Resources, Conservation and Recycling 52 (2007) 58–73

A review of the use of recycled solidwaste materials in asphalt pavements

Yue Huang ∗, Roger N. Bird 1, Oliver Heidrich 2

School of Civil Engineering and Geosciences, Newcastle University, UK Cassie Building,Claremont Road, Newcastle upon Tyne NE1 7RU, UK

Received 16 December 2006; accepted 20 February 2007Available online 6 April 2007

Abstract

The construction and maintenance of UK roads consume large amounts of quarried aggregates.The use of secondary (recycled), instead of primary (virgin), materials helps easing landfill pressuresand reducing demand of extraction. However, concerns over inferior road performance and addi-tional costs have hindered the widespread use of secondary aggregates in such applications. This isespecially the case in surface layers of asphalt pavements that may represent a value application forrecycled solid waste materials (SWM). Waste glass, steel slag, tyres and plastics are selected for thisstudy, which reviews standards and literature for technical requirements, as well as the performanceof asphalt pavements constructed using such recycled materials. Waste arising and management indi-cates that although there is a large potential for supplying secondary materials, a few factors haveeffectively depressed such recycling activities. Such barriers are described here and may also applyto the secondary use of other SWM. After identifying and quantifying such barriers a brief discussionsuggests ways of their removal.© 2007 Elsevier B.V. All rights reserved.

Keywords: Asphalt pavements; Recycling; Solid waste materials (SWM); Glass; Steel slag; Scrap tyres; Plastics

∗ Corresponding author. Tel.: +44 191 2226424.E-mail addresses: Yue.Huang@ncl.ac.uk (Y. Huang), R.N.Bird@ncl.ac.uk (R.N. Bird),

Oliver.Heidrich@ncl.ac.uk (O. Heidrich).1 Tel.: +44 191 2227681.2 Tel.: +44 191 2226854.

0921-3449/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.resconrec.2007.02.002

Y. Huang et al. / Resources, Conservation and Recycling 52 (2007) 58–73 59

Fig. 1. Structural layers of flexible and rigid pavements (*indicates optional).

1. Introduction

Around 95% of roads in the UK are paved with asphalt mixtures (IAT, 2000). Theconstruction and maintenance of these roads require large amounts of aggregates, whichtypically account for more than 90% by weight of the asphalt mixtures. It is estimated thatin 1999 the UK produced some 26 million tonnes (Mt) of hot mix asphalt (HMA) (EAPA,2004), which can lead to the assumption that some 20 Mt of aggregates were consumed.The Highways Agency alone uses about 15 Mt of aggregates annually on its managed trunkroads and motorways in England (Highway Agency, 2003). Meanwhile across Englandand Wales, some 48 Mt of industrial, 30 Mt of commercial, and 28 Mt of municipal wastewere generated, a considerable percentage (industrial: 47%; commercial: 66%; municipal:83%) was sent to landfills (DETR, 2000b). Such resource management does not seem to bein line with the country’s strategy for sustainable construction that requires for protectingthe environment and minimising the consumption of natural resources (DETR, 2000a).There is concern that high specification aggregates from UK permitted extractions couldbe exhausted as early as 2020 (Parker, 2004). The situation seems even more urgent forapproved landfill sites, as they are expected to run out of space in the next 5–10 years(Environmental Agency, 2006).1 Based on such pressures, the UK government introducedthe Landfill Tax in 1996 and the Aggregates Levy in 2002.

The use of secondary (recycled), instead of primary (virgin), materials helps easinglandfill pressures and reducing demand of extraction. This is one way of getting the roadconstruction industry on track towards sustainable construction practices. Current researchand practice tends to concentrate on the use of waste materials in the lower courses (base,sub-base, etc.) of the road as these absorb materials in larger quantities than the uppercourses. However, highway authorities in the UK are dealing more with the maintenanceand repair works rather than new construction of roads. Such works are affecting mainly theupper pavement layers (see Fig. 1). In addition, it can be argued that the cost of transportingand processing waste materials into desired properties can only be justified by using the recy-cled materials in value added applications such as asphalt surface layers. Thus, the property

1 Environmental Agency: http://www.environment-agency.gov.uk/yourenv/eff/1190084/resources waste/213982/207743/?version=1&lang= e accessed on 16 August 2006.

60 Y. Huang et al. / Resources, Conservation and Recycling 52 (2007) 58–73

requirements for these applications need to be understood to ensure that materials intendedfor recycling are able to meet relevant specifications, by using available technologies andfacilities, at a reasonable cost.

