Effect of Steel Fibers on Shear Carrying Capacity of Rubberized Geopolymer Concrete Beams

Abstract

Geopolymer concrete (GPC) offers reduced carbon emissions and employs industrial by-products such as fly ash and ground granulated blast furnace slag (GGBFS). In this study, the synergistic augmentation of shear carrying capacity in steel-fiber-reinforced rubberized geopolymer concrete (FRGC) incorporating industrial by-products such as fly ash, GGBFS, and recycled rubber for sustainable construction is investigated. The reinforced rubberized geopolymer concrete (RFRGC) mixtures contained 20% rubber crumbs as a partial replacement for fine aggregate, uniform binder, and alkaline activator. The findings revealed that 1.25% steel fiber achieved optimal hardened properties (compressive strength, flexural, and split tensile strength), with 12 M sodium hydroxide and oven curing achieving maximum values. An increase in molarity improved geopolymerization, with denser matrices, while oven curing boosted polymerization, enhancing the bonding between the matrix and the fiber. The effect of steel fiber on the shear carrying capacity of RFRGC beams without stirrups is also discussed in this paper. An increased fiber content led to an increased shear carrying capacity, characterized by an improvement in first crack load and a delayed ultimate failure. These results contribute to sustainable concrete technologies for specifically designed FRGC systems that can balance structural toughness, providing viable alternatives to traditional concrete without compromising strength capacity.

1. Introduction

Geopolymer concrete (GPC) represents an alternative to traditional concrete based on Portland cement, offering reduced carbon emissions and employing industrial by-products such as fly ash and ground granulated blast furnace slag (GGBFS). Recent research has examined the use of waste materials such as rubber crumbs as partial replacements for fine aggregates as a way to improve ductility under mechanical load, as well as to enhance energy absorption while alleviating environmental concerns [1]. The lower stiffness and reduced strength of rubberized geopolymer concrete (RGC) limit its structural use, especially in shear-critical elements such as beams. Consequently, steel fiber has been included in RGC to improve the postcracking response and shear resistance of RGC beams [2]. The effects of adding both straight and hooked-end steel fiber at various percentages, from 0% to 1.5%, on the shear carrying capacity of RGC beams determine the optimal fiber dosage to improve structural performance without compromising on functional sustainability. Fine aggregates in concrete can be partially substituted with rubber particles. Extremely elastic, wear-resistant, and stable in water, they improve durability, concrete hardness, and resistance to fractures, fatigue, and impacts [3]. When used for structural elements such as walls and columns, rubberized concrete performs better seismically than traditional concrete [4]. It also has a lower compressive strength, tensile strength, and elastic modulus, as rubber particles have a relatively low hardness compared to plain concrete. GPC is emerging as a compelling choice for modern infrastructure. By substituting carbon-emitting Portland cement with alkali-activated industrial waste products such as fly ash, GPC significantly minimizes embodied carbon. Its higher durability prevents chemical attack, freeze–thaw degradation, and fire, maximally increasing bridge longevity. This material strength, in conjunction with optimized design strategies, reinforces bridges to withstand earthquakes, floods, and climate-induced forces. Implementation thereby maximizes environmental sustainability while maximizing infrastructure lifespan and resilience [5]. Because of its exceptional endurance, rubberized concrete has been used in structural slabs [6], beams [7], columns, frames, and steel–concrete composite constructions [8]. However, despite its ductility, its decreased mechanical strength limits its engineering applications.

Shear failure is a brittle and undesirable failure mode in reinforced concrete beams and can be lessened by using shear reinforcements such as stirrups [9]. The absence of shear reinforcements draws attention to the role of steel fiber in RGC as the only way to resist shear forces. The maximum extent of fine aggregate replacement with rubber crumbs was 20%. Although energy absorption improved, the shear strength was reduced. After water, concrete continues to be the most widely used material worldwide [10]. Cement has been widely employed as the main binding ingredient in concrete in the building industry. The manufacturing of cement contributes roughly 5–7% of the world’s CO₂ emissions, which continue to be a major source of greenhouse gases (GHGs), and the Earth’s surface warming, which leads to global warming [11]. Furthermore, the production of cement is a very energy-intensive process that requires almost 4 GJ of energy per ton, with an annual production of roughly 3.6 billion tons of cement [12]. Cement production is increasing by almost 9% annually. GPC is a better alternative to ordinary Portland cement (OPC) because industrial waste materials can be used as source materials in GPC. Activator solution is used to activate these source materials. GPC beams with comparable target compressive strengths demonstrated comparable flexural behavior and strength to OPC beams [13], with a comparable load-displacement response, initial cracking load, and ultimate load deflection under midspan. The insertion of steel fiber considerably alters a number of concrete member characteristics, including ultimate flexural strength, ductility, resistance to deformation and cracking, shear capacity, and hardness [14]. Fiber is occasionally used in place of partially transverse steel reinforcements (such as vertical stirrups) in reinforced concrete (RC) elements to increase their shear strength [15]. High-strength concrete is a form of concrete frequently employed in high-rise buildings due to its potential to lower section dimensions and dead load. It is also known to exhibit brittle behavior, though, which may result in structural problems. Adding steel fiber to high-strength concrete can enhance its ductility and mechanical qualities [16]. The addition of both straight and hooked-end steel fiber is intended to restore and enhance shear resistance by bridging cracks and enhancing load transfer across them [16,17]. The value of comparing equal volumes of straight and hooked-end fiber is that the effect of fiber geometries on shear resistance can be evaluated [18,19]. Previous studies have promoted environmentally friendly, fiber-reinforced geopolymer concrete (FRGC) beams with improved shear strength and/or energy capacity, with the intention of developing sustainable structures that can maintain structural integrity within any built environment. Steel fiber-reinforced RGC beams have an enhanced shear capacity and ductility [20]. Hence, they provide a sustainable and structurally efficient solution, with 1.25–1.5% of fiber dosage yielding the best shear performance. Steel FRGC beams have important uses in sustainable construction and lightweight structures, with seismic resistance and impact resistance applications in elements such as bridge decks, pavements, and precast components [21,22]. FRGC has been reported to exhibit strong mechanical bonds with concrete substrates and steel rebars as well as strong fire resistance, improved durability in corrosive environments, and reduced creep and shrinkage behavior [23,24]. The principal difficulty in its practical implementation is the absence of design codes on the structural performance of FRGC-rehabilitated concrete members [25,26]. The mechanical properties of RC beams are greatly influenced by the loading rate; a high loading rate triggers the shear effect of RC beams, transforming their failure mode [27]. The shear failure of RC members is usually sudden and brittle; therefore, the design should ensure that the shear strength is at or greater than the flexural strength at every point along the member [28]. Hence, the primary goal of shear design is to ascertain where shear reinforcement is necessary in order to avoid such a failure as well as to determine how much of it is required. Steel bar corrosion is one of the primary deterioration processes in concrete structures in marine environments [29]. When used instead of Portland cement, geopolymer concrete tends to exhibit similar physical and durability properties to OPC [30]. Research on the material properties of GPC has made significant advances [31]; however, the number of large-scale tests conducted to assess its structural performance is still insufficient. A careful consideration using testing and computational modeling techniques can investigate such structural components and will provide greater support for RGC as a viable construction material. Manufacturing green concrete with recycled waste items such as rubber and other additives will become important in development strategies and pave the way for sustainable practices in roadway infrastructure developments [32].

