Scholarly article on topic 'The limits of partial life cycle assessment studies in road construction practices: A case study on the use of hydrated lime in Hot Mix Asphalt'

The limits of partial life cycle assessment studies in road construction practices: A case study on the use of hydrated lime in Hot Mix Asphalt Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — T. Schlegel, D. Puiatti, H.-J. Ritter, D. Lesueur, C. Denayer, et al.

Abstract Extensive published literature shows that hydrated lime improves Hot Mix Asphalt (HMA) durability. Its impact on the environmental impact of HMA has not been investigated. This paper presents a comparative Life Cycle Assessment (LCA) for the use of HMA without hydrated lime (classical HMA) and with hydrated lime (modified HMA) for the lifetime of a highway. System boundaries cover the life cycle from cradle-to-grave, meaning extraction of raw materials to end of life of the road. The main assumptions were: 1. Lifetime of the road 50years; 2. Classical HMA with a life span of 10years, maintenance operations every 10years; 3. Modified HMA with an increase in the life span by 25%, maintenance operations every 12.5years. For the lifetime of the road, modified HMA has the lowest environmental footprint compared to classical HMA with the following benefits: 43% less primary total energy consumption resulting in 23% lower emissions of greenhouse gases. Partial LCAs focusing only on the construction and/or maintenance phase should be used with caution since they could lead to wrong decisions if the durability and the maintenance scenarios differ. Sustainable construction technologies should not only consider environmental impact as quantified by LCA, but also economic and social impacts as well. Avoiding maintenance steps means less road works, fewer traffic jams and hence less CO2 emissions.

Academic research paper on topic "The limits of partial life cycle assessment studies in road construction practices: A case study on the use of hydrated lime in Hot Mix Asphalt"

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Transportation Research Part D

journal homepage: www.elsevier.com/locate/trd

S TRANSPORTATIO RESEARCH

The limits of partial life cycle assessment studies in road construction practices: A case study on the use of hydrated lime in Hot Mix Asphalt

T. Schlegela, D. Puiattib, H.-J. Ritterc, D. Lesueurd, C. Denayere, A. Shtizaf'*

a EESAC, 230 Impasse de Fergy, 74410 Duingt, France b DPST Consulting, 6 rue Adèle, 93250 Villemomble, France

c Federation of German Lime Industry (BVK), Annastrasse 67-71, 50968 Cologne, Germany d Lhoist Southern Europe, Tour W, 102 Terrasse Boieldieu, 92085 Paris La Défense Cedex, France e Carmeuse Group, Rue du Château 13a, 5300 Seilles, Belgium fEuropean Lime Association (EuLA), Rue de Deux Eglises 26/2, 1000 Brussels, Belgium

ARTICLE INFO

ABSTRACT

Article history:

Keywords:

Life Cycle Assessment (LCA) Hot Mix Asphalt (HMA) Road construction Road durability Hydrated lime

Green Public Procurement (GPP)

Extensive published literature shows that hydrated lime improves Hot Mix Asphalt (HMA) durability. Its impact on the environmental impact of HMA has not been investigated. This paper presents a comparative Life Cycle Assessment (LCA) for the use of HMA without hydrated lime (classical HMA) and with hydrated lime (modified HMA) for the lifetime of a highway. System boundaries cover the life cycle from cradle-to-grave, meaning extraction of raw materials to end of life of the road. The main assumptions were: 1. Lifetime of the road 50 years; 2. Classical HMA with a life span of 10 years, maintenance operations every 10 years; 3. Modified HMA with an increase in the life span by 25%, maintenance operations every 12.5 years. For the lifetime of the road, modified HMA has the lowest environmental footprint compared to classical HMA with the following benefits: 43% less primary total energy consumption resulting in 23% lower emissions of greenhouse gases. Partial LCAs focusing only on the construction and/or maintenance phase should be used with caution since they could lead to wrong decisions if the durability and the maintenance scenarios differ. Sustainable construction technologies should not only consider environmental impact as quantified by LCA, but also economic and social impacts as well. Avoiding maintenance steps means less road works, fewer traffic jams and hence less CO2 emissions.

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

In the past 20 years, due to the pressure on primary raw materials and increasing environmental concerns such as traffic jams due to maintenance works, the public authorities are considering the environmental impacts of different technical solutions using Life Cycle Assessment (LCA) tools. The extensive use of fossil fuels in European roads is responsible for more than 25% of the emissions from greenhouse gas (GHG) as stated in a European Commission report (2014). The commitment of various EU Member States to develop strategies for renewable fuels and more efficient car engines are already on-going,

* Corresponding author. E-mail address: a.shtiza@ima-europe.eu (A. Shtiza).

http://dx.doi.org/10.1016/j.trd.2016.08.005 1361-9209/® 2016 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

other actions are being undertaken to address the possibility to improve the overall efficiency of road infrastructure during the construction, maintenance and end-of-life phases (ECRPD, 2009, 2010). These initiatives aim to help the contributions of road infrastructure to overall sustainable development (Gschösser et al., 2012a,b). Given this context, sustainability has become a major concern in the field of civil engineering in recent years, as exemplified by the Green Public Procurement (GPP) for road construction and traffic signs. This is a European voluntary instrument being revised by the European Commission currently, whose aim is to change existing road construction methods favouring more environmental friendly and sustainable practices (EC, 2010a,b; Butt et al., 2015).

LCA is a tool to estimate the environmental impact of products throughout their entire life cycle (from cradle to grave) -from raw material extraction through transport, manufacturing, use and all the way to their end of life. The use of LCA standards (SETAC, 1993; ISO, 2006a,b) in civil engineering dates back to the 1990s. The following LCA studies have been completed on road construction:

1. Compare the environmental footprint of concrete versus asphalt pavements (Häkkinen and Mäkelä, 1996; Lundström, 1998; Rens, 2009; CIMBETON, 2011; Gschösser et al., 2012a,b).

2. Assess the impact of reusing by-products such as bottom ash (Mroueh et al., 2001; Birgisdottir, 2005; Olsson et al., 2006; Birgisdottir et al., 2006, 2007).

3. Evaluate the benefits of increasing the recycling rate of asphalt (Jullien et al., 2006; Gschösser et al., 2012a,b) as summarised in Table 1.

Performing rigorous LCA remains a tedious and costly task and its implementation in a systematic way at project level is sometimes unfeasible. Butt et al. (2015) addressed the proliferation of LCA tools to perform LCA of pavements, but also point out that very few have been adapted by the authorities. The road industry has developed simplified tools such as SEVE the software from the French Association of Road Contractors (USIRF) for the construction phase. ROAD-RES is another tool developed by Birgisdottir (2005), which covers the construction and the disposal stages. PaLATE, the Excel-based pavement LCA tool for environmental and economic effects was developed in 2003 by the University of California, Berkeley and covers the environmental and economic impacts of construction and maintenance of the pavement (Horvath, 2004). The aim of these simplified tools is twofold: 1. Develop partial LCA's to assess the impacts for only one phase (i.e. construction, maintenance or use phase); 2. Investigate a limited number of impact indicators, sometimes considering only GHG (Buisson et al., 2013). Although, partial LCA's remain easier to perform, they tend to eliminate solutions that have a higher environmental footprint at construction, even if those solutions might extend of the durability, with the possibility in some cases to compensate for the higher initial footprint (Newcomb et al., 2001; EAPA, 2007; Lesueur and Youtcheff, 2013).

