Replacement of stabilizers by recycling plastic in asphalt concrete

Replacement of stabilizers by recycling plastic in asphalt concrete

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Goutham Sarang Assistant Professor (Senior), School of Mechanical and Building Sciences (SMBS), Vellore Institute of Technology - Chennai Campus, Chennai, Tamil Nadu, India

14.1

Introduction

Road transportation is generally the most effective and preferred mode of transport, for both freight and passenger movement, due to its easy accessibility and adaptability to individual needs. Generally, roads can be laid with asphalt surfacing, known as flexible pavements, or with cement concrete, called rigid pavements. Most of the roads are flexible types with sub base, base, and surface course over the compacted subgrade layer. Asphalt concrete is conventionally used in the surface layer of flexible pavements. Asphalt concrete is the mixture of asphalt binder, crushed aggregates, and mineral filler in the suitable proportion. The mineral filler used in the mixture together with the binder fill the voids created due to the arrangement of different sizes of aggregates. Depending on the availability and the requirement of pavement, the type of aggregate is chosen, whereas, the expected pavement temperature is the major deciding factor in selecting the asphalt binder. The correct size and proportion of the aggregates and the quantity of binder are arrived through various mixture design procedures, including Marshall mix design, Superpave mix design, etc. The proportion of different sizes of aggregates (technically described as aggregate gradation) is more critical in asphalt concrete, when compared to cement concrete, since it has a significant effect on the performance. The aggregates in conventional asphalt mixture are dense or well graded to provide the maximum possible density, with the orientation of aggregates. Maximum density is achieved by packing the finer-sized aggregates and mineral filler in the voids between coarser sized aggregates. Gap-graded or open-graded mixtures are also used in the asphalt pavement surfaces for specific purposes. Gap-graded mixtures have higher proportion of coarser sized aggregates and mineral filler, and no or small amount of finer aggregates. When the amount of voids is almost the same in the mixtures using dense and gap-graded structures, it is very high in open-graded mixtures, since they generally do not have any mineral filler. This structure is used in porous pavements, where water can be removed easily from the pavement surface and may be used for storm water management. Typical gradation curves for dense, gap-, and open-graded mixtures are shown in Fig. 14.1.

Use of Recycled Plastics in Eco-efficient Concrete. https://doi.org/10.1016/B978-0-08-102676-2.00014-1 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Use of Recycled Plastics in Eco-efficient Concrete Dense graded

Gap graded

Open graded

100

80

% finer

60

40

20

0 0.01

0.1

1 Sieve size (mm)

10

100

Figure 14.1 Typical gradation curves for dense, gap- and open-graded mixtures.

The asphalt concrete mixtures are prepared by heating the constituent materials and mixing them properly at a temperature range of 150 Ce180 C, and these are known as hot mix asphalt (HMA) mixtures, generally used in the pavement surface course. The stipulated temperatures are maintained while laying the mixture in the field and while compacting it. Most of the flexible pavements constructed all around the world use this technology. High energy requirement to achieve the increased temperature, adverse effects on the environment, etc., lead to the development of mixtures which can be prepared at relatively lower temperature, called warm mix asphalt (WMA), and mixtures which do not require heating, known as cold mix asphalt.

14.2

Need for stabilization of asphalt concrete

Asphalt mixtures are intended to render a resilient, relatively waterproof, loaddistributing medium with considerable stability and durability. Being a viscoelastic material, asphalt behaves as an elastic solid at low temperatures and as a viscous liquid at high temperatures. Asphalt is generally susceptible to low temperature cracking, as well as excessive deformation at higher temperature, and hence, both should be addressed simultaneously. In order to use the asphalt binder in the pavement, it should be soft enough to control thermal cracking at low temperatures and stiff enough to control rutting (Jew et al., 1986; Prowell, 2000). Properties of asphalt and asphalt mixes can be improved by incorporating certain additives or a blend of additives. Asphalt treated with these additives or modifiers is known as “Modified asphalt” and is expected to provide mixtures with improved