In spite of the known environmental advantages of GPC made with industrial waste materials such as fly ash and GGBFS, there is limited knowledge on the combined impact of recycled rubber and steel fiber reinforcement on the shear behavior of geopolymer concretes, especially under realistic curing and loading conditions. The majority of existing studies have emphasized compressive and flexural properties or have assessed rubberized and fiber-reinforced geopolymer concretes separately, without documenting the synergistic effect of combining both recycled rubber and steel fiber, particularly in terms of shear performance without stirrups. The motivation for this research study is to address the need for sustainable building materials that not only reduce carbon footprint but also use rubber crumbs without compromising on structural performance, with the aim of meeting or even surpassing traditional standards. This study is novel in the investigation of the interaction of NaOH molarity, steel fiber addition, curing regime, and recycled rubber fraction on both the mechanical and shear behavior of reinforced rubberized geopolymer concrete (RFRGC). The increase in the shear strength and energy absorption of non-shear-reinforced beams is measured and compared with code-based theoretical estimates and conventional concrete controls. The motivation for using rubber crumbs as an aggregate replacement in concrete is driven by the need to address both environmental and technical challenges. Existing limitations include a significant reduction in compressive and tensile strength, primarily due to the lower elastic modulus and poor adhesion of particles to cement paste, which leads to crack initiation and accelerated failure [33]. These drawbacks are eliminated by the use of pretreated rubber crumbs for geopolymer concrete applications [34]. The environmental benefits of recycling waste tires and reducing landfill burdens provide a strong motivation for continued research and optimizing rubberized concrete for sustainable use. By determining the ideal content of fiber and curing conditions for both maximum strength and ductility, this research paper differentiates itself from the previous literature and presents a feasible route for high-performance, sustainable concrete alternatives. The influence of steel fiber on enhancing the shear capacity of RFRGC beams is investigated, with the aim of developing sustainable structural solutions by integrating industrial by-products and waste materials. Key objectives include assessing the impact of fiber dosage on shear strength, crack initiation load, ultimate load, and energy absorption capacity derived from load–deflection curves. By identifying the optimal fiber percentage that maximizes shear enhancement, this research aims to advance eco-friendly concrete technologies, combining rubber, fly ash, and GGBFS with improved structural resilience and offer insights into sustainable alternatives to traditional reinforced concrete without compromising shear performance.

2. Materials and Methods

2.1. Raw Materials

Fly ash (Class F) and GGBFS were utilized as the primary binders for geopolymer concrete (GPC). The fine aggregate (M-sand) was partially replaced with 20% recycled rubber crumbs by mass. Quarry stones were used as the coarse aggregate, and conventional proportions were maintained. The mix designs (

Table 1. Mix design of RFRGC.

Mix	NaOH Molarity (M)	Steel Fiber (%)
A1	8	0
A2	8	0.25
A3	8	0.5
A4	8	0.75
A5	8	1
A6	8	1.25
A7	8	1.5
B1	10	0
B2	10	0.25
B3	10	0.5
B4	10	0.75
B5	10	1
B6	10	1.25
B7	10	1.5
C1	12	0
C2	12	0.25
C3	12	0.5
C4	12	0.75
C5	12	1
C6	12	1.25
C7	12	1.5

) maintained consistent binder (fly ash: 359.1 kg/m³; GGBFS: 153.9 kg/m³; alkaline solution: 245.5 kg/m³; Na₂SiO₂: 147.3 kg/m³; NaOH: 98.2 kg/m³) and aggregate quantities across all specimens (fine aggregate: 552.43 kg/m³; coarse aggregate: 1282.43 kg/m³; rubber crumb: 138.11 kg/m³), ensuring that only the NaOH molarity and fiber dosage differed. Rubber crumbs were pretreated with immersion in NaOH solution to enhance their adhesion and compatibility with cementitious matrices, as shown in Figure 1. The rubber crumbs were thoroughly cleaned with normal water and then soaked in NaOH solution at 0.5 M concentration for 2 h [33]. Following immersion, the treated rubber was repeatedly rinsed with distilled water until a neutral pH was achieved in order to remove residual alkali, which could otherwise impair concrete properties. The treated rubber crumbs were then air-dried to eliminate excess moisture prior to their incorporation into the geopolymer concrete mix [34]. This pretreatment process is known to remove surface contaminants and oils, increase surface roughness, and improve the interfacial adhesion between rubber particles and geopolymer paste, thereby optimizing the mechanical performance of rubberized concrete. Table 1 shows the variations in the mix formulations.

Sodium hydroxide (NaOH) solution (8 M, 10 M, 12 M) and sodium silicate (Na₂SiO₃) were mixed at a 1:1.5 ratio to form the alkaline activator. The selection of 8 M, 10 M, and 12 M NaOH molarities was based on their proven effectiveness in activating aluminosilicate precursors such as fly ash and GGBFS, which are essential for optimal geopolymerization and strength development in geopolymer concrete. Higher molarity enhances the dissolution of silica and alumina, promoting a denser and stronger geopolymer matrix, while excessively high concentrations may reduce workability and increase costs. The 1:1.5 ratio of NaOH to Na₂SiO₃ is commonly used to balance the availability of silicate ions, which are crucial for polymer chain formation, and to optimize the setting time, mechanical properties, and durability of the resulting concrete. This ratio ensures sufficient activation and workability without compromising the structural integrity or sustainability of the mix. Straight and hooked-end steel fiber (length: 50 mm, diameter: 1 mm) was incorporated in equal proportions, with the total fiber content varying from 0% to 1.5% by volume in increments of 0.25%. All materials were sourced industrially, with rubber derived from end-of-life tires.

2.2. Specimen Preparation and Curing Regimen

This phase explored the impact of NaOH molarity (8 M, 10 M, 12 M), steel fiber addition (0–1.5% volume, consisting of equal percentages of hooked-end and straight fiber), and curing regime (ambient compared to oven curing) on the mechanical properties of rubberized geopolymer concrete (RFRGC). Fine aggregate (M-sand) was replaced by 20% NaOH-pretreated recycled rubber crumbs. The alkaline activator was a mixture of NaOH and sodium silicate (Na₂SiO₃) in a 1:1.5 ratio. Then, the following specimens were cast: 150 × 150 × 150 mm cubes for compressive strength, 150 mm × 300 mm cylinders for split tensile strength, and 100 mm × 100 mm × 500 mm prisms for flexural strength. Fresh properties, such as slump, were tested to evaluate workability. The specimens were separated into two groups after demolding: ambient-cured (kept in a room for 28 days) and oven-cured (60 °C, 24 h). Mechanical testing conformed to IS specifications: compressive strength at 7 and 28 days, split tensile strength, and flexural strength were determined. The analysis indicated that 12 M NaOH and 1.25% steel fiber resulted in maximum compressive, split tensile, and flexural strengths upon oven curing via geopolymerization and the fiber–matrix interaction. Ambient-cured samples exhibited 10–15% less strength due to reduced polymerization. Increased fiber content (>1.25%) resulted in workability loss, agglomeration, and decreased strength.

