Sustainable Development and Assessment of Low-Strength/High-Toughness Recycled Plastic Rebars for Structural Elements Under Light Loads

Abstract

The construction sector faces growing pressure to adopt sustainable alternatives amid the global plastic-waste crisis. This study presents a novel use of mechanically recycled high-density polyethylene (HDPE) and polypropylene (PP) to manufacture full-scale plastic rebars for mortar-free, light-load construction applications. A total of 48 samples, plain and ribbed, across three diameters (12 mm, 19 mm, and 25 mm) were fabricated and tested. Due to the absence of standardized protocols for recycled plastic rebars, tensile testing was conducted in reference to ASTM A615. Characterization techniques such as X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM) confirmed the material’s structural features and polymer integrity. XRD confirmed the crystalline phases of HDPE and PP, while SEM revealed ductile fracture in HDPE and brittle failure in PP. The 25 mm ribbed PP rebars demonstrated superior performance, achieving a maximum load capacity of 12.2 ± 0.6 kN, a toughness index of 19.3 ± 1.0, and energy absorption of 101.6 ± 5.0 N-m × 10. These results affirm their suitability for lightweight structural components such as boundary walls, partition panels, and mortar-free interlocking systems. Unlike prior studies that confined recycled plastics to filler roles in composites, this work validates their direct application as full-section, load-bearing members. Additionally, a polynomial-based empirical model was formulated to predict the tensile behavior of the recycled rebars. The findings underscore the potential of mechanical extrusion as a low-emission, scalable solution to convert plastic waste into durable construction materials that support circular economic principles.

1. Introduction

Plastic waste has become a critical environmental threat, with 79% found in landfills and marine environments in 2015, a figure projected to double by 2050 if current trends persist [1]. Recycling is widely endorsed as a key mitigation strategy, and the construction sector presents significant potential for large-scale reuse. HDPE and PP were selected based on their proven recyclability, structural potential, and prior validation in an earlier study on recycled plastics for construction [2]. Historically reliant on steel since Monier’s 1867 patent, reinforcement bars are now evolving through materials like fiber-reinforced polymers (FRP), bamboo, and coir fiber to address issues of cost and corrosion [3,4]. Despite advantages such as light weight and corrosion resistance, FRP rebars suffer from brittleness and anisotropy under dynamic loads [3,5], while bamboo and coir exhibit improved tensile and damping behavior in seismic settings [6,7]. Recent research has introduced hybrid rebars incorporating waste plastics (e.g., PET, PVC) with glass fibers, enhancing both strength and sustainability [8]. These innovations are increasingly used in mortar-free construction systems, where interlocking units replace cement joints, and recycled plastic rebars serve as low-cost seismic stiffeners [4]. Concurrently, microplastics pose rising health risks, with adults ingesting over 3000 particles annually [9,10]. Recycling is hampered by additive and heavy metal contamination, e.g., antimony from e-waste [11], yet solutions like micro-factories, HDPE-based reuse, and r-PVC/r-HIPS blends offer promise [12,13,14,15,16]. Advances in mechanical, chemical, thermal, enzymatic, and electrochemical recycling are improving recovery efficiency and polymer purity, with applications extending from plastic lumber to aerospace components [17,18,19,20,21,22]. These developments underscore the need for automation, improved sorting, and purification to scale up sustainable recycling systems [23]. A recent study examined recycled polyethylene from used products and found that contamination and material inconsistency can affect its reuse in new production processes [24]. Plastics have gained increasing attention in construction due to their corrosion resistance, low weight, and long service life, making them suitable for use in non-load-bearing structural components. Their reusability aligns with circular economic goals, particularly as concerns grow over the environmental impact of post-consumer plastic waste. Recent studies in Guayaquil, Ecuador, report that households generate approximately 1.64 kg of plastic waste per day, with HDPE and PP comprising around 20% and 10%, respectively [25,26]. Despite their recyclability, large portions remain unrecycled due to limited infrastructure and source separation [25]. These findings underscore the need for targeted recovery strategies for PP and HDPE in urban waste management systems.

The construction industry is actively seeking sustainable reinforcement strategies to address the limitations of conventional steel rebars, particularly their susceptibility to corrosion in aggressive environments such as marine and coastal regions [27]. Although epoxy-coated and stainless-steel variants have improved corrosion resistance, their high cost has stimulated the search for alternatives. Fiber-reinforced polymer (FRP) rebars, especially those made from glass, carbon, or aramid fibers, offer notable advantages including corrosion resistance, low weight, and reduced electromagnetic interference [3,5]. Nevertheless, FRP rebars exhibit brittle failure modes and anisotropic behavior, limiting their application under dynamic and shear loading conditions [28]. Recent developments in hybrid FRP composites, integrating jute and coconut fibers, have enhanced flexural and impact performance, providing greener reinforcement options [6]. Bamboo has also emerged as a cost-effective, renewable material for structural reinforcement, achieving satisfactory tensile strengths with proper treatment to mitigate biodegradability concerns [6,29,30]. Natural fiber ropes, such as jute, hemp, and coir, when chemically treated, have been effectively used to improve the ductility and seismic performance of concrete structures [7,31]. Ferrocement strips, thin mortar composites reinforced with mesh or wires, have proven effective in the flexural strengthening and crack control of slabs and walls [32]. Additionally, near-surface-mounted (NSM) techniques using steel wire strips have demonstrated substantial improvements in the shear capacity and ductility of retrofitted reinforced concrete beams [33]. Thermo-Mechanically Treated (TMT) rebars, known for their high strength and superior seismic performance, remain widely used in earthquake-prone regions [34]. Basalt Fiber-Reinforced Polymer (BFRP) rebars, produced from volcanic basalt rock, are gaining attention due to their excellent corrosion, chemical, and thermal resistance properties, making them ideal for marine and repair applications [35]. In parallel, recycled plastic rebars developed from PET, HDPE, and polypropylene fibers present a sustainable and corrosion-resistant alternative for non-critical applications, promoting circular economy principles [13,36]. These diverse innovations collectively signify a shift towards eco-efficient, resilient, and cost-effective reinforcement strategies in modern construction practices.

