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Effect of waste plastic bottles on the stiffness and fatigue properties of modified asphalt mixes
Materials and Design 61 (2014) 8–15
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Effect of waste plastic bottles on the stiffness and fatigue properties of modified asphalt mixes Amir Modarres ⇑, Hamidreza Hamedi Department of Civil Engineering, Babol Noshirvani University of Technology, Babol, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 22 October 2013 Accepted 15 April 2014 Available online 29 April 2014
Nowadays, the use of recycled waste materials as modifier additives in asphalt mixes could have several economic and environmental benefits. The main purpose of this research was to investigate the effect of waste plastic bottles (Polyethylene Terephthalate (PET)) on the stiffness and specially fatigue properties of asphalt mixes at two different temperatures of 5 and 20 °C. Likewise, the effect of PET was compared to styrene butadiene styrene (SBS) which is a conventional polymer additive which has been vastly used to modify asphalt mixes. Different PET contents (2–10% by weight of bitumen) were added directly to mixture as the method of dry process. Then the resilient modulus and fatigue tests were performed on cylindrical specimens with indirect tensile loading procedure. Overall, the mix stiffness reduced by increasing the PET content. Although stiffness of asphalt mix initially increased by adding lower amount of PET. Based on the results of resilient modulus test, the stiffness of PET modified mix was acceptable and warranted the proper deformation characteristics of these mixes at heavy loading conditions. At both temperatures, PET improved the fatigue behavior of studied mixes. PET modified mixes revealed comparable stiffness and fatigue behavior to SBS at 20 °C. However, at 5 °C the fatigue life of SBS modified mixes was to some extent higher than that of PET modified ones especially at higher strain levels of 200 microstrain. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Waste plastic bottles Stiffness Fatigue Asphalt mixes
1. Introduction During the recent years, engineers have been looking for new environmental friendly techniques in construction of roads pavement and much studies have been devoted to this research field (e.g. utilizing recycled asphalt pavement (RAP) materials, crumb rubber, construction debris, etc.) [1–3]. During the service life, many external factors might deteriorate the integrity of pavement. Among these factors, traffic loading is considered as the main factor which finally leads to fatigue cracking and permanent deformations especially in upper pavement layers. There are vast majority of cases which addressed the fatigue properties of conventional or modified asphalt mixes. Effects of many parameters and additives have been studied in this regard [4,5]. Different additive materials including fibers and polymers have been used to improve the fatigue resistance of asphalt mixes. Most of these materials were found to be effective with beneficial effects on the fatigue behavior of asphalt mixes [6–10].
⇑ Corresponding author. Tel.: +98 9111163215. E-mail addresses: (A. Modarres).
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http://dx.doi.org/10.1016/j.matdes.2014.04.046 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
The main reason of incorporating polymer modifiers in bitumens is to extend the range of service temperature or reduce the temperature sensitivity of them. These binders are visco-elastic materials. The degree to which their behavior is viscous or elastic is a function of temperature, loading period and loading duration. At high temperatures or long loading times, they behave like viscous liquids whereas at low temperatures or short loading times they behave as elastic (brittle) solids. Under intermediate conditions of the service period, they exhibit viscoelastic behavior in which the material’s response will be dependent upon temperature or loading velocity. For a polymer to be effective in road applications, it should be blend with bitumen and improve its efficiency at service temperatures without making it too viscous at mixing temperatures or too brittle at low temperatures. In other words, it must extent the range of service temperature while it improves the overall performance of pavement. Polymers that have been used for asphalt mix modification can be divided into three groups including thermoplastic elastomers (e.g. styrene butadiene styrene (SBS) and crumb rubber (CR)), plastomers (e.g. ethylene vinyl acetate (EVA) and polyethylene (PE)) and polymers with chemical reaction [11–13]. Thermoplastic elastomers such as SBS are usually used to extend both minimum and maximum service temperatures of
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bitumen, whereas plastomers are well known as effective additives at high service temperatures [11,12]. Although, the use of polymer modifiers has been recognized as an appropriate solution for promoting the engineering properties of bitumen and asphalt mixes, but it is relatively a costly procedure for paving roads [14,15]. From an environmental and economic point of view, the use of recycled instead of virgin materials could have several advantages such as help easing landfill pressures and reducing demands of extraction from natural quarries. Furthermore, this would be an alternative solution for environmental pollution by utilizing waste materials as secondary materials in road construction projects. As published in the literature, the waste of glass, rubbers, plastics and mineral productions were some popular materials used to modify the properties of bitumens and asphalt mixes [16]. Most researches have focused on using waste additives to improve the deformation and fatigue characteristics of asphalt mixes. According to research results, waste glass and waste rubber had a considerable contribution to fatigue resistance of these mixes [17,18]. Nowadays, many countries are seriously encountered with problems related to waste plastic materials. Plastic materials such as plastic bottles are mainly composed of Polyethylene Terephthalate (PET) polymer. PET is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers, beverage, food and other liquid containers, thermoforming applications and engineering resins often in combination with glass fiber [19]. PET is produced by the polymerization of ethylene glycol and terephthalic acid. Ethylene glycol is a colorless liquid obtained from ethylene, and terephthalic acid is a crystalline solid obtained from xylene. When heated together under the influence of chemical catalysts, ethylene glycol and terephthalic acid produce PET in the form of a molten, viscous mass that can be spun directly to fibers or solidified for later processing as a plastic. Based on previous studies PET has a great potential to be reused as modifier in asphalt mixture. Results indicated that adding PET to asphalt raised the mix resistance against permanent deformation and rutting [20,21]. During a laboratory study Mahrez & Karim examined the effect of different PET contents on the rheological properties of modified bitumen. They found that addition of PET to bitumen will increase the viscosity and reduce the temperature susceptibility of modified bitumen. Furthermore, the PET modified bitumen showed preferable elastic properties than the original one (i.e. higher complex modulus and lower phase angle) [22]. During a laboratory study about stone matrix asphalt (SMA) mixes the effect of PET was investigated using the cylindrical specimens. It was inferred that incorporating PET will reduce the bitumen loss which is one of the main SMA deficiencies. Furthermore, the effect of PET on the moisture susceptibility of these mixes was found to be negligible [23]. In a 2012 study Moghaddam et al. compared the stiffness and fatigue properties of PET modified mixes with conventional asphalt. Based on their report the fatigue life of modified mix containing 1% PET (by weight of aggregate) was twice than that of unmodified mix. However, the stiffness of modified mix was to some extent lower than conventional mix. The outcomes of this research indicated that the application of PET in SMA mixes could meet the various requirements of different environmental and loading conditions. Especially the results of stiffness test warranted the proper deformation characteristics of modified mixes at heavy loading conditions [24]. The addition of thermoplastic polymers (e.g. PET) to bitumen or asphalt mix enhances the material rigidity and restricts the permanent deformations under heavy loading conditions especially in upper pavement layers at higher temperatures [25]. The beneficial effects of PET on such high temperature characteristics of asphalt mixtures have been proved elsewhere [22,24]. However their performance in increasing the bitumen elasticity during drastic and
sudden temperature drops is not always satisfactory. In fact they might deteriorate the intermediate and low temperature characteristics of bitumen and asphalt mix (i.e. increasing the cracking potential of mix) [25]. Apart from abovementioned investigations, there is not enough information regarding to fatigue properties of PET modified mixes. For example the fatigue response of these mixes at various temperatures has not been well established. Since extending the range of service temperature is the main purpose of bitumen and asphalt modification it will be interesting to investigate the fatigue properties of PET modified mixes at various temperatures. Hence, in this study, the fatigue and stiffness properties of PET modified mixes have been investigated at intermediate and low temperatures. In this regard the effect of PET was compared with SBS which is a conventional polymer modifier in asphalt mixes and most of earlier researches have proved the beneficial effects of this additive on the technical characteristics of asphalt mixes [12,21,25]. The main objectives of this research were as follows: To investigate the effects of PET on stiffness properties of modified mixes at two testing temperatures. To evaluate the fatigue behavior of PET modified mixes in comparison with unmodified asphalt mixes. To compare the stiffness and fatigue properties of PET modified mixes with that of modified with SBS. 2. Materials and mix design 2.1. Bitumen and aggregate The original binder used in this study was 60/70 penetration grade bitumen that produced in Tehran oil refinery. Table 1, presents the basic properties of this bitumen. Also, as shown in Fig. 1 a 0–12.5 mm aggregate gradation was selected which was approximately in the middle limit of specifications. Table 2 summarizes the specifications of coarse and fine aggregate fractions and filler materials which were blended to achieve the final gradation. 2.2. Additives 2.2.1. PET In this study, waste plastic bottle (PET) was used as modifier additive in hot mix asphalt. To this end, PET bottles were cut into small pieces and crushed by a special crusher. Finally crushed particles were sieved to obtain the needed gradation. As indicated by previous researches, desired results were obtained by single size PET particles between the range of 0.425–1.18 mm [23,25]. Hence, in this research, PET chips were crushed and sieved to obtain the above-mentioned dimensions. Fig. 2 shows the image of the PET crumbs after the crushing and sieving process. PET consists of polymerized units of the monomer ethylene terephthalate, with repeating C10H8O4 units. The related components of studied PET were terephthalic and ethyleneglycol monomers. The physical
Table 1 Technical properties of original bitumen. Property (unit)
Standard
Value
Specific gravity Penetration (0.1 mm) Softening point (°C) Viscosity at 120 °C (cSt) Viscosity at 135 °C (cSt) Viscosity at 160 °C (cSt)
ASTM: ASTM: ASTM: ASTM: ASTM: ASTM:
1.013 65 50 966 467 168
D70 D5 D36 D2170 D2170 D2170
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Fig. 1. The aggregate gradation of studied hot mix asphalt.
