Bertrand Pouteau
email: bertrand.pouteau@eurovia.com

Kamal Berrada

Sandrine Vergne

Mai Lan Nguyen

Stéphane Trichet

Thierry Gouy

Pierre Hornych

Full scale testing of an energy harvesting test road integrating tubes

Keywords: Energy harvesting, tube network, full scale testing, durability

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

Introduction

French roads represent 1.2% of the territory (about 6000 km²). A large part of them are paved with black asphalt materials, which leads them to capture the sun's rays and increase their temperature. Road construction companies are developing innovative road solutions which integrate new functions, such as the ability to generate energy. A recent study shows that it is entirely appropriate to build pavements to harvest solar and geothermal heat energy [START_REF] Pouteau | Positive energy road: feedback on heat harvesting through pavement solution[END_REF].

In 2014, PIARC's report entitled "Alternative solutions for fossil fuels for the road system" listed the main limitations blocking the development of these innovations, among which the resistance to traffic [Comité technique AIPCR, 2014].

Since energy harvesting roads are in their early development, it is necessary to perform accelerated test at scale one to acknowledge the durability of new solutions.

The aim of the study is to assess the impact of the tubes on the durability of the overlay and structure at full scale.

This paper presents test performed on an experimental track that incorporates geothermal tube networks in the first 10 cm of the pavement. Descriptions of the track and of the experimental setup are provided in sections 2 and 3. The experimental results are discussed in section 4.

Experimental site description

Experimentations on Power Road solution, described in paragraph 2.1, are carried out with the support of ADEME within the framework of a national call for projects and the French government's Programme d'Investissement d'Avenir (PIA). In 2017, an experimental site was built on the Cofiroute network (Vinci Motorways). The paragraph 2.2 presents the entire site and underlines the part which was setup to enable accelerated testing. A description of the test track construction is provided in paragraph 2.3.

Power Road description

The tested solution is branded Power Road. The heat is harvested by the flow of a liquid (water and glycol) in tubes buried in the road. The tubes are laid in the asphalt material in the top 10 cm of the pavement under the wearing course. Tubes may weaken the pavement.

Two construction methods can be used to achieve tubes' integration. The indentation method consists of applying prefabricated elements of the exchanger (net-work of tubes) on the uncompacted hot asphalt mix, then compacting the mix (paving between the passage of pavers and compactors). The exchanger is integrated into the binder course layer during its installation as shown in figure 1. The grooving method consists of grooving the binder course using a specific milling machine, then laying the exchanger before installing a filling course and the wearing course. Only the indentation method was tested in the framework of this study.

Site location

The experimental site is located on the access road to the service area of the A10 motorway toll barrier in Saint-Arnoult-en-Yvelines [START_REF] Pouteau | Positive energy road: feedback on heat harvesting through pavement solution[END_REF]. Figure 2 shows the layout of the various components of the installation: 1/ 500 m² of pavement equipped with tubes under the wearing course; 2/ A technical room for the control system; 3/ A field of geothermal probes for energy storage; F/ an experimental area dedicated to performing an accelerated traffic test with the FABAC test machine.

Experimental track description

The experimental track was built on 11 th September 2017. It has a length of 18 m and a width of 2 m. It was divided in two zones (Z1 and Z2) with different thicknesses of layers (see figure 3). The wearing course and binder course are EB10 with 35/50 bitumen. HMA corresponds to the existing pavement.

Experimental setup description

To study the mechanical behavior of the Power road test sections, it was decided to perform full scale accelerated tests, using the FABAC traffic simulator of IFSTTAR [START_REF] Chabot | FABAC accelerated loading test of bond between cement overlay and asphalt layers[END_REF]]. 100m 200m

FABAC traffic simulator and test program

The FABAC machine is a small linear traffic simulator, which was developed initially for testing the fatigue behavior of concrete slab pavements. The FABAC machine has four wheel modules, driven by a chain and electric motor (see figures 4 and 5). Each wheel module can be equipped with single or dual wheels, and apply loads varying between 30 and 75 kN. The length of the loaded section is 2 m. The loading speed can vary between 1 and 5 km/h, and the maximum loading rate, under continuous operation, is 300 000 load cycles per week. The FABAC machine is 10m long and 3m high and was designed so that it can be transported by truck to test sites located on real roads. Thus, in this project, the FABAC machine was transported on the A10 test site, in order to test the two experimental sections (Z1 et Z2). The machine was installed on the site on September 14 th , 2017 and was moved back to IFSTTAR in May 2018. A view of the FABAC machine installed on the A10 test site is shown on figure 6.

Tests were performed on the two experimental pavement sections, with 65 kN dual wheel loads, and with a loading speed of 1 m/s (3.6 km/h). Section Z1 was tested between October 10, 2017 and February 1 st , 2018. 2 686 000 dual wheel loads were applied on this section. Then, section Z2 was tested between February 1 st , 2018 and april 15 th , and 447 000 load cycles were applied.

Survey methods

The monitoring of the experimental sections included:  visual distress surveys ;  measurements of transverse profile, carried out at 3 positions, located 50 cm apart;  the realization of photographs of the test sections.

The transverse profile measurements were performed with a profile measurement system consisting of a metal beam, supporting a mobile carriage, equipped with a laser displacement transducer. The system can measure the transverse profile over a length of 1.5 meter, and with a vertical measurement accuracy of 1 mm. A view of the profilometer is presented on figure 7.

Performance of section Z1

On section Z1, inspections and profile measurements were performed after 923 000, 1 521 000, 1 835 000 cycles, and at the end of the test, after 2 686 000.

Figure 8 presents a picture of the test section, at the end of the test, and a closer view of the aspect of the surface of the pavement, in the middle of the wheelpath. On this figure, it can be seen that at the end of the test, the test section presented some permanent deformation in the wheel path (the pattern of the tires is clearly visible), but no cracking, and no other significant deterioration. To analyze the evolution of the permanent deformations, figure 9 presents transverse profiles measured at different numbers of load cycles, and at two different positions on the test section : position P1, located at a distance of 45 cm from the beginning of the loaded area, and section P2, located at a distance of 95 cm (close to the middle of the loaded area). The results indicate that:  Permanent deformation developed mainly at the beginning of the test. After 923 000 cycles, the maximum permanent deformation reached 7 mm at position P1, and 6 mm at position P2.  Then, during subsequent loading, permanent deformation tended to stabilize, and only a small increase was observed; At the end of the test (2 686 000 loadings), permanent deformation attained 10 mm at position P1, and 6 mm at position P2.  Permanent deformation was higher at the beginning of the loaded area (position P1) than on the rest of the test section.

Because the test was carried out during a cold period (between end of October and end of January), and because the pavement surface presented only downward deformations, and no uplift along the rut, the observed permanent deformation was attributed to a post compaction of the asphalt layer, which could be due to the relatively high initial void content of the asphalt mix (initial mean value of 8.1%). No specific deformations were observed above the tubes located in the asphalt layer, indicating that there was no damage induced by the heating tubes.

The maximum final permanent deformation depth (10 mm) remains lower than the maximum acceptable rut depth for low to medium traffic pavements (15 mm).