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life, depending upon the degree of modifications and type of additives used. Tia et al. (1994) reported that Haas et al. (1983) defines these modifiers as: “An asphalt cement additive is a material which would normally be added to and/or mixed with the asphalt before mix production, or during mix production, to improve the properties and performance of the resulting binder and the mix, or where an aged binder is involved, as in recycling, to improve or restore the original properties of the aged binder.” Roberts et al. (1996) listed the advantages of using suitable additives in HMAs. It can provide the mixture with stiffness at high service temperatures as well as softness at low service temperatures. Additives can improve the asphalt-aggregate bonding and resistance to aging, fatigue, and abrasion of the mixture. They help in obtaining thicker asphalt film on aggregates improving the durability of the mixture. Reduction in the thickness of pavement layers and life cycle costs with an overall performance improvement can be achieved by using additives in asphalt concrete. The asphalt modifiers are generally classified into filler, extender, rubber, plastic, fiber, oxidant and antioxidant, hydrocarbon, antistripping agent, combination of plastic and rubber, other waste materials, etc. When crusher dust, cement, fly ash, lime, etc., are used as filler material in asphalt concrete to fill the voids, Sulfur and lignin can be used as extenders (Deme, 1978; FHWA, 2012; Terrel et al., 1980). Oxidation catalysts like manganese salts can be used to increase the stiffness of asphalt concrete mixtures. Sometimes asphalt binder undergoes oxidative hardening while placement, and this can be controlled by using antioxidants with suitable lead compounds. If the asphalt binder lacks the required properties, they can be achieved by incorporating harder or softer hydrocarbons. Stripping of asphalt and aggregates is a common issue with asphalt mixtures all around the world, and this can be controlled to a certain extent with hydrated lime and other antistripping agents (TRB, 2003). The most commonly used modifiers are polymers (both plastic and rubber) and fibers.

14.2.1 Dense graded asphalt concrete A modifier is generally not essential in dense graded asphalt concrete mixtures, but to achieve some specific purpose or to overcome certain deficiencies, a suitable stabilizer can be used. Alexander (1968) reported the usage of different types of modifiers by many researchers and practitioners since 1940s to improve the performance of asphalt binders (Clinebell and Stranka, 1951). In the early stages, asphalt modification using natural and synthetic polymers was more common in Europe than in the United States. When the high initial cost led the US agencies to be reluctant to adopt this technology, its improved life cycle cost made it popular among the contractors in Europe. Later with the development of new polymers and technologies, asphalt mixture modification became common in United States, as well as other countries (Roberts et al., 1996, Brule, 1996, Yildrim, 2007). Punith and Veeraragavan (2007) modified a paving grade asphalt binder using different proportions of reclaimed polythene from Low Density Poly-Ethylene (LDPE) carry bags shredded into size 22 mm. Asphalt concrete was prepared with the modified binder showed improved rutting resistance and temperature susceptibility compared to conventional mixtures and the authors suggested approximately 5%

