# Combined Performance of Polyethylene Terephthalate Waste Plastic Polymer and Crumb Rubber in Modifying Properties of Hot Mix Asphalt

Authors: Getnet Mekuria Alemu; Damtew Tsige Melese; Teyba Wedajo Mahdi; Adamu Beyene Negesa
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

Nowadays, the use of recycled waste materials in road pavement is regarded not only as a positive option in terms of sustainability but also as an appealing option in terms of providing improved service performance. The current study aimed at evaluating the performance of crumb rubber and polyethylene terephthalate plastic polymer in asphalt mixture in modifying the mechanical properties of asphalt pavement. Experimental tests were carried out both for asphalt binder and asphalt mixture. Different proportions of mix for crumb rubber and polyethylene terephthalate plastic polymer were used to systematically investigate the effect of mix ratio on performance of the asphalt material. The experimental analysis reveals that the combined application of 10% by weight of crumb rubber chips and 2% polyethylene terephthalate plastic polymer are the ideal mix ratios found effective in modifying properties of the asphalt mixture. The asphalt binder test results indicate that adding 10% crumb rubber to asphalt binder reduced penetration by 1.56% along with increment of the softening point by 4.33%. Furthermore, the indicated optimum mix amount resulted in 0.17% rise, 20.07% drop, and 20.71% increase in Marshal stability, flow, and stiffness, respectively. Besides, tensile strength of the asphalt mixture was enhanced with addition of the filler and binder materials. It was witnessed that the combined application of the additives performs better than their separate use in modifying properties of the asphalt mix.


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## Body

## 1. Introduction

Flexible pavements have a bitumen coating as the top most layer and other built-in layers made of earth materials [1, 2]. The highest stress intensity occurs at the top layers and relatively low stress is expected at lower layers. Hence, the pavement materials, especially asphalt mixture, have to be of superior strength [3–8]. Asphalt mixture is a heterogeneous material composed of asphalt binder, aggregates, and air voids. Crushed aggregates usually account for 85% of the total volume of the mix in approximate, and asphalt binder constitutes around 10% and the remaining part is air void [9–11].Because of extensive variation in climatic conditions, properties of construction materials, terrains, traffic volumes, and loading, the asphalt layer fail to withstand the pavement defects such as rutting, fatigue cracking, deformation potholing, and wear and tear. Hence, the asphalt binder and asphalt mixture ought to be improved to attain the safest requirement of flexible pavement via addition of polymer additives [6–8, 12]. As stated by Naskar et al. [13], polymer-made additives are proved effective in many aspects for modifying properties of the bitumen material in road pavement. This further contributes to increase in life span of the road and operation life. The most commonly practiced means of improving quality of the asphalt binder and asphalt mixture is via mitigating the rheological characteristics of binders and the asphalt mix. This can be executed by blending them with polymers like rubber and plastic. Plastic is a widely used material in modifying mechanical properties of the asphalt pavement [14–19]. According to Scott and Mashaan et al. [20, 21], huge amount of tyres and plastics produced around the world are the main causes of environmental pollution and other environment-related problems. In relation to this, the production of tyres is rising in Ethiopia particularly in the metropolis with accelerated demand for vehicles since recent years. As the demand for automobiles grows, so does the demand for tyres, resulting in an increase in tyre waste. Tyre debris is commonly discarded in the cities with no regard to the environment, and there are only a few recycling alternatives available in the country, including reuse, open-air burning, material recycling, and rethreading [22–24]. Many old tyres contain metallic substances and chemicals, which likely leach into the environment up on disintegration. Considerable amount of these chemicals are recognized to be carcinogenic and mutagenic [25]. The soil and environment around such old tyres are unconditionally subjected to contamination due to the direct contact with these harmful chemicals which are disposed without special precaution. In addition, the adverse impact of the tyre waste on water is not underestimated as it may cause massive groundwater pollution. With regard to plastic materials, of the massive number of plastic bottles consumed in the country, majority of them are not recycled [26, 27].Using crumb rubber as a modifier increases resistance of pavement to rutting and its overall performance in extreme climatic conditions under heavy traffic. Addition of crumb rubber into the asphalt binder increases the viscosity that higher mixing and compaction temperatures are required to meet the specifications [28–30]. The penetration and ductility of the bitumen modified with recycled plastic polymer chips decrease as the content of the plastic additives increases. This contributes to the expected substantial increase in life of pavement surfacing in comparison to using ordinary bitumen [31–37].Nobinur Rahman et al. [33] reported that waste polyethylene modifier up to 10% can be used in warmer areas from the view point of improving stability and void characteristics. Besides, Awwad and Shbeeb [38] and Casey et al. [39] investigated the modifying performance of low- and high-density polyethylene chips when added to asphalt bitumen. According to Awwad and Shbeeb [38], 12% by weight of high-density polyethylene is required to effectively modify rheological properties of the asphalt mix, whereas Casey et al. [39] proposed that only 4% is a sufficient amount. In relation to this, [35, 40] proposed that 6% and 4% of polyethylene terephthalate (PET) are the optimum amounts which best modify Marshal stability of the asphalt mix, respectively. As reported by Khan et al. [41], high-density polyethylene performs better than low-density polyethylene materials of the same amount in modifying stability and penetration resistance of the asphalt mix.One of the challenges faced in evaluating performance of pavement under traffic load is that it is too difficult to accurately solve problems exhibiting different loading conditions via analytical means [42]. Hence, the analytical equations merely used to predict the performance of pavement materials remains inaccurate. More specifically, in the case when properties of pavement materials change over the course of time and due to environmental factors, it is not convenient to decide on the performance of pavement materials only with the help of empirical methods [27]. In such a case, it is better to use the experimental approach to explore the critical effect of mechanical properties of the asphalt material on its overall performance. In relation to this, Gong et al. [36] pointed out that empirical approaches are less powerful than experimental methods due to limitation of a certain set of intrinsic material properties.Many studies have been conducted on applicability of crumb rubber and polyethylene terephthalate polymers in modifying asphalt properties. However, no common ground exists especially on the optimum amount of additives used for substantially modifying properties of the asphalt mix. Hence, the current study investigates the effect of polyethylene terephthalate plastic polymer as filler and crumb rubber on the overall performance of the asphalt mix. Accordingly, the optimum dosage of the additives to be used for modification is determined.


## 2. Materials and Methods

### 2.1. Materials Used

In the current study, four different materials were used to conduct the experimental analysis. These include bitumen, aggregate, crumb rubber, and waste polyethylene terephthalate plastic polymer (PET). Bitumen is a sticky, black and highly viscous liquid or semisolid, in some natural deposits. It is also the byproduct of fractional distillation of crude petroleum [43]. Accordingly, the asphalt binder with a grade of 80/100 was used. It was sourced from the hot mix plant located in Akaki Kality Sub-City, Addis Ababa, Ethiopia.Aggregate constitutes the granular part in the bituminous asphalt mixture that accounts for the largest percentage of the mix by weight. It is also the main source of load-bearing and strength characteristics of the mixture [44–51]. The physical properties of the aggregate used in the mixture were determined as per American Association of State Highway and Transportation Officials (AASHTO) and American Society for Testing and Materials (ASTM) specifications. The aggregate used was strong, tough, durable, and can be crushed into bulky particles without many flaky particles. In addition to the gradation requirements, the asphalt aggregate is also required to be strong enough to support and transmit the applied traffic loads [47–54]. Using low-strength aggregate material adversely impact the pavement performance. This might be in the form of improper gradation or poor abrasion resistance [55–58]. The use of marginal or substandard crushed aggregate for asphalt mix impairs functional significance of the pavement [57–64].Crumb rubber is one of the additives used for bitumen modification. It was obtained from locally available reclaimed passenger car tyres. Crumb rubber was prepared in Horizon Addis Tyre Company located in Addis Ababa, Ethiopia. Polymers most often used in modifying bitumen are categorized into two groups as elastomers and plastomers. Crumb rubber is one of elastomers’ polymer family which can possibly be stretched like a rubber band and easily recover its shape on releasing stretching force. This is a typical quality of elastomers that they can better resist permanent rutting in the case of asphalt [65].Polyethylene terephthalate (PET) is the chemical name for polyester and is categorized under thermoplastics. In the current study, polyethylene terephthalate plastic bottle chips were used as fillers in the asphalt mix. The material was collected from waste disposal sites. After cleaning by washing and drying, the filler material was prepared with the help of a mechanical grinder machine. Plastomers give high initial strength to the asphalt mixture to resist heavy loads [38]. Similarly, virgin PET is a superior material for some applications. It has various great features which make it superior to be used as a replacement material in the pavement. Some of its good qualities include good tensile strength, adequate thermal stability, chemical resistance, and processing capability [66]. Besides, it can be formed at normal temperatures and pressures [67, 68].