2. Waste arising and management of solid waste materials

Secondary aggregates can be defined as by-products from industrial processes or otherhuman activities. In 1999, industrial, commercial and municipal solid waste accountedfor approximately 13%, 6% and 8%, respectively, of total solid waste materials (SWM)produced in the UK (DETR, 2000b). Of those waste materials a large fraction might havethe potential to be used in road or building construction projects. Waste glass, steel slag,tyres and plastics are selected for this study; and surface layers (surface and binder course)of asphalt pavements are considered here as value application which are described in furtherdetail below.

2.1. Waste glass

It is estimated by Waste Resources Action Program (WRAP, 2004) that in 2003, some3.4 Mt of glass entered the UK’s waste stream of which some 2.4 Mt (71%) was containerglass, 0.76 Mt (23%) was flat (or window) glass and the remaining 0.24 Mt was other glass.The recycling rate for container and flat glass was 36% and 30%, respectively. In total, some1.1 Mt (33%) of waste glass was recycled, among which 0.73 Mt (66%) was fed to glasscontainer manufacturers and 0.14 Mt (13%) used as secondary aggregates. The majorityof 2.3 Mt (67%) of waste glass was disposed to landfills. The EU Directive on packagingwaste (EU, 1994) however, has set a UK recycling target of 60% by 2008 for waste glass(British Glass, 2004).

The lack of sufficient infrastructure for waste glass collection is thought to be the mainreason for sending the majority to landfills and recycling only a third in the UK (British Glass,2005).2 The recycling infrastructure serves not only as a passive container of recyclablewastes, but as a visual motivation that influences people’s recycling habit (Gonzalez-Torreet al., 2003). Currently in the UK, packaging recovery notes (PRNs) are issued as an incentiveto glass recycling, and the value of PRNs is suggested to be raised to cover the recycling cost(WRAP, 2004). Glass can be recycled indefinitely without loss of product quality (BritishGlass, 2004). Returning recycled cullet to a glassmaking plant saves energy and mineralresources in great quantity (Edwards and Schelling, 1999; Krivtsov et al., 2004). Using wasteglass as an aggregate might not save as much energy or mineral resources as it does withglass making (Grantthornton and Oakdenehollins, 2006), but the colour imbalance betweenglass production and waste arising may encourage seeking alternative markets for wasteglass in aggregates applications (WRAP, 2004). Attempts to use recycled glass in concrete,another value application, have to deal with the alkali-silica reaction (ASR) because of theabnormally high content (≥70%) of reactive silica in the glass (FHWA, 1997). In addition

2 British Glass: http://www.britglass.org.uk/Industry/Recycling.html accessed on 21 March 2005.

Y. Huang et al. / Resources, Conservation and Recycling 52 (2007) 58–73 61

to the described recycling processes, waste glass can be utilised as an aggregate in asphaltroad construction should the technical specification as described later are being met.

2.2. Steel slag

The amount of steel slag can be estimated based on the output from steel productionprocess, assuming that the process is stable and the rate of slag generation consistent.According to US NSA (National Slag Association), steel slag accounts for 7.5–15% ofsteel produced (NSA, 2001). The marketable slag is estimated by USGS (US GeologicalSurvey) at a rate of 10–15% steel production (USGS, 2001). One advantage of recyclingsteel slag is that it can be collected from a low number of steel plants, making the collectionmore efficient than that of most other solid waste materials. In addition, it is relatively easyto control and achieve a consistent quality of this waste material. UK transport researchlaboratory reported that some 1 Mt of basic oxygen steel (BOS) slag is produced annuallyin the UK, with about 4 Mt in stockpiles (TRL, 2003). Owing to decades of research andpractice, UK has now achieved a 100% recycling rate for steel slag, 98% of which are usedas aggregates, mainly in concrete and asphalt (ODPM, 2002). The UK’s steel productionsaw a decline from some 18 Mt in 1997 to not even reaching 12 Mt in 2002, before risingto 13.3 Mt in 2003, with further increase expected in the coming years (UK Steel, 2005).3

Although 100% of steel slag is recycled the application in asphalt pavements is valued dueto its properties as described later.