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(by weight of asphalt binder) polythene content in asphalt concrete mixtures. Along with Styrene Butadiene Styrene (SBS), two other elastomeric polymers (a cohesive product designated as OL and a reactive elastomeric terpolymer designated as EL) € were also used by Ozen et al. (2008) for asphalt binder modification. The possible advantages of binders and pavements with commonly used polymers include increase in softening point, viscosity, ductility, fracture toughness, elastic modulus, flexural strength, creep resistance, reduction in embrittlement by aging, rut susceptibility and low temperature cracking, enhanced Marshall stability, resilient modulus, tensile strength and traction, and overall improvement in performance both in the laboratory and field (Alexander, 1968; Shim-Ton et al., 1980; Denning and Carswell, 1981; Kortschot and Woodhams, 1984; Jew et al., 1986; Carpenter and VanDam, 1987; Lee and Demirel, 1987, Shuler et al. 1987, Nahas et al., 1990; Choquet and Ista, 1992; Dhalaan et al., 1992; Tia et al., 1994; Zaman et al., 1995; Hossain et al., 1999; Palit et al., 2004; Hamzah et al., 2006). Along with different types of polymers, various natural and synthetic fibers also perform as a good modifier in asphalt concrete. Asphalt mixture being weak in tension, McDaniel (2015) indicated that the incorporation of suitable fibers having good tensile properties results in the increase of the tensile strength of the mixture. This is accomplished by the transfer of stresses to the strong fibers, reducing the stresses on the relatively weak asphalt mix. Fibers were used in pavements as a reinforcement and crack retarding material from the beginning of 19th century. Researchers reported the treatment of fiber in the early years in the United States including the usage of asbestos fiber in the 1920s and cotton fibers during 1930s (Maurer and Malasheskie, 1989; Serfass and Samanos, 1996; Al-Qadi et al., 2008; McDaniel, 2015). Based on the availability and suitability, different types of fibers including asbestos, metallic wire, etc., were used in asphalt mixtures (Kietzman, 1960, Tons and Krokosky, 1960). Maurer and Malasheskie (1989) used different fabrics and polyester fiber in pavements and observed that the fiber-reinforced asphalt concrete performed well and the method of random inclusion of fibers was cost-effective, easy to apply, and not causing any delay in construction compared to the other methods adopted in that study. Polyester and polypropylene fibers were observed to increase the fracture energy by 50%e100% when incorporated in asphalt mixtures (Jenq et al., 1993) and similar improvement was observed with nylon fibers also by Lee et al. (2005). Huang and White (1996) concluded that asphalt overlays modified with polypropylene fibers were stiffer and had increased fatigue life compared to conventional overlays. Polypropylene and aramid fibers improved the performance of asphalt mixture by controlling major pavement distresses like permanent deformation, fatigue cracking, and thermal cracking (Kaloush et al., 2010). Glass fibers were also used successfully in asphalt concrete in combination with polypropylene fibers (Abtahi et al., 2013). Jahromi and Khodaii (2008) obtained improvement in mechanical properties like fatigue characteristics, deformation, etc., with the usage of carbon fibers in asphalt mixture. Tapkin (2008) observed that polypropylene fibers stabilized asphalt mixtures possessed increased Marshall and fatigue properties. Xu et al. (2010) studied the reinforcing effects and mechanisms of polyester, polyacrylonitrile, lignin, and asbestos fibers in asphalt concrete mixtures under temperature and water effects, and observed that fibers resulted in

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significant improvement in the rutting resistance, fatigue life, and toughness of mixture.

14.2.2 Gap- and open-graded asphalt concrete While stabilizers are incorporated in dense-graded mixtures for additional benefits, the aggregate structure of gap-graded and open-graded mixtures (as mentioned in the section 14.1 Introduction), and higher asphalt content necessitates the use of a stabilizing additive in these mixtures. Drain down, defined as the portion of the mixture (fines and asphalt) that separates itself from the mixture and flows downward during the elevated temperatures of production, transportation, and placement is a common issue in these two types of mixtures. The presence of increased fine content in gap-graded mixtures like stone matrix asphalt (SMA) and high air void content in open-graded mixtures elevate the drain down. In order to control drain down within limits, stabilizing additives, normally fiber stabilizers, are recommended in these mixtures (Brown and Manglorkar, 1993; Brown et al., 1997; Mallick et al., 2000; Root, 2009). Stabilizer materials may also improve the mixture performance even though generally they are incorporated in order to control the mastic drain down. Suitable fibers are commonly used for this purpose, and AASHTO (1990) reported the wide use of cellulose and rock wool mineral fibers, and less often certain polymers, to control drain down of SMA mixtures in Europe. Mallick et al. (2000) recommend the use of stiffer asphalt binders with polymers for asphalt mixtures with more than 20% voids, especially under medium to high volume traffic conditions. From laboratory observations, Shadman and Ziari (2017) found that to keep the formidability of asphalt in porous mixtures, suitable additives should be used. Most commonly cellulose and mineral fibers are recommended as a drainage inhibitor in asphalt mixtures (Lin et al., 2004, Chiu and Lu, 2007, Ramzanpour and Mokhtari, 2011). Researchers have reported the usage of different cellulose and mineral fibers and polymers (0.2%e0.5%) in various trials conducted in the United States (Brown, 1992; Brown and Manglorkar, 1993; Rademaker, 1996; Brown et al., 1997). Other than conventional ones, researchers have also tried some other types of fibers, including those based on polymers, in SMA mixtures. West (1995) conducted drain down test with different stabilizers including, 0.3% (by weight of total mixture) of cellulose, nylon, polyester, polypropylene fibers, 0.4% slag wool fiber, 12% (by weight of binder) ground tyre rubber, 5% (by weight of binder) Novophalt, and 7% (by weight of binder) Vestoplast. Brown and Cooley (1999) used different stabilizing additives, namely, cellulose fibers, mineral fibers (slag wool and rock wool), and polymers (SBS and polyolefin), and observed that the stabilizer type has significant effect in the low, intermediate, and high temperature performance of SMA fine mortar. Along with cellulose and mineral fiber stabilizers, Schmiedlin and Bischoff (2002) used thermoplastic and elastomeric polymer stabilizers at low and high contents in SMA test sections. The authors reported that the mix temperature should be properly maintained, especially in the case of mixtures with polymers. Putman and Amirkhanian (2004) found that waste fibers, produced from manufacturing processes such as scrap tyre processing and automotive carpet manufacturing, are successful in SMA, by comparing their performance

