Environmental benefits of microwave-assisted self-healing technology for pavements - A Life Cycle Assessment comparative study

1. INTRODUCTION

The transport sector is responsible of large amount of the global greenhouse gas (GHG) emissions, and, within this sector, road transport is by far the biggest emitter accounting for nearly three-quarters (74%) of all GHG (11. SLOCAT. 2021. Transport and climate change global status report 2nd edition- tracking trends in a time of change: the need for radical action towards sustainable transport decarbonisation.
). Adequate asphalt pavement maintenance represents an important opportunity for mitigating the GHG emissions derived from the road transport sector. In fact, it was observed that asphalt pavement conditions strongly affect the fuel consumption of vehicles, thus increasing the GHG emissions (22. Chatti K, Zaabar I. 2012. Estimating the effects of pavement condition on vehicle operating costs. Transportation Research Board. Washington, DC: The National Academies Press. https://doi.org/10.17226/22808.
). Furthermore, rehabilitation of deteriorated pavements requires the consumption of a great number of non-renewable resources and energy. Taking active preventive maintenance actions can effectively impede early disease and prolong the lifespan of asphalt pavements (33. Tian Y, B. Ma, Tian K, Li N, Zhou X. 2018. Study on components determination and performance evaluation of LS pre-maintenance agent. Appl. Sci. 8(6):889. https://doi.org/10.3390/app8060889.
). By delaying the need for rehabilitation, energy consumption could be saved as well as emissions, costs, and use of non-renewable resources (44. Flores G, Gallego J, Giuliani F, Autelitano F. 2018. Aging of asphalt binder in hot pavement rehabilitation. Constr. Build. Mater. 187:214–219. https://doi.org/10.1016/j.conbuildmat.2018.07.216.
, 55. Lou B, Liu Z, Sha A, Jia M, Li Y. 2020. Microwave absorption ability of steel slag and road performance of asphalt mixtures incorporating steel slag. Materials. 13(3):663. https://doi.org/10.3390/ma13030663.
).

Microwave (MW) assisted healing is an innovative technique to increase the durability of the pavement by healing cracks and restoring pavement performance without destroying the original structure (66. Liang B, Lan F, Shi K, Qian G, Liu Z, Zheng J. 2020. Review on the self-healing of asphalt materials: Mechanism affecting factors assessments and improvements. Constr. Build. Mater. 266(Part A):120453. https://doi.org/10.1016/j.conbuildmat.2020.120453.
). The technique takes advantage of the thermoplastic and dielectric properties of asphalt mixtures for closing cracks in damaged pavements. Asphalt mixture is a self-healing material and due to its thermoplastic nature, bitumen reduces its viscosity with the increase in temperature. Above a specific temperature, the bitumen behaves as a Newtonian fluid, filling the cracks inside the pavement in a sort of capillarity flow (77. García Á. 2012. Self-healing of open cracks in asphalt mastic. Fuel. 93:264–272. https://doi.org/10.1016/J.FUEL.2011.09.009.
).