2.3. Tyres

It is estimated by TRL that the UK generates over 0.44 Mt of waste tyres per annum.About 21% is shredded and used as raw materials for other processes, 22% sent for energyrecovery, and around 34% is disposed to landfills, stockpiles or illegal dumps where it ismixed with other waste making the recovery difficult (Viridis and TRL, 2002; Viridis andTRL, 2003). Approximately 40,000 t (or 9%) is combusted in cement kilns, as scrap tyreshave a comparable energy value to coal, and have been used as a cement making fuel in thelast decade or so (Bluecircle, 2003; UTWG, 2002). According to TRL, the high processingcost is responsible for the growth of unregulated tyres disposal (Viridis and TRL, 2003).European Tyre Recycling Association (ETRA) estimated the transport cost of waste tyresat about £1/tonne/km in average (Shulman, 2000). Use of scrap tyres in asphalt or otherpavement applications, although technically viable (see Section 4.3), needs to be subsidisedin order to compete with conventional aggregates (Washington DOT, 2003) in meeting thetechnical requirements for asphalt pavements.

2.4. Plastics

About 2.8 Mt of waste plastics is generated per annum in the UK. Most of those recycledare from industrial and commercial sources; recycling from domestic sources (e.g. bottles)

3 UK Steel: http://www.uksteel.org.uk accessed on 22 March 2005.

62 Y. Huang et al. / Resources, Conservation and Recycling 52 (2007) 58–73

is more difficult, for economic reasons (TRL, 2004). A future increase in recycling relieson the successful recycling of plastics mixed with other waste, and the support from robustenvironmental assessment method (British Plastics Federation, 2005,4 Patel et al., 2000).Similar to tyre rubber, a significant means of recovery of plastics waste is to retrieve thethermal content (38 MJ/kg), comparing favourably to that of coal (31 MJ/kg) and reducingenergy use as well as CO2 emissions (British Plastics Federation, 2005,5 Patel et al., 2000).

Data from UK WRAP indicate that about 0.4 Mt of waste plastics generated each yearis suitable for aggregates use. Presently only 0.008 Mt is being recycled for that purpose.Recycled plastics are mainly used in the form of street furniture, insulation, ducts and pipes,etc. Very little so far is used in pavement construction (WRAP, 2003a). Similar to glass, thelow PRN is blamed for such low recycling levels (DTI, 2004). Although plastic packagingaccounts for most waste plastics recycled in the UK, PVC (polyvinyl chloride) is amongthe main types that have the lowest recycling rate (WRAP, 2003c). Financial incentives arebelieved to be more effective than specifications in affecting the recycling activity (WRAP,2003b). Thus plastics used in asphalt pavements may provide an important outlet for suchmaterials.

3. Property requirements for materials in asphalt pavements

3.1. Property requirements for aggregates

A European standard (BSEN13043, 2002) for the specification of aggregates for usein asphalt was introduced in 2004 into the UK market. This standard specifies aggregatesin terms of technical requirements alongside relevant test methods. Therefore recycledmaterials that are intended for aggregates use in asphalt mixtures are subject to the samerequirements for property classification and testing as are virgin aggregates. Pavementengineers are now responsible for defining categories for aggregates properties relevantto their specific applications, as well as benchmarking the quarrying industry and othermaterial suppliers. Selected requirements for aggregates in surface layers asphalt are shownin Table 1.

3.2. Property requirements for asphalt

In order to withstand tyre and weather, pavement surface layers contain the strongestand most expensive materials in road structures. Characteristics they exhibit like friction,strength, noise and ability to drain off surface water are essential to vehicles’ safety andriding quality. Some are already associated with a standard test method (BSEN13036, 2002).Apart from the nature of component binder and aggregates, asphalt performance stronglydepends on the mixture type. Selection of a type for surface layers has to consider a multitude

4 British Plastics Federation: http://www.bpf.co.uk/bpfindustry/process plastics recycling.cfm accessed on 09February 2006.

5 British Plastics Federation: http://www.bpf.co.uk/bpfissues/Waste Management.cfm accessed on 08 February2006.

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Table 1Property requirements and test methods for aggregates in surface layers asphalt (PD6682-2, 2003)

Property category Test method Property requirements

Geometric BS EN933 Grading, fines content, flakiness indexPhysical and mechanical BS EN1097 Resistance to fragmentation, polished stone

value (PSV), aggregate abrasion value (AAV)Chemical BS EN1744 LeachingThermal and weathering BS EN1367 Water absorption, magnesium sulphate value

of factors including traffic, climate, condition of existing surface, and economics. No singlemixture type could provide all the desired properties, often some are improved at the expenseof others, making the selection difficult and contentious.

Stone mastic asphalt (SMA), porous asphalt or open graded friction course (OGFC) havea reputation for low tyre noise and high resistance to rutting and skidding, and are thereforepreferred to hot rolled asphalt (HRA) for road surface that is subject to heavy traffic interms of volume and loading (NAPA and FHWA, 2000). For both mixture types, a numberof properties are required of the component (particularly the coarse) aggregates such asPSV, resistance to fragmentation, affinity with bitumen, etc. Dense bituminous macadam(DBM) is commonly used in binder course and base.