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with conventional cellulose and other polyester fibers, which are specifically produced for use in HMA. SMA mixtures containing waste fibers showed similar resistance to permanent deformation and moisture susceptibility as that of conventional mixtures, and improved toughness. Xue et al. (2009) used polyester fiber extracted from recycled raw materials with 6.35 mm length, whereas Mahrez and Karim (2010) tried glass fiber in SMA with 80/100 penetration grade asphalt. Out of three natural fibers (coconut, oil palm fibers, and jute fibers), two waste fibers [fibers extracted from refrigerator door panels (FERP), and fibers extracted from old machinery belts [FEMB]), and an artificial fiber (glass fiber) used in SMA, Raghuram and Chowdary (2013) observed better drain down and performance characteristics for the jute fiber and FERP. Verhaeghe et al. (1994) reported that cellulose fibers provide greater stability and higher fatigue cracking resistance to open-graded mixtures. Punith et al. (2004), Punith et al. (2012), and Hassani et al. (2005) used cellulose fiber in open-graded mixtures and observed better results. Presence of fibers also resulted in the increased asphalt binder content, due to the absorption of binder by fiber particles, leading to improved mixture durability. Asphalt binders subjected to suitable modification can also prevent drain down in gap- and open-graded mixtures without any stabilizer, in addition to enhancing the mixture performance. Polymer-modified asphalts (PMA) are considered to provide additional resistance to bleeding, taking out some of the risk associated with high binder contents (Stuart et al., 2001; Shukla and Jain, 1989), and this prompted researchers to use different types of PMA in these mixtures. The most commonly used PMA type in SMA is with an elastomeric polymer SBS (Allen, 2006; Pasetto and Baldo, 2012; Cao and Liu, 2013). Brown and Cooley (1999) and Allen (2006) reported that SMA incorporating SBS PMA produced mixes that were more rut resistant with higher fatigue lives than SMA with unmodified binder. Researchers used SBS and other polymer-based PG binders including PG 70-28, 76-28, and 76-22 in SMA mixtures (Xie et al., 2005; Celaya and Haddock, 2006; Croteau et al., 2006, Vargas-Nordcbeck, 2007; Ishai et al., 2011) and in most of the cases, no other stabilizing additives were required to control drain down. Lin et al. (2004) used four types of commercially available PMA in SMA. The base binder was AC 20 and it was modified with two types of SBS polymers (linear and radial) in two proportions (3% and 6%). The authors used an approach of modified toughness for the evaluation and observed that PMA provided improved modified toughness, which indicates higher stiffness of SMA mixtures. Tayfur et al. (2007) observed least permanent deformation for SMA mix with SBS polymer, compared to mixtures with other polymer and fiber additives. Ghasemi and Marandi (2011) added different combinations of SBS polymer and recycled glass powder with penetration grade 60/70 asphalt and evaluated their advantages in SMA mixtures through Marshall stability, indirect tensile strength, and resilient modulus tests. The additives improved the performance and also provided better mechanical and physical characteristics of both binder and mixture. Hao et al. (2011) observed that the addition of SBS and Trinidad Lake Asphalt in SMA mixture satisfied the requirement, but a combination of both additives showed better performance. In an investigation, Al-Hadidy and Tan (2009) and Al-Hadidy and Yi-qiu (2010) compared SMA mixtures having SBS PMA and starch with control mixture.