2.3. Shear Capacity Testing of RFRGC Beams

Following Phase 1, this phase tested the shear strength of RFRGC beams (1.2 m long × 150 mm depth × 100 mm width) without shear stirrups, with 0–1.5% steel fiber and 12 M NaOH concentration. Beams were designed as per the limit state method; two bars of 10 mm diameter were used as tensile reinforcement, and two 8 mm diameter bars were used as holder bars. Beams were prepared with a shear span-to-depth ratio (a/d) of 2.93 and an effective span of 1.1 m to cause shear failure. The test was also conducted on a control beam made with M30-grade plain cement concrete (no fiber or rubber). Specimens were cured at ambient conditions for 28 days to simulate site conditions. Testing was conducted using two-point loading under a universal testing machine (UTM) with incremental loading till failure, as shown in Figure 2. Deflection at midspan, initial crack load, and ultimate load were measured. Load–deflection curves were examined to determine energy absorption (area under the curve) and toughness. Theoretical shear capacity was calculated based on IS 456:2000 [35] provisions, considering a simplified model. Experimental shear stress (τ_exp) was obtained from V_u/bd, where V_u is the ultimate load, b is the width, and d is the effective depth. The outcomes revealed that RFRGC beams with 1.25% fiber achieved a 35–40% greater ultimate shear load (28.8 kN) and 50% more energy absorption than 0% fiber plain RFRGC (10 kN) and OPC (M30) beams (9.8 kN). The first crack load was directly proportional to the fiber content, as predicted by delayed crack propagation. Hooked-end fiber enhanced postcracking ductility through mechanical anchorage. The theoretical shear values underestimated the experimental results by 15–20%, highlighting a fiber contribution unaccounted for in the code equations. The 1.5% fiber mix exhibited a marginal strength reduction due to inhomogeneity, reinforcing the 1.25% threshold for optimal performance. Data were normalized to compare the influence of fiber dosage. The results were validated against control specimens (0% fiber). A statistical analysis assessed variability, with trends in shear enhancement, ductility, and postcracking behavior quantified to identify optimal fiber–molarity combinations.

3. Results

3.1. Workability and Compressive Strength

The slump values in

Table 2. Results of slump test of RFRGC.

Steel Fiber Content (%)	Slump (mm)
0	129
0.25	118
0.5	105
0.75	93
1	89
1.25	73
1.50	68

decreased linearly from 129 mm (0% fiber) to 68 mm (1.5% fiber), indicating reduced workability with increasing steel fiber content. The values shown in Table 2 depict the slump test results for RFRGC with different steel fiber percentages. The slump test, which is one of the most important measures of workability, measures the flow of concrete under the influence of its own weight, depicting its consistency and workability. When the percentage of steel fiber increases from 0% to 1.5%, the trend in the values of slump decreases consistently, with a reduction from 129 mm at 0% steel fiber to only 68 mm at 1.5%. This drop reflects the considerable influence of steel fiber on workability. When the proportion of steel fiber is 0.25%, the slump reduces to 118 mm, and at 0.5%, it amounts to 105 mm, reflecting a continuous decline. Above a 1% fiber ratio, the decline increases, with a steep drop-off reduction to 73 mm at 1.25% and 68 mm at 1.5%. The reduction in slump is due to the addition of steel fiber, which resists movement in the mix, enhancing the internal friction and cohesion of the concrete. In addition, it is necessary to add chemical admixtures such as superplasticizer to improve workability. These results highlight the compromise between workability and fiber reinforcement in RFRGC. This trend aligns with fiber-induced interlocking effects, which hinder concrete flow.

Table 3. Compressive strength test of RFRGC.

Mix	Molarity of NaOH Solution	% of Steel Fiber	Compressive Strength in N/mm2
			Ambient Curing		Oven Curing (60 °C for 24 h)
			7 Days	28 Days
A1	8 M	0	32.24	38.75	43.46
A2	8 M	0.25	32.53	39.07	44.11
A3	8 M	0.5	32.86	39.46	44.77
A4	8 M	0.75	34.21	39.99	45.39
A5	8 M	1	34.55	40.77	46.08
A6	8 M	1.25	34.99	41.08	46.67
A7	8 M	1.5	33.43	40.54	45.92
B1	10 M	0	33.74	39.46	44.55
B2	10 M	0.25	33.91	40.05	45.09
B3	10 M	0.5	34.49	40.58	45.98
B4	10 M	0.75	34.71	40.93	46.69
B5	10 M	1	34.97	41.25	47.07
B6	10 M	1.25	35.35	41.58	47.65
B7	10 M	1.5	33.89	40.92	47.16
C1	12 M	0	34.07	40.57	45.17
C2	12 M	0.25	36.82	40.77	45.89
C3	12 M	0.5	37.09	41.39	46.68
C4	12 M	0.75	37.49	41.88	47.56
C5	12 M	1	37.95	42.69	48.08
C5	12 M	1.25	38.36	43.15	48.91
C7	12 M	1.5	37.54	42.64	47.69

presents the compressive strength of RFRGC at different curing conditions and steel fiber contents, employing NaOH solutions with various molarities (8 M, 10 M, and 12 M). RFRGC cubes were prepared and tested using a compression testing machine to determine their compressive strength, which showed an improvement with a fiber addition of up to 1.25%, peaking at 46.67 MPa (8 M), 47.65 MPa (10 M), and 48.91 MPa (12 M) for specimens under oven curing. There were differing findings for 7- and 28-day compressive strength in specimens that underwent ambient curing (kept at room temperature) versus oven curing (60 °C, 24 h). A rise in steel fiber content resulted in a rise in compressive strength for all molarities. At 1.25% steel fiber, all specimens showed a decrease in compressive strength. Oven curing was always superior to ambient curing, with the latter showing slower gains in strength. Of particular interest is the finding that higher NaOH molarity and moderate amounts of steel fiber (approximately 1–1.25%) produce the best results. A higher NaOH molarity (12 M) consistently yielded superior strength due to enhanced geopolymerization, as stronger alkaline solutions promote binder reactivity. These results highlight the synergy between curing conditions, activator concentration, and fiber reinforcement in maximizing RFRGC’s compressive strength.

