Vahan Baronian
email: vahan.baronian@cea.fr

Karim Jezzine

Nicolas Dominguez

Sylvain

New models for ultrasonic guided wave inspections in CIVA

Models for ultrasonic guided wave NDT are developed at CEA LIST and integrated into the CIVA platform. Models already available in CIVA 10 deal with plates and cylindrical guides, possibly multilayered. Mode computation, radiated field from different type of transducers (contact with or without wedge, surrounding or surrounded arrays in pipes) and response of a NDT examination in pulse-echo or pitch-catch configurations for a crack orthogonal to the guide axis are proposed. These models are now extended to deal with guides of arbitrary section with planar extrusion, modes being computed with the Semi-Analytical Finite Element (SAFE) method. A hybrid SAFE-Finite Element method has also been integrated to compute the response of a flaw of arbitrary shape, a weld or a complex junction. These new models as well as their validations are presented and discussed.

Introduction

Simulation tools for guided wave inspection are being developed at CEA LIST. These tools aim at designing and optimising NDT methods or probes, or interpreting experimental data. The developed models are based on a modal decomposition. In practice, the knowledge of mode behaviour and dispersion characteristics is an essential step for understanding complex phenomena arising in a guide. Measured or simulated signals are then interpreted in reference to modes. Typical questions concern the ability of modes to be transmitted through or reflected on a guide discontinuity, to be converted into other modes in the interaction. Simulations are thus performed in the frequency domain and are subsequently synthesized by Fourier transform over a limited spectrum.

At a given frequency, the elastodynamic field (particle displacement or velocity, stress) can be decomposed as a linear combination of eigenmodes computed in the crosssection of the structure whose guiding axis is taken as . The mode of this set is described at a given frequency by: i) its wavenumber , real for the finite number of propagative modes, imaginary for the finite number of evanescent modes or includes an imaginary part for the infinite number of inhomogeneous modes, ii) the corresponding particle displacement vector in the invariant cross-section of the waveguide, denotes the position in the cross section. The displacement at any position in the waveguide at the pulsation is expressed by: where denotes the amplitude coefficient in the decomposition.

A given mode may be transmitted through or reflected on a guide discontinuity or partially converted into other modes in the interaction. The modal description is used in the regular parts of the waveguide, whereas models for transduction or scattering by guide discontinuities or flaws are used locally. To compute modal solutions, the semianalytical finite element method has been implemented [START_REF] Dong | On Natural Vibrations and Waves in Laminated Orthotropic Plates[END_REF] . This method allows the computation of both wavenumbers , and modal displacements . It can deal with waveguides of arbitrary section and anisotropic materials.

The computation of modal amplitudes emitted by a transducer may be performed under the assumption that piezo-transducers can be modelled as sources of normal or tangential stresses over their active surface. For a transducer acting from the guiding surface, a Green function for a given mode is derived and a surface integration over the transducer performed [START_REF] Li | Excitation and Propagation of Non-Axisymmetric Guided Waves in a Hollow Cylinder[END_REF] .

The scattering problem is written in the form of a matrix of complex coefficients and , assuming that modal solutions in guiding structures connected to the local zone of scattering are known.

(resp.

) is the reflection (resp. transmission) coefficient for the incident mode and the reflected (resp. transmitted) mode. They stand for the scattering by an inhomogeneity of the guide. This matrix links an input vector constituted by the modal coefficients of the incoming wave, to output vectors constituted by those of the outgoing waves. For planar crack-like defects of arbitrary shape in an otherwise homogeneous guide, assuming that crack surface belongs to the guide cross-section, a mode matching method can be performed to compute the scattering matrix [START_REF] Jezzine | Simulation of Guided Wave Inspection Based on the Reciprocity Principle and the Semi-Analytical Finite Element Method[END_REF] .