Performance of section Z2

On section Z2, inspections and profile measurements were performed only at the end of the test, after 447 000 load cycles. The final aspect of section Z2, at the end of the test, is shown on figure 10. Like section Z1, section Z2 presented only limited rutting in the wheel path, and no other type of deterioration (no cracking, no deformations caused by the heating tubes). Figure 11 presents the transverse profile measurements made on section Z2, after construction and at the end of the test, at two different positions. On this section, which received 447 000 load cycles, the maximum permanent deformation depth attained 2 mm in the center of the section (profile P2), and was higher at the end of the section, attaining 5 mm. Like on section Z1, it is probable that this permanent deformation is due to post compaction of the asphalt layer. In conclusion, results on section Z2 confirmed those obtained on section Z1, indicating only a moderate permanent deformation, most probably due to a post-compaction of the asphalt material, due to its high initial void content.

Conclusions

After 2.6 million loading cycles of 65kN dual wheel loads applied on the experimental track, the following conclusions can be made:

-no cracks related to the presence of tubes introduced into the pavement by the process were observed. A very fine surface crack was observed around 2 million cycles, but it was no longer visible during the final observation; -no permanent deformation related to the presence of tubes in the pavement was found. -integration of tubes by the "indentation method" seems to have moderate to no impact on the resistance of the pavement to traffic loading.

This study also acknowledges that the FABAC machine remains a convenient device to investigate durability of innovative pavements.

Fig. 1

 1 Fig. 1 Installation of the tubes by indentation (left: positioning of a grid) (right: view of the pavement after indentation of all the grids)

Fig. 2

 2 Fig. 2 Layout of experimental site

Fig. 3

 3 Fig. 3 Experimental track structure description.

Fig. 4

 4 Fig. 4 Principle of the FABAC machine

Fig. 6

 6 Fig. 6 View of the FABAC machine installed on the A10 track

.

Fig. 7

 7 Fig. 7 View of the laser sensor profile measurement system

Fig. 8

 8 Fig. 8 View of section Z1 at the end of the full scale test, and close-up view of the deformations in the wheelpath

Fig. 9

 9 Fig. 9 Transverse profile measurements at different numbers of load cycles at two different locations on section Z1.

Fig. 10

 10 Fig. 10 View of section Z2 at the end of the full scale test, and detail of the wheelpath

Fig. 11

 11 Fig. 11 Transverse profile measurements at two different locations on section Z2, before and after loading

Acknowledgements

The work presented in the paper is part of a research carried out with the support of ADEME (the French Environment & Energy Management Agency) and the Investments for the future program of the French government.


Rodrigo Siroma
email: rodrigo-shigueiro.siroma@ifsttar.fr

Mai Lan Nguyen

Pierre Hornych

Emmanuel Chailleux

A literature review of bitumen aging: from laboratory procedures to field evaluation

Keywords: Bitumen, Aging, Laboratory aging, Field aging, End-of-life criteria

Bitumen age-hardening increases the risk of pavement cracking. A good understanding of this phenomenon is crucial for designing long-lasting pavements. In this context, the French national project "Modeling of Aging and Damage for Pavement Lifetime Assessment (MoveDVDC)" aims to characterize the residual mechanical properties of bituminous bases and their evolution over time to properly estimate the pavement remaining lifetime. As an initial part of this project, this paper presents an up-to-date literature review of neat bitumen aging. It focuses on investigating the evolution of properties observed in current accelerated laboratory procedures against those measured in field-aged samples. In addition, a wide set of indexes often used to estimate the aging level and end-of-life criteria of bitumen are analyzed and discussed.

Introduction

Despite being the principal binding agent in road construction, bitumen is highly prone to age-hardening-a combination of several mechanisms that increase the pavement cracking potential. During asphalt mix production and compaction, elevated rate of oxidation and lighter components evaporation occurs due to high-temperatures exposition [START_REF] Hunter | The Shell Bitumen handbook[END_REF]. From then on, progressive oxidation, steric hardening, and oil exudation take place throughout pavements' service life, under the combined effect of climate condition and traffic loading. One of the main mechanisms of aging [START_REF] Hunter | The Shell Bitumen handbook[END_REF][START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF], oxidation increases the concentration of polar functional groups resulting in intermolecular agglomeration (micelles) that increases the cracking susceptibility [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF]. Understanding bitumen aging is crucial for tailoring long-lasting asphalt mixes and, consequently, optimizing the use of such scarce resources. Hence, this paper aims to presents an indepth review of the relevant literature of neat bitumen aging.

Accelerated Laboratory Aging Protocols

Of several laboratory protocols [START_REF] Airey | State of the Art Report on Ageing Test Methods for Bituminous Pavement Materials[END_REF] proposed to reproduce field aging effects, most consist of high-temperature extended exposure as it accelerates the rate of bituminous oxidation, which is desirable [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF][START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF]. However, the use of excessively high temperature may diverge the aging effect in laboratory samples from that of the field [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF]. At binder level, usual procedures are based on relatively thin-film samples exposed to high temperatures to simulate the short-and long-term aging effects, such as Rolling Thin-Film Oven Test (RTFOT) and Pressure Aging Vessel (PAV), respectively. The Universal Simple Aging Test (USAT) is a 300μm film test proposed as an optimized alternative to the RTFOT and PAV procedures [START_REF] Kim | Asphalt Pavements[END_REF]. Although it is a static thinfilm test, which could yield inhomogeneous oxidation in the sample, the extremely thin film is comparable to the thickness of bitumen film on aggregates, in which oxidation can be considered as kinetics-controlled, i.e., absence of oxidation gradient [START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF].

At asphalt mix level, procedures can be classified according to the sample state (loose or compacted), accelerator factor (forced-draft oven, modified pressure, etc.), and aging factor (oxygen, enriched gas, actinic light, etc.). Compacted specimen aging can produce suitable samples for performance tests, whereas loose mix aging, such as [START_REF] De La Roche | Development of a laboratory bituminous mixtures ageing protocol[END_REF], yields more homogeneous aging with a high oxidation rate [START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF]. In the literature, some drawbacks are often stated: aging compacted samples is timeconsuming [START_REF] Steiner | Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen[END_REF], generates oxidation gradient, and can change the void content and geometry of the sample [START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF]; difficulty in ensuring sufficient adhesion and cohesion when compacting loose aged mix [START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF][START_REF] Steiner | Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen[END_REF]; and elevated conditioning temperature (above 100°C) may alter aging mechanisms [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF][START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF][START_REF] Steiner | Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen[END_REF]. An interesting approach to aging compacted samples, the Viennese Aging Procedure (VAPro) [START_REF] Steiner | Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen[END_REF] uses a triaxial cell with compressed air enriched with ozone and nitric oxides (highly oxidative gases present in the atmosphere) at 45°C-75°C, usually reached on surface layers in summer.

