Introduction

Concrete is widely regarded as the most prevalent construction material, favored for its strength, long-lasting nature, and adaptability, making it ideal for a variety of building projects across different load-bearing and environmental conditions. The extensive consumption of natural resources for concrete production leads to resource depletion, while the demolition of concrete structures adds to the growing issue of construction waste, intensifying the global waste management crisis. It is projected that Construction and Demolition Waste (CDW) could accumulate up to 27 billion tonnes by 20501,2. Incorporating CDW in concrete conserves natural resources and reduces landfill waste, promoting a more sustainable construction industry. Various research3,4 have incorporated CDW with supplementary cementitious materials such as fly ash, silica fume, and slag, combining them with fibers like basalt, polypropylene, and glass fiber, resulting in a notable increase in various strength parameters. However, despite these improvements, the use of CDW in concrete can also have adverse effects on performance. The presence of adhered mortar in recycled concrete aggregates often leads to increased water absorption, reduced density, and weaker bond strength, which can negatively impact the overall strength and durability of the concrete. Additional research and reviews5,6,7 have focused on enhancing the properties of adhered mortar in recycled concrete aggregates through various treatments and techniques, which have been found to be effective. Unfortunately, concrete is a brittle material and weak in tension. To overcome this plastic waste can be used that possesses high tensile strength and durability8. The majority of plastic does not break down naturally, posing significant environmental hazards. According to recent data, global plastic production has rushed to 368 million tonnes in 2019, and an estimated 8 million tonnes of plastic waste enter the oceans each year, and by 2050, the total accumulated plastic waste is projected to reach 12 billion tonnes9. As the cost of recycling plastics exceeds that of producing new, virgin plastics, around 93% of all plastic products end up in landfills10,11,12,13. The uncontrolled disposal and open burning of plastic waste led to significant environmental issues, such as the release of harmful air pollutants like dioxins, furans, and particulate matter. Additionally, the degradation of plastics increases microplastic and heavy metal levels in aquatic ecosystems, while also reducing water permeability and soil fertility in agricultural fields. This landfilled plastic can be recycled into fibers and used in concrete, enhancing its tensile properties while providing an efficient and sustainable solution for recycling plastic waste. Various studies14,15have focused on utilizing plastic waste as an alternative for aggregates, which improves aspects like abrasion and slip resistance by up to 20% and decreases porosity by less than 5%. In concrete, the inclusion of polystyrene waste, vehicle tires, and high-density polyethylene (HDPE) has been analyzed indicating a decrease in compressive strength, density, porosity, and water absorption. Some studies16 observed enhancement in mechanical properties like compressive, tensile, and flexural strength, while others reported a decline in mechanical properties with increased post-consumer plastic waste as coarse aggregates.

Previous research17,18 and reviews19 has focused on using plastic waste as a substitute for aggregates in concrete or as a binder in flexible pavements, employing materials such as polyethylene terephthalate (PET), electronic waste (E-waste), and granular plastic. The environmental consequences of plastic pollution such as land and water contamination, threats to wildlife, and the emission of harmful chemicals underscore the urgent need for creative solutions to integrate plastic waste into construction materials. Some researcher4,20 used plastic waste and disposal mask and found an consecutive increase in split tensile and flexural strength up to 26% and 60%.

This investigation aimed to enhance the mechanical and durability characteristics of M40 grade concrete by fully replacing the coarse aggregate with treated Construction and Demolition Waste (CDW) and incorporating plastic waste fibers from polyethylene bags and PET bottles. The plastic fibers were manually shredded and evaluated to ensure the appropriate aspect ratio, and were added in varying proportions, ranging from 0 to 1% by weight of cement, in increments of 0.25%. Microstructural properties were analyzed through SEM/EDS, and the concrete’s durability was tested under thermal and acid exposure. Sustainability assessments evaluated the economic and environmental benefits of using CDW and plastic waste fibers, providing important insights into sustainable construction practices. The novelty of this research lies in its innovative approach of manually recycling plastic waste, which is not only more environmentally friendly but also more efficient compared to traditional recycling methods that often produce harmful by-products21. Moreover, the use of treated CDW as a full replacement for coarse aggregates significantly enhances sustainability by reducing the carbon footprint22, This unique combination of recycled plastic fibers and treated CDW offers substantial improvements in both concrete performance and environmental impact, distinguishing it from conventional recycling and construction methods.