To allow the sealing phenomenon and to accelerate the crack-healing process, MW energy can be employed (88. Gallego J, del Val MA, Contreras V, Paez A. 2013. Heating asphalt mixtures with microwaves to promote self-healing. Constr. Build. Mater. 42:1–4. https://doi.org/10.1016/j.conbuildmat.2012.12.007.
). As a dielectric material, asphalt mixture can be polarized by an applied electric field. Under a high-frequency electromagnetic field (i.e., MW), the dipoles do not have sufficient time to respond to the oscillating field (99. Sun J, Wang W, Yue. Q. 2016. Review on microwave-matter interaction fundamentals and efficient microwave-associated heating strategies. Materials. 9(4).231. https://doi.org/10.3390/ma9040231.
, 1010. Metaxas A, Meredith R. 1983. Industrial microwave heating London: The institution of electrical engineers. SBN 10: 0906048893 ISBN 13: 9780906048894.
). The dielectric properties of the asphalt mixture govern the MW heating generation. Dielectric loss factor represents the ability of the material to convert electromagnetic radiation energy into heat (1111. Gallego J, Gulisano F, Trigos L, Apaza FR. 2021. Dielectric characterisation of asphalt mortars for microwave heating applications. Constr. Build. Mater. 308:125048. https://doi.org/10.1016/j.conbuildmat.2021.125048.
) and MW heating efficiency can be also enhanced by tuning the frequency of the MW generator. In most of the studies performed MW heating of the asphalt was implemented with a conventional MW oven (2.45 GHz). However, it has been revealed that the use of 5.8 GHz can led to an increase in the heating rate (calculated as the percentage of final Indirect Tensile Strength (ITS) versus the initial ITS) of asphalt mixtures (1111. Gallego J, Gulisano F, Trigos L, Apaza FR. 2021. Dielectric characterisation of asphalt mortars for microwave heating applications. Constr. Build. Mater. 308:125048. https://doi.org/10.1016/j.conbuildmat.2021.125048.
, 1212. Tang XW, Jiao SJ, Gao ZY, XL, Xu XL. 2008. Study of 5.8 ghz magnetron in asphalt pavement maintenance J. Electromagn. Waves Appl. 22(14–15):1975–1984. https://doi.org/10.1163/156939308787537883.
). The heating rate is useful to calculate the necessary heating times for reaching a specific temperature by the MW oven model used in the investigation or another oven with the same output power.

To reduce heating time and thus increase the energy efficiency of the MW heating process, some MW absorbers are often incorporated into the asphalt pavement slags from the steelmaking sector represent a promising solution for this purpose (1313. Liu J, Wang Z, Guo H, Yan F. 2021. Thermal transfer characteristics of asphalt mixtures containing hot poured steel slag through microwave heating. J. Clean. Prod. 308:127225. https://doi.org/10.1016/j.jclepro.2021.127225.
, 1414. Li C, Wu S, Chen Z. Tao G, Xiao Y. 2018. Enhanced heat release and self-healing properties of steel slag filler based asphalt materials under microwave irradiation. Constr. Build. Mater. 193:32–41. https://doi.org/10.1016/J.CONBUILDMAT.2018.10.193.
). Electric arc furnace slag (EAFS), also known as simply slags, comes from the steelmaking industry and is by-product, generated after the melting and the primary acid refining of liquid steel (1515. Arribas I, Santamaría A, Ruiz E, Ortega-López V, Manso JM. 2015. Electric arc furnace slag and its use in hydraulic concrete. Constr. Build. Mater. 90:68–79. https://doi.org/10.1016/j.conbuildmat.2015.05.003.
). Utilization of slags as an artificial aggregate provides several benefits in terms of Polishing Stone Value (PSV) (1616. Vaiana R, Balzano F, Iuele T, Gallelli V. 2019. Microtexture performance of EAF slags used as aggregate in asphalt mixes: a comparative study with surface properties of natural stones. Appl. Sci. 9(15):3197. https://doi.org/10.3390/app9153197.
), Los Angeles Abrasion coefficient (1717. Skaf M, Pasquini E, Revilla-Cuesta V, Ortega-López V. 2019. Performance and durability of porous asphalt mixtures manufactured exclusively with electric steel slags. Materials. 12(20):3306. https://doi.org/10.3390/ma12203306.
), angularity, and roughness (1818. Skaf M, Manso JM, Aragón Á, Fuente-Alonso JA, Ortega-López V. 2017. EAF slag in asphalt mixes: A brief review of its possible re-use. Resour. Conserv. Recycl. 120:176–185. https://doi.org/10.1016/J.RESCONREC.2016.12.009.
). Thanks to its excellent dielectric response, EAFS is also used for enhancing the MW heating efficiency of asphalt mixtures (1919. Gulisano F, Crucho J, Gallego J, Picado-Santos L. 2020. Microwave healing performance of asphalt mixture containing Electric Arc Furnace (EAF). Slag and Graphene Nanoplatelets (GNPs). Appl. Sci. 10(4):1428. https://doi.org/10.3390/app10041428.
, 2020. Trigos L, Gallego J, Escavy JI, Picado-Santos L. 2021. Dielectric properties versus microwave heating susceptibility of aggregates for self-healing asphalt mixtures. Constr. Build. Mater. 293:123475. https://doi.org/10.1016/j.conbuildmat.2021.123475.
). It was observed that by adding EAFS at 10% by volume of aggregates in the fine fractions (0.5/2 mm), the heating rate of asphalt mixtures passed from 4065 °C/kWh to approximately 7500 °C/kWh, for each kg of mixture.