3

Aging Indexes and Relationship with Field-Aging

Chemical Indexes

Fourier Transform Infrared (FTIR) Spectroscopy is widely used to track the oxygen uptake through two main functional groups: carbonyl (C=O), as it has a well-known linear relationship with the log viscosity over time [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF][START_REF] Petersen | Asphalt Oxidation Mechanisms and the Role of Oxidation Products on Age Hardening Revisited[END_REF]; and sulfoxide (S=O), which concentration in the past was considered unreliable due to its thermal decomposition at higher temperatures. However, especially in high sulfur content bitumen, sulfoxide reactions are the principal factor that leads to increased viscosity [START_REF] Petersen | Asphalt Oxidation Mechanisms and the Role of Oxidation Products on Age Hardening Revisited[END_REF]. The first stage of oxidation is marked by a rapid formation of both described indexes, where the growth rate of S=O is higher than that of C=O; then, S=O appears to stabilize while C=O amount increases at a constant rate [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF]. Despite being a handy and effective tool to track the oxygen uptake, the lack of standardization results in divergence of procedures as well as of calculation methods between laboratories [START_REF] Partl | Testing and Characterization of Sustainable Innovative Bituminous Mat. and Systems: State-of-the[END_REF][START_REF] Dony | MURE National Project: FTIR spectroscopy study to assess ageing of asphalt mixtures[END_REF].

Bitumen can be separated into four generic chemical groups-according to increasing polarity-known as SARA (Saturate, Aromatics, Resins, and Asphaltene) fraction. While nonpolar saturates content remains practically unchanged, the components of the other fractions tend to shift from aromatic through resins to asphaltene with aging [START_REF] Petersen | A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships[END_REF][START_REF] Guern | Physico-chemical analysis of 5 hard bitumens: Identification of chem. species and molec. organiz. before and after artif[END_REF]. The colloidal instability index (Ic) is the ratio between the sum of as-phaltene and saturate contents and the sum of aromatic and resin contents. The increase in Ic value over time represents a bitumen more prone to cracking. Bitumen extracted from cracked surface layers of European roads indicate as end-of-life criterion the limit value of 0.6 [12]. However, the bitumens studied were softer than those currently used, so this limit value should be adjusted based on bitumen consistence.

Differential Scanning Calorimetry (DSC) determines the glass transition temperature (Tg) and the amount of crystallizable fraction. The Tg value can be related to bitumen brittleness [START_REF] Kriz | Physical Aging in Semi-Crystalline Asphalt Binders[END_REF]. At temperatures above Tg, the crystallization that occurs may represent an undesirable cracking resistance performance After laboratory aging, the crystallizable fraction tends to increase [START_REF] Guern | Physico-chemical analysis of 5 hard bitumens: Identification of chem. species and molec. organiz. before and after artif[END_REF]. In [START_REF] Poulikakos | RILEM 252-CMB Symposium: Chemo-mechanical Characterization of Bituminous Materials[END_REF], bitumens with higher crystallizable fraction content display poorer low-temperature performance.

The Size Exclusion Chromatography (SEC) method classifies bitumens based upon their molecular size distribution. Laboratory [START_REF] Guern | Physico-chemical analysis of 5 hard bitumens: Identification of chem. species and molec. organiz. before and after artif[END_REF][START_REF]Short-term aging characterization of asphalt binders using gel permeation chromatography and selected Superpave binder tests[END_REF] aging leads to a remarkable increase in the large molecular size (LMS) region. The ratio between its value after and before aging [START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF][START_REF]Short-term aging characterization of asphalt binders using gel permeation chromatography and selected Superpave binder tests[END_REF] is often used as an aging index. In high-speed condition, it is possible to highlight asphaltene agglomeration [START_REF] Guern | Physico-chemical analysis of 5 hard bitumens: Identification of chem. species and molec. organiz. before and after artif[END_REF]. Extracted bitumens from a test road in Montana (US) show that the most cracked sections displayed a higher LMS population [17]. Through this test, it is possible to track the aging level of asphalt mixes by dissolving it in tetrahydrofuran (THF) without bitumen extraction [START_REF] Elwardany | Phenomenological Modeling and Laboratory Simulation of Long-Term Aging of Asphalt Mixtures[END_REF].

Physical Indexes

Empirical Tests

Classical tests such as Penetration and Softening Point have been used for decades as indexes to track neat bitumen aging. The bitumen age-hardening leads to a decrease in penetration and an increase in softening point value over time. For the Zaca-Wigmore Test Road (US), a limit value of 30 was stated for penetration (originally 100-200) as a pavement end-of-life criterion, although lower values were found in extracted bitumen from other test roads with little or no cracks [17]. Further, values lower than 30 can be found in hard bitumen currently used for high-performance asphalt mixes.

Mechanical Intrinsic Test

Complex modulus (G* = G'+iG" = |G*|e iφ ) of bitumen is measured by loading frequency sweep tests at multiple temperatures. The aging level can be tracked through either directly from test output or parameters of linear viscoelastic models, such as the analogical Huet modified and the Christensen-Anderson-Marasteanu (CAM) model. The analogical Huet modified model is composed of 1 string, 2 parabolic elements, and 1 dashpot (all assembled in series). Laboratory aged bitumen [START_REF] Yusoff | Predictability of complex modulus using rheological Model[END_REF] and bitumen extracted from loose aged asphalt mixes [START_REF] Partl | Testing and Characterization of Sustainable Innovative Bituminous Mat. and Systems: State-of-the[END_REF][START_REF] Themeli | A new method to quantify and evaluate ageing state of asphalt from viscoelastic measurement[END_REF] display the following trends with aging: the glassy modulus (spring G∞) and parabolic element (k) that represents the slope of the Cole-Cole plot at low temperatures and/or high frequencies remain unchanged; the parabolic element (h) that describe the slope at high temperatures and/or low frequencies decreases in [START_REF] Partl | Testing and Characterization of Sustainable Innovative Bituminous Mat. and Systems: State-of-the[END_REF][START_REF] Themeli | A new method to quantify and evaluate ageing state of asphalt from viscoelastic measurement[END_REF], but remains unchanged in [START_REF] Yusoff | Predictability of complex modulus using rheological Model[END_REF]; the dashpot (β) that represents the viscosity at higher temperatures and/or lower frequencies consider-ably increases; the δ parameter, balance between the two parabolics elements at intermediate temperatures, tends to increase; and the τ parameter that represents the relaxation time at the reference temperature considerably increases.

The CAM model is based on analytical equations whose parameters have physical meanings. During aging [START_REF] Yusoff | Predictability of complex modulus using rheological Model[END_REF][START_REF] Anderson | Continuous Models for Characterizing Linear Viscoelastic Behavior of Asphalt Binders[END_REF], the crossover frequency (ωc), frequency at which the phase angle is 45º, reduces; the v value that represents the velocity at which the phase angle converges towards the two asymptotes decreases; the glassy modulus (G∞) remains unchanged; and the R-value, which represents the relaxation spectra of the bitumen, increases, indicating a flattening of the master curve.