4. Performance of asphalt pavements containing recycled SWM

4.1. Waste glass

Satisfactory performance has been observed of asphalt pavements containing 10–15%crushed glass in surface course mixtures. 4.75 mm is the maximum size commonly acceptedconsidering a range of engineering properties including safety issues (skin cut, tyre puncture)for that application. Anti-strip agent, typically 2% hydrated lime, is added to retain thestripping resistance. Glass in asphalt of higher content and larger size is reported to have ledto a number of problems such as insufficient friction and bonding strength, and is consideredmore suitable for use in lower courses. In practice, the same manufacturing equipment andpaving method designed for conventional asphalt can be used for asphalt containing recycledglass (Airey et al., 2004; CWC, 1996; FHWA, 1997; Maupin, 1997; Maupin, 1998; Su andChen, 2002). RMC (now CEMEX) UK has been using recycled glass in DBM for bindercourse and base, with a 30% replacement rate. Twenty millimeters seems to be the maximumsize of processed glass particles. In 2002, hot mix asphalt (HMA) containing 10% recycledglass sand was used in a pilot resurfacing project by Tarmac Situsec. Economics in theseUK applications is reported to be ‘cost neutral’ (WRAP, 2005).6

4.2. Steel slag

The angular shape, hardness and roughly textured surface give steel slag the abilityto substitute coarse aggregates in asphalt where mix stability (resistance to rutting) and

6 Case studies from AggRegain, WRAP: http://www.aggregain.org.uk/index.html accessed on 10 March 2005.

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skid-resistance are concerned. Collaborative research was carried out by US StrategicHighway Research Program (SHRP) and University of Petroleum and Minerals in SaudiArabia. It was found that mix durability (resistance to moisture, fatigue) was improvedwhen coarse slag aggregates were supplemented with limestone filler and fine aggre-gates, and the bitumen prepared using polymer modification (Bagampadde et al., 1998;Khan and Wahhab, 1998). In 1994, trial section of asphalt containing 30% steel slag waslaid in Oregon, followed by a 5-year field inspection of ride and skid performance. Thetrial section did not exhibit as expected higher rutting and skid resistance than controlmixture. The report attributed the lack of measurable increase to the low content andsmall size (6.3–12.7 mm) of slag particles, and it mentioned the economic disadvantageof using slag aggregates due to increased mix density (implying higher transport cost)and mixing temperature (implying higher energy use) (Oregon DOT, 2000). UK TRLreported based on a 3-year investigation, that BOS slag produced from main UK sourcescan be used in pavement surface where a minimum PSV of 60 is required. Althoughthe report suggested that when assessing the anti-skid properties of asphalt made withslag aggregates, traditional PSV test should give way to ‘known in-service performanceunder comparable situations’ (TRL, 2003). Nottingham Centre for Pavement Engineering(NCPE) studied the mechanical (stiffness modulus, resistance to permanent deformation,resistance to fatigue cracking) and durability (aging susceptibility, moisture susceptibility)performance of asphalt containing slag aggregates. Seventy one percent coarse steel slagparticles were mixed with 21% fine BFS aggregates in SMA surfacing. Stiffness modu-lus was enhanced compared with control mixture made of gritstone, while mix densityand aging susceptibility also increased (Airey et al., 2004). Steel slag (≥9.5 mm) after3 years of aging (7 days expansion below 1%) replacing 62% of basalt aggregates wasused in SMA mixtures in China laboratory, resulting in improved surface performance(texture, friction, etc.), resistance to rutting and low temperature cracking (Wu et al.,2007).

The Research Association of Iron and Steel Slags (FEhS, Germany) studies confirmedthat BOS slag asphalt exhibit superiority in bearing and anti-polishing performance overasphalt made with established premium aggregates (basalt, flint gravel, etc.). Volumetricstability and leaching behaviour caused the most concerns. Precautionary treatment waspractised at the steel plant to reduce the free CaO/MgO content of steel slag before useas aggregates; and mandatory leaching test twice a year was required for use in roads andhydraulic structures (Motz and Geiseler, 2001). European standard permits the use of steelslag in asphalt provided the 7 days volumetric expansion is no more than 3.5% (BSEN13043,2002).