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The SBS modified binder resulted in mixes having lesser drain down and increase in stability, Marshall Quotient, rut resistance, and resilient modulus, in comparison with the other mixtures. Mokhtari and Nejad (2013) also made similar observations for SMA with SBS PMA, compared to control mix and mix with FischereTropsch wax added asphalt. Ramzanpour and Mokhtari (2011) observed that the effect of Rheofalt (added in three dosages 5%, 10%, and 15%) in SMA with ACd60/70 asphalt was more than that of SBS (5% by weight of binder) in terms of moisture resistance, whereas SBS was more capable of improving the Marshall and rutting properties. An evaluation of control and SBS SMA mixtures through Marshall Quotient approach, repeated creep test, indirect tensile strength test, and Wheel tracking tests showed the higher performance for polymer added mixtures (Sengul et al., 2013). SBS content of 6.5% was used in PMA by Khodaii et al. (2013) and Haghshenas et al. (2015). Researchers have also tried to use rubber-modified asphalt binders in SMA with an aim to avoid stabilizing fibers and to improve the mix properties (Jain et al., 2004). Generally, the required rubber is collected from used tyres, and they were observed to be performing better than conventional SMAs (Sharma and Goyal, 2006). The natural rubber improves the rutting resistance and ductility, whereas the processed tyre rubber reduces reflective cracking and rutting in SMA mixtures (Ahmadinia et al., 2012). Tyre processing includes punching, splitting, chopping, grinding, and cutting tyres into shredded or “crumb” rubber, as well as chemically altering tyres. Mechanical sizing, including chopping and grinding, is generally used to prepare crumb rubber (CR) by reducing the size of the tyres. Additional grinding and screening operations are carried out to obtain the desirable size range. The rubber modified binder prepared by wet process (by adding crumb rubber in the asphalt as binder modifier) is commonly named as “Asphalt Rubber” (AR) (Epps, 1994, Hossain et al., 1995, Chesner et al., 1998). Better drain down and resistance to deformation were obtained by Kumar et al. (2007) for SMA mixtures with crumb rubberemodified asphalt (CRMA) compared to mixes with natural and patented fibers. Chiu and Lu (2007) modified conventional asphalt binder using coarse and fine-ground tyre rubber after grinding, in different proportions for preparing SMA mixture. It was observed that, only fine rubber could produce a suitable mixture satisfying all volumetric requirements, and it showed better moisture and rutting resistance than the conventional SMA with mineral fiber as stabilizing additive. Dong and Tan (2011) reported excellent performance of SMA pavement with AR, compared to the other two SMAs frequently used in China. Punith et al. (2012) also observed that PG 64-22 asphalt modified with CR helps the SMA mixtures to meet the drain down requirements. Oda et al. (2012) observed improved fatigue behavior for SMA mixtures with AR compared to fiber-added mixes. Peralta et al. (2012) tried to characterize the interactions between asphalt and rubber in the production of AR and a good correlation between the rheological properties of the materials and the physical changes during the process was observed. Sarang et al. (2014a, 2015) successfully used polymer-modified and CRMA binders to prepare SMA mixtures, without any stabilizing additives. Polymer-added asphalt binders produced better results in open-graded mixtures than the ones using fiber stabilizers (Punith et al. 2004, 2012; Suresha et al., 2009;

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Hassani et al., 2005). Sainton (1990) reported constant draining properties, better fatigue and rutting performance under heavy truck traffic and improved resistance to shear stress and weathering for porous asphalt concrete with modified binders. The mixes with modified asphalt possessed higher bulk specific gravity and reduced their air voids and permeability, and improved stone-to-stone contact, compared to the mixes with neat binder. The mixture with CRMA showed better resistance to aged abrasion losses, whereas PMA added mixtures showed higher tensile strength values. For preparing porous asphalt mixtures, along with neat binder PG 64-22, Lyons and Putman (2013) used a SBS modified binder with 3% SBS and two CR modified binder with 5% and 12% CR contents added to neat binder, and emphasized the necessity of stabilizer in those mixtures.