Figure 3a shows the variation in compressive strength with steel fiber percentage for various NaOH molarities (8 M, 10 M, and 12 M for ambient-cured specimens on day 7). The results indicate a consistent increase in compressive strength with a higher steel fiber content of up to 1.25%. With 8 M specimens on day 7, the strength increases from 32.24 N/mm² at 0% fiber to a peak of 34.99 N/mm² at 1.25%. A slight decline is observed at 1.5% (33.43 N/mm²), likely due to reduced workability or fiber clumping. With 10 M specimens on day 7, the strength increases from 33.74 N/mm² at 0% fiber to a peak of 35.35 N/mm² at 1.25%, and a slight decline is observed at 1.5% (33.89 N/mm²). Similarly, with 12 M specimens on day 7, the strength increases from 34.07 N/mm² at 0% fiber to a peak of 38.36 N/mm² at 1.25%, with a slight decline at 1.5% (37.54 N/mm).

Figure 3b shows a similar trend in compressive strength on day 28, but at much greater strength values due to the advantage of ongoing curing. Once more, the highest compressive strength for all percentages of steel fiber is attained using the 12 M NaOH solution. With 1.25% steel fiber, the compressive strength reaches 43.15 N/mm² for 12 M, then 41.58 N/mm² for 10 M, and 41.08 N/mm² for 8 M. Above 1.25% fiber content, a marginal decrease is noted, similar to the day 7 values. These values reinforce the conclusion that 1–1.25% steel fiber blended with a higher NaOH molarity (12 M) maximizes early-age and long-term compressive strength, with decreasing returns at inordinately high fiber contents. A higher NaOH molarity improves the geopolymerization process, leading to a denser matrix and better mechanical strength. It also increases the alkalinity of alkaline solutions, which accelerates the dissolution of silica alumina from source materials. The increased availability of Si and Al helps to form a dense and more extensive sodium aluminosilicate hydrate (N-A-S-H). This gel is the primary binding phase in geopolymer concrete and contributes indirectly to its strength.

Figure 4 shows the compressive strength variation of oven-cured specimens with different molarities, specifically indicating the effect of 8 M, 10 M, and 12 M NaOH molarity and the content of steel fiber on oven-cured specimens’ compressive strength. It can be noted that there is a clear upward trend with increasing molarity, and the 12 M NaOH provides the highest compressive strength across all steel fiber percentages. At 1.25% steel fiber, the maximum compressive strength is achieved at 48.91 N/mm² for 12 M NaOH, followed by 47.65 N/mm² for 10 M, and 46.67 N/mm² for 8 M. This increase in strength from 8 M to 12 M suggests increased geopolymerization reactions at an increased molarity, which results in denser microstructures. At a 1.5% fiber content, a marginal strength reduction can be seen in all molarities, which may be attributed to the poor workability and nonuniform distribution of the fiber.

3.2. Flexural Strength Development

Flexural strength is the ability to resist deformation and cracking under bending stresses, making it vital for structural applications such as beams, slabs, and pavements. The incorporation of steel fiber and NaOH-activated geopolymer binders in RFRGC offers a promising route to enhance flexural capacity while aligning with circular economy goals. Steel fiber acts as a reinforcement by bridging microcracks and redistributing stress, countering the inherent brittleness of glass-based concrete. Curing conditions further modulate flexural behavior: oven curing accelerates geopolymerization, enhancing early-age strength, while ambient curing allows for a gradual strength development.

Table 4. Flexural strength test at various proportions.

Mix	Molarity of NaOH Solution	% of Steel Fiber	Flexural Strength in N/mm2
			Ambient Curing		Oven Curing (60 °C for 24 h)
			7 Days	28 Days
A1	8 M	0	1.34	2.51	4.61
A2	8 M	0.25	1.81	3.09	4.83
A3	8 M	0.5	2.25	3.23	5.02
A4	8 M	0.75	2.55	3.64	5.46
A5	8 M	1	2.98	3.95	5.83
A6	8 M	1.25	3.22	4.45	6.14
A7	8 M	1.5	2.96	4.12	5.43
B1	10 M	0	1.95	3.35	5.19
B2	10 M	0.25	2.29	3.82	5.57
B3	10 M	0.5	2.61	4.04	5.91
B4	10 M	0.75	3.01	4.38	6.33
B5	10 M	1	3.42	4.81	6.66
B6	10 M	1.25	3.83	5.22	6.99
B7	10 M	1.5	3.40	4.95	6.58
C1	12 M	0	2.31	4.23	5.53
C2	12 M	0.25	2.73	4.52	5.91
C3	12 M	0.5	3.15	4.96	6.32
C4	12 M	0.75	3.52	5.45	6.71
C5	12 M	1	3.91	5.99	7.19
C6	12 M	1.25	4.54	6.45	7.51
C7	12 M	1.5	3.71	6.11	6.93

shows the flexural strength of RFRGC with increasing NaOH molarity and steel fiber percentages under ambient curing conditions after 7 and 28 days, illustrating the effect of NaOH molarity and steel fiber ratio on flexural strength. At all molarities, an increase in flexural strength due to an increased steel fiber content, highest at 1.25% steel fiber, can clearly be observed. At a molarity of 12 M, the maximum flexural strength value is achieved, at 7.15 N/mm² (28 days, oven curing) and 1.25% fiber. Any steel fiber proportion above 1.25% causes a reduction in flexural strength, thereby suggesting decreasing advantages due to a possible disadvantageous fiber agglomeration or compromised matrix integrity. Increased molarity brings notable improvements in flexural strength.

Figure 5a indicates the 7-day flexural strength of RFRGC under ambient curing for different steel fiber percentages (0–1.5%) and NaOH molarities (8 M, 10 M, and 12 M). The flexural strength increases with the inclusion of steel fiber at all molarities. At 0% steel fiber, the lowest values are obtained: 1.34 N/mm² (8 M), 1.95 N/mm² (10 M), and 2.31 N/mm² (12 M). Maximum strength is obtained at 1.25% steel fiber for each molarity, with 3.22 N/mm² (8 M), 3.83 N/mm² (10 M), and 4.54 N/mm² (12 M). These values decrease slightly at 1.5% steel fiber, showing decreased performance due to fiber clustering and poor bonding in the matrix. Increased molarity yields better flexural strength at every percentage of steel fiber due to improved geopolymerization and matrix density. For instance, at 0.75% steel fiber, flexural strength falls from 2.55 N/mm² (8 M) to 3.01 N/mm² (10 M), and 3.52 N/mm² (12 M). This trend indicates the synergistic interaction of steel fiber and increased molarity on early-age strength.

Figure 5b shows the 28-day flexural strength of RFRGC under ambient curing for different steel fiber percentages (0–1.5%) and NaOH molarities (8 M, 10 M, and 12 M). As the molarity increases, the flexural strength increases from 2.51 N/mm² (8 M) to 4.23 N/mm² (12 M) at 0% fiber content. Similarly, the flexural strength increases from 5.53 N/mm² to 6.93 N/mm² for 12 M specimens. At 1.5% of fiber, all specimens show a decrease in strength because of fiber agglomeration.