### 2.2. Sample Preparation

Crushed basalt was used for preparation of the aggregate material. The aggregate material considered is of well graded type which proportionally contains coarse, fine, and filler dimensions. The aggregate material used was free from dust, clay, plant leaf, organic matters, and other impurities. The fine aggregate has a size ranging from 0.075 mm to 1 mm. Filler in the mix fills up the voids left between the coarse and fine aggregates. It plays a significant role especially in improving density and enhances strength of the mix. Suitability of the filler material is influenced by fineness of the particles [47–51, 69].The crumb rubber was produced by reducing scrap tyres until it passes 0.075 mm sieve size. First, the tyre was shredded and chipped using large machinery with crack mill of mechanical blades to obtain the required size of rubber shreds. Similarly, the reinforcing steel wires of the tyre material were removed by using magnets. To meet the gradation requirement, the crumb rubber was sieved and separated into categories. According to Qian et al. [65], crumb rubber particles with fine size are more beneficial to low-temperature cracking resistance and overall stability. Yigezu Tefera et al. [70] evaluated the effect of rubber modified bitumen on asphalt performance.Polyethylene terephthalate plastic wastes were mainly collected from roadsides and waste-disposal sites. The waste plastic bottles were washed to remove impurities; then, the clean PET plastic bottles were fed into mechanical shredders to prepare finer chips having particle size of less than 0.075 mm. Taherkhani and Arshadi [71] studied the effect of course and fine-graded PET on the asphalt mixture. The obtained findings reveal that the mixture containing fine-graded PET particles performs better than the mixture containing coarse-graded PET particles in improving resistance of the asphalt pavement to permanent deformation [71].


### 2.3. Experimental Design

In the current study, four dosages (5%, 10%, 15%, and 20%) of crumb rubber were used for the experimental analysis. The asphalt mixed with 15% crumb rubber achieved higher stability over the other dosages of crumb rubber. Similar range of crumb rubber dosage was considered to identify the well-performing mix ratio in modifying the asphalt bitumen. Similarly, comparison was made between the performances of 10% and 15% by weight dosages of crumb rubber.In this study, PET chips are used as filler materials. The effect of filler material on the asphalt mix was investigated by considering crumb rubber modified bitumen. In the current study, dosage of PET has varied in proportions of 0%, 2%, 4%, and 6% by weight of the filler material. According to ASTM, a minimum of three specimens are mandatorily required for the Marshall mix design to come up with convincing results. Fifteen samples were prepared (3 samples for each percentage of bitumen) for determination of the optimum bitumen content (OBC) with varying percentages of bitumen (4%, 4.5%, 5%, 5.5%, and 6%) without polymer modification. Conventionally, Marshall samples of 101.6 mm diameter and 63.5 mm height were used to calculate the density, quantity of air voids, Marshall stability, and flow and moisture damage by indirect tensile strength (ITS). The mixing process was carried out at a temperature of 150°C and five uncompacted asphalt mixture samples as per the number of the asphalt binder content were prepared for the maximum theoretical density test.So as to investigate the combined effect of crumb rubber and PET polymer, 12 samples of the modified asphalt mixture were prepared. The optimum PET polymer content (OPC) in the crumb rubber modified asphalt mixture was determined using three samples for each percentage of PET (0%, 2%, 4%, and 6%). Six compacted Marshal samples were used to determine moisture sustainability for the mix containing neat bitumen by using the optimum binder content (three samples for wet condition and the other three for dry condition). And a total of twenty-four compacted Marshal samples were used to determine moisture sustainability of the modified asphalt mixture using the three samples for each percentage of PET (0%, 2%, 4%, and 6%). The first twelve samples for wet conditions and the remaining twelve were used for dry conditions.


### 2.4. The Experimental Tests and Standards

#### 2.4.1. Tests and Standards for Aggregates

Sieve analysis was used to determine the particle size distribution of the course, fine aggregates and filler materials were used in the asphalt mix. The gradation test was performed in compliance with IS: 2386 Part 1 (1963) where a known weight of the aggregate (1 kg) is preferably poured into a well-arranged set of test sieves. The specific gravity of each of the aggregates was carried out using the pycnometer method in compliance with ASTM 1429. The conventional bulk density and moisture content determination tests were also performed. The water absorption test was carried out on natural aggregates in compliance with AASHTO T85 and IS: 2386 (Part III) 1963. Aggregate crushing value (ACV) is the mass of the material, expressed as a percentage of the test sample which is crushed into less than 2.30 mm under a compressive load of 400 kN. The test was carried out on natural aggregate under IS 2586 and in compliance with BS 487 (2009) and BS EN 932-1 (1999). An aggregate impact value (AIV) test was also carried out on natural aggregates under BS: 812 Part 112. The AIV is the percentage of fines produced from the aggregate sample after subjecting it to a standard amount of known weight, height, and a prescribed number of times. This test simulates the resistance to the impact of aggregates.


#### 2.4.2. Tests and Standards for Bitumen

Before preparing the modified asphalt mixture, the contribution of a single crumb rubber additive to bitumen property was evaluated. The asphalt binder with a penetration grade of 80/100 bitumen was heated to 160°C until it becomes a free-flowing semiviscous liquid melted. Using the wet process, the crumb rubber powder polymer was added to hot liquid bitumen. In this study, two proportions (10% and 15%) of crumb rubber by weight of asphalt binder were used to modify the asphalt binder. During the swelling process (at 160°C for 1 h), the crumb rubber polymer agents can adequately contact with the hot bitumen to make the agents expand and soft. The modified asphalt binder was evaluated using quality tests. The ductility of the asphalt binder was measured by the distance in mm to which a sample of bitumen will elongate before breaking when a standard briquette specimen of the material is pulled apart at a specified speed and a specified temperature of 25°C ± 0.5 as regulated by the ductility machine. The ring and ball tests were performed based on the ASTM D36 standard to evaluate the softening point of bitumen before and after modification. It measures the susceptibility of the blown asphalt binder to temperature changes. The penetration test was carried out as per the recommendation of ASTM D5 with the view of measuring the influence of modification of bitumen hardening. The penetration index test measures the distance in tenths of a millimeter, which a standard needle that would penetrate vertically into a sample/binder under standard conditions of temperature, load, and time. The average of three penetration readings was taken as the representative value.


#### 2.4.3. Tests and Specifications for Asphalt Mixture

The Marshall mixture design technique was used to calculate the optimum binder content (OBC) for a conventional asphalt mixture without polymer modification. In this method, the resistance of plastic deformation of a compacted cylindrical specimen of the asphalt mixture was measured when the specimen is loaded dramatically at a deformation rate of 50 mm/min [72]. Fabrication of cylindrical specimens for testing was done using the Marshall method, which followed the ASTM D1559 standard. The heated aggregate and binder were mixed for 5 minutes to ensure that the aggregate and binder were thoroughly combined. The mixture was poured into the mould, and each side was compacted with 75 blows. After 24 hours, the compacted specimens were taken from the moulds and stored until they were used in tests. Five samples of the uncompacted mixture were prepared for the determination of the maximum theoretical density. The bulk density was computed using the water volume displaced after samples are dry-weighted and subsequently saturated underwater. Following that, samples were conditioned at 60°C for 45 minutes for the Marshall test, as per the ASTM D1559 specification. The test samples were then put through their spaces in the Marshall press at a displacement rate of 50.8 mm/min until the maximum load is reached.Using the asphalt institution method, the mean of the bitumen content corresponding to maximum stability, maximum density, 4% air voids was computed as the optimal bitumen content (OBC). The modified asphalt mixture design included both dry and wet process methods. The heated aggregate and the modified asphalt binder with the governed crumb rubber content were mixed for 5 minutes, after which the required amount of PET particles was added to the mixture as a partial replacement for natural stone aggregate using a dry process and thoroughly mixed for 2 minutes until the aggregate and PET particles were completely coated with modified asphalt binder. This method of mixing ensures that PET remains semicrystalline and that only minor changes in the characteristics and structure of PET particles occur. The glass transition temperature for PET is roughly 70°C. The amorphous part of PET was melted at the mixing temperature, increasing binder cohesion, while the crystalline part does not change. The voids between aggregate particles are filled with the crystalline part of PET, increasing mixture stiffness.In this investigation, four proportions of PET (0%, 2%, 4%, and 6%) polymer by weight of the naturally crushed stone filler material were chosen to be investigated for preparation of the polymer-modified hot mix asphalt (HMA) samples. For adjusted combinations, the total percentage of binder (OBC) in the mix remained unchanged (5.5%).The indirect tensile strength (ITS) is basically used to determine the tensile properties of a bituminous mixture whose results are connected to the rutting and cracking qualities of the asphalt pavement [73]. The tensile characteristics of a bituminous mixture are evaluated by loading the Marshall specimen along with a diametric plane with a compressive load at a constant rate acting parallel to and along the vertical diametric plane of the specimen through two opposite loading strips. Marshall asphalt specimens with a thickness of 63.5 ± 1.3 mm and a diameter of 101.6 mm were manufactured for each category. The static indirect tensile strength of a specimen is determined using the procedure outlined in ASTM D 6931.According to the ASTM specification, the three samples available for each combination were divided into two groups for conditioning, one wet condition and the other dry condition with optimum asphalt binder content. Samples under wet conditioning underwent a period of hydrostatic vacuum with 50 mm Hg absolute pressure for 30 min, then the samples were introduced to a water bath at 40°C for 24 h and finally the samples were kept for two hours in a water bath at a temperature of 15°C before the test. Cylindrical specimens were inserted in two loading strips and subjected to a consistent compressive force at a speed of 50 mm/min till failure. The vertical compressive load was changed into homogeneous horizontal tensile tension in this method. Following conditioning, all samples were subjected to indirect tensile strength testing at a displacement rate of 50.8 mm/mi until the maximum load is reached was used to get the ITS value using the maximum applied load. The ITS test is a performance test that is often used to evaluate the moisture susceptibility of a bituminous mixture. The tensile strength ratio is a measure of water sensitivity. It is the ratio of the tensile strength of the water condition specimen (ITS wet) to the tensile strength of the unconditioned specimen (ITS dry). ITS is calculated using the following equation:(1)ITS=2Pπ∗t∗D,where ITS is the indirect tensile strength (MPa), P is the maximum applied load (N), t is the specimen thickness (mm), and D is diameter of the specimen (mm).


### 2.5. Material Quality Requirement

In addition to the physical properties, the chemical analysis was also undertaken for crumb rubber to identify its chemical constituents. X-ray diffraction approach was employed to chemically characterize the crumb rubber. The conducted XRD test reveals that the crumb rubber has less component of SiO2 which is less than 30%, and the percentage by weight of other oxides except Al2O3 and Fe2O3 is less than 1% (Table 1). The analysis report shows that the crumb rubber has a specific gravity of 1.15 g/cm³ and the moisture by weight was 0.54%. ASTM D6114 recommends crumb rubber having moisture less than 0.75% and specific gravity in the range of 1.15 ± 0.05.Table 1
Chemical composition of crumb rubber.
OxidesContent (%)SiO212.5Al₂O32.88Fe₂O36.54CaO<0.01MgO<0.01Na2O<0.01K2O<0.01MnO<0.06H2O0.54From the aggregate crushing test, the percentage grains less than 2.36 mm sieve size was 14.82%, which is below 25 as per British Standard Manual. The obtained result indicated that the sampled aggregate is strong enough to resist crushing under a gradually applied compressive load and withstand crushing under roller and traffic. The aggregate impact value test result indicated that the amount of the material finer than 2.36 mm was 10.33%, which is below 25 (Table2). Resistance to sudden shock of the sampled aggregate material has to be strong enough. The Los Angeles abrasion (LAA) test was performed for the coarse aggregate. The obtained percentage of the loss was far below the maximum value. Therefore, the sampled aggregate has good mechanical properties to resist crushing, degradation, and disintegration in order to produce a high quality of the hot mix asphalt.Table 2
Properties of aggregate materials.
Test typeTest methodTest resultSpecification19–25 mm6.3–19 mm3–6.3 mm0–3 mmACVBS 812, part 3 (1985)—14.82%——<25AIVBS 812, part 3 (1985)—10.33%——<25LAAASTM C131 and C535—17.84%——<30Water absorptionBS 812, part 2 (1975)1.81.71.71.8<2Bulk dry specific gravityAASHTO T85—912.562.572.582.64—Bulk SSD specific gravity2.612.622.622.68—Apparent specific gravity2.692.692.72.76—Besides, the specific gravity and water absorption test for both coarse and fine aggregates were conducted. To have well-performing asphalt material, the absorption capacity of the aggregate material has to be low as water absorbent materials are lower than the performance of the pavement. The test result indicates that the aggregate materials have less absorption of water. Hence, the good strength bond between the aggregate and asphalt binder develops. In addition, the specific gravity of the fine aggregate was found to be higher than that of the coarse aggregate.


## 2.1. Materials Used

In the current study, four different materials were used to conduct the experimental analysis. These include bitumen, aggregate, crumb rubber, and waste polyethylene terephthalate plastic polymer (PET). Bitumen is a sticky, black and highly viscous liquid or semisolid, in some natural deposits. It is also the byproduct of fractional distillation of crude petroleum [43]. Accordingly, the asphalt binder with a grade of 80/100 was used. It was sourced from the hot mix plant located in Akaki Kality Sub-City, Addis Ababa, Ethiopia.Aggregate constitutes the granular part in the bituminous asphalt mixture that accounts for the largest percentage of the mix by weight. It is also the main source of load-bearing and strength characteristics of the mixture [44–51]. The physical properties of the aggregate used in the mixture were determined as per American Association of State Highway and Transportation Officials (AASHTO) and American Society for Testing and Materials (ASTM) specifications. The aggregate used was strong, tough, durable, and can be crushed into bulky particles without many flaky particles. In addition to the gradation requirements, the asphalt aggregate is also required to be strong enough to support and transmit the applied traffic loads [47–54]. Using low-strength aggregate material adversely impact the pavement performance. This might be in the form of improper gradation or poor abrasion resistance [55–58]. The use of marginal or substandard crushed aggregate for asphalt mix impairs functional significance of the pavement [57–64].Crumb rubber is one of the additives used for bitumen modification. It was obtained from locally available reclaimed passenger car tyres. Crumb rubber was prepared in Horizon Addis Tyre Company located in Addis Ababa, Ethiopia. Polymers most often used in modifying bitumen are categorized into two groups as elastomers and plastomers. Crumb rubber is one of elastomers’ polymer family which can possibly be stretched like a rubber band and easily recover its shape on releasing stretching force. This is a typical quality of elastomers that they can better resist permanent rutting in the case of asphalt [65].Polyethylene terephthalate (PET) is the chemical name for polyester and is categorized under thermoplastics. In the current study, polyethylene terephthalate plastic bottle chips were used as fillers in the asphalt mix. The material was collected from waste disposal sites. After cleaning by washing and drying, the filler material was prepared with the help of a mechanical grinder machine. Plastomers give high initial strength to the asphalt mixture to resist heavy loads [38]. Similarly, virgin PET is a superior material for some applications. It has various great features which make it superior to be used as a replacement material in the pavement. Some of its good qualities include good tensile strength, adequate thermal stability, chemical resistance, and processing capability [66]. Besides, it can be formed at normal temperatures and pressures [67, 68].


## 2.2. Sample Preparation

Crushed basalt was used for preparation of the aggregate material. The aggregate material considered is of well graded type which proportionally contains coarse, fine, and filler dimensions. The aggregate material used was free from dust, clay, plant leaf, organic matters, and other impurities. The fine aggregate has a size ranging from 0.075 mm to 1 mm. Filler in the mix fills up the voids left between the coarse and fine aggregates. It plays a significant role especially in improving density and enhances strength of the mix. Suitability of the filler material is influenced by fineness of the particles [47–51, 69].The crumb rubber was produced by reducing scrap tyres until it passes 0.075 mm sieve size. First, the tyre was shredded and chipped using large machinery with crack mill of mechanical blades to obtain the required size of rubber shreds. Similarly, the reinforcing steel wires of the tyre material were removed by using magnets. To meet the gradation requirement, the crumb rubber was sieved and separated into categories. According to Qian et al. [65], crumb rubber particles with fine size are more beneficial to low-temperature cracking resistance and overall stability. Yigezu Tefera et al. [70] evaluated the effect of rubber modified bitumen on asphalt performance.Polyethylene terephthalate plastic wastes were mainly collected from roadsides and waste-disposal sites. The waste plastic bottles were washed to remove impurities; then, the clean PET plastic bottles were fed into mechanical shredders to prepare finer chips having particle size of less than 0.075 mm. Taherkhani and Arshadi [71] studied the effect of course and fine-graded PET on the asphalt mixture. The obtained findings reveal that the mixture containing fine-graded PET particles performs better than the mixture containing coarse-graded PET particles in improving resistance of the asphalt pavement to permanent deformation [71].


## 2.3. Experimental Design

In the current study, four dosages (5%, 10%, 15%, and 20%) of crumb rubber were used for the experimental analysis. The asphalt mixed with 15% crumb rubber achieved higher stability over the other dosages of crumb rubber. Similar range of crumb rubber dosage was considered to identify the well-performing mix ratio in modifying the asphalt bitumen. Similarly, comparison was made between the performances of 10% and 15% by weight dosages of crumb rubber.In this study, PET chips are used as filler materials. The effect of filler material on the asphalt mix was investigated by considering crumb rubber modified bitumen. In the current study, dosage of PET has varied in proportions of 0%, 2%, 4%, and 6% by weight of the filler material. According to ASTM, a minimum of three specimens are mandatorily required for the Marshall mix design to come up with convincing results. Fifteen samples were prepared (3 samples for each percentage of bitumen) for determination of the optimum bitumen content (OBC) with varying percentages of bitumen (4%, 4.5%, 5%, 5.5%, and 6%) without polymer modification. Conventionally, Marshall samples of 101.6 mm diameter and 63.5 mm height were used to calculate the density, quantity of air voids, Marshall stability, and flow and moisture damage by indirect tensile strength (ITS). The mixing process was carried out at a temperature of 150°C and five uncompacted asphalt mixture samples as per the number of the asphalt binder content were prepared for the maximum theoretical density test.So as to investigate the combined effect of crumb rubber and PET polymer, 12 samples of the modified asphalt mixture were prepared. The optimum PET polymer content (OPC) in the crumb rubber modified asphalt mixture was determined using three samples for each percentage of PET (0%, 2%, 4%, and 6%). Six compacted Marshal samples were used to determine moisture sustainability for the mix containing neat bitumen by using the optimum binder content (three samples for wet condition and the other three for dry condition). And a total of twenty-four compacted Marshal samples were used to determine moisture sustainability of the modified asphalt mixture using the three samples for each percentage of PET (0%, 2%, 4%, and 6%). The first twelve samples for wet conditions and the remaining twelve were used for dry conditions.


## 2.4. The Experimental Tests and Standards

### 2.4.1. Tests and Standards for Aggregates

Sieve analysis was used to determine the particle size distribution of the course, fine aggregates and filler materials were used in the asphalt mix. The gradation test was performed in compliance with IS: 2386 Part 1 (1963) where a known weight of the aggregate (1 kg) is preferably poured into a well-arranged set of test sieves. The specific gravity of each of the aggregates was carried out using the pycnometer method in compliance with ASTM 1429. The conventional bulk density and moisture content determination tests were also performed. The water absorption test was carried out on natural aggregates in compliance with AASHTO T85 and IS: 2386 (Part III) 1963. Aggregate crushing value (ACV) is the mass of the material, expressed as a percentage of the test sample which is crushed into less than 2.30 mm under a compressive load of 400 kN. The test was carried out on natural aggregate under IS 2586 and in compliance with BS 487 (2009) and BS EN 932-1 (1999). An aggregate impact value (AIV) test was also carried out on natural aggregates under BS: 812 Part 112. The AIV is the percentage of fines produced from the aggregate sample after subjecting it to a standard amount of known weight, height, and a prescribed number of times. This test simulates the resistance to the impact of aggregates.


### 2.4.2. Tests and Standards for Bitumen

Before preparing the modified asphalt mixture, the contribution of a single crumb rubber additive to bitumen property was evaluated. The asphalt binder with a penetration grade of 80/100 bitumen was heated to 160°C until it becomes a free-flowing semiviscous liquid melted. Using the wet process, the crumb rubber powder polymer was added to hot liquid bitumen. In this study, two proportions (10% and 15%) of crumb rubber by weight of asphalt binder were used to modify the asphalt binder. During the swelling process (at 160°C for 1 h), the crumb rubber polymer agents can adequately contact with the hot bitumen to make the agents expand and soft. The modified asphalt binder was evaluated using quality tests. The ductility of the asphalt binder was measured by the distance in mm to which a sample of bitumen will elongate before breaking when a standard briquette specimen of the material is pulled apart at a specified speed and a specified temperature of 25°C ± 0.5 as regulated by the ductility machine. The ring and ball tests were performed based on the ASTM D36 standard to evaluate the softening point of bitumen before and after modification. It measures the susceptibility of the blown asphalt binder to temperature changes. The penetration test was carried out as per the recommendation of ASTM D5 with the view of measuring the influence of modification of bitumen hardening. The penetration index test measures the distance in tenths of a millimeter, which a standard needle that would penetrate vertically into a sample/binder under standard conditions of temperature, load, and time. The average of three penetration readings was taken as the representative value.


### 2.4.3. Tests and Specifications for Asphalt Mixture

The Marshall mixture design technique was used to calculate the optimum binder content (OBC) for a conventional asphalt mixture without polymer modification. In this method, the resistance of plastic deformation of a compacted cylindrical specimen of the asphalt mixture was measured when the specimen is loaded dramatically at a deformation rate of 50 mm/min [72]. Fabrication of cylindrical specimens for testing was done using the Marshall method, which followed the ASTM D1559 standard. The heated aggregate and binder were mixed for 5 minutes to ensure that the aggregate and binder were thoroughly combined. The mixture was poured into the mould, and each side was compacted with 75 blows. After 24 hours, the compacted specimens were taken from the moulds and stored until they were used in tests. Five samples of the uncompacted mixture were prepared for the determination of the maximum theoretical density. The bulk density was computed using the water volume displaced after samples are dry-weighted and subsequently saturated underwater. Following that, samples were conditioned at 60°C for 45 minutes for the Marshall test, as per the ASTM D1559 specification. The test samples were then put through their spaces in the Marshall press at a displacement rate of 50.8 mm/min until the maximum load is reached.Using the asphalt institution method, the mean of the bitumen content corresponding to maximum stability, maximum density, 4% air voids was computed as the optimal bitumen content (OBC). The modified asphalt mixture design included both dry and wet process methods. The heated aggregate and the modified asphalt binder with the governed crumb rubber content were mixed for 5 minutes, after which the required amount of PET particles was added to the mixture as a partial replacement for natural stone aggregate using a dry process and thoroughly mixed for 2 minutes until the aggregate and PET particles were completely coated with modified asphalt binder. This method of mixing ensures that PET remains semicrystalline and that only minor changes in the characteristics and structure of PET particles occur. The glass transition temperature for PET is roughly 70°C. The amorphous part of PET was melted at the mixing temperature, increasing binder cohesion, while the crystalline part does not change. The voids between aggregate particles are filled with the crystalline part of PET, increasing mixture stiffness.In this investigation, four proportions of PET (0%, 2%, 4%, and 6%) polymer by weight of the naturally crushed stone filler material were chosen to be investigated for preparation of the polymer-modified hot mix asphalt (HMA) samples. For adjusted combinations, the total percentage of binder (OBC) in the mix remained unchanged (5.5%).The indirect tensile strength (ITS) is basically used to determine the tensile properties of a bituminous mixture whose results are connected to the rutting and cracking qualities of the asphalt pavement [73]. The tensile characteristics of a bituminous mixture are evaluated by loading the Marshall specimen along with a diametric plane with a compressive load at a constant rate acting parallel to and along the vertical diametric plane of the specimen through two opposite loading strips. Marshall asphalt specimens with a thickness of 63.5 ± 1.3 mm and a diameter of 101.6 mm were manufactured for each category. The static indirect tensile strength of a specimen is determined using the procedure outlined in ASTM D 6931.According to the ASTM specification, the three samples available for each combination were divided into two groups for conditioning, one wet condition and the other dry condition with optimum asphalt binder content. Samples under wet conditioning underwent a period of hydrostatic vacuum with 50 mm Hg absolute pressure for 30 min, then the samples were introduced to a water bath at 40°C for 24 h and finally the samples were kept for two hours in a water bath at a temperature of 15°C before the test. Cylindrical specimens were inserted in two loading strips and subjected to a consistent compressive force at a speed of 50 mm/min till failure. The vertical compressive load was changed into homogeneous horizontal tensile tension in this method. Following conditioning, all samples were subjected to indirect tensile strength testing at a displacement rate of 50.8 mm/mi until the maximum load is reached was used to get the ITS value using the maximum applied load. The ITS test is a performance test that is often used to evaluate the moisture susceptibility of a bituminous mixture. The tensile strength ratio is a measure of water sensitivity. It is the ratio of the tensile strength of the water condition specimen (ITS wet) to the tensile strength of the unconditioned specimen (ITS dry). ITS is calculated using the following equation:(1)ITS=2Pπ∗t∗D,where ITS is the indirect tensile strength (MPa), P is the maximum applied load (N), t is the specimen thickness (mm), and D is diameter of the specimen (mm).


## 2.4.1. Tests and Standards for Aggregates

Sieve analysis was used to determine the particle size distribution of the course, fine aggregates and filler materials were used in the asphalt mix. The gradation test was performed in compliance with IS: 2386 Part 1 (1963) where a known weight of the aggregate (1 kg) is preferably poured into a well-arranged set of test sieves. The specific gravity of each of the aggregates was carried out using the pycnometer method in compliance with ASTM 1429. The conventional bulk density and moisture content determination tests were also performed. The water absorption test was carried out on natural aggregates in compliance with AASHTO T85 and IS: 2386 (Part III) 1963. Aggregate crushing value (ACV) is the mass of the material, expressed as a percentage of the test sample which is crushed into less than 2.30 mm under a compressive load of 400 kN. The test was carried out on natural aggregate under IS 2586 and in compliance with BS 487 (2009) and BS EN 932-1 (1999). An aggregate impact value (AIV) test was also carried out on natural aggregates under BS: 812 Part 112. The AIV is the percentage of fines produced from the aggregate sample after subjecting it to a standard amount of known weight, height, and a prescribed number of times. This test simulates the resistance to the impact of aggregates.


## 2.4.2. Tests and Standards for Bitumen

Before preparing the modified asphalt mixture, the contribution of a single crumb rubber additive to bitumen property was evaluated. The asphalt binder with a penetration grade of 80/100 bitumen was heated to 160°C until it becomes a free-flowing semiviscous liquid melted. Using the wet process, the crumb rubber powder polymer was added to hot liquid bitumen. In this study, two proportions (10% and 15%) of crumb rubber by weight of asphalt binder were used to modify the asphalt binder. During the swelling process (at 160°C for 1 h), the crumb rubber polymer agents can adequately contact with the hot bitumen to make the agents expand and soft. The modified asphalt binder was evaluated using quality tests. The ductility of the asphalt binder was measured by the distance in mm to which a sample of bitumen will elongate before breaking when a standard briquette specimen of the material is pulled apart at a specified speed and a specified temperature of 25°C ± 0.5 as regulated by the ductility machine. The ring and ball tests were performed based on the ASTM D36 standard to evaluate the softening point of bitumen before and after modification. It measures the susceptibility of the blown asphalt binder to temperature changes. The penetration test was carried out as per the recommendation of ASTM D5 with the view of measuring the influence of modification of bitumen hardening. The penetration index test measures the distance in tenths of a millimeter, which a standard needle that would penetrate vertically into a sample/binder under standard conditions of temperature, load, and time. The average of three penetration readings was taken as the representative value.


## 2.4.3. Tests and Specifications for Asphalt Mixture

The Marshall mixture design technique was used to calculate the optimum binder content (OBC) for a conventional asphalt mixture without polymer modification. In this method, the resistance of plastic deformation of a compacted cylindrical specimen of the asphalt mixture was measured when the specimen is loaded dramatically at a deformation rate of 50 mm/min [72]. Fabrication of cylindrical specimens for testing was done using the Marshall method, which followed the ASTM D1559 standard. The heated aggregate and binder were mixed for 5 minutes to ensure that the aggregate and binder were thoroughly combined. The mixture was poured into the mould, and each side was compacted with 75 blows. After 24 hours, the compacted specimens were taken from the moulds and stored until they were used in tests. Five samples of the uncompacted mixture were prepared for the determination of the maximum theoretical density. The bulk density was computed using the water volume displaced after samples are dry-weighted and subsequently saturated underwater. Following that, samples were conditioned at 60°C for 45 minutes for the Marshall test, as per the ASTM D1559 specification. The test samples were then put through their spaces in the Marshall press at a displacement rate of 50.8 mm/min until the maximum load is reached.Using the asphalt institution method, the mean of the bitumen content corresponding to maximum stability, maximum density, 4% air voids was computed as the optimal bitumen content (OBC). The modified asphalt mixture design included both dry and wet process methods. The heated aggregate and the modified asphalt binder with the governed crumb rubber content were mixed for 5 minutes, after which the required amount of PET particles was added to the mixture as a partial replacement for natural stone aggregate using a dry process and thoroughly mixed for 2 minutes until the aggregate and PET particles were completely coated with modified asphalt binder. This method of mixing ensures that PET remains semicrystalline and that only minor changes in the characteristics and structure of PET particles occur. The glass transition temperature for PET is roughly 70°C. The amorphous part of PET was melted at the mixing temperature, increasing binder cohesion, while the crystalline part does not change. The voids between aggregate particles are filled with the crystalline part of PET, increasing mixture stiffness.In this investigation, four proportions of PET (0%, 2%, 4%, and 6%) polymer by weight of the naturally crushed stone filler material were chosen to be investigated for preparation of the polymer-modified hot mix asphalt (HMA) samples. For adjusted combinations, the total percentage of binder (OBC) in the mix remained unchanged (5.5%).The indirect tensile strength (ITS) is basically used to determine the tensile properties of a bituminous mixture whose results are connected to the rutting and cracking qualities of the asphalt pavement [73]. The tensile characteristics of a bituminous mixture are evaluated by loading the Marshall specimen along with a diametric plane with a compressive load at a constant rate acting parallel to and along the vertical diametric plane of the specimen through two opposite loading strips. Marshall asphalt specimens with a thickness of 63.5 ± 1.3 mm and a diameter of 101.6 mm were manufactured for each category. The static indirect tensile strength of a specimen is determined using the procedure outlined in ASTM D 6931.According to the ASTM specification, the three samples available for each combination were divided into two groups for conditioning, one wet condition and the other dry condition with optimum asphalt binder content. Samples under wet conditioning underwent a period of hydrostatic vacuum with 50 mm Hg absolute pressure for 30 min, then the samples were introduced to a water bath at 40°C for 24 h and finally the samples were kept for two hours in a water bath at a temperature of 15°C before the test. Cylindrical specimens were inserted in two loading strips and subjected to a consistent compressive force at a speed of 50 mm/min till failure. The vertical compressive load was changed into homogeneous horizontal tensile tension in this method. Following conditioning, all samples were subjected to indirect tensile strength testing at a displacement rate of 50.8 mm/mi until the maximum load is reached was used to get the ITS value using the maximum applied load. The ITS test is a performance test that is often used to evaluate the moisture susceptibility of a bituminous mixture. The tensile strength ratio is a measure of water sensitivity. It is the ratio of the tensile strength of the water condition specimen (ITS wet) to the tensile strength of the unconditioned specimen (ITS dry). ITS is calculated using the following equation:(1)ITS=2Pπ∗t∗D,where ITS is the indirect tensile strength (MPa), P is the maximum applied load (N), t is the specimen thickness (mm), and D is diameter of the specimen (mm).


## 2.5. Material Quality Requirement

In addition to the physical properties, the chemical analysis was also undertaken for crumb rubber to identify its chemical constituents. X-ray diffraction approach was employed to chemically characterize the crumb rubber. The conducted XRD test reveals that the crumb rubber has less component of SiO2 which is less than 30%, and the percentage by weight of other oxides except Al2O3 and Fe2O3 is less than 1% (Table 1). The analysis report shows that the crumb rubber has a specific gravity of 1.15 g/cm³ and the moisture by weight was 0.54%. ASTM D6114 recommends crumb rubber having moisture less than 0.75% and specific gravity in the range of 1.15 ± 0.05.Table 1
Chemical composition of crumb rubber.
OxidesContent (%)SiO212.5Al₂O32.88Fe₂O36.54CaO<0.01MgO<0.01Na2O<0.01K2O<0.01MnO<0.06H2O0.54From the aggregate crushing test, the percentage grains less than 2.36 mm sieve size was 14.82%, which is below 25 as per British Standard Manual. The obtained result indicated that the sampled aggregate is strong enough to resist crushing under a gradually applied compressive load and withstand crushing under roller and traffic. The aggregate impact value test result indicated that the amount of the material finer than 2.36 mm was 10.33%, which is below 25 (Table2). Resistance to sudden shock of the sampled aggregate material has to be strong enough. The Los Angeles abrasion (LAA) test was performed for the coarse aggregate. The obtained percentage of the loss was far below the maximum value. Therefore, the sampled aggregate has good mechanical properties to resist crushing, degradation, and disintegration in order to produce a high quality of the hot mix asphalt.Table 2
Properties of aggregate materials.
Test typeTest methodTest resultSpecification19–25 mm6.3–19 mm3–6.3 mm0–3 mmACVBS 812, part 3 (1985)—14.82%——<25AIVBS 812, part 3 (1985)—10.33%——<25LAAASTM C131 and C535—17.84%——<30Water absorptionBS 812, part 2 (1975)1.81.71.71.8<2Bulk dry specific gravityAASHTO T85—912.562.572.582.64—Bulk SSD specific gravity2.612.622.622.68—Apparent specific gravity2.692.692.72.76—Besides, the specific gravity and water absorption test for both coarse and fine aggregates were conducted. To have well-performing asphalt material, the absorption capacity of the aggregate material has to be low as water absorbent materials are lower than the performance of the pavement. The test result indicates that the aggregate materials have less absorption of water. Hence, the good strength bond between the aggregate and asphalt binder develops. In addition, the specific gravity of the fine aggregate was found to be higher than that of the coarse aggregate.


