Analysis of concrete properties with partial sand replacement by recycled plastic and its cost–benefit evaluation

The rising demand for sustainable and environmentally friendly construction materials has encouraged the exploration of alternatives to natural sand in concrete production. This study evaluates the feasibility of partially replacing fine aggregates with recycled polyvinyl chloride (PVC) and polyethylene terephthalate (PET) plastics in concrete. Concrete mixes of grades M15, M20, and M25 were prepared with plastic aggregate replacement levels of 0%, 5%, 10%, 15%, and 20%. The PET and PVC waste materials were melted, crushed, and incorporated into concrete mixtures, and their effects on workability and compressive strength were systematically examined. The results showed that in M15 concrete, workability remained adequate at 5% and 15% PET replacement; however, its compressive strength gradually decreased with increasing plastic content, showing a maximum reduction of 46.51% at 20% replacement. In contrast, both M20 and M25 mixes maintained acceptable workability across all replacement levels, and their compressive strengths consistently surpassed the standard minimum requirements of 20 N/mm² and 25 N/mm², respectively. Comparisons between PET and PVC replacements revealed slight reductions in workability (approximately 5%–8%) and compressive strength (approximately 5%–15%) as the substitution level increased. Although several studies have explored alternatives to natural sand, limited research has systematically examined the combined effects of PET and PVC plastics on the workability and strength of different concrete grades. This study addresses that gap by evaluating the performance of M15, M20, and M25 concretes with varying levels of PET and PVC replacements, identifying the feasible substitution range that maintains acceptable mechanical and workability properties. The findings demonstrate that up to 20% replacement is suitable for M20 and M25 grades without significant loss of strength, thereby contributing to sustainable construction practices through effective plastic waste utilization and conservation of natural resources.

GRAPHICAL ABSTRACT

GRAPHICAL ABSTRACT |

1 Introduction

The utilization of recycled materials in construction has garnered increasing attention in recent years as the world grapples with environmental concerns and seeks sustainable solutions (Zhang et al., 2023; Chen, 2024). One such endeavor is the exploration of recycled plastic as a partial replacement for sand in various construction applications (Ahmad et al., 2022; Zheng et al., 2024). Sand, a vital component in construction, is extensively used in various construction activities, including concrete production, mortar, and plastering (El-Aidy et al., 2024). However, the overexploitation of natural sand reserves has led to environmental degradation, resource depletion, and concerns regarding sustainability. Consequently, there is a pressing need to explore alternative materials that can mitigate the reliance on natural sand while addressing environmental challenges. Recycled plastic is a promising alternative to traditional construction materials (Skariah Thomas et al., 2022). With the burgeoning issue of plastic waste worldwide, recycling initiatives have gained traction as a means to curb pollution and conserve resources (Song et al., 2024). By repurposing plastic waste into construction materials, such as aggregates or fillers, the construction industry can contribute to both waste management efforts and sustainable development goals (Sabbrojjaman et al., 2024). Researchers have conducted few recent studies. Thienel et al. (2020) investigated the mechanical properties of concrete incorporating recycled plastic aggregates, highlighting its potential as a viable alternative to conventional sand. Additionally, Strieder et al. (2022) and El Afandi et al. (2023) explored the influence of recycled plastic on the durability and performance of concrete, emphasizing its role in mitigating environmental concerns associated with sand extraction while improving material properties.