The effectiveness of the MW-assisted healing technique has been widely assessed on the laboratory scale (21-2421. Lou B, Sha A, Barbieri DM, Liu Z, Zhang F. 2021. Microwave heating properties of steel slag asphalt mixture using a coupled electromagnetic and heat transfer model. Constr. Build. Mater. 291:123248. https://doi.org/10.1016/j.conbuildmat.2021.123248.
22. Norambuena-Contreras J, Gonzalez A, Concha JL, Gonzalez-Torre I, Schlangen E. 2018. Effect of metallic waste addition on the electrical thermophysical and microwave crack-healing properties of asphalt mixtures. Constr. Build. Mater. 187:1039–1050. https://doi.org/10.1016/j.conbuildmat.2018.08.053.
23. Tabaković A, O’Prey D, McKenna D, Woodward D. 2019. Microwave self-healing technology as airfield porous asphalt friction course repair and maintenance system. Case Stud. Constr. Mater. 10:e00233. https://doi.org/10.1016/j.cscm.2019.e00233.
24. Norambuena-Contreras J, García A. 2016. Self-healing of asphalt mixture by microwave and induction heating. Mater. Des. 106:404–414. https://doi.org/10.1016/j.matdes.2016.05.095.
). In laboratory assessments, asphalt mixture specimens are previously broken, then exposed to MW heating treatment, and finally broken again. The effectiveness of the MW heating treatment is typically evaluated through the recovery rate, which is calculated as the percentage of the original performance of the sample that is regained after the heating treatment. In addition, the feasibility of this technology was recently proved at full-scale (2525. Maliszewski M, Zofka A, Maliszewska D, Sybilski D, Salski B, Karpisz T, Rembelsk R. 2021. Full-scale use of microwave heating in construction of longitudinal joints and crack healing in asphalt pavements. Materials. 14(18):5159. https://doi.org/10.3390/ma14185159.
). Despite this, it has been studied that if only heat is applied it will not be possible to restore the asphalt pavement original behavior being recovery ratios between 65 -90%.

To overcome this limitation, it has been recently proposed a novel thermomechanical treatment, consisting of combining MW heating treatment and re-compaction energy for achieving complete healing of damaged asphalt mixtures (2626. Gallego J, Gulisano F; Contreras V, Páez A. 2021. Optimising heat and re-compaction energy in the thermomechanical treatment for the assisted healing of asphalt mixtures. Constr. Build. Mater. 292:123431. https://doi.org/10.1016/j.conbuildmat.2021.123431.
). In the field, the thermomechanical treatment would be implemented by heating the deteriorated pavement with a MW generator and then re-compacting the pavement (2727. Gallego J, Gulisano F, Contreras V, Páez A. 2021. The crucial effect of re-compaction energy on the healing response of hot asphalt mortars heated by microwaves. Constr. Build. Mater. 285:122861. https://doi.org/10.1016/j.conbuildmat.2021.122861.
). The effectiveness of this technology was also assessed for Half-Warm Asphalt Mixes (HWMA) containing EAFS and Reclaimed Asphalt Pavement (RAP) (2828. Lizárraga JM, Gallego J. 2020. Self-healing analysis of half-warm asphalt mixes containing Electric Arc Furnace (EAF). Slag and Reclaimed Asphalt Pavement (RAP). Using a novel thermomechanical healing treatment. Materials. 13(11):2502. https://doi.org/10.3390/ma13112502.
).

