1 Introduction

Concrete, as a fundamental construction material for infrastructure development, wields substantial potential for environmental impact. It ranks among the most extensively employed man-made substances on a global scale, prompting growing concerns about the over consumption of raw materials in concrete manufacturing. Comprising two pivotal constituents - an inert filler and a binding agent - concrete forms a chemically interlinked, rigid structure. Cement serves as the critical binding component, while coarse and fine aggregates provide the load-bearing strength. The worldwide construction industry is actively investigating alternative materials for concrete to mitigate reliance on conventional raw materials, given concrete’s diverse applications and adaptability. Recent research has demonstrated promising outcomes in employing polyethylene plastic waste as a feasible substitute for traditional aggregates in concrete [1, 2].

Annually, the global production of non-recyclable plastic waste amounts to millions of tons, with the majority ending up in landfills or contaminating the world’s oceans. In the context of India, plastic consumption has experienced a substantial upswing, rising from 6,800 kilo tonnes in 2010 to approximately 20,898 kilo tonnes in 2021. This signifies a remarkable 2.28-fold increase in consumption between 2010 and 2016, as illustrated in Fig. 1 [3].

Fig. 1
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Plastic consumption annually from year 1990 to 2021 [3]

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The adoption of plastic recycling entails energy consumption, thereby contributing to heightened environmental pollution on land, in the oceans, and in the atmosphere. The increased production of single-use plastic items and the management of waste disposal have emerged as significant global environmental challenges. Plastics are classified into two primary categories based on polymer properties: thermosetting plastics and thermoplastics [4]. Among them, PET (polyethylene terephthalate) polymer stands out for its extensive application in packaging, including bottled drinking water, and various products due to its durable semi-crystalline nature, exceptional mechanical strength, and robust resistance to hydrolysis, chemicals, and solvents, along with commendable tensile strength [5]. The ease of utilizing PET bottles has led to a surge in demand, consequently making their waste management a pressing concern [6].

In recent years, several experiments have explored the replacement of natural aggregates in cementitious composites with waste plastic aggregates. These experiments encompass various approaches, such as shredding and altering the geometric characteristics of heated and melted PET plastic. Incorporating these waste plastics in construction projects significantly reduces the density and overall weight of the resultant concrete structures [7]. Reusing waste plastics in construction is regarded as one of the most efficient disposal methods among the multitude of recycling management strategies. The integration of used PET bottles as filler materials in concrete offers opportunities for recycling these waste materials. Many studies have explored the application of PET bottle fibers as structural concrete reinforcements, albeit at the cost of a reduction in primary strength and a concurrent improvement in flexibility [8].

Numerous experimental investigations, employing life cycle assessment techniques [9, 10], have evaluated concrete incorporating varying proportions of powdered PET, indicating that a 5% substitution yielded the most improved strength [11, 12]. These investigations have also addressed performance, optimization [13], and chemical surface treatment of PET fibers, revealing their role in enhancing concrete properties [14]. Notably, the alteration of PET waste shape allows precise size matching with natural sand, with an increase in PET aggregate content leading to a decrease in mortar consistency [15]. The use of PET waste and recycled materials has become increasingly prevalent in lightweight mortars and lightweight concrete [16, 17]. The geometric attributes of PET fibers have a negligible impact on concrete workability, while the use of thinner-diameter fibers enhances workability [18]. Furthermore, studies incorporating 10% recycled PET granules as a replacement for fine aggregate have demonstrated improvements in concrete properties, especially up to approximately 10% volumetric replacement [19,20,21,22].

The incorporation of PET-aggregates in concrete, in any form, invariably leads to a reduction in compressive strength. However, the compressive strength development of concrete containing various PET-aggregates exhibits behavior akin to that of conventional concrete. No discernible differences in the mechanism of flexural failure were observed when PET waste concrete was used for monolithic and hybrid beams [23, 24]. The utilization of PET bottle shredding material as a substitute for fine aggregate yielded improved workability and strength at lower replacement percentages. Nevertheless, an increase in the percentage of replacement correlated with a decrease in strength [25,26,27]. The inclusion of Bakelite waste in concrete represents a sustainable approach to waste management and promotes the use of environmentally friendly construction materials. However, as the replacement ratio for coarse aggregate increases, the mechanical properties of the concrete deteriorate [28, 29].