Materials and methods

Materials

Recycled Construction and Demolition Waste (CDW) was sourced from the solid waste recycling plant in Perungudi, Chennai, Tamil Nadu, India. They are treated with adsorbent Polyethylenimine (PEI) and mechanical properties were obtained from previous research done by the current author23. Plastic fibers were fabricated from discarded single use polyethylene (PE) bags and PET bottles. The plastic fibers were manually shredded to a size of 1 mm x 10 mm, consisting of PE bags and single use PET bottles (Fig. 1). They were added to concrete as an additive at various percentage of 0%, 0.25%, 0.5%, 0.75%, and 1% by weight of concrete to form Plastic-Fiber Reinforced Concrete (PFRC)0.53-grade Portland pozzolana cement conforming to IS 12269:1987 with a specific gravity of 3.15 was used. Manufactured sand as fine aggregate and crushed stone of 10 mm as coarse aggregates of size 10 mm were used, respectively, for the conventional mix. The physical properties of aggregate are compared with the treated construction demolition waste (CDW) (Table 1) at various parameters and were tested in accordance with IS 2386:1963. M40 grade concrete with a mix ratio of 1:1.93:2.24, conforming to IS 10262:2019, and a water-cement (w/c) ratio restricted to 0.36 from previous research24, was used. Various mix proportions are tabulated in Table 2.

Table 1 Physical properties of aggregates.
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Table 2 Mix proportions.
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Fig. 1
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(a) Construction and Demolition waste, (b) Polyethylenimine (PEI).

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Methods

Fabrication of plastic fibers, preparation of CDW, and test procedures

Figure 2a depict the fabrication of plastic fiber from plastic pollution, The plastic pollution were sourced from a plastic waste dump and segregated into two categories: polythene plastic bags and PET plastic bottles. They were thoroughly cleaned to remove any organic debris and then processed to sizes ranging from 1 mm x 10 mm to ensure feasibility to utilize as additive in concrete. The fiber size was controlled to maintain a consistent aspect ratio of 10, as outlined by25. The tensile strength of the fabricated fibers is 540 MPa, with an elongation range of 7–10%. Recycled construction and demolition waste were pre-treated and preconditioned with polyethylenimine (PEI) to enhance their pore structure and surface texture, according to the study23. This treated CDW was then added to concrete. Plastic fibers were included as described in the “Materials” section. The mix proportions outlined in Table 2 were evaluated after a 28-day curing period to assess compressive strength Fig. 2b, split tensile strength Fig. 2c, and flexural strength Fig. 2d, in accordance with the guidelines set by IS 516:1959 and IS 5816:1999. Durability tests included acid attack and exposure to high temperatures. For the acid attack test, specimens were exposed to 10% concentrated sulfuric acid over three months. The acid solution was replaced with a freshly prepared solution at a constant interval of fifteen days. For the high-temperature exposure test, cube specimens were tested in an oven at a constant temperature of 350ºC for six hours. The weight of the specimens was checked every two hours by taking them out of the oven, and then they were returned to the oven to continue the thermal exposure.

The characteristics of the plastic fibers, both prior to and following exposure, were analyzed using Scanning Electron Microscopy (SEM), while their elemental composition was assessed through Energy Dispersive Spectroscopy (EDS). The SEM analysis was conducted at an acceleration voltage of 20 kV with a sample distance of 10 μm.

Fig. 2
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(a) Fabrication of plastic fiber, (b) Test setup of compressive strength, (c) Split tensile strength, (d) Flexural strength.

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Sustainability study (economic analysis and environmental impact)

The process of recycling coarse aggregate from construction and demolition waste involves several stages: acquiring the waste material, transporting it, sorting, crushing, cleaning, and air drying. The key expenses tied to this process include purchasing the waste, transportation, labor, fuel, water, energy, along with indirect fixed and operational costs. A similar recycling approach is applied to plastic waste, following comparable steps in its production. Energy is consumed at each phase of manufacturing construction materials, starting from raw material extraction or acquisition, followed by transportation to manufacturing facilities, and finally transforming them into finished products. This study evaluated the carbon content in blocks made from recycled plastic waste. Consequently, the energy used throughout these stages becomes “embodied” or “trapped” in the end product.

