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The concept of sustainability in the road construction sector is a complex issue because of the various steps that contribute to the production and release of greenhouse gas (GHG) emissions. Addressing this issue, the European Commission has put various policy initiatives in place to encourage the construction industry to adopt circular economy (CE) and industrial symbiosis (IS) principles e.g., the use of recycled materials.
Greenhouse gas (GHG) emissions in infrastructure projects are a key indicator when sustainability is being assessed (
Road construction is one of three main drivers of resource use in the European Union (
The European Commission (EC) has put various policy initiatives in place to encourage the construction industry toward circular economy (CE) principles. The overall idea is to reconsider the whole life cycle of resources, to make the European Union (EU) a “circular economy” based on recycling, and the use of waste as a resource (
The use of alternative materials for the construction and rehabilitation of roads would therefore be a strategy to be boosted, establishing regional industrial symbiosis (IS) agreements which can support companies to gain competitiveness and reduce the environmental impact associated to their day to day business activities (
Roads are built in layers and three main types of road pavements can be identified: flexible, semi-rigid, and rigid pavements. In Europe, the main pavement type is flexible (asphalt) ( surface, binder, and base courses, which consist of bituminous mixtures; road base and sub-base courses, which consist of cement bound or unbound aggregates.
Flexible pavement layer system (
Asphalt mixtures are typically composed of approximately 95% of mineral aggregates mixed with about 5% paving bitumen, with bitumen functioning as the glue that binds the mineral aggregates in a cohesive mix (
Some aggregates can, usefully, be created by recycling processes.
In this study, the use of steel slags obtained by an electric arc furnace (EAF), and reclaimed asphalt pavement (RAP) obtained by the deconstruction and milling of old asphalt pavement have been investigated. The HMA technology was used in the production process.
While recycling HMA results in a reusable mixture of aggregates and aged asphalt binders known as RAP (
As is widely known, steel slags, produced during the separation of molten steel from impurities in a steel-making furnace, are one of the most common industrial wastes and they can be used for several applications. Thanks to their high hardness and cementing properties, they are commonly used in the road sector (
As far as the recycling of RAP and of EAF steel slags are concerned, several studies have shown that the use of these materials is common in pavement construction because of their technical performances and economic value.
Similarly, other studies have demonstrated that steel slags with proper pre-processing and sufficient in-place quality control procedures can perform credibly well as asphalt aggregates (
Moreover,
The Life Cycle Assessment (LCA) is a considerable method to evaluate the environmental impacts of a system, a product, or a process. All the inputs (such as energy and resources) are identified, with the aim of quantifying the relevant emissions, the consumed resources, and the related environmental impacts. Considering a product, the impacts do not only arise during the manufacturing stage, but along its entire life cycle, including the extraction and transportation of raw materials, use and maintenance, possible reuse, and end of life. Therefore, the approach encompasses the whole life cycle of a product, “from cradle to grave,” as the first definition stated (
According to a circularity perspective, a new philosophy, referred to as “from cradle to cradle”, is taking hold: at their end of life, materials are not considered as waste to be discarded, but as secondary raw material, thanks to an appropriate recycling process. In this way, a cradle-to-cradle closed loop is outlined.
According to the ISO14040 standard, the four steps to perform a Life Cycle Assessment (LCA) are: the definition of the goal and scope of the analysis, the inventory analysis, the impact assessment, and finally, the interpretation of the results (
LCA phases (
The context of the study and its purpose are set. The goal of the LCA states the intended application and the reasons for carrying out the study, the intended audience, and whether the results are to be used for internal purpose or for disclosure to the public. The scope includes the following items: functional unit, system boundary, allocation procedure, data requirements, impact assessment method, assumption, and data quality. In particular, the functional unit, that defines the quantification of the identified function of the product, has the primary purpose to provide a reference to which the inputs and outputs are related, ensuring the comparability of the LCA results. The system boundary defines the unit processes to be included in the system. Criterions for the choice of the system boundaries are physical (description of the productive cycle), geographical (reference area), and temporal (reference period).
