Mélanie F C Denecker
email: melanie.denecker@ifsttar.fr

Laurent Gaillet
email: laurent.gaillet@ifsttar.fr

Chataigner Sylvain
email: sylvain.chataigner@ifsttar.fr

Jean- François David
email: jean-francois.david@ifsttar.fr

Richard Michel
email: richard.michel@ifsttar.fr

Yannick Falaise
email: yannick.falaise@ifsttar.fr

Corrosion and fatigue health monitoring systems appraisal for offshore steel structures

Keywords: condition monitoring, offshore, health monitoring, corrosion, mechanical fatigue, reliability, offshore structure

Offshore structures are faced to aggressive environmental conditions (sea salt, biocolonization), and to high mechanical fatigue due to cyclic wave loading. Because of these conditions, their maintenance plays a key role in their reliability over time. The use of monitoring in such a case could allow drastic decrease of maintenance costs provided safe and reliable systems may be applied on site. The presented work introduces our laboratory investigations concerning different electrochemical and nondestructive techniques used to monitor these damage mechanisms. Firstly, accelerated corrosion tests by artificial tidal cycles with natural seawater are described. They are carried out on carbon steel coupons with and without protective coating in order to monitor the evolution of corrosion versus time. Several parameters are measured during the test such as corrosion intensity, pH, corrosion potential and environmental parameters (temperature, seawater salinity). Secondly, fatigue sensors and more classical strain gauges have been used during mechanical fatigue tests to monitor damage under normalized fatigue cycles. The tests were carried out on T welded joints until failure. For each case, the comparison of the different techniques allows concluding on their suitability. Further investigations in laboratory are currently done to ensure such a conclusion, especially on durability issue. The final validation of these techniques will then be done on site with the help of a reduced-scale prototype at sea near French Atlantic coast.

INTRODUCTION

Offshore steel structures are faced to extreme service loads and aggressive environments. The safety of these structures may be affected by different damages among which corrosion and fatigue are the most prone to occur. In this context, IFSTTAR is currently working with several partners in a project called SURFFEOL (Survey and reliability of offshore wind turbines jackets) in order to give such solutions to offshore wind farms developers. One of the main objectives of this project is to carry out health monitoring of a wind turbine steel foundation to determine damage factors in relation with corrosion and fatigue damages. As inspection operations are particularly difficult to carry out in such environment, many non-destructives techniques have been investigated by various researchers to monitor corrosion and fatigue under offshore conditions [START_REF] Vs | Corrosion detection and monitoring: a review[END_REF]. In previous studies, the authors have precisely identified the locations where the risks of degradation were the highest and listed 8th European Workshop On Structural Health Monitoring (EWSHM 2016), 5-8 July 2016, Spain, Bilbao www.ndt.net/app.EWSHM2016 More info about this article: http://www.ndt.net/?id=19984 the available monitoring techniques for both corrosion and fatigue [START_REF] Gelebart | Optimization of health measurement of offshore structures[END_REF][START_REF] May | Structural Integrity Monitoring: Review and Appraisal of Current Technologies for Offshore Applications[END_REF][START_REF] Ciang | Structural health monitoring for a wind turbine system: a review of damage detection methods[END_REF]. Some of these sensors are now being assessed in laboratory conditions to study their adequacy with the studied phenomena and their reliability. Corrosion and fatigue phenomena can be monitored with different types of sensors such as acoustic emission techniques [START_REF] Ciang | Structural health monitoring for a wind turbine system: a review of damage detection methods[END_REF][START_REF] Guang | An acoustic emission method for the in service detection of corrosión in vertical storage tanks[END_REF], ultrasonic methods [START_REF] Lading | Fundamentals for remote structural health monitoring of wind turbine bladesa preproject[END_REF][START_REF] Sharma | Ultrasonic guides waves for monitoring corrosión in submerged plates[END_REF], optical fibers techniques [START_REF] Ciang | Structural health monitoring for a wind turbine system: a review of damage detection methods[END_REF][START_REF] Bennett | Monitoring of corrosion in steel structures using optical fiber sensors[END_REF], thermal imaging methods [START_REF] Avdelidis | Structural integrity assessment of materials by thermography[END_REF][START_REF] Marinetti | IR thermographic detection and characterization of hidden corrosion in metals: general analysis[END_REF], and others. Specific sensors for corrosion monitoring such as coupled multielectrode sensors are also reported [START_REF] Sun | Real-Time Monitoring of Crevice Corrosion Propagation Rates in Simulated Seawater Using Coupled Multielectrode Array Sensors[END_REF]. In order to assess the suitability of corrosion devoted sensors, an artificial tidal cyclic test with natural seawater, which simulates marine environment, has been set up. In parallel, fatigue tests have being carried out with different sensors. The first part will present thoroughly the led experimental investigations, regarding the studied materials, the corrosion process and related monitoring strategy, and the fatigue investigations with tested sensors. The second part will present the first obtained results, the investigations being still in progress.

EXPERIMENTAL METHODS

Materials

Laboratory investigations were conducted on S355G8+M carbon steel. According to standard NF EN 10225 [START_REF] Norme | Aciers de construction soudables destinés à la fabrication de structures marines fixes -Conditions techniques de livraison[END_REF], S355G8+M is a thermo-mechanically rolled structural grade steel used in offshore industry for the construction of fixed offshore structures. Two specimen dimensions were used: 100x100x8 mm 3 coupons for accelerated corrosion tests and T-welded joint samples for fatigue tests. Corrosion tests were carried out on 3 type of samples tested in artificial tidal and immersion conditions using natural seawater : i) as-received steel coupons, ii) coupons with protective coatings and iii) coupons with the same protective coating including a defect (based on the recommendation of ASTM G8 [START_REF] Astm G | Standard test methods for cathodic disbonding of pipeline coatings[END_REF]). The offshore coatings under investigation are threecomponent coating for tidal zone and two-component coating for immersion condition. The first system is made of Hempadur Multi-strength and Hempathane with a 830 µm total average thickness and the second of Hempadur Multi-strength with a 500 µm total average thickness. A carbon steel rod was fixed and isolated on each coupon to allow a secured connection between coupons and electrochemical instruments. Natural seawater was used and stored in closed tank during 2 months to let the solid particles settle to the tank bottom. Before tests, seawater was filtered with a glass microfiber filter.

Ten similar T-welded assemblies were realized with classical methods used in offshore steel construction. The fatigue class of such details is considered to be G according to BS specifications [START_REF]Code of practice for Fatigue Design and assessment of steel structures[END_REF] and F according to DNV specifications [START_REF]Fatigue design of offshore steel structures[END_REF].

Accelerated corrosion test and corrosion monitoring

The accelerated corrosion test was carried out by exposure of samples to two saline environment conditions: i) to an artificial tidal condition and ii) to an immersed condition in natural seawater. Coupons in the tidal zone were subjected to alternate conditions for 2h immersion in natural seawater and 2h drying at room temperature and relative humidity. The electrolyte solution was introduced into the vessel from an external storage tank through the inflow-pumping valve specifically programmed for our experiments. Coupons were immersed in seawater for 2 h and then seawater was drained through the outflow-pumping valve allowing samples to dry during 2 h at room temperature and relative humidity. Coupons in the immersed zone were subjected to immersion in seawater throughout the experiment duration. The experimental setup is described in Figure 1. During one month of artificial tidal cycles on coupons, the corrosion potential was monitored with a data logger EPC8 from NKE. Data logger EPC8 consists in measuring the corrosion potential between a coupon and a reference electrode every 1 minute. It contains 8 independent measuring channels; it is autonomous and designed to be installed in a humid marine environment. Additional measurements such as the corrosion potential, the polarization resistance R p using a potentiostat (VSP Bio-Logic) were performed to support the corrosion potential measurements from data logger EPC8. The corrosion current density is used instead of R p ; it is calculated from R p values by using Stern-Geary equation [START_REF] El-Mahdy | Electrochemical corrosion monitoring of galvanized steel under cyclic wet-dry conditions[END_REF]. A three-electrode cell configuration was employed for polarization measurement with a saturated calomel reference electrode (SCE) and a 316 L steel grid counter electrode.

