A Synthesis on the Effects of Two Commercial Recycled

sustainability

Article A Synthesis on the Eﬀects of Two Commercial Recycled Plastics on the Properties of Bitumen and Asphalt

Greg White School of Science and Engineering, University of the Sunshine Coast, Sunshine Coast, QLD 4561, Australia; [email protected]; Tel.: +61-400-218-048

Received: 7 September 2020; Accepted: 14 October 2020; Published: 16 October 2020

Abstract: The desire to develop sustainable infrastructure, including pavement structures and materials, is ever increasing in recent times. One opportunity is to partially replace high-cost bituminous binder with low-cost recycled plastic in asphalt mixtures. This synthesis combines the various research efforts to understand the effects of two commercially available recycled plastics, known as MR6 and MR10, on bituminous binders and asphalt mixtures. Using common test methods from the United Kingdom, the United States and Australia, generally consistent and significant effects were observed in various base bitumen grades and various common asphalt mixture types. Binder resistance to flow and binder elasticity both increased significantly and were associated with the three to four grade increases under the Performing Grading system. Similarly, mixture stiffness and mixture resistance to deformation increased significantly, while crack resistance and moisture damage resistance were not significantly affected. The effects of MR6 and MR10 were generally similar to the effects associated with conventional polymer modification of asphalt binders and asphalt mixtures, particularly those effects associated with plastomeric polymers.

Keywords: recycled; plastic; bitumen; asphalt

1. Introduction The desire to develop sustainable infrastructure, including pavement structures and materials, is ever increasing in recent times [1]. Given the diversity of pavement structures, which can include cement concrete, asphalt mixtures, granular crushed rock, and natural gravels, the opportunities for sustainable pavement construction are broad and many. When considering sustainability opportunities, it is important to take into account the eﬀect on the durability and expected life of the pavement, as well as the reduction in ﬁnancial or environmental cost of the more sustainable solution [2]. That is, an initiative that reduces the new pavement’s greenhouse gas emissions by 20% is not really sustainable if the pavement only lasts 50% of the life of the conventional solution [3]. It is also important to understand that sustainability initiatives are only viable if the cost to collect, process, and reincorporate a recycled material or product is less expensive than the cost of the material or product that it replaces [4]. For this reason, the replacement of high-cost materials, such as bituminous binder, and cement, with recycled or repurposed materials is a great interest. One opportunity is to replace high-cost bituminous binder with low-cost recycled plastic in asphalt mixtures [1]. However, there are many types and forms of recycled plastic and only a few of them are well suited to recycling in asphalt mixtures [5]. As detailed later, some researchers and practitioners have recycled plastic into asphalt mixtures in many diverse ways, ranging from simply shredding plastic containers, crates or car bumpers and adding them to asphalt mixture production, right through the depolymerization of waste plastic and blending of the recovered raw polymers into bituminous binder [6].

Sustainability 2020, 12, 8594; doi:10.3390/su12208594 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 8594 2 of 20

This paper presents a synthesis of research on two commercially available recycled plastic products for the modification and extension of bituminous binders in asphalt mixture production. The aim is to provide a summary of the effects of the two products on bitumen and asphalt and to consider the general trends found across various research efforts. The products, known as MR6 and MR10, are intended to be plastomeric and elastomeric, respectively. This synthesis is focused on the physical and mechanical properties of the bituminous binder and asphalt mixtures produced with both recycled plastic products. Environmental and practical issues, such as leaching, fuming, and storage stability, are not addressed here.

2. Background

2.1. Use of Recycled Plastic in Road Pavements Many countries have now reported the use of recycled plastic in asphalt mixture production, either as an aggregate extender, a bitumen extender, or a binder modifier. For example, Vancouver (Canada) incorporated plastic crate waste as a warm mixed asphalt wax additive in 2012 [7] and Rotterdam (The Netherlands) announced a plan to produce recycled plastic segments in a factory for road construction in 2015 [8]. Moreover, Janshedpur (India) reported reducing bitumen usage by 7% by mixing shredded recycled plastic during asphalt production [9]. More recently, a New Zealand asphalt contractor added shredded engine oil containers to asphalt at Christchurch Airport [10] and an independent asphalt producer includes recycled plastic as bitumen extended in every ton of asphalt produced. South Africa commenced recycled plastic trials in 2019, on a road near Jeffrey’s Bay [11]. In Australia, a comparative trial of three recycled plastic extenders and modifiers was constructed in May 2018, which was shortly followed by trials in Melbourne [12], Sydney [13], Adelaide [14], and Canberra [15]. Previous use of recycled plastic in the United Kingdom has recently been expanded to include roads and highways from Cumbria in the north, to Kent in the South, and many counties in between [16]. Even the United States has now performed trials in San Diego [17] and, more recently, in downtown Los Angeles [18]. It is clear that there is great interest in the use of recycled plastic in bituminous binders and asphalt mixtures. It is also clear that different trials have taken very different approaches to using different kinds of plastic and this can only lead to confusion about the technology.

2.2. Plastic Recycling in Asphalt Mixtures As stated above, there are many approaches to recycled plastic in asphalt mixtures. The polymer and form of plastic, the aim of the user and the local logistics will all inﬂuence the most appropriate approach to make. However, these same factors will also aﬀect the properties of the resulting asphalt mixture and the long-term performance of the road surface. In general, there are four approaches to recycled plastic in asphalt mixtures:

Uncontrolled waste disposal. Uncontrolled incorporation of variable plastic with only minimal • processing to reduce the particle size to be comparable to asphalt mixture aggregates. This offers minimal return on the investment and a higher performance risk. Aggregate extension. Hard plastic does not melt at typical asphalt production temperatures but • can partially replace or extend the aggregate in the mixture. However, the aggregate is much less expensive than the bituminous binder, so this approach provides a lower return on the investment. Some performance enhancement is often reported due to the reinforcing nature of the plastic, which is often flexible in nature. Binder extension. Soft plastic can be melted into the bituminous binder to partially replace or • extend the binder without necessarily enhancing its properties. This provides a greater return on the investment than aggregate extension because the binder is significantly more expensive than the aggregate. Sustainability 2020, 12, 8594 3 of 20

