João Santos
email: santosjmo@gmail.com

PhD Sarah Bressi

Veronique Cerezo

Michel Dauvergne

PhD Davide Lo Presti

PhD Joao Santos

PhD Véronique Cerezo

Life Cycle Assessment of low temperature asphalt mixtures for road pavement surfaces: a comparative analysis

Keywords: life cycle assessment (LCA), war mix asphalt (WMA), chemical additives, hot mix asphalt (HMA), reclaimed asphalt pavement (RAP), sustainable pavement construction and management

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Considerable amount of Greenhouse gases (GHG) and airborne pollutants are released into the atmosphere during the energy intensive asphalt mixtures production process [START_REF] Thives | Asphalt mixtures emission and energy consumption: a review[END_REF]. As GHG and their effect on the climate are increasingly in the spotlight with respect to policy, legislation and general public's concern, the pavement industry and scientific community have been challenged to improve conventional asphalt mixtures production processes by developing more sustainable technologies and behaviors.

One example of the specific engagement of research institutions and enterprises in developing and delivering multi-faceted and sound solutions meant to mitigate the environmental pressure originated by the sector's activities is the SUP&R ITN (Sustainable Pavement & Rail Initial Training Network) research project (http://superitn.eu/wp/) [START_REF] Lo Presti | SUP&R ITN: an international training network on sustainable pavements and railways[END_REF].

The SUP&R ITN is a training-through-research program, which through a multidisciplinary and multi-sectorial network, aims (1) to form a new generation of engineers versed in sustainable technologies for road pavement and railways and (2) to provide, to both academia and industry, design procedures and sustainability assessment methodologies to certify the sustainability of the studied technologies to the benefit of the European community. Some of the promising sustainable technologies commonly mentioned in the literature and studied in the framework of this research project are the asphalt mixes requiring lower manufacturing temperatures, such as (1) warm mix asphalt (WMA) [START_REF] Kristjánsdóttir | Assessing potential for warm-mix asphalt technology adoption[END_REF][START_REF] Hamzah | Evaluation of the potential of Sasobit ® to reduce required heat energy and CO2 emission in the asphalt industry[END_REF][START_REF] Tatari | Comparative sustainability assessment of warm-mix asphalts: a thermodynamic based hybrid life cycle analysis[END_REF][START_REF] Vidal | Life cycle assessment of hot mix asphalt and zeolite-based warm mix asphalt with reclaimed asphalt pavement[END_REF][START_REF] Mohammad | Louisiana's experience with WMA technologies: mechanistic, environmental, and economic analysis[END_REF][START_REF] Rodríguez-Alloza | Hybrid input-output life cycle assessment of warm mix asphalt mixtures[END_REF][START_REF] Almeida-Costa | Economic and environmental impact study of warm mix asphalt compared to hot mix asphalt[END_REF][START_REF] Stimilli | Warm recycling of flexible pavements: effectiveness of Warm Mix Asphalt additives on modified bitumen and mixture performance[END_REF], (2) halfwarm mix asphalt (HWMA) [START_REF] Rubio | Comparative analysis of emissions from the manufacture and use of hot and half-warm mix asphalt[END_REF] and (3) cold mix asphalt technologies (FHWA, 2016).

WMA is the name used to designate a set of technologies by which the traditional HMA is allowed to be manufactured, transported, placed and compacted at lower temperatures. Characteristically, the mixing temperatures of HMA vary from 150 to 180ºC [START_REF] Jones | Warm mix asphalt-a state-of-the-art review[END_REF], whereas for WMA and HWMA they are comprised between 100 and 140ºC, and between 60 and 100ºC, respectively. In addition to the mixing temperature reduction, the list of benefits that come with the use of these technologies is completed with the following items [START_REF] Rubio | Comparative analysis of emissions from the manufacture and use of hot and half-warm mix asphalt[END_REF]: (1) reduced emissions; (2) better working conditions due to the absence of harmful gases; (3) quicker turnover to traffic; (4) longer hauling distances; and (5) extended paving window. Furthermore, the potential sustainability of such solutions may be further broadened through the partial or full replacement of virgin and/or manufactured materials with recycled, co-product, or waste materials (RCWM), from which the reclaimed asphalt pavement (RAP), recycled concrete aggregate (RCA), recycled asphalt shingles (RAS), air-cooled blast furnace slag (ACBFS), steel furnace slag (SFS), foundry sand, etc., (Van Dam et al., 2016) are examples.

In order to prove quantitatively the theoretical environmental benefits to which the aforementioned technologies are associated with, the most significant environmental inputs and outputs over their life cycle, from raw materials production to the end of the technologies' life, should be assessed. This can be accomplished through life cycle assessment (LCA). LCA is a data-driven, systematic methodology, to investigate, estimate, and evaluate the environmental burdens caused by a material, product, process, or service throughout its life span [START_REF] Matthews | Life cycle assessment: quantitative approaches for decisions that matter[END_REF]. The life cycle begins at the acquisition of raw materials, evolves through several distinct stages (material processing, manufacturing and use), and terminates at the product end-of-life (EOL).

State-of-the-art of LCA studies on WMA technologies

Several research studies have been performed that apply the LCA methodology to measure the potential life cycle environmental impacts of the processes involving the production and placement of the WMA technologies in lieu of conventional HMA. [START_REF] Tatari | Comparative sustainability assessment of warm-mix asphalts: a thermodynamic based hybrid life cycle analysis[END_REF] developed a thermodynamicbased hybrid LCA model to evaluate the environmental impacts from an ecological resource accounting perspective of three types of WMA mixtures and compare them to those of a conventional HMA mixture. The following WMA technologies were assessed: Aspha-Min ® , Sasobit ® , and Evotherm ® WMA. [START_REF] Vidal | Life cycle assessment of hot mix asphalt and zeolite-based warm mix asphalt with reclaimed asphalt pavement[END_REF] performed a comprehensive LCA of road pavements including HMA and zeolite-based WMA, both with and without RAP content. The ReCiPe method was used to assess the environmental impacts according to two sets of impact categories: midpoint and endpoint categories. Additionally, the cumulative energy demand indicator was adopted to compare the mixtures in terms of energy consumption. Mohammad et al. (2014) compared the environmental performance of two WMA technologies, namely foaming and Sasobit ® additive, to that of a conventional HMA mixture, in terms of energy consumption at the asphalt plant and CO and CO 2 emissions monitored during their production and placement. [START_REF] Rodríguez-Alloza | Hybrid input-output life cycle assessment of warm mix asphalt mixtures[END_REF] performed a comprehensive hybrid inputoutput-based LCA of the production of Fischer Tropsch (F-T) wax-based WMA mixtures with and without crumb-rubber modified (CRM) binders. The potential benefits of that WMA technology in relation to a conventional HMA were quantified by accounting for the embodied energy requirement and GHG emissions in the supply chain. [START_REF] Giani | Comparative life cycle assessment of asphalt pavements using reclaimed asphalt, warm mix technology and cold in-place recycling[END_REF] carried out a process-based LCA in collaboration with an Italian asphalt-producing company with the objective of quantifying the potential environmental benefits resulting from constructing asphalt pavement using an unspecified type of WMA with the incorporation of up to 30% of RAP. Almeida-Costa and Benta (2016) quantified the potential benefits of two WMA technologies, Rediset ® and Sasobit ® additives, in relation to a conventional HMA mixture, by assessing the energy consumption and GHG emissions associated with their production. In turn, Yang et al. ( 2017) compared the environmental performance of crumb-rubber modified HMA and Evotherm ® WMA to that of a conventional HMA mixture, expressed in terms of energy consumption and hazard emissions associated with their production.

Aim and purpose of the study

Notwithstanding the merits of the studies listed previously in showing the potential environmental benefits of some WMA technologies, mostly in terms of energy consumption and emissions released during their production, several aspects can be pointed out which underpin the need of expanding the knowledge in this domain: (1) there is still a wide range of other WMA technologies equally worthy of being thoroughly analyzed; (2) the role of the upstream supply chain related to the production of chemical additives used in WMA mixtures is commonly excluded from the system boundaries; (3) the existing studies tend to narrow the system boundaries by focusing on a few life cycle phases, usually the materials extraction, mixtures production and construction phases, and thus excluding phases (i.e., work zone (WZ) traffic management, usage and EOL), which depending on the technical context, might drive the environmental performance of the system being analysed. Moreover, while the consideration of the last point is not methodologically wrong, provided that a given set of conditions are met, it constraints a more global view of the system and thereby opportunities for eventually more meaningful environmental improvements.

Given the issues abovementioned, this research study aims to perform a comprehensive and methodologically sounded pavement LCA of a road pavement section incorporating several WMA technologies (i.e., chemical additives and foamed-based), both with and without RAP content, which covers all the pavement life cycle phases, from raw material acquisition, via production and use phases, to the EOL phase.

The overall purpose is to increase the pavement community stakeholders' capacity to make more strategic and informed decisions regarding the construction and M&R of road pavement that would ultimately enhance the sustainability of pavement systems.

Methodology

A comparative process-based LCA study is performed taking into account the ISO 14040 series (ISO, 2006a;ISO, 2006b) and the Federal Highway Administration's (FHWA's) Pavement LCA Framework [START_REF] Harvey | Pavement Life-Cycle Assessment Framework[END_REF]. It calculates and compares the potential environmental impacts of different asphalt mixtures adopted in the construction and M&R of a road pavement section during its life cycle.

The stages adopted in this study include goal and scope definition, inventory analysis, impact assessment, and interpretation.

Goal and scope definition

Goal

The main goal of this paper is to quantify the potential life cycle environmental impacts of a flexible road pavement section throughout its life cycle. The road pavement section studied involves the use of conventional and low-temperature asphalt mixes, with and without RAP content, in the construction, maintenance and rehabilitation (M&R) of wearing courses of the flexible road pavements.