Fatigue test and monitoring

In order to assess the suitability of different monitoring devices, it was decided to realize 10 fatigue tests: 6 tests would be carried out at constant stress amplitude but for two different stress ratios R (corresponding to the minimum applied stress divided by the maximum applied stress), 3 tests would be carried out at varying stress amplitude, and 1 test would be dedicated to a preliminary study of durability of the proposed system. We will only present here first investigations related to tests at constant amplitude and for a stress ratio of 0.1 corresponding to the value adopted for the definition of fatigue classes in [START_REF]Code of practice for Fatigue Design and assessment of steel structures[END_REF][START_REF]Fatigue design of offshore steel structures[END_REF]. The investigations were done at a frequency of 7Hz and for stress amplitudes of 110 MPa and 140 MPa. Fatigue phenomenon is associated to three main steps [START_REF] Rolfe | Fracture and fatigue control in structures[END_REF]: cracking initiation, cracking propagation, and failure. Most of existing inspection or monitoring strategies are therefore associated to the ability of crack detection within steel elements [START_REF] Kuhn | Assessment of existing steel structures: recommendations for estimation of remaining fatigue life[END_REF]. As cracks were not accepted to occur in our application, our study focused on monitoring strategies adapted before cracking propagation has started. It is interesting to note in that case, that there is no clear evidence of the level of damage of the structure. The level may only be assessed through indication on the load history of the structure or the element.

Amongst the tested sensors, we will only present here the results given by six resistive strain gauges that were bonded at 15 mm of the weld toe (corresponding to a classical method of measurement) and by two Crackfirst TM sensors (provided by Strainstall UK) that were welded on each side of the reinforcement [START_REF] Zhang | Development of a fatigue sensor for welded structures[END_REF] but on the other side of the plate. The sensors location is shown on Figure 2. Ultrasonic inspection was also ponctually carried out during the experiments to detect crack initiation and propagation.

RESULTS AND DISCUSSION

Potential corrosion monitoring

Laboratory potential measurements were achieved in order to validate techniques that will be used on site. Potential (E corr ) and current density (i corr ) against time obtained for 2 samples in tidal zone and 3 samples in immersion zone are presented in Figure 3 and Figure 4 respectively. The calculation of i corr is given only for the active surface of samples in contact with the solution. In the case of coated coupons, the coating breakdown factor is taken into account according to the recommended practice DNV-RP-B401 [START_REF]Cathodic protection design[END_REF]. One month of measurements is shown for the two environmental conditions. During test, pH, salinity and temperature of seawater were determined in the range of 8.1-8.5, 29.5 -31 g/l and 18.5-20°C respectively. It is important to note that the corrosion potential monitored by data logger EPC8 was continuously measured, even when coupons were not immersed in seawater (i.e. for coupons in tidal zone). Measurements with the potentiostat were performed only during the immersion of all samples in seawater. For samples undergoing tidal cycles, E corr varies with tide between -0.70 V/SCE and -1.4 V/SCE (lowest values) for A1 sample (Figure 3). This variation of E corr is between -0.61 V/SCE and -1.4 V/SCE (lowest values) for A2 sample (Figure 3). The strongest values of E corr correspond to the immersion period and are supported by values obtained from the potentiostat. The corrosion potential decreases rapidly when coupons emerge from seawater as shown by Refait et al. [START_REF] Refait | Corrosion and cathodic protection of carbon steel in the tidal zone: Products, mechanisms and kinetics[END_REF] on unprotected steel coupons. These variations are difficult to explain because of the surface state of samples. Indeed, the imbibition/drying cycles and temperature variations on the surface sample can have an influence on the measured values. For A1 sample, E corr increases slightly from -0.72 to -0.68 V/SCE during the immersion period. This evolution is consistent with i corr decreasing from 3.1 to 0.3 µA.cm -2 that can be explained by the formation of corrosion products on surface sample which can be considered as a protection against corrosion. For A2 sample, E corr and i corr remain almost constant during all test duration at -0.61 V/SCE and 0.8 µA.cm -2 respectively. For samples in immersion zone, E corr provided by data logger EPC8 is more stable (Figure 4) and is consistent with values from the potentiostat. The E corr of unprotected coupon (B1) tends to increase slightly with time from -0.77 to -0.75 V/SCE. These data are in adequacy with the decrease of i corr from 4.7 to 0.9 µA.cm -2 and the formation of corrosion products on surface coupon. The E corr

B1 B2 B3

of B2 and B3 samples fluctuates slightly around -0.71 V/SCE and -0.69 V/SCE respectively. The i corr varies between 0.23 -1.7 µA.cm -2 and 0.05-0. 22 µA.cm -2 for B2 and B3 respectively. The difference between these two samples is the presence of defect that increases i corr of B2 sample. The i corr values of B2 are in the range of these of B1 coupon that can be explain by a corrosion concentration on the defect as observed for the crevice corrosion phenomenon.

In conclusion, the monitoring of the potential with data logger EPC8 gives indications on the evolution of corrosion versus time; data are consistent with those obtained with the potentiostat. These data are also supported by i corr values . Further investigations in laboratory are currently done under cathodic protection (by sacrificial anode and imposed current) to ensure such a conclusion. The final validation of this technique will then be applied on a reduced-scale prototype placed on site.

Fatigue damage assessment

The most common method to determine fatigue assessment on existing structure before cracking propagation relies on strain gage monitoring [START_REF] Kuhn | Assessment of existing steel structures: recommendations for estimation of remaining fatigue life[END_REF]. This strategy consists in recording the local strain variations and post-analyzing them in order to identify the stress ranges, the mean stress values, and the number of cycles for each. To this aim, the most widely used method is the Rainflow method [START_REF] Afnor A | Produits métalliques -Fatigue sous sollicitations d'amplitude variable -Méthode Rainflow de comptage des cycles[END_REF]. Miner's Rule can then be used to assess the final damage based on existing fatigue assessment on similar details [START_REF]Eurocode 3 -Design of Steel structures -Part1.9[END_REF]. Such method therefore requires a rather high amount of recorded data and of electric supply as rather high frequencies of sampling are used.

A novel method developed by Strainstall UK,Crackfirst TM sensor, consists in the fixation of a pre-cracked patch on the structure and on the monitoring of the rate of increase of the crack length of this patch. The method requires a precise calibration in laboratory that has been led with the help of The Welding Institute (TWI) [START_REF] Zhang | Evaluation of CrackFirst TM fatigue sensors[END_REF]. The crack propagation is followed through the failure of electrical links on both sides of the crack (Figure 5). One sensor gives therefore two measures of the length of the patch crack that may be related with the fatigue damage of the structure through the existing calibration. The advantage of such a sensor is that it requires much less energy as there is no required post-treatment and as it may be interrogated at any time. The amount of data is also much less important as the only information is the occurrence of failure of the electric wires. Of course, it has also some drawbacks, the main one being that it is impossible to get access to data concerning strain history of the studied detail. Though, the investigations are still in progress, first results are presented in Figure 6 for the sample stressed at an amplitude of 110 MPa (this sample was then stressed at 140 MPa to increase the damage rate) and 140 MPa at a ratio of 0.1. It can be stated that the results of Crackfirst sensor for that case are in close agreement with the expectations. The sensor even seems to give conservative indications close to the total design life. Such sensor may thus offer a reliable alternative to the commonly adopted strain measurement methodology that requires important supply and amount of data to be recorded. For all the tests that were carried out, the number of cycles for crack initiation determined from ultrasonic investigations and the total number of cycles at failure are reported in Table 1. This allows from the DNV specifications [START_REF]Fatigue design of offshore steel structures[END_REF] and the Miner's rule [START_REF]Eurocode 3 -Design of Steel structures -Part1.9[END_REF] the determinations of the fatigue degrees of damage at crack initiation and at failure. It can be observed that recommendations are very conservative for our case, as the determined fatigue damage degrees are above 2.5 for crack initiation.

CONCLUSIONS

In this study, two experimental approaches were used to assess the suitability of corrosion and fatigue sensors chosen to monitor the evolution of corrosion and fatigue on site. Potential data obtained with data logger EPC8 and the potentiostat are similar and consistent with the evolution of the current density. Data logger EPC8 allows to monitor the corrosion potential and give qualitative information on corrosion. Design fatigue life obtained for Crakfirst sensors is in adequacy with the DNV specifications [START_REF]Fatigue design of offshore steel structures[END_REF]. Crackfirst sensor can give some indications on the total design life of structure; it offers a good alternative to the classical strain measurement methodology. Further investigations are in progress to validate the choice of these sensors to study both damaging phenomena. The final validation of these techniques will then be applied on a reduced-scale prototype placed on site near French Atlantic coast.

