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Journal of Cleaner Production xxx (2014) 1e9

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Environmental evaluation of two scenarios for the selectionof materials for asphalt wearing courses

Ana Mladenovi�c a, Janez Turk a, *, Jaka Kova�c a, Alenka Mauko a, Zvonko Coti�c b

a Slovenian National Building and Civil Engineering Institute, Dimi�ceva ulica 12, 1000 Ljubljana, Sloveniab Structum d.o.o., Tovarni�ska cesta 26, 5270 Ajdov�s�cina, Slovenia

a r t i c l e i n f o

Article history:Received 21 July 2014Received in revised form6 October 2014Accepted 6 October 2014Available online xxx

Keywords:LCACarbon steel slag aggregateRoad constructionAsphalt wearing courseTransport sensitivity analysisSlovenia

* Corresponding author. Tel.: þ386 1 2804 250; faxE-mail addresses: ana.mladenovic@zag.si (A. M

(J. Turk), jaka.kovac@zag.si (J. Kova�c), alenka.maukocotic@structum.si (Z. Coti�c).

http://dx.doi.org/10.1016/j.jclepro.2014.10.0130959-6526/© 2014 Elsevier Ltd. All rights reserved.

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a b s t r a c t

Carbon steel slag is quite commonly used in road construction to replace natural aggregate. Since it isimportant to evaluate such a replacement from the environmental point of view, a Life Cycle Assessmentwas carried out in order to compare the environmental impacts of the construction of asphalt wearingcourses with the use of siliceous aggregates (the “conventional scenario”), and the use of alternative, steelslag aggregates (the “alternative scenario”). The main advantage of the alternative scenario is that areduction can be achieved in the consumption of natural aggregate, as well as in the quantity of slagdeposited on landfill sites. On the other hand larger amount of bitumen is needed as a binder. However, theresults of the Life Cycle Assessment (based on consequential modelling) revealed that the alternativescenario is to be significantly preferred if the following impact categories are taken into account: Acidifi-cation, Eutrophication, Photochemical Ozone Creation and Human Toxicity. In the case of the discussedindicators, the impacts are reduced to a level equal to about 80% of the conventional scenario impacts. Thisbenefit was additionally evaluated by means of a transport sensitivity analysis, which provided resultswhich could be useful for roadmanagers working on case studies using similar constructionmaterials. Thealternative scenario ismore sustainable than the conventional scenariowith regard to the discussed impactcategories evenwhen taking into account long delivery distances of the steel slag aggregate (~100 km) andminimal delivery distances of the siliceous aggregate. Considering similar delivery distances in both sce-narios, the alternative scenariowas found to be beneficial alsowith regard to the GlobalWarming, but onlywhen the delivery distance of the steel slag aggregates did not exceed 160 km.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Among those sectors which have a significant effect on energyconsumption and on the emission of greenhouse gases to the at-mosphere is the construction industry (Ortiz et al., 2009; Barandicaet al., 2013). This sector therefore needs to be actively involved inactivities which are aimed at reducing environmental impacts. Onesuch activity could be the replacement of non-renewable naturalaggregates by recycled aggregates (Blengini and Garbarino, 2010;Blackendaal et al., 2014; Turk et al., 2014), and in particular sec-ondary aggregates obtained from industrial wastes and by-products (Birgisd�ottir et al., 2006; Faleschini et al., 2014). Aggre-gates are an essential building material for the construction in-dustry, since they represent the main component of concretes and

: þ386 1 2804 484.ladenovi�c), janez.turk@zag.si@zag.si (A. Mauko), zvonko.