4.3. Scrap tyres

Use of tyre rubber in asphalt generally has two distinct approaches. One is to dissolvecrumb rubber in the bitumen as binder modifier, the other to replace a portion of fineaggregates with ground rubber that is not fully reacted with the bitumen. These are referredto as the ‘wet process’ and the ‘dry process’, respectively. Modified binder from the ‘wetprocess’ is termed ‘asphalt rubber’; asphalt made by the ‘dry process’ is ‘rubberised asphalt’(FHWA, 1997).

Y. Huang et al. / Resources, Conservation and Recycling 52 (2007) 58–73 65

4.3.1. The wet processIn the wet process, crumb rubber (0.15–0.6 mm) is blended with bitumen for a minimum

of 45 min at elevated temperature prior to contact with aggregates, usually in the rangeof 18–22% bitumen weight (Hicks, 2002). Light fractions of bitumen transfer into therubber making the rubber particles swell and the bitumen harden. The binder viscosity isincreased allowing for additional bitumen to be used, which in theory can help reduce top-down thermal cracking and bottom-up reflective cracking, and improve mix durability (e.g.resistance to moisture, oxidation and fatigue).

The modification effect can be influenced by a number of factors including the basebitumen composition, blending time and temperature, percentage and gradation of crumbrubber, and the grinding method (FHWA, 1997; West et al., 1998). These variables werestudied following the SUPERPAVE (SUperior PERforming asphalt PAVEment) methodat NCPE and US Texas DOT (Department of Transportation) (Airey et al., 2003; TexasDOT, 2000). FHWA believe that rubber particles in the ‘wet process’ will reduce resilientmodulus of the asphalt mixture, and therefore its resistance to permanent deformation(FHWA, 1997). The opposite was observed in Brazil and India where the asphalt rubbermixture had lower rutting potential because of higher stiffness and tensile strength at hightemperatures (Bertollo et al., 2004; Palit et al., 2004). As for low temperature performance, astudy at Kansas State University (KSU) suggested an 18–22% of rubber content, and statedthat a change within this range was less significant in affecting the tensile and fractureperformance of the asphalt than varying the binder content between 6 and 9% (Hossain etal., 1999). This was confirmed by Arizona State University (ASU) that longer fatigue lifeexhibited by asphalt rubber mixture came from the higher binder content (Zborowski et al.,2004). University of Liverpool had the permissible rubber (0.3–0.6 mm) content set at 10%of binder containing pen-50 or pen-100 bitumen. Resistance to rutting, fracture and fatiguewas increased as a result (Khalid and Artamendi, 2006).

Projects also revealed problems from the use of asphalt rubber in road surface. Bleed-ing and loss of coarse aggregates were observed on a Virginia SAM (stress absorbingmembrane) trial section containing 20% crumb rubber in the binder, and the SAM didnot hinder reflective cracking as expected (Maupin and Payne, 1997). A chip seal (or sur-face dressing) project in Iowa showed that the asphalt rubber compromised the frictionperformance (Iowa DOT, 2002). A project in Texas indicated that OGFC represented thebest application for asphalt rubber in terms of cost, resistance to cracking and raveling(Tahmoressi, 2001). NCPE suggested that asphalt rubber not be used in polymer modifiedbitumen (PMB), because the PMB-rubber interaction compromised the rheological prop-erties of the aged binder and as a result, the durability of asphalt mixtures (Airey et al.,2002).

The design method for conventional HMA can be used for asphalt rubber mixtures,with mix stability being the primary design factor. A rule of thumb is that if 20% crumbrubber is used in the binder, the binder content would be 20% higher than conventional.The binder content is recommended even higher in spray applications, for instance 45%higher in stress absorbing membrane interlayer (SAMI) than that required for conventionalasphalt. Placement of asphalt rubber mixtures can be accomplished using standard pavingmachinery except for pneumatic tyre roller as asphalt rubber will stick onto the roller tyres(Epps, 1994). The main concerns include the narrowed paving temperature window (e.g.

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no laying with ambient temperature below 13 ◦C), and potential toxic emissions (see theparagraph below) (Hicks, 2002).

Noise studies at Rubber Pavements Association (RPA) found that the use of tyre rubberin open-graded mixture binder reduced tyre noise by at least 50% (Rubber PavementsAssociation, 2006).7 Rubber particles of multiple sizes were believed to have a better soundabsorbing effect in spray applications (Zhu and Carlson, 1999). By 1995 there was nosuch sign that mixing and paving asphalt rubber materials impose additional environmentalburdens than conventional asphalt (Emery, 1995). More recent leaching test at Oregon StateUniversity (OSU) indicated that about 50% of leachate contaminants from asphalt rubbermixtures were released into surface and ground water system within the first few daysafter laying, with benzothiazole, aluminum and mercury being the elements detected atpotentially harmful concentration of 0.54, 1.5 and 0.02 mg/l, respectively (Azizian et al.,2003).