To scale up MW-assisted healing technology, Life Cycle Assessment (LCA) analyses are urgently required (2929. Gulisano F, Gallego J. 2021. Microwave heating of asphalt paving materials: Principles current status and next steps. Constr. Build. Mater. 278:121993. https://doi.org/10.1016/j.conbuildmat.2020.121993.
, 3030. Ayar P, Moreno-Navarro F, Rubio-Gámez MC. 2016. The healing capability of asphalt pavements: a state of the art review. J. Clean. Prod. 113:28–40. https://doi.org/10.1016/j.jclepro.2015.12.034.
). LCA is a tool for estimating and quantifying potential environmental impacts of products, processes, and services throughout their entire life cycle, from raw material extraction through transport, manufacturing, use, and the end of life. LCA studies for asphalt mixtures and roads has been carried out recently as many sustainable pavement technologies are being developed in the recent years due to the need to determine environmental impacts and their contribution to climate change.

A review of the state of the art of LCA for binders and asphalt mixtures has been previously made (31-3631. Santero NJ, Masanet E, Horvath A. 2011. Life-cycle assessment of pavements. Part I: Critical review. Resources Resour Conserv Recycl. 55(9–10):801–809. https://doi.org/10.1016/j.resconrec.2011.03.010.
32. Babashamsi P. Yusoff NI. Ceylan H, Nor NG, Jenatabadi HS. 2016. Sustainable development factors in pavement life-cycle: Highway/airport review. Sustainability. 8(3):248. https://doi.org/10.3390/su8030248.
33. Suwarto F, Tony Parry T, Airey G. 2023. Review of methodology for life cycle assessment and life cycle cost analysis of asphalt pavements. Road Mater. Pavement Des. https://doi.org/10.1080/14680629.2023.2278149.
34. Mattinzioli T, Sol-Sanchez M, Moreno-Navarro F, Rubio-Gamez MC, Martinez G. 2022. Benchmarking the embodied environmental impacts of the design parameters for asphalt mixtures. SM&T. 32:e00395. https://doi.org/10.1016/j.susmat.2022.e00395.
35. Mattinzioli T, Sol-Sanchez M, Jimenez del Barco Carrion A, Moreno-Navarro F, Rubio-Gamez MC, Martinez G. 2021. Analysis of the GHG savings and cost-effectiveness of asphalt pavement climate mitigation strategies. J. Clean. Prod. 320:128768. https://doi.org/10.1016/j.jclepro.2021.128768.
36. Mejía-Arcila J. Valencia-Saavedra W, Mejía de Gutiérrez R. 2020. Eco-efficient alkaline activated binders for manufacturing blocks and pedestrian pavers with low carbon footprint: Mechanical properties and LCA assessment. Mater. Construc. 70(340):e232. https://doi.org/10.3989/mc.2020.17419.
), however, very few had evaluated different pavement rehabilitation strategies or the benefits of using MW-assisted healing technology. In any case, the are promising results according to the research made.

A hybrid input-output-assisted Life-Cycle that compared a traditional road with one in which the self-healing technology was implemented concluded that self-healing roads can be an alternative as emissions could be decreased up to 16% as well as lower costs could be achieved over the road life (3737. Rodríguez-Alloza Heihsel M, Fry J, Gallego J, Geschke A, Wood R, Lenzen M. 2019. Consequences of long-term infrastructure decisions – the case of self-healing roads and their CO₂ emissions. Environ. Res. Lett. 14:114040. https://doi.org/10.1088/1748-9326/ab424a.
). Also, a pavement LCA model that was developed to evaluate energy use and GHG emissions from pavement rehabilitation strategies concluding that savings accumulated during the use phase due to reduced rolling resistance for roads with high traffic could be considerably higher than the impacts from material manufacturing and construction (3838. Wang T, Lee IS, Kendall A, Harvey J, Lee EB, Kim C. 2012. Life cycle energy consumption and GHG emission from pavement rehabilitation with different rolling resistance. J. Clean. Prod. 33:86–96. https://doi.org/10.1016/j.jclepro.2012.05.001.
). Besides, a recent study that considered bitumen and its self-healing capacity as a parameter and that developed an Open Life Cycle Assessment framework showed that a significant reduction in GHG emission and energy consumption could be achieved (3939. Butt AA, Birgisson B, Kringos N. 2012. Optimizing the highway lifetime by improving the self healing capacity of asphalt. Procedia - Social and Behavioral Sciences. 48:2190-2200. http://doi.org/10.1016/j.sbspro.2012.06.1192.
).