14.3

Addition of plastic in asphalt concrete

Even though polymer addition in asphalt mixtures is considered as a good option to improve pavement performance and life, the cost involved is a serious issue to be considered. Compared to the plain asphalt binder, polymer-modified asphalt binders are costlier and along with the increased energy requirement to prepare the HMAs. As a cost-cutting measure, waste or recycling plastic can be used for the same purpose, instead of virgin polymer. This has wide acceptance, not only due to the cost reduction, also due to its environmental friendliness. The plastic to be added is properly cleaned, and then shredded into sizes generally between 2.36 mm to 600 microns, as shown in Fig. 14.2. However, the method to add these plastics is an issue to be addressed.

Figure 14.2 Shredded waste plastic

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Incorporation of waste plastic in asphalt concrete is a hot topic of discussion even among the researchers. Normally plastics can be added in two methods, namely wet process and dry process.

14.3.1 Wet process In wet process, shredded plastics are directly added to the hot asphalt binder, and they are heated and mixed thoroughly to obtain a uniform mixture. Polymer-modified asphalt binders are prepared in this manner, generally with the help of a sophisticated equipment. High power stirring may be necessary to obtain a uniform blend with proper bonding between the constituent materials. If proper blending is not ensured, plastic materials may get separated from the binder and settle. In the preparation of large quantity of asphalt binder, this method can be adopted because in such cases required equipment can be used. But for small road projects, using sophisticated machineries to carry out proper blending may not be economical. Another issue associated with this process is regarding the shelf-life of the prepared asphalt binder. In many cases, storage stability will be less and the prepared binder may have to be used in a short span of time.

14.3.2 Dry process In the dry process, aggregates are heated as in the case of normal HMAs, and then the recycling plastic (preferably in shredded form) of required quantity is added and mixed properly until uniform mixing is ensured. In this case, the aggregates are coated with plastic, and then the asphalt binder is added to this blend, by maintaining the required temperatures. Indian Roads Congress (IRC) has issued a guideline for using waste plastic in hot asphalt mixes and suggested dry process, listing the following advantages (IRC SP 98, 2013). Plastic coating over stones is easy and the process improves the surface property and binding of aggregates. The temperature required is the same as the road laying temperature and no additional equipment is needed. The flexible films of all types of plastics can be used for this purpose. Still some researchers doubt the binding of asphalt with aggregates, since they are already coated with plastics.

14.4

Performance of asphalt concrete with plastics

Researchers modified asphalt concrete by adding recycling plastics in both wet and dry process methods. As reported by Little (1993), Felsinger Group from Austria conducted a study in 1989 and concluded that recycled low-density polyethylene (LDPE) can be added as a modifier to prepare asphalt binder with equal performance of binder produced by virgin polymer. Liang et al. (1993) observed that recycled polythene did not show much reduction in the quality of modified asphalt, but significant material cost saving was possible, when compared to the addition of virgin polymer. Addition of recycled or waste LDPE, high-density polyethylene (HDPE), plastics, and polyvinyl