Sustainability 2020, 12, x FOR PEER REVIEW 3 of 20 Binder extension and modification. When soft plastic is used to modify, as well as extend, • • theBinder bituminous extension binder, and modification. the effects can When be similar soft plas to thosetic is associatedused to modify, with traditional as well as polymersextend, the for binderbituminous and mixture binder, modificationthe effects can [19 be]. similar to those associated with traditional polymers for Thebinder use and of plastics mixture that modification can both modify [19]. and extend the bituminous binder is the most beneficial approachThe use to recycling of plastics plastic that can is both asphalt modify mixtures. and exte Thisnd isthe because bituminous the extended binder is the bituminous most beneficial binder isapproach expensive, to recycling but the polymersplastic is as thatphalt would mixtures. otherwise This is bebecause used th fore extended asphalt mixturebituminous performance binder is enhancementexpensive, but are the even polymers more expensive. that would otherwise be used for asphalt mixture performance enhancement are even more expensive. 2.3. The Products 2.3. BothThe Products MR6 and MR10 are developed and produced by MacRebur Ltd. [20]. In 2015, MR6 was developedBoth MR6 to: and MR10 are developed and produced by MacRebur Ltd. [20]. In 2015, MR6 was developed to: Productively consume a portion of waste plastic otherwise destined for landfill. • • ReduceProductively the cost consume of new roada portion construction of waste andplastic maintenance. otherwise destined for landfill. •• IncreaseReduce the strengthcost of new and road durability construction of local and roads. maintenance. •• Increase the strength and durability of local roads. MR6 was developed to improve deformation resistance via an increase in asphalt mixture MR6 was developed to improve deformation resistance via an increase in asphalt mixture stiffness. MR10 was developed to produce a more elastomeric and crack resistant bituminous binder. stiffness. MR10 was developed to produce a more elastomeric and crack resistant bituminous binder. Both products are manufactured from 100% recycled plastic that was selected for its physical properties, Both products are manufactured from 100% recycled plastic that was selected for its physical as well as being otherwise not economically recyclable. That is, the plastic used to produce MR6 properties, as well as being otherwise not economically recyclable. That is, the plastic used to produce and MR10 would otherwise be disposed of in landfill because there are no viable alternate recycling MR6 and MR10 would otherwise be disposed of in landfill because there are no viable alternate opportunities at this time. recycling opportunities at this time. The original MR6, was developed in a pelletized form. This included melting, extruding, The original MR6, was developed in a pelletized form. This included melting, extruding, and and cutting of the recycled plastic. This produced dense pellets with homogenous color (Figure1). cutting of the recycled plastic. This produced dense pellets with homogenous color (Figure 1). The The density of the pellets was determined to optimize the international shipping costs, which are based density of the pellets was determined to optimize the international shipping costs, which are based on the worse case of product volume and weight. By adjusting the packed density, the volume-based on the worse case of product volume and weight. By adjusting the packed density, the volume-based and weight-based shipping costs were equal, which is the most efficient for shipping. MR10 was also and weight-based shipping costs were equal, which is the most efficient for shipping. MR10 was also developeddeveloped asas aa pelletpellet (Figure(Figure1 1).). However,However, toto improveimprove thethe distributiondistribution of MR6 through the asphalt mixturemixture when when aa dry-mixingdry-mixing processprocess isis used,used, MR6MR6 waswas converted to a a shredded shredded form form in in 2019. 2019. Due Due to to thethe di differentfferent chemical chemical composition composition ofof MR10,MR10, itit isis stillstill producedproduced in a pelletized form (Figure 22).).

(a) (b)

FigureFigure 1.1. Original (a) MR6 pellets and and ( (bb)) MR10 MR10 pellets. pellets. Sustainability 2020, 12, 8594 4 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 4 of 20

(a) (b)

Figure 2. CurrentCurrent ( (aa)) MR6 MR6 shreddings shreddings and and (b (b) )MR10 MR10 (unchanged) (unchanged) pellets. pellets.

3.3. Research Methods The data presented in this this synthesis synthesis are are extracte extractedd from from existing existing publications publications on on the the effects eﬀects of of MR6MR6 andand MR10MR10 onon bituminousbituminous binder binder and and asphalt asphalt mixture mixture properties. properties. The The applicable applicable publications publications are summarizedare summarized in Table in Table1. In all1. cases,In all thecases, recycled the recycled plastic wasplastic added was at added a rate ofat 6%a rate of theof bituminous6% of the binderbituminous mass. binder mass.

Table 1. MR6 and MR10 research publications. Table 1. MR6 and MR10 research publications.

TitleTitle Focus Focus and Scope and Scope Year Year ReferenceReference SummarySummary of theof the products products and and the the effect RecycledRecycled waste waste plastic plastic for for extending extending and effonect asphalt on asphalt mixture mixture performance 20182018 [21][21] andmodifying modifying asphalt asphalt binders binders performanceproperties properties EvaluatingEvaluating recycled recycled waste plastic The eTheffect effect on typical on typical asphalt asphalt mixture mixture modificationmodification and exte andnsion extension of bituminous of performanceperformance properties, properties, as well as as well as2019 2019 [22][22] bituminous binder for asphalt environmental and safety issues binder for asphalt environmental and safety issues The effect on the Performance Grade Recycled waste plastic modification of The effect on the Performance Grade Recycled waste plastic modification of (PG) rating of two penetration grade 2019 [23] bituminous binder (PG) rating of two penetration grade 2019 [23] bituminous binder bituminous binders bituminous binders The improvement in base and Objective evaluation of the practical The improvement in base and surface Objective evaluation of the practical surface mixture modulus and benefits of asphalt binders modified mixture modulus and fatigue life and2019 [24] benefits of asphalt binders modified with fatigue life and the associated effect 2019 [24] with recycled plastic the associated effect on predicted recycled plastic on predicted pavement life pavement life LaboratoryLaboratory evaluation evaluation of asphalt EffEffectect of of common common bituminous bituminous binder containing recycled plastic as a binder properties and surface 2019 [25] containing recycled plastic as a bitumen properties and surface mixture 2019 [25] bitumen extender and modifier mixture performance properties extender and modifier performance properties Comparing wet mixed and dry mixed Comparison of the effect of wet and Comparing wet mixed and dry mixed Comparison of the effect of wet and dry binder modification with recycled dry production on bituminous 2020 [26] binder modificationwaste plasticwith recycled waste productionbinder properties on bituminous binder 2020 [26] plastic properties Summary of the effects on various Recycled plastic as an alternative to Summary of the effects on various Recycled plastic as an alternative to bituminous binder properties, conventional polymers for bituminous binder properties, including2021 [27] conventional polymers for bituminous including those used in the UK, 2021 [27] bituminous binder those used in the UK, Australia, and the binder Australia, and the USA USA Laboratory comparison of wet-mixing Comparison of the effect of the Comparison of the effect of the Laboratoryand dry-mixing comparison of recycledof wet-mixing waste and production process on the binder production process on the binder and2021 [28] dry-mixingplastic of recycled for binder waste and plastic for and mixture properties of a 2021 [28] asphalt modification mixturecommon properties surface mixture of a common surface binder and asphalt modification mixture Sustainability 2020, 12, 8594 5 of 20

The general approach taken in the various research eﬀorts detailed in Table1 was to test otherwise nominally identical samples of bituminous binder and/or asphalt mixtures, produced with and without recycled plastic. The binder and mixture samples were typical of materials used for local and highway road construction in the United Kingdom and Australia. The binders were either viscosity (in Australia) or penetration (in the United Kingdom) grade materials, while the asphalt mixtures were either dense graded asphalt (DGA) or stone mastic asphalt (SMA) of various nominal maximum aggregate size (NMAS) (Table2).

Table 2. Asphalt mixture properties.

Used by Research References Property [21,24][21,22,24][25][26,28] Mixture type Dense graded Stone mastic Dense graded Dense graded NMAS (mm) 20 10 10 10 Binder content (%) 4.8 6.3 4.9 5.2 Standard EN 13108-1 EN 13108-5 BCC Type 3 EN 13108-1

The test methods were all standard methods used in the United Kingdom, the United States, and Australia for measuring the characteristics and relative performance of binders and mixtures, as summarized in Table3, for bituminous binders, and Table4, for asphalt mixtures.

Table 3. Bituminous binder test methods.