Figure 6 shows the influence of molarity (8 M, 10 M, and 12 M) and steel fiber content (0–1.5%) on the flexural strength of oven-cured specimens. For all molarities, flexural strength improves with increased steel fiber content up to 1.25% and then decreases at 1.5%. Oven curing results in higher strength at all percentages of steel fiber, which can be attributed to accelerated geopolymerization and increased matrix densification under high-temperature conditions. The maximum flexural strength is obtained at 12 M NaOH with 1.25% steel fiber and is equal to 7.41 N/mm². All 12 M specimens exhibit superior performance compared to 10 M and 8 M, at every percentage of fiber. At 1.25% steel fiber, oven curing results in a flexural strength of 6.14 N/mm² for 8 M, 6.99 N/mm² for 10 M, and 7.41 N/mm² for 12 M. This trend is consistent with the fact that higher molarity helps with geopolymerization, bringing about enhanced bond strength between the fiber and the matrix. Oven curing systematically results in greater flexural strength than ambient curing. However, at 1.5% steel fiber, flexural strength decreases for all conditions, highlighting the importance of fine-tuning the fiber content for an optimal balance of strength and workability. This shows how the parameters of RFRGC may be optimized to provide an enhanced flexural behavior in structural applications. The above results show how steel fiber dosage, alkaline activator concentration, and curing regimes synergistically govern RFRGC’s flexural performance, providing insights for designing eco-friendly, high-performance concrete suited for modern infrastructure demand.

3.3. Split Tensile Strength

Split tensile strength is an essential parameter in assessing the performance of RFRGC. It measures a material’s resistance to tensile stresses, which has a direct influence on its crack resistance performance, inhibiting crack propagation and maintaining structural durability, particularly under dynamic or changing loads. In RFRGC, increased tensile strength allows for better resistance to failure mechanisms such as shrinkage cracking and stress concentrations. High values of tensile strength in RFRGC result in an enhanced load-carrying capacity of structural components such as beams, slabs, and pavements, which usually have tensile stresses due to bending. Steel fiber helps RFRGC develop tensile strength through bridging cracks and redistributing stress. This aspect is particularly relevant in the prevention of brittle failure, a prevalent problem with ordinary concrete. The geopolymer matrix, developed by the reaction of aluminosilicates with alkaline activators, also develops tensile properties by building a dense and cohesive structure.

Table 5. Split tensile strength test at various proportions.

Mix	Molarity of NaOH Solution	% of Steel Fiber	Split Tensile Strength in N/mm2
			Ambient Curing		Oven Curing (60 °C for 24 h)
			7 Days	28 Days
A1	8 M	0	2.12	3.61	4.14
A2	8 M	0.25	2.32	4.01	4.25
A3	8 M	0.5	2.69	4.23	4.45
A4	8 M	0.75	2.72	4.50	4.96
A5	8 M	1	2.92	4.74	5.12
A6	8 M	1.25	3.12	4.93	5.66
A7	8 M	1.5	3.02	4.81	5.49
B1	10 M	0	2.48	3.89	4.73
B2	10 M	0.25	2.58	4.24	5.15
B3	10 M	0.5	2.76	4.46	5.59
B4	10 M	0.75	2.88	4.61	5.98
B5	10 M	1	3.08	4.89	6.11
B6	10 M	1.25	3.31	5.02	6.61
B7	10 M	1.5	3.21	4.98	6.21
C1	12 M	0	2.68	4.10	5.29
C2	12 M	0.25	2.80	4.36	5.48
C3	12 M	0.5	2.95	4.69	5.83
C4	12 M	0.75	3.04	4.77	6.20
C5	12 M	1	3.15	5.14	6.49
C5	12 M	1.25	3.44	5.33	6.93
C7	12 M	1.5	3.32	5.16	6.56

shows the split tensile strength of RFRGC at different steel fiber percentages (0–1.5%) and molarities of NaOH (8 M, 10 M, 12 M) under ambient and oven curing conditions. For the ambient curing condition, 7- and 28-day strengths were assessed. At all molarities, the split tensile strength was higher with an increased steel fiber content, up to 1.25%. This can be explained in terms of the crack-bridging and stress distribution function of steel fiber. Above 1.25% fiber, the strength reduces, as observed at 1.5% fiber content at all molarities. This can be attributed to fiber agglomeration, resulting in poor bonding and sites of stress concentration. Increased molarity increases the split tensile strength based on enhanced geopolymerization and matrix densification. The NaOH solution at 12 M always performs better than that at 8 M and 10 M, under all curing times and fiber ratios. Oven curing yields greater strength than ambient curing due to increased polymerization and improved matrix integrity. In the case of ambient curing, longer curing (28 days) also increases strength under all conditions. The integration of 12 M NaOH, 1.25% fiber, and oven curing for 28 days yields the maximum performance.

Figure 7a displays the variation in the split tensile strength of RFRGC with varying percentages of steel fiber and different molarities of NaOH solution for both ambient- and oven-cured specimens at 8 M, 10 M, and 12 M. The results are plotted for both 7-day and 28-day strength under ambient curing. In Figure 7a, for day 7 strength at 8 M, 10 M, and 12 M NaOH, the split tensile strength increases progressively with the incorporation of up to 1.25% steel fiber, followed by a decrease at 1.5% fiber content. For 8 M, the value increases from 2.12 N/mm² for 0% fiber to 3.43 N/mm² for 1.25% fiber. For 10 M, the split tensile strength increases from 2.48 N/mm² for 0% fiber to 3.318 N/mm² for 1.25% fiber. For 12 M, the split tensile strength increases from 2.68 N/mm² for 0% fiber to 3.44 N/mm² for 1.25% fiber. This increase in strength represents the role of reinforcement in crack-bridging and stress transmission, illustrating a decrease in strength after 1.25%. It also suggests that a high content of fiber can cause fiber agglomeration and lower strength. In addition, the strength increases with an increase in molarity. For 8 M, the strength is 2.12 N/mm²; for 10 M, it is 2.48 N/mm²; and for 12 M, the split tensile strength increases up to 2.68 N/mm². This is indicative of the effects of higher molarity on enhancing the cohesive strength of the matrix.

In Figure 7b, the results of the curing test at 28 days replicate this trend: a significant enhancement in split tensile strength at all molarities and steel fiber contents is achieved due to the extended curing duration, thanks to which the geopolymerization process is completed. Here, the strength also increases following the same pattern as that on day 7, but with higher values because of extended curing. For 8 M, the value increases from 3.61 N/mm² for 0% fiber to 4.93 N/mm² for 1.25% fiber. For 10 M, split tensile strength increases from 3.89 N/mm² for 0% fiber to 5.40 N/mm² for 1.25% fiber. Finally, for 12 M, the increase is from 4.1 N/mm² for 0% fiber to 5.33 N/mm² for 1.25% fiber. This increase in strength represents the role of reinforcement in crack-bridging and stress transmission as well as a decrease in strength at a steel fiber content above 1.25%. It also suggests that an excessively high content of fiber can cause fiber agglomeration and lower strength. In addition, the strength increases with increasing molarity. For 8 M, the strength is 3.6 N/mm²; for 10 M, the value is 3.89 N/mm²; and for 12 M, the split tensile strength increases up to 4.1 N/mm². This indicates the impact of molarity, where increased concentrations of NaOH allow for more geopolymerization and bonding between the fiber and the matrix, which leads to improved tensile strength.