The comparative findings of this study are intended to be used by highway agencies and pavement practitioners to make more assertive judgments on the pros and contras associated with the use of emerging and commonly called sustainable strategies and practices for road pavement construction and M&R.

System description and boundaries

The system boundaries define the unit processes considered in the LCA study and were drawn to cover the pavement life cycle from a cradle-to-grave perspective, and to enable the performance of a parallel life cycle costs analysis in the near future.

Figure 1 presents the pavement life cycle phases and processes included within the system boundaries of the proposed pavement LCA model as well as their positioning towards relevant literature for the subject. Specifically, the system boundaries entail six pavement life cycle phases, modeled through individual but interconnected modules. They are the following: (1) extraction of materials and mixtures production, consisting of the acquisition and processing of raw materials, and the mixing process of asphalt mixtures in plant; (2) construction and M&R, including all construction and M&R procedures and related construction equipment usage; (3) transportation of materials, accounting for the transportation of materials to and from the construction site and between intermediate facilities (e.g., transportation of aggregates from the quarries to asphalt mixing plants, etc.); (4) WZ traffic management phase. The WZ traffic management costs consist of the additional costs borne by the road users (RUC) when facing a disruption of the normal traffic flow as a consequence of the constraints imposed by a WZ traffic management plan; (5) usage, which addresses the interactions of the pavement with vehicles and environment throughout the project analysis period (PAP); and (6) EOL, which models the destination of the pavement structure after the PAP.

The analysis boundaries for the road pavement were set at the sub-base and at the finished road surface. They include (1) the construction of all layers contained by the limits stated above and subsequent M&R activities; (2) the extraction of the materials needed to produce the mixtures used in those layers; and (3) the movement involved in hauling materials between facilities, between facilities and work site, and vice-versa. Furthermore, given the comparative nature of the study, it might have been decided to constraint the LCA system boundaries to the construction and M&R of the wearing course. However, the whole pavement structure was taken into account in order to enable the acquirement of knowledge on the relative contribution of each pavement life cycle phases to the total life cycle impact scores. As far as the system boundaries for RAP are concerned, it is assumed that prior to its utilization the material is processed via a crushing operation, which reduces the variable RAP fragments to uniform size in order to promote final blend consistency. The environmental burdens resulting from milling or removing the pavement and hauling the debris from the work site to the recycling unit were not included into the system boundaries on the basis of a "cut-off" approach (Schrijvers et al., 2016a), which is the most-widely used procedure to handle the EOL phase in pavements LCAs [START_REF] Aurangzeb | Hybrid life cycle assessment for asphalt mixtures with high RAP content[END_REF]. According to this approach, if a product is recycled at the EOL phase, no impacts for waste management are considered. The impacts of the recycling process are attributed to the second life cycle (Schrijvers et al., 2016b). Thus, the post-processing of the debris materials towards their transformation into usable RAP is only accounted for when considering the production of new mixtures which incorporate RAP into their composition. To accomplish the RAP processing task, a crusher unit located within the asphalt plant facility is considered, which consists of (1) a diesel-powered crusher, (2) a diesel-powered mobile screening plant, (3) an electrically-powered stackable conveyor and (4) a wheel loader.

The upstream emissions and resources consumption associated with the production of the energy sources used to power the different processes, construction equipment, and on-road vehicles were also included in the system boundaries. On the other hand, construction equipment, road-related safety and signaling equipment (including road marking), road accessories (fences, road lighting software, etc.), and the earthworks required to build the platform over which the pavement foundation will be built were not included in the system boundaries. The earthworks were excluded because the potential environmental impacts related to those works are better handled when performing a road LCA, as they are specific to a particular project. This fact makes it unsuitable for the general application of a pavement LCA model as it is intended in this case study.

Various supplementary sub-models that are attached to the corresponding modules, as well as the data required to run those models, are introduced and discussed later in this paper.

Functional unit

The functional unit is the central core of any LCA and forms the basis for comparisons between different systems with the same utility for the same function. In the pavement domain, this means a unit of pavement that can safely and efficiently carry the same traffic over the same project analysis period (PAP). Then, it is defined by their geometry, service life, and levels of traffic supported.

Case study features: traffic, service life, pavement structure and maintenance and rehabilitation strategy

The functional unit of the case study presented in this paper is a typical French highway section of 1-km length, composed of two independent roadways, each with 2 lanes with an individual width of 3.5m. The PAP is 30 years, starting in 2015. The initial two-way average annual daily traffic (AADT) was considered to be equal to 6500 vehicles/day, of which 33% are heavy duty vehicles (HDV) equally divided between rigid HDV and articulated HDV. The structure and composition of the French fleet of vehicles, expressed in terms of type of vehicles and European emissions standards, was that defined by CITEPA (Centre Interprofessionnel Technique d'Études de la Pollution Atmosphérique). The traffic growth rate was set equal to 1.5% per year [START_REF] Jullien | ECOLABEL. Deliverable D1.4: benchmarking exercise to assess the set of proposed indicators in diverse road case studies[END_REF]. The geometric characteristics of the pavement structure adopted in each of the independent roadways are presented in Figure 2. A flexible road pavement structure was selected because this type of pavement represents the overwhelming majority of the total extension of the French highway network.

As for pavement maintenance, a pavement M&R strategy derived from French practice was considered [START_REF] Jullien | Environmental assessment of road construction and maintenance policies using LCA[END_REF][START_REF] Jullien | ECOLABEL. Deliverable D1.4: benchmarking exercise to assess the set of proposed indicators in diverse road case studies[END_REF]. Figure 2 displays the maintenance tasks inherent to each M&R activity as well as the application timing. They were assumed to be the same irrespective of the type of mixture applied in the wearing course. This assumption is supported by research studies showing that HMA and WMA pavements have comparable long-term field performance in terms of structural durability (Washington State University et al., 2017) and the inexistence of solid scientific evidences that functional properties of HMA and WMA pavements will evolve distinctively over time.

Case study features: mixtures composition

In order to understand the potential environmental advantages and disadvantages related to the use, in wearing courses, of low-temperature asphalt mixes with and without RAP content, the reference pavement structure (Figure 2) constituted by layers made of conventional HMA without RAP content was compared to four alternative structures with equal geometry, but in which the wearing course of the initial structure, and subsequent M&R treatments, was made of WMA produced according with two different technologies (i.e., foaming and CECABASE ® additive) to lower the manufacturing temperature, and with and without the adding of RAP. Furthermore, the set of alternative mixtures was completed with the consideration of a conventional HMA with a RAP content equal to 50%, thus rising to 6 the total number of pavement sections to be analysed and compared. Table 1 presents the features of the several mixtures analyzed in the case study, which were investigated in the scope of the SUP&R ITN research project [START_REF] Lo Presti | SUP&R ITN: an international training network on sustainable pavements and railways[END_REF].

Table 1. Features of the bituminous mixtures used in all pavement structures studied.

Data sources and data quality requirements

The inventory data required to perform a LCA study are classified into two categories: primary and secondary data. Primary data are those specific to the production processes for the product or service studied in the LCA. In turn, secondary data represent generic or average data for the product or service subject to analysis. (EC, JRC -IES, 2010).

In this study the data sources were selected in order to be as much time, geographical and technological representative as possible. That means that the most recent and truthful data representing French processes and conditions were used as inputs and outputs when modelling the processes covered by the several sub-components integrating the system boundaries. Specifically, the primary data include mainly: (1) the composition of the mixtures; (2) the annual fuel consumption (FC), production and life period of asphalt mix plants; (3) transportation distances; (3) construction vehicles fleet composition; and (4) on-road vehicles fleet composition;

Regarding the secondary data, they are mainly related to the inventory analysis of (1) raw materials, (2) fuels, and (3) construction, transportation, and on-road vehicles operation, and they were obtained from existing publicly available reports and the ecoinvent database version 3.2, but modified whenever possible and suitable to best approximate French conditions by using French energy inputs/mixes.

Life cycle inventory

The life cycle inventory (LCI) stage consists of the real data collection and modelling of the system. In addition to the data sources, it relies on the several models selected for modelling the processes analyzed by the several considered sub-systems that make up the whole system.

Materials extraction and mixtures production phase

This pavement LCA phase addresses the environmental burdens arising from the acquisition and processing of the materials applied during the initial construction and future M&R of a road pavement segment. This includes all materials manufacturing processes, from extraction of raw materials to their transformation into a pavement input material (material extraction sub-phase), ending with the mixture production at a mixing plant (materials production sub-phase). The latter sub-phase accounts for the environmental burdens associated with the operation of the (1) mixing plant (i.e., dryer, hot screen, mixers, etc.), (2) wheel loader during the movement of aggregates from the stockpiles to the feed bins (3) electronic group of the asphalt plant setup and (4) RAP processing unit so that the RAP ensures the required properties to be incorporated into a new asphalt mixture.

Materials extraction sub-phase

The virgin aggregates required to produce the asphalt mixtures were modelled as gravel and the LCI data associated with their production were obtained from the unit process "gravel, crushed | gravel production, crushed" of the ecoinvent database. Also taken from the aforementioned database was the LCI data corresponding to the asphalt binder production "pitch | petroleum refinery operation". Regarding the bituminous emulsion production, the formulation and the consumption of energy resources defined in Eurobitumen report (Eurobitumen, 2011) were considered and combined with the LCI data associated with the production and transport of the corresponding items existing in the ecoinvent database version 3.2.

In one of the WMA production technologies considered in this study, CECABASE ® was used as a chemical additive. According to the scarce information available, this additive is made up of fatty acids, namely tetraethylenepentamine polyamides (http://www.cladding.com.au/Images/common/stakeholder-relations/asphalt-vic/Deer-Park-MSDS-Ceca-RT945.pdf.). Due to the inexistence in the literature of LCI data referring to its production, the ecoinvent database process "market for fatty acid | fatty acid | cut-off, U" was used as proxy.

As for the RAP processing, the common production rates of the several machines integrating the processing unit were considered when determining the energy requirements. The LCI data related to the production and distribution of those energy resources representing the French conditions were posteriorly taken from the ecoinvent database.