Property Method Description Measured in Research Penetration by a standard Penetration EN 1426 needle, over 5 s, into a binder [23,26,28] sample at 25 ◦C Percentage of elongation until Force ductility EN 13703 separation of a binder sample at [23] 25 ◦C Percentage recovery of a cut Elastic recovery EN 13398 binder sample after elongation [26,28] by 200 mm at 25 ◦C Softening temperature of a EN 1427 (UK) and Softening point binder sample according to the [23,25,26,28] AG:PT/T131 (A) Ring and Ball method Propensity of the binder to ﬂow Viscosity 60 AS 2341.2 [25] under load at 60 ◦C Percentage of torsional recovery Torsional AG:PT/T122 of a binder sample after rotating [25] recovery 180◦ at 25 ◦C (UK) denotes the method used in the UK while (A) denotes the test method used in Australia.

Table 4. Asphalt test methods.

Property Method Description Measured in Research Indirect tensile modulus at 20 C EN 12697-26 (UK) AS ◦ Stiffness (UK) or 25 C (A), an indicator of [22,24,25] 2891.13.1 (A) ◦ sample stiffness Marshall Stability of samples EN 12697-34 (UK) prepared by 50 blows to each side Stiffness [25,26,28] AS/NZS 2891.5 (A) by a standard Marshall hammer and tested at 60 ◦C Sustainability 2020, 12, 8594 6 of 20

Table 4. Cont.

Property Method Description Measured in Research Deformation following 10,000 passes of a Cooper’s wheel tracking EN 12697-22 (UK) Deformation resistance wheel of samples at a [21,22,25,26,28] AG:PT/T231 (A) pre-determined temperature, generally 40–60 ◦C Indirect tensile fatigue life of over a range of initial tensile strain magnitudes to develop a Cracking resistance EN 12697-24 [22,24,26,28] relationship between initial strain and cycles to failure, an indicator of sample fatigue life

Four-point bending at 20 ◦C and 200 Cracking resistance AG:PT/T274 µε sinusoidal repeated load, an [25] indicator of sample fatigue life Semicircular bending of notched samples under monotonic loading Cracking resistance EN 12697-44 [21,22] and tested at 0 ◦C, an indicator of sample fracture toughness Marshall Flow of samples prepared EN 12697-34 (UK) by 50 blows to each side by a Cracking resistance [25,26,28] AS/NZS 2891.5 (A) standard Marshall hammer and tested at 60 ◦C Ratio of indirect tensile strength of conditioned and unconditioned Moisture damage EN 12697-12 (UK) samples, where conditioning [21,22,25,26,28] resistance AG:PT/T232 (A) includes saturation and 72 h in 40 ◦C water (UK) denotes the method used in the UK, while (A) denotes the test method used in Australia.

4. Eﬀect on Bituminous Binders

4.1. Resistance to Flow

Viscosity at 60 ◦C measures flow potential, while penetration measures the resistance to flow. Generally, penetration and viscosity are inversely related, with viscosity increasing as penetration decreases for the same bituminous binder. The penetration of 100–150 penetration grade bitumen sourced and tested in the UK [23] reduced by 33% and 30%, with the addition of MR6 and MR10, respectively (Figure3). Similarly, for 70–100 penetration bitumen, MR6 [ 26] and MR10 [28] reduced the penetration by 54% and 36%, respectively (Figure4). Accelerated laboratory aging by the rolling thin film over (RTFO) reduced the unmodified binder penetration from 100 d.mm to 53 d.mm and the penetration of the MR6 and MR10 modified samples was 45% and 32% of the unmodified binder penetration after RTFO (Figure4). The reduced relative penetration after RTFO aging indicates that the addition of recycled plastic may also slow the binder aging process. However, specific studies are required to confirm this. Finally, the viscosity of Australian C170 (viscosity grade) bitumen [25] increased by 113% for MR6 and by 164% for MR10 (Figure5). All these results indicate the addition of recycled plastic to otherwise unmodified bitumen increases the resistance to flow of the bituminous binder. This is similar to the effect observed when conventional styrene-butadiene-styrene (SBS) and ethyl vinyl acetate (EVA) polymers are added to binders at comparable dosages. Sustainability 2020, 12, x FOR PEER REVIEW 7 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 7 of 20 Sustainability 2020, 12, 8594 7 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 7 of 20

Figure 3. Effect of recycled plastic on 100–150 bitumen penetration [23]. Figure 3. Effect of recycled plastic on 100–150 bitumen penetration [23]. Figure 3. Eﬀect of recycled plastic on 100–150 bitumen penetration [23]. Figure 3. Effect of recycled plastic on 100–150 bitumen penetration [23].

Figure 4. Effect of recycled plastic on unaged and aged 70–100 bitumen penetration [26,28]. FigureFigure 4. EffectEﬀect of of recycled recycled plastic on unaged an andd aged 70–100 bitumen penetration [26,28]. [26,28 ]. Figure 4. Effect of recycled plastic on unaged and aged 70–100 bitumen penetration [26,28].

Figure 5. Eﬀect of recycled plastic on C170 viscosity [25].

Sustainability 2020, 12, x FOR PEER REVIEW 8 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 8 of 20 Figure 5. Effect of recycled plastic on C170 viscosity [25]. Figure 5. Effect of recycled plastic on C170 viscosity [25]. Sustainability4.2. Elasticity2020 and, 12 ,Ductility 8594 8 of 20 4.2. Elasticity and Ductility Force ductility measures the degree of elongation before a bituminous binder sample is broken. 4.2.Both Elasticity Forcetorsional ductility and recovery Ductility measures and elastic the degree recovery of elongation measure thebefore degree a bituminous to which binda sampleer sample recovers is broken. after deforming,BothForce torsional ductility either recovery by measures rotation and elastic theor by degree elongation.recovery of elongation measure Unmodified beforethe degree 100–150 a bituminous to penetrationwhich binder a sample bitumen sample recovers isand broken. C320 after Bothviscositydeforming, torsional grade either recovery bitumen by rotation and have elastic ornegligible by recoveryelongation. elasticity measure Unmodified or ductility. the degree 100–150 However, to whichpenetration when a sample recycledbitumen recovers andplastic afterC320 is deforming,added,viscosity the grade eitherforce bitumenductility by rotation have[25] or (Figurenegligible by elongation. 6), elasticelasticity Unmodified recovery or ductility. [26,28] 100–150 However, (Figure penetration 7), when and bitumen torsionalrecycled and recoveryplastic C320 is viscosity[25]added, (Figure the grade force8) all bitumen ductilityincreased have [25] significantly. negligible(Figure 6), elasticityMR10 elastic had recovery or a ductility.greater [26,28] effect However, (Figure on the 7), elasticity when and recycledtorsional and ductility plastic recovery of is added,the[25] binder (Figure the forcethan 8) all MR6 ductility increased did. [The25 ]significantly. (Figureintroduction6), elastic MR10 of signific recovery had anta greater [ 26ductility,28] effect (Figure and on elasticity7), the and elasticity torsional is consistent and recovery ductility with [ 25the of] (Figureeffectthe binder of8 )moderate all than increased MR6 SBS did. significantly.or Thesignificant introduction MR10EVA conventi hadof signific a greateronalant polymer ductility e ffect on modification and the elasticity elasticity of is and bituminousconsistent ductility with binder of the the binder[19].effect of than moderate MR6 did. SBS The or introductionsignificant EVA of significant conventi ductilityonal polymer and elasticity modification is consistent of bituminous with the binder effect of[19]. moderate SBS or significant EVA conventional polymer modification of bituminous binder [19]. 2.5 2.5 2.0 ) 2 2.0 ) 2 1.5 1.5 1.0 1.0 0.5 Force ductility (J/cm ductility Force 0.5 Force ductility (J/cm ductility Force 0.0 0.0 Unmodified MR6 MR10 Unmodified MR6 MR10 Figure 6. Effect of recycled plastic on 100–150 bitumen force ductility [23]. Figure 6. Effect of recycled plastic on 100–150 bitumen force ductility [23]. Figure 6. Effect of recycled plastic on 100–150 bitumen force ductility [23]. 50 50 40 40 30 30 20 20