As the introduction of new pavement materials and technologies can improve the maintenance and rehabilitation process, further research on this topic is necessary. The novelty of this research is that no studies have assessed the feasibility of the thermomechanical treatment (MW heating and re-compaction) on the overall emissions and energy consumption of maintenance activities. This research aims to fill these gaps performing a LCA for evaluating the feasibility of MW-assisted healing technology evaluating different scenarios in which the heating energy and the addition of EAFS varies.

2. METHODOLOGY

2.1. LCA framework

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LCA is a tool for estimating and quantifying potential environmental impacts of products, processes, and services throughout their entire life cycle, from raw material extraction through transport, manufacturing, use, and the end of life. Life cycle is a term created by environmental evaluators to quantify the environmental impact of a material or product from when it is extracted from nature until it returns to the environment as waste. According to ISO 14040 (4040. ISO 14040. 2006. Environmental management – life cycle assessment – principles and framework: international standard 14040.
), the LCA framework has four steps that are iterative, as presented in Figure 1.

Figure 1.  LCA framework according to ISO 14040 and 14044.

LCA consists of four stages differentiated. To understand the sustainability of a new asphalt pavement technology requires a thorough evaluation of environmental impacts within all stages of a pavement’s life with the LCA framework. These main stages that allows to obtain results based on scientific rigor, traceability and transparency in its implementation are described in the following sections.

2.2. Definition of goal and scope

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In first stage a series of questions must be responded as this will determine the nature of the LCA and that constitute the objective, such as the reasons that lead to the study or the intended application. Objectives and scope must be defined in a consistent way. Also, the functional unit (FU) which is defined by ISO 14040 as ‘quantified performance of a product system for use as a reference unit’ must be included.

In this study the objective is to determine the potential environmental benefits of implementing the MW-assisted self-healing technology for asphalt pavements when compared to a conventional road with traditional maintenance strategies. Two different scenarios in which the energy applied to restore the pavement and the addition of slag varies has been studied.

The FU in this study is ‘m²’ of road and for the sensitivity analysis in which the road lifespan parameter is included the FU is ‘m² year’. Different time frames will be considered and compared since this is key of the purpose of the application of the MW-assisted self-healing technique: the extension of life service of asphalt pavements.

Different scenarios have been analyzed and they were selected and based on a previous laboratory research (2626. Gallego J, Gulisano F; Contreras V, Páez A. 2021. Optimising heat and re-compaction energy in the thermomechanical treatment for the assisted healing of asphalt mixtures. Constr. Build. Mater. 292:123431. https://doi.org/10.1016/j.conbuildmat.2021.123431.
). In that preceding study there were different scenarios with different repair rates. To assess this LCA only the scenarios in which the asphalt reaches a total repair of the pavement were selected and are the following:

In Scenario 1 (SC1), the asphalt mixture does not contain slags and the thermo-mechanical treatment was able to completely repair the pavement (being healing rates sometimes higher than 100%). The energy needed to heat this mixture without slag was 0.0098 kWh for each kg of bituminous mixture.

In Scenario 2 (SC2), slags are added (10% by volume of aggregates). MW heating is applied as well as a re-compaction treatment just like in SC1. In this case, to reach a completely repair the mixture energy required for heating was 0.0054 kWh for each kg of bituminous mixture. In this scenario, the use of slag made it possible to reduce the energy required for heating. In Table 1 a summary of the different scenarios studied for the MW self-healing treatment are presented:

Table 1.  MW-assisted self-healing different scenarios.

