into concrete enables diverse functional applications. Through optimized mix design, closed-pore GHBs serve as an effective thermal insulation material, facilitating the production of recycled aggregate thermal insulation concrete (RATIC), which demonstrates superior structural performance [31], ductility [32,33], resistance to salt freeze–thaw cycles [34], and long-term creep resistance [35]. The literature [36,37,38,39] indicates that compared to RAC without adding GHBs, the incorporation of GHBs at 130 kg/m³ significantly enhanced the chloride ion permeation resistance of RATIC with a reduced chloride ion diffusion coefficient of 11.49%, despite a 20.29% increase in the total internal porosity attributed to harmful pores. However, the increase in the RCA replacement ratio from 0% to 100% deteriorated the chloride resistance, increasing the diffusion coefficient by 18.73% and total porosity by 14.32% under identical conditions.

Although RACs exhibit inferior mechanical properties and durability compared to NCs, prior studies confirm that the incorporation of common functional additives, such as fly ash (FA), basalt fiber (BF), polypropylene fiber (PPF), and glass hollow beads (GHBs), can significantly modulate their mechanical performance and chloride permeability. The chloride resistance of RACs is governed by multiple factors, including the water-to-binder ratio, the recycled aggregate replacement rate, and the ITZ [40]. As a consequence, the individual effects of these additives on the strength and chloride permeability may be either beneficial or detrimental, depending upon their dosage, type, and the mix design. However, to the best of the authors’ knowledge, the quantitative synergistic effects of FA, BF, PPF, and GHBs on the mechanical properties and chloride penetration resistance of RACs have not been fully understood.

This study presents the first systematic quantification of the synergistic effects of four functional materials (GHBs, FA, BF, and PPF) on the mechanical properties and chloride permeability of RAC. In the experiment, the electric flux was selected as an indicator to evaluate the resistance to chloride permeability, while the compressive strength was employed as the main indicator to estimate the mechanical properties of RACs. A regression-based model was developed, and the Simple Additive Weighting (SAW) method was applied for the multicriteria evaluation and ranking of all mixtures. Finally, an optimal mixture formulation (15% GHB, 20% FA, 0.1% BF, and 0.1% PPF) that delivers a substantial improvement in the chloride penetration resistance without compromising the design strength of RACs is proposed.

2. Experimental Program

2.1. Materials

The production of recycled aggregate concrete (RAC) includes natural concrete aggregates (NCAs), RCAs, cement, steel slag (SS), sand, FA, silica fume (SF), GHBs, PPF, BF, water, and a water-reducing agent. The detailed information for these materials is provided below. The RCAs were procured from Wuhan Shengjingyang Environmental Protection Co., Ltd. (Wuhan, China) and had identical particle size ranges (5 to 10 mm) as the NCAs. As for the fine aggregate, natural river sand, having a fineness modulus of 2.55 and a particle size distribution ranging from 0.08 to 4.75 mm, was used. The SSA exhibited a particle size distribution of 5 to 10 mm, with an apparent density of 3290 kg/m³, a compacted bulk density of 1870 kg/m³, a compacted bulk porosity of 0.4%, and a crushing value index of 6.3%. The gradation curves of NCA, RCA, and sand are illustrated in Figure 1, while their physical parameters, as well as the SS properties, are given in

Table 1. Basic properties of NCA, RCA, river sand, and SSA.

Materials	Size (mm)	Apparent Density (kg/m3)	Compact Density (kg/m3)	Water Absorption Rate (%)	Clay Content (%)	Crush Value (%)
NCA	5–10	2560	1570	0.43	0.35	8.90
RCA	5–10	2450	1401	4.03	0.45	17.5
River sand	0.08–4.75	2640	1710	4.93	1.50	–
SSA	5–10	3290	1870	–	–	6.30

. The RCAs are categorized as Class II aggregate and adhere to the requirements within the Chinese Standard “Recycled Coarse Aggregate for Concrete (GB/T 25177-2010 [41])”.

The cement used was P.O 42.5 Ordinary Portland Cement (OPC), with a specific surface area of 345 m²/kg and fineness of 0.66. The FA was produced by the Xinliang Building Materials Processing Factory and met the requirements of the Chinese standard “Fly Ash Used for Cement and Concrete (GB/T 1596-2017 [42])”. As per the Chinese standard (GB/T 18736-2017 [43]), the SF supplied by Henan Wuhu Environmental Technology Co., Ltd. was characterized by an average particle size of 180 nm and a specific surface area of 23,700 m²/kg. The chemical compositions of the cementitious materials and SS are provided in

Table 2. Chemical components of cement, SF, FA, SSA, and GHBs.

Materials	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	MnO	P2O3
Cement	22.45	4.40	2.05	61.70	4.52	2.20	–	–
SF	95.73	0.72	4.25	0.35	0.70	0.65	–	–
FA	65.20	20.22	4.30	3.25	2.05	0.69	–	–
SSA	19.45	3290	22.32	37.53	6.04	–	1.90	1.38
GHBs	75.90	13.11	0.92	1.17	0.06	–	0.08	0.07

.

The GHBs, as an internal curing material, were sourced from Xinyang City, Henan Province, whose particle sizes ranged from 30 to 50 mesh. Table 2 and

Table 3. Physical and mechanical properties of GHBs.

Bulk Density (kg/m3)	Cylinder Compressive Strength (kPa)	Thermal Conductivity (W⋅m−1⋅K−1)	Water Absorption Ratio (g/g)
112.5	215.7	0.042	Tap water	Saturated lime water
			5.3	3.7

tabulate the chemical composition and physical properties of the GHBs, respectively. Two types of fibers, i.e., PPF and BF, were chosen as reinforcements to improve the weak mechanical properties and durability of RACs. Figure 2 presents the elemental composition and sample morphology of BF and PPF, as determined by energy-dispersive X-ray (EDX) spectroscopy, and the details of the material parameters of PPF and BF are given in

Table 4. Material properties of PPF and BF.