The evolution of the crack resistance at low temperatures can be outlined by graphs, such as the ωc versus R-value [START_REF] Rowe | ΔTc -Some thoughts on the historical development[END_REF] and the crossover modulus (G*C) versus crossover temperature (Tδ=45°) [START_REF] Widyatmoko | Mapping crack susceptibility of bituminous materials with binder durability[END_REF]. The risk of thermal cracking of asphalt mix may be evaluated by temperatures corresponding to the phase angle of 27º (Tδ=27°) or 45º (Tδ=45°), both at [START_REF] Ramond | Le module complexe des liants bitumineux[END_REF]24]. In [24], for 35-50 pen bitumens extracted from cracked road sections in France, Tδ=27° =12°C and Tδ=45° =35°C. The Glover-Rowe (G-R) parameter has been largely used to assess the cracking potential of asphalt mixes. Based on previous studies, [START_REF] Rowe | The Influence of Binder Rheology on the Cracking of Asphalt Mixes in Airport and Highway Projects[END_REF] determined the following two criteria: Damage onset at 180kPa and significant cracking at 450kPa (updated to 600kPa by [START_REF] Rowe | ΔTc -Some thoughts on the historical development[END_REF]). The G-R showed good accuracy in predicting the cracking susceptibility of aged sections [START_REF] Rowe | The Influence of Binder Rheology on the Cracking of Asphalt Mixes in Airport and Highway Projects[END_REF].

Creep modulus can be measured by Bending Beam Rheometer (BBR) test for evaluating the bituminous material's capacity to resist low-temperature cracking. At 60s loading time, the Ts is the temperature at which the stiffness (S) is ≤300MPa and the Tm, related to relaxation properties, is the temperature at which the slope m of the log-stiffness versus log-time curve presents a value ≥0.3 [START_REF] Glover | Development of a New Method for Assessing Asphalt Binder Durability with Field Validation[END_REF]. The ΔTc parameter is the difference between Ts and Tm [START_REF] Daniel | Relationships of Laboratory Mixture Aging to Asphalt Mixture Performance[END_REF]. As bituminous material ages, the curve flattens, and ΔTc becomes increasingly negative [START_REF] Rowe | ΔTc -Some thoughts on the historical development[END_REF]. A limit value of ΔTc≤-2.5°C was proposed by [START_REF] Anderson | Evaluation of the Relationship between Asphalt Binder Properties and Non-Load Related Cracking[END_REF] to indicate a possible risk of cracking, while [START_REF] Rowe | ΔTc -Some thoughts on the historical development[END_REF] set ΔTc≤-5°C for more urgent remediation. The Christensen-Anderson (CA) model can be used as a surrogate of BBR [START_REF] Rowe | New methods for assessing rheology data such as ΔTc and G-R Parameter and their relationship to performance of REOB in asphalt binders and other materials[END_REF], whose advantage is to reduce the amount of bitumen required.

Physical and Chemical Coupling Approaches

Several attempts and approaches have been proposed to build relationships between the chemical properties and physical performance of the bitumen. The δ-Method was proposed by [START_REF] Themeli | A new method to quantify and evaluate ageing state of asphalt from viscoelastic measurement[END_REF] to estimate the Apparent Molecular Weight Distribution (AMWD). It is based on the relationship between applied frequency and molecular relaxation time. According to the δ-Method, aging results in shifting from the lower molecular weight fraction (intermicellar environment) to a high weight population (asphaltene agglomeration) [START_REF] Partl | Testing and Characterization of Sustainable Innovative Bituminous Mat. and Systems: State-of-the[END_REF][START_REF] Themeli | A new method to quantify and evaluate ageing state of asphalt from viscoelastic measurement[END_REF]. The crossover frequency corresponds to the AMWD median value and evolves towards higher weight populations, which is in accordance with the literature. The δ-Method appears promising to connect rheological properties with the colloidal structure of bitumen.

Due to the good correlation stated in the evolution of chemical indices of laboratory and field aged material, [START_REF] Qin | Field aging effect on chemistry and rheology of asphalt binders and rheological predictions for field aging[END_REF] proposed an approach to predict the evolution of the rheological properties that occur in field pavements. A satisfactory relation was showed between chemical, such as FTIR carbonyl+sulfoxide and the total pericondensed aromaticity (TPA) determined by SAR-AD fraction, and mechanical indexes, such as the crossover modulus (G*c) and the characteristic relaxation time (λ). These latter parameters can be used to determinate the R-value and the ωc for different aging stages of bitumen, therefore, making possible the building of master curves through the Christensen-Andersen (CA) model. This procedure shows a good correlation with the results of extracted bitumen from the field. Similar to the previous study, [START_REF] Partl | Testing and Characterization of Sustainable Innovative Bituminous Mat. and Systems: State-of-the[END_REF] stated that the linear relationship between the four Huet modified model parameters (h, δ, β, and log(τ)) and the carbonyl index makes possible to estimate the evolution of G* during aging. However, no validation was performed during this study.

An alternative approach was proposed by [START_REF] Elwardany | Determination of Binder Glass Transition and Crossover Temperatures Using Four-mm Plates on a Dynamic Shear Rheometer: Significance and Applications[END_REF] to determining ΔTc through the Viscoelastic Temperature (TVE), represented by the temperature at the glassyviscoelastic behavior transition, and the crossover temperature (viscoelastic-viscous behavior transition), both determined by the 4mm Dynamic Shear Rheometer test. TVE was identified as the temperature corresponding to the maximum loss modulus at low temperature and display a satisfying correlation with the modulated DSC glass transition temperature (Tg). For some bitumen the peak is unclear. Despite the good correlation, this approach still needs to be further validated.

Conclusion

The main goal of this paper was to present a relevant literature review of neat bitumen aging. Since age-hardening makes bituminous material more prone to cracking, several techniques that better reproduce the field aging effects through increasingly optimized laboratory procedures have been proposed, such as the USAT and the VAPro methods. Despite using different approaches, the discussed indexes display a general coherence of the evolution of the bituminous material's behavior. According to the incidence and positive evaluation in the literature, mechanical and some chemical indexes-especially FTIR-are the most recommended, due to their sensitivity to track the aging evolution. Some end-of-life criteria proposed in the literature, summarized in Table 1, are no longer suitable to assess currently used bitumens, especially the harder ones for high-performance asphalt mixes. Finally, different attempts at physical and chemical coupling proposed by recent studies may be potential tools towards a better understanding of the aging phenomenon, thus enabling the construction of longer-lasting pavements. Table 1. End-of-life criteria found in the literature with comments Aging Index Estimated End-of-life criterion 5

Acknowledgments

The authors gratefully acknowledge the French National Research Agency (ANR) for the financial support of the MoveDVDC project.


Xuan Quy Le

Mai Lan Nguyen

Pierre Hornych

Tuan Quang

Nguyen

Quang Tuan Nguyen

Evaluation of Pavement Damage through the Analysis of Asphalt Layer Modulus and Strain Evolutions during an Accelerated Pavement Test

Keywords: Accelerated Pavement Test, Asphalt Pavement, Damage Analysis, Falling Weight Deflectometer, Strain, Temperature Correction

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

Introduction

The evaluation of pavement damage has been a major topic of in-service pavement assessment. Thanks to APT, the full-scale performance and deterioration of pavements can be monitored by carrying out different measurements with the repeated application of heavy traffic loads [START_REF] Autret | The circular test track of the Laboratoire Central des Ponts et Chaussées -First Results[END_REF][START_REF] Gramsammer | The LCPC's A.P.T. Facility: Evaluation of Fifteen Years of Experimentations[END_REF][START_REF] Perez Ayuso | Managing Data from Instrumentation in the CEDEX Test Track[END_REF][START_REF] Timm | Flexible pavement fatigue cracking and measured strain response at the NCAT test track[END_REF]. The Nantes Campus of Gustave Eiffel University (UGE) has been operating since 1982 a heavy traffic simulator, namely the fatigue carrousel, designed to test full-scale pavement structures up to failure under controlled loading conditions. This facility can apply up to 500 000 heavy vehicle loads per month [START_REF] Nguyen | Investigation of Crack Propagation in Asphalt Pavement Based on APT Result and LEFM Analysis[END_REF][START_REF] Hornych | The LCPC's APT facility contribution to pavement cracking knowledge[END_REF].