Results and discussion

Compressive strength

Figure 3 indicates that mix M7 exhibits compressive strength results comparable to those of control concrete. At 28 days, the compressive strength of M7, with 0.25% and 0.5% dosages of plastic fibers and 100% treated construction and demolition waste (CDW), was only 7% lower than the control specimen. Notably, all mixes with 0.25% PET showed results similar to the control specimen, with the maximum strength reduction observed at 23% in mix M10, which contained 0.25% PET and 1% polythene. Yu et al.26 demonstrated that incorporating hybrid plastic fiber waste and fly ash can significantly mitigate cracking in ultra-high-performance concrete specimens, resulting in notable enhancements in both compressive and flexural strengths. However, as the fiber content increased, a decline in compressive strength was observed27. This decrease in strength is primarily by the failure of bond behaviour between the plastic fibers and the concrete matrix28,29. When the quantity of plastic fibers increases, the likelihood of slippage at the fiber-matrix interface rises, resulting in load transfer through the fiber surfaces rather than through the bond, thereby reducing the overall bond strength. At the optimal fiber content, the compressive strength was increased by 11% compared to M2 mix, which consists of 100% treated CDW. This enhancement is attributed to the pretreatment and preconditioning of CDW with polyethyleneimine (PEI), which minimizes the pore structure of the CDW23. The reduced porosity strengthens the CDW, contributing to the overall improvement in compressive strength. A key factor affecting the mechanical properties is the weakened surface bond between the plastic fibers and the concrete matrix29. Unlike conventional reinforcement materials, plastic fibers do not adhere as strongly, and higher concentrations of these fibers can compromise the integrity of the concrete matrix30. This reduces the efficiency of the load transfer mechanism, thereby lowering the material’s resistance to compressive forces. The findings relay with Alphadavi et al.8, reported that a reduction in strength by up to 50% with increased plastic fiber content in concrete. Therefore, optimizing the fiber content is crucial to enhance the balance between the advantages of fiber addition and the disadvantages related with bond strength deterioration.

Fig. 3
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Compressive strength of plastic fiber concrete.

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Split tensile and flexural strength

Figure 4 illustrates the split tensile and flexural strengths of all the mixes. The optimum mix, M7, exhibited the highest split tensile and flexural strengths, measuring 5.27 MPa and 6.71 MPa, respectively. the strengths observed are 11.7% and 18.2% higher than the control mix, which recorded strengths of 4.73 MPa and 5.7 MPa. The notable improvement in split tensile and flexural strengths for mix M7 can be attributed to the ductile properties of the plastic fibers27. Incorporating plastic fibers at the optimal percentage enhances the ductility of concrete, enabling it to better absorb and redistribute stress. This improvement is consistent with Guler’s17 research, which noted an increase of 21.5% in the tensile strength. And Ganji et al.31 reported a 20% gain in tensile strength while using fibers from waste tire cords at a 0.45% fiber content. The enhanced ductility is particularly advantageous in areas prone to cracking, as the fibers help alleviate stress concentrations and slow down crack development32. Consequently, the concrete can support greater loads and perform better under tensile and flexural stresses. Additionally, the plastic fibers function as a bridging mechanism across cracks, providing extra resistance against crack widening and propagation33. This bridging action boosts the concrete’s overall toughness, making it more durable under mechanical stress. Reduced crack propagation not only strengthens the material but also improves its durability and lifespan34. Pretreating CDW with polyethyleneimine (PEI) fills the pores in the CDW, decreasing its porosity and enhancing its strength. This pore-filling effect results in a denser and more cohesive concrete matrix35. Libre et al.36 demonstrated increases in flexural strength of 23.8–34% with the addition of 0.2% and 0.4% polypropylene (PP) synthetic fibers.

The addition of plastic fibers in mix M7, combined with the use of treated CDW, significantly enhances its split tensile and flexural strengths by improving ductility, reducing stress concentrations in the cracking zone, and mitigating crack propagation. Together, these factors lead to the enhanced mechanical performance of the optimized mix in comparison to the control mix.