Elastic recovery (%) recovery Elastic 10

Elastic recovery (%) recovery Elastic 10 0 0 Unmodified MR6 MR10

Figure 7. EUnmodifiedﬀect of recycled plastic on 70–100 MR6 bitumen elastic recovery MR10 [26,28]. Figure 7. Effect of recycled plastic on 70–100 bitumen elastic recovery [26,28]. Figure 7. Effect of recycled plastic on 70–100 bitumen elastic recovery [26,28]. Sustainability 2020, 12, x FOR PEER REVIEW 9 of 20 Sustainability 2020, 12, 8594 9 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 9 of 20

Figure 8. Effect of recycled plastic on C170 torsional recovery [25]. Figure 8. Effect of recycled plastic on C170 torsional recovery [25]. Figure 8. Effect of recycled plastic on C170 torsional recovery [25]. 4.3.4.3. TemperatureTemperature Susceptibility Susceptibility 4.3. Temperature Susceptibility SofteningSoftening point point is ais globally a globally recognized recognized index property index forproperty indicating for theindicating temperature the susceptibilitytemperature ofsusceptibility polymerSoftening modified ofpoint polymer binders.is a modified globally In general, binders. recognized a In bituminous genera indexl, a binderbituminousproperty with for binder a higherindicating wi penetrationth a higherthe temperature penetration will have susceptibilityawill lower have softening a lower of polymer point.softening modified When point. recycled binders. When plastic recyclIn generaed was plasticl, addeda bituminous was to 100–150added binder to penetration 100–150 with a higher penetration grade penetration bitumen grade willinbitumen the have UK, ina the lowerthe softening UK, softening the softening point point. increased point When increased byrecycl 22%ed (MR6)by plastic 22% and (MR6) was 14% added and (MR10) 14% to 100–150(MR10) [23]. For [23].penetration Australian For Australian grade C170 bitumenbitumen,C170 bitumen, in the the softening UK, the thesoftening pointsoftening increased point point increased byincreased 45% by and 45by 32%,% 22% and for (MR6) 32%, MR6 forand and MR6 14% MR10, and(MR10) respectively MR10, [23]. respectively For [ 25Australian]. Finally, [25]. C170theFinally, softening bitumen, the softening point the softening of point 70–100 of point 70–100 penetration increased penetration binder by 45 bi% increasednder and increased32%, by for 40% MR6 by and40% and 20%,an MR10,d 20%, with respectively with the additionthe addition [25]. of Finally,MR6of MR6 [26 the ][26] and softening and MR10 MR10 [point28 [28],], respectively. of respectively. 70–100 penetration The The RTFO RTFO bi agingnder aging increased of of the the binder binder by 40% increased increased and 20%, the the with softening softening the addition point point ofbyby MR6 16%, 16%, [26] reflecting reflecting and MR10 the the oxidative [28],oxidative respectively. hardening hardening The of of RTFO the the bitumen. bi agingtumen. of After Afterthe binder RTFO,RTFO, increased thethee effectffect the of of recycledsoftening recycled plastic plasticpoint bywaswas 16%, reduced, reduced, reflecting with with the a a 27% 27%oxidative and and 22% 22% hardening increase increase of associated associat the bitumen.ed with with After MR6 MR6 andRTFO, and MR10, MR10, the effect respectively. respectively. of recycled This This plastic again again wasindicatesindicates reduced, that that with recycled recycled a 27% plastic plastic and may22% may increase reduce reduce the theassociat rate rate of edof aging agingwith ofMR6 of binders. binders. and MR10, respectively. This again indicatesBecauseBecause that the recycledthe softening softening plastic point point may test test reduce protocol protocol the is rateis almost almost of aging identical identical of binders. in in Australia Australia and and the the UK, UK, the the average average eeffectffectBecause waswas examined examined the softening by by combining combining point test theprotocol the various various is almost data data sets identicalsets (Figure (Figure in 9Australia). 9). Overall, Overall, and the thethe addition UK,addition the ofaverage of MR6 MR6 effectincreasedincreased was the theexamined binder binder softening softeningby combining point point by theby 33%, 33%,various while while data the the additionsets addition (Figure of of MR10 MR109). Overall, increased increased the the theaddition softening softening of pointMR6 point increasedbyby an an average average the binder of of 23%, 23%, softening and and the the point e effectffect by of of both33%, both waswhile was significant significant the addition ( p(-valuesp -valuesof MR10 0.01 0.01 increased and and 0.02, 0.02, the respectively). respectively). softening point by an average of 23%, and the effect of both was significant (p-values 0.01 and 0.02, respectively).

Figure 9. Figure 9.E Effectffect of of recycled recycled plastic plastic on on binder binder softening softening point point [ 23[23,25,26,28].,25,26,28]. Figure 9. Effect of recycled plastic on binder softening point [23,25,26,28]. SustainabilitySustainability2020 2020,,12 12,, 8594 x FOR PEER REVIEW 1010 of 20 20 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 20 4.4. Performance Grading 4.4. Performance Grading The PG rating and specification of bituminous binders in the USA were the result of the SuperpaveThe PGPG project rating rating andconducted and specification specification in the of1990s bituminous of [29].bituminous Alth bindersough binders there in the are USAin alsothe were USAlow-temperature the were result the of theresult parameters, Superpave of the projectSuperpavethe primary conducted project parameter inconducted the of 1990s interest in [29 the]. is Although1990s the high-temperature [29]. there Alth areough also there low-temperaturelimit. are The also original low-temperature parameters, PG grading parameters, the was primary based parametertheon theprimary DSR of parameterparameter interest is of theknown interest high-temperature as isG*/sin( the high-temperatureδ). However, limit. The since original limit. 2011, The PG the gradingoriginal DSR protocol wasPG grading based known on was the basedas DSR the parameteronmultiple the DSR stress known parameter creep as G*recovery /knownsin(δ). However,(MSCR)as G*/sin( hasδ since). beenHowever, 2011, used the tosince DSR measure 2011, protocol thethe parameter knownDSR protocol as theknown multiple known as Jnr(3.2), as stress the creepmultiplewhich recovery provides stress (MSCR)creep a best-practice recovery has been (MSCR)basis used of to high-temperaturehas measure been used the parameter to measurePG rating known the [30]. parameter as Jnr(3.2), known which as provides Jnr(3.2), a best-practicewhichBoth provides a 100–150 basis a best-practice of and high-temperature a 50–70 basis penetration of high-temperature PG rating grade [30 bi]. tumen PG ratingfrom the [30]. UK were modified with MR6 and BothMR10 a [23]. 100–150100–150 Both and MR6 a 50–70and50–70 MR10 penetrationpenetration increased grade the PG bitumenbitumen of both from bitumens the UK significantly were modifiedmodified (Figure withwith 10). MR6MR6 On andaverage, MR10 MR6 [23]. [23 ].was Both Both associated MR6 MR6 and and with MR10 MR10 four increased increased grade increases, the the PG PG of while both of both bitumensMR10 bitumens was significantly associated significantly with (Figure (Figure an average10). 10On). Onaverage,of three average, MR6grade MR6 was increases wasassociated associated (Figure with 11). four with Three grade four to gradeincreases, four increases,PG ratingwhile MR10 whileincreases was MR10 underassociated was the associated MSCR-basedwith an withaverage PG an averageofsystem three ofisgrade comparable three increases grade increasesto (Figure the effect (Figure 11). of Three conventi 11). Threeto foonalur to PG fourSBS rating and PG rating EVAincreases increasespolymers under underfor the bituminous MSCR-based the MSCR-based binder PG PGsystemmodification system is comparable is comparable[31]. to the to the effect effect of ofconventi conventionalonal SBS SBS and and EVA EVA polymers polymers for for bituminous binder modificationmodification [[31].31].