It lists all the inputs (materials and energy) and outputs (products, co-products, and emissions) to be used to compare standards and processes. Inventory analysis involves data collection and calculation procedures, aiming at quantifying the relevant inputs and outputs of a product system. The life cycle inventory uses both primary and specific as well as literature and secondary data from international databases.
The life cycle impact assessment (LCIA) includes the following mandatory elements: the selection of impact categories and characterization models; the assignment of LCI results to the selected impact categories (classification); and the calculation of category indicator results (characterization).
Finally, the life cycle interpretation aims at the identification of substantial issues, based on the results of the previous steps. The evaluation includes considerations about the completeness and the consistency of the study, conclusions, limitations, and recommendations.
In
Due to the high amount of GHG emissions generated during road construction, rehabilitation, and operation, the evaluation and reduction of the environmental impact related to the road sector have become an international challenge (
Therefore, LCA analysis performed by
The “Cooperativa trasporti Imola Scrl” (CTI) company has four plants for the production of asphalt mixes, three batch plants and one drum plant. In both the typologies, the mineral aggregates are dried and heated in a rotating drum. Nowadays, the predominant plant type in the U.S. and New Zealand is the drum-mix plant, while batch plants prevail in Europe, South Africa, and Australia (
Moreover, according to the European Commission, steel slags can be used in road construction, meeting the requirements of European and national legislations and standards, although a specific recycling target is not set (
In Italy, the steel slags resulting from steelmaking are considered by-products, whereas RAP, as a result of the milling operations of existing road pavements at their end of life stage, is not considered to be waste, as long it is re-used within the domain of the asphalt sector (
This study aims at testing the use of EAF steel slags and RAP in two mixtures, for wearing and binder courses, respectively. The physical and mechanical properties and the environmental performances have been evaluated. The objectives of the research study are summarized below: define a standard characterization of mixtures in order to evaluate the physical and mechanical performances related to the use of virgin and recycled materials; assess the environmental impacts associated with the mixtures and model a best-case scenario for the CTI batch plant with the maximum percentages of steel slags and RAP; identify practical implications of the use of recycled materials in new asphalt mixtures, from a life cycle and industrial symbiosis perspective.
The research study is divided into two phases: in the first phase, the effects of recycled materials on asphalt mixture proprieties are investigated. Two specific types of asphalt mixtures are produced with different compositions: 35% RAP and 16% steel slags for the wearing course, by weight; 40% RAP and 15% steel slags for the binder course, by weight.
Asphalt materials are characterized in terms of size distribution, strength modulus (indirect tensile strength), and volumetric properties (air voids content).
The second phase aims at evaluating environmental impacts by applying LCA methodology to the geographical context of the CTI company. The novelty of this study is the integration of the technical analysis of material characterization, assessed by laboratory experiments, with the analysis of the environmental impacts.
Four mixtures were analyzed: A control mixture for the wearing course (MixW0) An experimental mixture for the wearing course (MixW1) A control mixture for the binder course (MixB0) An experimental mixture for the binder course (MixB1)
A description of the four mixtures can be found in
Composition of asphalt mixtures, percentages of aggregates by weight.
Material | Fraction (mm) | MixW0 (control) | MixW1 (35% RAP, 15% EAF steel slags) | MixB0 (control) | MixB1 (40% RAP, 16% EAF steel slags) |
---|---|---|---|---|---|
Gravel | 14/20 | — | 15 | 6 | |
Gravel | 10/16 | — | — | 20 | — |
Gravel | 8/12 | — | — | 10 | — |
Gravel | 4/8 | 19 | 10 | 7 | — |
Gravel | 3/6 | 30 | 9 | 12 | 10 |
Sand | 0/4 | 45 | 30 | 32 | 25 |
Filler | — | 6 | 1 | 4 | 3 |
RAP | 0/8 | — | 35 | — | — |
RAP | 8/12 | — | — | — | 40 |
EAF slag | 4/8 | — | 15 | — | 16 |
The design of the aggregate distribution was based on gradation limits specified in the UNI 13108 Italian technical specification for bituminous layers, as shown in
MixW1 gradation and limits.