Mortar-free construction, or dry-stack masonry, is an emerging technique that eliminates the use of cement-based mortar by utilizing precisely manufactured interlocking units, such as concrete blocks, stabilized soil bricks, or recycled composites that fit together to ensure structural integrity through geometry and weight alone. This method offers numerous benefits, including faster construction, reduced material costs, and enhanced sustainability due to the absence of cement, which lowers carbon emissions. It also provides superior seismic performance, as the flexibility between interlocked units allows for better energy dissipation during earthquakes, making it ideal for disaster-prone regions and emergency shelters [37,38,39]. Advanced block designs now incorporate features like tongue-and-groove or dovetail joints, improving load transfer, ease of assembly, and structural strength [40,41]. Materials used in these systems include high-strength concrete and recycled composites, and researchers have integrated insulation elements to enhance energy efficiency [42]. The use of locally sourced materials further supports sustainability by reducing transportation emissions and costs [43,44]. Mortar-free construction has proven adaptable for both temporary and permanent structures in residential, commercial, and emergency contexts, with studies confirming its resilience to environmental stressors and comparable performance to conventional masonry [36]. Additionally, it simplifies maintenance by allowing for individual block replacement, reduces water ingress by eliminating mortar joints, and aligns well with green building initiatives. As urbanization increases and the demand for efficient, eco-friendly construction rises, mortar-free systems are poised to play a key role in the future of sustainable infrastructure development [44]. While this study focuses on construction applications, recycled plastics are also being reused in other sectors such as agriculture, packaging, and automotive manufacturing.

This study presents a pioneering approach by transforming municipal plastic waste, specifically HDPE and PP, into structural rebars for mortar-free construction. Unlike conventional studies that utilize recycled plastics as additives or fillers, this work develops full-scale, load-bearing rebars from waste polymers. A total of 48 rebar samples in three diameters (12 mm, 19 mm, 25 mm), both plain and ribbed, were fabricated through mechanical extrusion and subsequently tested. Due to the absence of standard methods for plastic rebars, tensile testing was conducted following ASTM A615 guidelines. XRD revealed distinct crystalline peaks of HDPE and PP, while SEM highlighted ductile tearing in HDPE and brittle fracture in PP. The mechanical results showed substantial tensile strength, with HDPE demonstrating better extrusion performance and recyclability. Earlier plastic waste is used in minimal quantities in research as a sustainable solution. This is the first study to validate recycled plastic rebars as standalone elements for sustainable construction. The findings offer a viable, low-carbon alternative to conventional steel reinforcement, reducing dependency on virgin raw materials. By employing mechanical extrusion and promoting the structural reuse of plastic waste, this research supports circular economy principles and contributes to sustainable material innovation, environmental resilience, and the broader goals of sustainable urban development.

Research Significance

This study presents an innovative approach to plastic-waste management by enhancing recycling efficiency through the development of construction-grade products. To address this gap, the present research explores the use of municipal plastic waste, specifically high-density polyethylene (HDPE) and polypropylene (PP), for the fabrication of novel reinforcement bars (rebars) suitable for mortar-free construction systems. Recycled plastic rebars developed in this study are suited for non-load-bearing or lightly loaded structural applications. Examples include partition walls, boundary fences, footpaths, and mortar-free interlocking units. These elements experience minimal stress, making them compatible with the mechanical limits of recycled HDPE and PP. Unlike previous studies that primarily evaluated recycled plastics as additives in concrete, roads, or soil stabilization, this research investigates their potential as independent structural elements. A total of 48 rebar samples were manufactured using mechanical extrusion, covering three different diameters (12 mm, 19 mm, and 25 mm), with both plain and ribbed surface textures. The use of mechanical extrusion not only reduces environmental hazards but also supports circular economic practices by converting plastic waste into durable, resource-efficient construction materials. Given the absence of specific ASTM standards for the tensile testing of recycled plastic rebars, mechanical behavior was evaluated following the guidelines of ASTM A615, traditionally used for steel reinforcement. The assessment also included the identification of material impurities and comprehensive analysis of the mechanical and microstructural properties. XRD patterns revealed crystalline peaks corresponding to HDPE and PP, confirming the retention of polymeric identity post-recycling. SEM images of fractured surfaces demonstrated ductile failure in HDPE and brittle fracture in PP, aligning with their known mechanical profiles. The results showed that both polymers possessed significant tensile strength, with HDPE displaying superior extrusion compatibility and recyclability. This study is among the first to validate the use of 100% recycled plastic for full-section rebars, offering a viable, corrosion-resistant, and cost-effective alternative to traditional steel reinforcement, particularly for non-critical and light-load applications. This research addresses critical gaps in Pakistan’s recycling infrastructure by demonstrating a scalable method to transform plastic waste into full-section recycled rebars, thereby expanding the structural use of polymers beyond conventional filler-based applications. These findings pave the way for further research on optimized polymer blends and large-scale implementation in sustainable infrastructure development.

2. Experimental Program

2.1. Collection and Synthesis of Recycled Plastic

The collection and sorting of waste plastic materials constituted a vital preliminary step to ensure the quality, consistency, and reliability of the samples used for mechanical recycling and testing in this study. Municipal solid waste (MSW) streams served as the primary source, with a targeted focus on isolating recyclable thermoplastics applicable for construction-related applications. Collected plastics underwent manual sorting to remove contaminants such as organic residues, paper, metals, and multilayer composites that could compromise material homogeneity. Municipal plastic waste was sourced from local waste collection centers and informal recyclers operating in the Islamabad and Rawalpindi regions. The collected waste primarily included post-consumer HDPE and PP products such as detergent bottles, food containers, and packaging materials. To ensure material purity, a multi-stage cleaning protocol was implemented: plastics were manually sorted to remove multilayer, PVC, PET, and heavily contaminated items, followed by soaking in a mild detergent solution and thorough rinsing with clean water. The washed materials were sun dried and visually inspected to ensure no residual organic matter or foreign particles remained. No chemical pretreatment was used. The sorting strategy prioritized the separation of high-density polyethylene (HDPE) and polypropylene (PP), with comparative material characterization guiding the selection. The sample preparation procedure in this research was followed, as per the methodology outlined in [2], ensuring standardized practices for cleaning, drying, and mechanical shredding. Palletization was performed, resulting in granules. For this research, rHDPE and rPP samples were prepared with grey and blue pallets, respectively. The recycled pallets of HDPE and PP are shown in Figure 1. Although handling PP is difficult in extrusion as compared to HDPE, on the other side, the engineering properties of PP are considerable for the construction industry. Research has also emphasized the importance of controlling processing parameters such as temperature and mixing ratio during multi-material blending to ensure consistency in product performance [2]. The experimental setup employed conventional extrusion-based recycling techniques to reprocess the sorted waste plastics into rebar forms, suitable for mechanical testing. Recent advancements in extrusion technology have significantly improved the sustainability and efficiency of recycling post-consumer and post-industrial plastics, particularly by enabling the processing of diverse polymers such as HDPE and LDPE with better output quality and reduced environmental impact. Furthermore, emerging catalytic extrusion processes offer new pathways to selectively upcycle polyolefin-based waste into high-value structural materials, aligning with the broader objectives of waste reduction and promoting circular economy principles [2].