Fig. 3. The viscosity–temperature diagram of bitumen used to select the mixing and compaction temperature ranges.
Table 2 Properties of coarse and fine aggregate fractions and filler materials. Property (unit)
Standard
Unit
Value
2.3. Mix design
Coarse aggregate Water absorption Bulk specific gravity Apparent specific gravity
ASTM: C127 ASTM: C127 ASTM: C127
(%) (gr/cm3) (gr/cm3)
2.2 2.498 2.663
Fine aggregate Water absorption Bulk specific gravity Apparent specific gravity
ASTM: C128 ASTM: C128 ASTM: C128
(%) (gr/cm3) (gr/cm3)
2.4 2.467 2.623
Filler Specific gravity
ASTM: D854
(gr/cm3)
2.665
Modifier additives are usually added to mixture under wet or dry conditions. During the wet process, additive first mixed with bitumen with a proper mixer until achieving a homogenous blend. Then blended materials are added to aggregates. In dry method, according to additive’s type and nature this material is mixed with aggregates before adding bitumen or added after mixing the bitumen and aggregates as a part of solid materials. Due to high melting point, it was not possible to mix the PET particles with bitumen in wet process. In fact it was impossible to achieve a homogeneous mixture in this process. The impossibility of achieving a desired blend through adding PET in wet process has been also mentioned in literature [26]. Therefore, in this study, dry method was followed and PET was added to mix with various quantities of 2%, 4%, 6%, 8% and 10% by the weight of bitumen. The optimum bitumen content for unmodified mix was equal to 5.7% which was determined with Marshall method. Based on previous studies the optimum bitumen content for PET modified mixes was almost equal to unmodified mix [24]. Therefore, in this research the same bitumen content was selected for modified and unmodified mixes. The mixing and compaction temperatures were determined by viscosity–temperature diagram. This diagram has been shown in Fig. 3. The viscosity of bitumen during the mixing and compaction process has been recommended between 150–190 and 250– 310 cst, respectively. On the basis of this criterion the mixing and compaction temperatures were kept constant between 157–162 and 145–150 °C, respectively. For each composition first aggregate and bitumen were mixed at abovementioned temperature range and then PET particles were added directly to mixture. Previous studies revealed that the viscosity of PET modified bitumen is to some extent higher than that of unmodified mixes. However, even for PET contents as high as 8% (by the weight of bitumen) the difference between the viscosity of the original and modified bitumen was negligible especially at mixing and compaction temperatures (i.e. higher than 130 °C) [22]. Therefore, PET modified mixes were mixed and compacted at similar temperature ranges of unmodified ones. SBS modified mixes were prepared by wet process. First, by the means of a high shear mixer, SBS was added to bitumen with the values of 4%, 5% and 6% by the weight of bitumen. Then, the prepared SBS-bitumen blend was added to aggregate. After the mixing and compaction process, prepared mixes were tested to measure the resilient modulus. Based on the criterion of maximum resilient modulus the optimum SBS content was selected equal to 5%. Finally, indirect tensile fatigue test was performed for mixes containing 5% SBS.
Fig. 2. Image of crushed PET particles.
properties of this additive were analyzed using related standard methods. Based on performed analysis the density of PET was equal to 1.08 gr/cm3. Also the glass transition and melting point temperatures were equal to 250 and 70 °C, respectively.
2.2.2. SBS The optimum SBS content was determined based on the results of resilient modulus test. Then asphalt mixes were prepared with the optimum SBS content and the fatigue tests were performed. Finally the fatigue response of SBS modified mixes were compared to PET modified ones.
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3. Experimental design The main laboratory program of this research consisted of the resilient modulus and fatigue tests. These tests were performed on cylindrical specimens using indirect tensile method, according to ASTM: D4123 & EN 12697-24, respectively [27]. In order to determine the stress level in above-mentioned tests indirect tensile strength (ITS) was measured according to ASTM: C496. All tests were accomplished at two testing temperatures of 5 and 20 °C. A universal testing machine (UTM-14) apparatus was used which had been equipped with a temperature control chamber. Chamber contained a reference specimen with two linear variable differential transducers (LVDTs) that measured and recorded the skin and core temperatures during the test. In order to achieve the intended temperature, specimens were put inside the chamber at least 5 h before testing. Before starting the test, the chamber, skin and core temperatures were controlled by related software. Test was started when the coefficient of variation of these three temperatures which automatically calculated by the controlling system software was less than 5%. 3.1. Resilient modulus (Mr) During the Mr test a haversine load was applied with the loading frequency of 1 Hz including 0.1 s loading and 0.9 s recovery times. Horizontal deformations were measured by two LDVTs that were installed along the sample’s diameter. At 5 °C resilient modulus test was done at two stress levels of 15% and 20% of ITS. Moreover, at 20 °C stress levels were fixed to 20% and 40% of ITS. For a dynamic load of P, resilient modulus was calculated by Eq. (1):
Mr ¼
Pðc þ 0:27Þ td
ð1Þ
where P: maximum dynamic load (N), c: poisson’s ratio, t: sample height (mm), d: horizontal deformation (mm). Possion’s ratio was calculated according to Eq. (2) [28]:
c ¼ 0:15 þ
0:35 1 þ eð3:18490:04233tÞ
ð2Þ
where t: temperature (°F) 3.2. Fatigue At each temperature fatigue test was performed at two stress levels utilizing indirect tensile loading method with a haversine loading [27]. Each load pulse consisted of 0.25 s loading and 1.25 s recovery times. Loading continued until complete splitting of samples. For indirect tensile fatigue test the maximum tensile stress and strain at the center of sample were calculated by Eqs. (3) and (4), respectively.