MixB1 gradation and limits.
The experimental program can be divided into three different phases. In order to evaluate the physical and mechanical performances of the designed mixtures, MixW1 and MixB1 were characterized in terms of particle size distribution (1), volumetric properties (2), and strength modulus (3) according to the standard
The sieve analysis was carried out in a laboratory to define the particle size distribution of MixW1 and MixB1. According to the EN 933-1 standard, a representative weighed sample for each mixture was separated on sieves of different sizes (Series 2). To find the percentage of the aggregate passing through each sieve,
In order to find the cumulative percentage of the aggregate retained in each sieve,
The % cumulative retained
To solve
Once the mix design for MixW1 and MixB1 was defined, the following step in the research program considered their physical analysis. The compactability and workability properties of the HMAs were evaluated against gyratory compactor samples (EN 12697-31). For both mixtures, three specimens per MixW1 were compacted up to 180 times more than the gyratory compactor, and three specimens per MixB1 were compacted up to 210 times more than the gyratory compactor. The air voids content (v) of each specimen was evaluated according to the EN 12697-8 standard.
Finally, for each mixture, according to the EN 12697-23 standard, the indirect tensile strength (ITS) was performed at 25°C.
The present study has assessed the impacts arising from the hot-mix batch plant by applying an LCA methodology to the geographical context of the CTI plant, located in the Emilia-Romagna region. As previously described, an LCA study consists of four stages: 1) goal and scope definition, 2) inventory analysis, 3) impact assessment, and 4) results and interpretation.
Quantitative and comparative life cycle assessment results on road construction materials are essential first steps toward making informed decisions and toward more sustainable practices in road construction ( - The raw material transportation from the mining site/quarry to the CTI plant; - The RAP transportation from road worksites to the CTI plant; - The RAP pre-processing, which includes crushing and screening; - The avoided production and transportation of natural aggregates (replaced by recycled aggregates), including extraction, processing, and transportation to the CTI batch plant; - The avoided production and transportation of virgin bitumen (replaced by recycled bitumen).
Diagram flow of recycling reclaimed asphalt pavement (RAP) and electric arc furnace (EAF) in the CTI batch plant.
The geographical scope is local. The study focuses on the conditions and CTI technologies used in 2018. The potential environmental impacts were evaluated using the software SimaPro®. This analytical tool works in accordance with the ISO 14040 standard (
Data regarding the core processes, i.e., transportation, hot recycling, and energy consumption, are primary data. For analyzing the CTI HMA batch plant, data were collected directly from the CTI company. Data related to other foreground processes, i.e., bitumen production, extraction of natural mineral resources, and pre-processes of waste asphalt, were instead taken from the LCA software SimaPro databases (Ecoinvent and Europe & Denmark databases). Therefore, the avoided impacts, due to the avoided consumption of natural virgin aggregates because of the EAF steel slags and RAP addition into hot mixes, are modelled using secondary data on quarry activities in Europe.
Inventory data about the transportation of the raw materials, asphalt waste, and bitumen are modelled using the primary data on CTI transports, as shown in
Inventory data about the transportation of the asphalt waste, the by-products, and the primary materials to the plant.
Material | Transport distance (km) | Description | Lorry type | Source |
---|---|---|---|---|
EAF slags | 150 | Road distance between company – CTI batch plant | 32 metric tons, EURO 6 | Ecoinvent 3.5 |
Asphalt waste | 40 | Road distance between RAP site – CTI batch plant | 32 metric tons, EURO 6 | Ecoinvent 3.5 |
Natural aggregates | 190 | Road distances between quarry site – CTI batch plant | 32 metric tons, EURO 6 | Ecoinvent 3.5 |
Virgin bitumen | 230 | Road distances between bitumen plants – CTI batch plant | 28 metric tons, EURO 6 | Ecoinvent 3.5 |
Inventory data about the energy consumption in the CTI batch plant.