Mixtures production sub-phase

This sub-phase addresses the LCI of the asphalt production process by considering different types of mixes, both with and without the incorporation of RAP in their formulations. In this case study it was assumed that all asphalt mixes were produced through a conventional heavy fuel oil (HFO)-fired batch mix plant. The asphalt plant operation takes into account the infrastructure and machinery used in mixes production. The period of life of the plant was estimated to be about 25 years. This value was obtained by considering the average yearly production (80,000 tons) and the average life time production (2,000,000 tons) of a typical French asphalt plant. The energy required for storing the binder in the asphalt plant, the fuel consumed by the wheel loader and the electricity consumed by the electric group of the asphalt plant were respectively 40 MJ, 0.194 liters and 5 MJ per ton of asphalt mix produced and correspond to the average French practices.

In order to capture the consequences in the energy requirements due to the variations in composition and manufacturing temperature of the several types of mixtures as well as the moisture content and initial temperature of the raw materials, the thermal energy (TE) required to produce the asphalt mixtures was determined through an energy balance represented by Equation ( 1). The values of the parameters in that equation are presented in Table 2 and were based on the literature and conditions commonly found in the real practice.

) 1 ( ) 100 ( ) 100 ( ) ( ) ( 0 0 0 0 0 0 0 CL t C W m W m L t C W m t t C m t t C m TE M i mix vap i i M i i i v M i w i i mix bit bit M i mix i i                                            (1)
Where TE is the thermal energy (MJ/tonne mixture) required to produce the asphalt mixtures, i m is mass of aggregates of fraction i, M is the total number of aggregates fractions, i C is the specific heat capacity coefficient of aggregates of fraction i, mix t is the mixing temperature of an asphalt mixture, o t is the ambient temperature, bit m is the mass of bitumen, bit C is the specific heat capacity coefficient of bitumen, Wi is the water content of aggregates of fraction i, w C is the specific heat capacity coefficient of water, v L is the latent heat required to evaporate water, vap C is the specific heat capacity coefficient of water vapor, CL is the casing losses factor. Casing losses are thermal energy used to heat plant iron (for example, the shell of the drum) and then radiated to the atmosphere, rather than being used to heat the mixture components (West et al., 2014). This factor was considered to be same for all mixtures studied and was calculated as the difference between the average value of the energy required to produce a conventional HMA in France and that calculated according to the energy balance described above. That means that the unexplained differences in the calculated energy are attributed to casing losses. In order to determine the process air emissions resulting from the mixes production, a methodology based on that developed by [START_REF] Santos | Environmental and economic assessment of pavement construction and management practices for enhancing pavement sustainability[END_REF] was applied. Firstly, the LCI data corresponding to the process "heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U" existing in the ecoinvent database was taken as reference when modeling the operation of the burner existing in the asphalt plant during the production of a conventional HMA with 0% RAP. Secondly, for the remaining mixes an emission factor (EF) multiplier was determined through the ratio between the thermal energy computed with Equation (1) for those mixes and that for the reference mix. Finally, the output flows expressed in terms of the GHG and the most common airborne substances released during mixes production were derived by multiplying the outputs flows taken as reference by the EF multipliers. The values of the EF multipliers as well as the thermal energy required to produce 1 ton of the mixtures studied are shown in Table 3. The fuel savings presented in this table are consistent with those commonly reported in the literature for both European [START_REF] D'angelo | Warm-mix Asphalt: European Practice[END_REF] and US (Mohammad et al., 2014) practices, according to which the reduction of the fuel consumption might amount to 11-35% and 12-14%, respectively, the latter expressed in terms of fuel costs. These ranges of values support the validity of the thermal energy model developed.

Table 3. Thermal energy (TE) required to produce 1 tonne of the mixtures studied and respective emission factor (EF) multipliers.

Construction and M&R phase

In the construction and M&R phase, the environmental burdens are due to the combustion-related emissions from construction machinery usage. Environmental impacts resulting from traffic congestion occurring during M&R interventions are dealt with in the WZ traffic management phase. The consumption-related emissions associated with the operation of each construction equipment were determined by combining the LCI data corresponding to the ecoinvent database process "machine operation, diesel, >= 74.57 kW, high load factor | machine operation, diesel, >= 74.57 kW, high load factor" with the typical productivity of each operation involved in pavement construction and maintenance activities.

In this section it is worth mentioning that the operating conditions of paving machines were considered the same regardless of the type of asphalt mix considered, i.e. HMA or WMA (with and without). Theoretically, a reduction in the number of roller passes needed to achieve a specified density is expected when WMA is used (Rubio et al., 2012;[START_REF] Zaumanis | Warm mix asphalt[END_REF]. However, there is no accurate and consistent scientific knowledge in the literature on the close relation between the reduction of the compactive effort required, in terms of roller passes, and the enhancement of WMA workability.

Transportation of materials phase

The environmental impacts resulting from the transportation of materials are due to the emissions released during the combustion process of the transportation vehicles when performing two-way trips. All materials and mixtures were assumed to be hauled by HDV, and a modified version of the ecoinvent database process "transport, freight, lorry >32 metric ton, EURO4 | transport, freight, lorry >32 metric ton, EURO4 | cut-off, U" was used to determine the environmental burdens associated with the transportation of materials movements. The original process was modified in order to disregard the inventory corresponding to the construction of the road infrastructure that is considered by the original process. The transportation distances considered for each material and mixture used in this case study are representative of the French conditions and are shown in Table 4.

Table 4. Transportation distances considered in the case study.

Work-zone traffic management phase

This pavement life cycle phase accounts for the marginal FC and emissions released by on-road vehicles due to traffic perturbations caused by M&R events in relation to those during normal road operation. The procedure adopted to calculate the environmental burdens arising during this phase was based on the two-step methodology developed by Santos et al. (2015aSantos et al. ( , 2015b)).

First, the COPERTv5.0 air pollutants and GHG emissions model (EMISIA, 2017) was run multiple times, each considering a different speed in the "Highway" driving mode, to compute a set of FC and emissions factors representing the French vehicle fleet characteristics per type of vehicle. Next, the FC and vehicle emissions calculated for the several discrete speed values are used to derive equations that allow to determine FC and emissions factors representative of the French vehicle fleet as a continuous function of the speed.

Secondly, changes in driving patterns were modelled using the capacity and delay models proposed by the HCM 2000 (TRB, 2000) to determine several outputs, such as the number of vehicles that traversed the WZ, the average queue length, the average queue speed in each hour, etc. Each section where there is a change in driving pattern was considered to be a new road "link". The characteristics of each link (length, number of vehicles and average speed) was combined with the equations previously determined to derive the environmental load of a WZ hour of a given M&R activity. Finally, the marginal FC and airborne emissions due to the WZ traffic management plan were calculated by subtracting FC and airborne emissions released during a WZ period from the results of an equivalent non-WZ period.

Usage phase

The usage phase addresses the pavement's environmental burden resulting from the interaction of the pavement with the vehicles, environment and humans throughout its PAP. Among the factors that have been identified in past research as being worthy of consideration during the usage phase of the pavement (i.e., pavement-vehicle interaction (PVI), traffic flow, albedo, leachate and runoff, carbonation and lighting) only the contribution from the PVI, namely that due to the pavement surface properties (i.e., macrotexture and pavement roughness), was taken into account in this analysis. The rationale for this decision lies in the fact that the remaining components either do not apply to the features of the case study under evaluation or lack well established and consistent scientific background. In order to determine the influence of the pavement surface properties on vehicle FC and tailpipe emissions, the Swedish National Road and Transport Research Institute (VTI)'s rolling resistance (RR) model [START_REF] Hammarström | A rolling resistance model, a fuel consumption model and the traffic energy saving potential from changed road surface condition[END_REF], developed within the European project MIRIAM (Models for rolling resistance In Road Infrastructure Asset Management systems), was combined with data from the COPERTv5.0 emissions model according to the two-step methodology proposed by Santos et al. (2015a).

In the first step, the VTI's RR model was used to calculate the additional FC due to the vehicles travelling over the rough pavement surface when compared to the FC of the vehicles travelling over a smooth surface. Then, instead of using the actual AADT in the COPERTv5.0 emissions model, an effective AADT (AADT E ) was used to relate the effect of pavement surface properties on the FC and emissions. The AADT E for a given macrotexture and roughness at time t, expressed in terms of the mean profile depth (MPD) and international roughness index (IRI), respectively, was calculated using Equation (2).

smooth t MPD t IRI i Veh N i i E i FC FC Veh t AADT t AADT ) ( ), ( _ 0 ) ( ) (      (2)
Where AADT is the annual average daily traffic value, N_Veh is the number of types of vehicles (in this case study it is equal to three, corresponding to passenger cars, rigid HDV and articulated HDV), i Veh is the percentage of vehicles of type i in the AADT,

) ( ), ( t MPD t IRI i FC
is the FC for the type of vehicle i travelling on a pavement with a specified IRI and MPD at time t, and

smooth i FC
is the FC of the same type of vehicle i travelling along a typical smooth pavement.

Estimating the influence of RR on FC and tailpipe emissions requires the prediction of the progression of the pavement surface properties over the PAP. In this case study, the pavement performance prediction model of the flexible pavement design method developed by AASHTO (1993) was adopted to predict the quality of the pavement over time, expressed in terms of PSI (Equation ( 3)). This model was posteriorly combined with the expression proposed by [START_REF] Al-Omari | Relationships between international roughness index and present serviceability rating[END_REF] to convert the PSI into IRI (Equation ( 4)). From the conceptual point of view, such conversion does not seem to represent an obstacle, as roughness is widely recognized as the main contributor to PSI. In turn, the model proposed by [START_REF] Lorino | Pavement surface characteristics evolution[END_REF] was adopted to predict the evolution of the macrotexture over the PAP (Equation ( 5)).