This paper presents the limits of the partial approach and shows the benefits of performing a rigorous complete LCA for the case of the HMA wearing layer of a highway with or without hydrated lime. In order to use realistic pavement data, the study was based on the same road structure and maintenance scenarios as those from an already published LCA from the French Road Industry Association (USIRF from its name in French - as described by Bilal et al., 2009).

2. Material & methodology

2.1. Literature data on hydrated lime in HMA

Lime is a product derived from limestone in an industrial process. Naturally occurring limestone, which is composed almost exclusively of calcium carbonate [CaCO3], transforms into quicklime (calcium oxide [CaO]) by applying heat (Boynton, 1980). When slaked with water, quicklime transforms into hydrated lime, which is a dry powder composed of calcium hydroxide [Ca(OH)2]. Lime products are versatile materials that are used in many different applications, e.g. in steel making, agriculture, environmental protection, civil engineering, etc. Hydrated lime for use in the application covered in this case study, are described in the European standard EN 459 (2011) or in the US standard ASTM C 1097 and AASHTO M 303 (2010). Hydrated lime has been known as an additive for Hot Mix Asphalt (HMA) in road construction from the late 19th century. However, its benefits became clearer in the 1970s when the roads in the USA experienced increased damage from moisture and frost, partly as a consequence of a general decrease in bitumen quality due to the petroleum crisis of 1973. After years of experience with the hydrated lime, the asphalt technologists observed other functionalities such as a decrease in bitumen ageing and an improvement in mechanical properties (such as strength, rutting resistance and fatigue), all contributing to extended road durability (Hicks, 1991; Little and Epps, 2001; Little and Petersen, 2005; Sebaaly et al., 2006; Lesueur, 2011). The physico-chemical mechanisms behind this improvement have been extensively and thoroughly mapped and reviewed by Lesueur (2011) and Lesueur et al. (2013) in approximately 110 papers covering different world regions. The North American State agencies estimate that hydrated lime at the usual range of 1-1.5% in the mixture (based on dry aggregate) increases the durability of asphalt mixtures by 2-10 years, or 25-50% (Hicks and Scholz, 2003). Because of these beneficial effects, hydrated lime is now specified in many states and it is estimated that every year, 40 Mt of asphalt mixtures are produced in the USA containing hydrated lime. If the technology has been used for decades in the USA, it is currently starting to be increasingly used in Europe. For example, the Netherlands made hydrated lime compulsory in porous asphalt (as defined in EN 13108-7), a type of mix that now covers 70% of the highways in the country equating to almost 1 Mt of

Summary of the life cycle studies carried in Europe covering roads/pavements reporting the functional unit and the environmental impacts investigated.

Studies by: EU countries Study type Standard followed Critical System boundaries Functional unit Environmental impacts investigated/

covered review reported

Construction Maintenance Use End Lifetime Length Road type Width Process Global Other Other

of energy warming environmental comments

life potential impacts

investigated

and reported

Earthworks Pavement Traffic [years] [km] [meter] [MJ] [kg CO2 eq.]

Häkkinen and Mäkelä (1996; Pereira et al.

(1997) Lundström

(1998)

Mroueh et al. (2001)

Mroueh et al. (1999)

France Finland

Stripple (2001) Sweden

Mroueh et al. (2001)

Rouwette and Schuurmans (2001)

Chappat and Bilal

(2003) Peuportier

(2003)

Ventura et al.

(2004)

Hoang (2005), Hoang et al.

(2005) Olsson et al.

(2006)

Jullien et al.

(2006)

Comparative LCA (concrete vs. asphalt pavement)

Comparative LCA (impact of traffic is NF X 30-300 investigated)

Comparative LCA (beton vs asphalt)

Comparative LCA (agregates vs various industrial by products such as: coal ash, crushed concrete waste and granulated blast furnace slag) Asphalt concrete

Comparative assessment of primary Eskola and Mroueh

raw materials versus secondary raw materials (by-products) Reinforced concrete

Comparative LCA (two asphalt pavements vs. one concrete pavement)

Comparative LCA (20 different road techniques)

Comparative LCA (6 variants asphalt concrete; cement concrete; asphalt vs. Cement concrete)

Comparative LCA (hot aggregate mix with 10%, 20%, 30% Recycled Asphalt Pavement)

Comparative LCA (asphalt concrete vs. reinforced concrete)

Environmental model to use municipal solid waste for road construction

Comparative LCA (asphalt without recycled material and with 10%, 20%, 30% Recycled Asphalt Pavement)

(1998) and Eskola et al. (1999)

SETAC, 1993 Guideline

NF FD X 30-021

Module routier élémentaire (MRE) based on ISO 14040 ISO 14040

ISO 14040 (NF EN ISO 14042)

Not in scope

reported

reported

LCA model developed as EcoGeo programme

SETAC 1993 Not

reported

reported

reported

reported

reported

reported

reported

reported

reported

x 40 1 m

x x 30

Motorway Pavement

reported

reported

Pavement 13 suburban

50 1 Motorway 17 x x

Pavement 9

reported

Road 13

1 m Interurban 1 x x

pavement 1 Interurban 14

pavement

Interurban 3, 8 x

pavement (7 cm thick)

Motorway 14 x

x x Not 150 m Asphalt 3.8

reported pavement

No copy, data from Sayagh et al., 2010

No copy, data from Sayagh et al., 2010

No copy, data from Sayagh et al., 2010

No copy, data from Sayagh et al., 2010

1; 2; 6; 20; 22

VOC, PAH and odors

Finland

Motorway

1; 4; 5; 7

Finland

Finland

1; 2; 8

Finland

Belgium

Stripple (2001)

Sweden

France

France

France

ISO 14040

France

Sweden

1; 2; 12

France

(continued on next page)

Table 1 (continued)

EU countries covered

Standard followed

Critical System boundaries

Functional unit

Environmental impacts investigated/ reported

Construction

Maintenance Use

Earthworks Pavement

Lifetime Length

[years] [km]

Road type Width

Process energy

Global

warming

potential

environmental impacts investigated and reported

Other comments

NTUA (2006;

Birgisdottir (2005)and Birgisdottir et al. (2006, 2007) Rens(2009)

Huang et al.

(2009) Sayagh et al.

(2010)

ECRPD (2010)

Bilal et al. (2008;

University of Biberach (2009) CIMBETON, 2011

Milachowski et al., 2011

Butt et al., 2014

Cyprus Denmark

Belgium, France UK

France

Five EU countries (Czech Republic, France, Ireland, Portugal, Sweden) France

Comparative LCA (road construction versus maintenance) Comparative LCA (municipal solid waste in landfill vs. road construction with asphalt concrete and cement concrete)

Comparative LCA (concrete vs asphalt)

Comparative LCA (natural agregates vs. waste glass)

Comparative LCA (between various pavement structures)

Comparative LCA (construction vs. maintenance)

Comparative LCA asphalt road vs. Concrete road

Comparative LCA study (on six different road types)

Comparative LCA (two concrete pavements and two asphalt pavements)

Comparative LCA (new vs recycled asphalts)

Comparative LCA (HMA without hydrated lime (classical) vs HMA with hydrated lime (modified)) Comparative LCA methods to consider feedstock energies for warm mixture additives and polymers