In the contemporary world, decision-making is of paramount importance for achieving optimal outcomes while balancing conflicting criteria to attain specific objectives. To tackle this challenge, advanced techniques such as Multi-Criteria Decision Making (MCDM) tools have been developed, leading to a paradigm shift in civil engineering applications [30]. These tools facilitate the evaluation of performance by considering technological, economic, and environmental factors. The objective of incorporating MCDM tools in civil engineering is to advocate for a sustainable approach to material design in the construction sector. These tools play a pivotal role in determining concrete mixtures by considering various inputs and selecting the most suitable alternative through different MCDM methods. The final decision-making process hinges on the alternatives presented [31]. Some studies introduce an intelligent multi-criteria decision-making model to handle complex construction scenarios, incorporating expert judgments. It proposes a decision hierarchy for method selection and demonstrates the models feasibility, providing a valuable tool for decision-making in underground construction [32]. Also a decision-making method for identifying optimal risk-control schemes for excavation engineering projects utilizing the fuzzy VIKOR method and triangular fuzzy sets was implemented based on engineering experience and environmental factors [33].

Numerous studies have assessed the properties of concrete incorporating PET-aggregates through experimentation. Typically, raw PET obtained from recycling programs has been manipulated or heated, and then shaped artificially using a specific mechanism or post-processing method. This, in turn, consumes energy and utilizes a minor amount of natural resources, contributing to pollution. The fundamental purpose and innovation of this study are to evaluate the benefits of using raw PET waste as a coarse aggregate in concrete, facilitating a sustainable construction process in the context of modern infrastructure development and smart cities. Replacement percentages of conventional coarse aggregates (CCA) with PET aggregates were set at 3%, 5%, and 8% without any preheat treatment or shredding. PET waste in the form of full bottle caps was adopted as a replacement for CCA, capitalizing on the natural shape of the bottle cap body and configuring it into half and quarter shapes. Additionally, the Analytical Hierarchy Process (AHP), a Multi-Criteria Decision Making (MCDM) tool, is employed to recommend the most suitable concrete mix based on strength properties. Analytic Hierarchy Process (AHP) is often favored over other Multi-Criteria Decision-Making (MCDM) tools due to its flexibility in structuring decision problems hierarchically, ease of use through pairwise comparisons ability to handle both quantitative and qualitative criteria, transparency in decision-making, and widespread acceptance and application across various applications. The findings aim to provide a more sustainable material for the production of environmentally friendly concrete with minimal alteration of waste and a reduction in post-processing treatment, thereby mitigating land and water pollution. The management of these substantial quantities of plastic waste, however, presents a significant environmental challenge.

2 Experimental program

2.1 Mixture materials

Portland pozolana cement (PPC) of 43 grade, the most common cementitious component for concrete, has been employed in the design. For concrete making, 20mm-sized quarry stones are employed as conventional coarse aggregates (CCA) together with natural river sand as fine aggregate (FA). Before being used in the concrete mix, the material was first cleaned to eliminate presence of any dirt. Prior test such as specific gravity, water absorption capacity was measured using Indian standard techniques using IS 2386 (Part IV) – 1963 and IS 1489 (Part I)-1991 [34, 35].

Table 1 represents physical properties of raw materials used for mix design of concrete.

Table 1 Physical Properties of raw materials
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2.2 Modified PETA

In this experimentation waste packaged drinking water bottle were collected from local restaurants and hotels and the bottle caps was segregated from collected waste. The segregated caps were then cleaned and open dried in an enamel try for 24 h. The full (F) caps were then cut into half (H) and quarter (Q) shape approximately 28mm in diameter and adopted for experimentation by a replacement of 3%, 5% and 8% with CCA with varying shapes. Figure 2 shows the geometry of modified PETA used in the experimentation according to shape alterations. Here the post processing, per-heating or shredding of waste plastic is completely eliminated and the collected PET bottle cap are directly used by making variation in the surface configuration of PETA. The detailed work chart illustrating the experimental flow is represented in Fig. 3.