Fig. 4
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Split tensile and flexural strength of plastic fiber concrete.

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Durability test

Thermal and acid resistance

Upon visual inspection, as shown in Fig. 5, both the control mix (M1) and PFRC specimens exhibited slight pale discoloration with no visible damage when exposed to a constant elevated temperature of 350 °C. This indicates good thermal stability for both specimen types at this temperature. The slight thermal deformation observed, manifesting as minor surface irregularities37. can be attributed to expansion and contraction within the matrix due to heat exposure. The variation in the color of the concrete, with some areas showing a more pronounced pale hue, is likely due to the heat-induced dehydration of the cement paste38, which causes chemical changes such as the breakdown of calcium hydroxide and water loss from the concrete matrix. Physical and visual inspection, as seen in Fig. 6, revealed significant erosion in both the control and optimum mix (M7) cubes when subjected to curing under sulfuric acid. The degree of erosion increased with the duration of acid exposure39. as sulfuric acid chemically reacts with the calcium hydroxide in the cement paste, forming calcium sulfate, which weakens the matrix. However, the M7 cubes exhibited comparatively less damage than the control cubes. This reduced damage can be attributed to the plastic fibers’ resistance to sulfuric acid, which provided additional protection to the concrete matrix by reducing acid penetration and slowing down the degradation process40. The plastic fibers’ chemical stability and non-reactive nature in acidic environments help maintain the concrete’s integrity. By bridging micro-cracks and reducing permeability, these fibers limit acid ingress, enhancing the concrete’s durability in acidic conditions. This highlights the beneficial impact of plastic fibers in refining the chemical resistance of concrete.

Fig. 5
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Thermal exposure of the specimens.

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Fig. 6
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Acid exposure of specimens.

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Variation in mass

Table 3 shows that the percentage of weight loss in the optimum mix M7 specimens was significantly higher, with a maximum reduction of 5.48%, compared to the control specimens, which only experienced a 3.5% mass loss when subjected to high temperatures. This increased weight reduction in the M7 specimens is due to the behavior of plastic fibers under elevated temperatures. As these fibers melt and degrade, they contribute to the overall mass loss. The melting of the plastic fibers, which lack resistance to high temperatures, results in the formation of voids within the concrete matrix41, thereby intensifying the weight reduction. This highlights a trade-off in the use of plastic fibers: while they enhance certain properties like chemical resistance, their low thermal resistance can result in significant mass loss and potential structural weakening when exposed to high temperatures. These findings align with Shaik’s42 conclusion that the thermal conductivity of concrete with plastic as aggregate is lower associated to conventional concrete due to the poor thermal conductivity and low resistance to thermal exposure of the plastic materials.

Table 4 indicates that the percentage of weight loss of the control specimens M1 was found to be more than that of the M7 specimens. The M7 specimens showed only 6.1% to a maximum of 13.67% weight loss from 15 days to 90 days, which is lower by 47.5% at 15 days and 87.3% at 90 days than the control mix M1, which experienced 9–25.6% weight loss respectively over the same duration of exposure to acids. This reduced weight loss in M7 specimens can be attributed to the resistance offered by plastic fibers to acid attack39. The plastic fibers are more resistant to acid degradation, thus improving the durability of the concrete matrix by reducing acid penetration and slowing down the erosion process. According to Sau et al.40, plastic exhibits significantly greater resistance to acid than traditional concrete, leading to minimal mass loss compared to conventional concrete. This enhanced acid resistance of plastic fibers results in a notable decrease in mass loss for the M7 mix than the control specimens, underscoring the benefits of using plastic fibers in concrete that is exposed to acidic conditions.

Table 3 Variation in weight after thermal exposure.
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Table 4 Variation in weight after acid exposure.
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Variation in compressive strength

Table 5 shows that, similar to the observed mass loss, the compressive strength of the M7 specimens decreased by 7% after being subjected to constant high temperatures, compared to the control mix M1. The reduction in compressive strength is linked to the degradation of plastic fibers at high temperatures40, which causes them to melt and degrade, forming voids and compromising the concrete’s overall strength43. However, the treated construction and demolition waste (CDW) in the M7 mix mitigates this strength reduction due to its treatment with polyethyleneimine (PEI)23,44. The PEI treatment improves the pore structure and enhances the bond strength between the CDW and the cement matrix, helping to counteract some of the negative effects of plastic fiber loss. This treatment enhances the concrete’s durability and structural integrity, partially offsetting the strength loss caused by the thermal degradation of plastic fibers.