Fiber Type	Length (mm)	Diameter (μm)	Elastic Modulus (GPa)	Tensile Strength (MPa)	Density (g/cm3)	Melting Point (°C)	Elongation at Break (%)
PPF	12	69	5.0	490	0.84	180	8.2
BF	12	15	99.0	3000	2.75	1500	3.7

. The water reducer was a polycarboxylate-based superplasticizer, achieving a water reduction rate of 28%.

2.2. Design of Mixture Proportions

Previous research [44] demonstrated that the incorporation of 7% SF significantly enhances the resistance of concrete to chloride penetration and that adding 25% steel slag aggregate (SSA) resulted in improvements in the compressive strength and permeability. Here, natural aggregate concrete (NAC) without adding RCAs, FA, PPF, BF, and GHBs was considered as the control specimen, where the mix design was developed with a water–cement ratio of 0.5 and a target strength grade of C40. A superplasticizer dosage of 0.15% by mass of cementitious materials was employed to maintain the workability of the concrete. To evaluate the influences on the mechanical and durability performance of RCAs from FA, PPF, BF, and GHBs, we designed and experimentally tested fourteen distinct mixtures with different RCA replacement ratios (0, 30, 50, and 100%), FA contents (0, 10, 20, and 30%), GHB dosages (0, 15, and 30%), and fiber types and volumes (polypropylene fibers and basalt fibers at 0.1 and 0.2%).

To differentiate the effects of individual and combined factors on the properties of concrete, fourteen mix formulations with different RCA replacements, GHB dosages, FA contents, and BF and PPF hybridizations were produced. The detailed formulations of all mixtures are shown in

Table 5. Detailed information on mix proportions of RAC.

Group	Group Purpose and Factorial Coverage	Specimen No.	Aggregate (kg/m3)				Binder (kg/m3)			Fiber Dosage (%)		GHB Dosage (%)
			NCA	RCA	SSA	Sand	Cement	SF	FA	PPF	BF
Control	Reference mix (50% RCA, no additives)	S-RA50-G0-FA0-BF0-PPF0	432	432	216	460	420	31	0	0	0	0
I	RCA ratio effect with varying RCA (30%, 50%, 100%)	S-RA30-G0-FA0-BF0-PPF0	605	259	216	460	420	31	0	0	0	0
		S-RA50-G0-FA0-BF0-PPF0	432	432	216	460	420	31	0	0	0	0
		S-RA100-G0-FA0-BF0-PPF0	0	864	216	460	420	31	0	0	0	0
II	GHB dosage effect with varying GHB (15%, 30%)	S-RA50-G15-FA0-BF0-PPF0	432	432	216	391	420	31	0	0	0	69
		S-RA50-G30-FA0-BF0-PPF0	432	432	216	322	420	31	0	0	0	138
III	FA content effect with varying FA (10%, 20%, 30%)	S-RA50-G0-FA10-BF0-PPF0	432	432	216	460	378	31	42	0	0	0
		S-RA50-G0-FA20-BF0-PPF0	432	432	216	460	336	31	84	0	0	0
		S-RA50-G0-FA30-BF0-PPF0	432	432	216	460	294	31	126	0	0	0
IV	Fiber hybridization—single vs. hybrid fibers	S-RA50-G0-FA0-BF0.2-PPF0	432	432	216	460	420	31	0	0	0.2	0
		S-RA50-G0-FA0-BF0-PPF0.2	432	432	216	460	420	31	0	0.2	0	0
		S-RA50-G0-FA0-BF0.1-PPF0.1	432	432	216	460	420	31	0	0.1	0.1	0
V	Synergistic effects—GHB + FA combination	S-RA50-G15-FA20-BF0-PPF0	432	432	216	391	336	31	84	0	0	69
	Synergistic effects—GHB + FA + Fibers combination	S-RA50-G15-FA20-BF0.1-PPF0.1	432	432	216	391	336	31	84	0.1	0.1	69

, where groups I, II, III, IV, and V are distinguished to evaluate the influences of RCA replacements, GNB dosages, FA contents, fiber types and volumes, and their composites on the mechanical properties and chloride permeability of RAC. For instance, S-RA50-G15-FA20-BF0.1-PPF0.1 denotes the RAC specimen modified by 50% RCA replacement, 15% GHB dosage, 20% FA content, and hybrid BF-PPF at 0.1% content. Since the objective of the present study is to address the challenge of enhancing the properties of RACs, we selected a control group comprising RAC specimens with 50% RCA replacement and without any additives. It is noted that the 14 mixtures were designed using a systematic group-based approach to efficiently isolate and compare the influences of individual factors and their key combinations. Since a full factorial design with ANOVA provides a rigorous statistical quantification of all factor interactions, the use of advanced statistical and optimization methods, such as Response Surface Methodology (RSM), is planned as a key focus of our subsequent research to build a predictive model for further mix proportion optimization.