St ¼
e¼
2p
ptd
2 DH 1 þ 3c D 4 þ pc p
11
break of sample. As seen in this figure for this definition fracture life corresponds to the point of vertical asymptote [27]. Likewise, Fig. 4 depicts the second definition of fracture life (N2) in indirect tensile method [29]. As it can be seen, the diagram of horizontal deformation is generally defined by the three zones. The accumulated permanent deformations rapidly increase in the primary zone. In the second zone, the rate of deformation increment gets stabilized and the fatigue curve has a linear trend. In the third zone, microcracks which formed in the second stage will progress. Finally the progress and combination of these cracks leads to complete splitting of specimen. According to Fig. 4 in the second definition (N2) the start point of crack progression is defined as the fracture life. As shown this point corresponds to the intersection point of the second and third zones slope.
4. Results and discussion 4.1. Indirect tensile strength (ITS) and resilient modulus (Mr) Fig. 5 shows the results of ITS test. As seen temperature had considerable effects on the ITS of specimens. Addition of 2% PET led to increase of ITS at both testing temperatures. After that, ITS continuously decreased by adding the PET content. At higher PET contents, bitumen accumulates on the surface of the PET particles. This issue results in the reduction of the bitumen film thickness around the aggregate particles and reduces the aggregate–bitumen adhesion and finally the tensile strength of the modified mix. However, at all PET contents the ITS values were in acceptable limit. Based on previous studies due to reduction of the bitumen film thickness, excessive amounts of PET will also reduce the moisture resistance of the modified mix [23]. Results of resilient modulus test at 5 and 20 °C, have been shown in Fig. 6. As seen, the stiffness of studied mixes reduced by increasing the stress level. However, similar to ITS test, at a constant stress level the highest stiffness quantity achieved at 2% PET content. At higher PET contents aggregates will be replaced by these particles which have less stiffness. Moreover, the reduction of the bitumen film around the aggregate particles might be another reason of stiffness reduction especially at higher PET contents. According to Fig. 6 the resilient modulus of studied mixes increased to twice by reducing the temperature from 20 °C to 5 °C. Similar to obtained results at 20 °C, at higher PET contents (i.e. more than 2%) there was a drop in the resilient modulus of studied mixes at 5 °C. However, due to noticeable stiffening of bitumen the dispersion of results were higher at this temperature. The interaction between bitumen and additive materials in modified mixes could have considerable effect on the behavior of
ð3Þ
ð4Þ
where p: maximum dynamic load (N), t: sample height (mm), D: sample diameter (mm), e: tensile strain at the center of samples, DH: horizontal deformation which measured by two LDVTs. During the fatigue testing horizontal deformations were automatically recorded and deformation–loading curve was plotted for each specimen by related software. Fig. 4 shows an example of horizontal deformation curve that obtained in this research. In this figure two fatigue life definitions were compared together (N1 & N2). For the first definition, according to EN12697-24, fatigue life is equal to the total number of cycles which leads to complete
Fig. 4. The load cycle–displacement curve and fatigue life definitions in ITFT method.
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Fig. 7. Relationship between Mr and temperature.
Fig. 5. Results of ITS test at 5 and 20 °C.
mixture. Studies indicated that this interaction changes with increasing the amount of additive. At high polymer contents the polymer phase becomes dominant, whereas, at optimum content of additive there will be an effective interaction which improves the mechanical properties of asphalt mix [30]. In order to evaluate the temperature sensitivity of studied mixes, the rate of resilient modulus (Mr) changes with an increase in temperature was investigated. Fig. 7 shows the effect of temperature on Mr of studied mixes. It should be noted that this test was performed at two temperatures and two stress levels at each temperature. Also at each condition (i.e. each temperature and stress level) the test was repeated twice. Therefore each line in Fig. 7 was drawn based on the results of 8 tests (i.e. 4 tests at each temperature). In this figure the slope values represent the temperature sensitivity of compared mixes. As seen higher slope values obtained for unmodified mixes and the mixes containing lower PET contents (e.g. 440 MPa/°C for unmodified and 445 MPa/°C for specimen containing 4% PET). By increasing the PET content up to 6% the slope reduced to 404 MPa/°C. After that increasing the amount of PET resulted in higher slope. At 10% the slope value was equal to 445 MPa/°C which was even higher than the unmodified one. Results indicated that for controlling the temperature susceptibility of asphalt mix if PET added in dry method there will
be an optimum content which in this research was equal to 6%. It is recommended to consider the temperature susceptibility as a design criterion during the production process of PET modified mixes especially in projects in which the modifier is added to mix via dry method. 4.2. Fatigue Fig. 8 shows the horizontal deformation curve of modified specimen containing 10% PET which tested at 20 °C. The initial strain level in this test was equal to 535 microstrain. Based on the fracture life definitions that presented in Section 3.2, the fracture life for the first and second definitions will be N1 = 65,664 and N2 = 58,500 cycles, respectively. Results of fatigue tests that performed at 5 and 20 °C have been shown in Fig. 9. It must be noted that in this figure the initial stress level at 5 and 20 °C were equal to 20% and 15% of ITS, respectively. Furthermore, this figure shows the results of fatigue test based on both fracture life definitions. Results indicated the beneficial effects of PET on the fatigue behavior of studied mixes. Therewith, at aforementioned stress levels, fatigue life increased upon reducing the test temperature. Unlike the results of ITS and Mr tests, adding the PET content even as high as 10% had profitable effects on the fatigue response of modified mixes.
Fig. 6. Results of resilient modulus test for various PET contents at 5 and 20 °C.
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Fig. 10. Comparison between the fatigue curves at 20 °C .
Fig. 8. Example of displacement curve obtained during the indirect tensile fatigue test.
Fig. 10 compares the fatigue curves of three specimens containing 0%, 8% and 10% PET which tested at 20 °C. With regard to this figure, for modified specimens the slope of deformation curve in the second zone (i.e. the zone with the constant rate of deformation increment) was less than unmodified one. Therefore, it could be concluded that PET modified mixes exhibited higher cracking resistance and flexibility than unmodified mixes. Finding a meaningful relationship between the mix stiffness and fatigue life has been a challenge for pavement scientists. Much research proved that this relationship is completely dependent on the method of fatigue testing. In controlled stress method, usually stiffer mixes revealed higher fatigue life whereas, in strain constant method the reverse was true [4,31–33]. Based on the findings of this research increasing the PET content resulted in lower stiffness and higher fatigue life. It might be due to higher energy absorbency of the PET particles than the bitumen phase which resulted in superior behavior against repeated loadings [7]. This phenomenon will postpone the crack propagation throughout the specimen diameter.
Fig. 11 shows the fatigue curves of both modified and unmodified specimens which attained at 5 and 20 °C. It should be noted that in all presented conditions there was a proper correlation between the initial strain and fatigue life (R2 values were more than 0.8). Fig. 11A compares the fatigue curves of modified and unmodified specimens based on the first fatigue life definition (N1). Similarly, Fig. 11B shows the fatigue laws of studied mixes based on the second definition (N2). According to this figure, at a constant strain level on average the fatigue life of PET modified mix was about 20% higher than unmodified one. Fig. 11 signifies the considerable effect of temperature on the fatigue response of studied mixes. As seen in this figure, the slope of fatigue curves noticeably reduced by decreasing the test temperature. As a result, the fatigue curves intersected each other at initial strain levels of 160–210 microstrain. It means that at higher strain levels of 210 microstrain the fatigue life of studied mixes reduced upon reducing the temperature. In contrast at lower strain levels of 160 microstrain specimens that tested at 5 °C revealed superior fatigue response than those tested at 20 °C. Between the initial strain levels of 160–210 microstrain there was an interference zone in which no meaningful relationship could be found between the testing temperature and fatigue life. Therefore it could be
Fig. 9. Results of fatigue tests at 20 °C (at 20% of ITS) and 5 °C (at 15% of ITS).