Fig. 2
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PETA normal and cut shaped configurations a Full, b Half, and c Quarter

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Fig. 3
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Flow chart of the experimentation work

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2.3 Design mix

Four mixture types with the natural CCA substituted with PETA by 3%, 5% and 8% were chosen in order to evaluate the performance of PET aggregate on the fresh and harden characteristics of concrete. Due to the difference in specific gravity between natural CCA and PETA, the volumetric design technique was adopted. Here, full cap PETA, half cap PETA, and quarter cap PETA concrete was compared to Normal M25 concrete, which served as the controls specimen.

In the concrete mix design, each combination had the same kind of Portland pozolana cement (PPC) of 43 grade and river sand as fine aggregate. Based on the findings of earlier tests on the core raw materials, an experimentation mix design for M25 concrete adopting IS10262:2009 [36] was designed and implemented. In Table 2, the weighted average mix proportions for each mix for a concrete volume of one cubic meter are listed. The concrete mix was created with a water cement ratio of 0.50 and a target strength of 25 MPa at 28 days. The proportions of cement, fine aggregate, coarse aggregate, and water/cement followed the formula 1:2.09:2.83:0.5 for the concrete mix design.

Table 2 Mix proportion details ingredients
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2.4 Test performed

  1. i.

    Workability- Utilizing the slump cone test method [37], flow ability of each freshly mixed mixture was assessed. Concrete was poured into a surface-leveled slump cone during the mixing of various concrete blends, and it was then tamped for every interval of concrete filling. The slump value was then calculated as the height difference between the cone and sample after the cone had been lifted.

  2. ii.

    Compressive (CS) and Splitting tensile strength (STS) - After 28 days of fully immersed water-curing, tests for concrete compressive strength and splitting tensile strength were conducted. Typical size molds were used for the experiments as per [38]. Loading was then induced gradually and constantly at a standard rate until the corresponding failure load occurs. The compressive strength and splitting tensile strength was then calculated for each mix blend adopted for experimentation.

  3. iii.

    Flexural strength (FS)-A total of 24 beam specimens (100mm × 100mm × 500mm) were cast, cured for 28 days and tested for two-point load and simply supported condition. The necessary failure load was then progressively and consistently applied at a standard rate. During testing, fracture patterns were noted and crack initiation and propagation were observed visually. The experimental summarized test findings are presented in Table 3 and its significance is explained in the discussion.

Table 3 Summarized test results at 28-day strength
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3 Results and discussions

3.1 Workability

Figure 4 illustrates the results of the slump test conducted on the investigated concrete mixtures. Notably, an increase in the percentage of PETA (Polyethylene Terephthalate Aggregates) replacement in the mix correlates with a reduction in the slump value. This observation aligns with prior research findings reported by Edmund et al. [7], Lee et al. [14], and Islam et al. [5]. The diminished slump is attributed to the sub optimal bonding between PETA and the concrete matrix, primarily caused by the smooth surface texture of PETA. Initially, a controlled mixture was meticulously formulated to achieve a 100 mm slump. However, as the PETA replacement percentage rises, both PEAT-F and PETA-H exhibit a declining slump. This decrease in workability can be ascribed to the larger diameter of PEAT-F and PETA-H aggregates and their inability to blend effectively with the other concrete matrix constituents. It becomes evident that an escalation in the surface area of PETA leads to a commensurate decrease in workability. Interestingly, the slump value for PETA-Q remains nearly identical to that of the controlled concrete, signifying excellent workability. This congruence with results obtained by Bamigboye et al. [4] suggests that the shape of PETA-Q closely matches that of the natural aggregate, thereby contributing to its favorable workability. Consequently, a smaller PETA surface area is associated with improved concrete flow, as evidenced in this study.

Fig. 4
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Observed slump variation for PETA concrete

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3.2 Compressive and splitting tensile strengths