Figure 10. Effect of recycled plastic on binder multiple stress creep recovery (MSCR PG) rating [23]. Figure 10. Figure 10. EffectEﬀect of recycled plastic on binder multiple stress creep recovery (MSCR(MSCR PG)PG) ratingrating [[23].23].

Figure 11. PG increase for recycled plastic modiﬁed binders [23]. Figure 11. PG increase for recycled plastic modified binders [23]. Figure 11. PG increase for recycled plastic modified binders [23]. SustainabilitySustainability2020 2020,,12 12,, 8594 x FOR PEER REVIEW 1111 of 20 20

5. Effect on Asphalt Mixtures 5. Effect on Asphalt Mixtures 5.1. Stiffness 5.1. Stiffness The stiffness, or modulus, of asphalt mixtures is used as a relative indicator of the structural contributionThe stiff ness,of the or layer modulus, to the ofstrength asphalt of mixtures the pavement. is used Modulus as a relative is the indicator only parameter of the structural routinely contributionmeasured on of asphalt the layer mixtures to the that strength is then of thedirectly pavement. related Modulusto pavement is the thickness only parameter design. The routinely stiffer measuredthe asphalt on mixture, asphalt the mixtures stronger that the is thenpavement directly will related be, or tothe pavement thinner the thickness pavement design. can be The without stiffer thecompromising asphalt mixture, its load-carrying the stronger ability. the pavement Although will not be, a ordirect the thinnermeasure the of pavementthe stiffness, can the be Marshall without compromisingStability is generally its load-carrying accepted as ability. an indicator Although of notthe abituminous direct measure binder’s of the contribution stiffness, the to Marshall asphalt Stabilitymixture stiffness is generally when accepted the composition as an indicator of the asphalt of the mixture bituminous is otherwise binder’s kept contribution constant. to asphalt mixtureA typical stiffness UK when DGA the with composition NMAS of of the20 mm, asphalt referred mixture to isas otherwise DGA 20, keptwas constant.produced with and withoutA typical recycled UK DGAplastic with modified NMAS asphalt of 20 mm, binder referred [21]. toIn as the DGA same 20, research, was produced an SMA with with and a without 10 mm recycledNMAS, referred plastic modified to as SMA asphalt 10, was binder produced [21]. In with the and same without research, recycled an SMA plastic with amodified 10 mm NMAS,asphalt referredbinder [21]. to as In SMA both 10, cases, was producedthe base asphalt with and was without the typical recycled UK plastic40–60 modifiedpenetration asphalt grade. binder The MR6 [21]. Inincreased both cases, the theDGA base 20 modulus asphalt was by 48%, the typical whereas UK the 40–60 SMA penetration 10 modulus grade. increased The MR6by 198% increased (for MR6) the DGAand 254% 20 modulus (for MR10), by 48%,as shown whereas in Figure the SMA 12. 10 modulus increased by 198% (for MR6) and 254% (for MR10), as shown in Figure 12.

14,000 12,000 10,000 8,000 6,000 4,000 2,000 Stiffness modulus (MPa) modulus Stiffness 0 40-60 MR6 MR8 DGA 20 SMA 10

Figure 12. Stiffness modulus increase for UK DGA 20 and SMA 10 [21]. Figure 12. Stiffness modulus increase for UK DGA 20 and SMA 10 [21]. In related work [26,28], a comparison of wet-mixing and dry-mixing of recycled plastic in otherwise nominallyIn related identical work 10 [26,28], mm NMAS a comparison DGA in theof UK,wet-mixing referred and to as dry-mixing DGA 10, significantly of recycled increasedplastic in theotherwise Marshall nominally Stability identical for the recycled10 mm NMAS plastic DGA modified in the asphalt UK, referred mixture to (Figure as DGA 13 ).10, Thesignificantly stiffness increaseincreased was the 80%–83% Marshall (forStability MR6) for and the 110%–161% recycled plastic (for MR10) modified and asphalt the effect mixture of the (Figure recycled 13). plastic The (plasticstiffness modified increase versuswas 80%–83% unmodified (for MR6) 50–70) and and 110% recycled–161% plastic (for MR10) type (MR6 and versusthe effect MR10) of the was recycled more significantplastic (plastic than modified the effect versus of the productionunmodified process 50–70) and (wet-mixing recycled versusplastic dry-mixing).type (MR6 versus MR10) was more significant than the effect of the production process (wet-mixing versus dry-mixing). Sustainability 2020, 12, x FOR PEER REVIEW 12 of 20 Sustainability 2020, 12, 8594 12 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 12 of 20 12 12 10 10 8 8 6 6 4 4

Marshall Stability (kPa) Stability Marshall 2

Marshall Stability (kPa) Stability Marshall 2 0 0 50-70 MR 6 W MR 6 D MR 10 W MR 10 D 50-70 MR 6 W MR 6 D MR 10 W MR 10 D

FigureFigure 13.13. Marshall Stability increaseincrease forfor UKUK DGADGA 1010 [[2266,28].,28]. W = Wet mixed and D = DryDry mixed. mixed. Figure 13. Marshall Stability increase for UK DGA 10 [26,28]. W = Wet mixed and D = Dry mixed. In similar research on a typical typical DGA DGA road road surface surface mixture mixture produced produced to to Australian Australian specifications, specifications, referredIn similar to as DGA research 10, theon a eeffect fftypicalect ofof DGA recycledrecycled road plasticpl surfaceastic modifiedmodified mixture binderbinderproduced was to compared Australian to specifications, unmodifiedunmodified viscosityreferred tograde as DGA (C320) (C320) 10, bitumen bitumenthe effect with with of recycledquadruplicate quadruplicate plastic samples modified samples [25]. [binder25 ].The The wasrecycled recycled compared plastic plastic to increased unmodified increased the theasphaltviscosity asphalt mixture grade mixture (C320)resilient resilient bitumen modulus modulus with by 12%quadruplicate by 12%(for (forMR6) MR6) samplesand and8% (for[25]. 8% MR10), (for The MR10), recycled as shown as plastic shown in Figure increased in Figure 14. The the14. Theincreaseasphalt increase mixturein stiffness in sti resilientffness was was significant, modulus significant, by with 12% with p-values (forp-values MR6) of of0.01 and 0.01 and 8% and <0.01,(for<0.01, MR10), for for MR6 MR6 as andshown and MR10, MR10, in Figure respectively. respectively. 14. The Inincrease the same in stiffnesswork, the was recycled significant, plastic with modified modified p-values as asphaltphalt of 0.01 binder and <0.01, produced for MR6 mi mixturesxtures and MR10, with a respectively. 32% (MR6) andIn the 8% same (MR10) work, higher the recycled Marshall plastic Stability, modified compared asphalt to unmodifiedunmodifiedbinder produced C320 mi asphaltxtures (Figure with a 1515).32%). Again, Again,(MR6) theand increase 8% (MR10) was higher significant,significant, Marshall withwith Stability,pp-values-values