 

                                      5.
+ SN M + SN S Z W t t R t 0 R t PSI PSI (3)          5 24 . 0 1 t t PSI Ln IRI (4) ) ln( 168 . 0 986 . 0 age MPD t    (5)
Where PSIt is the Present Serviceability Index in year t, PSI 0 is the Present Serviceability Index of a pavement immediately after construction (year 0), t W 80 is the number of 80 kN equivalent single axle load (ESAL) applications in year t (million ESAL/lane), ZR is the standard normal deviate, S 0 is the combined standard error of the traffic prediction and performance prediction, SNt is the structural number of a pavement structure in year t, MR is the sub-grade resilient modulus (pounds per square inch), t IRI is the International Roughness Index (m/km), t MPD is the mean profile depth (mm), age is the age of the surface course (years). In this case study the following parameters values were considered: a ZR value of -1.282, a S 0 value of 0.45 and a SN0 value of 5.13.

End-of-life phase

When a road pavement reaches the end of the PAP, it can be given two main destinations: (1) remain in place, or; (2) be removed. In this case study, it is assumed that the pavement remains in place and undergoes the M&R activity illustrated in Figure 2. The environmental burdens assigned to this phase are due to the materials extraction and mixtures production and the combustion-related emissions from the use of the construction equipment and transportation HDV. The environmental impacts resulting from the traffic disruption occurring during the EOL intervention are dealt with in the WZ traffic management phase.

Life Cycle Impact assessment

The life cycle impact assessment (LCIA) stage of the standardized LCA methodology comprises several steps, namely, classification, characterization, normalization, group and weighting (ISO, 2006a). Among these steps, only classification and characterization were undertaken in this study. The normalization, group and weighting are optional, and while they might be useful in translating the impact scores of different impact categories into a more understandable and somehow digestible form [START_REF] Dahlbo | Comparison of different normalised LCIA results and their feasibility in communication[END_REF], they also entail a risk of oversimplifying the results. Furthermore, in accordance with ISO 14040 series (ISO, 2006a;ISO, 2006b) no form of numerical, value-based weighting of the indicator results is permitted to be published, if the study is intended to support a comparative assessment to be disclosed to the public.

The calculation of the impact category indicator scores was performed at midpoint level by applying mainly the LCIA method CML 2001(Guinée et al., 2002). This method was selected because it is the primary base for the construction of the French environmental standards on building construction materials, the NF P 01-010 standard (AFNOR & French Standardisation Agency, 2004). Specifically, the following impact categories were considered: Climate Change (CC), Acidification (AC), Eutrophication (EU), Human toxicity (HT), Terrestrial ecotoxicity (TE), Photochemical oxidation (PO), Stratospheric ozone layer depletion (SOD), Abiotic resources depletion (ARD). Complementarily, the ReCiPe method's impact categories Particulate matter formation (PM) and Water depletion (WD) were also considered [START_REF] Goedkoop | ReCiPe 2008-a life cycle impact assessment method which comprises harmonized category indicators at the midpoint and the endpoint level[END_REF]. A time horizon of 100 years was considered for all impacts categories.

In addition, an energy analysis was carried out based on the cumulative energy demand (CED) indicator, computed according to [START_REF] Frischknecht | Implementation of Life Cycle Impact Assessment Methods[END_REF].

Finally, the OpenLCA software version 1.5.0 was used for modelling the processes analyzed in this case study [START_REF] Greendelta | Open LCA[END_REF].

Results and discussion

Baseline scenario

Total pavement life cycle impacts assessment results

The comparison between the environmental and energy indicators scores of the six pavement construction and M&R alternatives are illustrated in Figure 3. Table 1 in supplementary materials present their values. Based on LCIA results, one can say that a pavement construction and M&R scenario in which the mixture Foamed WMA, 50%RAP is employed in the wearing course throughout the pavement life cycle is the most environmentally friendly alternative among all the competing solutions, as it was found to present the best environmental performance in all environmental and energy indicators. For instance, for CC its overall impact score totals 1178377 kg CO 2 -eq, for AC 9630 SO 2eq, for HT 600713 kg 1.4-DCB-eq, for PO 543 kg ethylene-eq, and for ARD 27452 kg antimony-eq. These values mean a reduction of 6%, 7%, 5%, 8% and 9%, respectively, in relation to the impact scores associated with the use of the mixture HMA, 0%RAP. Indeed, the use of the mixture HMA, 0%RAP in the wearing course of a pavement throughout its life cycle is, in general, the most environmentally damaging alternative among all the competing solutions, as it was found to exhibit the worst environmental performance in 9 of the 12 environmental and energy indicators. The exceptions to the "12 of the 12" were observed for the impact categories TE, WD and renewable CED in which the alternative WMA-CECABASE ® , 0% RAP denoted the most expressive scores (i.e. 2035 kg 1.4-DCB-eq, 4123 m 3 and 625501 MJ), followed by its recycling-based counterpart, i.e. the mixture WMA-CECABASE ® , 50% RAP with impact scores equal to 2016 kg 1.4-DCB-eq, 3936 m 3 and 591512 MJ, respectively. Such results, which in the case of the mixture WMA-CECABASE ® , 0% RAP represent an increase of 233%, 70% and 19% relatively to those of the mixture HMA, 0%RAP, are explained by the process "market for fatty acid | fatty acid | cut-off, S -GLO", which with contributions greater than 71% and 41%, respectively for the impact categories TE and WD, was found to be main driver of such categories. Also, it was the responsible for the notorious consumption of biomass energy that is the root cause of the greater consumption of renewable energy. Similar contributions were found in the case of its recycling-based counterpart (i.e., the mixture WMA-CECABASE ® , 50% RAP).

On comparing pavement construction and M&R alternatives employing in the wearing course HMA or WMA mixtures without the addition of RAP, it was observed that the overall life cycle impacts of the alternatives using WMA mixtures were, in general, reduced by no more than 2% (WMA-CECABASE ® , 0% RAP) and 3% (Foamed WMA, 0%RAP). These tiny benefits agree well with the findings reported by [START_REF] Vidal | Life cycle assessment of hot mix asphalt and zeolite-based warm mix asphalt with reclaimed asphalt pavement[END_REF], although the type of WMA analysed in that study was different from those considered in the present study.

In addition, when comparing mixtures with a null content of RAP with their recycling counterpart (i.e. 50% RAP), it was observed that the life cycle environmental benefits mentioned above rose to values that can be as high as 9%. That is the case of the mixtures HMA, 50%RAP and WMA-CECABASE ® , 50% RAP for the impact categories ARD and SOD. Indeed, for those impact categories all the mixtures that do not contain RAP and those that contain RAP denote similar scores. In turn, the lowest life cycle environmental benefits are registered by the mixtures WMA-CECABASE ® , 50% RAP and Foamed WMA, 50%RAP for the impact category TE (1% and 2%, respectively).

(a) (b) 1,12E+06 1,14E+06 1,16E+06 1,18E+06 1,20E+06 1,22E+06 1,24E+06 1,26E+06 1,28E+06 1

Process contribution analysis

Figure 4 displays the contribution of the processes to the impact categories considered for all alternatives being compared. Overall, the environmental impacts are driven predominantly by the processes bitumen production and asphalt mixtures manufacturing. This general pattern is observed not only for all impact categories but also across all the alternatives. The share of the process bitumen production can be as high as 83% (SOD score for the alternatives WMA-CECABASE ® , 0% RAP and Foamed WMA, 0%RAP), whereas the highest contribution given by the asphalt mixtures manufacturing (around 57%) is observed in the impact category TE for the alternative conventional HMA, 0%RAP. The market for asphalt mixing plant and gravel production are other processes whose contributions cannot be neglected, although their maximum share does not go beyond 35% (HT score for the alternative conventional HMA, 0%RAP) and 25% (WD score for the alternative Foamed WMA, 0%RAP), respectively. In turn, the contributions of the transportation of materials and construction machinery operation are relatively reduced, under 7% (EU score for the alternatives conventional HMA, 0%RAP, WMA-CECABASE ® , 0% RAP, Foamed WMA, 0% RAP and WMA-CECABASE ® , 50% RAP) and 8% (EU score for the alternative WMA-CECABASE ® , 50% RAP), respectively.

Finally, it is worth mentioning that the length of the bars corresponding to the mixtures WMA-CECABASE ® , 0% RAP and WMA-CECABASE ® , 50% RAP for the impact categories TE and WD prove what was said in the previous section regarding the preponderance of the contribution given by the process "market for fatty acid | fatty acid | cut-off, S -GLO" for the scores of those impact categories.

Pavement life cycle phase contribution analysis

Figure 5 presents the contribution of each of the pavement life cycle phases to the 12 impact and energy indicators considered and for each of the pavement construction and M&R alternatives studied. The environmental performance of all alternatives for almost all impact and energy indicators are mainly driven by the phases related to the construction of the initial pavement structure, which comprises the materials extraction and production, the construction machinery operation and the transportation of the materials and mixtures. Its share varies between 29% and 57% for the indicator TE. The exception to the construction phase's dominance is observed in the TE indicator for the alternatives WMA-CECABASE ® , 0% RAP and WMA-CECABASE ® , 50% RAP, where the maintenance phase is the main contributor. The root cause of this outcome is the preponderance of the process "market for fatty acid | fatty acid | cut-off, S -GLO" in driving the environmental burdens of those alternatives with respect to the impact categories TE and WD, along with the fact that the total mass of those mixtures applied in all maintenance activities performed throughout the pavement life cycle is greater than that employed in the construction of the initial pavement structure. If the TE and WD indicators were not taken into account, then the construction phase would always be the main contributor for the remaining indicators with a minimum share not inferior to 49%, followed by the maintenance and EOL phases.