The evaluation of concrete quality places significant emphasis on its compressive strength, a pivotal and fundamental parameter. The results of the tests, as summarized in Table 3, indicate that the replacement of Conventional Coarse Aggregates (CCA) with Polyethylene Terephthalate Aggregates (PETA) leads to an enhancement in both compressive strength and splitting tensile strength, but only up to a 5% substitution level. This observation concurs with the findings of Umasabor and Daniel. After a curing period of 28 days, it is noteworthy that H-PA (Hybrid PETA Aggregates) exhibited the highest compressive strength, reaching 31.348 MPa. This superior performance is attributed to the efficient packing of PEAT-F (Full PETA) and PETA-H (Half PETA) in conjunction with CCA within the concrete matrix. In contrast, PETA-Q (Quarter PETA) displayed a decrease in compressive strength when compared to the control specimens and other combinations. These outcomes align with the experimental results reported by Bamigboye et al. [4], Islam et al. [5], and Edmund et al. [7]. The decrease in strength can be ascribed to the relatively smaller size of PETA particles within the concrete, which fails to effectively contribute to load shearing, consequently leading to early concrete crushing. The variation in compressive strength of the designed PETA-modified concrete mixes is graphically illustrated in Fig. 5. Regarding splitting tensile strength, a negative correlation with the surface area of PETA is observed, coupled with a marginal increase in response to changes in the shape of PETA. This relationship is depicted in Fig. 6. The finer surface texture and expanded surface area of PETA, which result in sub optimal bonding at the interface and the accumulation of free water, have also been identified as factors contributing to the decline in tensile strength. These observations align with the findings of Bamigboye et al. [4] and Islam et al. [16].

Fig. 5
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Compressive strength variation for PETA modified concrete

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Fig. 6
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Splitting tensile strength variation for PETA modified concrete

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In comparison to the controlled mixture, PETA-Q (Quarter PETA) exhibited a remarkable improvement in strength, ranging from 16 to 22%. After 28 days of curing, PETA-Q achieved a peak strength of 3.325 MPa. In this context, the smaller size and shape of PETA-Q particles serve as fibrous materials within the concrete mixture, augmenting its splitting behavior. A visual representation of the cylindrical specimen following the test is presented in Fig. 7. Unlike PETA-Q and PETA-H (Half PETA), no indications of sudden, brittle concrete failure were observed, suggesting an enhancement in the durability of the fractured concrete. These observations align with the findings of Fakhruddin et al. [24] and Aocharoen et al. [27].

Fig. 7
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Tested specimens for splitting tensile strength a M25 Controlled concrete b Full cap PETA concrete c Half cap PETA concrete and d Quarter cap PETA concrete

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3.3 Flexural strength

Figure 8 illustrates the flexural behavior of beams subjected to a two-point load system using a Universal Testing Machine, following 28 days of curing. The experiment involved untreated plain M25 grade concrete beam specimens, measuring 100 mm × 100 mm × 500 mm, without reinforcement. These beams were treated with PETA to evaluate the bending strength of the concrete. The results presented in Fig. 8 reveal that, after 28 days, PETA-Q (Quarter PETA) exhibited an impressive maximum strength of 4.452 MPa, nearly double the strength of the controlled beam. This enhancement can be attributed to the smaller size and shape of PETA-Q particles, which function as fibrous materials within the mixture, reducing the propagation of fractures and leading to improved bending strength. These findings contradict the results reported by Umasabor and Daniel [11], Islam et al. [16], and Fakhruddin et al. [24].

Fig. 8
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Flexural strength variation for PETA modified concrete

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When comparing PETA-Q to PETA-H (Half PETA) and PEAT-F (Full PETA), PETA-Q demonstrated a 26% and 90% increase in flexural strength, respectively. Notably, the flexural strength of PEAT-F was found to be comparable to that of the controlled beam, implying a higher likelihood of voids in the concrete, which compromise the cohesion of the matrix. Figure 9 provides a visual representation of the tested beam specimens, along with the fracture dispersion pattern induced during testing. It is evident that in the controlled beam, the fracture initiates at the center and follows a vertical bending crack, while in the PETA-modified concrete beam specimen, the crack moves toward the support as inclined crack. Furthermore, it can be observed that the crack initially displays a semi-vertical orientation up to one-third of the beam’s depth on the tension side before transitioning into a shear fracture-like behavior below the point load.

Fig. 9
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Crack initiation and pattern of failure a M25 Controlled concrete b Full cap PETA concrete c Half cap PETA concrete and d Quarter cap PETA concrete

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4 Application of analytical hierarchy process for selecting best concrete mix design

The Analytical Hierarchy Process (AHP) is a critical tool in the selection of the most suitable concrete mix parameter from a range of available options. Figure 10 illustrates the hierarchical structure of the AHP analysis used to identify the best mix parameter for PETA concrete, considering various criteria and alternatives. These tools are instrumental in prioritizing available options and facilitating the selection of the optimal decision from among the accessible alternatives. The AHP approach adheres to a hierarchical structure, with the ultimate goal positioned at the top, criteria at the second level, and alternatives at the lowest level. AHP stands out as the most widely employed mathematical model for resolving intricate and challenging decision-making problems. These tools excel in determining the highest priority among the offered alternatives. Notably, when conflicts arise in decision-making, one of the significant advantages of AHP is its ability to rank options while adhering to the defined objectives.