2.2. Sustainable Development of Recycled Plastic-Waste Rebars (RPR)

Palletization confirmed the production of recycled plastic to be used for the second round of extrusion. The extrusion temperature was maintained in the range of 150–170 °C to accommodate the melting characteristics of HDPE and PP, which have typical melting ranges of 130–171 °C depending on polymer grade and crystallinity [2,45]. Since the recycled plastic feedstock was unsorted and potentially contained minor impurities, including residual additives or other polyolefins, a slightly extended temperature range was necessary to ensure complete melt flow and avoid partial fusion. This approach enabled uniform extrusion without excessive thermal degradation, even in the presence of contaminants that could alter melting behavior. To form rebars of specific diameters and lengths, custom-designed sizing molds were employed, integrated with a water-cooling setup to ensure dimensional stability. In the initial phase, plain rebars without ribbed surfaces were produced. Figure 2a–d illustrate the typical layout of the extrusion and molding system, including the sizing molds used to fabricate the novel recycled plastic rebars (RPRs), while Figure 3 displays the finished rebar specimens. During operation, plastic material was fed through a hopper and conveyed by a motor-driven screw mechanism.

Heating elements, controlled via an electronic module and monitored by thermocouples, softened the plastic, which exited the die in a semi-solid state. The extrudate solidified within 10–15 min upon cooling, depending on machine temperature. Each rebar was labeled prior to testing. The resulting products were lightweight and displayed visibly ductile characteristics. The samples were developed after the second round of extrusion. The first round was palletization. The second round was for making plain bars from pallets. The ribbed bars were obtained after heating the plain bars and feeding them into the specialized rollers to create ribs on the surface of the rebars. The extrusion system featured an electronic control panel that allowed for precise adjustment of both temperature and screw speed settings. To maintain optimal operational conditions, the machine was housed in a closed, moisture-free environment. The motor operated at a maximum speed of 900 RPM, which was reduced through a gear mechanism to approximately 45–46 RPM. To produce all samples, the final extrusion speed was further controlled electronically and maintained within a range of 20–25 RPM to ensure consistent material flow and quality. The extruded plain bars that came out from the sizing molds were cooled down first from the custom-fixed water body, in which the water was continuously flowing at room temperature. The second cooldown was conducted by directly flowing water onto the extruded bars. The continuous flow of the extruded plastic was monitored, and, after the desired lengths were obtained, the plain bars were cut. At this speed, the recycled plastic extrude was workable and easily flowing through the extrusion machine. This arrangement was manually handled. The average time for making 10 feet was 15–20 min for rHDPE and rPP rebars. For rHPDE and rPP, the average weights of the samples are shown in

Table 1. Average weight of recycled plastic rebars.

	Weight (gms/m)
	Dia (25 mm)	Dia (19 mm)	Dia (12 mm)
Plain rPP	417.5 ± 2.1	225.0 ± 0.9	116.3 ± 0.7
Ribbed rPP	442.9 ± 1.7	250.0 ± 1.2	100.0 ± 0.8
Plain rHDPE	353.2 ± 1.5	214.7 ± 1.3	110.4 ± 1.4
Ribbed rHDPE	450.4 ± 1.1	236.0 ± 1.8	105.8 ± 1.8

.

The extrusion process was carried out through a screw system divided into four operational zones: the feed section, compression area, melting zone, and discharge outlet. Each stage maintained a specific temperature range, regulated by thermocouples, with values set between 50–55 °C, 100–110 °C, 120–130 °C, and 120–135 °C from the first to the fourth zone, respectively. To ensure consistent material flow and effective melting of recycled plastic, the screw speed was maintained between 40 and 50 RPM, depending on processing stability. The machine utilized six heating coils, each linked to a thermocouple, all managed through an automated control panel for accurate thermal adjustment. After shaping, the extruded plastic rebars were cooled in a water bath to retain their dimensional accuracy. A total of 48 specimens were produced for tensile testing, comprising both plain and ribbed bars in three different diameters: 12 mm, 19 mm, and 25 mm. Figure 2 illustrates the layout of the extrusion equipment and highlights the main sections used in the molding process. Figure 3 represents recycled plastic rebars developed from PP and HDPE, plain and ribbed, (a) 12 mm, (b) 19 mm, (c) 25 mm. The extrusion method applied in this study supports environmental sustainability by minimizing emissions and material waste compared to conventional construction material production techniques.

2.3. Test Setup

2.3.1. Tensile Tests

It is important to note that ASTM A615 is a tensile test standard developed for steel reinforcement bars and does not account for the viscoelastic and ductile nature of thermoplastics. However, due to the absence of established testing standards for full-scale recycled plastic rebars, ASTM A615 was used to facilitate structural-level comparisons with conventional reinforcement. The use of this standard may influence the observed stress–strain response, particularly regarding yield definition and post-yield behavior. The usual method for testing plastic for tensile properties is ASTM D638 [2,46]. The test setup available contains a servo-hydraulic universal testing machine, as shown in Figure 4. The A615 ASTM procedure was used to test tensile strength of recycled waste [47]. The weights of the samples are given in Table 1. Earlier, a sample of bamboo was also tested to check the tensile strength absorption [36]. The recycled plastic rebars (RPRs) were easy to bend and, therefore, a bend test was not required. Testing was conducted under controlled laboratory conditions, maintaining a room temperature of approximately 23 ± 2 °C and a relative humidity of 50 ± 5%. The recorded properties were analyzed for behavior under tensile loads. Future work may explore the adaptation or development of polymer-specific tensile testing protocols for large-diameter structural rebars.