Another noteworthy result emerging from the analysis of Figure 5 pertains to the almost consensual residual contribution given by the WZ traffic management and usage phases to all impact and energy indicators. Furthermore, in the usage phase the relative contributions are not only approximately 0%, but also negative, meaning that this phase plays a beneficial role for the environment. In the case of the WZ traffic management phase such outcome is entirely explained by the reduced traffic value carried by the road pavement throughout its life cycle, which is not enough to originate congestion when an M&R intervention is performed. As far as the usage phase is concerned, in addition to the low traffic volume, the explanation also lays on the way the pavement roughness and macrotexture impact RR and how such pavement surface properties evolve over time. In the VTI's RR model, while an increase of pavement roughness and macrotexture leads to an increase of the RR, and thus the vehicle's fuel consumption, the absolute effect of macrotexture is, in general, greater than that of roughness [START_REF] Bryce | Analysis of rolling resistance models to analyze vehicle fuel consumption as a function of pavement properties[END_REF]. Given that in this case study the pavement macrotexture is expected to decrease over time according to Lorino et al.'s model, its effect offset that of the roughness, notwithstanding the fact that the value of latter property is expected to increase throughout the pavement life cycle. Therefore, due to the combined effects of the aforementioned surface properties on vehicle's fuel economy and their evolution over the PAP, the usage phase turns out to be environmentally advantageous.

Scenario analysis

In order to assess the robustness of the results presented in the previous sub-section to changes in the methodological assumptions, a scenario analysis was performed by considering two alternative scenarios. In the first one, hereafter named AS1, it was assumed that the asphalt plant is run by natural gas rather than by HFO. This is a plausible scenario that stands the best chance of becoming the actual and near-term future practice, thereby replacing the baseline scenario, as new asphalt plants are increasingly switching to natural gas because of its general lower price and cleaner-burning properties. In the second one, hereafter named AS2, it was assumed that the road pavement structure is dismantled at the EOL and the debris transported to an asphalt plant where they will undergo the required processing operations to be considered as RAP. The allocation of the environmental impacts was made in accordance with the considerations underlying to the "cut-off" method described in subsection 2.1.2. The reasoning for the consideration of this alternative lays on the fact that it is commonly considered in the literature as the fate to be given to a road pavement when it reaches the end of its lifetime [START_REF] Van Dam | Towards sustainable pavement systems-a reference document[END_REF]. Another reason backing the choice of the alternative scenarios pertains to the fact that they evolve directly the pavement life cycle phases driving the global environmental performance of the system being analysed.

Figures 5 displays for each alternative the relative variation of the LCIA results in relation to the baseline scenario. They should be understood as follows: positive relative numbers mean that the alternative scenarios improve the LCIA results in relation to those associated with the baseline scenario while negative numbers represent a worsening of the environmental profile. The results show an almost unanimous reduction in all impacts and energy indicators for both alternative scenarios regardless the pavement construction and M&R alternative studied. Overall, AS2 leads to a uniform reduction of the scores (13-18%) of all impact categories across all alternatives being compared. In turn, the consequences resulting from considering the AS1 depend to a great extent on the impact category. The most expressive improvements in the environmental profiles are observed in the TE indicator (57%) for the alternative HMA, 50%RAP, while the most feeble improvements are observed in the ARD indicator (slightly lower 1%) for the alternative Foamed WMA, 0%RAP.

Notwithstanding the overall environmental benefits that can be achieved with both alternative scenarios, there is, however, an exception. It was observed in the alternative scenario AS1 for the energy indicator R-CED. This scenario increases the aforementioned indicator by 2-3% across the six alternatives, though it leads to a reduction of the total CED. The latter result is far more important than the consideration of the former isolatedly, as its explanation lays on the reduction of the NR-CED.

When comparing the extent to which the alternative scenarios affects the variation of the scores of a given impact category across the alternatives being compares, Figure 6 shows that AS1 originates environmental profiles that are slightly more prone to vary in relation to the baseline scenario, than those associated with the AS2. Also, among the six alternatives the greatest reductions in the environmental impacts scores are obtained for the alternative HMA, 50%RAP. Moreover, the environmental performances of the alternatives incorporating RAP seem to be less susceptible to variations when the AS2 is assumed to be implemented comparatively to the scenario in which the pavement structure is supposed to be dismantled at the end of its lifetime (AS2).

Finally, the results of the scenario analysis indicate that shifting the type of fuel consumed by asphalt plants from HFO to natural gas offers considerable potential environmental benefits and a more trustworthy option, as it refers to a decision made in the present with immediate consequences, whereas those associated with the AS2 depend on a decision which will be made far in the future, and thereby entails a great deal of uncertainty.

Summary and conclusions

In this paper, the results of a comprehensive process-based LCA of French road pavement sections incorporating alternatively two different WMA technologies, with and without RAP content, were presented and compared with those in which conventional HMA mixtures, also with and without RAP content, were adopted. The life cycle of the road pavement sections was divided into six main phases:

(1) materials extraction and mixtures production; (2) construction and M&R; (3) transportation of materials; (4) WZ traffic management; (5) usage; and (6) EOL. The LCI of inputs and outputs associated with the processes considered by the several pavement life cycle phases was determined by combining primary data, representing the current French practices and conditions, with secondary data taken primarily from the ecoinvent database version 3.2 and complemented with publicly available reports. Complementary, an energy balance model was developed to account for the energy required to produce the several mixtures studied, as well as the airborne emissions released during their production, as a function of the mixing temperature and the features of the mixtures components. The CML 2001 impact assessment method along with the CED indicator were adopted to characterize the environmental performance of the following asphalt mixtures: (1) conventional HMA, 0% RAP; (2) WMA-CECABASE ® , 0% RAP; (3) Foamed WMA, 0% RAP; (4) HMA, 50% RAP; (5) WMA-CECABASE ® , 50% RAP; and (6) Foamed WMA, 50% RAP.

The LCIA results of the case study showed that, for the mixtures studied and assumptions performed, the pavement construction and M&R scenario in which the mixture Foamed WMA, 50% RAP is employed in the wearing course throughout the pavement life cycle is the most environmentally friendly alternative among all the competing solutions. Regarding the results referring to the relative contribution of the several processes included in the systems boundaries, it was shown that, overall, the production of the bitumen is the main responsible for the life cycle environmental burdens of the systems studied. Moreover, when comparing the contribution given by the several pavement life cycle phases, the materials extraction and mixtures production was found to be the phase that plays the most important role in driving the life cycle environmental performance of the road pavement sections analysed. Finally, the results of a scenario analysis showed that the life cycle environmental impacts could be reduced if the asphalt plant was fueled by natural gas, or if the pavement structure was dismantled at the end of its lifetime and the debris recycled.

Acknowledgements and disclaimer

The research presented in this paper was carried out as part of the Marie Curie Initial Training Network (ITN) action, FP7-PEOPLE-2013-ITN. This project has received funding from the European Union's Seventh Framework Program for research, technological development and demonstration under grant agreement number 607524.

The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement. Moreover, this paper does not constitute a standard, specification, or regulation.

Figure 1 .

 1 Figure 1. Pavement life cycle phases and processes included in the system boundaries and their positioning towards relevant literature for the subject. (Acronyms: LCA-life cycle assessment; LCC-life cycle costs; M&R-maintenance and rehabilitation; WZ-work zone)

Figure 2 .

 2 Figure 2. Geometric characteristics of the flexible pavement structure and M&R strategy. (Acronyms: BBGA-bituminous bound graded aggregate; HMAC-hot mix asphalt concrete; STAC-super thin asphalt concrete; AC-asphalt concrete)

  Photochemical oxidation

Figure 4 .

 4 Figure 4. Contribution analysis by process: a) climate change; b) acidification; c) eutrophication; d) human toxicity; e) terrestrial ecotoxicity; f) photochemical oxidation; g) stratospheric ozone depletion; h) abiotic resources depletion; i)-particulate matter; j) water depletion. Key: Alternative 1: HMA, 0%RAP; Alternative 2: WMA-CECABASE ® , 0%RAP; Alternative 3: Foamed WMA, 0%RAP; Alternative 4: HMA, 50%RAP; Alternative 5: WMA-CECABASE ® , 50%RAP; Alternative 6: Foamed WMA, 50%RAP.

  market for concrete mixing factory | concrete mixing factory | cut-off, S -GLO machine operation, diesel, >= 74.57 kW, high load factor | machine operation, diesel, >= 74.57 kW, high load factor | cutoff, S -GLO transport, freight, lorry >32 metric ton, EURO4 | transport, freight, lorry >32 metric ton, EURO4 | cut-off, U (excluding road construction) -RER petroleum and gas production, on-shore | petroleum | cut-off, U -RoW market for fatty acid | fatty acid | cut-off, S -GLO market for electricity, medium voltage | electricity, medium voltage | cut-off,

Figure 6 .

 6 Photochemical oxidation; SOD-Stratospheric ozone layer depletion; ARD-Abiotic resources depletion; PM-Particulate matter formation; WD-Water depletion; NR-CED-Non-renewable cumulative energy demand; R-CED-Renewable cumulative energy demand.

References Road Pavement System (Foreground System) Background System

					Life Cycle Phases
	EN 15804:2012+A1	Product	Construction		Use
	FHWA's							End-of-Life
	Pavement LCA Framework Harvey et al. (2016)	Material Production	Design	Construction	Maintenance/ Preservation	-Air Emissions to:
		Transportation of Materials and Mixtures			Use	Transportation of Materials	-Water -Soil
	Inputs from
	Nature	Materials Extraction	Mixtures Production	Construction and M&R		WZ Traffic Management	End-of-Life
	Pavement	Agency		Agency		Agency and Road Users	Road Users	Agency and Road Users	Waste
	LCC-type LCA
	Santos et al.
	(2017)	Ø Pavement components manufacturing (i.e. raw materials, asphalt mixtures, etc.)	Ø Pavement components (i.e. § Construction equipment operation disposal/recycling wearing course, etc.) manufacturing/ installation and maintenance § Construction equipment operation Ø Pavement components removal &	Ø Traffic perturbation of on-road vehicles: § Going through § Detouring WZ	Ø Pavement vehicle interaction (PVI): § Rolling resistance	Ø Remain in place (or) Ø Pavement structure removal & disposal/ recycling § Construction equipment operation
					Fuel and Electricity Production

Table 2 . Values of the parameters considered in Equation (1).