Resilient modulus (MPa) modulus Resilient 1000 Resilient modulus (MPa) modulus Resilient 0 0 C320 MR6 MR10 C320 MR6 MR10 Figure 14. Resilient modulus increase for Australian DGA 10 [25]. Figure 14. Resilient modulus increase for Australian DGA 10 [25]. Figure 14. Resilient modulus increase for Australian DGA 10 [25]. Sustainability 2020, 12, x FOR PEER REVIEW 13 of 20 Sustainability 2020, 12, 8594 13 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 13 of 20

Figure 15. Marshall Stability increase for Australian DGA 10 [25]. Figure 15. Marshall Stability increase for Australian DGA 10 [25]. Figure 15. Marshall Stability increase for Australian DGA 10 [25]. The resultsresults ofof SMA SMA and and DGA DGA mixture mixture testing, testing, for for samples samples produced produced with with unmodified unmodified asphalt asphalt and asphaltand asphaltThe binder results binder modified of SMA modified withand recycledDGAwith mixturerecycled plastic, testing, plastic, generally for generally samples indicate indicate aproduced consistent a consistent with and unmodified significant and significantincrease asphalt inandincrease the asphalt sti ffinness the binder associatedstiffness modified associated with with MR6 withrecycled and MR6 MR10 plastic, and recycled MR generally10 plastic.recycled indicate This plastic. is a consistent Thisconsistent is consistent with and the significant with effect the of conventionalincreaseeffect of conventionalin the polymer stiffness polymer modification, associated modification, with particularly MR6 particularly and for MR the10 plastomeric recycledfor the plastomeric plastic. polymer This polymer EVA is consistent [19]. EVA [19].with the effect of conventional polymer modification, particularly for the plastomeric polymer EVA [19]. 5.2. Deformation Resistance 5.2. DeformationDeformation Resistance resistance isis measured inin thethe UKUK andand AustraliaAustralia usingusing thethe Copper’sCopper’s wheel tracking machineDeformation and is a well-established resistance is measured indirect in measure the UK and of asphalt Australia mixture using resistance the Copper’s to rutting, wheel shoving, tracking andmachine shearing.shearing. and is Other Othera well-established countries, countries, such such indirect as as the measure USA, use of asphaltdifferent different mixture wheel resistance tracking equipmentto rutting, shoving, or more fundamentalandfundamental shearing. deformationdeformation Other countries, resistanceresistance such tests,testas thes, suchsuch USA, asas FlowFlowuse different NumberNumber/Time /wheelTime [[32].32tracking]. equipment or more fundamentalThe Copper’sCopper’s deformation wheel resistance tracking test test s, was such performedperform as Flowed Number/Time onon otherwiseotherwise [32]. nominallynominally identicalidentical asphaltasphalt mixturesThe producedCopper’s withwheel and tracking without test recycled was perform plasticplastic modifiedmodifieded on otherwise binders innominally both the UK identical and Australia. Australia. asphalt ThismixturesThis included produced UK UK DGA DGAwith and20 20 and andwithout SMA SMA recycled10 10 [21], [21 ],UKplasti UK DGAc DGA modified 10 10[26,28], [ 26binders,28 and], and in Australian both Australian the UK DGA and DGA 10 Australia. [25]. 10 [ 25On]. OnThisaverage, average, included MR6 MR6 wasUK was DGAassociated associated 20 and with withSMA a 65% a 10 65% [21],reduction reduction UK DGA in in the the10 final final[26,28], wheel wheel and tracking tracking Australian depth, depth, DGA whereas whereas 10 [25]. MR10 On wasaverage,was associatedassociated MR6 was withwith associated aa 43%43% reductionreduction with a 65% (Figure(Figure reduction 1616).). TheThe in averagetheaverage final reductionreductionwheel tracking waswas statistically statisticallydepth, whereas significantsignificant MR10 forwasfor bothboth associated recycledrecycled with plasticplastic a 43% productsproducts reduction ((p p-values(Figure-values 16).<< 0.01).0.01). The averageThis significant significant reduction increase was statistically in asphalt significant mixturemixture deformationfor both recycled resistanceresistance plastic is is comparable productscomparable ( top-values thatto that associated < a 0.01).ssociated withThis with conventionalsignificant conventional increase polymers polymers in for asphalt asphalt for mixtureasphalt binder modification,deformationbinder modification, resistance such as such SBS is andascomparable SBS EVA and [19 EVA]. to that[19]. associated with conventional polymers for asphalt binder modification, such as SBS and EVA [19].

Figure 16. EffectEffect of recycled plastic on mixturemixture wheel trackingtracking depthdepth [[21,25,26,28].21,25,26,28]. Figure 16. Effect of recycled plastic on mixture wheel tracking depth [21,25,26,28]. SustainabilitySustainability2020 2020,,12 12,, 8594x FOR PEER REVIEW 1414 of 20 20 Sustainability 2020, 12, x FOR PEER REVIEW 14 of 20 5.3. Cracking Resistance 5.3.5.3. CrackingCracking ResistanceResistance Cracking resistance is a combination of the resistance to crack initiation (fatigue resistance) as well CrackingasCracking resistance resistance resistance to subsequent is is a combinationa combination crack propagation of theof the resistance resist (fractureance to crack to resistance). crack initiation initiation There (fatigue (fatigue are resistance) many resistance) laboratory as well as astestswell resistance asfor resistance asphalt to subsequent mixture to subsequent resistance crack crack propagation to propagationcracking, (fracture with (fracture fatigue resistance). resistance). tests Thereusually There are using many are a many laboratorycyclic laboratory load testsof a forregulartests asphalt for sample, asphalt mixture while mixture resistance fracture resistance totests cracking, usually to cracking, withuse monotonic fatigue with fatigue tests loading usually tests of usuallya using sample a using cyclic with a load pre-madecyclic of aload regular notch of a sample,orregular crack whilesample, initiation fracture while point. testsfracture Although usually tests usenot usually monotonica direct use measure monotonic loading of of fatigueloading a sample of of fracture a with sample a pre-maderesistance, with a pre-made notch the Marshall or cracknotch initiationFlowor crack test initiation point.is generally Although point. accepted Although not a as direct an not indicator measure a direct of ofmeasure the fatigue asphalt of of fatigue fracturebinder’s of resistance, relativefracture contribution resistance, the Marshall the to FlowtheMarshall crack test isresistanceFlow generally test is of acceptedgenerally an asphalt as accepted anmixture indicator as when an of indicator thethe asphaltmixtures of the binder’s areasphalt otherwise relative binder’s contributioncomparable. relative contribution to the crack to resistance the crack ofresistance anThe asphalt fracture of mixturean asphalt toughness when mixture theof a mixtures typicalwhen the UK are mixtures SMA otherwise 10 arewas comparable. otherwise measured comparable. for otherwise nominally identical mixtures,TheThe fracturefracture with and toughnesstoughness without of ofrecycl aa typicaltypicaled plastic UKUK SMASMA modified 1010 waswas asphalt measuredmeasured binder forfor otherwise otherwise[21]. Thenominally nominallyfracture toughness identicalidentical mixtures,increasedmixtures, withbywith 22% andand (for withoutwithout MR6) and recycledrecycl 16%ed (for plasticplastic MR10), modifiedmodified as shown asphaltasphalt in Figure binderbinder 17. [ In21[21]. similar]. TheThe fractureworkfracture [22], toughnesstoughness mixture- increasedspecificincreased fatigue by 22% 22% life (for (for relationships MR6) MR6) and and 16% were 16% (for developed (for MR10), MR10), as for shown as the shown same in Figure innominal Figure 17. In SMA similar17. 10 In workmixture, similar [22], work produced mixture- [22], mixture-specificwithspecific and fatigue without life fatigue recycled relationships life plastic, relationships were as well developed wereas for developeda typicalfor the sameUK for DGA thenominal same20 mixture SMA nominal 10 [24]. mixture, SMA Over 10 a produced mixture,practical producedrangewith and of induced withwithout and recycledstrain without magnitudes plastic, recycled as plastic, (100–500well as as for wellµ ɛa), typical asthe for fatigue aUK typical DGA life UK of20 the DGAmixture SMA 20 [24]. mixture 10 increasedOver [24 a]. practical Over by an a practicalaveragerange of 11% rangeinduced (for of MR6) inducedstrain and magnitudes strain 8% (for magnitudes MR10), (100–500 as (100–500 showµɛ), then inµε fatigue Figure), the fatiguelife18. Theseof the life improvements ofSMA the 10 SMA increased 10 in increased average by an byfatigueaverage an average life 11% increased (for 11% MR6) (for to MR6)and 52% 8% and (for 8%46% MR10), (for for MR10),the as showDG asAn 20 shownin mixture,Figure in Figure18. for These MR6 18. improvements Theseand MR10, improvements respectively in average in average(Figurefatigue fatigue19).life increased life increased to 52% to and 52% 46% and 46%for the for theDG DGAA 20 20mixture, mixture, for for MR6 MR6 and and MR10, MR10, respectivelyrespectively (Figure(Figure 19 19).).