LCA model, ROAD-RES

Not reported

Module routier élémentaire (MRE) based on ISO 14040 Not reported

NF P 01 010

DIN EN ISO 14040

(2006)

reported Not

reported

reported No

reported

reported

reported

reported

"Pavement Embodied Carbon Tool" from TRL - Transport Research Laboratory (UK)

ISO 14040-14044 (2006)

Not reported

reported

Urban road 13

Urban roada

reported

30,000 m2 Asphalt surface 1 Pavement

Pavementb 9,5;

11,5; 25,5; 27,5

Pavement highway

Pavement motorway

18 years 1

design

24 m (for 2 lines) 7 m (2 lines (3, 5 m each) + 1 m) 4 lines

Pavement 3.5

Pavement Not

reported

1; 2; 20; 21

1; 2; 3; 4; 5; 6; 15

1; 5; 16

1; 2; 4; 5; 20; 21 1

1; 2; 3; 4; 5; 6; 19; 20; 21

1; 2; 3; 20; 21

1; 2; 3; 6; 20; 21; 22

Studies by

Study type

Traffic

meter] [MJ]

[kgCO2eq

Not reported

LCA study

Pavement

Germany

France

ISO 14040

Pavement

Germany

ISO 14040

Nicuta, 2011

Romania

Pavement

EESAC (2012)

France

Sweden

1. Acidification potential [kgSO2 eq.]; 2. Abiotic depletion [kgSb eq.]; 3. Eutrophication potential [kgP eq.]; 4. Water [L]; 5. Waste [kg]; 6. Ecotoxicity; 7. Heavy metals, PM, VOC, NOx; 8. Heavy metals, leaching into land, land use, noise and dust, NOx, VOC, PM; 9. Leaching of heavy metals into the soil and the atmospheric emissions of NOx and CO2; 10. CH4, N2O; 11. Natural aggregates and bitumen; 12. Emissions to air (SO2, NOx, CO, CO2, HC, CH4, VOC, N2O and particles) and emissions to water (COD, N-tot, Oil, Phenol, As, Cd, Cr, Cu, Ni, Pb and Zn); 13. Volatile Organic Compounds (VOC); Polycyclic Aromatic Hydrocarbons (PAH) and odors; 14. Leaching to the ground; 15. Smells; 16. VOC, NOx; 17. CH4, HCl, HF, COD, metals, VOC; 18. HC, CH4, N2O; 19. Smog, smells; 20. Photo-oxidant formation; 21. Stratospheric ozone depletion; 22. Eutrophication. a 4400 tonnes of ash.

b 4 road sections (motorway, dual carriage way, wide single carriage way and single carriage way).

HMA per year. SANEF, the French Northern motorway company, currently specifies hydrated lime in the wearing courses of its network, since they observed asphalt mixtures durability extended with 20-25% due to the use of hydrated lime (Raynaud, 2009). As a consequence, countries like Austria, France, the Netherlands, United Kingdom and Switzerland now have a significant share of their HMA production including hydrated lime. In summary, and given the worldwide experience, road managers estimate that the presence of 1-2 wt% of hydrated lime in HMA increases its durability by typically 25% (Hicks and Scholz, 2003; Raynaud, 2009).

2.2. LCA methodology

Recent literature summarises the challenges posed by the research and LCA methodology in the sector of transportation and more particularly in the context of pavements (Muench, 2010; Santero et al., 2010; Carlson, 2011). From the literature it is clear that road pavement LCA studies can differ in terms of goal, scope, and system boundaries making comparison difficult if not impossible (Santero et al., 2011a; Carlson, 2011).

The differences in goal and scope, summarised in Table 1, comes from the fact that the published LCA's were not only intended to provide information on environmental performance of various products, but also to support marketing or environmental labelling. For example, the various raw materials (primary and/or secondary, such as waste) available to manufacture asphalt mixtures were largely investigated (Mroueh et al., 1999, 2001; Jullien et al., 2006; Huang et al., 2009; Nicuta, 2011; Gschosser et al., 2012a,b). On all these occasions system boundaries, functional units, construction practices, geotech-nical conditions, traffic load and intensity were different making it difficult to adopt a single representative structure (Carlson, 2011; Santero et al., 2011a). In addition, regional climate, local design practices, budget, service life, material availability, and other factors played a significant role in the design process (Santero et al., 2011a). In fact, these differences being inherent to any road project, a strict approach would require that calculations need to be done for each specific construction site in order to have a meaningful LCA (Stripple, 2001). Therefore, each LCA is representative of a given case-study and can only be extrapolated to other situations if the main limitations and assumptions are completely known (Carlson, 2011). In addition to differences in goal and scope, some of the published studies were either partial LCA studies covering only embankment and pavement construction (Olsson et al., 2006; Birgisdottir et al., 2007), or complete studies covering the entire life cycle of a road from cradle-to-end of life (Mroueh et al., 1999, 2001; Milachowski et al., 2011; EESAC, 2011). Moreover, goal and scope might affect other LCA characteristics, such as the functional unit (including the analysis period), environmental outputs, and data sources. For instance, a project-level comparative assessment may draw system boundaries that exclude lighting, traffic fuel consumption, carbonation, or other components that are assumed to be equal amongst competing alternatives.

Likewise, a policy-level assessment focusing on regional reduction strategies may exclude onsite equipment due to its relatively small impact. Similar difficulties are illustrated in the various studies already published, where the goal and scope were quite different: comparing a pavement material to another, investigating the use of waste or recycled materials, comparing construction to maintenance phases (see Table 1).

In this context, the scope of the study consisted of calculating the environmental footprint of classical HMA versus hydrated lime modified HMA. The LCA system boundaries covered the life cycle from cradle-to-grave for the HMA including:

Fig. 1. System boundaries. Note that "Bitumen" covers both the bitumen used in the asphalt mixture and the bituminous emulsion (refer to text for details).

used in the tack coat

raw material extraction, the different types of energy used during the construction and the maintenance (e.g. bitumen, diesel oil, natural gas, electricity), transportation, HMA production, road construction, road maintenance, HMA recycling, end-of-life. The boundaries of the system are specified in Fig. 1. The generation of waste is also displayed in Fig. 1 for completeness, but no reliable data could be found in literature.

2.3. Functional unit

The functional unit was a 1 km of French road surface (wearing course) with a width of 3.5 m (representing a road surface of 3500 m2) and a functional life of 50 years (corresponding to the expected life span of the whole road). Furthermore it was assumed that:

1. The quality of construction was identical for both options.

2. Since HMA with and without the inclusion of hydrated lime are very similar mixtures from a view of the constituent materials, the final evenness and skid resistance of the surface layer will be within an acceptance range.

3. For the same reasons, the pavement reflectance to lighting is identical. Table 2 provides respective percentages of the constituents for classical and modified HMA.