2.3.2. Characterization and Microstructural Analysis Procedure

a. SEM analysis procedure

The Scanning Electron Microscopy (SEM) analysis was performed using a ZEISS NCP instrument equipped with a secondary electron (SE2) detector. The micrographs were acquired under a high-vacuum environment, with the gun vacuum maintained between 5.74 × 10⁻¹⁰ and 5.82 × 10⁻¹⁰ mbar and the system vacuum ranging from 2.89 × 10⁻⁶ to 6.39 × 10⁻⁶ mbar. An accelerating voltage (EHT) of 15.00 kV was applied during imaging, which provided sufficient electron beam energy to resolve surface topography and morphological features of the polymeric samples. The working distance was set between 9.0 mm and 9.3 mm across the samples to optimize resolution and depth of field. Two magnification levels were used: 1.00 KX for broader surface observations (scale bar = 10 µm), and 5.00 KX for detailed morphological examination (scale bar = 2 µm) [48].

b. X-Ray Diffraction procedure

The X-ray diffraction (XRD) analysis was conducted utilizing a θ–2θ locked-coupled scan geometry to examine the crystallographic structure of polymer samples. A copper (Cu) anode was employed as the X-ray source, producing characteristic Cu Kα radiation with wavelengths of 1.5406 Å (Kα1) and 1.54439 Å (Kα2) and an average wavelength of 1.5418 Å, which is suitable for analyzing semicrystalline polymer matrices such as high-density polyethylene (HDPE) and polypropylene (PP). The operating conditions of the X-ray tube were set at a voltage of 40 kV and a current of 30 mA, providing adequate beam intensity for polymeric materials. The scan was performed using a goniometer (Model 512) with a 560 mm diameter. The primary and secondary Soller slits were fixed at 2.5°, ensuring minimized axial divergence and improved peak resolution. No monochromator or beam analyzer was applied, allowing for rapid data collection. The 2θ scan range initiated at 10° and continued up to approximately 32.2°, capturing the principal diffraction peaks relevant for HDPE and PP. Fine-step resolution was used, which enabled accurate peak identification and reliable estimation of crystallinity. The test setup reflects standard XRD measurement protocols for polymeric materials, providing sufficient resolution and data quality for distinguishing crystalline peaks associated with polymer phase identification and structural analysis [49].

2.4. Empirical Modelling Procedure for Maximum Load and Maximum Stress

The mechanical behavior of recycled plastic rebars was analyzed through an empirical modeling approach by fitting second-degree polynomial equations to the experimental data. This method allowed for the derivation of mathematical relationships between critical performance parameters (such as maximum load and maximum stress) and bar diameter. By determining the quadratic, linear, and constant coefficients for each material configuration, the modeling captures the non-linear trends inherent to polymeric materials under tensile loading. The use of polynomial fitting provides a robust framework to predict mechanical responses at intermediate diameters and facilitates comparative evaluation across different material types and surface structures [50].

3. Results

3.1. Tensile Performance of Recycled Plastic Rebars

3.1.1. Tensile Behavior

The load–elongation behavior of 12 mm diameter recycled plastic rebars, comprising both plain and ribbed configurations of HDPE and PP composites, is illustrated in Figure 5a. All specimens exhibited a non-linear increase in load with elongation, characteristic of polymeric materials transitioning from elastic to plastic deformation. Plain-HDPE rebars showed the lowest load-carrying capacity and a gradual, ductile failure mode, whereas plain-PP rebars displayed higher initial stiffness, a greater peak load, and earlier onset of softening. The ribbed-HDPE rebars demonstrated a slight improvement in load-bearing capacity compared to their plain counterparts, primarily due to enhanced surface interlocking, while retaining similar ductility profiles. Conversely, ribbed-PP rebars achieved the highest maximum load and elongation, demonstrating superior toughness and delayed necking, attributed to the inherent strength of polypropylene combined with the mechanical benefits of ribbed surface geometry. Yield points, identified using the 0.2% offset method and marked with red crosses, highlighted that PP-based rebars yielded at higher loads and lower elongations than HDPE rebars [51]. Overall, the ribbed configurations outperformed the plain ones, with ribbed PP rebars emerging as the most promising candidate for structural reinforcement applications requiring a balance of high strength and ductility, consistent with findings reported for surface-structured polymer reinforcements [52]. The load–elongation behavior of 19 mm diameter recycled plastic rebars, encompassing both plain and ribbed forms fabricated from HDPE and PP composites, is depicted in Figure 5b. All specimens exhibited typical non-linear load–elongation curves, associated with thermoplastic-based materials undergoing elastic deformation followed by plastic flow. Among the specimens, plain HDPE rebars recorded the lowest load-bearing capacity, characterized by a gradual and ductile failure behavior due to the material’s relatively low modulus of elasticity and high strain tolerance. In contrast, plain PP rebars demonstrated enhanced stiffness and achieved greater peak loads and elongations before failure, highlighting polypropylene’s superior mechanical properties [53]. Surface ribbing further improved the mechanical performance of both materials, with ribbed HDPE rebars showing increased early-stage stiffness relative to the plain HDPE, though overall ductility remained unchanged. Ribbed PP rebars achieved the highest load-bearing and elongation capacities, indicating significant toughness improvements and extended plastic deformation phases. Yield points determined by the 0.2% offset method occurred at higher loads for PP-based rebars, underscoring the material’s higher stiffness and yield strength [50]. The curves further illustrate that, despite a slightly lower maximum stress, plain PP rebars absorbed considerable energy through ductile deformation, consistent with prior studies on semicrystalline polymer matrices [54]. The load–elongation responses of 25 mm diameter recycled plastic rebars, including both plain and ribbed configurations of HDPE and PP composites, are presented in Figure 5c. All specimens demonstrated the characteristic non-linear behavior of thermoplastics, involving an initial elastic region followed by yielding and plastic deformation. Plain HDPE rebars showed the lowest peak loads and exhibited steep post-yield softening, reflecting the brittle behavior of HDPE at larger cross-sectional dimensions. Plain PP rebars outperformed HDPE with higher peak loads, improved stiffness, and better ductility prior to failure. Ribbed HDPE rebars exhibited enhanced early-stage load capacities, suggesting that ribbed geometries effectively delayed failure initiation by promoting mechanical interlocking, although ultimate ductility gains remained modest. Ribbed PP rebars achieved the highest overall load-bearing capacity and elongation, sustaining extended plastic deformation and demonstrating excellent toughness and energy absorption properties. The consistent shift of yield points to higher loads in PP-based rebars confirmed the mechanical superiority of polypropylene composites, while the ribbed geometry promoted better stress distribution and delayed crack propagation. These results align with previous findings, emphasizing the benefits of surface structuring in enhancing polymer composite performance [55]. The combination of larger cross-sectional area, extended elongation before failure, and superior energy absorption capacities makes ribbed PP rebars a highly promising sustainable alternative for reinforcing applications in structures demanding high impact resistance and durability.