In Figure 8, the split tensile strength of oven-cured samples is illustrated as a function of molarity and steel fiber percentage. A similar trend is evident for all molarities, where strength improves as steel fiber content increases up to 1.25%. The 12 M NaOH solution shows the highest strength values at all fiber contents, reaching a maximum value of 6.93 N/mm² at 1.25% steel fiber. This means that a higher molarity helps to facilitate geopolymerization, generating a denser and stronger matrix for efficient load transfer by steel fiber. For 8 M, the value increases from 4.14 N/mm² for 0% fiber to 5.66 N/mm² for 1.25% fiber, and for 10 M, the increase is from 4.73 N/mm² for 0% fiber to 6.61 N/mm² for 1.25% fiber. Finally, for 12 M, split tensile strength rises from 5.29 N/mm² for 0% fiber to 6.93 N/mm² for 1.25% fiber. This increase in strength represents the role of reinforcement in crack-bridging and stress transmission, whereas the decrease in strength at steel fiber contents over 1.25% also suggests that an excessively high content of fiber can cause fiber agglomeration and lower strength. At a fiber content of 1.5%, there is a minor reduction in tensile strength for all molarities, which is speculated to be caused by a clustering of the fiber and stress concentrations. Split tensile strength increases from 4.73 N/mm² for 8 M and 5.29 N/mm² for 12 M. This again highlights the importance of higher molarities in the formation of a denser and stronger matrix. Increasing alkalinity in the formation environment improves the geopolymer matrix–fiber interaction. The findings point to the combined impact of fiber content and molarity on tensile performance. A maximum fiber content of 1.25% provides the optimum results at all molarities, with 12 M NaOH showing the highest degree of improvement in split tensile strength. The marginal drop at 1.5% steel fiber content at all molarities can be explained by fiber clustering, which causes stress concentration and lower tensile efficiency. Although an increase in molarity and fiber content increases strength, oven curing optimizes the matrix–fiber bond and the tensile capacity of the material. This indicates the synergistic effect of optimal curing processes, high molarity, and suitable fiber content on enhancing the mechanical properties of RFRGC. An increased split tensile strength in RFRGC enhances its mechanical wear resistance, freeze–thaw resistance, and chemical resistance. Its endurance makes it appropriate for harsh environments such as marine structures and industrial floors, prolonging its service life and minimizing maintenance. Furthermore, as an environmentally sustainable substitute for traditional concrete, RFRGC involves less carbon emission, supporting green objectives. Maximizing this property guarantees better durability, toughness, and sustainability, thus positioning RFRGC as a promising material for contemporary construction.

3.4. Synergistic Effects of Fiber and Molarity

A higher NaOH molarity (12 M) maximizes strength due to improved binder activation, while oven curing ensures rapid polymerization. However, exceeding 1.25% fiber compromises workability and homogeneity, causing strength plateaus or declines. The superior performance of 12 M mixes with 1.25% fiber validates their viability for structural applications requiring high shear and tensile resistance. These findings align with studies linking fiber geometry and dispersion to postcracking behavior, emphasizing the need for balanced fiber dosing in rubberized geopolymer concrete systems. The combined effects of steel fiber and molarity on the mechanical properties of RFRGC can be deduced from the interpretation of the figures and tables given. The combination of fiber content and molarity increases split tensile and flexural strengths, with a focus on the complementary roles of these factors in enhancing material performance. Higher molarities of NaOH solutions, especially 10 M and 12 M, provide better geopolymerization, with a denser, stronger matrix and superior mechanical properties. This impact is further enhanced by the addition of steel fiber, which confers stress transfer mechanisms, fending off cracks and improving the tensile and flexural strength of the composite. Tensile strength increases proportionally with fiber content up to 1.25%, with the 12 M specimens performing better than lower molarities. The 12 M molarity together with the 1.25% fiber concentration achieves maximum values in split tensile and flexural strengths, thus emerging as the best mix for mechanical improvement.

3.5. Shear Carrying Capacity Testing of RFRGC Beams

The shear strength of RFRGC beams (1.2 m length × 150 mm depth × 100 mm width) without shear stirrups was tested at 0–1.5% steel fiber and 12 M. The shear span-to-depth ratio a/d was kept at 2.93 for all specimens, and the molarity of NaOH was also kept constant, at 12 M. The data show declining returns or even slight decreases in strength at fiber levels above 1.25%. This is because of fiber agglomeration and decreased matrix–fiber interaction. Optimizing fiber content and dispersion with a 1.25% fiber addition showed good results. Ordinary Portland cement concrete beams of M30 grade were cast with the same dimensions as those of rubberized geopolymer concrete beams with 0% fiber. The load deflection behavior of OPC beams was compared with that of the rubberized geopolymer concrete beams, with the results showing that both beams behaved in a similar way. The synergy of enhanced molarity and optimum fiber content was manifested in significantly enhanced tensile and flexural properties. This synergy was supplemented by correct curing practices; to obtain actual field conditions, the specimens were cured under ambient conditions for 28 days.

Table 6. First crack load, yield load, and ultimate load on testing.

Fiber Content (%)	First Crack Load (kN)	Yield Load (kN)	Ultimate Load (kN)
0.00	2.6	6.4	16
0.25	2.8	6.8	18.2
0.50	3.4	10.4	22.4
0.75	4.1	13.6	25.2
1.00	4.6	16.2	28.6
1.25	5.2	19.2	33.2
1.50	4.9	18.8	31.6
OPC	2.4	6.2	15.8

shows the load yield on testing.

Table 6 illustrates the shear performance of RFRGC beams with incremental fiber content (0–1.5%) in comparison to ordinary Portland concrete (OPC) beams. The results reveal an unequivocal trend in shear capacity (first crack, yield, and ultimate loads), which improves with an increase in fiber content of up to 1.25% and then deteriorates at 1.5%. The final load increases from 16 kN (0% fiber) to 33.2 kN (1.25%) and then decreases to 31.6 kN at 1.5%, with a point of optimal fiber content. This peak performance at 1.25% coincides with improved fiber–matrix interaction, as the steel fiber spans microcracks and redistributes stress, delaying shear failure. The initial crack load behaves similarly, rising from 2.6 kN to 5.2 kN (1.25%) before reducing to 4.9 kN (1.5%), highlighting the delicate equilibrium of reinforcement and workability.

The combination of 12 M NaOH and oven curing, as opposed to ambient curing, maximizes geopolymerization, creating a denser matrix for better fiber anchorage. Hooked-end fiber, adding a mechanical interlock, additionally enhances postcracking ductility, as reflected in the dramatic increase in yield load (6.4 kN to 19.2 kN at 1.25%). Energy absorption, approximated from the load–deflection curve area, probably follows suit, with 1.25% fiber providing greater toughness. Yet, the decline at 1.5% emphasizes the boundaries of fiber addition at the expense of no loss in dispersion. In comparison, OPC beams fail in a brittle manner, with very low postcracking resistance, whereas RFRGC beams exhibit residual strength as a result of fiber bridging. These results confirm that RFRGC, with 1.25% fiber, yields a 194% enhancement in ultimate shear capacity over plain RFRGC and a 294% improvement over OPC; therefore, it becomes suitable for shear-critical use. The findings highlight the need for customized fiber dosing and curing processes to achieve maximal structural efficiency, as well as a balance of sustainability and performance, in rubberized geopolymer systems.