Processes | Energy type | Energy consumption/ton (kWh/ton) | Methane (m3) | Source |
---|---|---|---|---|
Line 0 | Electricity | 6131 | 8.5 | Ecoinvent 3.5 |
Line 1 | Electricity | 273 | 8.5 | Ecoinvent 3.5 |
In the LCIA, the CML impact assessment baseline calculation method was adopted. The consumption of materials and energy as well as the emissions to air, water, and soil were gathered according to the effects they can have on the environment. According to
Therefore, this methodology aims to assess the environmental impacts of the processes identified in the inventory analysis. Hence, all substances were measured and assigned to an impact category. The results are represented by single midpoints.
In order to evaluate the physical and mechanical performances of the mixtures incorporating different recycled aggregate percentages for the wearing and binder courses, MixW1 and MixB1 were characterized in terms of air void content (v), indirect tensile strength (ITS), indirect tensile stiffness modulus (ITSM), and indirect tensile strength ratio (ITSR).
The determination of the air void content of MixW1 and MixB1 can be found in the
The LCA was chosen to evaluate the environmental impacts that affect the designed road life cycle (production and treatment processes and transportation of the involved materials). The overall environmental impacts related to the production of asphalt mixtures MixW1 and MixB1 in the CTI batch plant are shown in
Environmental impacts related to MixW1 and MixB1.
Impact categories | Unit | Total | |
---|---|---|---|
MixW1 | MixB1 | ||
Global warming potential | kg CO2 eq. | 4.60E + 04 | 5.80E + 04 |
Human toxicity | kg 1.4 - DB eq. | 1.82E + 04 | 2.62E + 04 |
Acidification | kg. SO2 eq. | 3.09E + 02 | 3.35E + 02 |
Eutrophication | kg PO4 eq. | 8.31E + 01 | 1.09E + 02 |
Ecotoxicity | kg 1.4 - DB eq. | 1.53E + 03 | 1.55E + 03 |
Photochemical oxidation | kg C2H4 | 1.53E + 01 | 1.72E + 01 |
Ozone layer depletion | kg CFC - 11 eq. | 3.18E − 02 | 3.39E − 02 |
Abiotic depletion | kg Sb eq. | 1.45E − 01 | 1.49E − 01 |
Abiotic depletion fossil fuels | MJ | 2.52E + 06 | 2.73E + 06 |
To discuss the results of the standard characterization of the designed mixtures, a comparison of the performances of the designed mixtures and control mixtures was first performed.
Mechanical and volumetric properties of MixW1 and MixW0.
Specimen | Avg. ITS (MPa) | Avg. void (%) | ||
---|---|---|---|---|
10 | 120 | 210 (v) | ||
MixW1 | 2.68 | 11.2 | 2.7 | 1.8 |
MixW0 | 1.19 | 13.2 | 4.0 | 2.5 |
Mechanical and volumetric properties of MixB1 and MixB0.
Specimen | ITS (MPa) | Void (%) | ||
---|---|---|---|---|
10 | 100 | 180 | ||
MixB1 | 1.88 | 9.6 | 4.6 | 1.6 |
MixB0 | 1.35 | 13.6 | 4.9 | 2.9 |
The mechanical analysis was supported by the ITS test in compliance with the EN 12697-23 standard. For each mixture, three samples were prepared with a gyratory compactor (180 and 210 times) and then conditioned at 25°C for 4 h before testing. According to the scientific literature, an ITS test is generally used to assess the level of tenacity of the aggregate-filler-bitumen bond (
To note, the average value of ITS recorded for both experimental mixtures was considerably higher than the limit suggested by the Italian technical specifications (
Similarly, if the Italian technical specification is taken into account, the air void content (v) was lower than the suggested one, which ranges between 3 and 8%.
The standard characterization of the mixtures evaluated the hardening effect of the old bitumen on the content blend. According to
Moreover, as the stiffness and the fatigue performance were not tested, further research might investigate these aspects.