Table 6 shows that the control specimens experienced a 7% greater strength loss compared to the M7 specimens when exposed to acids. The plastic fibers in the M7 mix offer better resistance to acid, leading to reduced strength loss45. Furthermore, the treated construction and demolition waste (CDW) in the M7 mix, which has been treated with polyethyleneimine (PEI), enhances the concrete’s acid resistance. The PEI treatment improves the chemical stability and bonding of the CDW, which in turn boosts the durability of the M7 specimens. From a sustainability perspective, the use of CDW and plastic fibers not only enhances performance but also addresses environmental issues46. Incorporating CDW helps reduce landfill waste and supports the recycling of construction materials, while using plastic fibers aids in reducing plastic pollution27. Thus, the combination of acid-resistant plastic fibers and PEI-treated CDW significantly lowers strength loss in the M7 specimens compared to the controls in acidic conditions, demonstrating a sustainable method for improving concrete durability.

Table 5 Variation in strength after thermal exposure.
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Table 6 Variation in strength after acid treatment.
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Micro-structural study (SEM/EDS)

The SEM images shown in Fig. 7a,b reveal distinct changes in the morphology of PE and PET fibers before and after exposure to sulfuric acid. The PE fibers, illustrated in Fig. 7c, display thread-like laminar formations and microcracks with minor degradation. In contrast, the PET fibers, depicted in Fig. 7d, exhibit a knot-like texture with microcracks and slight brittle fractures47. These differences underscore the varying effects of sulfuric acid on the two fiber types. EDS analysis, detailed in Tables 7 and 8, shows no significant changes in the elemental composition of PE and PET fibers before and after acid exposure18. This indicates that the fibers are not significantly damaged by the acid, maintaining their strength and exhibiting minimal weight loss48. The bond strength between the plastic fibers and the concrete matrix is crucial for the observed deformations on the plastic fiber surfaces17. This bond can create stress concentrations at the fiber-matrix interface, leading to microcracks and surface deformations. Furthermore, the treated demolition waste (CDW) with its enhanced surface also plays a significant role in strengthening this bond23. The pretreatment with PEI improves the surface roughness and chemical reactivity of the CDW particles (Fig. 7e), which contributes to a stronger bond with the concrete matrix. The improved surface of the treated CDW provides additional mechanical interlocking and chemical bonding sites, enhancing the adhesion between the plastic fibers and the concrete matrix35. This results in a more robust and durable composite material. The interaction between the fibers and the cement matrix ensures effective load transfer, supporting the fibers’ resilience and minimal damage even in acidic conditions. These observations align with Basha et al.42, who found that the bond between the concrete matrix and flake-type RPA is superior to that of other plastics. This strong fiber-matrix bond, strengthened by the treated CDW, is essential for consistent structural integrity and improving the overall performance of the composite material. Additionally, incorporating treated CDW not only enhances bonding but also supports sustainable construction practices through the recycling of waste materials.

Fig. 7
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(a) PE fiber before acid exposure, (b) PET fiber before acid exposure, (c) PE fiber after exposure, (d) PET fiber after exposure, (e) CDW in plastic fiber concrete.

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Table 7 Elemental composition of PE before and after exposure to acid.
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Table 8 Elemental composition of PET before and after exposure to acid.
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Sustainability study (economic analysis and environmental impact)

Economic analysis

An economic comparison between traditional coarse aggregates and treated Construction and Demolition Waste (CDW) highlights several advantages of using treated CDW. Traditional coarse aggregates, obtained from natural sources such as quarries, involve higher and more predictable costs due to extraction, processing, and transportation. These activities also contribute to considerable environmental consequences, including habitat annihilation and greenhouse gas emissions, which can lead to additional regulatory expenses. In contrast, treated CDW, which comes from processed demolition waste, generally has lower raw material costs. Although there are expenses associated with the collection, sorting, and treatment of CDW, these are often outweighed by savings from reduced landfill disposal and lower transportation costs, since CDW is typically sourced locally. Moreover, using treated CDW lessens environmental impact by decreasing the need for new aggregate extraction and reducing landfill waste, thus supporting sustainable construction practices. Quality control is essential to ensure that treated CDW meets industry standards, but research shows that its performance can be on par with traditional aggregates. Overall, treated CDW presents a cost-effective and eco-friendly alternative, offering substantial savings in raw material, transportation, and lifecycle costs while fostering long-term sustainability in construction.