vi�c, A., et al., Environmental eion (2014), http://dx.doi.org/

asphalts, and are also used for unbound layers in numerous engi-neering applications. The great majority of aggregates used in theconstruction industry are obtained from natural sources. Currently,about three billion tons of natural aggregate are produced annuallyin the countries of the European Union (Marinkovi�c et al., 2010).The mining of such aggregates is, firstly, apt to cause local envi-ronmental problems, and in some areas shortages of aggregate,especially high-quality aggregate, have already been observed(Huang et al., 2007; Carpenter and Gardner, 2009; Chowdhuryet al., 2010). The use of recycled and secondary aggregates,instead of natural aggregates, is therefore an important task. Ofcourse, it is crucial that such aggregates meet the same qualitystandards as natural aggregates, so that they are able to satisfy thesame performance standards. The use of recycled and secondaryaggregates exhibits certain advantages, such as the reduction ofenvironmental impacts associated with the mining of natural ag-gregates, and the reduction of the landfill space requirementsassociated with the need to landfill industrial wastes and by-products (Carpenter et al., 2007; Huang et al., 2009; Miliutenko

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et al., 2013). However, their use can generate some other environ-mental impacts, which eventually even out or even override theabove-mentioned benefits. For this reason, it is important tocompare different scenarios with regard to the use of conventionaland alternative materials, by means (for example) of Life CycleAssessments e LCA, in order to emphasize what the environmentalbenefits of the utilization of such materials are, as well as thepossible associated weaknesses.

According to the literature, different alternative materials can beused in road construction in order to replace natural aggregates.Several LCA studies about the utilization of alternative materialshave been carried out. The use of bottom ash in the sub-base layersof road, in order to replace gravel, was studied by Birgisd�ottir et al.(2006, 2007), and by Olsson et al. (2006). However, the benefits ofthe recycling scenario were found to be marginal, taking into ac-count the environmental burdens and resource consumption(Birgisd�ottir et al., 2006). It has also been shown that the results aresensitive to the transport distance for the roadmaterials and also toconditions affecting the leaching from roads. The differences be-tween energy uses in the two alternatives mainly originate fromthe production of crushed rock, and from the landfilling of bottomash (Olsson et al., 2006).

Mroueh et al. (2001), as well as Carpenter and Gardner (2009),studied the utilization of industrial by-products such as coal ash,crushed concrete waste, foundry sand and blast furnace slag (BFS),all as possible replacement materials for natural aggregate in thecase of road sub-base construction. Taking into account some lim-itations, the discussed by-products exhibit quite good potential forthe reduction of some of the environmental burdens.

Huang et al. (2009) presented a case study of pavement con-struction in which natural aggregates were partially replaced by acombination of waste glass (10%), incineration bottom ash (10%),and recycled aggregate pavement (RAP e 25%). Based on a sensi-tivity analysis, they found that replacement of natural aggregate byRAP has, relatively, the greatest effect on the reduction of green-house emissions, not only because of its tonnage. It does notmerelyreplace natural aggregate but also reduces the input of primarybitumen (an energy intensive product). However, the replacementof natural aggregate by glass causes more emissions, due to highconsumption of fuel in the collection of waste glass.

Among alternative materials, carbon steel slag (EAF C) is quitecommonly used in road construction. However, an overview of theexisting literature has shown that environmental evaluations havebeen performed for the use of blast furnace slag in road construc-tion (cf. Carpenter and Gardner, 2009; Sayagh et al., 2010), whereasthere is a lack of such studies in the case of the use of carbon steelslag obtained from an electric arc furnace (EAF). This latter kind ofslag is, however, a source of one of the best artificial aggregateswhich can be used for the replacement of the natural aggregates ofthe highest quality (Maslehuddin et al., 2003; Manso et al., 2006;Quasrawi, 2014). EAF C slag occurs as a waste or a by-product inthe production of low-carbon steel in an electric arc furnace. Theamount of steel slag produced in Europe in 2010 totalled about 21.8millions tons; about 31% of this amount was generated as EAF Cslag. About 76% of produced steel slag was used in qualified fields ofapplication, mostly in road construction (Euroslag and Eurofer,2012). Although recycling of slag is common practice in Europe,significant quantities of slag are still dumped in landfills or stockedfor long periods of time at steel plants (Gomes and Pinto, 2006). In2010, about 11% of produced steel slag was temporarily stored to beused at a later time, whereas 13% of the steel slag was taken to afinal deposit site (Euroslag and Eurofer, 2012).