Figure 1 :

 1 Figure 1: Experimental setup for the accelerated corrosion test

Figure 2 :

 2 Figure 2: Experimental setup of T welded joint plate for the fatigue test and location of sensors

Figure 3 :A2Figure 4 :

 34 Figure 3: Corrosion potential and current density vs. time for uncoated steel coupon (A1), coupon with protective coating including a default (A2) under tidal conditions, and zoom on A2 for time interval between 100-150h

Figure 5 :

 5 Figure 5: Photo of a Crackfirst sensor

Figure 6 :

 6 Figure 6: Monitoring results in fatigue for the test at R=0.1 and Δσ=140 MPa

Number of cycles (Δσ=110 MPa) Design life in number of cycles at Δσ=110 MPa Number of cycles (Δσ=140 MPa) Design life in number of cycles at Δσ=140 MPa Degree of fatigue

	Sample 1	Crack detection (US) Failure	991 486 991 486	538000 538000	190000 580000	261000 261000	2,6 4,1
	Sample 2	Crack detection (US) Failure			719000 1339430	261000 261000	2,8 5,1

1 and Δσ=140 MPa

Table 1 :

 1 Determination of the degree of fatigue damage for the realized investigations

ACKNOWLEDGMENT

SURFFEOL project is led by STX France and supported by the region of Pays de la Loire (France). STX France is acknowledged for kindly providing S355G8+M carbon steel samples for both tests. The region of Pays de la Loire is acknowledged for supporting this study.


Emilie Leprêtre
email: emilie.lepretre@ifsttar.fr

Sylvain Chataigner
email: sylvain.chataigner@ifsttar.fr

Lamine Dieng
email: lamine.dieng@ifsttar.fr

Laurent Gaillet
email: laurent.gaillet@ifsttar.fr

Arnaud Gagnon
email: arnaud.gagnon@cerema.fr

Jeremy Roth
email: jeremy.roth@cerema.fr

Corentin Leroy
email: corentin.leroy@cerema.fr

Experimental characterization of different adhesively bonded composite reinforcements for old steel structures

Keywords: CFRP, Adhesively bonded reinforcement, Old steel structures, Shear capacity LEPRETRE, Emilie, CHATAIGNER

Externally bonded composite reinforcements have proven their efficiency in the strengthening of existing concrete structures. Regarding their use in the case of old steel structures, there is however much less literature, more particularly concerning their use for old steel materials such as puddled iron and mild steel. For these steel structures, the adhesive joint behavior becomes more critical than in the case of concrete, as it would be the weak point of the assembly. This study investigates the bond behavior between CFRP (Carbon Fiber Reinforced Polymer) laminates and different old steel materials (mild steel and puddled iron). Different reinforcement processes (different composite elastic modulus and thicknesses, also with different adhesives) have been experimentally studied investigating the materials' behavior (steel, and adhesive) and the assembly behavior through the realization of single lap shear tests. Some of the shear samples were monitored using strain gages during the test in order to determine an equivalent interfacial behavior that may be used during the design process of the reinforcement or Finite element modelling. Additionally, some load/unload cycles have been applied to check their effect on the assembly capacity. Experimental results show different shear capacity and bond behavior for the different modulus of composite material but not for the different metallic substrates. Likewise, load/unload cycles underline the evolution of different behavior for both normal modulus and ultrahigh modulus CFRP.

Introduction

The potential of externally bonded FRP composites in strengthening steel structures has been clearly shown in some studies ( [START_REF] Xia | Behaviour of FRP-to-steel bonded joints[END_REF][START_REF] Yu | Experimental study on CFRP-to-steel bonded interfaces[END_REF][START_REF] Fawzia | Bond-slip models for double strap joints strengthened by CFRP[END_REF][START_REF] Zhao | State-of-the-art review on FRP strengthened steel structures[END_REF]). The excellent properties of the CFRP composite, such as non-corrosion, high strength, low weight and easy handing, make it an ideal choice for the strengthening of existing metallic structures. Regarding their use in the case of old steel structures, there is however much less literature, more particularly in the case of old steel materials such as wrought iron (puddled iron) and mild steel. Since CFRP composites are externally bonded to steel elements, a good understanding of bonding performance and mechanisms of the CFRP-to-old steel interface is thus essential. In this paper, the bond behavior between CFRP (Carbon Fiber Reinforced Polymer) laminates and different old steel materials (mild steel and puddled iron) is investigated. Single lap shear tests with three different reinforcement processes (CFRP laminate + adhesive) have been carried out. Normal Modulus CFRP laminates products and Ultra High Modulus CFRP laminates products as well as linear and nonlinear adhesives products were used. The first results deal with the assessment of the bond capacity and the failure mode of the different reinforcement processes. Comparison between S235 carbon steel plate and wrought iron (puddled iron) plate are done. Furthermore, some of the single lap shear samples were instrumented by strain gauges and some load/unload cycles have been applied to check their effect on the assembly capacity. The local bond-slip relationships for the different reinforcement process were also determined [START_REF] Chataigner | Characterization of composite to concrete bonded interface: Description of the single lap shear test[END_REF].

Experimental study

Specimen details and test procedure

The experimental program consisted of eighteen single lap shear coupon tests with identical geometrical form shown in figure 1. The principle of the single lap shear test is to load the adhesive joint in shear by applying tension at the free end of the bonded CFRP plate. The maximum value of the load that can be applied by the hydraulic jack is 100 kN. The testing machine was previously detailed in [START_REF] Chataigner | Characterization of composite to concrete bonded interface: Description of the single lap shear test[END_REF]. Table 1 gives the specimens tag and their characteristics. "S" refers to S235 carbon steel plate and "I" to wrought iron plate.

Specimen configuration

*manufacturer data e and w represents the thickness and the width of the CFRP laminate, respectively

Table 1: Specimen's designation and characteristics

Three coupon tests were realized for each specimen configuration. Among these three coupons test, only one was instrumented with strain gauges bonded on the CFRP laminates. The strain gauges arrangement is given in figure 1.b).

The non-instrumented coupon samples were tested until failure in order to assess the bond capacity and the failure mode of each reinforcement process, whereas the instrumented specimens were submitted to load/unload cycles before testing until failure.

For NM specimens the load/unload cycles realizes before testing until failure are the following: 10kN, 20kN, 30kN, 45kN. For UHM specimens the load/unload cycles are the following: 10kN, 30kN, 50kN, 70kN, 80kN, 90kN. Note that a five-minute time period is observed between each load/unload cycles.

Materials

S235 carbon steel grade was chosen due to its mechanical properties near those of mild steel currently present in old structures [START_REF] Bassetti | Application de lamelles précontraintes en fibres de carbone pour le renforcement d'éléments de ponts rivetés endommagés par fatigue[END_REF], while wrought iron material originates from the dismantling of an old riveted bridge.

Tensile tests were carried out to assess the mechanical properties of the S235 carbon steel and wrought iron materials used in the study. Wrought iron shows lower mechanical properties than S235 carbon steel. The results are listed in Table 2.

Material

(5.3) The values correspond to an average values from three test results, the standard deviation is indicated in brackets

Table 2: mechanical properties of metallic materials

Tensile tests were also carried out for adhesive A and adhesive C. For adhesive B, manufacturer data were used. The results are listed in Table 3 and the tensile stress-strain curves are shown in figure 2.

Adhesive

Tensile strength (MPa) Tensile Modulus (GPa) The three types of adhesives used in this study cover a wide range of elastic modulus and tensile strength (see Table 3). They also include both linear adhesives (adhesive A and B) and non-linear adhesive (adhesive C).

Experimental results and discussion

3.1

Failure modes and bond capacity

The failure modes and ultimate loads obtained for the non-instrumented specimens are presented in Table 4. The specimens after failure are shown in Figure 3.

For specimens with NM CFRP laminates the failure mode was mainly cohesive in the adhesive layer, whereas for specimens with UHM CFRP laminates the failure mode was mainly due to CFRP delamination, whatever the metallic material of the plate. These results indicate that the adhesive-to-steel interface is stronger than interface between carbon fibers and resin matrix for the UHM CFRP laminate. The ultimate capacity of the NM specimens (E=165 GPa and E=210 GPa) are similar, whatever the metallic materials. UHM specimens show very high ultimate capacity (one specimen exceeded the maximum load that can be applied by the testing machine).

Specimens

Effect of load/unload cycles

The failure modes and ultimate loads obtained for the instrumented specimens submitted to load/unload cycles are presented in Table 5. The load-displacement curves obtained for the different CFRP reinforcement process are shown in figure 4. A very short plateau is observed at the end of the test for NM specimens (more visible for reinforcement process B). The small size of the plateau is due to the use of a short bond length (120 mm) compared to the effective bond length. Based on this observation, reinforcement process B seems to have an effective bond length shorter than reinforcement process A.