## 3. Results and Discussion

### 3.1. Effects of Crumb Rubber on Asphalt Binder Properties

When the crumb rubber particle is added into the hot mix asphalt binder using the wet process, the aromatic oils of the asphalt are absorbed into crumb rubber forming gel-like materials. The experimental analysis reveals that the softening point of the crumb rubber modified asphalt binder is directly proportional to the percentage of the crumb rubber content applied (Figure1(a)). When the percentage of crumb rubber increases from 10% to 15%, the resulting value of softening point rises by 4.01% and 2.25%, respectively. This could be due to the increase in resistance to temperature susceptibility of the bitumen with addition of crumb rubber as pointed out by Tefera et al. [70]. As indicated in Figure 1(b), the penetration value of the crumb rubber modified asphalt binder decreases with an increment in dosage of waste crumb rubber. The penetration value of the additive-free bitumen went increasing by 1.56% and 6% with addition of 10% and 15% of crumb rubber chips, respectively.Figure 1
Mechanical properties of the asphalt binder: (a) softening point, (b) penetration value, (c) ductility, and (d) specific gravity.
(a)(b)(c)(d)Compared to the 0% and 10% crumb rubber modified asphalt binders, the asphalt binder modified by addition of 15% has a lower penetration value. This implies that the modified binder with 15% crumb rubber is more brittle and harder than the binder modified through blending of low dosage of crumb rubber. It can hence be inferred that introduction of additives in large amount to the asphalt binder enhances its resistance to needle penetration. The asphalt binder modified with addition of significant amount of additives is more preferred in hot climate conditions. Ductility is a key parameter which is a measure of tensile behaviour of the bituminous binder or homogeneity of the binder. Ductility of the asphalt binder decreases as the percentage of waste crumb rubber increases (Figure1(c)). Less magnitude of ductility was witnessed in the 15% by weight of the crumb rubber than the 10%. Similarly, the 10% crumb rubber modified asphalt binder demonstrated lower ductility in comparison with the additive-free bitumen. Addition of crumb rubber to the asphalt binder has negative contribution to the magnitude of the specific gravity (Figure 1(d)). The specific gravity of the asphalt binder falls with an increase in the amount of crumb rubber.The obtained results of the experimental tests are consistent with the findings of Yigezu Tefera et al. [70] in which it was reported that an increase in the rubber content from 0 to 20% resulted in a sharp decrease in penetration from 68.5 to 58.5 mm. Similarly, ductility of the asphalt binder was lessened from 100 to 75 cm for the same percentage rise in the rubber content. This is a good indication for the convincing capability of the 10% and 15% crumb rubber to improve the properties of the asphalt binder.


### 3.2. Marshall

In order to determine the best optimum binder content, a total of 15 samples were used with five different percentages of bitumen (ranging from 4 to 6% with a 0.5% increment). The result for each mixture was obtained by averaging the result on three replicate specimens. The optimum asphalt content was calculated as the average asphalt content value that corresponds to the maximum stability, maximum unit weight, and 4% air voids (AV).Stability is the highest required load under which specimens fail when applied at a steady rate of 50 mm/min at 60°C. The relationship between stability and various bitumen contents are presented in Figure2(a). Stability of the asphalt mix sharply increases for the bitumen content below 5.5% and begins falling progressively right after. This indicates that resistance of the specimen against the imposed load increases until the optimum amount of the additive is reached. However, beyond the stated optimum amount, the specimen is forced to experience low stiffness due to rise in plasticity characteristics.Figure 2
Marshall test results for the conventional asphalt mixture: (a) stability, (b) flow, (c) bulk specific gravity, (d) voids filled with bitumen, (e) air voids, (f) voids in mineral aggregate, and (g) Marshall quotient.
(a)(b)(c)(d)(e)(f)(g)The total amount of deformation observed at maximum load is referred to as flow. It is measured from the start of loading to the point at which stability begins to fall by 0.25 mm. As observed from Figure2(b), direct relationship developed between the dosage of bitumen and expected magnitude of flow. The 6% bitumen content indicates a plastic mix that will experience deformation under traffic load, whereas the 4% bitumen content indicates a mix with higher voids than normal. As a result of the high void, the asphalt material may prematurely experience cracks under traffic load which in turn impairs its durability.The effect of the bitumen content on bulk density is illustrated in Figure2(c). With increase in the bitumen content up to 5.5%, bulk density of the asphalt mix increases, whereas progressive decline is witnessed after the peak (2.413 g/cm3).Bitumen is intentionally used to fill the existing air void in the asphalt mix. Since the air void is replaced with the bitumen material, it significantly declines as the dosage of the bitumen material rises (Figure2(e)). Moreover, the percentage of voids filled with bitumen in the asphalt mix increases as the bitumen content increases. Hence, the maximum air void (Va) is detected at the lowest bitumen percentage (4%).Voids filled with bitumen (VFB) is the percentage of voids in mineral aggregates filled with the bitumen material. The relationship between VFB and bitumen concentrations is illustrated in Figure2(d). The lowest VFB content was observed at the lowest bitumen content (4%) which suggests that there are fewer asphalt films around the aggregate particles and are more susceptible to moisture and influences of weather. VFB percent increases as the bitumen content increases due to increase in the proportion of voids filled with the bitumen material.Voids in mineral aggregates (VMA) is the percentage of void volume in the aggregates before adding bitumen or the sum of the percentage of voids filled with bitumen and percentage of air voids remaining in the asphalt mix after compaction. In order for aggregate particles to be covered with a sufficient asphalt film, a minimum VMA must be maintained in the mix. As a result, a long-lasting asphalt paving mixture is produced [25]. In Figure 2(f), VMA% results for different bitumen contents are presented.The design bitumen content of the mix is determined based on the results of the Marshall test. The National Asphalt Pavement Association (NAPA) suggests that the bitumen content corresponding to 4% air voids is chosen as the design bitumen content. The Marshall test result reveals that the binder content resulting in 4% air voids is 5.5%. Accordingly, the 5.5% dosage of bitumen is considered to be the optimum binder content. The Marshall quotient (MQ) was determined by dividing the Marshall stability by the flow, which is commonly used as a measure of resistance to permanent deformation. According to Lo Presti [25], the mixture with higher MQ is stiffer and more resistant to permanent deformation. The variation in magnitude of MQ for a mixture with neat bitumen is shown in Figure 2(g). Bitumen binder content of 5% by weight of bitumen has more resistance to permanent deformation than the other binder contents.