EAF C slag aggregate is quite frequently used in road construc-tion since it exhibits excellent mechanical properties, which satisfyall the necessary mechanical requirements for its use in asphalt

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layers (including asphalt wearing courses) for all traffic loads. Thegreatest benefit shown by any of these properties is, however, thatof the much improved skid resistance of wearing courses (whencompared to that achieved when natural aggregates are used)(Liapis and Likoydis, 2012), although this improvement can only beensured when the slag is properly treated during the cooling pro-cess, so that a suitable microstructure is developed (Euroslag andEurofer, 2012). Taking into account the field experience of roadconstruction companies, the physical properties and surfacetexture of steel slag can provide a coefficient of friction in bitumi-nous surface courses which is higher than that which could beprovided by most natural aggregates. Furthermore, steel slag'sresistance to polishing means that its frictional benefits are main-tained over time. The permeability of an asphalt wearing courseprepared partly from EAF C slag aggregate is higher than that of acorresponding wearing course made from natural aggregate, whichmeans that less aquaplaning can be expected during heavy rainfall(Liapis and Likoydis, 2012). EAF C slag aggregates are inert; so thatthere are no concerns related to the leaching of hazardous sub-stances (cf. Gomes and Pinto, 2006; Arm et al., 2011).

The goal of this study was to compare the environmentalimpacts of two alternatives for the construction of asphaltwearing courses. One alternative is conventional, whereas theother is based on the utilization of EAF C slag aggregate. Here itshould be emphasized that several authors (Mroeh et al., 2001;Roth and Eklund, 2003) have demonstrated that the activitywhich predominantly contributes to the environmental burdenthroughout the life-cycle of a road is not related to the by-product, but to the energy needs for the manufacturing ofasphalt and binders (bitumen or cement). Additionally, environ-mental burdens are also highly dependent on the transport ofconstruction materials and the method of road construction. Inpractice this means that the result can be very case specificbecause of local specifications (i.e. the availability of raw mate-rials and their delivery distances) and cannot be generalized. Inthe case of the research described in this paper, this problem waseliminated by applying transport sensitivity analysis, in whichthe effect of the delivery distances, on the final results, of the twoalternative construction materials was evaluated. The obtainedresults are therefore more representative, and can thus be used asa tool to support decision making.

2. Materials and methods

The study was based on the Life Cycle Assessment (LCA)method, as supported by the international standard (ISO 14040,2006). LCA was carried out for the two investigated scenarios inorder to compare the environmental impacts and resourceconsumption:

(i) The conventional scenario - The use of conventional con-struction aggregates for the production of an asphalt wearingcourse (Table 1).

(ii) The alternative scenario -The use of EAF C slag aggregate (2/4and 4/8) instead of siliceous aggregate (2/4 and 4/8) in thewearing course (Table 1).

2.1. Case study characteristics

The data about the production of the two asphalt mixes (theconventional mix and the alternative mix), and about their placingon a road refer to a realistic case study, which was carried out inSlovenia. The construction works were performed on a regional

valuation of two scenarios for the selection of materials for asphalt10.1016/j.jclepro.2014.10.013

Table 1The construction materials used in the conventional and alternative scenarios. Theenergy requirements are also indicated. Data were provided by the Structumcompany.

Conventionalscenario

Alternativescenario

Material inflows (tons) (tons)Bitumen 15.660 16.500Carbonate sand (0/2) 107.793 114.000Carbonate filler 11.223 12.000Coarse grained aggregate (2/4 and 4/8) -

siliceous or steel slag126.324 157.500

Final product: ready-mixed asphalt 261.000 300.000

Energy inflows (MJ) (MJ)Natural gas 1750 2012Electrical power 4604 5292Diesel 1 1.05

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road, i.e. section 1072 between the villages of Kne�za and Podbrdo(between km 15.320 and km 15.950), where the annual averagedaily traffic is relatively light, i.e. around 1000 vehicles per day(88% of them being cars). The existing wearing course wasremoved and replaced by a new wearing course with a thickness of3 cm. An asphalt mix of type SMA 11 PmB 45/80-65 A2 was used,which was in accordance with the European standard SIST EN13108-1:2006/AC:2008 and the Slovene national standard SIST1038-1:2008.