Specimens

For UHM specimens, the delamination failure (inter-laminar failure of CFRP) was rather brittle which explains why the load-displacement curves of these specimens show no plateau. Moreover, for the load/unload cycled UHM specimens, the specimens failed by rupture of the CFRP plate in a brittle way (see figure 5). This must be due to fatigue failure closed to the grips used to apply the force. For NM specimens, a cohesive failure in the adhesive layer is still observed. NM specimens using reinforcement processes A and B, show similar load-displacement curves for both loaded and unloaded specimens. The behavior of the NM joints seems linear, which is not the case for the UHM specimens which display a nonlinear behavior for both loaded and un-loaded specimens. Moreover, UHM reinforcement process shows an ultimate load and displacement larger than for NM reinforcement process. This phenomenon is due to the materials properties (high elastic modulus for CFRP laminate and high ultimate strain for the adhesive). We can also note that the behavior of the joint do not seem to be much affected by the metallic material of the plate whatever the reinforcement process. For all specimens submitted to load/unload cycles, the initial stiffness is larger. Moreover, for UHM specimens, the increase of the initial stiffness of the loaded specimens is much higher than for NM specimens. However, UHM specimen submitted to load/unload cycles shows lower ultimate strength than the unloaded UHM specimens (a loss of 20% strength was observed, but it is important to note that the failure mode is modified and occurs within the CFRP plate). This is not the case for NM specimens for which a small increase of the ultimate strength is obtained for loaded specimens (around 10%).

Strain distribution

Axial strain distributions along the CFRP plate are shown in Figure 6 for the different specimens' configuration and at different load levels. The axial strain values are given by the strain gauges bonded to the top surface of the CFRP laminate. For each reinforcement process, values obtained from S235 carbon steel and wrought iron plates are similar. For this reason, the results for only one plate material are presented here for each reinforcement process. For NM specimens (reinforcement processes A and B), the strain distributions are very similar. Compared with UHM specimen, significant CFRP strains are developed in the NM specimens. In the case of UHM specimen, a much larger length of the CFRP laminate seems to be mobilized, which implies a larger effective bond length. This phenomenon can be due to the softer nonlinear behavior of the adhesive used in the UHM reinforcement process but also to the higher modulus of the CFRP (see Yu et al., 2012 [2]).

In Wu et al., 2012 [8], relationship of bond strength and bond length was studied for Araldite 420 and Sikadur 30 adhesives. The authors showed that the effective bond length of specimen with UHM CFRP laminate (E = 460 GPa) bonded to steel with Araldite 420 adhesive and Sikadur 30 adhesive was approximately 100-120 mm and 70-100 mm respectively, which is convenient with our results.

Bond-slip relationship

Bond-slip curve depicts the relationship between the local interfacial shear stress and the relative slip between the two adherents. The average experimental shear stress was calculated from the data given by the strain gauges bonded on the top surface of the CFRP laminates. The determination of the shear stress, τ, and the local slip, , between two strain gauges is made according to equations 1 and 2, [START_REF] Ghiassi | Numerical analysis of bond behaviour between masonry bricks and composite materials[END_REF].

. . . ( Equation 1)
. ∑ . ! (Equation 2)

Figure 7 shows the bond-slip curves obtained using equations 1 for all the specimens. NM specimens failed by cohesive failure in the adhesive that is not the case for UHM specimens for which the CFRP laminate rupture is observed near the grips. Thus, for UHM specimens, the bond-slip relationship is not entirely governed by the adhesive properties and the bond-slip curve obtained for both S235 carbon steel and wrought iron plate shall be valid only as long as the CFRP laminate is undamaged. The results obtained for both S235 carbon steel plate and wrought iron steel plate are similar, which implies that the bond-slip curve is not affected by the materials of the metallic plate. The bond-slip curves obtained for NM specimens have similar shapes and characteristic values to those obtained by Xia et al., 2005 [1] and [START_REF] Chataigner | Characterization of composite to concrete bonded interface: Description of the single lap shear test[END_REF], for CFRP laminate strengthened steel plate. The values of initial and maximal slips as well as the maximum shear stress value depend on the material properties of both CFRP laminate and adhesive. Thus for NM specimens, reinforcement process B shows smaller values for initial and maximal slips compared to reinforcement process A. This phenomenon can be due to the higher modulus of elasticity and the smallest ultimate strain value of the adhesive for reinforcement process B (see Fawzia et al., 2010, [3]). A bilinear bond-slip model can approximate the obtained experimental curves for NM specimens.

For UHM specimen, the bond-slip curve shape is different than for NM specimens and can be explained by the nonlinear behavior of the adhesive used and the failure mode of the specimens. In fact, as shown in the previous part, the failure of the UHM specimens is not entirely governed by the adhesive layer (delamination failure occurred for monotonically tested specimens and rupture of CFRP laminate occurred for load/unload cycled specimens). Local damages in the UHM laminate can thus explain this bond-slip curve shape (wide dispersion of the results). Nonetheless, the value of initial slip obtained for UHM specimens is convenient with those obtained by Wu et al., 2010 [8] for UHM CFRP laminate with a modulus of 460 GPa and Araldite 420 adhesive (nonlinear adhesive, see [START_REF] Yu | Experimental study on CFRP-to-steel bonded interfaces[END_REF]). Furthermore, the smallest initial slip for UHM specimens can be explained by the high stiffness of the CFRP laminate.

Conclusions

In this paper, the test results from single lap shear joint specimens were presented. Three types of CFRP reinforcement process (CFRP laminates + specific adhesives) were tested. Some of the tested specimens were instrumented by bonded strain gauges and submitted to load/unload cycles before testing until failure.

The findings from this experimental study are summarized as follows:

-For both monotonic and load/unload cycles testing, the results do not depend on the material properties of the metallic plate. -Load/unload cycles affect the behavior of the joint. For NM specimens, an increase of the initial stiffness and of the ultimate strength was observed, whereas for UHM specimens, a much higher increase of the initial stiffness is observed but the ultimate strength of the joint is reduced by around 20% (with a modification of the failure mode certainly due to our test setting). Additional investigations are needed concerning that last issue. -For NM specimens, cohesive failures in the adhesive layer are observed for both monotonic and load/unload cycles testing. For UHM specimens, delamination failure occurred for monotonically tested specimens while CFRP plate rupture occurred for load/unload cycled specimens. -Assessment of the effective bond length is difficult due to the short bonded length of the tested specimens. Nonetheless, based on the strain distributions for all reinforcement process, UHM specimens seem to have a higher effective bond length than NM specimens which show similar results

for the two NM reinforcement processes (in all case, the effective bond length seems to be less than 120 mm which is in agreement with [START_REF] Chataigner | Characterization of composite to concrete bonded interface: Description of the single lap shear test[END_REF] and [START_REF] Wu | Experimental study on bond behaviour between UHM CFRP laminate and steel[END_REF]). -Experimental bond-slip curves for NM specimens show a bilinear shape similar to that obtained by Xia et al., 2005 [3] and Chataigner et al., 2009 [5]. The key parameters values (maximum shear stress, initial slip and maximum slip) depend on the materials properties of the reinforcement process (CFRP plate and adhesive).

The results of these investigations are the starting point of a broader study aiming at assessing the ability of CFRP reinforcement to decrease the rate of crack propagation on both mild steel and wrought iron. It allowed designing the reinforcements and assessing the initial mechanical performances of the assembly.

Figure 1 :

 1 Figure 1: Single lap shear test specimens' geometry: a) side view; b) top view

Figure 2 :

 2 Figure 2: Tensile stress-strain curves of adhesives used in the study (N.B: data for adhesive B are from [2])

Figure 3 :

 3 Figure 3: Typical failure modes: a) I_NM_A_3 specimen; b) S_NM_B_1 specimen; c) I_UHM_C_3 specimen

Figure 4 :

 4 Figure 4: Load-displacement curves (unloaded and loaded specimens): a) NM_A specimens; b) NM_B specimens; c) UHM_C specimens

Figure 5 :

 5 Figure 5: UHM laminate failure for cycled specimen

Figure 6 :

 6 Figure 6: Strain distributions (variation with applied load level, F (kN), until the ultimate load Fu (kN)): a) I_NM_A_2; b) S_NM_B_2; c) S_UHM_C_2

Figure 7 :

 7 Figure 7: Experimental bond-slip curves for NM specimens: a) reinforcement process A; b) reinforcement process B; c) reinforcement process C

Table 4 : Test results

 4

		Metallic plate	CFRP reinforcement process	Ultimate capacity (kN)	Ultimate strength (MPa)	Failure mode
	S_NM_A_1 S_NM_A_3	S235	A	41,89 44,41	698,17 740,17	Cohesive failure
	S_NM_B_1 S_NM_B_3	S235	B	39,76 37,82	662,67 630,33	Cohesive failure
	S_UHM_C_1 S_UHM_C_3	S235	C	94,31 99,95	788,55 835,7	CFRP delamination
	I_NM_ A_1 I_NM_A_3	Wrought iron	A	35,77 39,89	596,17 664,83	Cohesive failure
	I_NM_B_1 I_NM_B_3	Wrought iron	B	35,71 44,48	595,17 741,33	Cohesive failure
	I_UHM_C_1 I_UHM_C_3	Wrought iron	C	>100 91,77	836,12 767,31	CFRP delamination

Table 5 : Test results

 5

		Metallic plate	CFRP reinforcement process	Ultimate capacity (kN)	Ultimate strength (MPa)	Failure mode
	S_NM_A_2	S235	A	47,1	785	Cohesive failure
	S_NM_B_2	S235	B	41,38	689,67	Cohesive failure
	S_UHM_C_2 S235	C	86,57	723,83	CFRP delamination
	I_NM_A_2	Wrought iron	A	46,01	766,83	Cohesive failure
	I_NM_B_2	Wrought iron	B	42,1	701,67	Cohesive failure
	I_UHM_C_2	Wrought iron	C	77,74	650	CFRP delamination

Acknowledgements

The authors would like to thank Freyssinet, Sika, Fyfe agencies and the French national rail company SNCF for having provided materials.