The following processes were not considered within the system boundaries:

1. Fuel consumption and related emissions of the vehicles using the road during the 50-year life span of the road. The major reason for this choice was that the smoothness of the surface layer is assumed to be equivalent for the two types of HMA (i.e. functionality is the same for both road types). This is very hard to document and outside of the scope of our paper. However, including traffic related emissions in pavement LCA, given their high level, tends to completely screen all differences due to infrastructure construction and maintenance, which was the focus of our work. In fact, it is documented that a big change of smoothness as measured by the International Roughness Index (IRI) from 2 m/km (considered as becoming very close to needing replacement) to 1 m/km (considered as the normal expected level for a new pavement) increased fuel consumption by 2.5% (Amos, 2006). However, we did not expect such a big difference between the section with and without lime. However, given that the lime-treated section will be in place 25% longer on the road, the better IRI of the lime section (if measurable) would be observed for the initial 10 years, but it would become worse than the reference for 2.5 yrs. once the reference has been maintained. So, the gains of the first 10 years would be dismissed by the losses in the next 2.5 yrs. The exact impact would have to be calculated based on hypothesis on the IRI vs. time evolution for both options (with and without lime) and the IRI vs. fuel consumption, this can produce huge variations depending on the selected scenario (Noshadravan et al., 2013; Azarijafari et al., 2016). There is not enough documented evidence on the effect of lime on surface smoothness to be incorporated at this stage in a LCA and not relevant if construction is the focus. Thus for both options, the rolling resistance and consequently the global fuel consumption (for similar driving conditions) will be the same.

2. The abrasion caused by vehicle tyres using the road during the 50 years. The reason is the same as described above.

3. The additional fuel consumptions and related emissions of vehicles due to the traffic jams caused by maintenance works. Despite thorough investigations, no detailed quantification of these impacts could be found in the literature. As a consequence, the impact of traffic jams on consumption and emissions could not be modelled. However it can be stated that the consumption and emissions due to the traffic jams raise with an increasing number of maintenance steps.

The production of HMA followed the usual procedure, consisting in drying the aggregate (sand and gravel) by heating it up to 180 °C, then mixing all the materials (bitumen, aggregate, filler and optionally hydrated lime). Depending on the technology used, this operation can be done in one (continuous plant) or two steps (batch plant see Lee and Mahboub, 2006). Once the HMA has been manufactured, it is transported by truck to the jobsite and spread hot with dedicated machines (finishers) over the surface of the road where it is compacted whilst still hot. After the HMA has cooled down, it can be readily trafficked. Fig. 2 illustrates the HMA production process.

The pavement structure data (type of materials and thicknesses) were taken from the LCA performed by USIRF (Bilal et al., 2008). In particular, the wearing course was a typical French bituminous pavement initially designed to withstand 5 million cumulated equivalent single-axle-loads over 30 years, corresponding to a high traffic road on the national network (structure

Table 2

Constituents of the classical and modified HMA.

Classical HMA (without addition of lime) (%)

Modified HMA (with lime addition) (%)

Bitumen Sand

Fine gravel Coarse gravel Filler

Hydrated lime

38 26 29 0.5 1.5

TC5 30 PF3 in the French design catalogue - Bilal et al., 2008). Note that French pavement design is based on a 13 t dual tyre single axle. Since the life span of the wearing course is much shorter than the life time of the road, the surface layer is maintained regularly. Based on information from the road constructors the life span of the surface layer typically varies between 7 and 12 years. A value of 10 years was selected for the time between maintenance operations, as proposed by Bilal et al. (2008), who provided additional information on the maintenance practices for roads built with the classical HMA. The same maintenance scenario was also applied for the modified HMA, but taking into account the 25% increase in durability as presented in the introduction (Hicks and Scholz, 2003; Raynaud, 2009), illustrated in Fig. 3.

2.4. Transportation distances

After comparison with other similar LCA's (University of Biberach, 2009; Sebben Paranhos, 2007), the assumptions taken into account in this LCA are summarised in Table 3. Because the French LCA (Bilal et al., 2008) has highlighted the large variability of the shipping distances of the minerals consumed for producing HMA, the impact of these transportation distances on the LCA results will be assessed in the sensitivity.

As far as waste generation was concerned, the scenario proposed by Bilal et al. (2008) for Reclaimed Asphalt Pavement (RAP - i.e. the HMA recovered after the milling operations) was used. More precisely, RAP was shipped from the construction/maintenance site (road) to the HMA plant where it was fully reused according to the following scheme: 50% of the RAP was reused as raw material in all new HMA and the other 50% was used as new base or sub-base material in other road building projects. The recycling of this asphalt is already taken into consideration, as a cheap substitute for new sand and gravel that would have otherwise been purchased from external production sites (Bilal et al., 2008). RAP replacing virgin bitumen, sand, gravel and filler is currently done all over Europe (Jullien et al., 2006; EAPA, 2011). The credit that was taken into account in the model included the production of sand and gravel materials as well as the shipping avoided. The end of life scenario was that the road would be used as the sub-base for a new pavement (Bilal et al., 2008).

2.5. Cut-off rules

The percentage of materials not traced back to the cradle is in this case directly related to the cut-off rules applied in the Life Cycle Inventories of the different materials used for the construction and the maintenance of the road. For the

Bitumen (production) Sand (production) Fine gravel (production) Coarse gravel (production) Filler (production) Hydrated Lime (production)

* i • i i > *

Shipping Shipping Shipping Shipping Shipping Shipping

» i i

Asphalt plant

Loading

HMA production

Fig. 2. Flow diagram of the HMA production process.

Fig. 3. Summary of the different steps of the construction and maintenance of the wearing course of the classical HMA (without hydrated lime, upper figure) and modified HMA (with hydrated lime, lower figure) as taken into account in the LCA model.

Table 3

Main assumptions for modelling the transportation.

Input flow

Main assumptions for modelling the transportation (base case) from production sites to HMA plant

Main assumptions for modelling the transportation (base case) HMA plant to construction site (construction and maintenance)

Bitumen Bitumen emulsion

Sand Fine Coarse Filler Hydrated gravel gravel lime

RAP (Reclaimed Asphalt Pavement)

Type of transportation Maximum payload Load factor Load factor

Truck for bulk goods, Euro norm IV 27 tonnes 100% (full)a 0% (empty)c

100% (full)b 0% (empty)" 0% (empty)d 100% (full)d

Average transportation distance (km) 500 500 50 50 50 150 250 500 500

Driving share urban (%) 0 0 0 0 0 0 0 0 0

Driving share interurban (%) 25 25 100 100 100 25 25 100 100

Driving share motorway (%) 75 75 0 0 0 75 75 0 0

Sulphur content of the fuel (ppm) 10

% Bio fuel in diesel 0

LCI database used ELCD/GaBi 4.4

a Load factor on the way back to the HMA plant. b Load factor on the way to the construction site. c Load factor on the way back from the HMA plant. d Load factor on the way back to the HMA plant.

production of HMA all inputs were fully considered. Based on the information gathered in the different databases used for this LCA study, the cut-off rules applied for each process unit are in general, the following:

- Coverage of at least 95% of the mass and energy of the input and output flows.

- The coverage of mass and energy for the input and output flows and the related environmental flows is at least 99% for the

production of hydrated lime.

The omission of some ancillary processes that consume small amounts of fuel compared to the overall energy consumptions (e.g. for shipping the mobile equipment like the finisher, the rollers and the water tank or for sweeping). A rough estimation of these consumptions and their related emissions shows that they represent far less than: 0.1% of the mass and energy of the input and output flows and 0.1% of their environmental relevance.

2.6. Allocation procedures

For the derivation of the Life Cycle Inventories of the different products entering in the composition of the HMA (i.e. bitumen, aggregates, hydrated lime, and fuels), an association between products and co-products could not be avoided. The inputs and outputs of the upstream processes were proportioned in accordance to the respective percentages of mass flows of the products and co-products. No other allocation procedure was used in the study.