Fig. 10
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Schematic diagram showing hierarchy of AHP

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The following steps were implemented to determine the weight of the mix for all the combinations tested in the experimentation:

  1. i.

    In the initial AHP step, a pairwise comparison of the experimental observations for attributes ‘i’ and ‘j’ as detailed in Table 3 is conducted, employing the normalization technique. When an attribute is compared to itself, it is invariably assigned the value 1, resulting in the main diagonal elements of the pairwise comparison matrix being equal to 1. An excerpt of the pairwise matrix for the compressive strength parameter is provided in Table 4 with matrix size M = 12.

  2. ii.

    The geometric mean of each row in the comparison matrix is computed, and the geometric means of the rows are normalized to ascertain the relative normalized weight of each attribute as given in Eqs. 1 and 2.

    $$\mathrm{Geometric \,mean \,}({{\text{GM}}}_{{\text{j}}})={[\prod\nolimits_{{\text{j}}=1}^{{\text{M}}}{\text{bij}}]}^{1/{\text{M}}}$$
    (1)
    $$\mathrm{Weight\, of \,each \,attribute}=\mathrm{ Wj }=\left[{{\text{GM}}}_{{\text{j}}} / \sum\nolimits_{{\text{j}}=1}^{{\text{M}}}{\text{GMj}}\right]$$
    (2)
  3. iii.

    Three PETA aggregate shapes were tested for compressive, splitting tensile, and flexural strength of concrete, with comparisons made to conventional concrete strength. By examining the weight obtained for the compressive strength of concrete, it becomes evident that PETA-H holds the highest weight (0.10) for the mix, positioning it as the top-ranking choice for compressive strength.

  4. iv.

    Analogous steps from i to iii were executed for splitting tensile and flexural strength evaluations of PETA concrete in comparison to conventional concrete. The most suitable alternatives for each test are highlighted in Tables 4, 5 and 6, and they are visually presented in Figs. 11 and 12.

  5. v.

    Figure 13 displays the maximum weights of alternatives for all tested mix combinations. The most appropriate mix parameter among the various combinations experimented with is ranked, and PETA-Q emerges as the top choice in the list.

  6. vi.

    As depicted in Fig. 13, it is notably evident that the highest weight is attributed to the 5% replacement of Conventional Coarse Aggregates (CCA) with PETA. Furthermore, out of the three tests conducted, the maximum weight is observed for PETA-Q and PETA-H types of aggregates. The triangular shape of PETA contributes to effective particle packing, rendering the concrete denser and subsequently enhancing its strength.

  7. vii.

    Here it is evident that, though the compressive strength of PETA concrete is improved by adding PETA-H the remaining two strengths have shown improvement using PETA-Q. Here the PETA of smaller shape acts like and fiber in the matrix resisting the crack initiation and thus improvement in the strength. If the weight for compressive strength of PETA-Q is observed from Table 4 and Fig. 11a the weight is equal to 0.096 which is almost equals with 0.10 and therefore concluded as the best suitable shape of PETA.

Table 4 Pair-wise matrix for compressive strength property for all mix types
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Table 5 Pair-wise matrix for split tensile strength property for all mix types
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Table 6 Pair-wise matrix for flexural strength property for all mix types
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Fig. 11
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Maximum weight of alternatives for a compressive strength and b split tensile strength

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Fig. 12
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Maximum weight of alternatives for flexural strength

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Fig. 13
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Ranking of most suitable concrete mix

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4.1 Field applications of AHP

The application of Multi-Criteria Decision-Making (MCDM) tools can provide significant advantages in the process of material selection for concrete production. These tools enable the consideration of a multitude of factors, encompassing aspects such as strength, sustainability, physical properties, shape and size, and feasibility, when making informed decisions. In particular, the Analytic Hierarchy Process (AHP) offers a systematic and structured approach to the comparative assessment and prioritization of different formulations of PET-based concrete, with a specific focus on its mechanical strength. This comprehensive evaluation involves the examination of various variables, including the shape of PET aggregates, the extent of substitution, and diverse mechanical strength properties. By harnessing the capabilities of AHP, the primary objective is to discern the most suitable concrete mixture from the available alternatives, taking into account a range of critical factors. Furthermore, the AHP methodology can be effectively deployed in practical field applications, facilitating the simplification of material selection processes and the refinement of material choices for concrete production.