Sylvain Chataigner

Juan Murcia-Delso

Clara Jiva Schulte

Presentation of the FASSTbridge project: FASt and effective Solution for STeel bridges life-time extension

Keywords: CFRP, Adhesively bonded reinforcement, Steel structures, Fatigue

One of the goals of sustainable development applied to bridge infrastructure is to provide bridge owners with strengthening solutions that may lead to an increase of existing structures service life. In the case of steel bridges, the assessment of the remaining service life is most often linked to the determination of the structural deterioration caused by corrosion and fatigue. Damage caused by fatigue is very difficult to assess before crack initiation, and is more bound to occur in old structures, for which the phenomenon was not taken into account in design before 1970. In addition, old steel materials present a more brittle behavior. The FASSTbridge project, financed through European Infravation call, is aimed at developing a pre-cracking fatigue assessment and strengthening methodology for steel bridges, based on the technology of adhesively bonded composite. The methodology will include an assessment method of remaining fatigue life of existing structures, the design and on-site application of a strengthening system, and the environmental and economical appraisal of the solution. A new strengthening technique based on CFRP plates will be developed. This technique will include the formulation and production of a specific adhesive, specifications for using commercial CFRP plates, and the definition of instrumentation plans to monitor the performance of the strengthening system. Finally an on-site application in an actual steel bridge will be carried out, to verify the quality of the proposed solution.

INTRODUCTION

Steel bridges serve as vital components in the transportation infrastructure. Bridges are a frequent cause of the major negative impact in densely populated areas with regard to both users' convenience (such as service disturbances, disruptions, accessibility problems, delays, traffic jams) and welfare (such as obstacles to safety and security, nuisance from noise, vibration dust, and air pollution). Moreover, problems derived from their improper functionality are also cause of important impacts in the economic activities of the affected area (transportation, job accessibility, development strategies). In Europe 15% of the 300 000 bridges are made of steel or they have concrete-steel composite structures [START_REF] Ye | P: A State-of-the-Art Review on Fatigue Life Assessment of Steel Bridges[END_REF]. Of this number, it is considered that approximately 68% need structural interventions. In the USA, 34% of the 599 000 bridges are made of steel. Of this number, approximately 9% are classified as structurally deficient, 15% are functionally obsolete and 9.5% are both structurally deficient and functionally obsolete [START_REF] Lee | Current State of Bridge Deterioration in the U[END_REF]. Many of these bridges were constructed using old standards and for a design service life of 50 years, which is coming to its end or has already been exceeded.

Fatigue is the second main cause of damage after corrosion for steel bridges, limiting their load-carrying capacity and residual life, and is one of the main causes involved in fatal mechanical failure of this kind of asset. The increase of traffic flows and loads in the last decades has a direct influence on this issue, especially on structures designed and erected many years ago for which fatigue was not taken into account during design [START_REF] Palmer | The development of welded steel fatigue design guidelines[END_REF]. Fatigue is a progressive and local weakening process in which structural damage is accumulated due to the continuous and repetitive application of external loads (vehicles and trucks in the case of steel bridges). The process of fatigue consists of three steps: crack initiation, crack propagation and failure. This phenomenon is extraordinarily dangerous and difficult to identify with a conventional structural stress analysis, which might lead to a misleading result of safety. In addition, there is no mean to measure fatigue damage on site before crack initiation. Damage can only be assessed using either fatigue design loading from the standards, or long-term on-site strain measurement [START_REF] Kühn | Assessment of Existing Steel Structures: Recommendations for Estimation of Remaining Fatigue Life[END_REF]. The corresponding methods, that allow the determination of the remaining service life, can be based on the characteristics of the studied asset (deterministic methods) or in existing statistics (probabilistic). The latter require a considerable amount of data, which is still scarce. Therefore, deterministic methods are commonly used.

Currently, the mainstream strategy concerning fatigue has been a reactive strategy. Maintenance or repair indeed has mainly occurred after the appearance of cracks in the structure. With a bridge stock that is inevitably ageing, it is necessary to widely adopt a preventive strategy to enable road administrations to enlarge the service life of steel and composite steel bridges in a cost-effective and sustainable manner to avoid the high economic and environmental costs of following the current strategy in the years to come. The prevention of fatigue is thus a high priority. To provide such a strategy, it is essential to seek an easyto-apply solution which includes an engineering analysis methodology for assessing the fatigue damage status and the application of fast, cost-effective and sustainable retrofitting strengthening techniques that enable the wide adoption of the preventive approach, avoiding difficult, resource-consuming and costly retrofitting and repair interventions and demolitions of the steel and composite steel bridges stock.

Once fatigue damage has been identified, normally by visual inspection for crack detection, various methods are frequently used to strengthen steel bridges. The most popular among engineers is the attachment of steel plates to the tension flange of the girders (FHWA, 2013) (Fig. 1). However, this method has several disadvantages: plates are usually bulky, heavy, difficult to fix and prone to corrosion and fatigue. In general, conventional strengthening techniques are labor-intensive and disruptive to traffic flows, thus limiting their application in the wide network of European and American steel bridges. Carbon Fibre Reinforced Polymers (CFRP) composites, although more expensive than steel plates in terms of price per unit/m 2 , have several and relevant advantages that make them suitable and cost effective for steel bridges retrofitting: their application is less time consuming than traditional solutions (i.e. from one month to a few days, therefore less traffic disruptive), have a high strength-to-weight ratio, excellent fatigue properties, low space disruption, high durability and versatility, and are easy to transport, handle and apply (without heavy equipment) [START_REF] Tavakkolizadeh | Fatigue strength of steel girders strengthened with carbon fiber reinforced polymer patch[END_REF][START_REF] Bocciarelli | Fatigue performance of tensile steel members strengthened with CFRP plates[END_REF] (Fig. 1). Despite their wide application on indoor concrete structures, where CFRP is progressively replacing the traditional techniques, CFRP has not been broadly used for steel structures yet, especially in bridges. The main technical barrier for the adoption of CFPR is the durability of existing commercial adhesives in outdoor environments, which is still limited for this kind of interventions, where high functional periods with minimum maintenance are expected. When exposed to outdoor environments and fatigue conditions, in order to ensure cost-effective and durable strengthening interventions, high quality adhesives are necessary [START_REF] Zhao | FRP-strengthened metallic structures[END_REF].

OBJECTIVES OF FASSTbridge project

FASSTbridge aims at drastically reduce the economic and environmental costs of ownership of the steel bridges stock in Europe and the USA by providing a reliable preventive, cost-effective and sustainable solution for steel bridges life-time extension. The preventive nature of the solution is the key to costeffectiveness and sustainability, since it will allow the timely design and implementation of innovative, competitive CFRP-based strengthening actions that will reduce the overall costs and environmental impact of life-time extension.

This solution will stand on two pillars. The first will be dedicated to the proposal of a FASSTbridge methodology to prevent the evolution of irreversible fatigue derived problems at a pre-cracking scenario. The second will concern the development of a FASSTbridge strengthening system to preventively extend life-time of steel bridges.

FASSTbridge methodology

The methodology will address the development and validation of a preventive and easy-to-apply procedure to assess the remaining life of steel bridges. Such a procedure should be in agreement with existing methods presented in (AASHTO, 2012;[START_REF] Kühn | Assessment of Existing Steel Structures: Recommendations for Estimation of Remaining Fatigue Life[END_REF], and should propose a clear strategy between the classical nominal stress method and the local stress approach. It should also be supported by indicators from non-destructive or monitoring methods.

If insufficient remaining service life is determined, a proposal will then be made regarding the design and the application of adhesively bonded composite reinforcement for fatigue strengthening operations. This will be based on existing guidelines and previous investigations (CNR, 2007;DNV, 2012;[START_REF] Schnerch | Design guidelines for the use of HM strips: strengthening of steel concrete composite bridges with high modulus carbon fiber reinforced polymer (CFRP) strips[END_REF][START_REF] Cadei | Strengthening metallic structures using externally bonded fibre-reinforced polymers[END_REF].