 2

	Name

  0%RAP production -heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U -Europe without Switzerland foamed WMA, 0%RAP production-heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U -Europe without Switzerland HMA, 50%RAP production -heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U -Europe without Switzerland WMA-CECABASE, 50%RAP production -heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U -Europe without Switzerland foamed WMA, 50%RAP production-heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other than natural gas | cut-off, U -Europe without Switzerland gravel production, crushed | gravel, crushed | cut-off, S -CH

		6						6
		5						5
		4						4
	Alt.	2 3					Alt.	2 3
		1						1
		0%	20%	40%	60%	80%	100%	0%	20%	40%	60%	80%	100%
		petroleum refinery operation | pitch | cut-off, S -Europe without Switzerland
		conventional HMA production-heat production, heavy fuel oil, at industrial furnace 1MW | heat, district or industrial, other
		than natural gas | cut-off, S -Europe without Switzerland WMA-CECABASE,

renewable cumulative energy demand Figure 5. Contribution analysis by pavement life cycle phase: a) climate change; b) acidification; c) eutrophication; d) human toxicity; e) terrestrial ecotoxicity; f) photochemical oxidation; g) stratospheric ozone depletion; h) abiotic resources depletion; i) particulate matter; j) water depletion; k) renewable cumulative energy demand; l) non-renewable cumulative energy demand. Key: Alternative 1: HMA, 0%RAP; Alternative 2: WMA-CECABASE ® , 0%RAP; Alternative 3: Foamed WMA, 0%RAP; Alternative 4: HMA, 50%RAP; Alternative 5: WMA-CECABASE ® , 50%RAP; Alternative 6: Foamed WMA, 50%RAP.

			Terrestrial ecotoxicity				Photochemical oxidation
	100%	Construction Maintenance WZ Usage EOL	100%	Construction Maintenance WZ Usage EOL
	80%							80%
	60%							60%
	40%							40%
	20%							20%
	0%	1	2	3 Alternatives	4	5	6	0% -20%	1	2	3 Alternatives	4	5	6
				(e)							(f)
			Stratospheric ozone depletion				Abiotic resources depletion
	100%		Construction Maintenance WZ Usage EOL	100%	Construction Maintenance WZ Usage EOL
	80%							80%
	60%							60%
	40%							40%
	20%							20%
	0%							0%
		1	2	3	4	5	6		1	2	3	4	5	6
	-20%							-20%
				Alternatives						Alternatives
				(g)							(h)
			Climate change Particulate matter						Acidification Water depletion
	100% 100%	Construction Maintenance WZ Usage EOL Construction Maintenance WZ Usage EOL	100% 100%	Construction Maintenance WZ Usage EOL Construction Maintenance WZ Usage EOL
	80% 80%							80% 80%
	60% 60%							60% 60%
	40% 40%							40% 40%
	20%							20%
	20%							20%
	0%							0%
	0%	1	2	3	4	5	6	0%	1	2	3	4	5	6
	-20%	1	2	3	4	5	6	-20%	1	2	3	4	5	6
	-20%			Alternatives				-20%			Alternatives
				Alternatives (i)							Alternatives (j)
			Renewable cumulative energy demand		Non-
	100% 80% 100%	Eutrophication Construction Maintenance WZ Usage EOL Construction Maintenance WZ Usage EOL	80% 100% 100%	Human toxicity Construction Maintenance WZ Usage EOL
	80% 60%							80% 60%
	60% 40%							60% 40%
	40% 20%							40% 20%
	20%							20%
	0%							0%
	0%	1	2	3	4	5	6	0%	1	2	3	4	5	6
	-20%	1	2	3	4	5	6	-20%	1	2	3	4	5	6
	-20%			Alternatives			-20%			Alternatives
				Alternatives (k)							Alternatives (l)

Construction Maintenance WZ Usage EOL


Farid Miah

Emmanuel De-Luycker

Frederic Lachaud

Yann Landon

Robert Piquet

EFFECT OF DIFFERENT CUTTING DEPTHS TO THE CUTTING FORCES AND MACHINING QUALITY OF CFRP PARTS IN ORTHOGONAL CUTTING ─ A NUMERICAL AND EXPERIMENTAL COMPARISON

The necessity of understanding the influence of cutting variables in orthogonal cutting of Carbon Fiber Reinforced Polymer (CFRP) is vital because of their significant influences to the quality of manufactured parts. In this present research work the influences of different cutting depths to the cutting and thrust forces have been analyzed and a comparison between an equivalent homogeneous material (EHM) macro-model and experimental results have been made. The reasons of the beginning high cutting and thrust forces have been studied and explained. The post analysis of the experimental machined surfaces has been done to analyze the generated surface roughness and fiber-matrix interface crack generation. Five different cutting depths and four individual fiber orientations have been tested both numerically and experimentally. Significant influence of cutting depths to the cutting force has been found and the surface quality of newly generated machined part is discovered as a function of cutting depth and fiber orientation.

INTRODUCTION

Because of the high mechanical strength with respect to the density, good resistance to corrosion and to fatigue, composite materials can play an important role to the aeronautic and automotive structures. Nowadays, the extension use of composite structures in aeronautics and automotive is raising many complications in the manufacturing processes, including orthogonal cutting process as these materials contain multiple phases. Orthogonal cutting is a type of material cutting technique in which the cutting tool generates a plane surface which is parallel to the original planar surface of the machined material [START_REF] Merchant | Mechanics of the metal cutting process. i. orthogonal cutting and type 2 chip[END_REF]. This surface is machined with a cutting edge perpendicular to the direction of the relative displacement between the tool and the workpiece.

Recently the research activities in orthogonal cutting of composite laminates have been increased indeed the numerical modeling of composite cutting is still poorly developed [START_REF] Cantero | 2D and 3D Approaches to Simulation of Metal and Composite Cutting[END_REF]]. According to [START_REF] Lasri | Modelling of Chip Separation in Machining Unidirectional FRP Composites by Stiffness Degradation Concept[END_REF], the machining of fiber reinforced polymer (FRP) materials differs from machining conventional metals and their alloys due to the heterogeneity and anisotropy of FRP materials. [START_REF] Liu | A Review of Mechanical Drilling for Composite Laminates[END_REF] noted that composite laminates are regarded as hard-tomachine materials, because of low machining efficiency and undesirable machining-induced delamination. But the simple condition of orthogonal machining in studying chip formation permits to gather knowledge about different components like chip shape and size, shear stress and strain in the chip, friction conditions, cutting forces, cutting temperatures, etc. [START_REF] Olga | Characterization of Friction Properties at the Workmaterial/cutting Tool Interface during the Machining of Randomly Structured Carbon Fibers Reinforced Polymer with Carbide Tools under Dry Conditions[END_REF].

Regarding the orthogonal cutting of Carbon Fiber Reinforced Polymer (CFRP) many answers in literature are limited, especially concerning how different cutting depths affect the cutting and thrust forces and the machining quality of manufactured parts. On the subject of cutting depth influence [START_REF] Zitoune | Experiment-calculation Comparison of the Cutting Conditions Representative of the Long Fiber Composite Drilling Phase[END_REF] analyzed the cutting forces using 0.05 to 0.25mm cutting depth and 0°, 45°, 90° and 135° fiber orientation of unidirectional T2H/EH25 materials . In their numerical model only 0° fiber orientation has been used. However, their research does not talk about the thrust forces and the effect of a cutting depth greater than 0.25mm. [START_REF] Florent | Etude de la coupe en perçage par le biais d'essais élémentaires en coupe orthogonale : application aux composites carbone-époxy[END_REF] analyzed the resultant force of F c (cutting force) and F t (thrust force) and the machined surface quality with respect to the cutting speed from quasi-static to 120 m/min. He used four different fiber orientations but kept the cutting depth constant throughout the whole research. [START_REF] Wang | An Experimental Investigation into the Orthogonal Cutting of Unidirectional Fibre Reinforced Plastics[END_REF] focused on the machining forces with respect to the rake angle and fiber orientation but in their research the maximum cutting depth used was 0.25 mm which does not disclose the influence of higher cutting depth. Several analytical models [START_REF] Bhatnagar | ON THE MACHINING OF FIBER-REINFORCED PLASTIC (FRP) COMPOSITE LAMINATES[END_REF][START_REF] Zhang | A Force Prediction Model for Cutting Unidirectional Fibre-Reinforced Plastics[END_REF]] [Jahromi and Bahr, 2010] have been found to predict the cutting forces but the concern of cutting depth's affect is missing. The numerical models mentioned here to predict the cutting and thrust forces mainly used an equivalent homogeneous material (EHM) which, basically, deviate the numerical result from experimentation. [START_REF] Nayak | Machining Studies of Ud-Frp Composites Part 2: Finite Element Analysis[END_REF] tried to amend it by modeling the fiber and matrix separately without taking matrix damage in calculating the cutting forces. It was found that both the cutting and thrust forces matched quite well with the experimental results.

In this present research work a comparison between numerical (Finite Element Analysis) and experimental research have been carried out on the influences of different cutting depth to the cutting and thrust forces, surface roughness, subsurface damage and fiber-matrix interface debonding. Five different cutting depths (0.1mm, 0.25mm, 0.50mm, 0.75mm and 1.0 mm) and four different fiber orientations (0°, 45°, 90° and 135°) have been used in orthogonal cutting of CFRP both numerically and experimentally. Each cutting depth has been tested with four fiber orientations while the cutting speed was kept constant during all tests. The cutting force and thrust force have been analyzed for every cutting depth and fiber orientations.