Figure 17. Effect of recycled plastic on SMA 10 fracture toughness [21]. Figure 17. Eﬀect of recycled plastic on SMA 10 fracture toughness [21]. Figure 17. Effect of recycled plastic on SMA 10 fracture toughness [21].

Figure 18. Eﬀect of recycled plastic on SMA 10 fatigue life [22,24]. Figure 18. Effect of recycled plastic on SMA 10 fatigue life [22,24]. Figure 18. Effect of recycled plastic on SMA 10 fatigue life [22,24]. Sustainability 2020, 12, x FOR PEER REVIEW 15 of 20 SustainabilitySustainability2020 2020,,12 12,, 8594 x FOR PEER REVIEW 1515 of 20 20

6.0 6.0 5.5 5.5 5.0 5.0 4.5 4.5 4.0 4.0 Log fatigue (cycles) 3.5 Log fatigue (cycles) 3.5 3.0 3.01.7 1.9 2.1 2.3 2.5 1.7 1.9 2.1 2.3 2.5 Log initial strain (µɛ) 40-60Log initialMR6 strain (µɛ)MR10 40-60 MR6 MR10

Figure 19. EffectEffect of recycled plastic on DGA 20 fatigue life [[22,24].22,24]. Figure 19. Effect of recycled plastic on DGA 20 fatigue life [22,24]. In similar work, the fatigue life of a typical UK DGA 10 was measured for otherwise nominally identicalIn similar mixtures work, produced the fatigue with life unmodifiedunmodified of a typical 50–70 UK DGA penetrationpenetration 10 was measuredasphalt, as for well otherwise as with nominally recycled plasticidentical incorporated mixtures produced byby dry-mixing with andunmodified wet-mixing 50–70 processesprocesses penetration [[28].28]. Theasphalt, fatigue as life well relationships as with recycled were generallyplastic incorporated similar (Figure (Figure by dry-mixing 20) 20) and and the the and associated associated wet-mixing fati fatiguegue processes life life models models [28]. were The were fatigue not not significantly significantlylife relationships different different were (p- (valuesgenerallyp-values 0.33 0.33similar to to0.74). 0.74).(Figure Furthermore, Furthermore, 20) and the the associatedthe production production fati guemethod method life models(wet-mixing (wet-mixing were notversus versussignificantly dry-mixing) dry-mixing) different had had no(p- nosignificantvalues significant 0.33 effect to e0.74).ffect on on theFurthermore, the fatigue fatigue lives livesthe measuredproduction measured (p (methodp-value-value 0.49). 0.49).(wet-mixing The The same same versus research research dry-mixing) also measuredmeasured had no Marshallsignificant Flow effect and on the the results fatigue indicated lives measured only minor (p-value didifferencesfferences 0.49). The in the same results research associated also measured with the unmodifiedunmodifiedMarshall Flow and and recycled the results plasticplastic indicated modifiedmodified asphaltonlyasphalt minor bindersbinders differences (Figure(Figure 2121).in). the results associated with the unmodified and recycled plastic modified asphalt binders (Figure 21).

Figure 20. EffectEﬀect of recycled plastic on UK DGA 10 fatigue life [[26,28].26,28]. Figure 20. Effect of recycled plastic on UK DGA 10 fatigue life [26,28]. Sustainability 2020, 12, x FOR PEER REVIEW 16 of 20

Sustainability 2020, 12, 8594 16 of 20 Sustainability 2020, 12, x FOR PEER REVIEW 16 of 20 4 4 3 3 2 2

Marshall Flow (mm) Marshall 1

Marshall Flow (mm) Marshall 1 0 50-70 MR 6 W MR 6 D MR 10 W MR 10 D 0 50-70 MR 6 W MR 6 D MR 10 W MR 10 D Figure 21. Effect of recycled plastic on UK DGA 10 Marshall Flow [26,28]. Figure 21. Effect of recycled plastic on UK DGA 10 Marshall Flow [26,28]. In Australia,Figure the 21.fatigue Effect lifeof recycled of otherwise plastic on nominally UK DGA 10 identical Marshall FlowDGA [26,28]. 10 was measured (in In Australia, the fatigue life of otherwise nominally identical DGA 10 was measured quadruplicate) for asphalt mixtures with and without recycled plastic modified asphalt binder [25]. (in quadruplicate) for asphalt mixtures with and without recycled plastic modified asphalt binder [25]. AlthoughIn Australia, the mixtures the fatiguewith recycled life of plasticotherwise had anominally lower average identical fatigue DGA life 10(Figure was 22),measured the results (in Although the mixtures with recycled plastic had a lower average fatigue life (Figure 22), the results quadruplicate)were not statistically for asphalt significant, mixtures with with p-values and without of 0.32 (for recycled MR6) plasticand 0.67 modified (for MR10). asphalt That binder is, recycled [25]. were not statistically significant, with p-values of 0.32 (for MR6) and 0.67 (for MR10). That is, recycled Althoughplastic modified the mixtures asphalt with binder recycled did not plastic significantly had a lower change average the fatigue fatigue life life of (Figure the asphalt 22), the mixtures. results plastic modified asphalt binder did not significantly change the fatigue life of the asphalt mixtures. wereThe same not statistically research also significant, found no with significant p-values differen of 0.32ce (for in the MR6) Marshall and 0.67 flow (for values MR10). for That the unmodifiedis, recycled The same research also found no significant difference in the Marshall flow values for the unmodified plasticand modified modified mixtures asphalt (Figure binder 23),did withnot significantly p-values of 0.61 change (for theMR6) fatigue and 0.39 life of(for the MR10). asphalt mixtures. Theand same modified research mixtures also found (Figure no 23 significant), with p-values differen ofce 0.61 in the (for Marshall MR6) and flow 0.39 values (for MR10). for the unmodified and modified mixtures (Figure 23), with p-values of 0.61 (for MR6) and 0.39 (for MR10). 120,000 120,000 100,000 100,000 80,000 80,000 60,000 60,000 40,000 Fatigue life (cycles) life Fatigue 40,000 Fatigue life (cycles) life Fatigue 20,000 20,000 0 C320 MR6 MR10 0 Figure 22. Effect of recycledC320 plastic on average MR6 Australian DGA 10 fatigue MR10 life [25]. Figure 22. Effect of recycled plastic on average Australian DGA 10 fatigue life [25].