The methodology will also include a strategy regarding the maintenance of the strengthening, and the monitoring to verify the efficiency of the structural reinforcement and to contribute to databases for the assessment of steel bridges in fatigue.

FASSTbridge strengthening system

The proposed strengthening system will rely on the development of a specific adhesive for the considered application. The adhesive should be durable and allow a correct force transfer between CFRP plate and steel adherend. It should also have good rheological performance before curing to allow an easy on site application. The obtained mechanical properties of the cured adhesive should be durable and in agreement with existing standards. Associated to this adhesive, a commercial CFRP plate will be chosen according to the requested technical and economical properties needed for the project.

The compatibility of both parts (adhesive and CFRP plate) will be verified through different tests on the assembly and under different environmental conditions relevant with the design codes recommendations (AASHTO, 1989;EN1990, 2002EN 1991EN , 2003)). The experiments will also check the compatibility of different monitoring devices with the assembly. The led experimental investigations will be followed by the definition of a pre-certification plan for the studied system.

FASSTbridge solution

In addition to the assessment tools and strengthening system, the FASSTbridge solution will be completed with a method to evaluate the effectiveness and efficiency of the interventions based on costbenefit analysis and life cycle analysis.

An on-site application will be carried out at the end of the project to demonstrate the applicability of the complete solution. This will be done on a composite steel/concrete bridge from the community of Madrid. It will include the application of complete methodology (determination of the remaining life-time, design of the reinforcement, cost-benefit and life-cycle analyses, application of the reinforcement and monitoring on site) (Fig. 2).

Demonstration

To ensure the achievement of the stated objectives a work plan, including 6 work packages, has been defined for the project (Fig. 3). The first package is dedicated to the overall management and coordination of the project. The last package aims at ensuring good dissemination of the results. The other four packages are the technical ones and will be thus more deeply described in the following paragraphs. The work package 2 will be dedicated to design the FASSTbridge methodology regarding the assessment of the remaining fatigue life of the structures, the design of the reinforcement and its maintenance. A review of existing methods regarding fatigue will be carried out for both US and European cases (Fig. 4). This should lead to a proposal of an assessment method placed between the classical nominal stress method and the local stress approach [START_REF] Kim | Novel hot spot stress calculations for welded joints using 3D solid finite elements[END_REF]. If the assessment proves one or more detail to be prone to fatigue damage, inspection should be carried out to check the existence of cracks.

In the case of existing cracks, calculation methods based on fracture mechanics will be recommended.

Adhesively bonded CFRP may be used to slow crack propagation in such a case [START_REF] Bassetti | Crack repair and fatigue life extension of riveted bridge members using composite materials[END_REF][START_REF] Liu | Experimental study of fatigue crack growth behaviour in adhesively reinforced steel structures[END_REF][START_REF] Lepretre | Reinforcement of a cracked steel plate using CFRP bonding[END_REF], which will not be included in the project since the aim of FASSTbridge is to define a preventive solution.

In the case where no cracking occurred, the second part of WP2 will be dedicated to the definition of the design of strengthening, using adhesively bonded CFRP plates and based on existing recommendations (CNR, 2007;DNV, 2012;[START_REF] Schnerch | Design guidelines for the use of HM strips: strengthening of steel concrete composite bridges with high modulus carbon fiber reinforced polymer (CFRP) strips[END_REF][START_REF] Cadei | Strengthening metallic structures using externally bonded fibre-reinforced polymers[END_REF]. This will also include a specific review on the topic of monitoring and the definition of a specific strategy for the studied case. Work package 3 aims at defining the FASSTbridge reinforcement system, i.e. developing the adhesive, choosing the CFRP plate and assessing their compatibility. The adhesive will be developed by Collanti according to the specifications of the adhesive defined at the beginning of the and dedicated to obtain durable and reliable performance. The rheological behavior of the adhesive will be assessed as in [START_REF] Benzarti | Accelerated ageing behaviour of the adhesive bond between concrete specimens and CFRP overlays[END_REF][START_REF] Houhou | Analysis of the non-linear creep behavior of FRP-concrete bonded assemblies[END_REF] to check that the developed adhesive meets the requirements in terms of ease of application, mechanical properties, thermal properties (glass transition temperature) and durability.

The technical and economical properties of the requested CFRP plate will then be determined and a commercial product chosen. Its compatibility with the developed adhesive will be assessed using first prototype testing on the assembly.

Fig. 5: Experimental results (glass transition temperature and mechanical tensile behavior) regarding the influence of hydrothermal ageing on two commercial epoxy resins [START_REF] Benzarti | Accelerated ageing behaviour of the adhesive bond between concrete specimens and CFRP overlays[END_REF] 3.3 WP4: Strengthening system process qualification This work package is entitled to validate the adopted strengthening system through an extensive experimental campaign, designed according to the requirements of the system from the literature (Lignhoff et al., 2009;[START_REF] Zhao | State-of-the-art review on FRP strengthened steel structures[END_REF][START_REF] Ghafoori | Design criterion for fatigue strengthening of riveted beams in a 120-year-old railway metallic bridge using pre-stressed CFRP plates[END_REF]Hesmati et al., 2015;[START_REF] Dawood | Environmental durability of a CFRP system for strengthening steel structures[END_REF].

The influence of the execution process and the temperature will be investigated by static tests as the one defined in [START_REF] Chataigner | Use of a single lap shear test to characterize composite-to-concrete or composite-to-steel bonded interfaces[END_REF]. Additionally, specific campaigns dedicated to the study of the durability of the reinforcement process, will also be carried out similarly to campaigns done in [START_REF] Chataigner | Adhesively bonded composite reinforcements for steel structures: durability of the stress transfer[END_REF] (Fig. 6). The experimental campaign will also be designed to assess the possibility to integrate sensing capabilities to the reinforcement in agreement with the defined methodology.

Work package 4 will also lead to the definition of the cost-benefit and the life-cycle analyses. It will describe the proposed adopted methodology for both cases and the requested information, so that both may be carried out. The last technical package aims to demonstrate the applicability of the whole proposed solution on a real bridge. For this purpose, the community of Madrid has proposed to analyze one of the bridges within their area of responsability (Fig. 7). The chosen bridge is a composite bridge erected in the 1960s. It has suffered a considerable increase of traffic flow that was not considered during design, but does not present damages. The FASSTbridge methodology will be applied on this bridge to assess its remaining fatigue life. A design of an adhesively bonded CFRP reinforcement will then be done for the most critical details. This will allow to carry out both cost-benefit and life cycle analyses of the solution in that particular case and to apply the designed reinforcement system and the chosen monitoring strategy on site.

The FASSTbridge project aims at developing a fast and effective solution for steel bridges fatigue lifetime extension. The project intends to propose: -a methodology of structural assessment of remaining service life regarding fatigue, and design of a reinforcement based on adhesively bonded CFRP with additional monitoring, -in addition to a reinforcement process that will be developed. This one should combine a specifically designed adhesive with a chosen commercial CFRP product.

The overall proposed solution will be completed with cost-benefit and life-cycle assessment tools so that the advantage of the proposed procedure may be evaluated. The project includes a specific work package dedicated to the application of the whole solution on site on a composite steel-concrete bridge in Madrid.

To achieve the comprehensive and extensive work proposed in this project, the consortium of the project is composed by public and private research and expertise organisms, consulting engineering firms, a construction company, a public structure owner, an adhesive production company.