Since the machined surface quality and subsurface damage have been known to be highly dependent on many process parameters, there have been many previous studies focused separately on the effect of fiber position, cutting speed, rake angle on the surface quality, eg: [START_REF] Florent | Etude de la coupe en perçage par le biais d'essais élémentaires en coupe orthogonale : application aux composites carbone-époxy[END_REF][START_REF] Iliescu | Approches experimentale et numerique de l'usinage a sec des composites carbone /epoxy[END_REF][START_REF] Shchurov | SPH-Simulation of the Fiber-Reinforced Composite Workpiece Cutting for the Surface Quality Improvement[END_REF][START_REF] Arola | Chip Formation in Orthogonal Trimming of Graphite / epoxy Composite[END_REF][START_REF] Arola | Finite Element Modeling of Edge Trimming Fiber Reinforced Plastics[END_REF][START_REF] Alaiji | 3D Finite Element Modeling of Chip Formation and Induced Damage in Machining Fiber Reinforced Composites[END_REF][START_REF] Wang | An Experimental Investigation into the Orthogonal Cutting of Unidirectional Fibre Reinforced Plastics[END_REF]. Different failure models, Hashin, Maximum stress and Hoffman criterion have been used by [START_REF] Lasri | Modelling of Chip Separation in Machining Unidirectional FRP Composites by Stiffness Degradation Concept[END_REF] focusing on progressive failure of unidirectional glass fiber-reinforced polymer composite (FRP). The outputs answered many questions regarding the machining surface quality; nevertheless, the effect of different cutting depth is not answered yet. The present research carried out a post analysis of newly generated machined surfaces to analyze the surface roughness and crack generation and propagation in the fiber-matrix interface as function of cutting depth. Simultaneously, the work also shows in-depth findings regarding the influence of fiber orientation to the surface quality of machined part.

Initially, once the tool touches the workpiece the cutting and thrust force get higher and become unstable which later decreases and stabilized. Any analytical description of this phenomenon has not been found in the literature. In this research work the reason of this initial high force has been analyzed and explained.

It has been found both numerically and experimentally that the cutting force increases if the cutting depth increases but the thrust force varies mostly depending on fiber orientations. Surface quality and fiber-matrix interfacial crack of machined part mostly depends on the fiber orientation rather than cutting depth.

NUMERICAL MODEL

Geometry and Boundary Conditions

Contact Modeling

It should be noted that in numerical model it is very difficult to simulate ideal friction behavior. In this model a penalty friction formulation with friction coefficient 0.3 has been used following Coulomb friction model, Eq. 1, to define the interaction between the cutting tool and the workpiece. It permits some relative motion; i.e. elastic slip, of the actual surface when they should remain sticking.

(1)

(2)

Here ! "#$ is the limiting shear stress, τ is the equivalent shear stress, µ, P and b are the friction coefficient, contact pressure and cohesion sliding resistance with zero normal pressure respectively.

Material and Failure Criteria

An equivalent homogeneous anisotropic material has been used to predict the machining forces. Both the fiber and matrix materials are assumed to be isotropic. By using the homogenization procedure and the FEM, the global constitutive behavior of the corresponding orthotropic material has been framed by symmetric stiffness matrix. C3D8R 8-node linear brick hexagonal elements with reduced integration has been used for the workpiece throughout the simulations. The cinematic split has been controlled by the average strain. 4 node 3D bilinear rigid quadrilateral elements, R3D4, has been used for cutting tool.

The failure modes have been managed by a Continum Damage Mechanics (CDM) type constitutive law, which was first developed by [START_REF] Matzenmiller | A constitutive model for anisotropic damage in fiber-composites[END_REF] and later modified by [START_REF] Muhammad | An analytical method for predicting cutting forces in orthogonal machining of unidirectional composites[END_REF] and [START_REF] Florent | Etude de la coupe en perçage par le biais d'essais élémentaires en coupe orthogonale : application aux composites carbone-époxy[END_REF]. From the different modes of rupture 5 damage criteria have been defined; 1-tensile failure criterion in fiber direction, 2-compressive failure criterion in fiber direction, 3-matrix crushing criteria, 4-matrix failure criteria and 5-delamination damage criteria. This constitutive law was integrated by a VUMAT sub-routine. Arbitrary Lagrangian-Eulerian (ALE) adaptive meshing has been used to retain high-quality mesh by maintaining large deformations and losses of material throughout the analysis. ALE combines the features of pure Lagrangian analysis and pure Eulerian analysis [Abaqus User Manuel, 2016] Fig. 2. It allows the nodes to move while the topology remains unchanged according to the Eq.3.

Adaptive Mashing and Distortion Control

(3) Here, X is the original position and X i+1 is the new position of a node, u i+1 its nodal displacement, x # % is neighboring nodal positions and N N is weight functions.

The Frequency used was 10 with remeshing sweep per increment 6 and the boundary region smoothing has been used as default values which are 30 for initial feature angle, 30 for transition feature angle and 60 for mesh constraint angle.

EXPERIMENTAL CONFIGURATION

Machine Setup

The experimental configuration have been carried out with a single axis, quasi-static to 2m/sec cutting speed setup supported by a conventional milling machine, Fig. 3. The cutting tool is fixed and the workpiece is clamped in a movable device. The cutting speed is transmitted from a motor to the device containing the specimen via a ball screw module. The motor speed is computer controlled via a COMPAX 3 C3 S150 V4 F11 I12 T11 M00 case. A position sensor is integrated in the motor which allows recording the position and speed of the motor.

X !&' = X + " !&' = N # x ! # Fig.3. Experimental setup
The motor and the module for converting the rotational movement into translation movement are fixed on the table of the milling machine frame. The cutting tool is placed by a holder putting a KISLER 9257B triaxial force sensor in between the machine and the tool holder. The force sensor is tied to the fixed part of the milling machine by a chamber. The force sensor is designed to measure the forces in X, Y and Z directions where X represents the horizontal force (cutting force), Y represents the vertical force (thrust force) and Z represents the transverse force (e.g. reaction of vibration). Transverse force has not been taken into account as it is near zero for orthogonal cutting.

Materials and Cutting Conditions

An intermediate modulus unidirectional Carbon/Epoxy (T800S/M21) material has been used as a test specimen. Each specimen contains only one specific fiber orientation and 16 fiber layers. The Representative Elementary Volume (REV) of the workpiece is 80 mm * 4 mm * 45 mm. For the cutting tool, K20 micro grain tungsten carbide has been used. The cutting parameters have been shown in Tab.1.

Table.1. Cutting parameters.

Near the tool tip 7° clearance angle has been kept for 4 mm longer distance to maintain high stiffness to resist tip breaking. After this distance the angle is 15°.

RESULTS AND DISCUSSION

Cutting Force

In Fig. 4. the cutting forces are separated according to fiber orientation and compared with respect to the cutting depths. It is clearly seen that there is a strong influence of cutting depths on the cutting forces. Regardless of fiber orientation, it is found that if the cutting depth increases, the cutting force increases as well both numerically and experimentally. At 0° fiber orientation, Fig. 4 (a), the simulated force at every cutting depth is greater than the experimental forces whereas it is the contrary at 90° (Fig. 4[c]) and 135° (Fig. 4[d]) fiber angles. At 45° fiber orientation, Fig. 4 (b), the experimental force gets higher at 0.5mm cutting depth. This result is comparable with [START_REF] Alaiji | 3D Finite Element Modeling of Chip Formation and Induced Damage in Machining Fiber Reinforced Composites[END_REF] which found that if the fiber orientation is less than 40°, the experimental cutting force remains less than numerical value and if the orientation is more than 40° the experimental value gets higher than the numerical value. The reason for this phenomenon is because at 0° fiber orientation the tool needs comparably less force to complete the chip formation as the fibers are parallel to the cutting direction. Moreover, at this angle the fibers are not cut crosss-sectionally and there is minimum or no elastic return effect from the fiber. By the same token, at 135° fiber orientation, Fig. 135 (d), a large deviation between the numerical cutting force and experimental values is found indeed both forces have the same rising tendency with respect to the cutting depth. It is because at 135° fiber orientation the fibers are cut in big bunches rather than smooth cutting, causing an increase of the average cutting force. This phenomenon is less realistic in EHM model which is the cause of this deviation. Nevertheless, the trend of cutting forces predicted by FE simulations matched well with the experimental results.

Thrust Force

By the recorded thrust force history, Fig. 5, it is learned that thrust force can be positive as well as negative depending on the cutting depth and fiber orientation (but cutting force stands always positive). It should be reported that thrust force strongly depends on the elastic return phenomenon; the same was found by [START_REF] Lasri | Modelling of Chip Separation in Machining Unidirectional FRP Composites by Stiffness Degradation Concept[END_REF] and [START_REF] Patrick | Contribution à L'étude de L'effet des Conditions D'usinage D'éprouvettes en Composites à Matrice Polymère sur Leur Réponse Mécanique[END_REF]. Due to this phenomenon, the real and nominal depths of cut vary. Thrust force causes deflection of the tool and reduces the depth of cut and affects tolerances. But it fluctuates rapidly if the cutting depth is changed, and a small variation in experimental setup can change the result dramatically.

The thrust force drops if the cutting depth increases regardless of fiber orientation, which means it has inverse relationship with cutting depth. At 0.1mm cutting depth of 0° and 90° fiber position the thrust force is maximum seeing minimum at 1mm, Fig. 5 (a) and (c). For all cutting depths at 45° fiber orientation, Fig. 5 (b), the experimental values remain always positive and at 135° orientation, Fig. 5 (d), it is always negative. Additionally, a significant deviation between the numerical and experimental forces is found. The reason why is that, at 45° fiber orientation fiber experiences high downward pressing from the tool tip during cutting. Once the fiber is cut, due to elastic return characteristics it pushes upward on the clearance surface of the cutting tool more than any other fiber orientation. It is, as, at 45° fiber orientation that the fibers are inclined towards the cutting direction; consequently the clearance surface of the tool faces comparably more surface contact from the cross-sectional cut surface of fiber. More contact means more friction and high vertical force. On the other hand, at 135° fiber orientation the fibers are inclined at the opposite angle of cutting direction. Fiber does not face any downward pressing action from the tool tip and there is almost zero elastic return phenomenon. But as on the rake surface the tool faces downward vertical force from the ongoing cutting chip the total vertical force (thrust force) gets negative for all cutting depths. Here it should be remarked that according to the cutting mechanics, no matter how much the cutting depth is, at 45° fiber orientation the thrust force will always be positive and at 135° fiber orientation it will always be negative.