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chloride (PVC) with asphalt improves the stability, tensile strength, stiffness, void characteristics, Marshall quotient, and moisture resistance of asphalt mixtures (Panda and Mazumdar, 2002; Hınıslıo glu and A gar, 2004; Bose et al., 2005; Rahman et al., 2013). Polacco et al. (2005) and Garcı’a-Morales et al. (2006) studied the rheology of asphalt binder modified with different recycled polymers and explained the compatibility of each polymer. Asphalt binder modified with shredded waste polythene caused increase in storage stability and resistance to aging, viscosity, degradation, and temperature susceptibility, compared to the unmodified binder, and this modified binder was observed to be improving the performance of asphalt mixture based on the results from dynamic creep test, indirect tensile test, resilient modulus test, and Hamburg wheel track test (Punith and Veeraragavan, 2007, 2010a, 2010b). Even though many research works have been carried out using wet process, comparatively limited studies are reported with the method of dry process for incorporation of waste polymer in asphalt mixtures. Zoorob and Suparma (2000) used recycled plastic pellets with 5.00e2.36 mm size in dense graded asphalt mixture as a replacement to the same sized aggregates. The mixture named as “plastiphalt,” was observed to have increased strength and improved deformation capacity. Hassani et al. (2005) replaced different percentages of 4.75e2.36 mm aggregates with polyethylene terephthalate (PET) granules in asphalt concrete and determined the volumetric and Marshall properties. Some researchers have observed that coating of shredded plastic over the hot aggregate (by dry process) provides the mixture better strength and performance than blending it with asphalt (wet process), and also it helps in the usage of higher quantity of polymers (Vasudevan et al., 2006, 2010, 2012; Ravi Shankar et al., 2013; Shankar et al., 2014). Awwad and Shbeeb (2007) have observed that the polymer coating over aggregates provides a rougher surface structure and better adhesion between aggregates and asphalt, and this improves the engineering properties of the mixture. Addition of waste plastics and waste polymeric packaging material using dry process was reported to be improving the impact value, abrasion value, and water absorption of aggregates (Sabina et al., 2009), along with increasing the stability, tensile strength, moisture susceptibility, and rut resistance thereby improving the pavement performance (Jain et al., 2011). A study conducted by Aslam and Rahman (2009) showed that most of the commonly used polymers do not cause any evolution of gas around 130e140 C and at this temperature, plastic will be in the molten form having well-binding property. IRC also suggests the usage of waste plastics shredded into size between 2.36 mm and 600 mm, by the dry process method (IRC SP 98, 2013). Little (1993) conducted experiments on two types of SMA mixtures with recycled LDPE additives, and they were observed to be performing better than mixes without polymer. Casey et al. (2008) modified asphalt binder by adding some commonly available recycled polymers in different proportions, to use in SMA mixes, and the binder and mixture performances were assessed. Punith et al. (2010) incorporated reclaimed polyethylene obtained from carry bags in SMA by blending with penetration grade 60/70 asphalt (5% by weight of asphalt) and also by shredding and mixing

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with aggregates (0.3% by weight of mixture). Both methods controlled the mixture drain down and performed better than conventional SMA with cellulose fiber additive. Incorporation of waste plastic bottles (PET) at various percentages (2%, 4%, 6%, 8% and 10% by weight of asphalt) in aggregates-asphalt blend was effective in retarding the drain down and improved mixture’s Marshall characteristics, stiffness, and resistance against permanent deformation (Ahmadinia et al., 2011, 2012). Similar observation was made by Moghaddam and Karim (2012) and Moghaddam et al. (2014) with the addition of waste PET flakes (at dosages 0.2%, 0.4%, 0.6%, 0.8%, and 1% by weight of aggregates), obtained from PET bottles, in SMA using dry process method. Sarang et al. (2014b, 2016) prepared SMA mixtures by adding 4%, 8%, 12%, and 16% (by weight of asphalt) of shredded waste plastics in dry process method to plain asphalt binder. Mixture with 8% plastic content performed well, and the results were comparable with the SMA mixture with polymer-modified binder.