Projects in the late 1980s showed that asphalt rubber in dense-graded mixtures helpedreduce the asphalt layer thickness by 20–50% without compromising its performance (Kirk,1991). The thickness reduction was confirmed by accelerated load testing (ALT) at Uni-versity of California Berkeley and South Africa (Hicks, 2002). Another benefit of usingasphalt rubber is to prolong the pavement life. A project in Brazil having 15% rubber in theHMA overlay binder found that cracking was developed 5–6 times slower than in conven-tional asphalt; also the asphalt rubber mixture outperformed in terms of surface deflection,interface strain and rut depth (Nunez et al., 2005). Similarly, binder of 15% rubber (size of0.2/0.4/0.6 mm) was used in dense-graded asphalt in Japan. The mixture exhibited improvedperformance in dynamic stability, 48 h residual stability, flexural strength and strain value;and asphalt containing 0.2/0.4 mm-sized rubber showed the best laboratory results (Souzaet al., 2005). On the other hand, FHWA confirmed that the production of crumb rubber mod-ified asphalt is normally 50–100% more expensive than producing conventional (FHWA,1997). Practice by individual State DOT revealed a range of cost increase: 21% in Col-orado (Harmelink, 1999), 50–100% in Virginia (Maupin, 1996), 25–75% for gap-gradedand 80–160% for open-graded in Arizona (Way, 1998), $10–$15/tonne in Oregon (Hicks,2002), $16/tonne in California (Caltrans, 2003), to name but a few. However, life cyclecost analysis (LCCA) was recommended by all practitioners for assessing the cost effec-tiveness of the use of asphalt rubber, taking an analysis period of 30–40 years includingthe maintenance and user cost. LCCA was conducted at ASU and OSU using the WorldBank’s Highway Development and Management model (HDM-4) and the FHWA’s LCCAmethod (FHWA, 1998), respectively. According to their results, the use of asphalt rubberwas ‘cost effective’. Meanwhile, they recognised that this is not always the case, and theresults depend on many input variables which need to be studied on an individual basis(Jung et al., 2002; Hicks and Epps, 2000).

4.3.2. The dry processIn the dry process, ground rubber (0.85–6.4 mm) substitute for fine aggregates in the

asphalt, at typically a 1–3% replacement rate.

7 Rubber Pavements Association: http://www.rubberpavements.org/library/noisereduction.asp accessed on 26January 2006.

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Asphalt properties of particular interest in the dry process include resilient modulusand noise reduction. Where there was a 10–20% increase of binder content as required,the resilient modulus of the rubberised asphalt was reduced implying an increase of layerthickness, compared with conventional mixtures (FHWA, 1997). Some other laboratoryresults showed a reduced permanent deformation (Reyes et al., 2005; Selim et al., 2005).Acoustic analysis and field measurement confirmed that rubberised asphalt paving is effec-tive in reducing traffic noise from light-duty vehicles (Sacramento County, 1999). Leachingtest indicated that rubber in sand-based root zones (typically seen in sports and recreationfields) reduced by more than half the nitrate concentration of leachate into ground water,by replacing traditional gravel of comparable size in the drainage layer (Lisi et al., 2004).

The design method for conventional mixtures can be used to design rubberised asphaltcontaining 1–3% of ground rubber particles. A target air void of 2–4% is the primary designfactor (FHWA, 1997). The time and temperature at which the bitumen reacts with rubberparticles need to be controlled with care, to retain the physical shape and rigidity requiredfor the dry process. A project in Turkey found that when Marshall Stability, flow, VMA(voids in the mineral aggregate), unit weight and VFA (voids filled with asphalt) all weretaken into consideration, the optimum technical parameters were: 0.95 mm for tyre rubbergradation, 10% for tyre rubber ratio, 5.5% for binder ratio, 155 ◦C for mixing temperature,15 min for mixing time and 135 ◦C for compaction temperature (Tortum et al., 2005).