Table 2. Summary of tensile performance metrics for recycled plastic rebars (RPRs), including maximum stress, yield load, elongation at yield, energy absorption, toughness index.

Sample	Max Load (kN)	Max Elongation (mm)	Energy Absorption (N-m × 10)	Max Stress (MPa)	Yield Load (kN)	Elongation at Yield (mm)	Energy Absorption up to Yield (N-m)	Toughness Index
Dia (12 mm)
Plain HDPE	1.2 ± 0.1 (6.2)	123.9 ± 6.2 (6.8)	8.4 ± 0.4 (11.9)	10.3 ± 0.5 (5.3)	0.7 ± 0.01 (2.1)	15.7 ± 0.8 (4.2)	6.9 ± 0.3 (8.1)	12.3 ± 0.6 (3.7)
Plain PP	2.0 ± 0.1 (7.2)	39.7 ± 2.0 (7.9)	5.6 ± 0.2 (10.5)	17.6 ± 0.9 (3.8)	1.0 ± 0.1 (7.8)	7.1 ± 0.4 (3.5)	4.0 ± 0.2 (8.0)	14.2 ± 0.7 (2.4)
Rib PP	1.7 ± 0.1 (7.2)	140.8 ± 7.0 (7.3)	19.6 ± 0.1 (13.5)	15.1 ± 0.8 (3.1)	1.1 ± 0.1 (8.6)	18.1 ± 0.9 (3.5)	13.0 ± 0.7 (7.7)	15.1 ± 0.8 (2.2)
Rib HDPE	1.7 ± 0.1 (4.2)	129.8 ± 6.5 (4.3)	19.7 ± 0.9 (11.2)	15.3 ± 0.8 (4.2)	1.3 ± 0.1 (17.1)	16.8 ± 0.8 (1.5)	14.0 ± 0.7 (5.9)	14.1 ± 0.7 (2.9)
Dia (19 mm)
Plain HDPE	3.4 ± 0.2 (5.2)	28.8 ± 1.4 (5.6)	5.9 ± 0.3 (10.2)	12.1 ± 0.6 (4.5)	1.7 ± 0.1 (27.1)	6.7 ± 0.3 (7.5)	6.6 ± 0.3 (9.5)	9.0 ± 0.5 (2.1)
Plain PP	4.9 ± 0.2 (3.2)	45.5 ± 2.3 (9.1)	16.5 ± 0.8 (11.4)	17.2 ± 0.9 (2.2)	2.3 ± 0.1 (3.1)	7.6 ± 0.4 (8.8)	10.2 ± 0.5 (9.5)	16.2 ± 0.8 (2.9)
Rib HDPE	4.2 ± 0.2 (7.2)	31.9 ± 1.6 (5.3)	8.9 ± 0.4 (9.9)	14.9 ± 0.7 (11.2)	2.1 ± 0.1 (3.1)	7.2 ± 0.4 (4.2)	8.7 ± 0.4 (8.7)	10.2 ± 0.5 (2.4)
Rib PP	6.6 ± 0.3 (4.2)	63.4 ± 3.2 (4.6)	32.6 ± 1.6 (5.9)	23.3 ± 1.2 (14.3)	3.4 ± 0.2 (7.1)	9.1 ± 0.5 (4.3)	18.1 ± 0.9 (3.9)	18.0 ± 0.9 (2.1)
Dia (25 mm)
Plain HDPE	3.0 ± 0.1 (6.2)	18.9 ± 0.9 (5.1)	4.0 ± 0.2 (3.9)	6.1 ± 0.3 (11.2)	2.3 ± 0.1 (8.2)	4.6 ± 0.2 (4.9)	6.3 ± 0.3 (3.9)	6.4 ± 0.3 (3.7)
Plain PP	8.0 ± 0.4 (3.2)	47.0 ± 2.3 (6.6)	25.6 ± 1.2 (17.9)	16.3 ± 0.8 (13.2)	4.1 ± 0.2 (4.6)	8.1 ± 0.4 (4.8)	18.7 ± 0.9 (2.8)	13.7 ± 0.7 (8.7)
Rib HDPE	9.3 ± 0.5 (2.2)	32.8 ± 1.6 (3.6)	20.5 ± 1.0 (11.9)	19.0 ± 1.0 (17.2)	4.3 ± 0.2 (4.1)	6.8 ± 0.3 (7.5)	16.0 ± 0.8 (3.4)	12.8 ± 0.6 (8.7)
Rib PP	12.2 ± 0.6 (9.2)	100.6 ± 5.0 (6.1)	101.6 ± 5.0 (13.9)	24.9 ± 1.2 (6.2)	7.1 ± 0.4 (6.1)	12.8 ± 0.6 (6.5)	52.6 ± 2.6 (5.6)	19.3 ± 1.0 (9.7)