2.7. Data quality and validation

Sources of data that were used in this study were selected in order to be as much time, geographical and technological representative as possible. Thus whenever possible, the most recent and accurate data representing French conditions were used. The processes used for producing HMA, building and maintaining the surface layers was taken from reports and LCA's studies performed or validated by the French and German Trade Associations of road contractors (Bilal et al., 2008, 2009; University of Biberach, 2009), a PhD study by Sebben Paranhos (2007) and LCI of bitumen (Biolntelligence, 2011). The remaining data used in the LCA was extracted from well-known databases (ELCD, 2008-2009; Ecoinvent; GaBi), except for the LCI datasheet of hydrated lime commissioned by EuLA (EESAC, 2011), which was approved by an independent critical reviewer. This section presents the data sources and the assumptions used in relation to the modelling of the construction and maintenance of the wearing course as summarised in Table 4.

Table 4

List of the databases used for this LCA study.

Input flow Name of the inventory Source Description Remark

Bitumen Bitumen at refinery (RER) Ecoinvent 2,0 Production of bitumen in Europe 1

Bitumen at refinery (EU-15) PE International/Gabi 4 4 Production of bitumen in EU-15 2

Bitumen Bitumen emulsion at refinery (RER) Biolntelligence/ Production of bitumen emulsion (without the 3

emulsion Eurobitume flows associated with the building of the

infrastructure required to produce, transport

and refine crude oil)

Sand Sand 0/2; wet and dry quarry; production ELCD/PE International Standard mineral production used as natural 3

mix, at plant; undried (RER) aggregates in the construction industry

Fine gravel Gravel 2/32; wet and dry quarry; production ELCD/PE International according to the applied technology 3

mix, at plant; undried (RER)

Coarse gravel Crushed stone 16/32; open pit mining, ELCD/PE International 3

production mix; at plant; undried (RER)

Filler Limestone flour (50 im) from Germany PE International/Gabi Production of limestone filler with an average 3

4,4 grain size of 50 im in Germany

Hydrated lime Hydrated lime at plant (RER) EuLA/ESSAC Production of hydrated lime in EU-27 3

Electricity LCI electricity mix; AC consumption mix. At ELCD/PE International AC electricity, low voltage used by industry and 3

consumer; <1 kV (RER) SME - energy mix EU-27

Natural gas Natural Gas; from onshore and offshore ELCD/PE International Natural gas that is used by power plants, 3

production including pipeline and LNG industries and end consumers

transport; consumption mix, at consumer;

desulphurised (RER)

Diesel oil - Diesel; from crude oil; consumption mix, at ELCD/PE International Production of Diesel oil 3

production refinery; 200 ppm (RER)

Diesel oil - Adapted ELCD : PE Combustion of Diesel oil with a sulphur 3

combustion International/ content of 10 ppm (instead of 200 ppm)

for mobile Ecoinvent datasets for

equipment diesel combustion

Transportation Articulated lorry transport; Euro 4; 401 total ELCD/PE International Transport by road 3

with truck weight; 27 t maximum payload (RER)

1. Dataset uses for the base scenario; 2. Dataset use for the sensitivity analysis; 3. Dataset use for the base and alternative scenario.

3. Results

The methodology of ISO 14040-44 standard series (ISO, 2006 a,b) has been applied to the LCA study, consisting of: data gathering, life cycle modelling (in the Gabi 4.4 software), LCI calculation, Life Cycle Impact Assessment (LCIA) analyses and results. Since the study was conducted during 2011 and the external critical review finalised in 2012, the updating of the software tools such as Gabi is not accounted for.

LCIA method used to transform LCI results into impact categories shows the LCI results by presenting the impact in terms of damage to the natural environment, human health, or natural resources. LCI results are converted to one or more midpoint categories (e.g., climate change) through characterisation factors (e.g., global warming potential), which normalise similar pollutants to a single metric (e.g., CO2, CH4, N2O, and other GHGs to CO2 eq). Various models, databases, and reports are available to assist in the characterisation process, including IPCC (2007), and CML (2011). To this end, relevant impact categories must be identified. Based on the recommendations of the SETAC (1993) working group on impact assessment, the impact categories listed in Table 5 were selected.

When preparing this LCA study, it appeared that water consumption and waste generation could not be evaluated accurately because the data on production of different materials or processes used for the construction of the wearing layer was lacking or unreliable. Therefore they are not included in the present study. Furthermore the impact category relating to land use/land competition was not included since no reliable data were available for the production of minerals and hydrated lime. Table 5 presents a summary of the results of the environment impact assessment that are relative for the selected functional unit (FU).

3.1. Energy and elementary raw materials

Fig. 4 presents the primary total energy consumed during all the different life cycle stages. It is clear from the graph that road maintenance was the main contributor in terms of total energy consumption. This confirms that a partial LCA only focusing on construction, maintenance or use phase would neglect important contributing factors from other stages and would therefore have a high risk of leading to unsound decisions.

Table 5

Results of the LCIA.

Environmental impact Calculation method Unit

Wearing course with Variation

Classical HMA without Modified HMA with % addition of lime lime addition

Primary energy feedstock (including the feedstock energy of the bitumen) Total primary energy Sum of total/non renewable energy MJ

consumption consumption

Non renewable energy MJ

consumption

Primary energy consumption (excluding the feedstock energy of the bitumen)

Total primary energy

consumption Non renewable energy consumption

Climate change Greenhouse gas emissions

Environment and health Air acidification

Photochemical oxidant

formation Stratospheric ozone

depletion Human toxicity

Eco-toxicity Freshwater toxicity

Terrestrial eco-toxicity

Nature and biodiversity Eutrophication Resource consumption Abiotic resource consumption

Sum of total/non renewable energy consumption

CML 2001-Nov 2009 based on IPCC 2007

3,903,380 3,886,191

1,588,567 1,571,378

kg eq. CO2 95,476

kg eq. SO2

CML 2001-Nov 2009 based on RAINS model IIASA 2007 CML 2001-Nov 2009 based on UNECE kg eq.

trajectory model

CML 2001-Nov 2009 based on WMO model

CML 2001-Nov 2009 based on USES model RIVM

kg eq. CFC11 kg eq. 1, 4-DCB

CML 2001-Nov 2009 based on USES kg eq. 1, 4

model RIVM DCB

CML 2001-Nov 2009 based on USES kg eq. 1, 4

model RIVM DCB

CML 2001-Nov 2009 kg eq. PO4

CML 2001 Dec 2007 kg eq. Sb

707 109 0.025 11,429

3707 199

196 1797

2,243,208 2,232,704

960,446 949,942

73,655

396 63

0.014 6448

2002 133

107 1032

-42.5 -42.5

-39.5 -39.5

-43.9 -42.6 -45.5 -43.6

-46.0 -33.0

-45.6 -42.6

10 000 000

8 000 000

2 6000000 —.

c 4 000 000 o

a -2000000 ,o

-4 000 000

-6 000 000 -I-

■ I Road construction 11 Rood maintenance Recycling of RAP ■ End-of-life Total life cycle

Fig. 4. Primary total energy consumption [MJ/FU].

Based on the end-of-life scenario that was selected in this LCA study, it confirmed that the solution with the modified HMA required about 43% less primary total energy compared to the solution with the classical HMA. Even if one considers that no credit would be granted to the existing wearing layers at their end-of-life, the saving in primary total energy would be around 23% for the modified HMA. If the focus of the study was only on the construction stage alone the energy consumption will be similar, however for the maintenance and recycling stages the energy savings will be 30% and 50% respectively with the modified HMA as compared to classical HMA.