2.3. Specimen Preparation and Testing

The preparation of tested specimens was conducted in accordance with the relevant requirements of the Standard for Test Methods of Concrete Physical and Mechanical Properties (GB/T 50081-2019 [45]) and ASTM C39/C39M-23 Standard Test Method [46]. The concrete was mixed using an HJW-60 compulsory concrete mixer, and compacted in molds via a vibrating table. Due to the incorporation of GHBs as an internal curing material in recycled concrete, a three-stage mixing process was adopted. Initially, GHBs were mixed with a portion of the mixing water for 1.5 min, followed by the addition of aggregates for another 1 min. Subsequently, cement, functional materials, superplasticizer, and the remaining water were added and mixed for 3.5 min. After preparation, specimen surfaces were covered with plastic film for curing. After 24 h, the specimens were demolded and transferred to a standard curing room (temperature: 20 ± 2 °C; RH ≥ 95%) until the testing age of 28 days, after which subsequent experiments were conducted. Compressive strength tests were performed using cubic specimens (100 × 100 × 100 mm³) after 28 days of standard curing as per Chinese standard GB/T 50081-2019 [45]. As shown in Figure 3, a hydraulic universal testing machine (SYE-2000BS) applied a constant loading rate of 0.5 MPa/s, with final strength values averaged from three duplicate samples.

In the present study, the electric flux method was utilized to determine the resistance to chloride permeability. This procedure was conducted on the specimens subsequent to the measurement of their carbonation depth. The test required cylindrical specimens with a diameter of 100 mm and a height of 50 mm via Chinese Standard GB/T 50082-2009 [47], which is the same as in ASTM C1202-2010 [48]. Consequently, the original 100 mm cubic specimens were processed to meet these specifications. As shown in Figure 3, each cube was cut into a 100 mm × 100 mm × 50 mm rectangular prism, preserving the surface that had been exposed to CO₂. This prism was then cored to produce the final 100 mm diameter by 50 mm high cylindrical specimen. To ensure a proper seal during testing, the side surfaces of each cylinder were coated with epoxy resin. Following the curing of the epoxy, the specimens underwent vacuum saturation. The internal pressure was reduced to below 5 kPa within five minutes and maintained for three hours. Deionized water was then introduced to submerge the specimens completely. After 1 h of soaking under vacuum, pressures were gradually restored to atmospheric level and the samples were left to soak for an additional 18 h.

After saturation, each specimen was secured in a test cell, which was initially filled with distilled water to check for leaks over a 10 min period, confirming an effective seal. For the test itself, the chamber connected to the negative electrode was filled with a 3.0% NaCl solution, while the positive electrode chamber contained a 0.3 mol/L NaOH solution. After connecting the electrodes to the flux meter, the test was initiated and run for 6 h, during which the total electric flux was recorded. Ultimately, the electric flux value (unit: C) for each original prismatic specimen was determined by calculating the average of the results from its three parallel cylindrical cores. Figure 4 illustrates the schematic diagram of the testing process of the electric flux method. It should be noted that the ASTM C1202 test measures ionic conductivity, which can be influenced by changes in pore solution chemistry resulting from the incorporation of fly ash. While the electric flux values in this study provide a practical basis for comparative assessment under controlled conditions, future research should employ direct chloride diffusion tests (e.g., the ASTM C1556 ponding test) to derive fundamental diffusion coefficients, enabling a more accurate evaluation of chloride transport that is independent of pore solution effects.

The total electric flux (Q) through each specimen was calculated using the following equation:

$$Q=900\left(I_0+2I_{30}+2I_{60}+\cdots +2I_t+2I_{300}+2I_{330}+2I_{360}\right)$$

(1)

where Q is the total electric flux in Coulombs (unit: C), I₀ is the initial electricity (unit: A) measured to a precision of 0.001 A, and I_t is the current (A) recorded at time t, also to a precision of 0.001 A. Although the nominal diameter of the standard specimen is 100 mm, the effective area for current flow is considered to be a 95 mm diameter circle due to edge effects. Consequently, for specimens with an actual diameter (x) exceeding 95 mm, the measured charge (Q_x) should be corrected to a standardized value (Q_s) corresponding to a 95 mm diameter specimen [see Equation (2)]. The chloride ion penetrability of RAC was then evaluated based on the classification criteria outlined in ASTM C1202 [48], as detailed in

Table 6. Chloride penetrability classification based on electric flux (ASTM C1202-2010 [48]).

Electric Flux (C)	Chloride Penetrability	Typical Concrete Example
>4000	High	High w/c ratio (>0.6) concrete
2000 to 4000	Moderate	Moderate w/c ratio (0.5–0.6) concrete
1000 to 2000	Low	Low w/c ratio (<0.5) concrete
100 to 1000	Very low	Low w/c ratio concrete with 5–10% silica fume
<100	Negligible	Polymer-modified concrete with 5–10% silica fume

.

$$Q_s=Q_x\times {\left(95/x\right)}^2$$

(2)

3. Results and Discussion

3.1. Compressive Strength

The mean and standard deviation values of the compressive strength and electrical flux are summarized in

Table 7. Test results of the concrete mixtures (average ± standard deviation).

Specimen No.	Compressive Strength (MPa)	Electrical Flux (C)
RA0-G0-FA0-BF0-PPF0	46.565 ± 1.871	939.891 ± 30.654
RA30-G0-FA0-BF0-PPF0	44.037 ± 1.974	1193.411 ± 31.735
RA50-G0-FA0-BF0-PPF0	39.765 ± 1.506	1354.743 ± 40.906
RA100-G0-FA0-BF0-PPF0	38.172 ± 1.114	1625.597 ± 59.588
RA50-G15-FA0-BF0-PPF0	36.016 ± 0.923	980.281 ± 25.956
RA50-G30-FA0-BF0-PPF0	33.642 ± 1.402	893.822 ± 26.838
RA50-G0-FA10-BF0-PPF0	41.376 ± 1.855	1170.340 ± 34.555
RA50-G0-FA20-BF0-PPF0	42.351 ± 1.907	1032.113 ± 21.857
RA50-G0-FA30-BF0-PPF0	37.245 ± 0.931	980.202 ± 20.021
RA50-G0-FA0-BF0.2-PPF0	42.156 ± 1.122	1153.198 ± 25.194
RA50-G0-FA0-BF0-PPF0.2	41.647 ± 1.729	1130.077 ± 20.945
RA50-G0-FA0-BF0.1-PPF0.1	43.516 ± 1.025	980.262 ± 21.103
RA50-G15-FA0-BF0-PPF0	36.016 ± 1.223	980.244 ± 24.898
RA50-G15-FA20-BF0-PPF0	37.157 ± 1.436	865.029 ± 29.985
RA50-G15-FA20-BF0.1-PPF0.1	40.227 ± 1.147	807.471 ± 23.182