The functional unit is the construction of a surface course of aroad pavement over a length of 600 m (with a thickness of 3 cm),and it refers to this actual case study.

Fig. 1. Schematic representation

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2.2. System boundaries

Not all stages of the Life Cycle Assessment of the asphalt wearingcourse construction are discussed in this study, only the productionand placing stages (Fig. 1). The reason for this is the lack of relevantdata. However, it was assumed that when steel slag aggregate isused instead of siliceous aggregate there are no measurable effectson the life expectancy of the asphalt layers (around 20 years), or onthe technical constraints. It was also assumed that, during theoperation and maintenance stage, the environmental burdens donot differ significantly for the two different types of asphalt wear-ing course. Both of these two assumptions are supported by theresults of laboratory tests and field experience mentioned in theintroduction.

In both scenarios, all the construction materials are producedand assembled at the asphalt production plant, where the asphaltready-mix is manufactured. The material requirements for theproduction of conventional and alternative ready-mixes are pre-sented, together with the energy requirements, in Table 1.Manufacturing of the asphalt mixwith the slag aggregates is energymore intensive, due to its higher total weight (300 tons in thealternative scenario versus 261 tons in the conventional scenario),and the emissions, too, are higher (Table 2). The transport of theready-mixed asphalt to the location of the road construction is alsotaken into account, since the mass of the ready-mixes differs in thetwo comparable scenarios. The energy requirements for the placingof the asphalt as a wearing course are the same in both scenarios(Table 3).

of the system boundaries.

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Table 4Processes which are included in system (their source and the country to which theyrefer). For more process information, see the GaBi database documentation.

Process Geographicalcode

Source andreference year

Production of limestone flour asa proxy for carbonate filler(in a quarry)

Germany PE 2005

Production of limestonecrushed stone fines as aproxy for carbonate sand (ina quarry)

Germany PE 2005

Production of crushed stone asa proxy for coarse grainedsiliceous aggregate (in aquarry)

Europe PE 2006

Production of slag granulate asa proxy for steel slagaggregate (via secondarytreatment)

Germany PE 2007

Bitumen production at refinery EU-15 PE 2003Power grid mix production at

power plantEU-27 PE 2002

Diesel fuel production atrefinery

EU-15 PE 2003

Natural gas production atprocessing plant

EU-27 PE 2002

Transport by truck Global PE 2005Process water production Europe PE 2005Asphalt production at the

asphalt plantSI (casespecific data)

Structumcompany 2013

Placing of the asphalt as awearing course

SI (case specificdata)

Structum company 2013

Slag disposal on landfill forsteel, as non-hazardouswaste (avoided impact)

Europe PE 2005

Table 2Emissions related to the operation of the asphalt plant during the production of 261tons of the conventional asphalt mix and a corresponding 300 tons of the alternativeasphalt mix. Data were provided by the Structum company.

Emissions to the air Conventional scenario (kg) Alternative scenario (kg)

Carbon dioxide 4883 5613.5Carbon monoxide 3 3.45Hydrocarbons 0.52 0.59Nitrogen oxides 2.57 2.96

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2.3. System inventory analysis

Inventory analyses ideally include an assessment of all possibleflows into and out of a system. Inputs are related to energy con-sumption and the consumption of natural resources, whereasoutputs are related to releases into the air and water, and theproduction of any kind of solid waste (Horne et al., 2009). Dataabout the environmental burdens and energy requirements asso-ciated with the production of construction materials, fuels andelectricity, and with transport, are already stored in the“professional þ extended” dataset, which is a part of the GaBisoftware (PE international, 2010).

The EAF C slag impacts are reflected in the processes of itsproduction as an alternative aggregate, i.e. cooling, quenching,ageing, two-stage crushing, magnetic separation and sieving. Datafrom the GaBi dataset were applied in order to evaluate the energyrequirements and the related environmental burdens associatedwith above-mentioned processes. Slag utilization in the road con-struction industry means that dumping on landfill is avoided. Thisis an avoided impact, which can be taken into account as an envi-ronmental credit in the alternative scenario (Heijungs and Guin�ee,2007). Such a consequential Life Cycle Inventory method isdesigned to generate information about the consequences of ac-tions (Ekvall and Weidema, 2004; Escobar et al., 2014). Avoidedburdens that might be related in the case of the disposal of slagwere evaluated based on data that can be found in the GaBi dataset.