Simulations based on hybrid SAFE-FE method

Theory

To deal with arbitrary flaw shapes, guide inhomogeneities or junctions between several guides, a finite element (FE) scheme has been developed with the further goal to limit the computation zone to a minimal size for efficiency [START_REF] Baronian | A Numerical Method for the Simulation of NDT Experiments in an Elastic Waveguide[END_REF] . The computation relies on the use of artificial boundary conditions endowing transparency. Radiation conditions at infinity are brought back to the artificial boundaries by building an operator coupling the finite elements inside the FE zone to modal solutions in guides. An original mixed formulation has been derived whose unknowns are the displacement field in the bounded domain and the normal component of the normal stresses on the artificial boundaries. The scattered field is then projected on modal solutions in guides through the use of bi-orthogonality relations. This method has been established for 2D and 3D waveguides in Cartesians coordinates. It can deal with the scattering by a junction made up of an arbitrary number of waveguide [START_REF] Baronian | Hybrid SAFE/FE Simulation of Inspections of Elastic Waveguides Containing Several Local Discontinuities or Defects[END_REF] . In the next release of CIVA GW software, only the 2D (dealing with the scattering of Lamb wave) and axisymmetric version are implemented. The main steps of the hybrid method are recalled, in particular the building of transparent boundaries conditions by using the bi-orthogonality relation of Fraser expressed in terms of the so-called mixed vectors X and Y, first introduced by Pagneux and Maurel [START_REF] Pagneux | Lamb wave propagation in inhomogeneous elastic waveguides[END_REF] . This relation is the key point of the method since it allows projecting the scattering field on the modal decomposition. A complex junction between N waveguides is represented on Figure 1. A bounded domain connected to an arbitrary number N of different waveguides on its boundaries is considered.

Figure 1. A complex scattering junction connected to N uniform waveguides.

The aim of the coupling method is to build modal transparent boundary conditions on the interfaces of the computational domain for the diffracted waves. Let's consider an incident right-going mode n0 on the interface at :

The diffracted displacement field can be expressed by a sum over the outgoing waves using the modal decomposition in each uniform waveguide k: ) as a Lagrange's multiplier is subsequently introduced. This leads to a mixed variational formulation where unknowns correspond to the total displacement field and the Lagrange's multipliers. Moreover, as we have introduced k Lagrange's multipliers, we need k more equations to complete the variational system, which is done by expressing a weak variational formulation of the remaining equations issued from the coupling operator ( for or for ). More precisely this corresponds to the compatibility between the corresponding displacement component (axial or transverse ) given by the finite element representation and its modal representation (on the artificial boundaries). Scattering coefficients can finally be expressed in term of reflection and transmission coefficient:

Simulation of the interaction of Lamb wave with a complex junction

A first example is proposed to illustrate the capabilities of the hybrid SAFE-FE method. The junction of six steel plates is shown on figure 2. A wedge transducer is positioned on a 50mm thick branch and radiates Lamb wave through the junction. A similar transducer acts as a receiver on the other side of the junction 700 mm from the emitter. The emitter is driven by a narrow band excitation of 20 cycles modulated by a Hanning window at 110 kH. The angle of both wedge transducers is set arbitrarily at 31°.The mesh used for the FE computation, as well as the distribution of energy among the different existing modes in the excitation branch are also shown on Figure 2.

Figure 2. Inspection of a complex junction connecting six uniform waveguides (top left). FE meshing of the junction (bottom left). Distribution (percentage) of incident energy in the left branch among symmetric and antisymmetric modes around the transducer centre frequency (right).

Snapshots of the displacement wavefield inside the FE box are represented on Figure 3. A comparison between received signals obtained in presence of the complex junction and in the case of a simple plate of 50mm thickness shows that the junction induces a loss of 13 dB (Figure 4).

Figure 3. Snapshots of displacement wavefield inside junction at t=350µs (a), t=385µs (b), t=420µs (c), t=455µs (d), t=490µs (e) and t=525µs (f).