. Figure 5 presents the 28-day compressive strength (f_cs) and the corresponding net gain for all tested specimens. As expected, increasing the RCA replacement resulted in a significant decrease in f_cs up to 18.0%. This strength loss occurs because RCA has higher porosity and pre-existing microcracks. These defects compromise the integrity of the concrete matrix. To assess the effects of GHBs, FA, and fibers, the RAC specimen with 50% RCA replacement (S-RA50-G0-FA0-BF0-PPF0) was designated as the control. This choice aligns with practical engineering applications and the Chinese Standard JGJ/T 240-2018 [49], which conservatively limits RCA replacement to 50%. The control specimen exhibited an f_cs of 39.77 MPa, against which all other mixtures were compared.

In comparison to the control specimen, the incorporation of GHBs significantly reduced the f_cs by up to 15.2%, which is mainly attributed to the lower intrinsic strength and density of the GHB particles. The influence of FA on f_cs was unexpected: replacements of 10% and 20% FA enhanced the f_cs by around 5%, whereas a 30% FA content resulted in a 6.2% reduction. This dual effect stems from the competition between the pozzolanic activity of FA, which refines the microstructure, and the dilution effect from the reduced cement content. Regarding fiber reinforcement, the addition of 0.2% BF or 0.2% PPF alone resulted in a marginal increase in f_cs (up to 6.2%). In contrast, the hybridization of 0.1% BF and 0.1% PPF produced a more substantial improvement of approximately 10% in f_cs. This demonstrates the synergistic potential of hybrid fibers, which effectively bridge microcracks and transfer tensile stresses, thereby reinforcing the weak RAC matrix more efficiently than single fibers.

While the incorporation of GHB compromises the RAC strength, this adverse effect can be effectively mitigated by the synergistic use of FA and hybrid fibers. This is clearly demonstrated in Figure 5b, where the composite specimen S-RA50-G15-FA20-BF0.1-PPF0.1, containing 15% GHBs, 20% FA, and a 0.1% BF −0.1% PPF hybrid, transformed the net strength gain from −9.3% to a positive 1.3%. This enhancement is due to a fundamental shift in the failure mode of the RAC under compression, as illustrated in Figure 6. It can be found that the control specimen devoid of any additives exhibited a classic brittle failure (Figure 6a). It failed abruptly upon reaching its peak load, characterized by the formation of a major vertical crack and significant spalling of concrete chunks from the surface. This behavior revealed the inherent weak matrix and poor crack resistance of RAC. In contrast, the hybrid-reinforced specimen S-RA50-G15-FA20-BF0.1-PPF0.1 displayed a markedly improved failure mode. Instead of a catastrophic fracture, it developed a network of well-distributed microcracks and showed no signs of concrete spalling. Consequently, the specimen maintained significant post-peak integrity, retaining its structural coherence even after ultimate failure. This observation confirms the efficacy of the hybrid BF-PPF system in bridging macrocracks and suppressing unstable crack propagation. These distinct failure patterns for both the plain RAC and the hybrid fiber-reinforced RAC are in strong agreement with findings reported in the recent literature [30,50,51].

3.2. Chloride Permeability

Figure 7 illustrates the electric flux and net gain for all tested specimens. The control specimen (S-RA50-G0-FA0-BF0-PPF0) with 50% RAC replacement exhibited an electric flux of 1358 C, the second lowest among all mixtures. This is primarily attributed to the inherent inferior quality and relatively higher content of RCA. As expected, the electric flux peaked at 1627 C when the RAC replacement increased to 100%. Figure 7b demonstrates that, compared to the control specimen, the incorporation of GHBs, FA, and single and hybrid fibers significantly reduced the chloride permeability by up to 34.0%, 27.7%, and 27.7%, respectively. The enhancement mechanism of each additive can be explained as follows. The porous structure of GHBs hinders moisture and chloride ion ingress. More importantly, their internal curing effect significantly improves the impermeability of RAC. Furthermore, the incorporation of FA reinforces this resistance through a dual mechanism. As hydration progresses, the pozzolanic reaction of FA generates additional low-alkalinity calcium silicate hydrate (C-S-H) gel within the cement matrix. This secondary hydration product, renowned for its superior strength and stability, fills micro-voids and thus improves resistance to chloride ion penetration. From a physical perspective, FA particles are typically finer than cement particles and therefore induce a micro-filler effect. This not only reduces the total porosity of the cement paste but also refines the pore size distribution, thereby obstructing potential permeation pathways.

In terms of mechanical enhancement, the hybridization of BF and PPF yielded a more significant improvement of 27.7% in the chloride permeability resistance compared to single fibers, with improvements of 14.9% for BF and 16.6% for PPF. The reason for this can be explained as follows. The crack-bridging and stress-transfer capabilities of BF and PPF enable the concrete matrix to sustain greater tensile stresses. Consistent with the published literature [52,53,54,55,56], this enhancement is attributed to the positive synergy between a low-strength, highly ductile PPF and a high-strength, high-modulus BF. Therefore, the hybrid BF-PPF effectively resists the formation and propagation of multi-scale cracks from the micro to macro levels, ultimately enhancing the chloride permeability resistance of RAC. It should be pointed out that the mechanistic explanations for the reduced chloride permeability are inferred from macroscopic experimental results. Direct microstructural validation using techniques such as SEM, MIP, and XRD will be essential in future work to conclusively confirm the proposed mechanisms of pore structure refinement and internal curing.