In this study, a full-scale experiment on a low traffic pavement is used to analyze and compare different methods, of evaluation of pavement damage under traffic: deflection measurements, asphalt strain measurements and evolution of surface cracking.

Presentation of the full scale test

The full-scale test presented in this paper was carried out from 28 th of May 2018 to 8 th of February 2019 on six different pavement sections to evaluate the in situ performance of the reinforcement on their lifetime. However due to limited space, only results obtained on one section are presented in this paper. The 22m-long pavement structure consisted of two asphalt layers, with a total thickness of 11cm, an unbound granular subbase (30cm thick) and a sandy subgrade, with a total thickness of 260cm. A total of 2.2 million load cycles were applied, with the 65kN French standard dual-wheel load. The temperature in the asphalt layers (at 6cm-depth) was recorded continuously, every ten minutes, by a temperature probe. To monitor pavement performance, three types of tests were performed during the APT: (1) FWD tests, carried out on the pavement surface before the start of loading, and after 0.5, 0.75, 1 and 2.2 million load cycles; (2) Longitudinal strain measurements, at the bottom of the asphalt layers. These measurements were made using TML strain gauges (model KM-100A). Two longitudinal strain gauges, L1 and L3, were placed at the bottom of the asphalt layers at two different longitudinal positions; (3) Distress survey of surface cracks.

3

Pavement damage evaluation from FWD tests

Backcalculation of asphalt layer moduli

The FWD measurements made at different stages of the experiment were used to backcalculate pavement layer moduli, using the Alizé-LCPC pavement design software, based on a multi-layer linear elastic model. The actual thicknesses of asphalt layers, obtained by levelling measurements during pavement construction, were used in these calculations. Due to space limitations, only results from FWD tests made at one position on the pavement surface, above strain gauge L1, are taken into account.

Evaluation of pavement damage from asphalt layer moduli

Following the back-calculations, the variation of asphalt layer moduli with the number of load cycles was used to evaluate pavement damage [START_REF] Mateos | Evolution of Asphalt Mixture Stiffness under Combined Effects of Damage, Aging, and Densification under Traffic[END_REF]. For that purpose, a damage parameter noted 𝐷 𝐸 𝑇 (Eq. ( 1)), was introduced, to take into account the decrease of asphalt layer moduli with loading, at a fixed temperature.

𝐷 𝐸 𝑇 = 𝐸 𝑁0 𝑇 -𝐸 𝑁𝑖 𝑇 𝐸 𝑁0 𝑇 (1)
Where: 𝐷 𝐸 𝑇 is the damage ratio of the asphalt layer at temperature T; 𝐸 𝑁0 𝑇 and 𝐸 𝑁𝑖 𝑇 are respectively asphalt layer moduli at load cycle zero (N0) and load cycle Ni, and at temperature T.

Due to low and intermediate temperature conditions during the experiment (from 12 °C to 19 °C), it was assumed that the damage ratio 𝐷 𝐸 𝑇 is the same for different tested temperatures [START_REF] Van Rompu | New fatigue test on bituminous binders: Experimental results and modeling[END_REF]. The calculation of this damage ratio for a given temperature T requires determining the initial layer modulus 𝐸 𝑁0 𝑇 at the same temperature. As the initial FWD tests were performed at 15 °C, only a modulus at 15 °C (𝐸 𝑁0 15°) could be determined exper-imentally. Therefore, to calculate the initial moduli for other test temperatures, a correction was applied, based on laboratory complex modulus tests. The results of these tests were fitted using the Huet-Sayegh viscoelastic model [START_REF] Huet | Etude par une méthode d'impédance du comportement viscoélastiques des matériaux hydrocarbonnées[END_REF][START_REF] Sayegh | Contribution à l'étude des propriétés viscoélastiques des bitumes purs et des bétons bitumineux[END_REF], and the model was used to calculate moduli at different temperatures. The calculated initial moduli, at different temperatures, and the corresponding damage ratios 𝐷 𝐸 𝑇 at different numbers of load cycles are presented in Table 1. The calculated damage attains 73% at the end of the experiment. As mentioned above, strain levels at the bottom of the asphalt layers were measured using two longitudinal strain gages, L1 and L3. The strains measured during the first part of the experiment, where the asphalt layers presented no damage (up to 600 000 cycles), are plotted in Fig. 1, in function of the measured temperature. Calculations were made with the Alizé-LCPC software, to compare the measured strains with theoretical values. For these calculations, it was necessary to take into account the actual depth of the gauges in the asphalt layer, which can vary slightly, due to the installation process. For that purpose, strains were calculated using Alizé-LCPC, for four different depth levels (0, 0.5, 1.0, and 1.5 cm above the asphalt layer interface), and compared with the experimental values. The results show that for gauge L1, the best match is obtained for a level of the gauge h = 1.5cm above the layer interface. For gauge L3, the best match is obtained for a level h = 0.5cm.

After defining the most probable positions of the gauges, calculations were carried out with Alizé-LCPC, considering four different conditions of the asphalt layers interface: bonded, semi-bonded, fully de-bonded and partially bonded. In this last case, the interface was modelled as a thin layer (2mm thick) of bitumen [START_REF] Duong | Continuous strain monitoring of an instrumented pavement section[END_REF], with a complex modulus representative of a 30/50 grade pure bitumen. The strains calculated with these four interface conditions are present in Fig. 1. The results show that at low and intermediate temperatures (less than 26 °C for strain gauge L1 (Fig. 1a) and less than 22 °C for strain gauge L3 (Fig. 1b)), the measured strains are close to those corresponding to the fully bonded condition. However, at higher temperatures (30 °C at position L1 and 25-30 °C at position L3), the measured strains are lower than those predicted with a fully bonded interface, and are closer to those obtained with partially bonded conditions. This means that at high temperature, some sliding occurs at the interface, leading to a redistribution of strains in the two asphalt layers. This interesting result is in accordance with other research on the influence of layer interface conditions on pavement behavior, in particular work of the RILEM TC 272-PIM.

Evaluation of pavement damage from strain evolution

In order to evaluate asphalt layer damage from strain data, a damage parameter 𝐷  𝑇 was introduced, to describe the increase of strains with traffic (Eq. ( 2)).