Secondly, a comparison of LCA results between the two mixtures for the wearing course (MixW0 and MixW1) and the two mixtures for the binder course (MixB0 and MixB1) was performed. Hence,
Contribution analysis related to recycling of RAP and EAF slags in CTI batch plant for wearing course
Environmental impacts related to MixW0 and MixW1, MixB0 and MixB1.
Impact categories | Unit | Total | |||
---|---|---|---|---|---|
MixW0 | MixW1 | MixB0 | MixB1 | ||
Global warming potential | kg CO2 eq. | 7.51E + 04 | 4.60E + 04 | 9.37E + 04 | 5.80E + 04 |
Human toxicity | kg 1.4 - DB eq. | 2.86E + 04 | 1.82E + 04 | 4.02E + 04 | 2.62E + 04 |
Acidification | kg. SO2 eq. | 5.30E + 02 | 3.09E + 02 | 6.36E + 02 | 3.35E + 02 |
Eutrophication | kg PO4 eq. | 1.30E + 02 | 8.31E + 01 | 1.73E + 02 | 1.09E + 02 |
Ecotoxicity | kg 1.4 - DB eq. | 2.30E + 03 | 1.53E + 03 | 2.33E + 03 | 1.55E + 03 |
Photochemical oxidation | kg C2H4 | 2.61E + 01 | 1.53E + 01 | 3.25E + 01 | 1.72E + 01 |
Ozone layer depletion | kg CFC - 11 eq. | 4.74E − 02 | 3.18E − 02 | 5.97E − 02 | 3.39E − 02 |
Abiotic depletion | kg Sb eq. | 2.44E − 01 | 1.45E − 01 | 2.23E − 01 | 1.49E − 01 |
Abiotic depletion fossil fuels | MJ | 3.74E + 06 | 2.52E + 06 | 4.73E + 06 | 2.73E + 06 |
These results are supported by the ones obtained in a preliminary and more general study, previously undertaken (
Diagram flow of recycling RAP and EAF in the CTI batch plant for hot-mix asphalt, with the inclusion of the avoided products for wearing course (MixW0 and MixW1) and binder course (MixB0 and MixB1).
The inert nature of RAP, and the excellent mechanical properties of the EAF slags make them two potentially useful materials in a wide variety of applications, including re-use or recycling in new asphalt pavements. This case study demonstrates the high potential for recycling RAP and EAF steel slags in the road construction sector, as a secondary raw material and a by-product, respectively.
As a result of testing the use of EAF steel slags and RAP in new bituminous mixtures, the physical and mechanical properties as well as the environmental performances of the two mixtures have been evaluated for wearing and binder courses, respectively. In order to maximize the environmental sustainability of the road pavement, the use of RAP and EAF steel slags can be recommended.
Moreover, the authors believe that LCA results and indicators are appropriate tools to compare and communicate the environmental performances of different asphalt mixtures in road construction.
By reducing the global environmental impact and recycling by-products, the CTI and the co-located companies are a real case study of industrial symbiosis at the meso-level.
The authors believe that the development of industrial symbiosis projects provides the opportunity to promote waste reduction, reuse, and recycling, while reducing the environmental impacts, as well as increasing companies’ competitiveness, in particular in countries like Italy, where there are already several large industrial clusters. Moreover, information sharing among stakeholders would facilitate the development of industrial symbiosis networks.
Future research efforts could focus on investigating other recycled materials, for the same applications as virgin ones, with the purpose of reaching the same quality level and performances. In this issue, no economic evaluation was carried out. As a future research direction, the economic sustainability will be evaluated.
The original contributions presented in the study are included in the article/
AB and AE designed the study. AE collected information and materials from the company. AB and AE conceived and planned the experiments for the characterization of asphalt mixtures. AE carried out the experiment. All the authors contributed to the interpretation of the results. AE and CM designed the LCA model and analysed the data. All the authors contributed to the interpretation of the LCA results. AB took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors gratefully acknowledge contributions from the CTI company (Imola, Italy).
The Supplementary Material for this article can be found online at: