Evaluating the Effect of Plastomer Modified Asphalt Mixture on High/Low Temperature Performance

Asphalt pavement’s surfaces deteriorate over time due to combined effect of traffic and surrounding environment. Fatigue and rutting are the major distresses which cause failures in flexible pavements. Different temperature control computer operated equipment’s are being used worldwide to predict the performance of asphalt mixtures at approximately same condition to those in-service pavements. Similarly, different types of polymers such as elastomer and thermoplastic have been used all over the world in Hot Mix Asphalt (HMA) for the improvement of asphalt mixtures. But little attention has been taken to evaluate the effect of plastomer on hot mix asphalt performance. Moreover, the initial cost of elastomer is higher than other types of polymers such as plastomer. The aim of this research study is to check the effect of various plastomers on high/low temperature performance of asphalt mixture. Four performance tests like Cooper wheel tracker, dynamic modulus, uniaxial repeated load and four-point bending beam test are used to evaluate the effect of different type of plastomers such as polyethylene terephthalate, high density and low density polyethylene with limestone aggregate quarry and 60/70 pen grade asphalt binder. This research study concludes that plastomer increases flexibility and hardness of asphalt mixtures and improves the rut resistance, dynamic modulus and fatigue life of asphalt mixtures. Plastomer modification shows significant benefits as compared to neat binder for high/low temperature performance. Moreover, it can be concluded that plastomer provides an efficient and economical blend of asphalt mixture.


INTRODUCTION
utting and fatigue cracking are the normally occurring distresses in asphalt pavements. Bitumen is a material which is used for the construction of asphalt pavements and it has viscoelastic and thermoplastic properties, i.e. it behaves as an elastic solid at low temperature and under rapidly moving traffic conditions, similarly it behaves as viscous fluid at high temperature and under slow moving traffic conditions [1,2]. So asphalt mixture which is used for road construction has specific parameters and temperature negatively influences these parameters and decreases the lifespan of pavement structure [3]. Rutting (Permanent deformation) and cracking (fatigue) are the major distresses which deteriorate the pavement at high and intermediate temperature. Permanent deformation and cracking due to fracture damage are the distresses in asphalt pavement which appear in the wheel track along the driving direction [4]. Due to increase of temperature in summer season and heavily loaded vehicles especially slow moving vehicles, the effect of rutting increases significantly. Flexible pavements undergo permanent deformation due to heavy and channelized traffic loading. Annihilation of flexible pavement due to occurrence of rutting in parallel direction of moving traffic may cause many traffic accidents if the rut depth and area are beyond the limited range. To overcome the problem of weak asphalt mixes, it is necessary to enhance asphalt properties to control rutting and fatigue cracking. Preparation of high performance asphalt mixture with increased service life is essential. So, modification of asphalt is important to improve the performance of bituminous layer against heavy traffic load and harsh climatic conditions.
Asphalt mixtures with additives such as thermoplastic, elastomer and plastomer are usually used to change the phase composition and improve the engineering properties of asphalt to bear the heavily loaded traffic in slow and rapidly moving traffic conditions. In this research study different plastomer modifiers are used to improve hot mix asphalt performance against fatigue cracking as well as permanent deformation.
Modified asphalt can improve the properties of asphalt mixture. Most commonly used polymers are Styrene-Butadiene-Rubber (SBR), Styrene-Butadiene-Styrene (SBS), Polyethylene and Ethylene Vinyl Acetate (EVA). These polymers enhance the performance of asphalt mixture against rutting, fatigue cracking and moisture damaging [5,6]. SBS acts as an elastoplastomeric polymer and can increase the dynamic performance of asphalt mixture under varying load conditions [7,8]. The basic function of elastomers is to improve rheological and chemical properties of bituminous binder [9]. Similarly, plastomers are used in asphalt mixture for enhancement of properties of asphalt mixture [10]. Plastomers such as waste plastic are introduced into asphalt mixture which increase the performance properties of asphalt concrete mixture and also increase the pavement service life [11,12]. Use of waste plastic in asphalt concrete mixture may increase pavement service life 2.81 times [13] and may reduce the quantity of asphalt binder and consequently may reduce the total cost of whole project [14]. Other type of plastomer such as plastic scrap is used in asphalt for performance improvement. Addition of plastic scrap shows greater indirect tensile strength, higher air void content, reduced moisture damage and a 2% decrease in the ratio of conserved tensile strength [15]. Crumb rubber and polymer modified asphalt mixtures can be used for a specific condition in extreme climatic condition areas. Some enhanced properties can be achieved e.g. more resistance against deformation at high temperature, less susceptible due to change in temperature, increase in long term resistance properties, improvement of compatibility between aggregate and bitumen, increase in fatigue life and prevention of rutting and cracking [16]. Polymer modified asphalt mixtures have been used for road construction [17,18]. The modification of asphalt mixture with waste plastic carries advantages of improvement in conventional binder performance in a cheap way and also with utilization of waste plastic [19]. Similarly, when we utilize low density polyethylene in conventional mixture, it decreases the penetration and phase angle, increases the complex modulus, softening point, and rotational viscosity which indicate the high/low temperature rheological characteristics of the asphalt mixture [20]. When we utilize ground tire rubber with low density polyethylene, it increases the conventional and rheological properties of binders [21][22][23]. Similarly, utilization of motor oil and waste polymers in bitumen have a good effect on high/low temperature performance and enhance the rut resistance and fatigue life of asphalt mixture [24]. [25] reported that modification of asphalt mixture with high density polyethylene reduces the temperature and moisture susceptibility of the mixture. Fatigue performance of asphalt mixture can be improved by using Styrene-Butadiene-Rubber (SBR) [26][27].
In developing an investigational testing method for evaluating the characterization of HMA mixes, most researchers have used different latest techniques for performance testing by using different types of modifiers and study the individual effect of each modifier on HMA performance. But previous research studies have not deliberated more about the effect of plastomer on asphalt mixture and have not described the comparative effect of different plastomers i.e. lowdensity polyethylene, high density polyethylene and polyethylene terephthalate on asphalt mixture.
The primary objective of this research study is to evaluate the effect of plastomer on HMA and determination of optimized doses of plastomer for best results. Furthermore, this research study evaluates the comparative effect of different types of plastomer on HMA performance.