𝐷  𝑇 =  𝑁𝑖 𝑇 - 𝑁0 𝑇  𝑁𝑖 𝑇 (2)
Where: 𝐷  𝑇 is a damage ratio considering strain at the bottom of the asphalt layer at temperature T;  𝑁0 𝑇 and  𝑁𝑖 𝑇 are respectively strains measured at the start of the APT (at cycle N0) and at load cycle Ni, at temperature T. Calculated strains were also determined using Alizé-LCPC, based on the backcalculated moduli of asphalt layers obtained from FWD tests. To illustrate this approach, only measurements of strain gauge L1 are considered. Fig. 2a presents the evolutions of asphalt layer temperatures, and experimental and calculated strains with the number of loading cycles. As the strains were all measured at a temperature close to 12 °C, the calculations were done for this temperature. Fig. 2a shows that the calculated strains are very similar to the experimental values up to nearly 0.75 million cycles. After 0.75 million cycles, and up to 1.5 million cycles, the measured strains increase much more than the calculated values. This can be explained by the fact that the experimental values represent local measurements, at the position of the strain gauge, where cracks probably appeared [START_REF] Van Rompu | New fatigue test on bituminous binders: Experimental results and modeling[END_REF], whereas the calculated strains, determined from back-calculated moduli of different layers, represent global values.

The damage ratio 𝐷  𝑇 is plotted in Fig. 2b and compared with 𝐷 𝐸 𝑇 (see Eq. ( 1)). The values of 𝐷  𝑇 follow very well the evolution of 𝐷 𝐸 𝑇 , up to 1 million load cycles. After 1.1 million cycles, 𝐷  𝑇 starts to increase more rapidly. As already mentioned, this can be explained by the development of local cracks at the strain gauge position. Fig. 2b also presents the evolution of surface cracking of the pavement with the level of traffic. The percentage of cracking on Fig. 2b corresponds to the percentage of the length of the test section affected by cracks.

By comparing the three damage indicators presented on Fig. 2, it can be concluded that:

-Up to million cycles, the structure presents practically no damage: there is no significant evolution of strains or modulus, and no surface cracking. -Between 0.6 million and 1.1 million load cycles, the two damage indicators, based on strain and modulus give similar results, and indicate a significant increase of damage, but no surface cracking is observed. This second phase probably corresponds to a phase of damage initiation, where micro-cracks started to develop at the bottom of the asphalt layer but did not propagate to the pavement surface [START_REF] Nguyen | Investigation of Crack Propagation in Asphalt Pavement Based on APT Result and LEFM Analysis[END_REF]. After 1.1 million cycles, the level of damage is close to 50%. -In the last part of the experiment, after 1.1 million cycles, cracks start to propagate to the pavement surface, and this is also associated with a rapid increase of the strain-based damage ratio. The increase of the modulus-based parameter 𝐷 𝐸 𝑇 is more gradual. It can be noted that the onset of surface cracks corresponds to a damage level of about 50%. The same observation is made by Mateos [START_REF] Mateos | Evolution of Asphalt Mixture Stiffness under Combined Effects of Damage, Aging, and Densification under Traffic[END_REF].

Conclusions

Different methods of evaluation of pavement damage, in a full-scale flexible pavement, based on the evolution of asphalt layer modulus, asphalt layer strains and surface cracking were analyzed and compared. By combining these indicators, it is possible to better understand the damage mechanisms. In a first phase, up to about 1.1 million load cycles, no cracks were observed on the pavement surface; during this phase, damage ratios based on measured strains, and on measured asphalt layer moduli gave very similar results. After 1.1 million load cycles, corresponding to a threshold level of damage of about 50%, cracks started to propagate to the pavement surface, and this was also detected by an increase of the strain-based damage indicator.

Fig. 1 .

 1 Fig. 1. Calculated and measured longitudinal strain values at the bottom of the asphalt layers, as a function of temperature: a) gauge L1, b) gauge L3.

Fig. 2 .

 2 Fig. 2. Evolutions of temperature, strains (a) damage ratio and surface cracking (b) versus number of load cycles.

Table 1 .

 1 Asphalt layer moduli and damage ratios at different numbers of load cycles.

		Loading	Asphalt layer modulus at temperature T (Mpa) Damage ratio
		cycles	12 °C	14 °C	15 °C	19 °C	𝐷 𝐸 𝑇
		0	13609	12537	11711	9482	0%
		500 000	-	-	-	9103	4.0%
		750 000	-	8324	-	-	33.6%
		1 000 000	7113	-	-	-	47.7%
		2 200 000	-	3389	-	-	73.0%
	4	Pavement

damage evaluation from strain data analysis 4.1 Comparison of measured and calculated strains in the asphalt layer

Table 2 .

 2 Comparison of measured and calculated strains, to determine the actual depth of the gauges in the asphalt layer. Table 2 compares the measured and calculated strains, for a temperature of 20 °C.

							
	Asphalt layer	Strain	measured	calculated	calculated	calculated	calculated
	thickness	gauge	at 20 °C	for	for	for	for
				h = 0cm	h = 0.5cm	h = 1.0cm	h = 1.5cm
	(cm)		(µm/m)	(µm/m)	(µm/m)	(µm/m)	(µm/m)
	11.5	L1	138.26	187.5	169.0	151.3	134.5
	11.2	L3	171.28	192.7	173.2	154.6	136.6


Juliette Blanc

Emmanuel Chailleux

Pierre Hornych

Chris Williams

Zahra Sotoodeh-Nia

Laurent Porot

Simon Pouget

François Olard

Jean-Pascal Planche

Davide Lo Presti

Zahra So- Toodeh-Nia

Ana Jimenez

Del Barco

From laboratory mixes to full scale test: rutting evaluation of bio-recycled asphalt mixes

Keywords: Bio-materials, Reclaimed asphalt, Accelerated pavement test, Rutting test, Laboratory test

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

Tested materials

The aim of the project was to maximise the use of RAP with bio-based materials. For this purpose, three mixes were designed incorporating 50% RAP content, in association with three different and complementary innovative bio-materials [START_REF] Chailleux | BioRePavation: innovation in bio-recycling of old asphalt pavements, comparison between EU and US mix design specification systems[END_REF]:

-Mix1: designed with a bio-based rejuvenator, SYLVAROAD™ RP1000, a performance Additive, from Kraton Chemical used to treat the RAP. It was specifically designed to increase RAP content up to 100 % or to reuse very hard, low quality RAP. -Mix2: designed with a bio-binder, Biophalt®, from Eiffage Infrastructure, used for total replacement of bituminous binder in recycling techniques. -Mix3: designed with a bio-based additive from Iowa State University, an Epoxidized Methyl Soyate (EMS) aimed to compatibilize virgin binder and aged binder from RAP. Aggregate grading curve and binder content were chosen using aggregate packing optimisation concepts (GB5® type material) in order to maximize mix density and particle interlock [START_REF] Pouget | GB5® Mix Design: A New Approach for Aggregate Grading Optimization for Heavy Duty Flexible Pavements[END_REF]. The properties of these three GB5-type mixes (mix 1, mix2 and mix3) were compared with the reference high modulus asphalt mix (EME2), commonly used in France.