Similar results were found for non-renewable energy as this energy represents more than 99% of the primary total energy consumed. The contribution of the different life cycle stages in the primary energy consumption without inclusion of the feedstock energy of the bitumen is shown in Fig. 5. The above results confirm those already observed for the primary total

Fig. 5. Contribution of the different processes in the primary total energy consumption [MJ/FU].

energy consumption, i.e. the modified HMA leads to energy savings of about 40% (with the end-of-life scenario used in this LCA). In order to identify the origin of the energy consumptions, it is worth analysing the contribution of the main processes used for the construction and the maintenance of the road. Fig. 5 illustrates the results of this analysis. The main contributors in the primary total energy consumption were (by decreasing order of importance): 1. The production of bitumen and bitumen emulsion; 2. The production of HMA; 3. The fuel used by the trucks used for the transportation (raw materials, HMA, RAP) and by the mobile equipment (construction and maintenance); 4. The production of the minerals and the hydrated lime.

3.2. Abiotic depletion

The abiotic depletion potential is linked to the scarcity of the resources used. As can be seen from Table 5 it is clear that the solution with the modified HMA consumed much less resources than the option with classical HMA. The savings were similar to those observed for the energy consumptions. This can be easily explained by the fact that fossil fuels (in particular the crude oil used for the productions of bitumen and diesel oil consumed by the trucks and the mobile equipment) contribute to almost 99% to the abiotic resources depletion index. The results indicate that the most important contributor to the abiotic depletion potential in this LCA was by far the crude oil, i.e. about 77-78%. This result confirms that most of the energy consumed in the system, i.e. the production of bitumen and the use of diesel oil is based on crude oil.

3.3. Emissions to air

Similar to the energy consumption, road maintenance was the life cycle stage that contributed the most to the GHG emissions, in order of magnitude as follows: road maintenance, end-of-life, construction (Fig. 6).

For the chosen end-of-life scenario, the option with the modified HMA led to 23% less GHG emissions than the solution with the classical HMA. Even if another end-of-life scenario that would not provide any credit would be assumed, the saving in GHG emissions would be around 14%. According to the LCI results, 90-93% of the GHG emissions were due to CO2 emissions.

Again if the focus of the study was only on the construction stage alone the global warming potential for the modified HMA will be 18% higher. For the maintenance and recycling stages the energy savings will be 25% and 50% respectively with the modified HMA as compared to classical HMA. The global warming potential savings of the total life cycle for all the stages covered by the study are around 35% for the modified HMA. Therefore not taking some of the stages into account, as would be the case for a partial LCA, would lead to erroneous environmental decisions.

Fig. 7 shows the contribution of the main processes used for the construction and the maintenance of the road in the GHG emissions. Hence the main contributors to the GHG emissions are (by decreasing order of importance): 1. The fuel used by

I Road maintenance Recycling of RAP ■ End-of-life

Fig. 6. Global warming potential [kg eq. CO2)/FU].

Fig. 7. Contribution of the different processes in the global warming potential [kgeq. CO2/FU].

the trucks used for the transportation (raw materials, HMA, RAP) and by the mobile equipment (construction and maintenance); 2. The production of hydrated lime (for the modified HMA); 3. The production of bitumen and bitumen emulsion; 4. The production of the HMA; 5. The production of the minerals.

The difference in the acidification potential between the two solutions was around 44% in favour of the solution with the modified HMA (Table 5). In the present case, the acidification potential was due to two types of emissions: 1. NOx emissions which contribute to about 51% to the acidification potential; 2. SO2 emissions (approximately 47% of the acidification potential).

The photochemical oxidant formation is mostly attributable to the Volatile Organic Compounds (VOC) that are emitted mainly during the production of bitumen, by the combustion of diesel oil and by the HMA plant when bitumen is heated. The difference in the photochemical oxidant formation indexes between the two solutions was around 43% (in favour of the modified HMA as shown in Table 5).

The ozone layer depletion potential is typically affected by the processes that consume electricity (e.g. manufacturing of bitumen, aggregates, hydrated lime, and HMA plant). The difference in the ozone layer depletion potential between the two solutions was around 45% (in favour of the modified HMA, as shown in Table 5).

3.4. Emissions to water

In the present case, the eutrophication potential is mainly attributable to the NOx emissions that lead to the formation of nitrates in the surface water. Therefore all major combustion processes that emit nitrogen oxides (as listed previously) contribute indirectly to the eutrophication. As can be derived from Table 5, the difference in the eutrophication potential between the two solutions is around 45% in favour of the modified HMA.

3.5. Sensitivity analysis

Based on the ISO 14040-14044 requirements and in order to validate the outcome of the study the sensitivity analysis was conducted using four parameters from the main assumptions for the classical (base case). The selection of these parameters was made based on the major impact they play in the overall life cycle assessment and the availability of data. The critical review considered that the sensitivity analysis was sound and comprehensive.

1. Use another LCI dataset for bitumen in order to investigate the impact of lower energy consumption on the LCA results. Thus the Ecoinvent database (used for the classical HMA) was replaced by the Gabi 4.4/PE International database. Results are shown in Table 6. Clearly, changing the bitumen dataset affected all indicators, especially primary energy consumption, but the comparisons between modified and classical HMA remained similarly favourable to the modified HMA.

Table 6

Sensitivity analysis: summary of the impact of the LCI dataset for the production of bitumen on the final results.

Environmental impact

Unit Base case: Ecoinvent database for bitumen Base case: GaBi4/PE International database production for bitumen production

Classical HMA without addition of lime

Modified HMA Classical HMA with lime addition without addition of lime

Modified HMA with lime addition

Variation

Primary total energy MJ

consumption

Primary energy consumption MJ

(without feedstock energy)

Climate change kg eq.

Air acidification kg eq.

Photochemical oxidant kg eq.

formation C2H14

Stratospheric ozone depletion kg eq.

Human toxicity kgeq. 1,

Freshwater toxicity kgeq. 1,

Terrestrial eco-toxicity kg eq. 1,

Eutrophication kg eq.

Abiotic resource consumption kg eq.

(ADP) Sb

3,903,380

I,588,567 95,476 707

109 0.025

II,429 3707 199 196 1797

2,243,208

960,446

73,655

3,593,014 1,278,201 87,733 523 82 0.002 4451 216 101 97

2,076,088 793,326 69,485 297 48 0.001 2691 122 80 54 971

2. Change energy consumption and type of fuels in the HMA plant as this process represents an important contributor in the global energy consumption, global warming and acidification potentials. As mentioned before detailed information about the consumptions and the emissions of HMA plants are extremely scarce and LCI datasets do not exist. Therefore for both parameters, the sensitivity analysis was limited to a qualitative analysis.

The values for the HMA production were based on a specific energy consumption of the HMA plant of 345 MJ/t HMA. Following the bibliographical review (Bilal et al., 2008, 2009; Natural resources Canada; Sebben Paranhos, 2007; Lunser, 1999; Carmeuse personal communication), this consumption typically varies between 220 and 370 MJ/t HMA. From these lower and upper values, the new contribution of the HMA plant and the total energy consumption can be easily calculated for both solutions. The results are summarised in Table 7 and show that the absolute values of the energy consumptions were obviously changed. In any case, the solution with the modified HMA required globally less energy.