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Fig. 12. Comparison between the results of Mr test for PET and SBS modified specimens.
Fig. 11. Fatigue laws of studied mixes at various temperatures.
concluded that at higher strain levels the softer mix which tested at 20 °C exhibited superior fatigue response than the stiffer mix which tested at 5 °C. Hence, under heavy loading conditions fatigue failure becomes critical at lower temperatures. In contrast at lower strain levels of 160 microstrain fatigue failure is expected to occur sooner under moderate climatic conditions. However, apart from the initial strain level the addition of PET to studied mixes led to an increase in fatigue life at both testing temperatures.
Fig. 13. Comparison between the fatigue curves of PET and SBS modified mixes at 20 °C.
4.3. Comparison of PET and SBS Fig. 12 compares the results of resilient modulus (Mr) test for PET and SBS modified mixtures. The stress levels at 5 and 20 °C were equal to 20% and 40% of ITS, respectively. As it can be seen at 20 °C both additives had similar effects on the stiffness of studied mixes. At this temperature, on average the Mr of unmodified mixes was about 4700 MPa. Incorporating 5% SBS increased the Mr value for about 9%. However, as previously mentioned at higher contents of 2%, PET reduced the stiffness of studied mixes. For specimens which tested at 5 °C, SBS had higher efficiency than PET on reducing the stiffness of unmodified mixes. At this temperature the Mr of unmodified mix was about 12,700 MPa which reduced to about 8850 MPa with adding 4% SBS. At low temperatures the asphalt mix tends to behave like brittle material. Therefore at these temperature ranges the lower stiffness is desired due to higher deformability and higher resistance against the detrimental effects of repeated loadings. Therefore, PET modified mixes showed intermediate behavior in comparison to other mixes. SBS is a thermoplastic elastomer which usually could improve both low and high temperature characteristics of modified mixes [34]. Although some authors claim that a decrease in strength and resistance to penetration is observed at higher temperatures, but most of previous documents confirmed the proper effects of this additive at various testing conditions [34].
Fig. 14. Comparison between the fatigue curves of PET and SBS modified mixes at 5 °C.
The SBS content selected for preparing the fatigue test specimens was 5% which corresponded to the SBS content which resulted in maximum resilient modulus. Fig. 13 compares between the obtained fatigue curves of PET and SBS modified mixes at 20 °C. As it can be seen both additives improved the fatigue response of studied mixes. However, SBS modified mixes showed to some extent better fatigue behavior than PET modified ones. As shown in Fig. 14 overall at 5 °C modified mixes revealed predominant fatigue behavior than unmodified asphalt. However, at higher strain levels (i.e. more than 300 microstrain), higher differences were found between the fatigue life of SBS and PET modified mixes. At lower strain levels the differences between the fatigue
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curves of modified mixes containing 6% PET and the SBS modified ones reduced. It could be inferred that at lower strain levels of 200 microstrain modified mixes with 6% PET exhibited longer fatigue lives than SBS modified mix. Finally, it could be concluded that PET had comparable effects to SBS on the stiffness and fatigue behavior of studied asphalt mixes. Since PET is a recycled material and is cheaper than original polymer modifiers like SBS, the use of it in asphalt mix is desired in both economical and environmental points of view. 5. Conclusions This paper presents the results of a laboratory study about the effects of waste plastic bottles (PET) on the stiffness and fatigue properties of asphalt mixes. Likewise, similar tests were performed on SBS modified mixes and the acquired results were compared. Based on the obtained results the following conclusions can be drawn. (1) Addition of more than 2% PET reduced the resilient modulus at both testing temperatures of 5 and 20 °C. However, at all PET contents the resilient modulus quantities were in acceptable limit. (2) PET improved the fatigue behavior at both testing temperatures. Unlike the results of ITS and Mr tests the addition of the PET content up to 10% had beneficial effects on the fatigue response of modified mixes. (3) There were some intersection points between obtained fatigue curves at 5 and 20 °C. These points were between the strain levels of 160–210 microstrain. At higher strain levels of 210 microstrain, adding temperature resulted in higher fatigue life. However at lower strain levels of 160 microstrain, stiffer mixes which tested at 5 °C showed better fatigue response than those tested at 20 °C. (4) At 20 °C PET and SBS had similar effects on the stiffness of studied mixes. However, at 5 °C, SBS reduced the mix stiffness. The later outcome is ideal because it improves the material flexibility at low temperatures. (5) Both additives improved the fatigue response of studied mixes. Anyway, SBS modified mixes showed to some extent better fatigue behavior than PET modified mixes especially at higher strain levels of 200 microstrain.
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[6] NCHRP APPENDIX II-1. Calibration of fatigue cracking models for flexible pavements, USA; 2004. [7] Ye Q, Wu S, Li N. Investigation of the dynamic and fatigue properties of fibermodified asphalt mixtures. Int J Fatigue 2009;31:1598–602. [8] Tapkın S. The effect of polypropylene fibers on asphalt performance. Build Environ 2008;43:1065–71. [9] Jahromi S, Khodaii A. Carbon fiber reinforced asphalt concrete. Arab J Sci Eng 2008;33:355–64. [10] Attaelmanan M, Pei Feng Ch, Al-Hadidy AI. Laboratory evaluation of HMA with high density polyethylene as a modifier. Constr Build Mater 2011;25(5):2764–70. [11] Ameri M, Mansourian A, Sheikhmotevali AH. Laboratory evaluation of ethylene vinyl acetate modified bitumens and mixtures based upon performance related parameters. Constr Build Mater 2012;40:438–47. [12] VuralKök B, Çolak H. Laboratory comparison of the crumb-rubber and SBS modified bitumen and hot mix asphalt. Constr Build Mater 2011;25:3204–12. [13] Al-Hadidy AI, Yi-qiu T. Effect of polyethylene on life of flexible pavements. Constr Build Mater 2009;23(1):1456–64. [14] Chui-Te Chiu, Li-Cheng Lu. A laboratory study on stone matrix asphalt using ground tire rubber. Constr Build Mater 2007;21:1027–33. [15] Ahmed LA. Improvement of Marshall properties of the asphalt concrete mixtures using the polyethylene as additive. Eng Technol 2007;25(3):383–94. [16] Huang Y, Bird RN, Heidrich O. A review of the use of recycled solid waste materials in asphalt pavements. Resour Conserv Recycle 2007;52:58–73. [17] Arabani M, Mirabdolazimi SM, Sasani AR. The effect of waste tire thread mesh on the dynamic behaviour of asphalt mixtures. Constr Build Mater 2010;24:1060–8. [18] Arabani M, Mirabdolazimi SM, Ferdowsi B. Modeling the fatigue behaviors of glasphalt. Sci Iranica A 2012;19(3):341–5. [19] Li JM, Liu C, Qiao HC, Zhu LY, Chen G, Dai XD. Hot embossing/bonding of a poly(ethylene terephthalate) (PET) microfluidic chip. J Micromech Microeng 2008;18-015008:10. [20] Wan Mohd Nazmi, Wan Abdul Rahman, AchmadFauzi Abdul Wahab. Green pavement using recycled polyethylene terephthalate (PET) as partial fine aggregate replacement in modified asphalt. Proc Eng 2013;53:124–8. [21] Baghaee Moghaddam T, Soltani M, RehanKarim M. Evaluation of permanent deformation characteristics of unmodified and poly-ethylene terephthalate modified asphalt mixtures using dynamic creep test. Mater Des 2013;53:317–24. [22] Mahrez A, Karim MR. Rheological evaluation of bituminous binder modified with waste plastic material. In: 5th International Symposium on Hydrocarbons & Chemistry (ISHC5), Sidi Fredj, Algiers, May 23–25; 2010. [23] Ahmadinia E, Zargar M, RehanKarim M, Abdelaziz M. Performance evaluation of utilization of waste polyethylene terephthalate (PET) in stone mastic asphalt. Constr Build Mater 2012;36:984–9. [24] Baghaee Moghaddam T, RehanKarim M, Syammaun T. Dynamic properties of stone mastic asphalt mixtures containing waste plastic bottles. Constr Build Mater 