### 3.3. Effects of PET on Crumb Rubber Modified Asphalt Mix

To investigate the effect of PET as a filler material on performance of the asphalt mix, four percentages by weight of PET (0%, 2%, 4%, and 6%) were considered. Three Marshall specimens were tested for each percentage of PET. Accordingly, total of 12 specimens were tested at the optimum binder content (5.5%) to evaluate the effect of adding PET filler to crumb rubber modified asphalt mixture.Modified asphalt mixtures have a better level of stability than conventional asphalt mixtures. As can be seen, the Marshal stability of all asphalt mixtures containing 10% crumb rubber modified asphalt mixture with varying percentages of filler PET particles is higher than that of the mixture lacking crumb rubber and PET polymers. The highest Marshall stability was 12.38 kN and it is associated with the 2% filler PET. As observed from Figure3(a), stability of the modified asphalt mix abruptly declines when the PET content exceeds 2%. This is may be due to inclusion of fine particles into voids of the mixture which results in compact volume. The asphalt mixture modified with 10% crumb rubber and 2% PET was found to be more stable than the mixture without PET, and 9.94% margin of difference was observed between the two cases. This happens since the fine PET chips replace the available air void and hence increases stiffness of the mixture. Besides, the fine PET particles have low melting rate, resulting in more crystalline parts which positively contributes to material stiffness. [71] in the study on investigation of the effect of fine and course PET polymer on asphalt mixture concluded that fine PET particles are more effective in generating good stability than the coarse one. Hence, the improvement performance of the PET polymer and crumb rubber modified asphalt mixes is mainly the result of good adhesion developed between the bitumen binder and PET polymer  which enhance stability and durability of the asphalt mix.Figure 3
Properties of the PET modified asphalt mix: (a) stability (KN), (b) flow, (c) bulk SG (Gmb), (d) air void, (e) VMA, (f) VFB, and (g) Marshall quotient.
(a)(b)(c)(d)(e)(f)(g)Generally, the flow of PET modified asphalt mix is higher than the conventional asphalt mix for some dosages of PET. However, it remains less for some amount of PET. The flow magnitude abruptly drops as the content of PET rises up to 4%, whereas further increment in the content of the PET filler beyond 4% leads to an increase in flow magnitude (Figure3(b)). The higher values of flow falling within the accepted range indicate that the modified asphalt mixture exhibits greater flexibility which enhances the ability of the hot mix asphalt pavement to undergo deformation without cracking.PET polymer and crumb rubber modified asphalt mix has a lower bulk density than the conventional asphalt mix. Bulk density of the mix rises as the PET content increases (Figure3(c)). With introduction of the 2% PET, bulk density of the mix becomes 2.305 g/cm3, whereas it was reduced to 2.267 g/cm3 when the mix is tested in the absence of PET polymer. Highest bulk density was witnessed at 2% PET and relatively low values were observed for the 4% and 6% dosages. The presence of microfine particles of PET in the hot mix asphalt causes the mix to be denser resulting in an increase in the unit weight of the mix [39, 40].The modified asphalt mixes experience higher proportion of air voids than the conventional asphalt mixes. The PET-free mix, however, has higher air void than the 2% and 4% contents of PET. Beyond that, continuous increment in air void is observed (Figure3(d)). Air voids in the asphalt mix (Va) and voids filled with bitumen (Vb) affect the percentage of voids in mineral aggregates (VMA) for the asphalt mix. The 4% and 6% PET contents resulted in higher VMA than the 2% PET. In general, the VMA percent of modified asphalt mixes is higher than that of normal asphalt mixes. Figure 3(e) depicts the relationship between VMA and PET concentration. With regard to voids filled with bitumen, the maximum value of VFB is attained at 4% PET. The lowest value, on the other hand, is observed at 6% PET (Figure 3(f)). It can, hence, be inferred that the 6% PET modified asphalt mix give rise to the development of high resistant to plastic deformation which in turn enhances the mix stiffness.Figure3(g) illustrates variation of Marshall quotient (MQ) with a change in the dosage of PET polymer for the mixtures containing crumb rubber modified asphalt binder. The magnitude of MQ increases with the increase in the content of PET up to 2% (highest MQ is achieved). However, further rise in the PET content beyond 2% leads to reduction in magnitude of MQ. The asphalt mixture is made of the 10% crumb rubber modified asphalt binder and 2% by weight of PET polymer. The critical combination of these additives contributes more to resistance of the mix to plastic deformation than other combinations. In the study conducted by Shinde et al. [74], it was concluded that the asphalt mixture modified with 20% crumb rubber at the optimum binder content of 4% improved Marshall stability by 2.14%. Similarly, the Marshal stability increased by 4.6% with combined application of 10% crumb rubber and 2% PET polymer in the current study. To acquire the optimum modifier dosage that produces an asphalt mix with the highest mechanical properties. The percentage of PET polymer which satisfies the highest required magnitude of stability is considered as an optimum amount. Accordingly, the 2% by weight of PET was identified as the optimum amount.


### 3.4. Tensile Strength and Ratio

One of the most important features of asphalt mixes is tensile strength, which is directly associated to fracture resistance and persistent deformation. Tensile strength of the mixes was determined at temperature of 25°C. Figure4(a) depicts the effect of the PET content on indirect tensile strength (ITS) of the asphalt mixes. The experimental test reveals that ITS increases as the PET content increases up to 2% after which the tendency reverses and the ITS drops as the PET level rises. Hence, it can be stated that ITS of the asphalt mixtures modified with 2% PET filler particles are higher than the mixtures without the additive. Introducing the 2% PER polymers in the asphalt mix enhances the moisture susceptibility and pavement resistance to fatigue cracking than other dosages. The obtained results also reveal that magnitude of the indirect tensile strength in the dry condition is by far greater than that of the wet condition. Figure 4(b) shows the relationship between the indirect tensile ratio (ITR) and dosage of PET polymer.Figure 4
(a) Indirect tensile strength and (b) indirect tensile ratio (ITR).
(a)(b)


## 3.1. Effects of Crumb Rubber on Asphalt Binder Properties

When the crumb rubber particle is added into the hot mix asphalt binder using the wet process, the aromatic oils of the asphalt are absorbed into crumb rubber forming gel-like materials. The experimental analysis reveals that the softening point of the crumb rubber modified asphalt binder is directly proportional to the percentage of the crumb rubber content applied (Figure1(a)). When the percentage of crumb rubber increases from 10% to 15%, the resulting value of softening point rises by 4.01% and 2.25%, respectively. This could be due to the increase in resistance to temperature susceptibility of the bitumen with addition of crumb rubber as pointed out by Tefera et al. [70]. As indicated in Figure 1(b), the penetration value of the crumb rubber modified asphalt binder decreases with an increment in dosage of waste crumb rubber. The penetration value of the additive-free bitumen went increasing by 1.56% and 6% with addition of 10% and 15% of crumb rubber chips, respectively.Figure 1
Mechanical properties of the asphalt binder: (a) softening point, (b) penetration value, (c) ductility, and (d) specific gravity.
(a)(b)(c)(d)Compared to the 0% and 10% crumb rubber modified asphalt binders, the asphalt binder modified by addition of 15% has a lower penetration value. This implies that the modified binder with 15% crumb rubber is more brittle and harder than the binder modified through blending of low dosage of crumb rubber. It can hence be inferred that introduction of additives in large amount to the asphalt binder enhances its resistance to needle penetration. The asphalt binder modified with addition of significant amount of additives is more preferred in hot climate conditions. Ductility is a key parameter which is a measure of tensile behaviour of the bituminous binder or homogeneity of the binder. Ductility of the asphalt binder decreases as the percentage of waste crumb rubber increases (Figure1(c)). Less magnitude of ductility was witnessed in the 15% by weight of the crumb rubber than the 10%. Similarly, the 10% crumb rubber modified asphalt binder demonstrated lower ductility in comparison with the additive-free bitumen. Addition of crumb rubber to the asphalt binder has negative contribution to the magnitude of the specific gravity (Figure 1(d)). The specific gravity of the asphalt binder falls with an increase in the amount of crumb rubber.The obtained results of the experimental tests are consistent with the findings of Yigezu Tefera et al. [70] in which it was reported that an increase in the rubber content from 0 to 20% resulted in a sharp decrease in penetration from 68.5 to 58.5 mm. Similarly, ductility of the asphalt binder was lessened from 100 to 75 cm for the same percentage rise in the rubber content. This is a good indication for the convincing capability of the 10% and 15% crumb rubber to improve the properties of the asphalt binder.


## 3.2. Marshall

In order to determine the best optimum binder content, a total of 15 samples were used with five different percentages of bitumen (ranging from 4 to 6% with a 0.5% increment). The result for each mixture was obtained by averaging the result on three replicate specimens. The optimum asphalt content was calculated as the average asphalt content value that corresponds to the maximum stability, maximum unit weight, and 4% air voids (AV).Stability is the highest required load under which specimens fail when applied at a steady rate of 50 mm/min at 60°C. The relationship between stability and various bitumen contents are presented in Figure2(a). Stability of the asphalt mix sharply increases for the bitumen content below 5.5% and begins falling progressively right after. This indicates that resistance of the specimen against the imposed load increases until the optimum amount of the additive is reached. However, beyond the stated optimum amount, the specimen is forced to experience low stiffness due to rise in plasticity characteristics.Figure 2
Marshall test results for the conventional asphalt mixture: (a) stability, (b) flow, (c) bulk specific gravity, (d) voids filled with bitumen, (e) air voids, (f) voids in mineral aggregate, and (g) Marshall quotient.
(a)(b)(c)(d)(e)(f)(g)The total amount of deformation observed at maximum load is referred to as flow. It is measured from the start of loading to the point at which stability begins to fall by 0.25 mm. As observed from Figure2(b), direct relationship developed between the dosage of bitumen and expected magnitude of flow. The 6% bitumen content indicates a plastic mix that will experience deformation under traffic load, whereas the 4% bitumen content indicates a mix with higher voids than normal. As a result of the high void, the asphalt material may prematurely experience cracks under traffic load which in turn impairs its durability.The effect of the bitumen content on bulk density is illustrated in Figure2(c). With increase in the bitumen content up to 5.5%, bulk density of the asphalt mix increases, whereas progressive decline is witnessed after the peak (2.413 g/cm3).Bitumen is intentionally used to fill the existing air void in the asphalt mix. Since the air void is replaced with the bitumen material, it significantly declines as the dosage of the bitumen material rises (Figure2(e)). Moreover, the percentage of voids filled with bitumen in the asphalt mix increases as the bitumen content increases. Hence, the maximum air void (Va) is detected at the lowest bitumen percentage (4%).Voids filled with bitumen (VFB) is the percentage of voids in mineral aggregates filled with the bitumen material. The relationship between VFB and bitumen concentrations is illustrated in Figure2(d). The lowest VFB content was observed at the lowest bitumen content (4%) which suggests that there are fewer asphalt films around the aggregate particles and are more susceptible to moisture and influences of weather. VFB percent increases as the bitumen content increases due to increase in the proportion of voids filled with the bitumen material.Voids in mineral aggregates (VMA) is the percentage of void volume in the aggregates before adding bitumen or the sum of the percentage of voids filled with bitumen and percentage of air voids remaining in the asphalt mix after compaction. In order for aggregate particles to be covered with a sufficient asphalt film, a minimum VMA must be maintained in the mix. As a result, a long-lasting asphalt paving mixture is produced [25]. In Figure 2(f), VMA% results for different bitumen contents are presented.The design bitumen content of the mix is determined based on the results of the Marshall test. The National Asphalt Pavement Association (NAPA) suggests that the bitumen content corresponding to 4% air voids is chosen as the design bitumen content. The Marshall test result reveals that the binder content resulting in 4% air voids is 5.5%. Accordingly, the 5.5% dosage of bitumen is considered to be the optimum binder content. The Marshall quotient (MQ) was determined by dividing the Marshall stability by the flow, which is commonly used as a measure of resistance to permanent deformation. According to Lo Presti [25], the mixture with higher MQ is stiffer and more resistant to permanent deformation. The variation in magnitude of MQ for a mixture with neat bitumen is shown in Figure 2(g). Bitumen binder content of 5% by weight of bitumen has more resistance to permanent deformation than the other binder contents.