Before construction, laboratory rutting tests were performed on mixes produced in lab following the European method (French wheel tracking test) and US method (flow number test). Results are presented in Table 1. All three innovative materials gave results well below the European and US specifications for resistance to rutting. According to the European method, the results are similar for the 3 mixes, taking into account the repeatability of the tests. The full scale experiment was realized in two phases: The first phase was performed between July and September 2017, to evaluate rutting resistance under a 65 kN dual wheel load, with a speed of 43km/h and a narrow transversal wandering (+/-26cm). The second phase was performed between November 2017 and March 2018, to evaluate fatigue resistance of the pavements, also under 65 kN load. This second phase will not be detailed here, but is presented in [START_REF] Blanc | Full-scale validation of bio-recycled asphalt mixtures for road pavements[END_REF]. During the rutting test, loads were applied only when the pavement surface temperature exceeded 30°C, which is the procedure generally used for rutting tests on the IFSTTAR APT. 200 000 loads were applied. The pavement temperatures varied between 30 °C and 40 °C, with some short periods with higher temperatures, especially at the start of the test, where a maximum temperature of 53 °C was recorded on the surface of the pavement. Four different structures corresponding to materials described above, mix1, mix2, mix3 and EME2, were tested simultaneously.

3

Representativeness of the rutting test

A histogram of the surface temperature distribution during the loading phases of the rutting test is presented on Fig. 1. Most of the loads (81 %) were applied when the surface temperature in bituminous layer was between 30 and 54°C. An analysis was conducted to evaluate how many years of traffic at high temperature, on a real pavement, could correspond to the 162 205 loads applied in the APT experiment, with a surface temperature higher than 30°C.

To do this, we used: -Typical pavement surface temperature variations recorded during one year in Nantes on a bituminous pavement (from February 22th 2017 to February 21th 2018) -A reference traffic: a traffic of 150 heavy vehicles/day (class of traffic T3, in France) was considered, which corresponds to a medium traffic level, on a secondary road [START_REF] Corte | Design of Pavement Structures : The French Technical Guide[END_REF]. -A typical mean daily distribution of traffic, representative of traffic on French roads, determined using data from several vehicle counting stations [START_REF] Mauduit | Evaluation of the concept of equivalent temperature for pavement design[END_REF]. With the data presented above (number of heavy vehicles per day, mean daily distribution of traffic and surface temperature variations), it is possible to calculate a histogram of the number of heavy vehicle passes, as a function of the surface temperature, for the studied pavement. This histogram was calculated for a period of 20 years, which corresponds to the usual pavement design life in France. In this calculation, it was considered that the traffic and the temperature variations are the same each year.

This simplified analysis leads to the conclusion that, with the Nantes climate, a daily traffic of 150 heavy vehicles per day applied during 20 years corresponds, to approximately 177 500 heavy vehicle loads applied when the surface temperature exceeds 30°C. Therefore, it can be considered that the rutting evaluation made on the carrousel represents approximately 18 years of traffic (almost 20 years) during hot periods, on a road with a daily traffic of 150 heavy vehicles per day. This result is obtained for the Nantes climate, which is a mild oceanic climate, with air temperatures varying typically between about 5 °C in the winter and 25 °C in the summer. Of course, the number of years of traffic simulated would be lower for a hotter climate. The same type of analysis could be easily applied to another climate, providing that the necessary pavement surface temperature data are available.

Results and conclusion

Rut depth measurements were made with a profilometer developed at IFSTTAR, equipped with a laser sensor, which can measure the vertical rut depth with an accuracy of 1 mm. The transversal profile of the pavement was measured on a width of about 1.4m. The maximum rut depth value was then determined for each profile. The measurements (4 or 5 measurements per section) were performed on each section. Rut depth was measured at 0, 10 000, 20 000, 40 000 load cycles, and then each 40 000 cycles up to 200 000. Temperature measurements, at the bottom, in the middle of the bituminous layer and at the surface, were also performed during the whole duration of the experiment, at time intervals of 10 minutes. The evolution of mean rut depths (in %) during the test is presented on Fig. 2, as well as the recorded temperatures. The final percentages of rutting are quite the same for the Mix 1 and EME2 sections (about 5%). Percentages of rutting of the Mix 2 (10.9%) and Mix 1 sections (10.0%) are more important. At the end of the test, it could be concluded that:

-Rut depths increased rapidly on all sections during the first 10 000 cycles. This rapid increase could be due to post compaction, and also to the higher temperatures observed during these first 10 000 cycles (up to 53 °C).

-After 10 000 cycles, rutting continued to increase, but at a much lower rate, about 1 % of increase, until 200 000 loads for the EME2 and Mix 3 sections, and 2 % for the Mix 1 and Mix 2 sections. This indicates good performance of all the materials. It is important to note that, in France, for pavements with medium trafic (150 trucks per day), rutting is considered like a severe damage if the rut depth exceeds 20 mm (in our case, about 20% of rut depth).

-The results obtained on the test track are consistent with the laboratory rutting tests. The materials presenting the best performance on the test track (EME2 and Mix 3 sections) also presented the best performance in the laboratory. During the deconstruction, trenches were cut in the pavements to evaluate the deformations of the different pavement layers and conclude about the origin of the rutting. The pavement profiles at the end of the rutting test are presented on Fig. 4. Rutting of the EME2 and Mix 3 sections could be explained by a post compaction of the granular layers. The rut depth is about 6mm for these 2 sections. For Mix1 and Mix2, measured rut depths are respectively 11 and 12.5mm. It seems that for these two sections, rutting could be explained by a post compaction of the granular layer (estimated at 6mm, like for the other sections) and a shear flow mechanism, limited to the top layer, of about 5 and 6.5 mm, which can be considered very low for 20 years of trafic with high temperatures on a pavement. It is possible to conclude that the carrousel test shows that all the bio-recycled asphalt mixtures performed very satisfactory and that the predictions obtained with the laboratory wheel tracking tests were confirmed by the results obtained during the full scale experiment.

Fig. 1 .

 1 Fig. 1. Histogram of surface temperature distribution during the loading phases of the rutting test

Fig. 2 .

 2 Fig. 2. Evolution of rutting with the number of load cycles

Fig. 4 .

 4 Fig. 4. Pavement transverse profiles at the end of the test

Table 1 .

 1 Results of laboratory tests for resistance to rutting. In this experiment, the thickness of the bituminous layer is about 90mm. The subgrade includes a stone bed (50/120 mm), and an unbound granular (UGM) subbase. The bearing capacity of the subgrade was measured at different positions on each structure by means of dynamic plate load test, which gave values between 63 and 85 MPa for the stone bed, and between 94 and 103 MPa on top of the UGM layers. The bituminous mixes were produced and the pavements were built on the 30 th and 31 st of May 2017 by Eiffage. The in situ void contents of the mixes were close to the laboratory values.