As shown in Figure 7, the specimen S-RA50-G15-FA20-BF0.1-PPF0.1 showed the greatest improvement in chloride resistance, which incorporated a hybrid mixture of 15% GHBs, 20% FA, and 0.1% BF-PPF hybridization. This makes sense both theoretically and experimentally, owing to the synergistic effects of hybrid GHB-FA-BF-PPF. Moreover, the use of mixed additives enhances the RAC matrix by preventing early-stage microcrack initiation and later resisting macro-level water penetration. In addition, the specimen S-RA50-G15-FA20-BF0-PPF0, which excluded fibers, exhibited a comparable improvement of 36.2% in the chloride permeability resistance, while the fiber-reinforced specimen achieved 40.4% enhancement. This suggests that the main contribution of fibers to RAC performance lies in strength enhancement rather than chloride permeability resistance, which is attributable to their multi-fiber reinforcement and crack-bridging abilities, as already demonstrated in Figure 5.

3.3. Effect of RCA Replacement on Compressive Strength and Chloride Permeability

Figure 8a,b present the influences of RCA replacement on the compressive strength (f_cs) and electrical flux (Q). It is found that as the RCA replacement increases, the value of f_cs decreases significantly, while that of Q shows an increasing tendency. This means that the NAC incorporating 0% RCA exhibited the highest compressive strength and optimal resistance to chloride penetration. Specifically, as the RAC replacement increased to 30%, 50%, and 100%, the compressive strength of RCA decreased by 5.5%, 14.6%, and 18.1% compared to NAC, respectively. Increasing the RCA replacement to 30%, 50%, and 100% resulted in increases in the electrical flux of 26.8%, 44.2%, and 72.7%, respectively. Consequently, the permeability grade exhibits a deterioration from ‘very low’ to ‘low’ as per the widely accepted guideline ASTM C1202-2010 [48]. Equations (3) and (4) present the regression models for compressive strength and electric flux against the RCA replacement ratio (R). These models fit the experimental data well, with correlation coefficients (R²) of 0.890 and 0.975, respectively.

$$f_{cs}=0.00067\cdot {\left(100R\right)}^2-0.156\cdot \left(100R\right)+46.904$$

(3)

$$Q=6.790\cdot \left(100R\right)+972.9$$

(4)

The reductions in both strength and chloride permeability due to the addition of RCA are consistent with well-documented micro-level mechanisms reported in the literature, which typically include increased porosity, a weakened ITZ, and the presence of inherent microcracks [13]. First, the incorporation of RCAs increases the overall porosity and alters the pore size distribution within the concrete matrix. This more porous and interconnected structure provides additional pathways for chloride ion ingress, consequently increasing the electric flux and reducing the resistance to chloride permeability. Second, the ITZ in RAC is inherently more complex and porous. Unlike NCAs, RCAs are coated with adhered old mortar, creating multiple and weaker interfaces, e.g., the new mortar–old mortar and mortar–aggregate interfaces. These interfaces exhibit lower bond strengths and higher porosities, leading to the most vulnerable zones for chloride transport. The capillary pores within the old mortar layer facilitate the ingress of chloride-containing solutions, accelerating diffusion through RACs. Furthermore, it is widely recognized that the mechanical crushing process producing RCAs can induce initial microcracks within aggregate particles [13]. Increasing the RCA replacement amplifies the number and connectivity of these defects, forming interconnected pathways that degrade chloride resistance. Therefore, mitigating the adverse effects of RCA on both the strength and chloride permeability necessitates the integration of functional admixtures, including GHBs, FA, and fibers, to refine the microstructure and restore performance.

3.4. Effect of GHB Dosage on Compressive Strength and Chloride Permeability

As already mentioned in Section 3.1 and Section 3.2, the incorporation of GHBs in RACs presents a promising strategy to enhance the resistance to chloride permeability. Prior to evaluating its effects on permeability, it is crucial to ascertain that this additive does not adversely compromise the mechanical properties of concrete. The influence of GHB dosage on the f_cs is depicted in Figure 9. The results reveal that the incorporation of GHB generally leads to a reduction in the f_cs. Specifically, at GHB dosages of 15% and 30%, the f_cs decreased by 9.4% and 15.0%, respectively, compared to the control mixture without GHBs. Despite this, in stark contrast to the moderate loss in the f_cs, the impact of GHBs on the chloride penetration resistance is substantially positive. The improvement in chloride resistance of GHB-modified RAC is achieved through a synergistic mechanism. Chemically, the internal curing water from GHBs facilitates prolonged hydration and secondary reactions, generating additional C-S-H gel and ettringite that densify the cement matrix and the critical ITZ, thereby obstructing chloride transport pathways. This microstructural refinement is identified as the primary mechanism. Physically, GHBs regulate internal humidity, reducing early-age cracking, and their unsaturated state under service conditions can adsorb invasive chloride ions, providing an additional protective measure against permeation [39].

Figure 9b illustrates the variation in the electric flux with different GHB dosages. The addition of GHBs resulted in a remarkable reduction of up to about 35% in electric flux, indicating significant enhancements in the resistance to Cl⁻ penetration. This pronounced improvement is attributed to the internal curing effect of the pre-soaked GHB. The water gradually released from the GHBs optimizes the late-stage hydration environment of the cement paste. The significant reduction in electric flux suggests that the incorporation of GHBs may lead to a refined pore structure and a densified ITZ, which warrants verification through direct microstructural analysis in future studies.