Concerning the type of fuel used, it was assumed in the base scenario that the HMA plant was burning natural gas. This minimized CO2 and SO2 emissions, because the emission factors for natural gas are quite low (respectively 56 kg CO2/GJ and 30 mg SO2/GJ). A HMA plant with the same specific energy consumption but burning Heavy Fuel Oil (HFO) with a sulphur content ranging between 0.5% and 3% (in mass), would emit 39% more GHG (78kgCO2/GJ) and 4-24 more SO2 (120740 mg/GJ).

Clearly, switching to HFO instead of natural gas would increase CO2 and SO2 emissions per tonne of produced HMA. Hence, the more HMA is needed for road maintenance, the higher the GHG emissions and the acidification potential will be (if one assumes that the NOx emissions of the HMA plant remain nearly identical). Since the solution with classical HMA consumes more HMA that the option with the modified HMA, the difference between both solutions would increase

Table 7

Sensitivity analysis: summary of the energy consumptions calculated in alternatives cases.

Contribution in the energy consumption

Specific energy consumption of the HMA plant (MJ/tHMA)

Solution with classical HMA

Primary total Primary energy energy excluding feedstock

Solution with modified HMA

Primary total Primary energy energy excluding feedstock

HMA production

220 370

371,036 624,015

371,036 624,015

196,431 330,362

196,431 330,362

Sum for all processes 220 370

3,691,564 1,377,599 3,945,453 1,630,730

2,131,599 848,837 2,265,530 982,768

when substituting HFO for natural gas, for both the indicators, global warming potential and acidification potential. The solution with modified HMA will thus remain the most environmental friendly solution.

3. Modify transport distances for aggregates and sand. As can be derived from the contribution analysis, the use of Diesel oil, especially for shipping the above raw materials (that represent 93% of the materials consumed for producing HMA) is an important contributor to several indicators. Therefore the transportation distance was increased from 50 km (the assumption made in the base case) to 200 km. Results are shown in Table 8 and clearly show that changing the transport distance affects all indicators, especially GHG emissions, but the comparisons between modified and classical HMA remained similarly favourable to the modified HMA.

4. Change the maintenance intervals for the modified HMA (for shorter or longer periods). In the base case, it was assumed that the modified HMA increases the durability of the surface layers so that road maintenance had to be performed every 12.5 years instead of 10 years with the classical HMA. Because the 25% increase in the life span does represent an average value, it is worth considering the changes that may occur if this life time increases only by 15% (pessimistic scenario) or if it increases by up to 35% (optimistic scenario).

The increase of the life time of the surface layers is 35% with the modified HMA (compared to classical HMA). As shown in upper Fig. 8, the maintenance of the road will then become necessary every 13.5 years. As the life time of the whole road is assumed to be 50 years, this timeline will be attained before the life span of the surface layers spread during the last maintenance step is reached. As the benefit of the prolonged life time is lost, there will be consequently no difference between this alternative and the base scenario (lower Fig. 8). Consequently there are no changes in the LCA results between this alternative scenario and the one used in the classical HMA (base case) scenario. The increase of the life time of the surface layers is only 15% with the modified HMA (compared to classical HMA). As shown Fig. 8 (lower part), the maintenance of the road occurs then every 11.5 years with the consequence that an additional maintenance step is required to reach the end-of-life of the road.

This scenario becomes similar to the base scenario with classical HMA as shown in Fig. 3, except that classical HMA is replaced by modified HMA. The gap between both solutions is in particular important for the energy consumptions and the GHG emissions. Since the maintenance scenario is now identical for the classical and the modified HMA, these results can be explained by the use of hydrated lime in the modified HMA that replaces the less energy and less carbon intensive filler in the classical HMA. However, the end-of-life scenario would probably have to be adapted since after 50 years, the road would still have 7.5 years of good riding condition.

Since the study is comparing the use of HMA with and without lime, other techniques were not assessed (for example microsurfacing) because the use of lime is not well documented for them.

Since the filler and hydrated lime products in the HMA formulations investigated are both produced by lime manufacturers members, there was no need for a panel critical review. Therefore, following the requirements of the ISO 14040-14044

Table 8

Sensitivity analysis: impact of the transportation distances for aggregates and sand on the final results.

Environmental impact

Base case: d = 50 km

Base case: d = 200 km

Variation

Classical HMA Modified HMA Classical HMA Modified HMA (MO - CL)/

without addition of with lime addition without addition of with lime addition CL lime lime

Primary total energy MJ

consumption

Primary energy consumption MJ

(without feedstock energy)

Climate change kg eq.

Air acidification kg eq.

Photochemical oxidant kg eq.

formation C2H14

Stratospheric ozone depletion kg eq.

Human toxicity kg eq. 1,

Freshwater toxicity kg eq. 1,

Terrestrial eco-toxicity kg eq. 1,

Eutrophication kg eq.

Abiotic resource consumption kg eq. Sb

3,903,380

I,588,567 95,476 707

109 0.025

II,429 3707 199 196 1797

2,243,208

960,446

73,655

4,047,037

I,732,224 105,633 737

114 0.025

II,740 3719 204 103 1866

2,320,562

1,037,800

79,128

Fig. 8. Maintenance scenario of wearing course consisting of modified HMA with an increase in the life time of 35% (upper figure) 15% (lower figure).

series, the study underwent a post critical review. Two external critical reviewers from TNO (Utrecht, The Netherlands) confirmed that the study was conducted in accordance with the IS014040-14044 standard series.

4. Discussion

Apart from a direct comparison of two technical options, i.e. asphalt mixture with or without hydrated-lime, the results of this study aim to address two key issues barely addressed in former LCA studies on road pavements, namely the methodological challenges as well as sustainability issues in road construction.

4.1. Methodological challenges

The methodology used to conduct a pavement LCA is itself a valuable contribution, regardless of the numerical results or conclusions. Documenting assumptions, disclosing data sources, and clearly defining goals and scope serve to establish the framework, or individual methodology, that is used for a particular study. This study uses LCA standard to investigate the pavement life cycle, emphasizing impacts associated with HMA pavements covering all the phases of the life cycle (raw materials, transport, construction, maintenance and end of life) and provides a comprehensive analysis of the environmental burden of the infrastructure system investigated. Making use of the ISO standards is possible to quantify the key environmental impacts and assess the local availability of raw materials and supports policy maker's decisions in road construction and maintenance.

Häkkinen and Mäkelä (1996), Stripple (2001), Athena (2006), Bilal et al. (2008), EESAC (2011), and Celauro et al. (2015) are examples of pavement LCAs that provide reasonably transparent methodologies and cover the majority of the life cycle

steps. While the boundary decisions, functional units, and other study-specific decisions are subject to availability of data, a transparent methodology allows the various audiences (LCA practitioners, policy makers, technicians) to understand the rationale behind the selection of the assumptions, leading to more robust conclusions and allow the replication of the results for other studies.

Generally, the data for LCAs come from a wide variety of sources, including government databases, industry reports, system models, and first-hand collection. Since the entire life cycle is being analysed, the volume of necessary data is often large and overwhelming. LCA software packages, such as GaBi (PE International), ELCD QRC), SimaPro (PRe Consultants), not only provide a modelling framework, but also include an abundance of life-cycle data on materials and industrial processes. These types of packages are generally proficient at quantifying upstream impacts for commodities by including large databases, but third-party information is often necessary to complement these data in appropriate ways and to evaluate niche products and processes that are not included in the databases. External models, such as those describing building energy consumption, vehicle dynamics, or electricity generation, are commonly used to complement the core LCA model and provide spatial, temporal, and system-specific data. Such models are particularly useful when characterizing the operation phase of the life cycle (Santero et al., 2011a,b).