and Figure 6 summarize the tensile behavior of recycled plastic rebars (RPRs). In all diameter groups, polypropylene (PP)-based rebars exhibited superior mechanical performance compared to their high-density polyethylene (HDPE) counterparts, with ribbed PP rebars consistently achieving the highest values of maximum load, elongation, energy absorption, and toughness index. For the 12 mm diameter group, plain HDPE rebars displayed the lowest maximum load 1.2 kN and energy absorption 84.9 N-m, alongside a lower yield load 0.7 kN and energy absorption up to a yield of 6.9 kN-mm. Conversely, ribbed PP rebars attained the highest elongation 140.8 mm and a substantially improved toughness index 15.1, underscoring the positive influence of ribbing and material choice. At 19 mm diameter, similar trends were observed. Ribbed PP rebars exhibited a maximum load of 6.6 kN, a toughness index of 18.0, and the highest energy absorption up to yield 18.1 N-m, demonstrating a pronounced enhancement compared to plain rebars. The ductility, as measured by elongation at yield, remained relatively stable across configurations but was highest for ribbed PP rebars 9.1 mm. In the 25 mm diameter group, the mechanical advantage of ribbed PP rebars became even more pronounced. They achieved a maximum load of 12.2 kN, an elongation at failure of 100.6 mm, and a remarkable toughness index of 19.3. In contrast, plain HDPE rebars at the same diameter exhibited significantly lower values across all categories, including maximum load 3.0 kN and toughness index 6.4. Overall, the data indicate that increasing diameter generally enhanced the load-carrying capacity and energy absorption for all specimens; however, the degree of improvement was notably higher in PP-based rebars, particularly those with ribbed surfaces. Ribbing consistently contributed to improvements in both strength and toughness across all material types and diameters, validating the design strategy of incorporating surface structuring to enhance the mechanical performance of recycled plastic composites used for structural reinforcement [2].

3.1.2. Empirical Modelling for Maximum Load and Maximum Stress

The given set of behavior can be trended into empirical modelling using second-degree polynomial equations for the given data for the values of each diameter of the rebar shown in Figure 7. The relationship between maximum load (P) and bar diameter d for recycled plastic rebars, including plain HDPE, plain PP, ribbed HDPE, and ribbed PP, is illustrated through second-degree polynomial fitting, following the general equation:

P = ad² + bd + c

(1)

where P is the maximum load in kN, d is the diameter of the bar in mm, and a, b, and c are the quadratic, linear, and constant coefficients, respectively. For the materials studied, the quadratic coefficients a were determined as 0.0181 for ribbed PP, 0.0084 for plain PP, 0.0384 for ribbed HDPE, and −0.0303 for plain HDPE rebars. The corresponding linear coefficients b were 0.1370, 0.1505, −0.8339, and 1.2621, respectively, while the constant terms c were found to be −2.5392, −1.0306, 6.2057, and −9.6179, respectively. The positive values of the quadratic coefficients for ribbed PP, plain PP, and ribbed HDPE indicate that the maximum load capacity increases with diameter, whereas the negative quadratic coefficient for plain HDPE suggests a peak load at an intermediate diameter (around 19 mm), followed by a decline at 25 mm [56].

Figure 8 shows the empirical relation of the maximum stress and diameter of the bar. The relationship between maximum stress σ and bar diameter d for recycled plastic rebars, including plain HDPE, plain PP, ribbed HDPE, and ribbed PP configurations, is represented by second-degree polynomial fitting, following the general equation:

σ = ad² + bd + c

(2)

where σ is the maximum stress in MPa, d is the bar diameter in mm, and a, b, and c are the quadratic, linear, and constant coefficients, respectively. The values obtained for the quadratic coefficient (a) are −0.0962, −0.0068, 0.0384, and −0.0692 for plain HDPE, plain PP, ribbed HDPE, and ribbed PP rebars, respectively. The corresponding linear coefficients b are 3.2382, 0.1515, −0.8339, and 3.3079, while the constant terms (c) are −14.714, 16.758, 6.2057, and −14.591 for the respective materials. The negative quadratic coefficients for plain HDPE, plain PP, and ribbed PP indicate a slight decrease or saturation in maximum stress at larger diameters, while the positive coefficient for ribbed HDPE suggests a consistent increase with diameter. The fitted polynomial curves closely align with the experimental data points at 12 mm, 19 mm, and 25 mm diameters, illustrating the combined influence of material type and surface structuring on the stress-bearing capacity of recycled plastic rebars. The polynomial regression model was used to capture the relationship between rebar diameter and mechanical properties such as maximum load and tensile strength. While the fitting offers a statistically valid representation, its practical utility lies in providing an early-stage estimation tool for designers and engineers. This model enables performance prediction for intermediate diameters not explicitly tested, supports trend visualization for future material scaling, and offers insight into how dimensional variations affect load-bearing capacity. Such modeling is especially valuable during the material optimization phase or for rapid assessment in pilot construction scenarios using recycled plastic reinforcement.

3.2. Microstructural Behviour

3.2.1. SEM Analysis

The SEM image of HDPE in Figure 9 reveals a highly irregular and rugged surface morphology, indicating that the material has undergone substantial mechanical stress or fracture. The surface appears fragmented with signs of delamination and flaking, suggesting localized failure zones. The texture is notably rough, featuring a mixture of sharp ridges and more rounded depressions, which points to a combination of abrasive and erosive degradation mechanisms. A prominent crack runs centrally across the image, branching into several fine, interconnected sub-cracks. The tortuous path of these cracks implies they propagated through a heterogeneous matrix, possibly containing multiple phases or inclusions. The sharp crack tips are indicative of brittle fracture behavior, which may have been initiated or accelerated by the material’s internal structure. Scattered pores are visible, some of which are clustered near the crack regions. These voids likely acted as stress concentrators and played a role in the initiation and growth of cracks. Additionally, areas of particle pull-out and surface detachment are apparent, along with hints of embedded second-phase particles or inclusions that could have influenced crack path deflection and surface irregularity. Overall, this image suggests that the material experienced significant structural degradation due to combined mechanical and microstructural factors. The SEM image displays a complex and uneven surface, characterized by substantial topographical variation. The morphology indicates a combination of coarse and fine features, with regions that appear compact and others that are more fractured and open. The texture is highly rugged, showing signs of intense surface disruption likely caused by mechanical loading or environmental exposure. Several long and interconnected cracks are observed traversing the surface. These cracks exhibit branching behavior and irregular paths, hinting at propagation through a structurally non-uniform or multiphase material. The edges of the cracks are sharp and clean, consistent with brittle fracture characteristics, although minor plastic deformation may be present at localized points where the material seems slightly smeared. Pores are visible across the image, particularly concentrated around crack intersections and defect-rich zones. These pores vary in size and appear to be both isolated and clustered, suggesting possible gas entrapment during processing or post-fabrication degradation. Additionally, surface spalling and material pull-out are noticeable, particularly in the upper and mid-regions of the image, reinforcing the presence of mechanical damage.