Scenario	EAFS addition	MW treatment	Re-compaction treatment	Energy required for heating a kg of mix	Repair rate
Scenario 1 (SC1)	no	yes	yes	0.0098 kWh	100%
Scenario 2 (SC2)	yes	yes	yes	0.0054 kWh	100%

According to ISO 14040, in this study, the system boundaries include all the significant life cycle phases: construction, rehabilitation road opeations and a final road dismantling. This covers the production and transportation of materials, their placement in the road structure, the road maintenance operations, and the demolition of the infrastructure. The analysis period considered included the entire life cycle of the road (‘‘cradle-to-grave” LCA), from the extraction of raw materials.

Figure 2 shows the different operations/strategies considered during the life service of of the conventional road and the MW-assisted self-healing roads (SC1 and SC2 has the same system boundaries) in which the MW treatments are applied during the life service to restore the asphalt pavement.

Figure 2.  System boundaries for each type of road during life service.

2.4. Life Cycle Impact Assessment

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In this third stage of the LCA the amount of impact associated with the data collected in the inventory phase of the life cycle is determined. The inventory data will be related to the impact categories selected in the study and their corresponding indicators.

To carry out the calculations in this research, the professional tool SimaPro^® was used, which is widely used by the international scientific community. The ecoinvent database and the updated ILCD 2011 midpoint characterization method (EF 2.0) have been included, among others.). The sixteen selected environmental impact categories and their units can be seen in Table 2.

Table 2.  Environmental impact categories.

Impact category	Unit
Climate change (CC)	kg CO2 eq
Ozone depletion (OD)	kg CFC11 eq
Ionizing radiation (human health) (IR)	kBq U-235 eq
Photochemical ozone formation (PO)	kg NMVOC eq
Respiratory inorganics (RI)	disease inc.
Human toxicity (non-cancer effects) (Hhnc)	CTUh
Cancer human health effects (Hhc)	CTUh
Acidification (AC)	mol H+ eq
Eutrophication (freshwater (Euf)	kg P eq
Eutrophication marine (Eum)	kg N eq
Eutrophication terrestrial (Eut)	mol N eq
Ecotoxicity (freshwater) (EC)	CTUe
Land use (LU)	Pt
Water scarcity (WS)	m3 depriv.
Resource use, energy carriers (Ruec)	MJ
Resource use, mineral, and metals (Rumm)	kg Sb eq

2.4.1. LCA results of the conventional road

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The results of the conventional road by stages and/or operations and by materials and/or processes are presented in Figure 3 and Figure 4, respectively.

Figure 3.  LCA results of the conventional road by stages and/or operations per m².

In Figure 3 it can be observed the construction stage of the road the main responsible of all the category impacts: 41%-56%. Also, the recycling is responsible of the 18-31% total environmental impacts. It can also be noted that the slurry maintenance operation is the maintenance operation that hardly contributes to the impacts.

Figure 4 shows that the main impacts are due to the production of bitumen, the consumption of diesel and the transportation. Electricity use hardly contributes to the impacts. However, it can be also observed that the contribution of the production of bitumen is different depending on the environmental category impact. For ozone depletion (OD) is 96% of total impacts while for respiratory inorganics (RI) is only 5%. Besides, for RI diesel is 81% while for OD is 2%. And gravel contributes 52% for land use (LU) but only 4% for climate change (CC).

Figure 4.  LCA results of the conventional road by materials and/or processes per m².

2.4.2. LCA results of the MW-assisted self-healing roads

 ⌅

In Figure 5 and 6 the results by stages and/or operations by m² with the two different scenarios of the MW-assisted self-healing roads are presented.

Figure 5.  LCA results of SC1 self-healing road by stages and/or operations per m².

Figure 6.  LCA results of SC2 self-healing road by stages and/or operations per m².

It can be observed that in both cases construction still has an important role regarding the impacts in all the categories. Besides, the MW and re-compaction treatment stage is only responsible for the 2-3% of all the impacts and for some categories the impacts the value is negligible. This shows that it is possible to recover the original asphalt properties with very low environmental impacts. Also, likewise the conventional road, construction stage is the responsible of the greatest number of impacts, varying from 46-77% for SC1 and from 52-76% for SC2.