The production and maintenance of transport vehicles (trucks)was not taken into account. The use of Euro 4 type trucks, with acapacity of 27 tons, was assumed for the transport of all the con-struction materials in both scenarios. The diesel consumption ofsuch trucks is around 35 L per 100 km, according to the generallyaccepted specifications (PE international, 2010). Twoway distanceswere taken into account in the LCA analysis, the trucks returningempty.

In general, a search was performed for those data which reflectan average European production process, in order to make thisstudy more widely representative. However, some data (e.g. rockextraction) refer to Germany, since some of the processes stored inthe GaBi dataset refer to specific countries only (Table 4).

Table 3Emissions from machinery during the placing of the asphalt mix over a 600 m longsection of a road. The same emissions were assumed for the conventional and thealternative scenario. Data were provided by the Structum company.

Emissions to the air Conventional/Alternative scenario (kg)

Ammonia 5.4-E-03Carbon dioxide 1044Carbon dioxide 55Carbon monoxide 1.14Dust (PM2.5) 4.2-E-02Methane 1.1-E-02Nitrogen oxides 6Nitrous oxide 7.1-E-03NMVOC 4.7-E-01Sulphur dioxide 3.5-E-02

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Data on the energy and water consumption at the investigatedasphalt plants (batch plant technology, the capacity of the plantbeing 100 tons per hour), as well as data on emissions occurringfrom this plant, and the corresponding data related to the placing ofthe asphalt, were gathered by the company which produced andplaced the asphalt mix. However, it was not possible to obtainexactly all the data. Emissions related to fuel (natural gas) com-bustion were considered, whereas no data about emissions fromthe hot mix asphalt were obtained. Data of the same kind as thatobtained at the asphalt plant were obtained during the road con-struction process. It was not possible to measure the emissionswhich occurred during the cooling and compacting of the asphalt,so these data were neglected in the model. However, it wasassumed that, if the two scenarios are compared, the neglectedemissions did not differ significantly.

2.4. Comparison of the two scenarios

In the first stage of the LCA analysis, the burdens of the twoscenarios were compared with regard to the manufacturing/extraction of raw materials and the manufacturing of ready-mix atthe asphalt plant (Tables 1 and 2). The effect of the delivery of theraw materials to the asphalt plant was omitted, considering somehypothetically ideal conditions, where all the materials occur in thedirect vicinity of the plant. The transport of ready-mix from theplant to the road works was taken into account in this comparison,since the mass of the ready-mix is significantly greater in thealternative scenario than in the conventional scenario. The deliverydistance was set to 70 km (one-way). By taking into account suchan approach, the burdens that can be attributed to the conventionaland alternative scenarios can be quite representatively comparedfrom the environmental and the human health point of view.

valuation of two scenarios for the selection of materials for asphalt10.1016/j.jclepro.2014.10.013

Fig. 2. The distribution of the studied impacts between the different processes included in the model of the construction of an asphalt wearing course. The two scenarios arecompared.

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In next stage of the LCA analysis, the delivery of constructionmaterials to the asphalt plant was additionally taken into consid-eration. The sensitivity of the results to the delivery distances of thetwo alternative coarse-grained aggregates (i.e. the siliceousaggregate and the steel slag aggregate) was evaluated. The deliverydistance of these two types of aggregate was set as a variableparameter, and the two scenarios were than compared, consideringthe energy requirements, as well as the environmental and humanhealth burdens. Depending on the delivery distances, preferencecan be given either to the conventional scenario or to the alterna-tive scenario. The effect of a variable delivery distance of the asphaltmix (from the asphalt production plant to the road works) was alsostudied.