RESEARCH METHODOLOGY
Research methodology is divided into three phases shown in Fig. 1. Phase-I consists of selection of material i.e. aggregate, bitumen and plastomer. Aggregate is taken from Margallah quarry which has been most frequently used aggregate in HMA mixes throughout the Pakistan. Bitumen with 60/70 penetration grade used in this study is taken from National Oil Refinery (NRL), as this is most commonly used bitumen in road construction material in Pakistan. The plastomers such as Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE) and Polyethylene Terephthalate (PET) are taken from local sources. Phase-II consists of mix design preparation. Marshal method of mix design as per ASTM 1559 is taken on to get Optimum Bitumen content (OBC) and volumetric properties of mixture.
Phase-III consists of performance testing of asphalt mixture. Four HMA performance tests such as Cooper Wheel Tracker Test (CWTT), Dynamic Modulus (DM) test, Four-Point Bending Beam Test (FPBB), Uniaxial Repeated Load Permanent Deformation (URLPD) test are selected to study the effect of plastomers on HMA performance at high/low temperature. Physical and chemical properties of material are also conducted according to standard conditions.

Aggregate
Aggregate used for bituminous mixture is taken from local quarry i.e. Margallah which contains limestone minerals. It is the best mechanically fractured aggregate producing quarry of Pakistan. Physical and Mechanical properties of selected aggregates are measured according to standard specification of AASHTO, ASTM and BS. Engineering properties of aggregates have been summarized in Table 1. Rounded and river bed particles are not used in this research study.   This denotes interlocking of coarse aggregate without compaction effort. Bailey method offers better interlocking characteristics as compared to conventional gradation [18]. For the purpose of getting relatively better interlocking of aggregate, Bailey gradation method is used in this research study. Tabular and graphical representation of aggregate gradation is reported in Table 2 and Fig. 2 respectively.

Bitumen
A 60/70 penetration grade bitumen containing paraffinic as well as asphaltic hydrocarbon chain with Performance Grade (PG) 58-22 is selected in this research study. The basic properties are given in Table  3.
RTFO aged binders are used for frequency sweep test to determine the time and temperature dependency of bitumen by using Dynamic Shear Rheometer (DSR). A range of frequency from 0.1 to 10 and temperature from 22 to 82 is used for development of master curve. Master curve is plotted between reduced frequencies taken on abscissa and complex shear modulus (G * ) taken on ordinate by using sigmoidal parameters. Master curve and sigmoidal coefficient has been presented in Fig. 3.

Modifiers
Three different types of plastomer have been used in this research study i.e. Low-Density Polyethylene (LDPE), High Density Polyethylene (HDPE) and Polyethylene Terephthalate (PET) as shown in the Fig.4 (a-c).
These plastomers have been taken from locally available resources. The modification of asphalt mixture with plastomer improves the permanent deformation resistance and dynamic modulus of asphalt mixture [19].  Table 4.     The two methods of mixing are normally used for mixing of selected modifiers or additives into asphalt mixture i.e. dry method and wet method. In this study, dry method of mixing has been used and modifiers are blended with aggregate prior to addition of bitumen. [21].
After attaining 180±5°C temperature of aggregate, polyethylene is mixed at design quantity. After that temperature is allowed to dropdown that is 160°C to avoid aging of asphalt binder. Then bitumen is added into the aggregate.

Sample Preparation
Marshal method of mix design is adopted at 4% air voids for optimization and prepare 10.2x6.3 cm test specimen to get optimum bitumen content (OBC). Specific gravities and volumetric analysis are also conducted on design optimum bitumen content (OBC). Design volumetric parameters for each mixture type are summarized in Table 5.
Based on optimum asphalt content, the samples are prepared for performance testing. Super pave gyratory compacted samples, (with 150mm diameter) by adopting procedure stipulated in AASHTO PP 35, sre prepared for Cooper wheel tracker, uniaxial repeated load and dynamic modulus test. Slabs (with 50mm thickness) are prepared at 6±0.5% air voids (AV) by using roller compactor for four-point bending   Four performance tests (CWTT, DM, URLPD, FPBB) were carried out and results obtained from these tests were used for analysis. Description of test and dimensions of used specimen are given in Table 6.