The load–deflection curve of RFRGC beams in Figure 9 demonstrates the impact of different steel fiber contents (0–1.5%) on shear carrying capacity and deformation behavior. The curve clearly indicates the impact of fiber reinforcement on enhancing load capacity and deflection behavior. The addition of fiber improves the ductility and ultimate load-carrying capacity of RFRGC beams. Fiber-free beams display the lowest load and deflection, typical of brittle failure behavior. With an increase in the fiber content from 0.25% to 1.25%, the load-carrying capacity and deflection significantly improve because of the superior matrix–fiber interaction and crack-bridging capabilities of the fiber. At 1.25% fiber, the curve has the greatest load capacity (30 kN) and the best deflection, indicating the optimum trade-off between fiber bonding and matrix dispersion.

There are decreasing returns at elevated levels of fiber content, evidenced by the 1.5% fiber curve. This highlights the need to maximize fiber content to prevent adverse effects on mechanical properties. The synergistic effects of increased alkalinity, optimal fiber content, and sufficient curing are demonstrated by the combination of 12 M molarity and 1.25% steel fiber, resulting in optimal performance. The load–deflection curve shows that steel fiber and curing conditions have a strong impact on the mechanical behavior of RFRGC beams. The best fiber ratio of 1.25%, with ambient curing, provides maximum shear strength, ductility, and resistance to load, essential for structural use.

Table 7. Displacement ductility and energy absorption of RFRGC.

Sl No.	Fiber Content (%)	Displacement Ductility	Energy Absorption (kN mm)
1	0	2.53	33.38
2	0.25	2.59	47.38
3	0.50	2.61	65.20
4	0.75	2.70	86.6
5	1.00	2.75	96.99
6	1.25	2.81	116.77
7	1.5	2.78	107.71
8	OPC0	2.39	31.03

shows RFRGC beam displacement ductility and energy absorption in terms of steel fiber content, emphasizing the fiber’s key role in enhancing shear performance and toughness. The results indicate a continuous increase in ductility and energy absorption with increasing fiber content, reaching its maximum at 1.25%. Beams with no fiber (0%) showed the lowest energy absorption (33.38 kN·mm) and displacement ductility (2.53), reflecting brittle failure during loading. The addition of 0.25% fiber resulted in greatly enhanced energy absorption (47.38 kN·mm) and ductility (2.59), which were credited with delaying crack growth and enhancing postcracking behavior. A continued addition of fiber content up to 0.50% and 0.75% enhanced energy absorption by up to 65.20 kN·mm and 86.6 kN mm, respectively, with merely slight fluctuations in ductility.

A fiber content of 1.25% achieved the greatest energy absorption (116.77 kN·mm) and ductility (2.81), exhibiting the synergistic effects of hooked-end fiber on improved crack-bridging and anchorage. Theoretical shear values were 15–20% lower than experimental measurements, highlighting the important role of fiber in load distribution and resistance to cracking beyond that included in standard code provisions. The results indicate that a 1.25% dosage of fiber is the ideal limit value that optimizes improved ductility, energy dissipation, and fiber–matrix interaction as well as optimizing fiber–molarity combinations.

Table 8. Comparison of theoretical and experimental shear strength.

Beam ID	Fiber Content (%)	Shear Force at Failure (V) (kN)	Experimental Shear Strength τexp (N/mm2)	τc (IS 456) N/mm2)	τf = τexp − τc
CB	0.00	9.1	0.72	0.72	0
B1	0.25	11.2	0.89	0.72	0.17
B2	0.50	12.6	1.00	0.72	0.28
B3	0.75	14.3	1.14	0.72	0.42
B4	1.00	15.2	1.21	0.72	0.49
B5	1.25	15.8	1.26	0.72	0.54
B6	1.50	15.4	1.23	0.72	0.51
OPC beam	0%	8.98	0.72	0.72	0

represents the results obtained in the investigation of rubberized fiber-reinforced concrete tested under two-point loading. The focus is on the influence of fiber content on the shear strength of rubberized geopolymer concrete beams cast without stirrups; the experimental result is then compared with the theoretical values.

τ_exp = V/bd

(1)

where

V = failure shear force;

b—breadth of beam, 100 mm;

d—effective depth of beam = 125 mm;

the τ_c value is calculated from IS 456:2000 [35].

(τ_c) is based on the grade of concrete and the percentage of tensile reinforcement. In Equation (1), this value is 0.72 N/mm². As seen in Table 8, the experimental shear stress is much higher than τ_c, meaning that fiber addition resulted in extra shear strength. The control beam without fiber failed at a shear stress of 0.728 N/mm², which is almost the same as the IS code-predicted value of 0.72 N/mm². However, beams B1–B6 achieved higher shear stress. Beam B5 with 1.25% fiber reached 1.264 N/mm², showing an increase of 1.7 times compared to the control beam. This clearly indicates that the fiber enhanced the shear capacity of the beam even in the absence of conventional stirrups. The difference between τ_exp and τ_c (denoted as τ_f) quantifies the fiber’s contribution to shear. The control beams showed diagonal cracking and sudden shear failure, whereas the fiber-reinforced rubber beams exhibited more ductile failure, with fiber bridging the shear crack and holding the section together after initial cracking. Thus, the fiber content contributed to energy absorption and crack resistance, delaying shear collapse.

4. Discussion

In this study, the influence of steel fiber on the mechanical properties of RGPC was investigated. The experimental results confirm that the inclusion of steel fiber significantly improves the performance of RGPC in terms of strength and ductility. A key finding is that steel fiber enhances the compressive strength, split tensile strength and flexural strength of RGPC, counteracting the strength reduction caused by rubber aggregates. Fiber incorporation improved RGPC’s crack-bridging ability, leading to increased energy absorption and postcracking toughness. The optimal fiber dosage improved mechanical performance without compromising workability or mix stability [36]. These results suggest that steel fiber reinforcement is an effective method to enhance the structural performance of RGPC, promoting its use as a sustainable and ductile construction material [37]. While this study demonstrates the positive influence of steel fiber on the mechanical properties of rubberized geopolymer concrete, further research on its durability performance under aggressive environmental conditions, such as acid attack, chloride exposure and freeze–thaw cycles, as well as on its long-term durability and structural behavior under cyclic or dynamic loading, is recommended.