Figure 4. Simulated waveform measured on the receiver in a uniform plate (left) and after crossing the junction (right) 2.3 Simulation of the interaction of Lamb wave with corrosion-pitting

In this section, the interaction of Lamb modes with corrosion pitting in a 2mm thick aluminium plate is simulated. Material properties used are =2700kg/m 3 ., Lame's coefficient =0.2662x10 11 Pa and =0.5597x10 11 Pa. A similar study has been performed by Terrien et al. [START_REF] Terrien | A combined finite element and modal decomposition method to study the interaction of Lamb modes with micro-defects[END_REF] to detect the presence of corrosion in aircraft structures. The basic idea is to use Lamb modes at relatively high frequency to detect early corroded areas. Indeed, at high frequencies, Lamb modes are more sensitive to corrosion microstructure than to thickness variations, which may allow corrosion to be discriminated from plate thinning. Corrosion pits are modelled by a distribution of cracks of 100 µm depth and width (Figure 5). Two identical wedge transducers are positioned on each side of the corroded area, 200 mm from each other. The emitter is driven by a narrow band excitation composed of 20 cycles modulated by a Hanning window at 2 MHz. As the angle of the wedge transducers are adjusted to match the phase velocity of the S0 mode at 2MHz, only the fundamental symmetric mode S0 is incident on the corrosion area and the receiving probe detects essentially the same mode. This is confirmed by the simulation of the distribution of energy carried by the five existing modes in the transducer frequency bandwidth (S0, S1, S2, A0 and A2) shown on Figure 6. Simulations of inspections have been performed with a length of corroded area ranging from 2mm to 10mm, its depth being set to 100µm. The amplitude of the received signal, as shown on Figure 7, decreases with the length of the corroded area. These results are in accordance with Terrien et al. [START_REF] Terrien | A combined finite element and modal decomposition method to study the interaction of Lamb modes with micro-defects[END_REF] . Indeed, the presence of a corroded area induces a conversion from S0 mode to A0 mode, as shown on Figure 7. This conversion is found to be proportional to the length of the corroded area. The receiver being only sensitive to S0 mode, this explains the decrease of the received signal with the length of the corroded area. The interaction of Lamb wave with a thickness variation has also been investigated. For a 10 mm long and 100µm deep plate thinning corresponding to the same thickness as corrosion pittings considered previously, the received signal (Figure 8a) is very close to the one obtained for a healthy plate (Figure 7a). Indeed, nearly no mode conversion occurs in this case (Figure 8c). This allows a 10 mm long corroded area (Figure 8b and 8d) to be discriminated from a plate thinning of similar extent.

Mode and field computation in guides of arbitrary section

Theory

The semi-analytical finite element method (SAFE) [START_REF] Dong | On Natural Vibrations and Waves in Laminated Orthotropic Plates[END_REF] allows the computation of both wave vectors and modal displacements in the section as being the eigenvalues and eigenvectors (resp.) of a quadratic system of equations. This system is the discrete form of a variational problem in the guide section. Since it is based on a finite element discretisation of the section, it allows one to deal with all sorts of guide characteristics (shape, constitutive materials). Specifically, multilayered structures, anisotropy and viscoelasticity can be easily accounted for. We have implemented this method to deal with various guiding structures. Guide section is meshed either by 1D linear element for plates, cylinders or tubes, or 6-noded triangular finite elements in the general case of arbitrary section. As the mesh is restricted to the section, the method is computationally very efficient

Simulation of mode and field computation in a rail

The wavefield emitted by a piezo transducer in a steel rail structure has been computed with the SAFE method (see configuration on Figure 9). The emitter is driven by a narrow band excitation composed of 10 cycles modulated by a Hanning window at 40 kHz.

Phase and group velocities dispersion curves up to 100 kHz are shown on Figure 10 as well as displacement and stress for arbitrary mode and frequency.