5 Conclusions

This experimental study aimed to enhance the performance of materials and structures while retaining the use of conventional coarse aggregates by integrating discarded PET bottle caps into structural concrete. A significant impetus behind this investigation was the promotion of sustainable concrete production and the realization of environmental benefits through the effective utilization of this waste material. Furthermore, the study sought to validate the experimental findings by employing a multi-criteria decision-making optimization tool to address conflicting alternatives. The following conclusions are drawn from this research:

  1. 1.

    PETA (Polyethylene Terephthalate Aggregates) with a smaller surface area exhibits workability comparable to that of the controlled mix. This enables optimal flow and improved compaction, resulting in denser concrete with enhanced strength properties. PETA-Q, in particular, maintains a similar slump as the controlled mix, signifying that a larger surface area and a higher percentage of PETA have a detrimental effect on concrete workability, leading to harsher mixes and reduced strength in the final product.

  2. 2.

    Compressive strength (CS) improves up to a 5% replacement of conventional coarse aggregate (CCA). In this context, PETA-H demonstrates the highest CS value of 33.190 MPa. When combined with CCA, PETA-H offers a mix with improved workability, reduced PETA surface area, and enhanced CS. Since the CS values for PETA-H and PETA-Q are nearly identical, it can be inferred that PETA-F decreases concrete strength by reducing density, aggregate content, and ultimately resulting in lighter concrete.

  3. 3.

    The addition of PETA has a marginal impact on splitting tensile strength (STS) and flexural strength (FS). In this scenario, PETA-Q outperforms PETA-F and PETA-H. A maximum STS of 3.325 MPa, which is 22.24% higher than the standard concrete cylinder after 28 days, was observed. Additionally, the maximum FS is approximately double that of CCA concrete, measuring 4.45 MPa. This increase in strength is attributed to the fact that smaller PETA shapes function as fibrous materials, enhancing particle packing and overall strength.

  4. 4.

    The application of a Multi-Criteria Decision-Making (MCDM) tool facilitated the ranking of the most suitable mix type among the tested trial mixes, taking into consideration various test parameters. It is concluded that PETA-Q, with an optimal 5% replacement of CCA, significantly enhances concrete strength and is ranked as the most suitable mix.

This study underscores the positive impacts of incorporating PETA in concrete, leading to improved structural properties, addressing CCA scarcity, and mitigating pollution stemming from discarded plastic bottles through sound waste management practices.

6 Limitations and scope for future work

The integration of PET waste in concrete production may not be universally suitable for all types of cement. The results of the experimental investigation emphasize that the replacement of conventional coarse aggregate with PET aggregates is most effective up to a 5% replacement ratio. Beyond this threshold, a noticeable decline in the mechanical properties of the concrete is observed. It’s worth noting that the tested concrete in this study was plain cement concrete, and variations may arise when dealing with reinforced cement concrete (RCC). In light of the limitations identified in this research, the following areas merit consideration for future studies.

  1. 1.

    Further experiments are recommended to explore the generalization of these findings to reinforced cement concrete (RCC). This extension of the research can provide insights into the behavior of PET aggregates in a more complex and structurally demanding environment.

  2. 2.

    The influence of different shapes of PET aggregates on the mechanical properties of concrete can be systematically investigated by varying the shapes and conducting comprehensive tests. This approach can yield a deeper understanding of how the geometry of PET aggregates impacts concrete performance.

  3. 3.

    Exploring the long-term durability of concrete incorporating PET aggregates and evaluating the economic feasibility of this approach for large-scale applications are excellent avenues for further investigation. This area of study can delve into the long-term performance, resilience, and resistance of PET-modified concrete in various environmental conditions, providing valuable insights into its potential for sustainable construction.