Figure 22. Effect of recycled plastic on average Australian DGA 10 fatigue life [25]. SustainabilitySustainability2020 2020, 12, 12, 8594, x FOR PEER REVIEW 17 of of 20 20 Sustainability 2020, 12, x FOR PEER REVIEW 17 of 20

FigureFigure 23. 23.E Effectffect ofof recycledrecycled plastic plastic on on average average Australian Australian DGA DGA 10 10 Marshall Marshall Flow Flow [ 25[25].]. Figure 23. Effect of recycled plastic on average Australian DGA 10 Marshall Flow [25]. BasedBased onon thethe resultsresults of of various various asphaltasphalt mixturemixture typestypes commonlycommonly usedused inin AustraliaAustralia andand thethe UK,UK, thethe addition additionBased on of of MR6the MR6 results and and MR10 ofMR10 various were were either asphalt either associated mixturassociatede with types with a moderatecommonly a moderate increase used increase in in Australia crack in resistance,crack and resistance, the or UK, no significanttheor noaddition significant change of MR6 inchange and crack MR10 in resistance, crack were resistance, either when associ compared whenated compared with to otherwise a moderate to otherwise identical increase unmodifiedidentical in crack unmodified resistance, mixtures. Thisormixtures. no is significant comparable This is comparablechange to the in eff ectcrack to ofthe modificationresistance, effect of modification when with compared conventional with conventionalto otherwise plastomeric plastomericidentical polymers, unmodified polymers, such as EVAmixtures.such [ 19as]. EVA InThis contrast, is[19]. comparable In conventional contrast, to the conventional elastomeric effect of modification polymers,elastomeric suchwith polymers, asconventional SBS, are such generally plastomericas SBS, associated are polymers,generally with significantlysuchassociated as EVA with greater [19]. significantly improvementIn contrast, greater conventional in crackingimprovement resistance elastomeric in cracking of asphalt polymers, resistance mixtures. such of asphalt as SBS, mixtures. are generally associated with significantly greater improvement in cracking resistance of asphalt mixtures. 5.4.5.4. MoistureMoisture DamageDamage ResistanceResistance 5.4. Moisture Damage Resistance MoistureMoisture damage damage resistance resistance is is commonly commonly expressed expressed as as the the tensile tensile strength strength ratio ratio (TSR), (TSR), which which is is thethe ratioratioMoisture of of the the damage indirect indirect resistance tensile tensile strength strength is commonly for for conditioned conditioned expressed and asand the unconditioned unconditioned tensile strengthsamples, samples, ratio (TSR), whichwhich which isis also also is knowntheknown ratio as as of the the the Lottman, Lottman, indirect or ortensile modified modified strength Lottman Lottman for conditioned test, test, depending depending and on unconditionedon the the conditioning conditioning samples, protocol. protocol. which The The is TSR alsoTSR wasknownwas calculated calculated as the Lottman, fromfrom tensiletensile or modified strengthstrength Lottman measurementsmeasuremen test, dependingts forfor UKUK SMASMA on the 1010 andconditioningand DGADGA 2020 [protocol.21[21],], forforUK UKThe DGA DGA TSR 10,was10, by bycalculated dry-mixing dry-mixing from and and tensile wet-mixing wet-mixing strength [26[26,28], measuremen,28], and and for forts Australian Australianfor UK SMA DGA DGA 10 10 and10 [ 25[25]. DGA]. InIn 20 somesome [21], cases,cases, for UK thethe DGA TSRTSR was10,was by lowerlower dry-mixing forfor thethe recycledand wet-mixing plastic modified modified[26,28], and mixtures, mixtures, for Australian whereas whereas DGAin in othe other 10r cases,[25]. cases, In the thesome TSR TSR cases,was was higher the higher TSR for forwasthe the recycledlower recycled for plastic the plastic recycled modified modified plastic mixtures modified mixtures (Figure mixtures, (Figure 24). Ov 24 whereaserall,). Overall, the in effect othe the rof cases, e ffrecycledect the of recycledTSR plastic was on plastichigher TSR was onfor TSRthenot recycled significant, was not plastic significant, with modified p-values with mixturesofp-values 0.72 and (Figure of 0.52, 0.72 for24). and MR6 Ov 0.52,erall, and for theMR10, MR6 effect andrespectively. of MR10, recycled respectively. That plastic is, onthe TSR addition That was is, thenotof recycled additionsignificant, plastic of recycledwith did p-values not plastic significantly of did0.72 not and change significantly 0.52, for the MR6 moisture change and MR10, damage the moisture respectively. resistance damage of That the resistance is,various the addition asphalt of the variousofmixtures. recycled asphalt plastic mixtures. did not significantly change the moisture damage resistance of the various asphalt mixtures.

FigureFigure 24. 24.E Effectﬀect ofof recycledrecycled plastic plastic on on mixture mixture TSR TSR value value [ 21[21,25,26,28].,25,26,28]. Figure 24. Effect of recycled plastic on mixture TSR value [21,25,26,28]. Sustainability 2020, 12, 8594 18 of 20

6. Summary and Conclusions The commercially available recycled plastic products MR6 and MR10 for asphalt binder and asphalt mixture modification have been added to various unmodified bitumens and to various asphalt mixture types, and the effects measured using a range of tests commonly specified in Australia and the UK, as well as for PG rating system commonly used in the USA. Overall, the results were generally positive (Table5). Recycled plastic products increased the resistance to asphalt binder flow and asphalt mixture deformation resistance. Significant ductility and elasticity were introduced to the asphalt binders, but the effect on asphalt mixture cracking resistance was either moderate or not significant. There was no significant difference in the asphalt mixture moisture damage resistance, but the mixture stiffness increased two- to three-fold. The effects of MR6 and MR10 were generally similar to the effects associated with conventional polymer modification of asphalt binders and asphalt mixtures, particularly those effects associated with the plastomeric polymer EVA [19].

Table 5. Summary of eﬀects of recycled plastic on binder and mixtures.

Property Effect Binder properties Resistance to flow Significant increase in resistance to flow, based on penetration and viscosity Incorporation of substantial elasticity and ductility that were negligible in the Elasticity and ductility unmodified asphalt Performance grading Three (MR6) to four (MR10) grade increases based on the MSCR test protocol Mixture properties Two- to three-fold increase across various mixture types, based on various Stiffness measures of mixture modulus Significant 65% (MR6) and 43% (MR10) reduction in wheel tracking rut depths for Deformation resistance various mixture types No significant reduction in fatigue life or fracture prorogation resistance, using Crack resistance various test methods No significant difference in resistance to moisture damage across various mixture Moisture damage resistance types, based on the Lottman test

Funding: This research received no external funding. Acknowledgments: The support of MacRebur Ltd. is gratefully acknowledged and greatly appreciated. Conﬂicts of Interest: The author declares no conﬂict of interest.

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