2012;34:236–42. [25] Ahmadinia E, Zargar M, Karim MR, Mahrez A, Shafigh P. Using waste plastic bottles as additive for stone mastic asphalt. Mater Des 2011;32:4844–9. [26] Casey D, McNally C, Gibney A, Gilchrist MD. Development of a recycled polymer modified binder for use in stone mastic asphalt. Resour Conserv Recycle 2008;52:1167–74. [27] EN 12697–24. Bituminous mixtures-test methods for hot mix asphalt – Part 24: resistance to fatigue. European committee for standardization; July 2004. [28] Witczak, MW., Kaloush, KT. pellinen, K. El-basyouny, M. Simple performance test for superpave mix design. NCHRP 465; 2002. [29] Thiago F, Kim YR, Lee J. Research on fatigue of asphalt mixtures and pavements in Nebraska. Report No. P579, University of Nebraska Lincoln; February 25; 2008. [30] Sengoz B, Isikyakar G. Evaluation of the properties and microstructure of SBS and EVA polymer modified bitumen. Constr Build Mater 2008;22:1897–905. [31] Modarres A. Investigating the toughness and fatigue behavior of conventional and SBS modified asphalt mixes. Constr Build Mater 2013;47:218–22. [32] Mahrez A, Karim MR. Fatigue characteristics of stone mastic asphalt mix reinforced with fiber glass. Int J Phys Sci 2010;5(12):1840–7. [33] Tayebali A, Rowe AGM, Sousa JB. Fatigue response of asphalt aggregatemixtures. J Assoc Asphalt Paving Technol 2010;61:333–60. [34] Yildirim Y. Polymer modified asphalt binders. Constr Build mater 2007;21:66–72.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of waste plastic bottles on the stiffness and fatigue properties of modified asphalt mixes Amir Modarres ⇑, Hamidreza Hamedi Department of Civil Engineering, Babol Noshirvani University of Technology, Babol, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 22 October 2013 Accepted 15 April 2014 Available online 29 April 2014
Nowadays, the use of recycled waste materials as modifier additives in asphalt mixes could have several economic and environmental benefits. The main purpose of this research was to investigate the effect of waste plastic bottles (Polyethylene Terephthalate (PET)) on the stiffness and specially fatigue properties of asphalt mixes at two different temperatures of 5 and 20 °C. Likewise, the effect of PET was compared to styrene butadiene styrene (SBS) which is a conventional polymer additive which has been vastly used to modify asphalt mixes. Different PET contents (2–10% by weight of bitumen) were added directly to mixture as the method of dry process. Then the resilient modulus and fatigue tests were performed on cylindrical specimens with indirect tensile loading procedure. Overall, the mix stiffness reduced by increasing the PET content. Although stiffness of asphalt mix initially increased by adding lower amount of PET. Based on the results of resilient modulus test, the stiffness of PET modified mix was acceptable and warranted the proper deformation characteristics of these mixes at heavy loading conditions. At both temperatures, PET improved the fatigue behavior of studied mixes. PET modified mixes revealed comparable stiffness and fatigue behavior to SBS at 20 °C. However, at 5 °C the fatigue life of SBS modified mixes was to some extent higher than that of PET modified ones especially at higher strain levels of 200 microstrain. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Waste plastic bottles Stiffness Fatigue Asphalt mixes
1. Introduction During the recent years, engineers have been looking for new environmental friendly techniques in construction of roads pavement and much studies have been devoted to this research field (e.g. utilizing recycled asphalt pavement (RAP) materials, crumb rubber, construction debris, etc.) [1–3]. During the service life, many external factors might deteriorate the integrity of pavement. Among these factors, traffic loading is considered as the main factor which finally leads to fatigue cracking and permanent deformations especially in upper pavement layers. There are vast majority of cases which addressed the fatigue properties of conventional or modified asphalt mixes. Effects of many parameters and additives have been studied in this regard [4,5]. Different additive materials including fibers and polymers have been used to improve the fatigue resistance of asphalt mixes. Most of these materials were found to be effective with beneficial effects on the fatigue behavior of asphalt mixes [6–10].
⇑ Corresponding author. Tel.: +98 9111163215. E-mail addresses: (A. Modarres).
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http://dx.doi.org/10.1016/j.matdes.2014.04.046 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.
The main reason of incorporating polymer modifiers in bitumens is to extend the range of service temperature or reduce the temperature sensitivity of them. These binders are visco-elastic materials. The degree to which their behavior is viscous or elastic is a function of temperature, loading period and loading duration. At high temperatures or long loading times, they behave like viscous liquids whereas at low temperatures or short loading times they behave as elastic (brittle) solids. Under intermediate conditions of the service period, they exhibit viscoelastic behavior in which the material’s response will be dependent upon temperature or loading velocity. For a polymer to be effective in road applications, it should be blend with bitumen and improve its efficiency at service temperatures without making it too viscous at mixing temperatures or too brittle at low temperatures. In other words, it must extent the range of service temperature while it improves the overall performance of pavement. Polymers that have been used for asphalt mix modification can be divided into three groups including thermoplastic elastomers (e.g. styrene butadiene styrene (SBS) and crumb rubber (CR)), plastomers (e.g. ethylene vinyl acetate (EVA) and polyethylene (PE)) and polymers with chemical reaction [11–13]. Thermoplastic elastomers such as SBS are usually used to extend both minimum and maximum service temperatures of
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bitumen, whereas plastomers are well known as effective additives at high service temperatures [11,12]. Although, the use of polymer modifiers has been recognized as an appropriate solution for promoting the engineering properties of bitumen and asphalt mixes, but it is relatively a costly procedure for paving roads [14,15]. From an environmental and economic point of view, the use of recycled instead of virgin materials could have several advantages such as help easing landfill pressures and reducing demands of extraction from natural quarries. Furthermore, this would be an alternative solution for environmental pollution by utilizing waste materials as secondary materials in road construction projects. As published in the literature, the waste of glass, rubbers, plastics and mineral productions were some popular materials used to modify the properties of bitumens and asphalt mixes [16]. Most researches have focused on using waste additives to improve the deformation and fatigue characteristics of asphalt mixes. According to research results, waste glass and waste rubber had a considerable contribution to fatigue resistance of these mixes [17,18]. Nowadays, many countries are seriously encountered with problems related to waste plastic materials. Plastic materials such as plastic bottles are mainly composed of Polyethylene Terephthalate (PET) polymer. PET is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers, beverage, food and other liquid containers, thermoforming applications and engineering resins often in combination with glass fiber [19]. PET is produced by the polymerization of ethylene glycol and terephthalic acid. Ethylene glycol is a colorless liquid obtained from ethylene, and terephthalic acid is a crystalline solid obtained from xylene. When heated together under the influence of chemical catalysts, ethylene glycol and terephthalic acid produce PET in the form of a molten, viscous mass that can be spun directly to fibers or solidified for later processing as a plastic. Based on previous studies PET has a great potential to be reused as modifier in asphalt mixture. Results indicated that adding PET to asphalt raised the mix resistance against permanent deformation and rutting [20,21]. During a laboratory