2.5. Impact categories

Environmental indicators based on the midpoint model (CML2001) were used, as proposed by Hauschild et al. (2013). Attentionwas paid to a number of impact categories, i.e. Global Warmingover 100 years (GW in kg CO2 equivalent), Abiotic Depletion (AD inkg Sb equivalent), Acidification (A in kg SO2 equivalent), Eutro-phication (E in kg PO4 equivalent), Photochemical Ozone Creation(POC in kg Ethene equivalent), and Renewable and Non-renewableEnergy Requirements - net (MJ). Human Toxicity (HT in kg DCBequivalent) was also evaluated. The environmental and humanhealth burdens in the above-discussed categories were comparedfor both scenarios.

3. Results

A hypothetical (ideal) case study, where the delivery distancesof the road materials between the source and the asphalt plant areassumed to be minimal, and thus can be neglected, is firstdiscussed.

Taking into account such a hypothesis, the main benefit ofalternative scenario is that the consumption of natural aggregate(which is a mineral resource) is reduced. The processes involved inthe production of EAF C slag aggregate are energyewise relativelyless intensive than the extraction of siliceous aggregate from aquarry, and the related emissions are also much lower. For thisreason, the production of slag aggregate instead of siliceousaggregate shows a significantly lower burden in all the impactcategories.

On the other hand, the alternative scenario exhibits three dis-advantages with regard to material and energy needs.

(i) A larger amount of bitumen is required, as a binder, whichalso has an impact on natural (fossil) resources. Fossil-basedbitumen production takes place in petroleum refineries,where a large number of energy intensive processes areinvolved, which have a relatively high negative environ-mental impact (Blankendaal et al., 2014). This impact ishigher in the alternative scenario, where the bitumenrequirement is higher than in the conventional scenario(Fig. 2, see also Table 1).

(ii) Moreover, the production of asphalt mixes from slag aggre-gates is energymore intensive than the production of asphaltmixes from natural aggregates. A greater amount of naturalgas and electrical power is needed in the asphalt plant for theproduction of the alternative asphalt mix (Table 1), whichresults in higher emissions taking place not only directly atthe asphalt plant (Table 2), but also at exploitation fields andat refineries where natural gas is explored and processed,and at power plants where electricity is generated. All these

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burdens are otherwise attributed to the asphalt plant, wherethe discussed energy sources are consumed (Fig. 2).

(iii) Because of the slag aggregate, the mass of the ready mix ishigher, and more trucks are needed for the transportation tothe construction site (over the same distance, which is, inthis case, 70 km).

However, another important fact has to be taken into account. Ifslag is used as an alternative aggregate in wearing courses, thelandfilling of slag is avoided. It should be taken into considerationthat the landfilling of non-hazardous or inert waste (such as slag) isassociated with emissions (mostly carbon dioxide) to the sur-rounding environment, because of the treatment of the non-hazardous matter, such as compaction, the use of sealing mate-rials, and leachate treatment. These impacts are avoided when theslag is recycled. For this reason, avoided landfilling has a negativevalue on the graphs, as it is considered as a credit (Fig. 2). The totalimpact is partly reduced (Fig. 2), because the discussed credit istaken into consideration in the alternative scenario. For example,the impact on some of the environmental categories is reduced byabout 20% (Photochemical Ozone Creation, Acidification andEutrophication) whereas the impact on Global Warming is reducedby about 10%.

As shown in Fig. 2, the LCA analysis therefore reveals thebeneficial use of steel slag aggregate considering several of thestudied impact categories: Acidification, Eutrophication, Photo-chemical Ozone Creation and Human Toxicity. A relatively smallbenefit was shown also in the case of Global Warming. The con-ventional scenario was found to be beneficial only with regard toAbiotic Depletion. Considering the energy needs, both scenarios arequite similar - even if a slight preference is given to the alternativescenario, difference is still negligible (0.2%) and thus not significant.

3.1. Transport sensitivity analysis

The delivery distance of the siliceous aggregate can, in practice,differ significantly from the delivery distance of the replacementmaterial - steel slag aggregate (i.e. delivery from the source to theasphalt plant). Moreover, not the same amounts of these two typesof coarse grained aggregates are used in the discussed scenarios,which means that the results may be very sensitive to the deliverydistance. For this reason, a test was carried out considering thedelivery distance of coarse grained aggregate as a variable param-eter. The delivery distance of other construction materials refers tothe Slovenian case study. The one-way delivery distance forbitumen is 280 km, and 50 km for the carbonate aggregate andcarbonate filler. The ready mix is transported for a distance of70 km to the construction site.