4.3.3. Other applications in pavement structureTyre shreds have applications in road foundation. Compared with compacted soil, tyre

rubber is of: (1) light weight, (2) low thermal conductivity, (3) high hydraulic conductivityand, (4) high shear strength at large strains. Leaching potential seems to be the main concern.ASTM-D6270 and EN12457 procedures are followed in the States and Europe, respectively,to measure and characterise the leachate. Constituent analysis of tyre sample indicated thatalthough it contained leachable hydrocarbons (e.g. PAH), metals (e.g. zinc) and respira-tory dust, the released concentration was not of a concern to human health or surroundingenvironment under normal operating conditions (e.g. open air, neutral pH value) (Edeskar,2004). Tyre rubber used in lower pavement layers can help reduce the depth of frost pene-tration in winter time. Processing of scrap tyres has a by-product: waste fibre, which wasadded into SMA mixtures to prevent the ‘drain down’ of bitumen from aggregates, withoutcompromising deformation resistance or moisture susceptibility of the mixture in whichtraditional stabilising additives like cellulose or mineral fibre are commonly used (Putmanand Amirkhanian, 2004).

4.4. Plastics

Similar to tyre rubber, recycled plastics can either replace a portion of aggregates, orserve as a binder modifier. DBM with recycled plastics, mainly low density polyethylene(LDPE) replacing 30% of 2.36–5 mm aggregates, reduced the mix density by 16% andshowed a 250% increase in Marshall Stability; the indirect tensile strength (ITS) was alsoimproved in the ‘Plastiphalt’ mixtures (Zoorob and Suparma, 2000). Recycled LDPE ofa size between 0.30 and 0.92 mm replacing 15% aggregates in asphalt surfacing nearlydoubled the Marshall quotient, and increased the stability retained (SR) by 15%, implying

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improved rutting and water resistance. A 20% increase of binder content was required inthis case (Qadir and Imam, 2005). The blending of recycled LDPE to asphalt mixturesrequired no modification to existing plant facilities or technology (FHWA, 1997). Flexuralbehaviour of asphalt containing recycled plastics (PVC bottle) was studied. Bending strengthwas increased by adding 2–6% mixture weight of plastic particles, with further investi-gation suggested to depict the ‘bending strength against plastics content’ curve (Ergunet al., 2005). Recycled plastics (PE film) used at 0.4% of mixture weight (or about 8%of binder weight) as bitumen modifier, increased the Marshall Stability before and afterwater logging (60 ◦C, 24 h) by 3.3 and 2.6 times, respectively (Justo and Veeraragavan,2002).

5. Discussion

The use of recycled materials in roads varies across the UK (TRL, 2001). This is prob-ably due to the difference in access to suitable natural aggregates and in the capacity oflocal landfills. Other than technical barriers may exist, as for example, lack of collectinginfrastructure, alternative use of recycled SWM, limited market information and additionalcost all may inhibit the waste from being recycled into pavement asphalt. The governmentencourages recycling by legislation, purchasing power and grants that are offered to com-panies to help initiate recycling locally (QPA, 2004). The use of recycled SWM in asphaltpavements must have a value-added prospect and is likely to be practical where there is aconsistent supply.

From a technical perspective, asphalt with well crushed glass (e.g. ≤4.75 mm) replacinga few percent (e.g. 10–15%) of fine aggregates should not be excluded from use in asphaltsurface layers, as glass particles are ground too finely to present any safety risks, and PSVand AAV requirements apply only to coarse aggregates in the mixtures. However, this maypose a non-technical barrier as fine aggregates are only used in moderate amount in SMAand OGFC, where recycled SWM that can be used in larger size (e.g. steel slag) makes abetter choice because of less processing requirements and a higher replacement rate. It isrecognised that the replacement rate should be allowed to vary to the size of glass particles,and vice versa (Maupin, 1998).

Steel slag should be used in place of coarse aggregates in surface asphalt, to make bestuse of its mechanical strength and skid resistance. Large particle size and high content arerecommended by laboratory and trial results. The main drawback is the high specific gravityof steel slag (3.2–3.6), if used in stone-dominated mixtures like SMA or OGFC, will driveup the overall mix density, implying an increase of transport cost. The presence of freeCaO/MgO in slag makes it liable to expand in humid condition and therefore unsuitable foruse in structures vulnerable to volumetric expansion. The common approach is to exposethe slag to spray water or natural weathering for a period of between 12 and 18 months(Airey et al., 2004; FHWA, 1997). The time span could be reduced if chemical treatmentis performed before the slag leaves the steel plant as is the practice in Germany, althoughthe associated cost and environmental implication need to be further investigated. Leachingpotential is one of the main environmental concerns over the use of secondary materialsin road structures (Mroueh et al., 2001). Research in Germany has identified pH-value,

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Table 2Waste arising in the UK and application in asphalt pavements

Waste arising(Mt/year)

Recycling rate(%)

Aggregatesuse (%)

Use in asphalt pavements

Aggregates Replacerate (%)

Binder Replacerate (%)

Glass 3.4 33 4.1√

10–30 XSteel slag 1.0 100 98

√30–62 X

Scrap tyre 0.44 21 N/A√

1–3√

18–22Plastics 2.8 5 0.29

√15–30

√8

√indicates an option; X indicates not an option.

electrical conductivity and Chromium concentration in the leachate as the main concernsfor using slag aggregates (Motz and Geiseler, 2001).