The relationship between the GHB dosage and the compressive strength and electric flux can be expressed as follows:

$$f_{cs}=0.202\cdot \left(100G\right)+39.478$$

(5)

$$Q=0.640\cdot {\left(100G\right)}^2-34.584\cdot \left(100G\right)+1354.9$$

(6)

The good agreement between the predictions and the experimental data demonstrates that Equations (5) and (6) can be effectively utilized to estimate the f_cs and electric flux of RACs with reasonable accuracy. Overall, while the incorporation of GHBs leads to a moderate decrease in the f_cs, its substantial enhancement of chloride penetration resistance offers a significant net benefit to the durability of RAC. Given that the water amount is a critical factor influencing the performance of RACs, future studies should be conducted to elucidate the combined and interactive effects of RCA replacement, GHB dosage, and water amount on the mechanical behavior of RACs.

3.5. Effect of FA Content on Compressive Strength and Chloride Permeability

Figure 10 presents the influence of FA content on the f_cs and electrical flux of RACs. Compared to the control specimen without adding FA, the f_cs increased by 4.5% and 6.6% for a given FA content of 10% and 20%, respectively. However, as the FA dosage further increases to 30%, there is a marginal reduction of about 6.0% in the f_cs. The reason can be explained as follows. The primary role of FA particles is to act as a pozzolanic binder. At elevated FA replacement levels, the dilution effect from increased FA and the inherent inferior properties of the RCAs become dominant, leading to a reduction in overall strength. This observation is also consistent with the observation obtained from Kurda et al. [54], where they suggested that the incorporation of high-volume FA has a more detrimental effect on the compressive strength of RAC than on that of coarse RCA.

As shown in Figure 10b, the incorporation of FA significantly enhances the chloride penetration resistance of RAC, with the electrical flux decreasing sharply by approximately 30% as the FA content increases from 0 to 30%. This improvement is primarily attributed to the pozzolanic reaction between FA and calcium hydroxide [Ca(OH)₂], which consumes the latter to form additional calcium–silicate–hydrate (C-S-H) gel. These secondary hydration products fill the capillary pores and microcracks, thereby densifying the concrete matrix and reducing permeability. This mechanism is also consistent with findings from previous studies, where FA was shown to mitigate the detrimental effects of RCA on chloride resistance [55]. The regression equations for compressive strength and electric flux versus the FA content (A) are given in Equations (7) and (8). It is shown that the predictions are in good agreement with the experimental results, with correlation coefficients (R²) of 0.810 and 0.981, respectively.

$$f_{cs}=-0.2020\cdot {\left(100A\right)}^2+0.449\cdot \left(100A\right)+39.422$$

(7)

$$Q=0.346\cdot {\left(100F\right)}^2-23.155\cdot \left(100F\right)+1362.2$$

(8)

Indeed, the carbonation of the remaining Ca(OH)₂ into calcium carbonate (CaCO₃) contributes to this enhanced resistance because this transformation produces insoluble products that further block pore networks. While the inherent porosity of RCAs typically compromises chloride penetration resistance, the synergistic effects of the pozzolanic reaction and carbonation induced by FA effectively counteract this weakness. Saravanakumar and Dhinakaran [56] reported that even at high replacement levels, such as the 60% FA and 100% RCA mixes, RACs still exhibited a moderate level of chloride penetration resistance.

3.6. Effect of Single and Hybrid Fibers on Compressive Strength and Chloride Permeability

It is generally agreed that incorporating fibers into RAC can effectively improve its mechanical properties and durability. However, the extent of such improvement remains underexplored, and the comparative efficiency of single versus hybrid basalt–polypropylene fibers (BF-PPF) has not been quantitatively evaluated.

Figure 11 presents the variation in the f_cs and the electrical flux for different fiber composites, including specimens with 0% fibers (F0), single 0.2% BF (BF0.2), 0.2% PPF (PPF0.2), and hybrid 0.1% BF + 0.1% PPF (BF0.1PPF0.1). It is found that the addition of fibers can slightly improve the compressive strength, while significantly reducing the electrical flux, with hybrid BF-PPF yielding optimal performance. Specifically, compared to the specimen F0, the specimens BF0.2, PPF0.2, and BF0.1PPF0.1 exhibited increases in the f_cs by 6.2%, 5.0%, and 10.3%, alongside electrical flux reductions of 15.0%, 17.0%, and 28.0%, respectively. These results confirm that BF, PPF, and their hybridization positively influence both the strength and chloride permeability of RAC. Notably, PPF contributed less to strength enhancement than BF, attributable to its inferior material properties and weaker interfacial bonding with the concrete matrix, as reported in recent studies [30].

In fact, the limitations of PPF were mitigated through hybridization with BF, which amplified f_cs by about 10% and decreased the electrical flux by around 28%, and therefore enhanced the permeability grade from ‘low’ to ‘very low’. This synergistic effect stems from multi-scale crack resistance: PPF improves crack control and toughness by inhibiting crack initiation and propagation, thereby blocking chloride ion ingress channels, while BF enhances the tensile strength and chemical corrosion resistance, bolstering structural stability. Collectively, these fibers optimize the concrete microstructure, reduce porosity, densify the interfacial transition zone, and concurrently improve strength and impermeability. Coupled mechanisms between fiber bridging and chloride transport inhibition have been established. According to Li et al. [57], fibers transfer stress across cracks, thereby mechanically restraining crack opening. This reduction in microcrack width limits the primary pathways for chloride ingress. Given the high sensitivity of chloride transport to crack width, fiber-induced crack control directly retards chloride penetration in cracked concrete [58]. Furthermore, fiber incorporation enhances the complexity and uniformity of the pore structure [57], increasing the tortuosity of transport paths and further inhibiting chloride diffusion. Additionally, high-performance admixtures such as composite superplasticizers and retention agents improve paste cohesion and refine the pore structure [59], which strengthens the fiber–matrix interface and supports chloride transport resistance. Thus, fiber bridging acts both mechanically by controlling crack width and microstructurally by pore structure refinement to significantly inhibit chloride transport in cracked concrete. A detailed micro-mechanical model linking fiber bridging and chloride transport needs to be developed in future studies.