Fig. 1 :

 1 Fig. 1: Photo of a repair using additional bolted steel plate on the left (FHWA, 2013), and adhesively bonded CFRP on the right (On site application in France)

Fig. 2 :

 2 Fig.2: Scheme of the objectives of FASSTbridge project

Fig. 3 :

 3 Fig.3: Implementation of FASSTbridge project

Fig. 4 :

 4 Fig.4: Fatigue classes according to AASHTO on the left and Eurocode on the right

Fig. 6 :

 6 Fig.6: Failure modes in shear of a CFRP to steel bonded assembly before and after hydrothermal ageing[START_REF] Chataigner | Adhesively bonded composite reinforcements for steel structures: durability of the stress transfer[END_REF]

Fig. 7 :

 7 Fig.7: Photos of the bridge in Madrid for the on-site application

ACKNOWLEDGEMENTS This work is part of the FASSTBRIDGE project. This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 31109806.0008. FASSTbridge is co-funded by Funding Partners of The ERA-NET Plus Infravation and the European Commission. The Funding Partners of the Infravation 2014 Call are: MINISTERIE VAN INFRASTRUCTUUR EN MILIEU, RIJKSWATERSTAAT BUNDESMINISTERIUM FÜR VERKEHR, BAU UND STADTENTWICKLUNG, DANISH ROAD DIRECTORATE, STATENS VEGVESEN VEGDIREKTORATET, TRAFIKVERKET -TRV, VEGAGERÐIN, MINISTERE DE L'ECOLOGIE, DU DEVELOPPEMENT DURABLE ET DE L'ENERGIE, CENTRO PARA EL DESARROLLO TECNOLOGICO INDUSTRIAL, ANAS S.p.A., NETIVEI, ISRAEL -NATIONAL TRANSPORT INFRASTRUCTURE COMPANY LTD, FEDERAL HIGHWAY ADMINISTRATION USDOT


Tarek Marray

Philippe Jacquet

Mohammed Mansori

Agnès Fabre

Laurent Barrallier

A Thermodynamic and experimental study of low-alloy steels after carbonitriding in a

Keywords: carbonitriding in low-pressure atmospheres, vacuum carbonitriding, carbides, carbonitrides, alloy steel

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

INTRODUCTION

In many practical cases the performance of a structural component depends on the mechanical properties of the surface [START_REF] Bocchini | Overview of surface treatment methods for PM parts[END_REF]. Many methods of thermochemical treatment are used to raise the properties of the surface, such as gas carburizing [2 -4], carbonitriding [START_REF]Metals Handbook, Heat Treating[END_REF][START_REF] Stratton | Gaseous carburizing and carbonitriding: the basics[END_REF], and low-pressure carburizing (vacuum carburizing) [5 -7].

Conventional methods of thermochemical treatment of steel are based on diffusion of carbon and (or) nitrogen in surface layers. Carbonitriding, which is known to be more advantageous than standard carburizing, especially at a low thickness of the hardened layer, is commonly performed in conventional atmospheres [START_REF] Stratton | Gaseous carburizing and carbonitriding: the basics[END_REF][START_REF] Ghiglione | Nitruration, nitrucarburation et derives[END_REF]. Published information on low-pressure carbonitriding (LPCN) is quite scarce [START_REF] Herring | Vacuum carburizing of P/M steels[END_REF][START_REF] Altena | Low-pressure carbonitriding using acetylene and ammonia -a novel diffusion process for casehardening[END_REF].

Due to the intrinsic nonequilibrium of the low-pressure treatment the conventional methods [START_REF] Jacquet | A novel technique to monitor carburizing processes[END_REF][START_REF] Kawata | Atmosphere control during low-pressure carbonitriding processes[END_REF] are inapplicable for controlling such processes. For this reason, we used computer simulation to predict the relation between the effective thickness of the hardened layer and the accurate profiles of the distribution of carbon and nitrogen in steels [13 -18].

On the whole, the process of carbonitriding (developed on the basis of carburizing) is performed in two stages. The first stage, which is usually termed a boost one, provides delivery of a high content of carbon and nitrogen from the atmosphere to the surface. The second stage, i.e., diffusion, consists in keeping the parts at a constant temperature in vacuum (after cutting off the gas flow to the atmosphere).

Such division of the process of LPCN makes it possible to avoid transition through the maximum solubility of carbon in the austenite and hence to prevent precipitation of undesirable phases such as M 23 C and M 7 C 3 carbides and M(C, N) carbonitrides. Earlier studies of alloys of the Fe -Cr -C system treated in a conventional manner have revealed formation of microstructures represented by a-ferrite and M 3 C, M 7 C 3 and M 23 C 6 complex carbides depending on the alloying of the metal [START_REF] Berns | Microstructure of Fe -Cr -C hardfacing alloys with additions of Nb, Ti and B[END_REF][START_REF] Lu | Structural materials: properties, microstructure and processing[END_REF].

The aim of the present work was an experimental and computational study of the effect of the composition of steels on undesirable precipitation in the process of low-pressure carbonitriding.

METHODS OF STUDY

Studied Steels

The samples for carbonitriding were fabricated from two low-alloy steels the chemical compositions of which are given in Table 1. The low content of carbon in the steels guarantees a tough core in the parts after carbonitriding.

All the samples had a cylindrical shape and were AE16 ´20 mm in size. Both end faces were ground before the treatment.

Low-Pressure Carbonitriding

Most of the results presented were obtained using a commercial BMI Fours Industriels furnace that had earlier been used for developing a sensor for monitoring processes of low-pressure carburizing [START_REF] Grafen | Applications of low-pressure carburization with high temperatures (1000°C to 1050°C) in industrial practice[END_REF]. The samples were subjected to LPCN at 860°C in a BMIBMicro vacuum furnace. Carbonand nitrogen-bearing atmospheres were created by feeding ethylene and ammonia in the stages of carburizing and carbonitriding, respectively (Fig. 1). The pressure, the gas flow, and the duration of such treatments were optimized in order to provide a saturated layer with a thickness of 0.9 mm on the basis of results of the experiments described in detail in [START_REF] Marray | Carbonitruration basse pression d'aciers et de pièces obtenues par la Technologie MIM[END_REF][START_REF] Marray | Carbonitriding treatment applied to PIM elaborated parts[END_REF]. The thickness of the saturated layer was determined from the data obtained by measuring the hardness; the hardness of the layer should exceed 550 HV after quenching.

The concentration profiles of carbon and nitrogen were measured with the help of a LECO analyzer and by the method of glow discharge optical emission spectroscopy, respectively. The metallographic analysis of the carbonitrided layers was performed after etching the metallographic specimens in Nital and Murakami reagents. The hardness profiles were obtained using a Buehler Micromet 2100 device at a load of 1 N. The maps of distribution of the chemical elements in the carbonitrided samples were obtained with the help of scanning electron microscopy and x-ray diffraction analysis. To determine the phase composition the x-ray diffraction spectra were obtained in cobalt K a radiation with wavelength 0.1789 nm.

RESULTS AND DISCUSSION

Experimental Studies

Figure 2 presents the microstructure of the studied steels after LPCN (with duration exceeding that in Fig. 1) and oil quenching. The structure of steel B is represented first by martensite and then by bainite in the direction from the surface to the core (Fig. 2a ). The surface zone of steel 16MnCr5 (with a thickness of 200 mm) contains retained austenite. Martensite and bainite are detectable at a distance h = 200 -500 mm from the surface. The core of the sample contains perlite and ferrite (Fig. 2b ).

The curves of hardness distribution presented in Fig. 3 match the observed structures. In steel B the hardness is approximately constant (800 HV 0.1 ) at h = 0 -400 mm and then decreases gradually. In steel 16MnCr5 the hardness decreases in a layer with a thickness of 200 mm, which corresponds to the presence of a "dark component." A maximum hardness (750 -800 HV 0.1 ) is attained at h = 200 -400 mm. Then the hardness decreases upon the appearance of equilibrium components (ferrite and perlite) in the layer at h = 400 -700 mm.

To determine the presence or absence of chromium-bearing carbides we used the Murakami reagent. Such precipitation has not been detected in carbonitrided steel 16MnCr5. This cannot be said about steel B, a typical microstructure of which is presented in Fig. 4. The surface layer of the steel exhibits four zones that correspond to the detected distribution of carbides over the thickness of the sample. From the surface to h = 90 mm the fraction of the chromium-bearing carbides is 1.65%. At a distance h = 90 -200 mm the proportion of these carbides attains a maximum (6.4%) and then de- creases gradually to 1.3% (h = 200 -310 mm) and 0.4% (h = 310 -400 mm).

To understand this trend, the profiles of carbon and nitrogen concentrations were imposed on the micrographs obtained after etching steel B in the Murakami reagent (Fig. 4).

A carbon content exceeding 1 wt.% was observed in a surface layer up to 310 mm thick. A nitrogen content exceeding 0.6 wt.% was preserved in the next layer with a thickness of up to 20 mm and decreased progressively to 0.2 and 0.07% in the layers 100 and 200 mm thick, respectively.

The imposition of the profiles of C and N concentrations on the micrographs reflects the probable effect of nitrogen on the formation of chromium-bearing carbides. Indeed, only the nitrogen concentration varies between the surface and h = 310 mm. When the nitrogen content exceeds 0.2%, etching with the Murakami reagent reveals chromium-bearing carbides in an amount of 1.65%. This concentration increases by a factor of 4 (to 6.4%) when the nitrogen content falls below 0.2%.

Finally, when nitrogen is not detected in the surface layer, the fraction of the precipitates containing chromium is the lowest (< 1%).