14.5

Field investigations

Positive results obtained from the laboratory investigations, in terms of better strength and durability, by using recycling plastics in asphalt concrete, have given confidence to the contractors and agencies to use the technology in field. In Hong Kong, Wang et al. (2017) conducted studies on laboratory prepared asphalt binder samples, and SMA pavements constructed using neat asphalt and different combinations of PMA (30, 85 and 100 %) with neat asphalt, mainly to establish test methods to forensically check the type and quality of PMA. A study conducted in Switzerland on porous asphalt concrete using laboratory specimens and core samples collected from eight pavement sections showed that the mix type not made with PMA was the most water sensitive and the same had poor fatigue behaviour also (Poulikakos and Partl 2009, 2012). In India, different states had constructed roads with waste plastic added asphalt concrete mixtures, even before the release of the IRC guidelines, and after that many states started trying the same. A stretch in the city of Bengaluru, some stretches in the Tamil Nadu state, etc., laid in 2002, are some of the first plastic roads in India (Sapna, 2012; Sribala, 2016). For some stretches with plastic added asphalt concrete, laid during 2002, evaluation was done during 2007e08 period, and compared with a road without plastic (Central Pollution Control Board, 2008). The following parameters were examined for each road: the roughness of the pavement surface, the resistance offered by the pavement surface against skidding of vehicles, the pavement macrotexture for the geometrical deposition, the field density, the structural evaluation for the strength of the pavement, the gradation of the materials in the laid road, different tests on recovered bitumen, the condition of the road (cracks, raveling, potholes, rutting, corrugation edge break, etc.). From the study it was concluded that there was no pothole formation, rutting, or raveling for those roads after 5e6 years of construction. Another interesting development is “100% plastic road” idea by a company, and the same is planned in Rotterdam by the city council (Volkerwessels, 2015; Sims, 2015).

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14.6

Conclusion

Based on the study conducted, the following conclusions can be drawn: • • • • • • • •

Neat asphalt binders may not be sufficient to withstand the increasing traffic load; they have to be improvised using suitable stabilizer/modifier. Among different methods of stabilizing asphalt concrete mixtures, usage of different types of polymers (both rubbers and plastics) and natural and synthetic fibers is very common and effective. Stabilizers are generally a part of the mix design in gap-graded and open-graded mixtures in order to control drain down. Stabilizing additive may not be required if the asphalt binder used in gap- or open-graded mixture is polymer-modified one, or other suitable modifier is included. Most of the researchers observed that, recycling or waste plastic shows similar performance of virgin polymer in stabilizing the asphalt concrete and controlling drain down. Even though many researchers and some design standards consider dry process as the preferable method to incorporate waste plastic in asphalt concrete, the bonding between asphalt binder and aggregates is an issue to be addressed. A few field evaluations reportedly showed better performance of “plastic roads” compared to roads without plastics. Waste or recycling plastics can be recommended as an efficient stabilizer and modifier in asphalt concrete to enhance the mixture stability performance, since they are easily available, economic, and moreover have environmental benefits.

References AASHTO, 1990. Report on the 1990 European Asphalt Tour. American Association of State Highway and Transportation Officials, Washington DC, USA. Association of Asphalt Paving Technologists, 69, pp. 391e423. Abtahi, S.M., Esfandiarpour, S., Kunt, M., Hejazi, S.M., Ebrahimi, M.G., 2013. Hybrid reinforcement of asphalt-concrete mixtures using glass and polypropylene fibers. Journal of Engineered Fibers and Fabrics 8 (2), 25e35. Ahmadinia, E., Zargar, M., Karim, M.R., Abdelaziz, M., Shafigh, P., 2011. Using waste plastic bottles as additive for Stone Mastic Asphalt. Materials and Design 32 (10), 4844e4849. Ahmadinia, E., Zargar, M., Karim, M.R., Abdelaziz, M., Ahmadinia, E., 2012. Performance evaluation of utilization of waste Polyethylene Terephthalate (PET) in stone mastic asphalt. Construction and Building Materials 36, 984e989. Al-Hadidy, A.I., Tan, Y., 2009. Mechanistic analysis of ST and SBS-modified flexible pavements. Construction and Building Materials 23 (8), 2941e2950. Al-Hadidy, A.I., Yi-qiu, T., 2010. Comparative performance of the SMAC made with the SBSand ST-modified binders. Journal of Materials in Civil Engineering 22 (6), 580e587. Al-Qadi, I., Morian, D., Stoffels, S., Elseifi, M., Chehab, G., Stark, T., 2008. Synthesis on Use of Geosynthetics in Pavements and Development of a Roadmap to Geosynthetically-modified Pavements, Report. Federal Highway Administration (FHWA), USA. Alexander, J.A., 1968. Effects of Rubber Additives on Properties of Asphaltic Materials. MS Thesis. Massachusetts Institute of Technology, USA.

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