Evaluation of the properties of tuff pavers incorporating Polyethylene Terephthalate (PET) waste as binding material

ABSTRACT

The study focuses on the potential use of Polyethylene Terephthalate (PET) plastic waste as a substitute for cement replacement for binding properties in paver blocks. This study will be helpful in getting rid of plastic pollution and can be applied to construction industry including water logged areas, pedestrian tracks, kerb stones and light traffic applications. Physical properties, mechanical properties, chemical composition of PET plastic including microstructural properties and statistical analysis are the main focus in this research study. Results of mechanical testing show that by adding more coarse aggregates than PET plastic reduces the compressive strength but increases the tensile strength by keeping the sand content less. The mix ratio incorporating 36% PET plastic, 28% sand and 36% crush gives the optimum results in terms of compressive strength i.e., 43 MPa and also shows good split tensile strength of 6.45 MPa. These blocks also exhibit less water absorption and are temperature resistant up to 100 ^oC. Regression analysis shows the validity of the experimental results, and SEM images show the significant improvement in the microstructure properties. The analysis of cost revealed the enhancement of economic feasibility and are up to 57% economical as compared to conventional cement-based pavers. Based on the findings of this research study, it is concluded that PET plastic waste can be utilized as a replacement of cement to overcome plastic pollution, to promote sustainable solution to construction industry and contribute towards solid waste management.

Keywords:
PET plastic; Cement-less paver block; Binding agent; Plastic pollution; Sustainable solution

1. INTRODUCTION

Cement is an essential building material extensively used in constructing concrete structures such as buildings, roads, bridges, and other infrastructure projects. Typically, concrete comprises around 10 to 15% cement content. A significant source of carbon dioxide emissions is the production of cement, accounting for a considerable portion of global greenhouse gases emissions [¹]. According to the International Energy Agency (IEA), an annual increase of CO₂ emissions by the production of cement is about 1.5% on average from 2015 to 2021 [²]. In comparison to the above reports, an annual decrease of 3% is required until 2030 to stay on pace with the scenario of reaching net zero emissions by 2050 [³].

The rapid development of modern industries, a growing population, and economic progress have undoubtedly improved life worldwide. However, a downside of this progress is that more and more manufactured items, like plastic products, are being used [⁴]. Meanwhile, the issue of plastic waste has emerged as a growing environmental concern.

The widespread use of plastic materials has caused a rapid generation of waste causing harm to the environment [⁵]. It was expected that the plastic production will reach 390.7 million metric tons globally in 2021, representing an annual increase of 4% as reported by Statista Research Department [⁶]. Since 1950, almost half of all plastic waste has ended up in landfills or dumped into the oceans [⁷]. As per the report of World Economic Forum in 2023, the ocean is expected to contain more plastic than fish by weight if current trends continue by 2050 [⁸]. As a result, there has been increasing interest in exploring alternative materials that can replace cement in construction while also addressing the problem of plastic waste.

One of these alternatives is to use plastic waste in place of some or all of the cement in concrete mixes [⁹]. This can be done either partially or entirely. Even though using plastic waste might have minor change in strength but they can be used for pedestrian applications such as footpaths, parks and yards [¹⁰]. This strategy has the potential to not only cut down the amount of plastic waste that is disposed of in landfills or that causes pollution to the environment, but also has the potential to cut down the environmental impact caused by the production of cement by making use of recycled materials. Due to the slow degradation rate, the plastic waste cannot be dumped in to landfill [¹¹].

Recent researches have demonstrated that plastic waste can be used in place of cement in concrete without severely affecting the materials strength. Plastics can be used to increase cracking resistance, mechanical strength and impact resistance by utilizing in concrete mixture [¹²,¹²,¹³,¹⁴,¹⁵,¹⁶]. By diverting plastic waste from landfills and repurposing it for construction, the cost of product and waste management can be reduced [¹⁷]. As a result, the construction industry can become more sustainable due to plastic good mechanical and thermal properties [¹⁸,¹⁹,²⁰].

Plastic waste can be used in different forms, such as shredded or granulated, and can replace varying percentages of cement in the concrete mix. However, it is essential to note that the replacement percentage must be carefully considered to ensure that the resulting material meets the required strength and durability standards for the intended application. By using recycled plastic as a partial replacement for cement in concrete can lead to a 15% reduction in the materials carbon footprint [²¹].