In addition, the replication of the data is yet another key point. LCA studies generally rely on national averages and other generalities, project-level assessments should be performed using specific design inputs and location-based data. Assessments that compare design alternatives should pay close attention to the functional unit and data sources to ensure that the design equivalent and the data sources are accurately represent the structures being compared (Stripple, 2001; Carlson, 2011; Santero et al., 2011a,b). If any of the parameters of the assumptions is changed, the results will likely differ. It is clear from this paper and from the literature review (Table 1) that LCA studies represent one design, with sensitivity analysis carried out for the specific design. Extrapolation of the results and conclusions to other case studies shall be undertaken cautiously to ensure that assumptions are adapted to local conditions and or specific case study needs as already discussed extensively by Stripple (2001), Carlson (2011), and Santero et al. (2011a,b).

4.2. Sustainability pillars as part of the road construction practices

Sustainable development is only achieved when environmental, social and economic pillars are jointly favourable. Environmental assessment is the most developed pillar which quantifies various environmental impacts for a specific case study via the ISO standards. However, some authors focus only on partial LCA's, and it is clear from this work that not considering the whole life time of the pavement may change radically the conclusions. This statement is not restricted to the current case study, and the same conclusion would have been reached for any other technology for which the construction stage would lead to a higher environmental impact but the maintenance scenario would give a lower impact than the reference. Therefore, it is very important that road pavements remain evaluated over their whole life cycle if a holistic environmental picture is to be assessed. In other words, performing partial LCA must be an exception rather than a rule if the environmental impact of the road industry is really to be decreased. In order to do so, documenting durability becomes the biggest challenge. In the present case, a thorough literature review of around 110 articles (Lesueur, 2011), could support that adding hydrated lime to the HMA, it was possible to increase the durability of the road by 25%. Hence, the search for more durable techniques must become one of the key actions on the political agenda for sustainable practices, therefore lowering the pressure on primary raw materials. This is a major way to improve sustainability because road construction projects consume large quantities of materials (Mroueh et al., 1999, 2001; Jullien et al., 2006; Birgisdottir et al., 2007; Rajendran and Gambatese, 2007; Huang et al., 2009; Butt et al., 2015).

Due to the extension of the road durability, the overall cost for maintenance will be reduced as the cost is essentially linked to the total thickness of the asphalt layers. As a brief estimate, a saving of 4 cm of HMA over a total thickness of 13 cm for the reference scenario (Fig. 3) would represent a 30% cost reduction. Note that the maintenance cost which represents in the end 13 cm of additional HMA, would supersede construction cost that only considered 8 cm of HMA. These very rough estimates do not take into account that some of the HMA was recycled, but they highlight that the savings can be quite significant when the whole life time of the pavement is taken into account. They would clearly compensate for the extra cost of 2-3% per ton of HMA due to hydrated lime addition. Therefore, the lower environmental footprint will more likely be associated to a lower overall cost, showing the economic benefit of the modified HMA.

Finally, traffic jams due to maintenance operations will likely be reduced thanks to the longer durability of the hydrated-lime modified HMA. This would limit the societal impact of traffic jams and should therefore give a positive impact on the societal aspects as well. So, if this study and other published LCA (Table 1) focus on the environmental footprint alone, the need to take into account the other pillars of sustainable development is obvious in order to support the industry efforts towards durable and sustainable road construction practices.

5. Conclusions

The scope of the study consisted in calculating the environmental footprint of classical HMA (no hydrated lime) versus modified HMA (with hydrated lime). The LCA system boundaries covered the entire life cycle from cradle-to-grave for the HMA. Based on this LCA study and for the above mentioned assumptions, HMA with hydrated lime had a lower environmen-

tal footprint for the majority of environmental impact categories than classical HMA. The primary total energy consumption was 43% less when the modified HMA was employed compared to the classical HMA. Road maintenance was the main contributor in terms of total energy consumption; The contribution of different processes in the primary total energy consumption were (in decreasing order of magnitude): The production of bitumen, the production of HMA, the fuel used by the trucks for the transportation (raw materials, HMA, RAP) and the fuel used for the mobile equipment (construction and maintenance). The modified HMA consumed much less resources than classical HMA. The savings were similar to those observed for the energy consumptions. This could be easily explained by the fact that fossil fuels (in particular the crude oil used for the productions of bitumen and diesel oil consumed by the trucks and the mobile equipment) contributed to almost 99% to the abiotic resources depletion index. As already observed for energy consumption, the road maintenance was the life cycle stage that contributed the most to GHG emissions. For the chosen end-of-life scenario, the option with the modified HMA led to 23% lower GHG emissions than the solution with the classical HMA. Even if another unrealistic end-of-life scenario for waste re-use would be assumed, the saving in GHG emissions would still be around 14%. According to the life cycle inventories, 90-93% of the GHG emissions were due to CO2 emissions. The difference in the acidification potential between the two solutions was around 44% in favour of the solution with the modified HMA. In the present case, the eutrophication potential was mainly attributable to the NOx emissions leading to the formation of nitrates in the surface water. Therefore all major combustion processes that emit nitrogen oxides (as listed previously) contributed indirectly to the eutrophication. The difference in the eutrophication potential between the two solutions was around 45% in favour of modified HMA.

This study was performed using a pavement structure and maintenance scenario typical of France. In principle, it would be necessary to conduct a similar work in other technical contexts in order to study the influence of the choice of wearing course and maintenance scenario. Still, the principal difference between modified HMA and classical HMA is due to one maintenance operation less over the 50 years of road life time, thanks to the increased durability provided by hydrated lime. Therefore, the conclusions should remain unchanged in another technical context, as long as the benefits of hydrated lime would materialise in gaining one or more maintenance operations.

Different conclusions would have most likely been reached if the study had been limited to the construction stage only. This is a critical issue, since partial LCA studies are increasingly used at the project level in some countries (Buisson et al., 2013). Therefore, partial LCAs must be used with great care since they would favour technologies with low construction impact, even if they generate a higher impact during maintenance operations. This is all the more perilous as maintenance is seen to be the main contributor for most environmental impact factors as reported in various reports where the various life stages were investigated (Häkkinen and Mäkelä, 1996; Mroueh et al., 2001). In addition, durability data for all possible solutions must be carefully documented in order to rigorously assess the environmental footprint of the road infrastructures across Europe.

Finally, if the LCA study helped quantify the environmental impact of using hydrated lime in HMA, it was still focusing on only one pillar of sustainable development. As stressed out in the so-called Brundtland report (World Commission on Environment and Development, 1987), sustainable development will be achieved only when all the three pillars (i.e. economic, environmental and social) find a balanced equilibrium. Ideally, the impact of hydrated lime in HMA on the other two pillars should be investigated in the future. However, such work would require societal indicators related to the impact of traffic jams as a consequence of roadworks, which are unfortunately quite difficult to quantify at the present time.

References

AASHTO M 303 AASHTO M 303-89,2010. Standard Specification for Lime for Asphalt Mixtures. American Association of State and Highway Transportation Officials, pp. 1-3.

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