Taking into account delivery distances of coarse grained aggre-gate up to 250 km (500 km, twoways), the alternative scenario stillexhibits benefits in the case of Acidification Potential, Eutrophica-tion Potential, Photochemical Ozone Creation Potential and HumanToxicity Potential. However, if the delivery distances of the coarsegrained aggregate are longer than ~160 km (~320 km, two ways),the conventional scenario is beneficial with regard to the impact onGlobal Warming (Fig. 3). The limit delivery distances of coarsegrained aggregate, at which the benefit of one scenario is rejectedagainst another scenario, are shown in Table 5.

The results shown in Fig. 3 can also be used in a different way.For example, if the distance to the nearest source of EAF C slag is70 km (140 km, twoways) but only 20 km (40 km, twoways) to thenearest quarry with virgin resources, then the conventional sce-nario is beneficial with regard to Global Warming Potential. How-ever, the alternative (EAF C slag) scenario is beneficial with regardto some other environmental indicators (Acidification Potential,

valuation of two scenarios for the selection of materials for asphalt10.1016/j.jclepro.2014.10.013

Fig. 3. Comparison of two scenarios based on the results of the performed transport sensitivity analysis. The delivery distance of the coarse grained aggregate (siliceous or EAF Cslag) was set as a variable parameter.

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Table 5The results of the transport sensitivity analysis showed the limit delivery distancesof the steel slag aggregate (157.500 kg per functional unit) as a replacement forsiliceous aggregate (126.324 kg per functional unit). A limit delivery distance wasdefined for each of the studied impact categories.

Indicator Beneficial scenario Beneficial scenario rejectedat a delivery distance of(km e two ways)a

Energy net Alternative 115ADP Conventional NeverGWP Alternative 320POCP Alternative 1660AP Alternative 2370EP Alternative 770HTP Alternative Never

a The delivery distance refers to the delivery of coarse aggregate (silicate aggre-gate in the conventional scenario and steel slag aggregate in the alternativescenario).

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Eutrophication Potential), as can be seen from Fig. 3. Even if asignificantly long delivery distance of the steel slag aggregate istaken into account (more than 250 km, one-way), the alternativescenario exhibits a lower impact on Human Toxicity than theconventional scenario with some minimal delivery distance ofsiliceous aggregate (i.e. 1 km) (Fig. 3). With regard to the impact onAbiotic Depletion, the delivery distance of the steel slag aggregateshould be at least 120 km shorter (one-way) than delivery distanceof the siliceous aggregate in order to reduce the impact of thealternative scenario to a value comparable to that in the conven-tional scenario.

However, the results are also sensitive to the delivery of ready-mix from the asphalt production plant to the road works. In prac-tice, the transport of ready-mix also depends on the extent of theroad constructionworks, the longer the road section, the longer thedistance over which the ready-mix has to be transported. Asmentioned above, the emissions related to this transport are higherin case of the alternative scenario, where more trucks are neededfor the delivery of the asphalt mix. However, it was found that thediscussed burdens of the alternative scenario do not significantlyexceed those of the conventional scenario, regardless of the de-livery distance of the ready-mix. Moreover, this contribution isminor when compared to the total burdens, and has almost noimpact on the final results.

4. Discussion

In some previous studies, the benefits of recycling scenario werefound to be marginal, taking into account environmental burdensand resource consumption (Birgisd�ottir et al., 2006), whereas otherstudies have indicated quite a good potential for reducing some ofenvironmental burdens (Mroueh et al., 2001; Carpenter andGardner, 2009). Here the results of the discussed study have indi-cated that the use of EAF C slag results in a significant improvementin some of the impact categories in comparison with the conven-tional scenario. This can be partly attributed to source of the data.Previous studies rely mostly on data gathered in the field and fromvarious contributions to the literature. In this study, most of thedatawere gathered from available datasets. Such are the data aboutthe production of natural aggregate (which can be replaced by steelslag aggregate as an alternative). Production of natural aggregatereflects an average regional (European) situation, so that the dif-ference between two compared scenarios could be less (in coun-tries with more advanced technologies for the production ofsiliceous aggregate), or even more (in countries with relatively lessadvanced technologies for such production). It was also found outthat the credit related to the avoidance of slag disposal has a

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significant impact on the results. This credit was also evaluated byusing data from the GaBi dataset, as above-mentioned, and refers toan average European landfill site for steel slag.