To obtain diffractograms at different distances from the surface (surface; h = 100, 200, 300, 500 mm; core) we removed layers with a specified thickness from carbonitrided samples. It can be seen from Fig. 5 that the diffractograms reflect the presence of M 23 C 6 and V 8 C 7 carbides on the surface in addition to the a-phase. At a certain distance from the surface there appear an M 7 C 3 carbide. Carbides of types M 23 C 6 and M 7 C 3 have also been detected in carbonitrided steels in [22 -26], where the eutectic (Fe, Cr) 7 C 3 carbides have exhibited a relative resistance to the heat treatment [START_REF] Laird | Solidification and solid state transformation mechanisms in Si alloyed high-chromium white cast irons[END_REF]. On the contrary, the authors of [START_REF] Wu | Microstructural features of an iron-based laser coating[END_REF][START_REF] Pearce | Duplex nature of eutectic carbides in heat-treated 30% chromium cast iron[END_REF] report a transformation of hexagonal M 7 C 3 carbides with a very high chromium content into cubic M 23 C 6 carbides upon an appropriate heat treatment. Carbide M 23 C 6 can also be formed due to decomposition of M 7 C 3 [START_REF] Wu | Microstructural features of an iron-based laser coating[END_REF].

Giving data on the types of the carbides (M 23 C 6 and M 7 C 3 ) these studies do not reflect their composition. However, it is obvious that they do contain chromium.

We used the maps of the distribution of elements obtained by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to determine the compositions of the precipitates. The measurements were made at different distances from the surface. Figure 6a presents a map of elements on the surface, which shows the presence of chromium-rich zones. The chromium-enriched zones coincide with the zones enriched with molybdenum. Figure 6b presents a map of the distribution of elements at a distance h = 200 mm. Comparative analysis shows that the regions enriched with chromium and molybdenum are similar to the regions detected on the surface. Moreover, the same zones contain manganese and vanadium.

We suggest two hypotheses explaining the results, namely, (1 ) different types of carbides (V 8 C 7 , M 23 C 6 and M 7 C 3 ) coexist at this depth and (2 ) the phase composition of the precipitate varies over the thickness.

With allowance for the experimental results we may be positive that carbonitriding of steel B causes formation of chromium-bearing carbide precipitates in the surface layer, which are absent in steel 16MnCr5. However, the results of the x-ray diffraction phase analysis should be accepted with caution (the limit of detection of a phase is at least 3 -5%). The map of the distribution of the chemical elements widens the information on the composition of the surface layers. At least we may be positive that the diffusion layer, according to the data of the x-ray diffraction analysis, contains different carbides (V 8 C 7 , M 23 C 6 and M 7 C 3 , Fig. 5).

Thermodynamic Simulation

It impossible to determine exactly the chemical composition of the metallic component M in the M 23 C 6 and M 7 C 3 carbides detected with help of x-ray diffraction, which justifies the attention to thermodynamic computational tools. We used the Thermo-calc Ò classical software (version R ) to make thermodynamic computations.

Phase Distribution. We started with thermodynamic computations for determining the distribution of the phases over the thickness. The computation was based on the concentration profiles of carbon and nitrogen and gave us only the presence of an austenitic phase. This phase in steel 16MnCr5 is enriched with carbon and nitrogen. No other phase is predictable, which agrees with experimental results.

A similar approach was applied to steel B. In contrast to the results obtained for 16MnCr5, we predicted the appearance of many other phases for this grade. Indeed, the curves of the phase distribution (Fig. 7) reflect the presence of carbides of types M 23 C 6 and M 7 C 3 and an MC carbide with an fcc lattice in addition to the austenite.

The weight fraction of the precipitates varies over the thickness of the layer (and thus depends on the content of carbon and nitrogen). At a distance of about 100 mm from the surface the weight percent of the precipitates is the highest (8.25%).

The surface contains only an MC carbide in an amount of 4.33 wt.%. The content of this carbide decreases to 1 wt.% almost linearly until h = 50 and then remains invariable. Starting with h = 10 mm the weight fraction of carbide M 23 C 6 increases rapidly and attains 7.3 wt.% at h = 100 mm. Then the concentration of this phase decreases and should be zero at h > 350 mm according to the computation. The decrease in the content of M 23 C 6 coincides with precipitation of an M 7 C 3 carbide, which appears after h = 200 mm. The content of this carbide attains 1.87 wt.% and is preserved at this level at h > 250 mm.

Phase Composition. The variation of the phase composition over the thickness of the specimens was estimated simultaneously with computing the distribution of the concentration profiles of the alloying elements. The content of the fcc austenite phase exceeds 91.7 wt.%. It also contains the whole of the nickel over the whole of the thickness of the diffusion layer. The same concerns the manganese until the appearance of an M 7 C 3 carbide that contains a little more than a half of the manganese of the matrix. The MC phase with an fcc lattice contains the whole of the vanadium of the steel. Being the only precipitate at h = 10 mm it also contains the entire chromium, molybdenum, tungsten and nitrogen. At this depth the stoichiometry of the MC phase corresponds to a carbonitride with composition Cr 0.68 V 0.22 Mo 0.05 W 0.03 Fe 0.02 (C 0.15 N 0.85 ).

At h > 20 mm the precipitated M 23 C 6 carbide takes the whole of the chromium, molybdenum, tungsten and, especially, carbon from the MC carbonitride with fcc lattice. The composition of the MC phase shifts gradually to a vanadium nitride and becomes V 0.91 Fe 0.05 Cr 0.037 W 0.002 Mo 0.001 (C 0.03 N 0.97 ) at h = 50 mm. It should be noted that whatever the thickness of the diffusion layer, nitrogen is contained in the MC phase exclusively. At h = 100 mm the M 23 C 6 carbide takes virtually the whole of tungsten, molybdenum, chromium and carbon and is describable by the formula (Fe 0.75 Cr 0.21 Mo 0.027 W 0.013 ) 23 C 6 . At h > 200 mm the precipitated M 7 C 3 carbide takes carbon and chromium from the M 23 C 6 carbide, the proportion of which decreases progressively over the thickness of the layer. Thus, the elements contained initially in the M 23 C 6 carbide are redistributed and go into carbides of types MC and M 7 C.

We assume that the presence of nitrogen hinders the precipitation of the M 7 CC 3 carbide appearing at h = 250 mm, after which the concentration of nitrogen is reduced to zero. Starting with h = 350 mm the layer does not contain precipitates of type M 23 C 6 . The M 7 C 3 carbide becomes a prevalent chromium carbide and has composition (Cr 0.499 Fe 0.452 V 0.037 Mn 0.007 W 0.005 ) 7 C 3 .

CONCLUSIONS

1. After low-pressure carbonitriding of low-alloy steels with 0.16 -0.18% C the concentration of the alloying elements (Cr, Mn, V, W, Mo) in their diffusion layer increases, which causes the appearance of chromium-bearing carbides of types MC, M 23 C 6 and M 7 C 3 .

2. The prevalent carbide in the diffusion layer of the carbonitrided steels is MC, which is replaced gradually by an M 23 C 6 carbide, while an M 7 C 3 carbide appears in the deeper layers free of nitrogen.

3. The data of the layer-after-layer phase analysis and the maps of the distribution of the elements agree well with the thermodynamic computations of the phase composition of the diffusion layer in the steels studied after low-pressure carbonitriding.

Fig. 1 .

 1 Fig. 1. Scheme of a cycle of low-pressure carbonitriding (LPCN).

Fig. 2 .

 2 Fig. 2. Microstructure of steels B (a) and 16MnCr5 (b ). Etching with Nital (light microscopy).

2 Fig. 3 .Fig. 4 .Fig. 5 .

 2345 Fig. 3. Distribution of the hardness over the thickness of samples of steels B (1 ) and 16MnCr5 (2 ) (h is the distance from the surface).

Fig. 6 .

 6 Fig. 6. A map of distribution of elements in carbonitrided steel B: a) on the surface; b ) 200 mm from the surface.

Fig. 7 .

 7 Fig. 7. Computed distribution of phases (C ph is the phase content) over the thickness of carbonitrided layer for steel B.

TABLE 1 .

 1 Nominal Chemical Compositions of the Steels of Grades B and 16MnCr5

					Content of elements, wt.%
	Steel
		C	C r	M n	V	W	M o	N i	S i
	B	0.18	3.12	0.34	0.7	0.45	0.46	0.4	0.19
	16MnCr5	0.16	1.00	1.00	-	-	-	-	0.40
									5 bar
									1 bar
						Pressure 25 mbar
						Òåmperature 860°Ñ
						C H 2 4	Diffusion	NH 3	N 2
						33	12	45	t, min

Laboratoire de Science des Matériaux, ECAM, Lyon, France.

Arts et Métiers ParisTech, MSMP, Aix en Provence, France.

Arts et Métiers ParisTech, LaBoMaP, Cluny, France.

Laboratoire de Chimie des Matériaux et Evironnement, Université Cadi Ayyad, Marrakech, Maroc.

The authors are sincerely grateful to the BMI Fours Industriels, St. Quentin Fallavier, France, for the help rendered and for the financial support.