study Mahrez & Karim examined the effect of different PET contents on the rheological properties of modified bitumen. They found that addition of PET to bitumen will increase the viscosity and reduce the temperature susceptibility of modified bitumen. Furthermore, the PET modified bitumen showed preferable elastic properties than the original one (i.e. higher complex modulus and lower phase angle) [22]. During a laboratory study about stone matrix asphalt (SMA) mixes the effect of PET was investigated using the cylindrical specimens. It was inferred that incorporating PET will reduce the bitumen loss which is one of the main SMA deficiencies. Furthermore, the effect of PET on the moisture susceptibility of these mixes was found to be negligible [23]. In a 2012 study Moghaddam et al. compared the stiffness and fatigue properties of PET modified mixes with conventional asphalt. Based on their report the fatigue life of modified mix containing 1% PET (by weight of aggregate) was twice than that of unmodified mix. However, the stiffness of modified mix was to some extent lower than conventional mix. The outcomes of this research indicated that the application of PET in SMA mixes could meet the various requirements of different environmental and loading conditions. Especially the results of stiffness test warranted the proper deformation characteristics of modified mixes at heavy loading conditions [24]. The addition of thermoplastic polymers (e.g. PET) to bitumen or asphalt mix enhances the material rigidity and restricts the permanent deformations under heavy loading conditions especially in upper pavement layers at higher temperatures [25]. The beneficial effects of PET on such high temperature characteristics of asphalt mixtures have been proved elsewhere [22,24]. However their performance in increasing the bitumen elasticity during drastic and
sudden temperature drops is not always satisfactory. In fact they might deteriorate the intermediate and low temperature characteristics of bitumen and asphalt mix (i.e. increasing the cracking potential of mix) [25]. Apart from abovementioned investigations, there is not enough information regarding to fatigue properties of PET modified mixes. For example the fatigue response of these mixes at various temperatures has not been well established. Since extending the range of service temperature is the main purpose of bitumen and asphalt modification it will be interesting to investigate the fatigue properties of PET modified mixes at various temperatures. Hence, in this study, the fatigue and stiffness properties of PET modified mixes have been investigated at intermediate and low temperatures. In this regard the effect of PET was compared with SBS which is a conventional polymer modifier in asphalt mixes and most of earlier researches have proved the beneficial effects of this additive on the technical characteristics of asphalt mixes [12,21,25]. The main objectives of this research were as follows: To investigate the effects of PET on stiffness properties of modified mixes at two testing temperatures. To evaluate the fatigue behavior of PET modified mixes in comparison with unmodified asphalt mixes. To compare the stiffness and fatigue properties of PET modified mixes with that of modified with SBS. 2. Materials and mix design 2.1. Bitumen and aggregate The original binder used in this study was 60/70 penetration grade bitumen that produced in Tehran oil refinery. Table 1, presents the basic properties of this bitumen. Also, as shown in Fig. 1 a 0–12.5 mm aggregate gradation was selected which was approximately in the middle limit of specifications. Table 2 summarizes the specifications of coarse and fine aggregate fractions and filler materials which were blended to achieve the final gradation. 2.2. Additives 2.2.1. PET In this study, waste plastic bottle (PET) was used as modifier additive in hot mix asphalt. To this end, PET bottles were cut into small pieces and crushed by a special crusher. Finally crushed particles were sieved to obtain the needed gradation. As indicated by previous researches, desired results were obtained by single size PET particles between the range of 0.425–1.18 mm [23,25]. Hence, in this research, PET chips were crushed and sieved to obtain the above-mentioned dimensions. Fig. 2 shows the image of the PET crumbs after the crushing and sieving process. PET consists of polymerized units of the monomer ethylene terephthalate, with repeating C10H8O4 units. The related components of studied PET were terephthalic and ethyleneglycol monomers. The physical
Table 1 Technical properties of original bitumen. Property (unit)
Standard
Value
Specific gravity Penetration (0.1 mm) Softening point (°C) Viscosity at 120 °C (cSt) Viscosity at 135 °C (cSt) Viscosity at 160 °C (cSt)
ASTM: ASTM: ASTM: ASTM: ASTM: ASTM:
1.013 65 50 966 467 168
D70 D5 D36 D2170 D2170 D2170
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Fig. 1. The aggregate gradation of studied hot mix asphalt.
Fig. 3. The viscosity–temperature diagram of bitumen used to select the mixing and compaction temperature ranges.
Table 2 Properties of coarse and fine aggregate fractions and filler materials. Property (unit)
Standard
Unit
Value
2.3. Mix design
Coarse aggregate Water absorption Bulk specific gravity Apparent specific gravity
ASTM: C127 ASTM: C127 ASTM: C127
(%) (gr/cm3) (gr/cm3)
2.2 2.498 2.663
Fine aggregate Water absorption Bulk specific gravity Apparent specific gravity
ASTM: C128 ASTM: C128 ASTM: C128
(%) (gr/cm3) (gr/cm3)
2.4 2.467 2.623
Filler Specific gravity
ASTM: D854
(gr/cm3)
2.665
Modifier additives are usually added to mixture under wet or dry conditions. During the wet process, additive first mixed with bitumen with a proper mixer until achieving a homogenous blend. Then blended materials are added to aggregates. In dry method, according to additive’s type and nature this material is mixed with aggregates before adding bitumen or added after mixing the bitumen and aggregates as a part of solid materials. Due to high melting point, it was not possible to mix the PET particles with bitumen in wet process. In fact it was impossible to achieve a homogeneous mixture in this process. The impossibility of achieving a desired blend through adding PET in wet process has been also mentioned in literature [26]. Therefore, in this study, dry method was followed and PET was added to mix with various quantities of 2%, 4%, 6%, 8% and 10% by the weight of bitumen. The optimum bitumen content for unmodified mix was equal to 5.7% which was determined with Marshall method. Based on previous studies the optimum bitumen content for PET modified mixes was almost equal to unmodified mix [24]. Therefore, in this research the same bitumen content was selected for modified and unmodified mixes. The mixing and compaction temperatures were determined by viscosity–temperature diagram. This diagram has been shown in Fig. 3. The viscosity of bitumen during the mixing and compaction process has been recommended between 150–190 and 250– 310 cst, respectively. On the basis of this criterion the mixing and compaction temperatures were kept constant between 157–162 and 145–150 °C, respectively. For each composition first aggregate and bitumen were mixed at abovementioned temperature range and then PET particles were added directly to mixture. Previous studies revealed that the viscosity of PET modified bitumen is to some extent higher than that of unmodified mixes. However, even for PET contents as high as 8% (by the weight of bitumen) the difference between the viscosity of the original and modified bitumen was negligible especially at mixing and compaction temperatures (i.e. higher than 130 °C) [22]. Therefore, PET modified mixes were mixed and compacted at similar temperature ranges of unmodified ones. SBS modified mixes were prepared by wet process. First, by the means of a high shear mixer, SBS was added to bitumen with the values of 4%, 5% and 6% by the weight of bitumen. Then, the prepared SBS-bitumen blend was added to aggregate. After the mixing and compaction process, prepared mixes were tested to measure the resilient modulus. Based on the criterion of maximum resilient modulus the optimum SBS content was selected equal to 5%. Finally, indirect tensile fatigue test was performed for mixes containing 5% SBS.