		Mixes	Rut depth EU method NF EN 12697-22+A1 30 000 cycles at 60°C Rut Depth (%)	Rutting resistance (flow number) At 7% air void, T=54°C AASHTO TP-79 (cycles)
	Requirements for a high modulus Mix Requirements for a GB4 Asphalt base course material	< 7.5% < 10%	Requirement medium traffic level >190
		Mix 1	5.6% (void content = 4.4%)	609
		Mix 2	4.3% (void content = 3.5%)	578
		Mix 3	3.7% (void content = 5.5%)	668
		EME2	3.1 % (void content = 4.8%)	863
	2	Accelerated pavement test
	The fatigue carrousel of IFSTTAR is an outdoor road traffic simulator designed to
	study the behaviour of real scale pavements under accelerated heavy traffic. The fa-


Diana Khairallah
email: diana.khairallah@ifsttar.fr

Juliette Blanc

Pierre Hornych

Louis-Marie Cottineau

Olivier Chupin

Jean-Michel Piau

Diego Ramirez Cardona

Alain Ducreau

Frederic Savin

Diego Ramirez

Monitoring of railway structures of high-speed line BPL with bituminous and granular sublayers

Keywords: Ballasted track, asphalt concrete, monitoring, sensors, data acquisition

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

1

High-speed line BPL

The phenomena of packing and wear of the ballast, under dynamic stresses, lead to high frequencies of maintenance, and high costs, on high-speed lines. It has been demonstrated that the settlements in the ballast are linked to the high accelerations produced in this layer by the passage of high-speed trains. The solution with bituminous underlay was used since the 1980s in several countries like the United States, Italy, Spain, especially on high-traffic and high-speed lines (HSL). In France, the interest in this technique is recent. Following a first test on the East European HSL, which led to satisfactory behavior, structures with an asphalt concrete sublayer under the ballast were built on a high scale lane, the Bretagne-Pays de la Loire (BPL) highspeed lane. This solution is intended, among other things, to reduce the amplitude of the accelerations produced at the passage of the HST. On HSL BPL, the railway underlayment is made of granular materials on 77 km west of the route and with asphalt concrete on 105 km east of the route as shown on Figure 1. The asphalt concrete (GB) is a class 4 (GB4) according to the NF EN 13-108-1 standard and the unbound granular material (UGM) is of UGM A type with particle size 0 / 31.5 mm.

In the innovative track structure, the upper part of the earthworks (PST) is treated with lime and hydraulic binders and is surmounted by a 15 cm thick unbound granular layer (UGM) and a 12cm thick bituminous layer. The standard structures include a 37 cm thick granular layer under the ballast with a treated PST layer as well.

Instrumenting several sections of the BPL line appeared to be an opportunity to study the behavior of different sections with granular underlayment and with bituminous layers and to compare their behavior. Accelerometers, weather stations, anchored displacement sensors, horizontal and vertical strain gauges, moisture probes and temperature sensors were installed on each section. The instrumentation is detailed in [START_REF] Khairallah | Monitoring of railway structure with bituminous underlayment[END_REF][START_REF] Khairallah | Monitoring of railway structures of the high-speed line BPL with bituminous and granular sublayers[END_REF]. The sensors are connected by cables to the data acquisition systems. The acquisition systems are based on PEGASE boards, developed by IFSTTAR [START_REF] Cam | Improving wireless sensor behavior by means of generic strategies[END_REF]. In this article, only the measurements made with accelerometers and anchored displacement sensors are presented. All the measurement systems are powered by batteries and solar panels, for continuous year-round operation without interruption. All the data collected on each section is transferred continuously via a 4G network (within a few hours maximum) to a remote server. Thus, each system is autonomous in energy and data transmissions. The developed processing routines make it possible in particular to calculate the maximum and minimum values of the sensor signals (Fig. 3), which can then be used to compare the responses obtained on different sections, or for different speeds. The highest peak of deflection is determined for each train passage on each section. For accelerometer signals, a positive and a negative peak (corresponding to upward and downward accelerations respectively) are determined. The different signal treatment procedures were applied to the sensor signal recorded during a period of 23 months, after the opening of the line to commercial traffic. More than 16,000 HST passes were recorded during this period. All these passages were exploited and the maximum deflection values recorded by the anchored deflection sensors were determined. Measurements obtained on section 2 with granular sublayer and on sections 1 and 4 with bituminous sub-layers, presenting different soils were compared. Soil on Section 1 is composed of yellowish fine sand with water inflow detected at 4.5 m depth. Section 4 is built on a yellowish clay soil with sand and scattered blocks. No water was detected on this site. Figure 4 compares the maximum deflection values recorded for all double 30-bogie trains that circulated on the three mentioned sections starting from the track opening to commercial traffic (July 2017) up to May 2019. It can be seen that sections 2 and 4 have similar maximum deflection values, ranging between 25 and 42 mm/100, but with different evolutions in time: on the granular section, deflection peaks present seasonal variations, values increasing between September and May and decreasing during the summer months. On bituminous section 4, the deflection evolution is quite different: a decrease in values during the first 7 months of track service is observed, followed by a stabilization of deflection values, regardless of the season.

On section 1 with bituminous sublayer and sandy soil with water infiltrations, unlike on sections 2 and 4 with clay soil, the maximum deflection values are significantly higher, of the order of 48 to 70 mm/100 (for double trains) . Seasonal variations during the first 7 months of service are also more pronounced on this section.

Acceleration measurements

In a second step, we compare accelerometers measurements obtained on section 2 with granular sub-layer and on section 4 with bituminous sub-layer, during the commercial phase. Figure 5 shows a comparison of the minimum and maximum accelerations for two accelerometers installed below the rail axis at the top of the sub-layer: AS2 on section 2 with granular sub-layer and A3 on section 4 with an asphalt concrete sublayer. The results clearly show that acceleration peaks at the top of the GB sublayer are much lower than those measured at the top of the granular sublayer, both for upward and downward accelerations. It can be that, during the commercial phase, the presence of the GB layer reduces by a factor of about 2 the accelerations measured under the ballast.

Conclusion

High-speed line BPL consists of two different structures, with granular and bituminous sublayers. It was instrumented with more than 100 sensors during construction, to monitor the response of the two types of structures. The commercial phase started on July 1, 2017, and was followed during nearly 2 years, until May 2019. Traffic during this phase averages 23 HST per day, and consists of 4 HST types, single trains with 13 and 15 bogies, and double trains with 26 and 30 bogies. The analysis of this phase therefore required the processing of a large number of measurement files. These data have been processed only partially in this paper, focusing on the evolution of maximum deflections and maximum and minimum accelerations over the duration of the observation period.

The deflection analysis showed deflection levels higher on section 1 than on sections 2 and 4, probably due to differences in soil type and water intervention. Deflections seasonal variations were also highlighted, on sections 1 and 2, but not on section 4. This confirms the large influence of the subgrade soil on the track deflections. No change in the average level of deflection (which may reflect a degradation of the track) was observed during the 2 years follow-up.

The analysis of accelerations measured at the top of the sub-layers indicated a very significant reduction of accelerations on the bituminous section, in comparison with the UGM section, by a factor of about 2. As for the deflection, no evolution of accelerations was observed during the 2-year period, indicating a stable behaviour of the track.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Two structures of the BPL high-speed line

Fig. 3 .

 3 Fig. 3. Retained peaks from (a) deflection signals (b) acceleration signals

Fig. 4 .

 4 Fig. 4. Comparison between the maximum deflection values measured on section 2 with granular sublayer and sections 4 and 1 with bituminous sublayer

Fig. 5 .

 5 Fig. 5. Comparison of positive and negative vertical accelerations peaks measured by accelerometer AS2 on section 2 with granular sub-layer and accelerometer A3 on section 4 with bituminous sub-layer, during the commercial phase