## 3.3. Effects of PET on Crumb Rubber Modified Asphalt Mix

To investigate the effect of PET as a filler material on performance of the asphalt mix, four percentages by weight of PET (0%, 2%, 4%, and 6%) were considered. Three Marshall specimens were tested for each percentage of PET. Accordingly, total of 12 specimens were tested at the optimum binder content (5.5%) to evaluate the effect of adding PET filler to crumb rubber modified asphalt mixture.Modified asphalt mixtures have a better level of stability than conventional asphalt mixtures. As can be seen, the Marshal stability of all asphalt mixtures containing 10% crumb rubber modified asphalt mixture with varying percentages of filler PET particles is higher than that of the mixture lacking crumb rubber and PET polymers. The highest Marshall stability was 12.38 kN and it is associated with the 2% filler PET. As observed from Figure3(a), stability of the modified asphalt mix abruptly declines when the PET content exceeds 2%. This is may be due to inclusion of fine particles into voids of the mixture which results in compact volume. The asphalt mixture modified with 10% crumb rubber and 2% PET was found to be more stable than the mixture without PET, and 9.94% margin of difference was observed between the two cases. This happens since the fine PET chips replace the available air void and hence increases stiffness of the mixture. Besides, the fine PET particles have low melting rate, resulting in more crystalline parts which positively contributes to material stiffness. [71] in the study on investigation of the effect of fine and course PET polymer on asphalt mixture concluded that fine PET particles are more effective in generating good stability than the coarse one. Hence, the improvement performance of the PET polymer and crumb rubber modified asphalt mixes is mainly the result of good adhesion developed between the bitumen binder and PET polymer  which enhance stability and durability of the asphalt mix.Figure 3
Properties of the PET modified asphalt mix: (a) stability (KN), (b) flow, (c) bulk SG (Gmb), (d) air void, (e) VMA, (f) VFB, and (g) Marshall quotient.
(a)(b)(c)(d)(e)(f)(g)Generally, the flow of PET modified asphalt mix is higher than the conventional asphalt mix for some dosages of PET. However, it remains less for some amount of PET. The flow magnitude abruptly drops as the content of PET rises up to 4%, whereas further increment in the content of the PET filler beyond 4% leads to an increase in flow magnitude (Figure3(b)). The higher values of flow falling within the accepted range indicate that the modified asphalt mixture exhibits greater flexibility which enhances the ability of the hot mix asphalt pavement to undergo deformation without cracking.PET polymer and crumb rubber modified asphalt mix has a lower bulk density than the conventional asphalt mix. Bulk density of the mix rises as the PET content increases (Figure3(c)). With introduction of the 2% PET, bulk density of the mix becomes 2.305 g/cm3, whereas it was reduced to 2.267 g/cm3 when the mix is tested in the absence of PET polymer. Highest bulk density was witnessed at 2% PET and relatively low values were observed for the 4% and 6% dosages. The presence of microfine particles of PET in the hot mix asphalt causes the mix to be denser resulting in an increase in the unit weight of the mix [39, 40].The modified asphalt mixes experience higher proportion of air voids than the conventional asphalt mixes. The PET-free mix, however, has higher air void than the 2% and 4% contents of PET. Beyond that, continuous increment in air void is observed (Figure3(d)). Air voids in the asphalt mix (Va) and voids filled with bitumen (Vb) affect the percentage of voids in mineral aggregates (VMA) for the asphalt mix. The 4% and 6% PET contents resulted in higher VMA than the 2% PET. In general, the VMA percent of modified asphalt mixes is higher than that of normal asphalt mixes. Figure 3(e) depicts the relationship between VMA and PET concentration. With regard to voids filled with bitumen, the maximum value of VFB is attained at 4% PET. The lowest value, on the other hand, is observed at 6% PET (Figure 3(f)). It can, hence, be inferred that the 6% PET modified asphalt mix give rise to the development of high resistant to plastic deformation which in turn enhances the mix stiffness.Figure3(g) illustrates variation of Marshall quotient (MQ) with a change in the dosage of PET polymer for the mixtures containing crumb rubber modified asphalt binder. The magnitude of MQ increases with the increase in the content of PET up to 2% (highest MQ is achieved). However, further rise in the PET content beyond 2% leads to reduction in magnitude of MQ. The asphalt mixture is made of the 10% crumb rubber modified asphalt binder and 2% by weight of PET polymer. The critical combination of these additives contributes more to resistance of the mix to plastic deformation than other combinations. In the study conducted by Shinde et al. [74], it was concluded that the asphalt mixture modified with 20% crumb rubber at the optimum binder content of 4% improved Marshall stability by 2.14%. Similarly, the Marshal stability increased by 4.6% with combined application of 10% crumb rubber and 2% PET polymer in the current study. To acquire the optimum modifier dosage that produces an asphalt mix with the highest mechanical properties. The percentage of PET polymer which satisfies the highest required magnitude of stability is considered as an optimum amount. Accordingly, the 2% by weight of PET was identified as the optimum amount.


## 3.4. Tensile Strength and Ratio

One of the most important features of asphalt mixes is tensile strength, which is directly associated to fracture resistance and persistent deformation. Tensile strength of the mixes was determined at temperature of 25°C. Figure4(a) depicts the effect of the PET content on indirect tensile strength (ITS) of the asphalt mixes. The experimental test reveals that ITS increases as the PET content increases up to 2% after which the tendency reverses and the ITS drops as the PET level rises. Hence, it can be stated that ITS of the asphalt mixtures modified with 2% PET filler particles are higher than the mixtures without the additive. Introducing the 2% PER polymers in the asphalt mix enhances the moisture susceptibility and pavement resistance to fatigue cracking than other dosages. The obtained results also reveal that magnitude of the indirect tensile strength in the dry condition is by far greater than that of the wet condition. Figure 4(b) shows the relationship between the indirect tensile ratio (ITR) and dosage of PET polymer.Figure 4
(a) Indirect tensile strength and (b) indirect tensile ratio (ITR).
(a)(b)


## 4. Conclusions

In this study performance of the PET polymer as filler material and crumb rubber in improving mechanical properties of the asphalt mix was investigated. Based on findings of the experimental study, the following conclusions are drawn:(i)
The asphalt mix modified with the combination of the PET polymer and crumb rubber has higher stability and stiffness than the conventional asphalt mix, while other attributes of the modified mix are still within the requirements’ allowable range. The adjusted asphalt mix has a slight increase in flow and air voids, although the VMA percent and bulk density of the two mixes are nearly identical.(ii)
The optimum amount of crumb rubber required for modification of the asphalt binder keeping the binder properties within the specification limit was found to be 10% by weight. Crumb rubber can conveniently be used as a modifier for the asphalt binder as well as for improving performance of the asphalt mix. Application of crumb rubber at its optimum amount significantly improved the critical properties of the asphalt binder which are an integral part of the pavement performance. The low density of added polymers is the cause of the acute drop in bulk density of the asphalt mix.(iii)
The optimum content of the polyethylene terephthalate polymer to be added to bring about maximum modification in properties of the asphalt mix is 2% by weight(iv)
The best enhancement in properties of the hot mix asphalt is achieved via combined application of the 10% crumb rubber modified asphalt binder and 2% polyethylene terephthalate polymer as a filler material. The crumb rubber modified asphalt binder in combination with the plastic polymer of the stated amount improved the indirect tensile strength by increasing asphalt strength which in turn lessens occurrence of rutting.



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*Source: 6320490-2023-01-27.xml*