Fig. 2. Image of crushed PET particles.
properties of this additive were analyzed using related standard methods. Based on performed analysis the density of PET was equal to 1.08 gr/cm3. Also the glass transition and melting point temperatures were equal to 250 and 70 °C, respectively.
2.2.2. SBS The optimum SBS content was determined based on the results of resilient modulus test. Then asphalt mixes were prepared with the optimum SBS content and the fatigue tests were performed. Finally the fatigue response of SBS modified mixes were compared to PET modified ones.
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3. Experimental design The main laboratory program of this research consisted of the resilient modulus and fatigue tests. These tests were performed on cylindrical specimens using indirect tensile method, according to ASTM: D4123 & EN 12697-24, respectively [27]. In order to determine the stress level in above-mentioned tests indirect tensile strength (ITS) was measured according to ASTM: C496. All tests were accomplished at two testing temperatures of 5 and 20 °C. A universal testing machine (UTM-14) apparatus was used which had been equipped with a temperature control chamber. Chamber contained a reference specimen with two linear variable differential transducers (LVDTs) that measured and recorded the skin and core temperatures during the test. In order to achieve the intended temperature, specimens were put inside the chamber at least 5 h before testing. Before starting the test, the chamber, skin and core temperatures were controlled by related software. Test was started when the coefficient of variation of these three temperatures which automatically calculated by the controlling system software was less than 5%. 3.1. Resilient modulus (Mr) During the Mr test a haversine load was applied with the loading frequency of 1 Hz including 0.1 s loading and 0.9 s recovery times. Horizontal deformations were measured by two LDVTs that were installed along the sample’s diameter. At 5 °C resilient modulus test was done at two stress levels of 15% and 20% of ITS. Moreover, at 20 °C stress levels were fixed to 20% and 40% of ITS. For a dynamic load of P, resilient modulus was calculated by Eq. (1):
Mr ¼
Pðc þ 0:27Þ td
ð1Þ
where P: maximum dynamic load (N), c: poisson’s ratio, t: sample height (mm), d: horizontal deformation (mm). Possion’s ratio was calculated according to Eq. (2) [28]:
c ¼ 0:15 þ
0:35 1 þ eð3:18490:04233tÞ
ð2Þ
where t: temperature (°F) 3.2. Fatigue At each temperature fatigue test was performed at two stress levels utilizing indirect tensile loading method with a haversine loading [27]. Each load pulse consisted of 0.25 s loading and 1.25 s recovery times. Loading continued until complete splitting of samples. For indirect tensile fatigue test the maximum tensile stress and strain at the center of sample were calculated by Eqs. (3) and (4), respectively.
St ¼
e¼
2p
ptd
2 DH 1 þ 3c D 4 þ pc p
11
break of sample. As seen in this figure for this definition fracture life corresponds to the point of vertical asymptote [27]. Likewise, Fig. 4 depicts the second definition of fracture life (N2) in indirect tensile method [29]. As it can be seen, the diagram of horizontal deformation is generally defined by the three zones. The accumulated permanent deformations rapidly increase in the primary zone. In the second zone, the rate of deformation increment gets stabilized and the fatigue curve has a linear trend. In the third zone, microcracks which formed in the second stage will progress. Finally the progress and combination of these cracks leads to complete splitting of specimen. According to Fig. 4 in the second definition (N2) the start point of crack progression is defined as the fracture life. As shown this point corresponds to the intersection point of the second and third zones slope.
4. Results and discussion 4.1. Indirect tensile strength (ITS) and resilient modulus (Mr) Fig. 5 shows the results of ITS test. As seen temperature had considerable effects on the ITS of specimens. Addition of 2% PET led to increase of ITS at both testing temperatures. After that, ITS continuously decreased by adding the PET content. At higher PET contents, bitumen accumulates on the surface of the PET particles. This issue results in the reduction of the bitumen film thickness around the aggregate particles and reduces the aggregate–bitumen adhesion and finally the tensile strength of the modified mix. However, at all PET contents the ITS values were in acceptable limit. Based on previous studies due to reduction of the bitumen film thickness, excessive amounts of PET will also reduce the moisture resistance of the modified mix [23]. Results of resilient modulus test at 5 and 20 °C, have been shown in Fig. 6. As seen, the stiffness of studied mixes reduced by increasing the stress level. However, similar to ITS test, at a constant stress level the highest stiffness quantity achieved at 2% PET content. At higher PET contents aggregates will be replaced by these particles which have less stiffness. Moreover, the reduction of the bitumen film around the aggregate particles might be another reason of stiffness reduction especially at higher PET contents. According to Fig. 6 the resilient modulus of studied mixes increased to twice by reducing the temperature from 20 °C to 5 °C. Similar to obtained results at 20 °C, at higher PET contents (i.e. more than 2%) there was a drop in the resilient modulus of studied mixes at 5 °C. However, due to noticeable stiffening of bitumen the dispersion of results were higher at this temperature. The interaction between bitumen and additive materials in modified mixes could have considerable effect on the behavior of
ð3Þ
ð4Þ
where p: maximum dynamic load (N), t: sample height (mm), D: sample diameter (mm), e: tensile strain at the center of samples, DH: horizontal deformation which measured by two LDVTs. During the fatigue testing horizontal deformations were automatically recorded and deformation–loading curve was plotted for each specimen by related software. Fig. 4 shows an example of horizontal deformation curve that obtained in this research. In this figure two fatigue life definitions were compared together (N1 & N2). For the first definition, according to EN12697-24, fatigue life is equal to the total number of cycles which leads to complete
Fig. 4. The load cycle–displacement curve and fatigue life definitions in ITFT method.
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Fig. 7. Relationship between Mr and temperature.
Fig. 5. Results of ITS test at 5 and 20 °C.
mixture. Studies indicated that this interaction changes with increasing the amount of additive. At high polymer contents the polymer phase becomes dominant, whereas, at optimum content of additive there will be an effective interaction which improves the mechanical properties of asphalt mix [30]. In order to evaluate the temperature sensitivity of studied mixes, the rate of resilient modulus (Mr) changes with an increase in temperature was investigated. Fig. 7 shows the effect of temperature on Mr of studied mixes. It should be noted that this test was performed at two temperatures and two stress levels at each temperature. Also at each condition (i.e. each temperature and stress level) the test was repeated twice. Therefore each line in Fig. 7 was drawn based on the results of 8 tests (i.e. 4 tests at each temperature). In this figure the slope values represent the temperature sensitivity of compared mixes. As seen higher slope values obtained for unmodified mixes and the mixes containing lower PET contents (e.g. 440 MPa/°C for unmodified and 445 MPa/°C for specimen containing 4% PET). By increasing the PET content up to 6% the slope reduced to 404 MPa/°C. After that increasing the amount of PET resulted in higher slope. At 10% the slope value was equal to 445 MPa/°C which was even higher than the unmodified one. Results indicated that for controlling the temperature susceptibility of asphalt mix if PET added in dry method there will
be an optimum content which in this research was equal to 6%. It is recommended to consider the temperature susceptibility as a design criterion during the production process of PET modified mixes especially in projects in which the modifier is added to mix via dry method. 4.2. Fatigue Fig. 8 shows the horizontal deformation curve of modified specimen containing 10% PET which tested at 20 °C. The initial strain level in this test was equal to 535 microstrain. Based on the fracture life definitions that presented in Section 3.2, the fracture life for the first and second definitions will be N1 = 65,664 and N2 = 58,500 cycles, respectively. Results of fatigue tests that performed at 5 and 20 °C have been shown in Fig. 9. It must be noted that in this figure the initial stress level at 5 and 20 °C were equal to 20% and 15% of ITS, respectively. Furthermore, this figure shows the results of fatigue test based on both fracture life definitions. Results indicated the beneficial effects of PET on the fatigue behavior of studied mixes. Therewith, at aforementioned stress levels, fatigue life increased upon reducing the test temperature. Unlike the results of ITS and Mr tests, adding the PET content even as high as 10% had profitable effects on the fatigue response of modified mixes.