The replacement of cement with the plastic waste has gained considerable attention as a sustainable solution for the construction industry. All the studied literature did not research on complete replacement of cement with PET plastic showing the limited research on this topic. Most of the literature investigated the potential use of PET plastic as a partial replacement of aggregates. There needs to be more research on the feasibility and potential of using plastic waste as a complete replacement for cement in various products. Based on the report published by the United Nations Development Program (UNDP), Pakistan produces an estimated annual quantity of 20 million tons of solid waste, with plastic waste constituting approximately 5 to 10 percent of this total [²²].

In Pakistan, the predominant composition of the 250 million tons of waste is comprised of plastic bags, PET bottles, and food scraps [²³]. The Pacific Garbage Patch, which encompasses approximately 1.8 trillion plastic wastes, has now expanded to a size three times that of France. The escalating plastic pollution in Pakistan is presenting a significant environmental hazard [²⁴]. The production of cement-based pavers contributes to CO₂ emissions and the depletion of natural resources, making using plastic waste as a replacement material an attractive option [²⁵].

However, mechanical properties and environmental analysis of pavers made entirely from plastic waste have yet to be thoroughly evaluated. In addition, the cost-effectiveness and feasibility of using 100% plastic waste as a replacement material in large-scale paving projects still need to be discovered.

Overall, the use of plastic waste as a replacement for cement in construction has the potential to address the environmental and economic challenges facing by the construction industry. By providing an overview of the current use of cement in construction, its environmental impact, and the magnitude of the plastic waste problem, this research aims to contribute towards solid waste management, to overcome environmental pollution and to promote sustainable development.

2. MATERIALS

Plastic waste from PET bottles in shredded form (size <4.75 mm) used as a binding material, Lawrence-purr silica sand as fine aggregates and stone crush as coarse aggregates. Physical properties of aggregates are presented in Table 1 and gradation curve of fine and coarse aggregates is shown in Figure 1.

Table 1
Physical properties of aggregates.

	TEST VALUES
DESCRIPTION	FINE AGGREGATES	COARSE AGGREGATES
Type	Silica (SiO2)	Stone Crush
Size of particle	< 4.75 mm	4.75–10 mm
Fineness Modulus	2.91	–
Silt Content	3.78%	–
Specific Gravity	2.80	2.90
Compacted Bulk Density	1.58 g/cm3	1.65 g/cm3
Loose Bulk Density	1.39 g/cm3	1.41 g/cm3
Water Absorption	1.87%	4.89%

Figure 1
Gradation curve of aggregates.

The thermal properties of PET plastic are presented in Table 2 while the chemical analysis of PET plastic in terms of oxides and chemical elements shown in Table 3 respectively as presented in past research [²⁶].

Table 2
Thermal properties of PET plastic.

PROPERTIES	TEST VALUES
Softening Point	239 ℃
Melting Point	243 ℃
Flash Point	300 ℃
Fire Point	353 ℃

Table 3
Chemical analysis of PET plastic.

CHEMICAL COMPOUND	CONSTITUENTS	%
Polyethylene Terephthalate (PET)	(C10H8O4)n	99.524
Silicon Dioxide (Silica)	SiO2	0.223
Sulfur Trioxide (Sulfuric oxide)	SO3	0.198
Calcium Oxide (Lime)	CaO	0.017
Chlorine	Cl	0.015
Iron Oxide (Hematite)	Fe2O3	0.013
Thulium Oxide	Tm2O3	0.006
Copper Oxide	CuO	0.003

3. EXPERIMENTAL SETUP

3.1. Mix design

PET plastic, sand and coarse aggregates are used as key ingredients for the development of sustainable plastic pavers as shown in Figure 2. No water is required for mixing and curing purposes as there is no cement present in it so no hydration process involved.

Figure 2
Ingredients of plastic paver block.

A total of seven (07) mixes are designed including pilot studies to determine the influence of fine and coarse aggregates. Each mix ratio is tested against nine (09) samples. Out of seven (07) mix ratios, three (03) mixes are selected based on the pilot studies. The mix proportion of these mixes for 1 m³ volume, in comparison of control mix, are presented in Table 4. Mix ID is based on PET plastic, sand and coarse aggregates with same mixing pattern. For example, P36,S28,Cr36 represents paver mix having 36% plastic content with 28% sand and 36% crush while the conventional cement-based paver (control mix) represented as C57,S14,Cr29 which shows 57% cement involved instead of plastic with 14% sand content and 29% crush.

Table 4
Mix proportion for plastic paver block.

MIX ID	CEMENT (kg/m3)	PET PLASTIC (kg/m3)	FINE AGGREGATES (kg/m3)	COARSE AGGREGATES (kg/m3)
P44,S22,Cr34	0	613	351	493
P36,S28,Cr36	0	502	431	537
P31,S31,Cr38	0	425	486	569
C57,S14,Cr29 (Control Mix)	823	0	226	422

3.2. Sampling procedure

Weigh all the materials according to the mix proportion and place it in a heating container in such a way that the coarse aggregates laid down first following the fine aggregates (sand) and on the top is waste PET bottles plastic in shredded form. All the materials should be uniformly distributed in the container according to the sequence given above. Place the sample in the heating oven for melting the shredded plastic. Oven should be pre-heated at 300 ^oC before placing the container. Place the container in the pre-heated oven for approximately 2–3 hours to melt the plastic. When the plastic is completely melted or converted into viscous form, take out the sample from the heating oven and mix it manually with a trowel within one minute otherwise it will become a hard mass. Lubricate enough the mold (200 × 100 × 60 mm) from inside for easily demolding. After mixing the material, shift mixed material into the mold and compact it manually with a compacting flat plate so that there should be no air bubbles remain in the material. Now provide a smooth finishing from the top surface using a flat plate. Demold the sample after 5 minutes and allow it to cool down for 5–10 minutes only. As there is no cement involved in this casted paver, so no curing is applicable. As a result, the sample needs no further time taking process which means that the sample is ready to use after 5–10 minutes. Only the top layer of paver block cools down rapidly. The internal side cools down slowly takes approximately 10 minutes depending upon room temperature. This will develop strong bond between plastic and aggregates inside which cause long term dimensional stability and make it durable. The sampling procedure in pictorial form is given below in Figure 3.

Figure 3
Flowchart of casting plastic paver block.

The potential challenge associated with manual mixing is that every time manual mixing does not give uniformity. Due to non-uniformity, the aggregates do not disperse properly and there is an inconsistency in the mixture.

The use of safety gadgets including mask, heat proof gloves, goggles, safety issues etc. will reduce the potential risk of safety for operators.

The use of mixing machine with built-in heating oven of 300 ^oC and compacting hammer will serve the purpose of more controlled mixing and production on large scale.

4. METHODS

4.1. Compressive strength

The specimens of plastic paver blocks were tested under uniaxial compression loading for crushing as per ASTM C67 to ensure the accuracy and reliability of the test results. The test was performed on automated Compression Testing Machine (CTM) manufactured by CONTROLS, UK applying a uniform loading rate of 20 MPa/s to prevent from localized failure caused by uneven loading.

4.2. Split tensile strength

The specimens of plastic paver blocks were tested for split tensile strength as per ASTM C67. To assess the tensile strength of concrete and other materials, there is an indirect method available which is known as split tensile test. The test was also performed on CTM under uniform loading rate. This test is crucial to measure the resistance of block to tensile stresses.

4.3. Water absorption

The specimens of plastic paver blocks were tested for water absorption as per ASTM C936. The specimens were soaked for 1, 7, 14, 28, 56 days in a water bath. Water absorption test helps us to determine the amount of water absorbed by the specimen. Water enters pores in the cement paste and even in the aggregate. One of the most important properties of a good quality concrete is low water absorption and low permeability. A concrete with low permeability resists ingress of water and is not as susceptible to freezing and thawing.

4.4. Temperature effect

Temperature is one of the most important points in the case of using plastic in any kind of material or product. The main objective of this test is to investigate the temperature effect on the plastic paver in controlled environment.

As the samples were casted after melting the plastic at a higher temperature, so there is a need to check the pavers back at high temperature to see any deterioration. For this purpose, the samples were placed in the heating oven for 24 hours which is pre-heated at a temperature of 100 °C.

4.5. Regression analysis

A regression analysis is a technique used for statistical analysis. It is a mathematical way of sorting out the variable which does have an impact. It also shows a relationship between dependent and independent variables. This technique creates an equation to predict the outcomes if independent variable is changed. This allows to check the validity of experimental data and results gathered.

There are different software and tools to perform regression analysis like SPSS, STATA and MS Excel etc. In this research, MS Excel was used to perform regression analysis to check the validity of experimental results and predict the behavior of end results if variables are changed.

4.6. Scanning electron microscope (SEM)

SEM is used to observe the micro-structure of the casted samples. The phenomenon of SEM is the electron beam which scans the sample in a raster-pattern. Electrons get reflected on the surface or even ionize atoms within the sample by liberating electrons instead of passing through the specimen. These backscattered and secondary electrons can be used as a signal to build up the final image. The morphology of a specimen represented by SEM images and can also reconstruct quasi-three-dimensional views of the sample surface. So, this technique is used to obtain high-resolution images of surface features and told about the different chemical elements distribution within the sample.

5. RESULTS

5.1. Compressive strength

Three different types of paver blocks were casted by varying proportions of PET plastic, fine and coarse aggregates.

The compressive strength of mix containing 44% PET plastic, 22% sand and 34% crush shows the lowest value of 25 MPa at all ages. This is due to more quantity of plastic than fine and coarse aggregates which results in segregation. The sand mixes with the melted plastic but the coarse aggregates settled down during the time of material shifting into the mold causes non-uniformity and loose bond between the aggregates and plastic.

On the other hand, mix containing 36% PET plastic, 28% sand and 36% crush shows the highest value of 43 MPa at all ages which is due to uniform bonding between plastic and coarse aggregates. The strong bonding and improved particle packing indicate effective integration of plastic and coarse aggregates, minimizing the potential weak points and ensuring consistent strength properties. Similar results were also reported the incremental trend of compressive strength and improved particle packing when PET plastic used as a partial replacement of fine aggregates [²⁷].

The melting time increases as the size of shredded PET plastic increases beyond 4.75 mm. With increasing heating time, the lower part of PET flakes melted but upper part remains in the solid state due to pan temperature. It takes time to melt the upper part which results in the breaking of bonding particles in already melted lower side causing less viscosity. As a result, mechanical properties show lesser results.

In comparison to the cement-based pavers (control mix), the average compressive strength observed to be 26.73 MPa after 7 days, 36.35 MPa after 14 days and 40.93 MPa after 28 days of curing. As there is cement used in these conventional pavers, curing is required to hydrate the cement completely. This is the reason that the compressive strength of these cement-based pavers increases with time and gain maximum strength by 28 days. As a result, these conventional cement-based pavers are ready to use after 28 days of casting which is a time taking process as compared to the plastic pavers which are ready to use on the same day of casting.

Several factors contribute to the observed strength disparities, including variability in plastic-to-aggregate ratios, mixing consistency, and bonding quality between materials. These factors collectively influence the overall structural integrity and load-bearing capacity of the pavers.

There seems to be a clear trend of failure pattern for all mixes specimens. The failure pattern of plastic paver looks like not crushed exactly but the thickness reduces due to applied compression load and the sides of paver expand. This is due to the melted plastic behaves like a flexible material which expands the length and width of specimens. This means that the plastic paver does not burst suddenly but have flexible properties as well. In all cases, the cracks appear from the edges of plastic paver and center remains in contact as shown in Figure 4.

Figure 4
Failure pattern under uniaxial compressive load.

5.2. Split tensile strength

The split tensile strength of all plastic pavers ranges from 14% to 24% as compared to compressive strength which is greater than the conventional cement-based pavers (control mix) of approximately 8% to 12%. This shows that the plastic pavers also outperform the cement-based pavers in tensile strength at all ages.

Due to increase in coarse aggregates than PET plastic, rough surface aggregates provide better bonding with the cement matrix and increases stiffness. As a result, localized cracking reduces and tensile strength improves.

The compressive and split tensile strength results at different ages are shown in Figure 5 where CS7 shows Compressive Strength at 7 days and STS7 shows Split Tensile Strength at 7 days and similarly at 14, 28 days.

Figure 5
Mechanical test results at different ages.

The result shows good mechanical properties with the addition of PET plastic flakes [²⁸].

5.3. Water absorption

Plastic paver blocks show very less water absorption as compared to other mix ratios and conventional pavers. Lesser the percentage of water absorption, greater will be the strength.

The average water absorption test results of plastic paver block specimens on 1, 7, 14, 28, 56 days showed in Table 5. According to the results, mix ratio containing 36% PET plastic, 27% sand and 36% crush absorbs an average of 1.70% water, whereas other mix ratios show higher percentage of 2.09% and 1.85% water absorption respectively. This shows that when the number of aggregates increases the percentage of water absorption increases. The specimen which has maximum quantity of plastic shows least water absorption and least amount of plastic specimen shows maximum water absorption in all of the mix ratios. This water absorption is primarily due to the moisture content of the ingredients used. Plastic is a very dense material in the form of melted state results very negligible absorption.

Table 5
Water absorption of plastic paver blocks.

MIX ID	WATER ABSORPTION (%)
P44,S22,Cr34	2.09
P36,S28,Cr36	1.70
P31,S31,Cr38	1.85
C57,S14,Cr29 (Control Mix)	6.49

The Water Absorption shown by concrete paver is 6.49%. It is observed that recycled plastic made pavers performs much better than the conventional cement-based pavers. Lower water absorption rates indicate reduced susceptibility to moisture related degradation. This characteristic is crucial as higher water absorption can lead to weakening of the material over time in terms of strength due to chemical reactions and other environmental factors. The main reason of reinforced concrete deterioration is due to the corrosion of reinforcement caused by chloride ions attack [²⁹]. This relationship observed where higher aggregate content correlates with increased water absorption suggests that aggregates may contribute to pathways for water ingress into the pavers. This insight underscores the importance of optimizing aggregate-to-plastic ratios to minimize water absorption and enhance overall durability.

5.4. Temperature effect

The samples were placed in a pre-heated oven at 100 ^oC for 24 hours to check the temperature effect. The measurements were taken after taking out samples from the oven to check any deterioration from heat. The samples show no change in the shape and size. The only change occurs in the samples was the weight loss. All the mix ratios show very minimal decrement in weights i.e., 0.44% for mix ratio P44,S22,Cr34, 0.30% for mix ratio P36,S28,Cr36 and 0.51% for mix ratio P31,S31,Cr38 respectively. This shows that the pavers are workable at those places and areas where temperature is too high.

5.5. Regression analysis

It is noted from the results shown in Table 6, R square value is very close to 1.00 which means that the experimental data results are very near to regression equation showing the validity of experimental results. In this analysis, multiple independent variables are considered including fine aggregates and coarse aggregates while the dependent variable is the compressive strength. According to the results, the average difference found to be 4-5% only. The equation developed from linear regression analysis is given below:

Table 6
Summary output of regression statistics.

REGRESSION STATISTICS
Multiple R	0.98269
R Square	0.96568
Adjusted R Square	0.96283
Standard Error	1.43264
Observations	27

(1) $$fp\text{’ = 773}\text{.51 + 1}\text{.54}\left(F\text{.}A\text{.}\right)\text{− 2}\text{.61}\left(C\text{.}A\text{.}\right)$$

fp’ = Compressive strength of plastic pavers at 28 days

F.A. = Content of Fine Aggregates in kg/m³

C.A. = Content of Coarse Aggregates in kg/m³

The negative coefficient for coarse aggregates implies the inverse relation with the compressive strength. The magnitude of coefficient shows the expected change in compressive strength by 2.61 MPa when increase in coarse aggregates.

5.6. Scanning electron microscope (SEM)

The SEM images showed that using plastic instead of cement led to a significant improvement in the microstructure of the paver blocks, resulting in a denser structure compared to the conventional paver block. As the plastic fills the gap between aggregates, the overall voids also reduce [¹²]. The comparison of SEM images between plastic and cement-based paver blocks reveals notable differences in microstructure, highlighting the advantages of using plastic over conventional cement. Plastic-based tuff pavers exhibit a denser microstructure in comparison to the cement-based tuff pavers as shown in Figure 6. This densification can be attributed to several factors inherent to plastic materials. Plastics can be processed to have a more uniform and compact internal structure during manufacturing, which interprets into a tighter packing of molecules at the microscopic level. This structural uniformity is often lacking in cement-based materials, where variability in particle size and distribution can lead to more porous microstructures. Additionally, physical and chemical properties of plastic contribute to their ability to form a denser microstructure. Plastics are generally less prone to forming voids or air voids during processing compared to cement, where hydration and curing processes can sometimes result in uneven packing and porosity as shown in Figure 6. The absence of these voids in plastic-based tuff pavers enhances the overall density, strength, and making them less susceptible to cracking or degradation under load.