The present study attributes enhanced performance to hybrid fibers based on macroscopic evidence and the well-established principle of multi-scale crack bridging: stiff BF restrains microcracks, while ductile PPF bridges macro-cracks. Therefore, future studies should employ detailed microscopic characterization, such as SEM, to directly visualize fiber dispersion and pull-out mechanisms, providing definitive quantitative evidence.

Figure 12 presents a comparison between the predicted compressive strengths and electrical fluxes obtained from Equations (3)–(8) and the experimental data. It can be found that the model predictions are in good agreement with the experimental results, with relative errors less than 10%, demonstrating the effectiveness of the proposed model.

4. Performance Assessment and Ranking of RAC

Considering the individual contributions of the functional materials (GHBs, FA, and fibers), as discussed in Section 3, it is of vital importance to quantitatively evaluate their synergistic effects on the compressive strength and chloride permeability of RAC. Figure 13 shows comparisons of the f_cs and electrical flux of RAC for different mix designs: (1) RAC with 50% RCA replacement (control group), (2) RAC hybridized with 15% GHBs, (3) RAC hybridized with 15% GHBs and 20% FA, and (4) RAC hybridized with 15% GHBs, 20% FA, 0.1% BF, and 0.1% PPF. The results indicate that the specimen with a fully hybridized mix (GHB + FA + BF + PPF) exhibits optimal performance, achieving a 1.3% increase in compressive strength and a marked enhancement of about 40% in the chloride penetration resistance, which upgrades the permeability grade from “low” to “very low”. This hybridization strategy effectively balances strength and durability. Practically, this mix design offers a useful approach to significantly improving the chloride penetration resistance while maintaining the design strength of RACs with 50% RCA replacement, aligning with engineering guidelines for sustainable construction.

To comprehensively evaluate and rank the experimental mix formulations based on the RAC’s compressive strength and chloride penetration resistance, a multicriteria decision-making (MCDM) approach was utilized. To this end, the Simple Additive Weighting (SAW) method, also known as the Weighted Sum Method (WSM), was adopted. Pioneered by Fishburn and MacCrimmon [60], the SAW method is one of the most established and straightforward linear scoring techniques, and is particularly applicable to the multicriteria evaluation of fiber-reinforced concrete [61].

The performance score (P_s) for each mix formulation was computed via the following equation:

$$P_s={\displaystyle \sum _{j=1}^m{w}_j{\left(x_{ij}\right)}_{normal}}$$

(9)

In Equation (7), P_s represents the overall performance score, w_j is the assigned weight for each criterion (a constant weight of 0.143 was uniformly assigned to the two indicators [30]), and x_ij is the normalized value of the selected performance indicators, including the compressive strength (ƒ_cs) and electrical flux (Q). For beneficial criteria, where higher values are desirable (i.e., ƒ_cs), the normalized value was determined as follows:

$${\left(x_{ij}\right)}_{normal}={\displaystyle \frac{x_{ij}}{Max\left(x_{ij}\right)}}$$

(10)

Conversely, for non-beneficial criteria, where lower values are preferred (i.e., Q), the normalization was performed using the following equation:

$${\left(x_{ij}\right)}_{normal}={\displaystyle \frac{Min\left(x_{ij}\right)}{x_{ij}}}$$

(11)

Table 8. Performance score (P_s) and ranking of RAC with different mixtures.

Mix ID	Ps	Rank
S-RA0-G0-FA0-BF0-PPF0	1.859	2
S-RA30-G0-FA0-BF0-PPF0	1.622	8
S-RA50-G0-FA0-BF0-PPF0	1.450	13
S-RA100-G0-FA0-BF0-PPF0	1.316	14
S-RA50-G15-FA0-BF0-PPF0	1.597	11
S-RA50-G30-FA0-BF0-PPF0	1.627	6
S-RA50-G0-FA10-BF0-PPF0	1.58	12
S-RA50-G0-FA20-BF0-PPF0	1.692	5
S-RA50-G0-FA30-BF0-PPF0	1.624	7
S-RA50-G0-FA0-BF0.2-PPF0	1.605	10
S-RA50-G0-FA0-BF0-PPF0.2	1.609	9
S-RA50-G0-FA0-BF0.1-PPF0.1	1.758	3
S-RA50-G15-FA20-BF0-PPF0	1.732	4
S-RA50-G15-FA20-BF0.1-PPF0.1	1.864	1

details the comprehensive performance scores and rankings of all mix formulations as determined by the SAW method. The fully hybridized RAC, containing hybrid BF-PPF, GHBs, and FA, achieved the highest overall rating due to its exceptional results in compressive strength and chloride penetration resistance. The subsequent positions were held by the conventional NAC without RCAs and additives and the RAC with hybrid GHB-FA (i.e., without fibers), ranking second and third, respectively. From Table 8, a key observation is the superior performance of the hybrid fiber system (BF-PPF) over formulations containing only BF or PPF. This indicates that fiber hybridization offers a synergistic advantage, optimizing both mechanical properties and chloride permeability. In contrast, the RAC mixtures lacking fibers and GHBs showed moderate performance, with the 20% and 30% FA contents ranking fifth and seventh, while the 10% FA mix was among the three lowest-performing formulations.