Rut Performance of Mixes
The rut performance of neat and modified asphalt mixtures was determined with Cooper wheel tracker test. The wheel tracker system which is used for prediction of rut performance of bituminous mixture was created by repetitive wheel passes. The test was used to study the rut resistance of bituminous mixture at standard condition of load, temperature and at standard condition speed. The results of rut resistance cab be work out from rate of rutting during test and rut depth was obtained by using Linear Variable Displacement Transformer (LVDT). Cooper wheel tracker load assembly js shown in Fig 6.

Rut Performance of Mixes
The rut performance of neat and modified asphalt    7 shows the deformation curve of modified mixtures. Rut depth which is measured in millimeter (mm) was plotted as ordinate while number of wheel passes were plotted as abscissa. It has been observed that plastomers modified mixture performed better as compared to virgin mixture. In plastomers, polyethylene terephthalate has maximum resistance against rutting and improve up to 70% resistance against rutting. As from previous studies, [21] reported that asphalt mixture modification with polymers, improve stiffness as well as temperature susceptibility of the mixture, which, in turn, decreases the rut depth of asphalt mixture which is common issue in high temperature area. The result shows that none of the tested specimen has reached at its failure rut value of 12.5mm at 10,000 wheel passes.

Dynamic Modulus Test
It is a linear viscoelastic test which is used for the determination of complex dynamic modulus |E * |. The |E * | is a stress-strain relationship of linear viscoelastic material under a sinusoidal loading condition. Dynamic modulus is a component which is used for the estimation of stiffness of bituminous material at a given frequency and temperature condition. Neat and modified asphalt mixtures in replicates were tested at standard condition of frequency and temperature. The obtained date such as |E * | and phase angle which describe the visco-elastic behavior of material were used for development of master curve. The master curve simulates pavements with different possible loading as well as climate conditions. In development of master curve, time-temperature superposition principles were used. In time-temperature superposition principle, all data of various frequencies and temperature should be shifted to a reference temperature and get a single curve which is known as |E * | master curve.

Creep Performance of Mixes
Creep performance test has performed in Universal Testing Machine (UTM) which is a closed loop control testing device which is used for asphalt material testing. UTM transmit energy to the sample with high pressure through double sided piston. For creep performance test by UTM, a repeated load of definite magnitude with a definite cyclic duration was applied to the sample. In this test, strain correlates with permanent deformation which simulate to the rutting in the field. In this study, conditioning stress and time of 10 kPa and 100 second respectively, were applied. Similarly, a pulse loading of 3600 cycles at 500 kpa stress condition with pulse width of 500 milli-seconds and pulse period of 2000 milli-second were applied. Results obtained from this test are given in Fig. 9. In Fig. 9, accumulated strain was taken on primary ordinate and pulse cycles were taken on abscissa. The result shows that polyethylene terephthalate has minimum strain at high temperature. The sequence of performance is polyethylene terephthalate, then low density polyethylene, high density polyethylene and virgin binder has maximum strain.  Fig. 10.   11 shows the improvement in performances due to polyethylene terephthalate. PET has considerable effect on rut resistance, dynamic modulus and creep performance.  Fig. 11: Percent Improvement in Performances due to PET Fig. 12 shows the improvement in performance of asphalt mixture due to low density polyethylene. LDPE increases rut resistance, dynamic modulus and creep performance to that of neat binder. LDPE also has a considerable effect on fatigue performance of the mixture. Fig. 12: Percent improvement in performances due to LDPE Fig. 13 shows the improvement in performances due to high density polyethylene. HDPE also increases rut resistance and creep performance but its improvement in dynamic modulus is lower than other plastomers. HDPE also increase fatigue performance of the asphalt mixture up 7% to that of neat binder.

CONCLUSIONS
The aim of this research study was to evaluate the effect of plastomer modifiers on performance of asphalt mixture and also provide performance ranking of different plastomers at high/low temperature. Based on all laboratory test results it can be concluded that, (i) Plastomer modified asphalt mixture enhances the viscoelastic behavior of asphalt mixtures and provides harder, stiff and flexible pavements. (ii) The plastomer such as polyethylene terephthalate has greater influence on asphalt mixture at high temperature as compared to other plastomers. It improves the rut resistance up to 70% to that of conventional asphalt mixtures. (iii) The plastomer such as high-density polyethylene improves rut resistance up to 50% to that of conventional asphalt mixture. (iv) The plastomer such as low-density polyethylene improves rut resistance up to 60% and fatigue performance up to 13% to that of conventional asphalt mixture.

RECOMMENDATION
It can be recommended that use the polyethylene terephthalate in high temperature areas perform better as compared to LDPE and HDPE.