However, challenges such as optimizing the mix proportions, assessing long-term durability, and addressing potential compatibility issues between plastic and cementitious materials remain areas of ongoing research (Daneshvar et al., 2022). Despite these challenges, the collective findings underscore the promising prospects of utilizing recycled plastic as a sustainable alternative to sand in construction applications, highlighting the need for further investigation and standardization to facilitate its widespread adoption (Zikang et al., 2024). Ahmadi et al. (2023) examined the use of plastic in cement mortar and concrete. They examined the impact of using plastic aggregates in concrete and found that the workability of concrete decreases when angular plastic aggregates are used, whereas smooth aggregates improve workability. Although plastic aggregates tend to reduce compressive strength, the loss in tensile and flexural strength is less pronounced. Althoey et al. (2023) investigated the use of non-biodegradable plastics, such as polycarbonate (PC) and polyethylene terephthalate (PET), as partial replacements for natural aggregates in mortar. Their study showed a reduction in compressive strength as the percentage of plastic aggregates increased, with varying volume fractions of sand replaced by plastic (3%, 10%, 20%, and 50%). Goh et al. (2022) explored the incorporation of plastic bags as fibers into concrete. They tested concrete by varying fiber proportions (0.2%, 0.4%, 0.6%, 0.8%, and 1% by weight) and found that compressive strength decreased with higher plastic content, although tensile strength improved, peaking at 0.8% addition. Similarly, Mohe et al. (2022) performed experiments on plastic aggregates in concrete. They replaced coarse aggregates in concrete with plastic at levels of 25%, 50%, 75%, and 100%. They observed a reduction in both strength and density of the concrete, concluding that replacing more than 36% of the aggregates with plastic is unsuitable for structural applications. They also suggested using plastic to create lightweight concrete.

The reviewed literature indicates that plastic waste has been widely explored in construction materials, particularly as a replacement for fine aggregates, fibers, and even coarse aggregates. However, there remains a clear research gap in the use of recycled plastic, specifically as a fine aggregate replacement, as most studies have not fully examined its potential in this role. Therefore, this study aims to investigate the viability of utilizing recycled plastic as a partial replacement for sand in construction applications. By assessing factors such as mechanical properties, durability, and environmental impact, this research seeks to elucidate the feasibility and effectiveness of incorporating recycled plastic into construction materials. The study will also provide insights into the challenges, opportunities, and potential implications associated with adopting recycled plastic in construction practices. Unlike many studies that focus on a single concrete grade, this study systematically compares low-, medium-, and high-strength concrete to assess how recycled aggregate affects performance.

2 Methodology

Materials used and testing methods, including the instruments used and analysis procedures, are discussed in this section, and a flow diagram of the study is provided.

2.1 Materials used

In this study, polyethylene terephthalate (PET) and polyvinyl chloride (PVC) plastics were investigated as potential substitutes for sand at varying percentages: 0%, 5%, 10%, 15%, and 20%. PET, commonly found in beverage bottles and food packaging, possesses desirable properties such as durability, lightweight, and resistance to moisture, making it a potential candidate for sand replacement in construction materials like concrete and asphalt (Pan et al., 2023). Similarly, PVC, widely used in pipes, flooring, and window frames, exhibits attributes like versatility, chemical resistance, and ease of processing, rendering it suitable for applications where sand traditionally serves as a filler or aggregate. Thus, PET and PVC plastics were melted, crushed, and subsequently employed as a replacement for fine aggregate in concrete for the study. Different grades of concrete were prepared, that is, M15, M20, and M25, with mix ratios of cement:fine aggregate:coarse aggregate of 1:2:4, 1:1.5:3, and 1:1:2, respectively. The mix proportions used for different grades of concrete are shown in Table 1. The table summarizes the material requirements for producing 1 m³ of concrete for the M15, M20, and M25 grades. As the grade of concrete increases, the cement quantity and strength increase, while the water/cement ratio decreases to achieve higher strength. Sand and aggregate quantities vary according to the mix ratios, ensuring proper workability and desired compressive strength. The fine aggregates were replaced with recycled PET and PVC at varying percentages from 0% to 20%. Both PET and PVC raise concerns regarding their environmental impact, including issues related to recycling, pollution, and end-of-life disposal.

Table 1

Table 1. Mix proportions used for different concrete grades for 1 m³ of concrete.

2.2 Testing methods

Two tests were conducted: a workability test on the concrete using the slump cone method and a compressive strength test using the universal testing machine (UTM). Other major instruments, including an oven, a curing tank, and a casting mold, were used to analyze the properties of concrete.