Figure 8 illustrates a cost comparison for the production of natural aggregate (NA), Construction and Demolition Waste (CDW), and treated CDW, including transportation, handling, and processing costs as outlined in the schedule of rates49. The costs for CDW also include treatment and labor. The figure shows that in 2017, the cost of CDW was approximately 4.6 times lower than that of NA, and it is now about 4 times lower. In 2024, treated CDW remains 1.2 times more affordable than NA50. This demonstrates that the utilization of treated CDW not only reduces costs but also promotes sustainable construction practices. Using treated CDW in concrete contributes to sustainability by minimalizing the requirement for virgin aggregate extraction, protecting natural resources, and minimizing landfill waste. Furthermore, it lowers the carbon footprint associated with aggregate production and transportation, thereby enhancing the environmental performance of concrete.

Fig. 8
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Economic comparison of natural aggregate and CDW.

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Environmental impact

The energy required for processing and embodied carbon in materials used for plastic fiber and Construction and Demolition Waste (CDW) are illustrated in Fig. 9. Energy consumption is measured in MJ per unit, considering transportation energy for cement within a 120 km radius and aggregates transported over distances ranging from 25 to 45 km. Energy usage increases from 2040.532 MJ/m2 to a peak of 4579.27 MJ/m2 from M1 to M18, primarily due to the incorporation of plastic fiber additives. The energy consumption for utilizing plastic fiber could significantly reduce the energy requirement compared to landfill disposal51. Significant energy savings are achieved when Natural Aggregates (NA) are substituted by CDW. Optimal replacements result in 1.3 times less energy at optimum mix M7 compared to control specimens, highlighting CDW’s lower energy demand. Similarly, carbon emissions from transportation, measured as CO2 emissions amount to 0.077 kg of CO2 per MJ/unit of energy consumed52. This reduction trend is similar in embodied carbon, with reductions of 1.2 times the conventional concrete M1. Managing energy consumption and embodied carbon as part of pollution control requires effective systems involving substantial manpower and machinery. Lawler53 outlines steps to develop frameworks for managing the disposal of CDW underlining their current disposal challenges in landfills.

Fig. 9
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Embodied carbon and energy of plastic fiber concrete.

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Conclusion

Incorporating non-decomposable plastics as fibers in concrete mixtures, along with treated Construction and Demolition Waste (CDW) as aggregate, provides an effective solution for waste disposal while enhancing the mechanical properties of concrete. The study tested M40 grade concrete containing manually shredded plastic fibers from polyethylene bags and PET bottles in various proportions.

After 28 days, the mixes demonstrated comparable compressive strength to conventional concrete, with notable improvements of 11.41% in flexural strength and 17.72% in tensile strength at optimal fiber dosages of 0.25% and 0.5% by weight of binder, and with 100% replacement of natural aggregate with treated CDW.

Under elevated temperature exposure, both control (M1) and PFRC (M7) specimens exhibited minimal visual changes, though the plastic fibers melted and showed limited resistance to high temperatures. In acid exposure tests, PFRC demonstrated superior resistance, resulting in lower strength and weight loss compared to the control mix. SEM and EDS analyses confirmed minimal damage to the fibers from acid exposure, highlighting their effectiveness in maintaining strength and minimizing weight loss.

From a sustainability perspective, the integration of plastic waste fibers and treated CDW in concrete results in 1.3 times lower energy consumption and 1.2 times lower embodied carbon emissions compared to the control mix. This underscores the environmental benefits of these materials, promoting sustainable construction practices through reduced resource consumption and a lower environmental impact of concrete structures.