The worst effective mixtures were the unmodified RAC specimens with 50% and 100% RCA replacement. Their performance scores were merely 77.8% and 70.6% of that achieved by the optimal hybrid composite, which can be linked to their inadequate strength and chloride penetration resistance. Consequently, it is reasonably concluded that for RACs incorporating 50% or more RCAs, the addition of functional materials is essential to overcome intrinsic weaknesses in their strength and permeability resistance. Although the SAW method assumes constant equal weights for all criteria, the ranking of RAC mixtures is sensitive to the assigned weights. As was insightfully demonstrated by Asadoullahtabar et al. [62], a more comprehensive evaluation that incorporates impact weights for different sustainability criteria (such as environmental impact and cost) can significantly refine the decision-making process. Therefore, a sensitivity-based or dynamic weighting approach should be adopted for more accurate performance ranking. This will be investigated in future studies.

While the macroscopic experimental results demonstrate a significant synergistic enhancement in the mechanical and durability properties of RAC, it should be noted that this study has several limitations. First, the microstructural interaction among the four additives (GHBs, FA, BF, PPF) within the interfacial transition zone (ITZ) has not been directly analyzed. Therefore, further studies are certainly essential, performing SEM, MIP, or XRD analyses to provide a mechanistic explanation of the macroscopic findings presented in this paper. Second, although a series of regression models have been developed to quantify the individual influence of each parameter, the nonlinear coupling effects between parameters were evaluated primarily through direct comparison of individual versus combined modifications. To more comprehensively capture these complex interactions, further studies need to be conducted to employ advanced modeling tools such as Response Surface Methodology (RSM) or Artificial Neural Networks (ANNs). Third, the presence of non-conductive fibers (BF, PPF) may distort current flow during the RCPT, potentially resulting in a conservative estimate of the absolute chloride permeability. However, as this effect is systematic across all fiber-containing mixtures, the reported relative improvements in electric flux remain a valid indicator of performance enhancement. Further work is currently underway to employ chloride diffusion ponding tests in conjunction with microstructural analysis to directly quantify chloride diffusion. Fourth, the specific surface area–to–cement paste ratio of the fibers, a key factor influencing the compressive strength enhancement, was not explicitly characterized. While this ratio was indirectly controlled by keeping fiber geometry and dosage constant, future research could systematically vary fiber morphology to isolate and quantify its specific role in the performance of hybrid-modified RAC. Finally, some mechanical properties critical for structural design, such as the tensile strength and modulus of elasticity, were not experimentally investigated. Future research will be conducted to systematically evaluate these parameters to fully verify the structural integrity of GHB-RAC for practical engineering applications.

5. Future Work

While the manuscript demonstrates notable experimental findings on the synergistic modification of recycled aggregate concrete, its broader structural engineering implications remain to be fully explored. Future research should focus on integrating the proposed hybrid RAC in high-rise buildings, particularly addressing its expected performance in core walls, columns, or shear walls under combined mechanical and environmental loads. Such applications hold considerable promise for advancing sustainable construction without compromising structural safety.

As highlighted by Bagheri et al. [63], it is essential to evaluate the durability, ductility, and energy dissipation capacity of sustainable materials when used in conjunction with complex foundation systems, such as long–short combined piled rafts. Based on the present findings and limitations, the following research directions are recommended:

(i)

Long-term mechanical and durability performance of hybrid RAC under combined axial, cyclic, and environmental loading.

(ii)

Seismic behavior of RAC shear walls and columns in dual structural systems, accounting for soil–structure interaction.

(iii)

Development of design guidelines and constitutive models for RAC in high-rise applications, incorporating life-cycle assessment and resilience-based metrics.

(iv)

Further experimental investigations should be performed to evaluate the structural performance of modified RAC under realistic loading scenarios (e.g., bending, shear, or combined loading) to demonstrate its applicability in engineering structures.

These studies are expected to help bridge the gap between material-level innovations and structural system performance, promoting the adoption of sustainable materials in high-rise buildings, such as for seismic-resistant design.

6. Conclusions

This paper has presented a comprehensive evaluation of the strength and chloride permeability of RAC using fourteen mixtures containing GHBs, FA, BF, and PPF. The chloride permeability was assessed via the electric flux method, following the Chinese standard GB/T 50082-2009 and ASTM C1202. Based on the experimental results, the main conclusions are as follows:

(1)

Increasing RCA replacement reduced both the compressive strength and chloride permeability of RAC. Notably, 100% RCA replacement showed an 18.1% strength reduction and a 72.7% increase in electric flux, deteriorating its permeability grade from ‘very low’ to ‘low’ per ASTM C1202.

(2)

Single GHB or FA additions enhanced chloride resistance but compromised compressive strength. A 30% GHB dosage reduced electric flux by 34.1% but decreased strength by 9.4%, whereas a 30% FA content offered a 16.3% flux reduction with a smaller 6.0% strength loss.

(3)

FA content exhibited a nonlinear effect on strength, peaking at a 6.6% increase with 20% FA dosage but decreasing at 30%. Conversely, chloride resistance was improved consistently, with electric flux decreasing by approximately 30% at 30% FA content.

(4)

Multilinear correlations were established among the compressive strength, electric flux, and dosages of RCAs, GHBs, and FA. The proposed regression models enable the prediction of compressive strength and electric flux based on the known values of a single additive.

(5)

Hybrid BF-PPF significantly improved both strength and permeability more effectively than single BF or PPF. The optimal hybrid mix (0.1% each) yielded the highest enhancements with a 10.3% increase in compressive strength and a 28.0% reduction in electric flux.

(6)

According to the overall performance score of multicriteria assessment, RAC (50% RCA replacement) reinforced with 15% GHB dosage, 20% FA content, and 0.1% dosage of hybrid BF-PPF achieved the highest overall performance score, demonstrating a significant increase of 40.7% in electric flux without any losses in compressive strength.