It can be observed from Figure 7 and Figure 8 that the bitumen contributes to the impacts depends on the environmental category: bitumen is responsible of 96% of the OD while for respiratory inorganics (RI) is only 4-6%. Same as diesel, which has hardly impact for resource use, mineral, and metals (Rumm) impact, only 4-5% but 82-84% for RI. Hence, it can be seen the importance of LCA which yields different impact results depending on the category. Besides, it can be noted that addition of slag does not contribute to the environmental impacts, only 1% for cancer human health effects (Hhc).

Figure 7.  LCA results of SC1 self-healing road by materials and/or processes per m².

Figure 8.  LCA results of SC2 self-healing road by materials and/or processes per m².

2.4.3. Comparison results

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The comparison results of the impacts of the total environmental impacts for the conventional road and for the two different scenarios of the MW-assisted self-healing road are presented and compared in Figure 9.

Figure 9.  Percentual comparison environmental impact categories results per m².

The results shows that the best scenario for all the category impacts is SC2 (in which MW heating is applied as well as a re-compaction treatment for a pavement with slag addition) when compared to the conventional road. In SC1 the lack of slag leads to an increase in the energy needed for heating which is translated in higher environmental impacts. The addition of slag allowed to reduce the heating energy required which, in additions to the re-compaction made after, a healing rate of 100% can be achieved. The decrease in all environmental categories impacts varies from 17-25%. when SC2 is compared with the conventional road. It can be observed that a decrease of 20% is achieved for the CC category. It must be noted that the lifespan parameter has not been considered.

2.4.4. Sensitivity analysis

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Sensitivity analysis is a tool through which the changes that occur in a variable are studied when certain variations are introduced into the model. In this study the variable is the lifespan of each road as the potential of the MW-assisted self-healing technology is to repair cracked and deteriorated asphalt pavements so the road durability could be increased, maintenance operations reduced, hence, environmental impacts could be decreased during the road life span.

Sensitivity analysis is also known as a hypothetical analysis, since it is essential to determine how the different values that parameter can adopt affect a dependent variable. In the sensitivity analysis performed in this LCA all impact category indicators have been divided by year, so the lifespan of each road can be now considered, being the FU ‘m² year’.

Pavements systems has a long-life span: the expected lifespan of a typical road with double or triple asphalt layers is 20-40 years (4141. Merill D. 2005. Guidance on the development assessment and maintenance of long-life flexible pavements. Transport Research Laboratory. ISBN 1-84608-638-8.
). Hence, in this research the life service of the conventional road studied considered is 30 years. On the other hand, it is expected that if MW-assisted self-healing technology is implemented road maintenance operations could be delayed and even some of them not necessary.

In this research different life spans have been considered for the self-healing road (to simplify, the results of the best scenario (SC2) are the ones compared with the conventional road). Lifespans are: 30 years (the same as the conventional one so both can be compared (SC2 30)) and 32, 35 and 42 years (SC2 32, SC2 35 SC42, respectively) to determine the benefits of MW-assisted self-healing emerging technology as the lifespan increases. The comparison results are presented in Table 3.

Table 3.  Environmental impacts results per m² year.