The quality of the data stored in GaBi datasets (regarding theprocesses and their related emissions) is considered to be good.However, using some of the proxy data from the dataset could be alimitation of the study. Problems may occur to find some datarelevant for an exactly specific process. Data for the disposal of slagon landfill, and for the secondary treatment of EAF C slag, refer toproxy processes, which may not be completely relevant for thisstudy. For this reason, the results would be more reliable if the data(i.e. about emissions) were gathered directly at the landfill site,where slag is disposed of. Detailed primary data would be requiredfor the secondary treatment of EAF C slag in order to obtain aproper aggregate, but it was not possible to obtain such datawithin the scope of this research. By gathering good quality data inthe field (which is always a challenge), the reliability of the studyand of its results would be improved. Despite the above-mentioned limitations, our study has confirmed the results ofsimilar past surveys, and has also emphasized the sensitivity of thesource of the data.

5. Conclusions

As shown in this study, the utilization of EAF C steel slagaggregate (alternative scenario) results in lower environmentalimpacts on Acidification, Eutrophication, Photochemical OzoneCreation, and HumanToxicity and conditionally on GlobalWarming(depending on the delivery distance of steel slag aggregate).Considering a similar delivery distance of the conventional and thealternative aggregate, the impacts of the alternative scenario arereduced by about 20% in comparison with the impacts of the con-ventional scenario. With regard to impact on Human Toxicity, onthe one hand, the reduction appears to be even greater. On theother hand, the conventional scenario shows a relatively lowerimpact on the Abiotic Depletion. There are three reasons for this: (i)a larger amount of bitumen is required in alternative scenario; (ii)the energy demands (for fossil fuels) at the asphalt plant, for thepreparation of an asphalt mix with slag aggregates, are also higher;and (iii) a larger amount of diesel fuel is consumed during thetransport of the asphalt mix to the construction site (i.e. a greatermass quantity because of slag aggregate).

Taking into account the limitations mentioned in the discussion,one of the most important results of this study is that the roadmanagers will be able to use the results of the presented transportsensitivity analysis in order to make decisions that are sustainablefrom the environmental point of view. The results of the performedtransport sensitivity analysis show that preference for the alter-native scenario can, with regard to Acidification Potential, Eutro-phication Potential and Photochemical Ozone Creation Potential, berejected when the delivery distance of the alternative aggregateexceeds a delivery distance of the conventional aggregate by about100 km or even more, which represents quite a large difference.However, a degree of caution needs to be taken into account,especially when the impacts of two compared scenarios do notdiffer significantly.

Even if the obtained results are set on one side, the discussed useof steel slag can certainly be relevant also from other points of view,which could be strategic for countries without (or with rare)sources of aggregate, but rich in industry. There is no source of highquality siliceous aggregate in Slovenia, so this kind of aggregate hasto be imported from other countries. However, Slovenia doespossess a steel plant which produces EAF C slag. If the slag is notused in other industries, it would accumulate on waste depositsites, which can be problematic.

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A. Mladenovi�c et al. / Journal of Cleaner Production xxx (2014) 1e9 9

Acknowledgements

The survey was carried out within the framework of the ReBirthproject (LIFE10 INF/SI/138). The authors would like to express theirgratitude to DRSC (the Slovenian Roads Agency), to the Munici-pality of Tolmin, and to the CPG Company (Cestno podjetje NovaGorica, d.d.) for the maintenance and construction of the road testfields. The authors are grateful to Mr Peter Sheppard for carefulediting of the text and to anonymous reviewers for theirconstructive suggestions.

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