Fig. 6. Results of resilient modulus test for various PET contents at 5 and 20 °C.
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Fig. 10. Comparison between the fatigue curves at 20 °C .
Fig. 8. Example of displacement curve obtained during the indirect tensile fatigue test.
Fig. 10 compares the fatigue curves of three specimens containing 0%, 8% and 10% PET which tested at 20 °C. With regard to this figure, for modified specimens the slope of deformation curve in the second zone (i.e. the zone with the constant rate of deformation increment) was less than unmodified one. Therefore, it could be concluded that PET modified mixes exhibited higher cracking resistance and flexibility than unmodified mixes. Finding a meaningful relationship between the mix stiffness and fatigue life has been a challenge for pavement scientists. Much research proved that this relationship is completely dependent on the method of fatigue testing. In controlled stress method, usually stiffer mixes revealed higher fatigue life whereas, in strain constant method the reverse was true [4,31–33]. Based on the findings of this research increasing the PET content resulted in lower stiffness and higher fatigue life. It might be due to higher energy absorbency of the PET particles than the bitumen phase which resulted in superior behavior against repeated loadings [7]. This phenomenon will postpone the crack propagation throughout the specimen diameter.
Fig. 11 shows the fatigue curves of both modified and unmodified specimens which attained at 5 and 20 °C. It should be noted that in all presented conditions there was a proper correlation between the initial strain and fatigue life (R2 values were more than 0.8). Fig. 11A compares the fatigue curves of modified and unmodified specimens based on the first fatigue life definition (N1). Similarly, Fig. 11B shows the fatigue laws of studied mixes based on the second definition (N2). According to this figure, at a constant strain level on average the fatigue life of PET modified mix was about 20% higher than unmodified one. Fig. 11 signifies the considerable effect of temperature on the fatigue response of studied mixes. As seen in this figure, the slope of fatigue curves noticeably reduced by decreasing the test temperature. As a result, the fatigue curves intersected each other at initial strain levels of 160–210 microstrain. It means that at higher strain levels of 210 microstrain the fatigue life of studied mixes reduced upon reducing the temperature. In contrast at lower strain levels of 160 microstrain specimens that tested at 5 °C revealed superior fatigue response than those tested at 20 °C. Between the initial strain levels of 160–210 microstrain there was an interference zone in which no meaningful relationship could be found between the testing temperature and fatigue life. Therefore it could be
Fig. 9. Results of fatigue tests at 20 °C (at 20% of ITS) and 5 °C (at 15% of ITS).
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Fig. 12. Comparison between the results of Mr test for PET and SBS modified specimens.
Fig. 11. Fatigue laws of studied mixes at various temperatures.
concluded that at higher strain levels the softer mix which tested at 20 °C exhibited superior fatigue response than the stiffer mix which tested at 5 °C. Hence, under heavy loading conditions fatigue failure becomes critical at lower temperatures. In contrast at lower strain levels of 160 microstrain fatigue failure is expected to occur sooner under moderate climatic conditions. However, apart from the initial strain level the addition of PET to studied mixes led to an increase in fatigue life at both testing temperatures.
Fig. 13. Comparison between the fatigue curves of PET and SBS modified mixes at 20 °C.
4.3. Comparison of PET and SBS Fig. 12 compares the results of resilient modulus (Mr) test for PET and SBS modified mixtures. The stress levels at 5 and 20 °C were equal to 20% and 40% of ITS, respectively. As it can be seen at 20 °C both additives had similar effects on the stiffness of studied mixes. At this temperature, on average the Mr of unmodified mixes was about 4700 MPa. Incorporating 5% SBS increased the Mr value for about 9%. However, as previously mentioned at higher contents of 2%, PET reduced the stiffness of studied mixes. For specimens which tested at 5 °C, SBS had higher efficiency than PET on reducing the stiffness of unmodified mixes. At this temperature the Mr of unmodified mix was about 12,700 MPa which reduced to about 8850 MPa with adding 4% SBS. At low temperatures the asphalt mix tends to behave like brittle material. Therefore at these temperature ranges the lower stiffness is desired due to higher deformability and higher resistance against the detrimental effects of repeated loadings. Therefore, PET modified mixes showed intermediate behavior in comparison to other mixes. SBS is a thermoplastic elastomer which usually could improve both low and high temperature characteristics of modified mixes [34]. Although some authors claim that a decrease in strength and resistance to penetration is observed at higher temperatures, but most of previous documents confirmed the proper effects of this additive at various testing conditions [34].
Fig. 14. Comparison between the fatigue curves of PET and SBS modified mixes at 5 °C.
The SBS content selected for preparing the fatigue test specimens was 5% which corresponded to the SBS content which resulted in maximum resilient modulus. Fig. 13 compares between the obtained fatigue curves of PET and SBS modified mixes at 20 °C. As it can be seen both additives improved the fatigue response of studied mixes. However, SBS modified mixes showed to some extent better fatigue behavior than PET modified ones. As shown in Fig. 14 overall at 5 °C modified mixes revealed predominant fatigue behavior than unmodified asphalt. However, at higher strain levels (i.e. more than 300 microstrain), higher differences were found between the fatigue life of SBS and PET modified mixes. At lower strain levels the differences between the fatigue
A. Modarres, H. Hamedi / Materials and Design 61 (2014) 8–15
curves of modified mixes containing 6% PET and the SBS modified ones reduced. It could be inferred that at lower strain levels of 200 microstrain modified mixes with 6% PET exhibited longer fatigue lives than SBS modified mix. Finally, it could be concluded that PET had comparable effects to SBS on the stiffness and fatigue behavior of studied asphalt mixes. Since PET is a recycled material and is cheaper than original polymer modifiers like SBS, the use of it in asphalt mix is desired in both economical and environmental points of view. 5. Conclusions This paper presents the results of a laboratory study about the effects of waste plastic bottles (PET) on the stiffness and fatigue properties of asphalt mixes. Likewise, similar tests were performed on SBS modified mixes and the acquired results were compared. Based on the obtained results the following conclusions can be drawn. (1) Addition of more than 2% PET reduced the resilient modulus at both testing temperatures of 5 and 20 °C. However, at all PET contents the resilient modulus quantities were in acceptable limit. (2) PET improved the fatigue behavior at both testing temperatures. Unlike the results of ITS and Mr tests the addition of the PET content up to 10% had beneficial effects on the fatigue response of modified mixes. (3) There were some intersection points between obtained fatigue curves at 5 and 20 °C. These points were between the strain levels of 160–210 microstrain. At higher strain levels of 210 microstrain, adding temperature resulted in higher fatigue life. However at lower strain levels of 160 microstrain, stiffer mixes which tested at 5 °C showed better fatigue response than those tested at 20 °C. (4) At 20 °C PET and SBS had similar effects on the stiffness of studied mixes. However, at 5 °C, SBS reduced the mix stiffness. The later outcome is ideal because it improves the material flexibility at low temperatures. (5) Both additives improved the fatigue response of studied mixes. Anyway, SBS modified mixes showed to some extent better fatigue behavior than PET modified mixes especially at higher strain levels of 200 microstrain.
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