Figure 6
SEM images of PET plastic and cement paver blocks.

5.7. Cost analysis

Cost of everything depends on its raw material. If the raw materials cost is high then the cost of the whole process increases. The cost significantly decreases by the use of plastic waste [³⁰].

In cement-based pavers, the main cost depends on the cement. The price of one (01) kg of cement in Pakistan is around PKR 30 as of February 2025. As there is the usage of PET plastic waste instead of cement, the cost of plastic waste is very minimal i.e., PKR 12 per specimen including shredding charges. The electricity cost for melting the plastic is approximately PKR 5-6 per specimen so the total cost of the plastic paver is approximately PKR 18.

Rest of the raw materials including sand and aggregates remains the same for both of the products i.e., PKR 44 for sand per cft and PKR 135-150 for aggregates per cft. The cost of pavers of all ratios and cement-based paver for one unit is shown in Figure 7.

Figure 7
Cost analysis comparison.

5.8. Life cycle assessment (LCA)

LCA is an environmental assessment method of products used to quantify the impacts throughout the entire life cycle. This process includes a life cycle of the products starting from the extraction of raw materials, production and ends towards the waste management including disposal and waste treatment.

This assessment was done on an open-source LCA software named as OpenLCA in which impact values was calculated by ReCiPe Midpoint LCIA method. LCI data was imported through eco-invent database.

It can be noted from the analysis that Plastic-based paver reduced Global Warming by 61% which means that the emissions of Greenhouse Gases (GHG) in plastic pavers is only 39% compared with the 100% value of cement-based paver. Whereas emissions of particulate matter in the atmosphere were also reduced by 58%. A decrease of 52% in Freshwater eutrophication is observed while that of marine eutrophication is also the same.

6. CONCLUSIONS

The findings of this study lead to the following conclusions.

• PET plastic waste can be utilized completely instead of cement or bitumen as a binding agent offers a sustainable solution for the construction industry.

• Mix ratio P36,S28,Cr36 claimed the highest compressive strength of 43 MPa and also with a split tensile strength of 6.45 MPa, making it suitable for light traffic applications per ASTM C902.

• Other mix ratios also showed good results in terms of compressive and split tensile strength. These paver blocks can be used for pedestrians, kerb stones etc. as per ASTM C902.

• Mix ratio P36,S28,Cr36 exhibits lowest rate of water absorption i.e., 1.70% which is within the limits of ASTM C936 and can be used in water logged areas.

• The plastic paver block also showed temperature resistance at 100 ^oC with no change in dimensions and the weight loss is less than 1%, the paver block can be confidently utilized in the areas where the temperature may rise up to 100 ^oC.

• The regression analysis also showed the validity of experimental results which means the results are satisfied and the average difference found to be 4-5% only which is within the acceptable limits.

• SEM images of plastic paver block showed the tightly packing of particles results in less number of pores due to which water absorption is less and mechanical strength is higher.

• The cost of plastic paver block is PKR 18 per specimen and PKR 63 per square feet which is 57% economical than the conventional cement-based paver. This shows that the overall construction cost with the plastic paver blocks also reduces significantly.

7. LIMITATIONS

The durability aspect of these pavers may be increased by abrasion and freeze-thaw test to use them in diverse climatic conditions for widespread adaptability which is not monitored in this study due to limited scope.

8. FUTURE SCOPE

In this research, manual mixing and compaction was performed. With the inclusion of mechanical based mixing and compaction, the usage of these paver blocks will be applicable to heavy traffic areas also. Moreover, the creep and softening behavior are also important phenomena to study long-term thermal stability assessment.

9. ACKNOWLEDGMENTS

The research is supported by Higher Education Commission Pakistan through National Research Grant Program for Universities (NRPU-16783).

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Publication Dates

• Publication in this collection
  17 Oct 2025
• Date of issue
  2025

History

• Received
  17 Mar 2025
• Accepted
  16 Sept 2025