In general, tyre rubber is used in asphalt mixtures to reduce cracking, improve durabilityand mitigate noise. Depending on the application, different variables need to be consideredwhen assessing the technical performance of asphalt containing tyre rubber: binder prop-erties in the wet process, and mixture properties in the dry process. So far, most laboratoryand field work has been focused on the ‘wet’ trial. It is generally agreed that asphalt rubbermixtures improves durability and low-temperature performance. On high-temperature per-formance however, there are mixed views in the United States ranging from better, similaror comparable, to worse. Results from the ‘dry’ trial so far are of limited number, andare as well far from conclusive. Generally, rubberised asphalt does not show significantlyimproved performance to offset the additional cost (FHWA, 1997; Oregon DOT, 2002). Thewet process is more tolerant, whilst the dry process requires extra care in materials selection,mix design and asphalt manufacture. The economic break-even point in both processes iswhether the increased cost (e.g. waste processing, higher binder usage) can be warrantedby a return through longer pavement life. Life cycle cost analysis can be helpful to find outwhen and where the use of tyre rubber in asphalt is cost effective.

Recycled LDPE can substitute a portion between 15 and 30% of aggregates dependingon its particle size and if properly designed, the rutting, cracking and aging performanceof the mixture may improve as a result. Recycled PE accounting for 8% of the binder as abitumen modifier, can also increase the mixture’s Marshall Stability. Similar to tyre rubberin the ‘dry process’, a number of asphalt properties when using recycled plastics are yetto be reported, nor are certain the cost and environmental implications, due to the limitedpractice so far (Table 2).

6. Conclusion and recommendation

The use of recycled materials in pavement asphalt represents a valuable outlet for suchmaterials. Yet value applications usually come with additional property requirements andtechnical restriction. These are liable to drive up the processing cost, often higher thanthat of purchasing virgin aggregates. Overcoming the many barriers, technical as well asnon-technical, requires the commitment from all stakeholders to act accordingly (WRAP,2006). Government efforts, in most cases are important, if not essential, to provide research

70 Y. Huang et al. / Resources, Conservation and Recycling 52 (2007) 58–73

and information service free from commercial restriction, and enhance the profitability ofrecycling through legislation or financial incentives.

When assessing the performance of asphalt pavements containing recycled SWM, someconditions need to be studied and specified unambiguously such as: (1) Mixture type, asdifferent mixtures (e.g. SMA, HRA) impose different property requirements for componentaggregates. (2) Particle size of recycled SWM and the replacement rate. (3) Nature andprocessing techniques of the SWM (e.g. weathering for steel slag).

It is obvious that each recycled SWM will have more than one potential use. Recyclingmaterials back into its initial use (e.g. recycled glass cullet to glass making) often aremore sustainable rather than finding new applications. The responsibility for the asphaltindustry is to find the right source of SWM and use as the right components in pavementasphalt that make sense in both technical and financial terms. Aggregates consumed bythe asphalt industry alone outweigh the total arising of the four waste materials discussedabove (around 7.6 Mt/year). Diverting other SWM to pavement use is therefore worth theongoing efforts that have given approval to an expanding list of recycled SWM for use inroad construction (TRL, 2004), to further ease landfill pressures and reduce the demand forquarrying minerals.

Apart from technical and economic factors, concerns over the use of recycled SWM inpavement asphalt also come from their potential of causing environmental burdens in roadstructures, such as run-off pollutants and leaching (CIRIA, 1997; Mroueh et al., 2001).Transport and processing of SWM into desired properties implies additional energy useand emissions. Conflicting statements require objective environmental assessment tools thatcan quantify and compare the various environmental burdens for the different constructiontechniques, materials in use and maintenance options. This is highlighted when SWMreplace primary resources to achieve for example sustainable construction methods. Lifecycle assessment (LCA) emerges as a promising tool for the road sector to investigate theenvironmental impacts throughout pavement life, and present the results for communication.

Acknowledgements

An earlier version of this paper was presented at the 4th International Conference onMaintenance and Rehabilitation of Pavements and Technological Control (MAIREPAV4),18-21 August 2005, Belfast, UK. Financial support from Aggregate Industries UK Ltd isgreatly appreciated.

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