This particular thrust force phenomenon cannot be found by any HEM FEM model which is the fact in present research work too. The same cause was mentioned by [START_REF] Rao | Micro-Mechanical Modeling of Machining of FRP Composites -Cutting Force Analysis[END_REF]. The HEM models found on literature are not realistic to represent the thrust force. To predict this phenomenon numerically the only way is to develop micro mechanical models with separate fiber and matrix properties. But it will not be easy as the model might have several hundreds of thousands of elements which may deserve large computer memory.

Eventually, it can be said that the trends of the predicted cutting and thrust forces at all cutting depths matched to the experimental results in spite of the fact that there are some deviations.

Force When the Tool Touches the Workpiece

It has been found that initially once the tool tip touches the workpiece the cutting force gets much higher and thrust force remains lower than the average value while both of the forces become unstable, Fig. 6. These initial contact forces, induced due to the cutting tool, is a function of contact pressure, friction parameter and shear stress. While the tool touches the workpiece it faces a strong reaction force which lasts from initial tool-workpiece contact point to the initiation of fiber cut. This incident generates a momentary high vibration to the cutting tool and workpiece that affect the initial unstable contact forces. At the point of fibers separation (cut), the force reaches its peak values; once the front fibers start to be separated (cut), the force reduces until the total separation of the first chip (the first peak zones of the zoomed area of cutting and thrust forces of Fig. 6.). The duration of this phenomenon was measured 0.0165 second (3.3mm cutting length at 200mm/sec cutting velocity) which is interestingly equal to the length of the first chip. Concurrently, the thrust force fluctuates both in positive and negative direction unstably; as in this very initial cutting zone, the fibers experience both pressing and cutting phenomenon. The same phenomenon is repeated during the course of the second chip generation (the second peak zones of the zoomed area of cutting and thrust forces of Fig. 6.) though the second chip's length has been found around 4mm, longer than the first one. By the end of second chip separation (7.3mm total cut) the generated initial high vibration and instability in the tool and workpiece reduce considerably as both of them are held tightly in the holders.

SURFACE STATE

The quality of machined surface relies on many surface parameters. [START_REF] Pramanik | An FEM Investigation into the Behavior of Metal Matrix Composites: Tool-particle Interaction during Orthogonal Cutting[END_REF] mentioned that newly generated surfaces remain under residual compressive stress and these surfaces are damaged due to cavities left by the pullout of fibers; whereas [START_REF] Shchurov | SPH-Simulation of the Fiber-Reinforced Composite Workpiece Cutting for the Surface Quality Improvement[END_REF] said it is the interfacial debonding between fibers and matrix in the area under machined surface which is responsible. In this paper the surface roughness has been analyzed from experimental results

following a numerical and experimental crack generation and propagation comparison.

Surface Roughness

In order to evaluate the surface roughness after machining, Alicona INFINITEFocus SL with standard straight edge 10x has been used. A cross-sectional measurement of the machined surface on the workpiece has been done taking a profile of 100 micrometer width. Typical surface profiles recorded for all cutting conditions. The parameter discussed here is mainly the average roughness of the profile, R a which if formulated according to the Eq.4.

(4)

It is defined as the average absolute deviation of the roughness irregularities from the mean line over one sampling length as shown in Fig. 7. Fig. 7. Arithmetic average height, R a [START_REF] Gadelmawla | Roughness parameters[END_REF].

It has been found that the surface roughness height variation is dominated by the fiber orientation rather than the cutting depth.

verage absolute deviatio

A strong influence of cutting depth is seen at 135° fiber position, Fig.8 (d). As the fibers are inclined toward the opposite way of cutting direction, the apex of the cut surface of fibers get debonded from the matrix by the tool tip. Additionally, micro-cracks are generated along the fiber-matrix interface which gets longer depending on cutting depth. It is because the cross-sectional stiffness of fiber is higher than the adhesive force of the fiber-matrix interface. As a result, cracks are generated and propagated along the fiber-matrix interface. This argument well supports the findings of [START_REF] Wang | An Experimental Investigation into the Orthogonal Cutting of Unidirectional Fibre Reinforced Plastics[END_REF]] and [START_REF] Koplev | The Cutting Process, Chips, and Cutting Forces in Machining CFRP[END_REF] which show that if the fiber orientation gets grater than 90°, the roughness increases rapidly, that means the fiber orientation plays an important role. Moreover, fibers are very abrasive which causes sharpness loss of cutting tool. The more the tool is used, the more it loses its sharpness, and, as a result, the radius of the tool tip increases, hence increased springback and surface roughness.

Crack Generation and Propagation

Subsurface damage or crack generation can be of different types according to [START_REF] Lasri | Modelling of Chip Separation in Machining Unidirectional FRP Composites by Stiffness Degradation Concept[END_REF]; fiber-matrix interface debonding, inter-ply delamination, transverse matrix cracking and fiber rupture. The trend of rising crack and damage during cutting is found to be a function of cutting depth and fiber orientation. At 0° and 45° fiber position, at all cutting depths, no crack has been found, neither numerically nor experimentally. Thus the remarkable point is that the cutting depth has no influence on the surface damage at these fibers' positions. At 0.1 mm and 0.25 mm cutting depth of 90° fiber orientation no crack has been found experimentally indeed a number of cracks were seen numerically at 0.25 mm cutting depth. At 0.50 mm, 0.75 mm and 1 mm depth several noticeable cracks have been marked having 4mm long in experimentation and 2.1 mm in numerical model.

The significant crack generation and propagation have been found at 135° fiber orientation. A numerical and experimental comparison of generated cracks for different cutting depths is shown in Fig. 10. On the figure it can be clearly seen that if the cutting depth increases, the crack extends. At 0.1 mm depth the fiber matrix interface crack is 0.85 mm in simulation and 4.5 mm in experimentation, whereas, at 1mm cutting depth the simulated crack is 10.2 mm and the experimental one is 14 mm. The simulated cracks are shorter than the experimental ones.

The reason behind it is that in CFRP there are three different phases; fiber, matrix and fiber-matrix interface. Each of the phases contains distinct characteristics. As the cross-sectional stiffness of fiber is higher than the adhesive force between fiber and matrix, the fibers get debonded from the matrix which generates crack and permits to propagate depending on the amount of force exerted by the forwarding tool; similar note was found from [Shchurova et al., 2016]. Unlikely, the EHM model does not hold quietly those discrete characteristics which are the causes of varying result from experimentation.

Fig. 10. Numerical and experimental fiber-matrix interface crack as a function of cutting depth at 135° fiber position.

CONCLUSION

In orthogonal cutting depending on the type of material, the parts quality and generated defects can vary; and it is not because of the mechanics of cut but because of the individual characteristics of materials. Understanding the relationship of cutting variables with cutting quality and required cutting efforts is crucial to secure the best cutting parameters and to well understand their influences. A range of cutting depths has been used to analyze their influences to the machining forces and produced surface quality. The numerical results of cutting forces provide a good agreement with the experimental cutting forces, which reveals that the cutting force increases if the cutting depth increases; but the thrust force is mainly dominated by fiber orientation rather than cutting depths.

Cutting depths has significant influence on surface roughness and crack generation principally at 90° and 135° fiber orientation. For any cutting depth at 135° fiber orientation the thrust force will always be negative (vertically downward) and at 45° fiber orientation always positive (vertically upward). The initial unstable high forces are functions of initial compression of fibers by the tool and generated vibration and instability in the cutting tool and the workpiece. This present work provides a framework for studying the cutting depths effects with micromechanical model later in future.

Fig. 1 .

 1 Fig.1. Schematic 2D view of numerical model. A three-dimensional (3D) finite element (FE) model has been developed to analyze the machining forces and cutting induced damages on the machined surface. The workpiece has been modeled as a 3D deformable solid part. The bottom of the rectangular workpiece has been fixed by considering U x = U y =

Fig. 2 .

 2 Fig.2. Arbitrary Lagrangian-Eulerian (ALE) method.

Fig. 4 .

 4 Fig.4. Comparison of numerical and experimental cutting forces: (a) 0° fiber orientation (b) 45° fiber orientation (c) 90° fiber orientation and (d) at 135° fiber orientation.

Fig. 5 .

 5 Fig.5. Comparison of numerical and experimental thrust forces: (a) 0° fiber orientation (b) 45° fiber orientation (c) 90° fiber orientation and (d) at 135° fiber orientation.

Fig. 6 .

 6 Fig.6. Initial contact forces (experimental).

  At 0° fiber orientation, Fig.8(a), the average roughness remained from 0.33 µm to 0.38 µm while at 45° orientation, Fig.8(b), it is always around 0.8 µm for all cutting depths. It should be noted that when the fibers are positioned at 0° they are parallel to the cutting direction; as a result they do not get cut cross-sectionally which gives smooth surface regardless of cutting depth. Comparably, at 45° fiber position the roughness is mainly generated by the elastic return of the fibers which previously get compressed by the tool tip during cutting. Consequently the roughness profile remains almost the same whatever the cutting depth. At 90° fiber orientation, Fig.8 (c), there is little gradual increase of the roughness as the cutting depth increases; indeed, the maximum value does not pass over 2 µm. Two typical roughness profiles of 90° fiber angle can be seen at Fig.9.

Fig. 8 .

 8 Fig.8. Roughness (experimental): (a) 0° fiber orientation (b) 45° fiber orientation (c) 90° fiber orientation and (d) at 135° fiber orientation.

Fig. 9 .

 9 Fig.9. Typical roughness profile of 0.5 mm and 0.75 mm at 90° fiber orientation.