2.2.1 Workability test

Concrete slump or slump cone tests were used to determine the workability or consistency of the concrete mix prepared during the progress of the work. This test was carried out in agreement with the guidelines of Indian Standard (IS) 1199 (1959): Methods of sampling and analysis of concrete (Garcia-Troncoso et al., 2022). The apparatus used in the slump test was a mold in the form of the frustum of a cone having a height of 30 cm, a bottom diameter of 20 cm, and a top diameter of 10 cm. The tamping rod is a 16-mm-diameter, 60-cm-long steel rod that is rounded at one end. The concrete mixes with suitable water/cement ratios were prepared in the laboratory, as were the nine cubes cast after conducting the slump test. The testing procedure mentioned in Section 5.1 (Tests for Workability-Slump Test) of the above-mentioned IS code was followed. The slump was determined by measuring the difference in height between the cone and the highest point of the slumped specimen in millimeters (mm).

2.2.2 Compressive strength test

For the compressive strength test, cube specimens of 15 × 15 × 15 cm³ were prepared and tested in compliance with IS 516 (1959): Method of Tests for Strength of Concrete (de et al., 2023). The procedures for mixing, sampling, and curing were followed as per the code. After mixing, the fresh concrete was placed into cube molds and compacted in three layers, each layer receiving at least 35 strokes using a tamping rod (16 mm diameter, 60 cm long, bullet-shaped end). The specimens were stored in moist air for 24 h and then demolded and submerged in clean water until testing. The compressive strength test was performed using a UTM after 7 days, 14 days, and 28 days of curing, with a minimum of three specimens tested at each age. The load was applied gradually at 140 kg/cm² per minute until failure. The compressive strength was calculated by dividing the failure load by the specimen’s cross-sectional area of (225 cm²), and the average value of the three specimens was taken as the compressive strength (N/mm²).

2.3 Analysis of the study

Figure 1 shows the analysis procedure and pictorial representation phases of the research, starting with the collection of PET and PVC plastic samples. The plastic samples underwent a variety of procedures, including washing and drying, before being melted. After the melting process, the recycled plastic was cooled and then crushed into finer particles. The plastic components were prepared following these procedures to eventually be melted and crushed for use in concrete mixtures. After preparation of different grades of concrete by partial replacement of sand with recycled plastic, the testing was conducted as per Indian Standards. The slump test, a basic process for determining the workability of a concrete mix, was conducted, and nine concrete cubes were cast for each grade. After 1 day of drying, the cube samples were removed from the mold and cured. Curing is an important stage in ensuring that the concrete achieves appropriate qualities. After the requisite number of days of curing, these concrete cubes were tested for compressive strength with a universal testing machine (UTM), which assesses the compressive strength of concrete. This systematic approach emphasizes the entire technique employed in the study to investigate the impacts of adding plastic components to concrete.

Figure 1

Figure 1. (a) Plastic samples, (b) melted PET, (c) melted PVC, (d) crushed PET, (e) crushed PVC, (f) slump test, (g) concrete cubes, (h) curing of concrete cubes, and (i) testing of concrete cubes.

3 Results and discussion

The changes in the properties of concrete (i.e., workability and compressive strength) after replacing sand with recycled plastic (PET & PVC) in various proportions (0%, 5%, 10%, 15%, and 20%) in different grades of concrete (M15, M20, and M25), have been discussed and analyzed in this study.

3.1 Grain size distribution of PET and PVC

The sieve analysis shown in Table 2 revealed that PET and PVC particles are slightly coarser and better graded than natural sand, as reflected by the consistently higher retained percentage in most of the sieve sizes. All three materials show similar behavior at coarser sieves between 4.75 mm and 1.18 mm, but marginally more material is retained by PET and PVC. As the size of the sieves decreases, the difference becomes noticeable, especially at 300 µm and 150 µm, where the retention was significantly higher for PET and PVC than sand, indicating larger fine fractions. Pan residue also showed that small proportions of very fine material are present in PET (2%) and PVC (3%). No very fine material was found in the sand. PET and PVC particles are relatively coarser with more fines than natural sand, which might influence their performance as aggregate replacement in construction materials.

Table 2

Table 2. Grain size distribution of fine aggregates, that is, sand, PET, and PVC.

3.2 Replacement of sand with PET plastic

The sand was replaced with crushed PET plastic in M15, M20, and M25 concrete grades. The results of workability and compressive strength for different percentages of plastic are discussed.

3.2.1 M15 grade concrete with PET replacement

Figure 2 shows the workability and compressive strength of M15 grade concrete with different percentages of PET at a consistent W/C ratio of 0.60. It is observed that the workability generally increases at 5% and 15% PET replacement, indicating improved workability. The 10% and 20% mixes showed comparatively lower slump values, indicating medium workability. This improvement at moderate replacement levels is due to the lightweight and smooth surface of PET, which reduces internal friction. However, at 20% replacement, workability decreases because excess PET leads to reduced cohesiveness and possible segregation. Overall, concrete remains workable up to 15% PET replacement. Therefore, we can say that it is easy to work with concrete after partially replacing sand with recycled PET plastic at 5% and 15%. The graph shows that the compressive strength at all curing ages peaks approximately 5% PET replacement. At 5%, the PET particles likely fill voids effectively without significantly weakening the cement matrix, resulting in better compactness and strength. Beyond 5%, the comprehensive strength begins to decrease gradually. PET has a lower stiffness and poor bonding with cement paste, which reduces strength. At 20% replacement, the strength reduces sharply, confirming that excessive PET content compromises structural performance. The percentage decrease of compressive strength from 0% to 20% replacement is 46.51%. Results show that the compressive strength is M15 (15 N/mm²) after replacing sand with 0%, 5%, 10%, and 15% PET plastic, whereas the strength at 20% replacement is slightly lower. Almutairi et al. found that the strength of concrete decreases across different grades when fine aggregates are replaced with other construction materials (A Research On Green Concrete, 2023). For M15 grade concrete, 5% PET replacement provides the best balance between workability and strength. While PET improves workability at moderate levels, higher replacement percentages reduce compressive strength due to weak bonding and increased voids. Therefore, PET can be used as a partial fine aggregate substitute up to approximately 5% in M15 concrete, especially for non-structural and low-load applications.

Figure 2

Figure 2. Workability and strength comparison graph of M15 grade concrete with different percentages of PET replacement.

3.2.2 M20 grade concrete with PET replacement

Figure 3 illustrates the workability and compressive strength of M20 grade concrete at various percentages of PET plastic replacement at a consistent W/C ratio of 0.60. The concrete becomes less workable with increasing PET content. This reduction in workability is due to the hydrophobic nature and smooth surface of PET, which limits water absorption and weakens the bonding between aggregates and cement paste. At 15% and 20% replacement, the slump increases again slightly, which may be due to excess PET causing reduced packing density, allowing more free water in the mix. The graph shows that at all curing ages (7 days, 14 days, and 28 days), compressive strength decreases gradually as PET replacement increases. This reduction occurs because PET is less rigid than natural sand and has poor bonding characteristics, which introduces weak zones within the concrete matrix and increases voids. Even at 20% PET replacement, the concrete still retains a compressive strength above the minimum requirement for M20 grade (20 N/mm²), indicating that the mix is still structurally acceptable for certain applications. Imtiaz et al. (2021) demonstrated that using geopolymer concrete as a substitute for OPC concrete can lower its global warming potential by up to 53.7%, albeit with a slight reduction in strength. For M20 grade concrete, increasing PET content results in a gradual decrease in comprehensive strength, due to weaker bonding and increased voids. Workability decreases to approximately 10% replacement and then slightly improves at higher replacement percentages because of reduced cohesiveness. PET can be safely used up to approximately 10% replacement without significantly compromising strength or workability, making it a feasible option for sustainable and non-critical concrete applications.

Figure 3

Figure 3. Workability and strength comparison graph of M20 grade concrete at different percentages of PET replacement.

3.2.3 M25 grade concrete with PET replacement

Figure 4 illustrates the workability and compressive strength of M25 grade concrete at various percentages of PET plastic replacement at a consistent W/C ratio of 0.60. The slump shows a moderate variation across the replacement levels. At 5% PET, the slump slightly increases, indicating improved workability due to the smooth and lightweight nature of PET, which reduces internal friction. At 10% and 15% PET, the slump values decrease, suggesting reduced cohesiveness because PET does not bond well with cement paste. At 20% PET, the slump increases again, likely due to excess PET reducing the aggregate interlock, creating a more open mix with more free water movement. The 28-day compressive strength remains relatively stable across the replacement levels, with only slight reductions as the PET percentage increases. Even at 20% PET replacement, the compressive strength remains above the characteristic strength of 25 N/mm², which means the concrete still meets the M25 grade requirements. The reason strength does not decrease sharply is that M25 concrete has a higher cement content, which helps maintain matrix integrity even when part of the fine aggregate is replaced. However, PET still has poor bonding characteristics compared to natural sand, so there is a gradual decline in 14-day and 28-day strengths with increasing PET content. Hong et al. (2023) reviewed the properties of lightweight concrete and concluded that replacing up to 20% of the aggregates results in only a slight change in strength. For M25 grade concrete, PET replacement up to 20% does not compromise the required compressive strength, although slight reductions occur at higher levels due to weaker bonding and increased voids. Workability remains acceptable across all replacement percentages but varies due to changes in cohesion and particle packing. Thus, PET can be effectively used as a partial fine aggregate replacement in M25 concrete, offering suitable utilization without significantly affecting structural performance.

Figure 4

Figure 4. Workability and strength comparison graph of M25 grade concrete at different percentages of PET replacement.

3.3 Replacement of sand with PVC plastic

3.3.1 M15 grade concrete with PVC replacement

Figure 5 illustrates the workability and compressive strength of M15 grade concrete at various percentages of PVC plastic replacement at a consistent W/C ratio of 0.60. It is observed that the slump values show moderate variation across the replacement levels. At 5% PVC replacement, the slump increases slightly, indicating improved workability. This is due to the smooth and lightweight nature of PVC, which reduces friction among aggregate particles. At 10% replacement, the slump decreases, suggesting reduced cohesiveness and lower mix flow. At 15% and 20%, slump values increase again slightly. This may be attributed to excess PVC reducing the packing density, causing more free water movement in the mix. The compressive strength decreases consistently with the increase in PVC replacement across all curing ages. At 0% replacement, strength is highest because natural sand provides strong interlocking and bonding. At 5% replacement, the strength slightly decreases but remains acceptable for the M15 grade. Beyond 10%, compressive strength drops more noticeably, as PVC particles do not bond well with the surrounding cement matrix and introduce voids, weakening the internal structure. At 20% replacement, the strength reduction becomes significant, indicating that such high substitution is not desirable for structural applications. Yasser et al. (2023) observed that partial replacement of aggregates up to 20% in lightweight concrete resulted in only a minor reduction in compressive strength, which is consistent with our results for M15 grade concrete using recycled PVC plastic. For M15 grade concrete, replacing sand with PVC reduces compressive strength gradually as the replacement percentage increases, due to poor bonding and lower stiffness of PVC compared to natural sand. Workability remains within acceptable limits but shows variations based on the amount of PVC used. PVC replacement up to approximately 20% can be used in non-structural applications without major performance compromise, while higher percentages lead to progressively weaker concrete.

Figure 5

Figure 5. Workability and strength comparison graph of M15 grade concrete at different percentages of PVC replacement.

3.3.2 M20 grade concrete with PVC replacement

Figure 6 illustrates the workability and compressive strength of M20 grade concrete at various percentages of PVC plastic replacement at a consistent W/C ratio of 0.60. The slump initially decreases at 5% PVC replacement, indicating a slight reduction in workability. This happens because PVC is smooth and hydrophobic, reducing bonding and limiting water distribution in the mix. From 10% to 20% replacement, the slump gradually increases, showing improved flowability at higher PVC content. This increase is likely due to reduced internal friction and lower packing density caused by excess lightweight PVC, which allows more free water movement. The compressive strength decreases consistently with increasing PVC replacement across all curing periods. At 0% replacement, strength is highest because natural sand provides good packing and strong bonding with the cement paste. At 5% PVC, the strength remains close to that of the control mix, showing that small PVC amounts have minimal adverse effect. Beyond 10%, the strength reduction becomes more noticeable. This occurs because PVC does not bond well with cement, and its lightweight nature introduces voids and weak zones in the concrete. The lowest compressive strength values are observed at 20% PVC, although the 28-day strength remains above the minimum requirement for M20 concrete, making it potentially usable in non-structural applications. For M20 grade concrete, increasing PVC content leads to a gradual reduction in compressive strength due to weaker bonding and increased voids in the concrete matrix. Workability first decreases and then increases with higher PVC content due to changes in mix cohesion and particle packing. PVC can be used up to approximately 5%–10% replacement without significantly compromising performance, providing a sustainable and economical alternative in non-structural moderate strength applications.

Figure 6

Figure 6. Workability and strength comparison graph of M20 grade concrete at different percentages of PVC replacement.

3.3.3 M25 grade concrete with PVC replacement

Figure 7 illustrates the workability and compressive strength of M25 grade concrete at various percentages of PVC plastic replacement at a consistent W/C ratio of 0.60. The slump value is highest at 0% PVC and gradually decreases as the PVC content increases. This reduction indicates lower workability with increasing replacement. The decrease occurs because PVC particles are smooth, rigid, and hydrophobic, which reduces the adhesion of cement paste, making the mix less cohesive. At 20% replacement, the slump is the lowest, showing that excess PVC leads to poor mix consistency and reduced flow. The compressive strength decreases consistently with increasing PVC replacement at all curing ages. The highest strength is recorded at 0% PVC, where natural sand provides good particle interlocking and strong bonding with cement paste. At 5% PVC, a slight reduction in strength is observed, but it remains close to the control mix, meaning that the concrete structure is still acceptable. Beyond 10%, the strength decreases more sharply because the PVC does not chemically bond with the cement paste, which introduces voids and weak zones in the matrix. Concrete made with PVC has lower stiffness than that made with natural sand. At 20% replacement, strength is significantly reduced at all curing ages, making such mixes unsuitable for structural applications. For M25 grade concrete, replacing fine aggregate with PVC results in a gradual decrease in both compressive strength and workability. While the mix remains functional up to approximately 5% replacement, higher levels significantly reduce performance due to poor bonding and increased porosity. Therefore, PVC can be used up to a limited percentage (5%) in M25 concrete, primarily for non-structural or low load-bearing applications, while still contributing to sustainable waste utilization.

Figure 7

Figure 7. Workability and strength comparison graph of M25 grade concrete at different percentages of PVC replacement.

3.4 Comparison of different grades of concrete with PVC replacement

Figure 8 illustrates the relationship between different percentages of PVC replacement in concrete and the resulting compressive strength over 7, 14, and 28 days, as well as the slump values indicating workability. Across all three sets of data, there is a consistent decline in compressive strength as the percentage of PVC replacement increases, with the most significant drop observed in the 7-day strength. However, the 28-day strength is approximately similar (<5%) to the strength of concrete without replacement. Furthermore, the slump values also generally decrease with higher PVC content, indicating reduced workability. However, one result also shows a slight increase in slump at 5% PVC replacement before it starts to decline. Overall, the graph demonstrates that increasing the PVC content in concrete decreases the compressive strength by 5%–15% and workability by 5%–8%, with the effects becoming more pronounced as the replacement percentage rises. Belaïd (2022) found that the compressive strength of concrete decreased by approximately 10%–15% when plastic aggregates were used, aligning closely with the 5%–15% reduction in compressive strength reported in this study, particularly at higher replacement percentages.

Figure 8

Figure 8. Comparative graph of compressive strength and slump values for PVC replacement.

3.5 Comparison of different grades of concrete with PET replacement

Figure 9 shows the compressive strength of concrete at various percentages of PET replacement over different curing periods (7 days, 14 days, and 28 days), along with corresponding slump values. As PET replacement increases, there is a slight decrease in compressive strength across all curing times. For example, at 0% PET replacement, the compressive strength after 28 days reaches 41 N/mm², while at 20% PET replacement, it ranges from 22 N/mm² to 41 N/mm², depending on the mixture. However, the reduction in strength is minimal, suggesting that PET can still be effectively used as a partial replacement in concrete. Additionally, the slump values, which indicate the workability of the concrete mix, generally decrease as the PET content increases, suggesting that higher PET percentages might reduce the fluidity of the mix. The results imply that while PET can be used as a partial replacement in concrete, there is a trade-off between strength and workability, particularly at higher PET replacement levels. Sayed Mohammad Akid et al. (2023) examined PET plastic as a replacement in concrete and found that up to 10%–15% PET replacement had a negligible effect on compressive strength, similar to our findings, where the 28-day strength ranged from 22 N/mm² to 41 N/mm² even at 20% PET replacement. Their study also showed that higher PET content reduced the fluidity and workability of the mix, consistent with the decrease in slump values observed in our study. Overall, this study’s findings are in line with previous research, demonstrating that increasing PET content in concrete leads to a slight decrease in compressive strength and workability.

Figure 9

Figure 9. Comparative graph of compressive strength and slump values for PET replacement.

The degree to which recycled plastic aggregate addition affects strength, durability, and workability varies depending on the initial strength of the concrete, even though the overall trend may be the same for all grades. Concrete of higher quality might retain strength better, whereas concrete of lower quality might lose strength more quickly. Different structural applications call for different grades of concrete. For example, M25 is used for reinforced concrete structures. Its safe use in suitable construction scenarios is ensured by knowing how plastic aggregate affects each grade. The way cement paste and plastic aggregates interact can change depending on the mix proportions, even with comparable patterns. Because higher-strength concretes have denser matrices and lower water-to-cement ratios, plastic aggregates may have different effects on durability, strength reduction, and porosity. Sustainability is frequently the objective when adding recycled plastic aggregate. Researchers can optimize the best grade for various use cases by examining several grades and figuring out the best balance between environmental benefits and strength retention. Engineers base their design choices on experimental data. In practical applications, where safety margins and structural integrity are essential, even slight variations in strength matter. Guidelines for using aggregates in various situations can be improved by comparing grades. Usage of recycled plastic in concrete will be beneficial for the environment as it will provide economic viability for construction, encourage recycling of plastic trash, and lessen reliance on sand.

4 Cost–benefit analysis

The cost–benefit analysis of using recycled PET and PVC plastics as partial sand replacements in concrete, evaluated through the benefit–cost ratio method, reveals important insights into material performance across different concrete grades (M15, M20, and M25). The primary benefits include conserving natural resources by reducing sand usage, promoting environmental sustainability by recycling plastic waste, and maintaining adequate compressive strength and workability for construction purposes (Almutairi et al., 2021). Table 3 presents a detailed economic and performance assessment of the replacement of natural sand with PET and PVC plastics in various concrete grades. For all the different grades, the relative cost–benefit calculations are made based on the quantity of sand removed, the monetary savings from reduced sand usage, and the processing cost required for turning plastic waste into usable aggregate. In M15 concrete, PET replacements at 5% and 10% depict reasonable cost increases of 287 rupees and 574 rupees, respectively, and still achieve an acceptable compressive strength; hence, these can be used for practical purposes.

Table 3

Table 3. Summarizing the cost–benefit analysis for the partial replacement of sand with PET or PVC plastics in different concrete grades (M15, M20, and M25) for 1 m³ of concrete.

At 15%, though cost increase is higher (861 rupees), the environmental benefit increases with increased utilization of wastes, while the strength decreases. It has been observed that at 20% replacement, M15 concrete containing PET not only meets but actually exceeds the target strength of 15 MPa, which signifies that a still higher amount of plastics can still meet the performance requirements while offering all the cost benefits associated with reduced extraction of sand. In spite of the modest increase in processing cost (574–1,148 rupees), the strength performance coupled with environmental benefits like reduced sand mining and diversion of plastic waste makes the use of PET and PVC an economically and ecologically viable option. The entire table depicts that the replacement of PET and PVC up to 20% can offer an optimum blend of cost efficiency, sustainability, and adequate compressive strength for all grades of concrete. Consequently, the benefit–cost ratio favors up to 20% replacement in higher-grade concretes, effectively balancing cost, performance, and environmental impact.

5 Conclusion

This study investigated the feasibility of using recycled polyethylene terephthalate (PET) and polyvinyl chloride (PVC) plastics as partial replacements for sand in different grades of concrete (M15, M20, and M25). The key findings provide insights into the impact of plastic aggregates on the workability, compressive strength, and overall sustainability of concrete mixtures as follows.

• For M15 concrete, 5% and 15% PET replacement resulted in slightly higher workability, while 10% and 20% PET replacement showed medium workability.

• In M20 and M25 concrete, PET replacement up to 20% maintained sufficient workability, aligning with practical application standards.

• Similarly, PVC replacement up to 15% retained medium workability, with 20% replacement showing increased slump values.

• PET replacement led to a slight decrease in compressive strength with increasing plastic content. However, up to 15% replacement maintained acceptable strength for M15 and M20 grade concretes, while M25 grade concrete retained structural integrity even at 20% replacement.

• PVC replacement also exhibited a marginal decline in strength, with reductions in the range of 5%–15%. Nonetheless, strength values remained above the required thresholds for structural use.

• Across all concrete grades, higher PET and PVC content led to a trade-off between strength and workability.

• A 20% PET replacement in M15 grade concrete resulted in a 46.51% reduction in compressive strength, while M20 and M25 grades exhibited minor decreases without compromising usability.

• PVC replacement led to a similar pattern, with strength reductions becoming more pronounced at higher percentages.

• The use of recycled PET and PVC in concrete contributes to reducing plastic waste accumulation and sand depletion, supporting sustainability in construction.

• The cost–benefit analysis highlights the economic feasibility of incorporating plastic waste into concrete, particularly in non-load-bearing and lightweight applications.

The study demonstrates that incorporating PET and PVC plastics as partial sand replacement in concrete enhances sustainability by reducing plastic waste and conserving natural resources. It also shows economic feasibility, especially for non-load-bearing and lightweight applications, with M20 and M25 grades maintaining acceptable workability and compressive strength even at 20% replacement. However, the use of plastic aggregates leads to a moderate reduction in compressive strength at higher replacement levels, particularly in M15 concrete, indicating a trade-off between strength and workability. Additionally, the weaker bonding between plastic particles and the cement matrix may affect long-term performance. Future research should therefore focus on improving the interfacial bonding of plastic aggregates through surface modification or chemical treatments, and on evaluating long-term durability, thermal resistance, and performance under field conditions to ensure the reliability and structural integrity of plastic-modified concrete in practical applications.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

Author contributions

PA: Methodology, Investigation, Supervision, Writing – original draft. MM: Formal Analysis, Data curation, Writing – review and editing. MV: Methodology, Investigation, Writing – review and editing. UF: Data curation, Conceptualization, Writing – original draft. AK: Writing – review and editing, Formal Analysis, Investigation.

Funding

The authors declare that no financial support was received for the research and/or publication of this article.

Acknowledgements

The authors would like to acknowledge the administrator of Baba Ghulam Shah Badshah University, Rajouri, J & K, India.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The authors declare that no Generative AI was used in the creation of this manuscript.

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