The SEM image of PP exhibits a distinctly brittle fracture surface with radial crack propagation patterns, indicative of a stress concentration origin [57]. A circular fracture zone in the center of the image features multiple radiating cracks, suggesting a failure initiated by a point load or localized impact [58]. The overall morphology is smooth in the undamaged regions, contrasting sharply with the rough, fragmented zones around the crack front. The fracture pattern is dominated by trans-granular cracking, evidenced by the uninterrupted cracks traversing through the material without significant deflection [59]. This crack morphology is characteristic of brittle failure, where the material lacks sufficient plasticity to arrest crack growth. The well-defined crack tips and branching behavior further indicate that the crack growth was rapid and unstable. Such features are typically observed in polymer-based or composite materials under tensile stress conditions [57,59]. Several pores and voids are distributed near the crack origin and along the radial fracture lines. These features likely served as crack initiation sites or weakened the local structure, facilitating crack propagation [59]. The relatively clean background and absence of significant particle pull-out suggest a uniform matrix composition in this region, with minimal filler reinforcement or secondary phase presence. Overall, the image illustrates a classic case of brittle failure influenced by stress concentration and intrinsic material weakness. The presence of radially expanding cracks and micro-voids reflects a sudden fracture event likely exacerbated by pre-existing flaws or environmental embrittlement [58]. The fracture behavior is predominantly trans-granular, as the cracks pass directly through the bulk of the material without significant deviation or deflection. This is characteristic of brittle materials, particularly those with low fracture toughness and high stiffness [59].

The sharp and clean crack edges, coupled with the absence of plastic flow lines or necking, reinforce this interpretation. Additionally, the crack network displays tortuosity and branching, suggesting a heterogeneous internal structure or the presence of micro-defects that influenced crack trajectory [60]. Pores and voids are visible throughout the image, particularly along the crack path. These features act as stress risers and contribute to premature crack initiation and propagation [59]. In some zones, fragmented material and evidence of particle pull-out can be observed, which further signifies localized failure due to interfacial weaknesses or the breakdown of bonding between matrix and filler phases [58]. Overall, the image portrays a material that failed under brittle fracture conditions, influenced by microstructural heterogeneity and stress concentration points. The features suggest that the material lacks sufficient ductility to absorb applied energy, leading to catastrophic failure upon crack initiation [57]. The presence of brittle fracture features, including sharp crack propagation, minimal plastic yielding, and absence of fibrillation, indicates that the material possesses limited energy absorption capacity and fails rapidly upon crack initiation.

3.2.2. XRD Analysis

Figure 10a depict polymeric samples with distinct crystallinity features of high-density polyethylene (HDPE). Both spectra exhibit the characteristic HDPE diffraction peaks at approximately 21.6° and 23.9° 2θ, corresponding to the (110) and (200) crystallographic planes, respectively. In the top spectrum, these peaks are sharp and intense, indicating a high degree of crystallinity and structural order, suggesting a minimally processed HDPE sample. In contrast, the bottom spectrum shows broader and less intense peaks, implying reduced crystallinity due to thermal degradation and mechanical processing. The XRD spectra shown in Figure 10b illustrate the crystallographic behavior of polypropylene (PP) in two different samples. Both charts reveal prominent diffraction peaks at approximately 14.1°, 16.8°, 18.6°, 21.3°–21.9°, and 25.5° 2θ, corresponding to the PP crystallographic planes (110), (040), (130), and (111)/ (041), respectively. The upper spectrum features relatively sharp and distinct peaks, indicating a well-ordered crystalline structure typical of isotactic polypropylene.

The presence of multiple well-defined peaks suggests high purity and minimal structural disruption. In comparison, the lower spectrum also displays the same PP peaks but with slightly sharper intensities and a marginally broader baseline, which may suggest subtle differences in molecular orientation, degree of crystallinity, or presence of minor additives or processing effects. Overall, both samples confirm the presence of crystalline PP, with the second spectrum potentially representing a purer or less processed form [1]. The overlapping nature of the peaks may also indicate minor polymorphic transitions or a heterogeneous polymer blend, as similarly reported in studies investigating nucleating agents and crystallization kinetics in polypropylene systems [2,3]. These findings are critical when evaluating the structural integrity of recycled polypropylene used in composite or construction applications, where crystallinity significantly influences material strength and durability. Overall, the XRD analysis provides robust evidence of the semicrystalline nature of both HDPE and PP within the respective samples. It also highlights the subtle structural variations that can arise due to differences in processing, recycling stages, or additives. Such crystallographic evaluations are vital for assessing the suitability of these materials in advanced applications such as polymeric reinforcement in construction or infrastructure development, particularly when derived from post-consumer waste streams [4]. The mechanical property comparison in Figure 6 strongly aligns with the structural observations from SEM and XRD analyses. Ribbed PP rebars consistently exhibit the highest values in maximum load, stress, and energy absorption, especially evident in the 25 mm samples, correlating with their higher crystallinity observed in XRD and brittle fracture morphology in SEM. Conversely, HDPE samples, particularly the plain ones, show moderate strength but superior elongation and toughness indices, which is supported by ductile fracture patterns and moderate crystallinity, confirming their suitability for ductility-driven, light-load applications.