	Conventional lifespan	MW-assisted self-healing road (SC2) lifespan
Category impact	30 years	30 years	32 years	35 years	42 years
CC	3.989400507	3.215537824	3.01456671	2.756175278	2.296812732
OD	1.31688E-05	9.90026E-06	9.2815E-06	8.48594E-06	7.07162E-06
IR	0.497189466	0.393283155	0.368702958	0.337099847	0.280916539
PO	0.050016211	0.039891137	0.037397941	0.034192403	0.028493669
RI	5.2697E-07	4.37189E-07	4.09865E-07	3.74734E-07	3.12278E-07
Hhnc	2.31934E-07	1.85988E-07	1.74364E-07	1.59418E-07	1.32849E-07
Hhc	4.21924E-08	3.4194E-08	3.20569E-08	2.93092E-08	2.44243E-08
AC	0.026644056	0.02183952	0.02047455	0.018719588	0.015599657
Euf	0.000224758	0.000183229	0.000171777	0.000157053	0.000130878
Eum	0.011324645	0.009286474	0.008706069	0.007959835	0.006633196
Eut	0.122940376	0.100909324	0.094602491	0.086493706	0.072078089
EC	2.2723725	1.785754879	1.674145199	1.530647039	1.2755392
LU	7.780079291	6.295015283	5.901576828	5.395727385	4.496439488
WS	204.8744969	164.0181118	153.7669798	140.586953	117.1557941
Ruec	144.6522779	111.4508233	104.4851469	95.52927713	79.60773094
Rumm	1.16311E-05	8.94524E-06	8.38617E-06	7.66735E-06	6.38946E-06

Table 3 shows than even when the lifespan is the same the MW-assisted self-healing technology decreases all the environmental category impacts. In Figure 10 a percentual comparison is presented so the benefits can be better observed. The more the lifespan of self-healing road increases, the more the environmental benefits will increase as well.

Figure 10.  Percentual comparison of the conventional and SC2 road with different life spans.

In Table 4 a percentage comparison of the environmental benefits of implementing the MW-assisted self-healing technology are presented when lifespan increases due to the implementation of this technology.

Table 4.  Percentage of environmental benefits by impact category.

	SC4 30	SC4 32	SC4 35	SC4 42
CC	19.40%	24.44%	30.91%	42.43%
OD	24.82%	29.52%	35.56%	46.30%
IR	20.90%	25.84%	32.20%	43.50%
PO	20.24%	25.23%	31.64%	43.03%
RI	17.04%	22.22%	28.89%	40.74%
HHnc	19.81%	24.82%	31.27%	42.72%
HHc	18.96%	24.02%	30.53%	42.11%
AC	18.03%	23.16%	29.74%	41.45%
Euf	18.48%	23.57%	30.12%	41.77%
EUm	18.00%	23.12%	29.71%	41.43%
EUt	17.92%	23.05%	29.65%	41.37%
EC	21.41%	26.33%	32.64%	43.87%
LU	19.09%	24.15%	30.65%	42.21%
WS	19.94%	24.95%	31.38%	42.82%
RUmm	22.95%	27.77%	33.96%	44.97%

Table 4 shows that there is a substantial reduction of the environmental impacts in all categories with respect to a road with conventional maintenance strategies even when the life span is hypothetically the same.

By adding slags to the asphalt pavement (SC2), further reduction of the environmental impact for each category is attained. This is due to the fact the addition of EAFS allows for a more efficient heating process, reducing energy consumption and showing the benefits of applying both MW and re-compaction of the environmental impact. It can be noted that CC category impact is reduced by 42% with respect to the conventional strategy.

CONCLUSIONS

Determining accurately the benefits of the actual emerging technologies will lead towards successful paths of sustainable decision-making and to achieve climate change goals. Microwave (MW)-assisted self-healing technology can repair the pavement by heating with MW energy the cracked asphalt surface as bitumen can melt and flow through the cracks sealing them. To determine how sustainable this technology could be it is necessary to carry out a Life Cycle Assessment (LCA) so environmental impacts could be compared with a traditional road with conventional maintenance operations, hence, environmental benefits determined.

In this study, to determine how sustainable this technology could be, a ‘‘cradle-to-grave” (LCA) has been carried out to determine the potential environmental benefits by comparing a conventional road with traditional maintenance operations with different self-healings road scenarios in which the use of Electric Arc Furnace slag (EAFS) and different MW heating energy were considered.

From this research, the following findings were obtained:

In terms of future development, supporting the implementation of this innovative pavement technology and conducting LCA studies are necessary as it will help engineers, project managers as well as stakeholders and policy makers to take decisions and actions to develop a more sustainable pavement sector. Also, in future research, full-scale studies and further LCA with different assumptions are necessary as well as it should be also evaluated the effect of using different materials to enhance the MW heating efficiency.