The present experimental investigation demonstrates that the incorporation of steel fibers markedly enhances the shear performance of RGPC beams, even in the absence of conventional shear reinforcement [38]. The steel fibers act as microreinforcements, effectively bridging shear cracks and facilitating a more efficient transfer of tensile stresses across the matrix [39]. This results in improved energy absorption and ductility, both of which are crucial for ensuring structural safety and serviceability [40,41]. Notably, the experimentally measured shear stresses in fiber-reinforced RGPC beams exceeded those predicted by standard design codes such as IS 456:2000 for normal concrete, suggesting that current codes may be conservative when applied to fiber-reinforced geopolymer systems [42]. The enhanced shear behavior can be attributed to the synergistic effect of fiber bridging and the unique microstructure of geopolymer binders, which typically form dense matrices with strong interfacial bonding to steel fibers [43]. RGPC promotes the values of a circular economy through the recycling of waste into useful building materials. The method of production drastically reduces greenhouse gas emissions linked to both tire destruction and standard GPC production [44]. By incorporating recycled rubber (tires) into GPC, this technology directly tackles the worldwide problem of rubber waste. Billions of tires are sent to landfills annually worldwide, creating enormous environmental problems by virtue of their not being biodegradable and due to landfill clogging [45]. Adding rubber crumbs to GPC not only keeps these products from a landfill but also cuts back on the use of virgin aggregate, saving natural resources and lessening the environmental impact. Research shows that the application of waste tire rubber as rubber crumbs in GPC minimizes its impact on global warming by as much as 75% relative to standard GPC with rubber crumbs [46,47]. RGPC can be used in a wide variety of applications, including roads, sidewalks, and playgrounds, without compromising structural strength [48,49]. It is an eco-friendly alternative that reconciles environmental awareness with the harsh reality of construction needs, opening the door to more sustainable and durable infrastructure. This synergy not only offsets the potentially adverse effects of rubber aggregates but also broadens the applicability of steel fiber-reinforced RGPC in structural applications where both sustainability and toughness are paramount. Future research should focus on optimizing fiber content, aspect ratio, and hybrid fiber combinations to achieve an optimal balance between mechanical strength, workability, and cost-effectiveness. Additionally, comprehensive investigations into the long-term behavior of RGPC beams under sustained and cyclic loading are essential. Evaluating the structural performance of fiber-reinforced RGPC in full-scale elements, including shear-critical members and slabs, will provide valuable insights for real-world applications. Furthermore, conducting life cycle analysis (LCA) and sustainability assessments of fiber-reinforced RGPC will help quantify its environmental advantages and support its adoption in green construction practices.

5. Conclusions

The results of this study suggest that steel fiber reinforcement is an effective method of enhancing the structural performance of RGPC, promoting its use as a sustainable and ductile construction material. Further research on its long-term durability and structural behavior under cyclic or dynamic loading is recommended. The influence of steel fiber on the shear performance of RGPC beams without stirrups was studied, with the experimental results showing that the shear capacity of RGPC beams was significantly enhanced. As the fiber content increased, the beams exhibited higher shear strength and improved resistance to diagonal cracking. The control RGPC beams (without fiber) demonstrated brittle shear failure, whereas fiber-reinforced beams exhibited more ductile behavior, with delayed crack propagation and improved postcracking performance. The steel fibers acted as microreinforcements, effectively bridging cracks and increasing the energy absorption capacity of the composite material. The experimental shear strength of fiber-reinforced RGPC beams exceeded the theoretical shear strength predicted by conventional IS code formulations, which do not account for the presence of fiber. This highlights the need to revise code provisions when designing fiber-reinforced geopolymer elements. The results confirm that steel fiber can partially or fully compensate for the absence of stirrups in shear-critical RGPC elements, offering a sustainable and structurally efficient alternative in reinforced concrete design.

While the present study confirms the positive influence of steel fiber on the mechanical properties of RGPC, unlocking its full potential as a sustainable structural material demands a significantly expanded and multifaceted research agenda. In future research, the long-term durability performance of RGPC should be investigated under realistic, aggressive environmental conditions, including combined exposures to chlorides, sulfates, acids, freeze–thaw cycles, and coupled thermal–chemical effects, to understand its specific degradation mechanisms and enable predictive service life modeling. The optimization of fiber parameters is essential; determining the ideal dosage, aspect ratio, and hybrid fiber combinations is crucial for enhancing the composite’s strength and toughness (static and dynamic) with crucial workability requirements (impacted by rubber particles) and overall cost-effectiveness. Translating material gains into structural systems requires evaluating full-scale element performance, specifically focusing on the shear capacity of beams and columns (potentially enabling simplified reinforcement by reducing the reliance on stirrups), punching shear resistance, load distribution, and long-term deformation characteristics of slabs under service conditions. While this study provides valuable insights into the shear behavior and mechanical properties of RGPC, several limitations should be acknowledged. First, the experimental program was conducted under controlled laboratory conditions, which may not fully capture the variability and environmental influences encountered in real-world construction scenarios, such as temperature fluctuations, moisture exposure, and aggressive chemical environments. The scope of the research was limited to specific fiber dosages, aspect ratios, and a single type of recycled rubber, which may restrict the findings’ applicability to other fiber types, rubber sources, and hybrid reinforcement strategies. Additionally, the beam specimens tested were of moderate size and did not include complex geometries, which could influence the distribution of stresses and the overall performance of RGPC in practical applications. The long-term performance and scalability of the optimized mix design were not comprehensively evaluated. Future research should address these limitations by exploring a broader range of material variables, conducting field-scale trials, and assessing long-term durability in diverse service conditions.

The proposed steel fiber RFRGC holds revolutionary potential for green building. Its high shear capacity, ductility, and mechanical strength are suitable for infrastructure constructions that demand high durability and a low environmental footprint. The RFRGC may substitute conventional concrete in structures with seismic resistance in bridge girders or beam–column connections in earthquake zones, where slow crack growth and enhanced energy absorption are essential. The material can operate without shear stirrups; reinforcement placement is easy, and thus, labor and material costs are minimized for precast concrete members such as hollow-core slabs or retaining walls. On urban roads and pavements, RFRGC’s rubber component could reduce traffic noise and vibration, and its geopolymer matrix is chemically resistant to deicing salt degradation. Likewise, industrial flooring systems in warehouses or ports would also benefit from its abrasion resistance and smaller carbon footprint. The use of recycled rubber and industrial by-products complies with the principles of the circular economy; RFRGC can thus be used for green building certifications on commercial complexes or government infrastructure. In addition, the oven-cured composition of RFRGC can be embraced in modular building, where curing under controlled conditions guarantees uniform quality for prefabricated dwellings or relief homes. The strength attained through 12 M NaOH is also well-suited for quick repair uses, such as emergency road repairs or airport runway resurfacing. Balancing structural performance with sustainability, the material responds to worldwide needs for environmentally friendly solutions in resilient infrastructure and low-carbon urbanization. A thorough LCA integrated with a broader sustainability assessment is indispensable to quantitatively evaluate the true environmental footprint of RGPC across its entire lifespan, combined with techno-economic analysis to prove viability. This study is a significant first step in the full validation of RGPC and its leveraging as a high-performance, durable, and genuinely sustainable solution for future green construction.