4. Proposed Practical Utilization of Developed Rebars for Structural Elements Under Light Loads

This study presents a novel approach to advancing sustainable development in construction by repurposing post-consumer plastic waste, specifically high-density polyethylene (HDPE) and polypropylene (PP), into low-strength recycled plastic rebars (RPRs) intended for elements subjected to light loads. Utilizing mechanical extrusion, plain and ribbed rebars were fabricated in diameters of 12 mm, 19 mm, and 25 mm, specifically designed for mortar-free construction systems that eliminate the need for cementitious bonding agents. Material characterization through XRD and SEM confirmed the structural integrity of the recycled polymers, revealing ductile fracture modes in HDPE and comparatively brittle behavior in PP. Addressing critical gaps in Pakistan’s recycling infrastructure, this research offers a scalable, high-value application for plastic waste by converting it into durable, load-bearing structural elements. Practical application areas for these mortar-free systems include non-load-bearing partition walls, boundary walls, pedestrian pathways, lightweight modular shelters, and prefabricated furniture, where moderate strength and high ductility are essential [31]. While PE and PP are prone to degradation through UV exposure and environmental aging, our previous study [2] demonstrated that such degraded plastics can still be mechanically recycled for structural use. Building on that work, the current study focuses on transforming unsorted HDPE and PP waste into full-section rebars, showing that even moderately degraded plastics can be repurposed for light-load construction, supporting circular economy goals.

The introduction of ribbed profiles significantly enhanced stress distribution and mechanical interlocking, critical for ensuring stability in mortar-free construction. Ribbed PP rebars demonstrated superior mechanical properties, achieving higher tensile strengths, energy absorption, and toughness indices across all tested diameters. Empirical modeling using second-degree polynomial fitting successfully captured the relationship between bar diameter and mechanical behavior, confirming their suitability for scalable applications. These mechanical advantages make recycled plastic rebars particularly attractive for dry-joint modular systems, seismic-resilient structures, prefabricated wall panels, modular floor systems, and roof trusses, especially in construction methodologies that avoid the use of wet concrete or mortar. Their inherent ductility and capacity for energy dissipation also make them ideal for rapidly deployable infrastructure in disaster-prone or resource-constrained regions, where quick, mortar-free assembly is necessary. While the developed recycled plastic rebars exhibit promising mechanical and structural properties, their application is presently limited to light-load scenarios. These include non-critical infrastructure such as footpaths, modular shelters, fencing, and boundary walls. The rebars are not yet suitable for load-bearing structural applications, where high compressive and flexural demands exist. Moreover, several challenges must be addressed for large-scale adoption. These include thermal deformation under elevated temperatures, long-term creep behavior, bonding performance with concrete in hybrid applications, and the lack of recognized design codes for plastic-based reinforcement. Commercialization also requires scalable production processes, material consistency across waste streams, and alignment with national construction standards. Addressing these factors will be essential to transition from pilot-scale research to practical implementation.

From a sustainability perspective, the mechanical extrusion of municipal plastic waste into functional rebars represents a significant advancement toward low-carbon, mortar-free construction practices. Unlike previous studies that confined recycled plastics to secondary roles, this work establishes recycled HDPE and PP as primary structural reinforcements capable of supporting modular, dry-assembly construction techniques. The energy-efficient and scalable extrusion process minimizes environmental impacts associated with both plastic disposal and cement production, addressing two major sources of carbon emissions simultaneously. The successful development and validation of RPRs across multiple diameters and surface configurations not only promote material-efficient strategies but also set a new benchmark for integrating recycled materials into structural applications. By enabling durable, ductile, and eco-friendly mortar-free construction systems, this research supports the broader vision of sustainable urbanization, circular economy adoption, and resilient infrastructure development [2].

5. Conclusions

This study introduces a novel method for the use of municipal plastic-waste HDPE and waste PP as standalone structural rebars for mortar-free construction. Unlike previous works limited to fillers, this research develops and tests full-scale recycled plastic rebars. A total of 48 plain and ribbed samples in three diameters were evaluated using ASTM A615 guidelines. XRD and SEM analyses confirmed the polymers’ chemical integrity, crystallinity, and fracture modes. The findings establish a sustainable, low-impact alternative to other rebars in the industry, advancing circular economy goals in the construction sector. The following conclusions are drawn from this study:

• Ribbed polypropylene (PP) rebars displayed the highest mechanical performance among all samples, achieving a maximum load of 12.2 ± 0.6 kN and a toughness index of 19.3 ± 1.0. The inclusion of ribs enhanced the stress distribution, delaying failure and improving ductility, making ribbed PP the most effective option for light-load reinforcement.
• SEM analysis revealed brittle fracture patterns in both polymers. HDPE exhibited irregular fracture surfaces with signs of crack branching and delamination, while PP showed smoother, trans-granular fractures, indicating rapid failure under localized stress.
• XRD analysis verified the semicrystalline structure of both materials. HDPE showed clear peaks at 2θ ≈ 21.6° and 23.9°, and PP exhibited distinct peaks near 14.1°, 16.9°, and 18.6°, reflecting structural differences influenced by polymer type and processing conditions.
• Regression modeling showed positive load trends for ribbed PP and HDPE rebars across increasing diameters, while plain HDPE showed reduced performance at larger sizes. These trends highlight the effect of geometry and material on tensile behavior.
• This study successfully demonstrated that recycled HDPE and PP can be processed into structural rebars suitable for non-critical, mortar-free construction systems. Their application in boundary walls, partition panels, and modular systems offers a sustainable alternative to conventional materials, promoting waste valorization and circular economy practices.

Although the above experiments confirm the feasibility of using recycled plastics in construction, challenges such as material contamination and quality inconsistency must be addressed. Advanced sorting and purification techniques are essential to ensure uniformity in recycled inputs. Future research should explore a broader range of modifications like polymer mixes, rib arrangements and construction application items, including interlocking blocks, corrugated sheets, and beams for mortar-free systems. Evaluating their static, dynamic, and thermal performance will be key to validating their structural reliability. This innovative method contributes to circular economic objectives by reducing plastic waste and offering an alternative to traditional materials like steel. The research also opens prospects for future work, including the optimization of composite formulations, durability testing under environmental exposures, and scaling up production for full-scale structural trials.