Conclusions

Models for GW / NDT simulation have been described. A specific FE method has been derived for computing the scattering by arbitrary flaw shapes, guide inhomogeneities or junctions between several guides. It includes exact transparent artificial boundaries for minimizing the size of the FE zone, thus reducing computation costs. A 2D version for plates and an axisymmetric version for pipes has been implemented in CIVA. The SAFE method has also been implemented to deal with guided propagation in uniform guides of arbitrary section. In addition, the simulation of pipe inspection can now handle the presence of inner fluid and an algorithm for the computation of amplitude and delay laws for phased array in pipes is now proposed in CIVA.

  where denotes the sign (+ or -) of propagation for the diffracted waves, the magnitude of the outgoing mode, the local transverse coordinates, the local axial coordinate and the local axial position of the boundary , for each uniform waveguide . is composed by the transverse component of the normal stress and the axial component of the displacement field, whereas (the dual of ) is composed of the transverse component of the displacement field and the opposite of the axial component of the normal stress: , Fraser's relation expressed in term of and corresponds to an orthogonality relation between two modes n and m that propagate in the same direction: , where (.)S denotes the integration over the cross-section of the waveguide and nm is the Kronecker symbol. The mixed variables and for the diffracted field can be similarly expanded thanks to the modal decomposition: Using the bi-orthogonality relation, the amplitude of the mode can be recovered as follow: Introducing the expression of in the previous modal expansion, one expresses two coupling operators and (similar to DtN, for Dirichlet-to-Neumann) mapping the trace of to the trace of on each boundary or vice-versa: Now using one of these so-called boundary maps, it is possible to formulate the diffraction problem in the bounded domain where the transparent boundary conditions on the section will be expressed by the means of the chosen coupling operator or . They correspond to an outgoing wave condition for the computational domain . This condition represents a modal transparent boundary condition for guided waves. The diffraction problem is then formulated by a variational formulation where the unknown is the total displacement field . To express the boundary terms on the section corresponding to the work of the external stresses on the boundaries , the part of the coupling operator related to the stresses components ( for or for ) is introduced. The corresponding missing stress component ( for or for

Figure 5 .

 5 Figure 5. Inspection of a 2-mm thick corroded aluminium plate. In this pitch-catch configuration, corrosion pits are modelled with a distribution of 100µm square notches. The distance between emitter and receiver is 200mm.

Figure 6 .

 6 Figure 6. Distribution (percentage) of energy among existing modes in the emitting wedge transducer frequency bandwidth. At 2Mhz, around 90% of the energy is carried by the S0 mode.

Figure 7 .

 7 Figure 7. Simulated waveform on the receiver for a healthy 2mm thick plate (a) and a plate with a corroded area of 2mm length (b), 5mm length (c), 10mm length (d). Distribution of transmitted energy towards the receiver (percentage) among existing modes for a healthy plate (e) and a plate with a corroded area of 2mm length (f), 5mm length (g), 10mm length (h).

Figure 8 .

 8 Figure 8. Simulated waform on the receiver and distribution of transmitted energy towards the receiver after diffraction by a 100µm deep and 10 mm long plate thining (a and c) or 100µm deep and 10mm long corroded area (b and d)

Figure 9 .

 9 Figure 9. Inspection configuration of a rail and FE mesh used for the SAFE computation. Rail dimensions are 150 mm width and 172 mm height. The computation is positioned 1m from the piezo emitter

Figure 11 .

 11 Figure 11. Maximum of displacement along X (a), Y (b) and Z (c) in the computation zone defined on Figure 9. Simulated waveforms near the centre of the rail, corresponding to the displacement along X (d), Y(e) and Z (f).


Imen Brini
email: imen.brini@ird.fr

Denis Feurer

Riadh Tebourbi

Damien Raclot

Fabrice Vinatier

Riadh Abdelfattah

Contribution of InSAR coherence images to evaluate surface elevation change detected from multi-temporal photogrammetric imagery

Introduction

The landscape pattern and topography are affected by natural events and anthropic processes. Detecting and estimating these topographic changes is crucial for understanding the underlying processes.

The combination of various datasets obtained from different remote sensing approaches, each with specific resolution, enables the detection of 3D surface elevation change.

The aim of this work is to explore the potential of InSAR data to validate the detected changes in the 3D Earth surface, extracted from the photogrammetric approach.

The study area is the Lebna watershed, located in the coastal plain in northeastern Tunisia, in the Cap Bon peninsula. This watershed is prone to soil erosion mostly enhanced by agricultural practices and Mediterranean climate (Mekki et al., 2018).

The tri-stereo Pléiades satellite images were acquired in 2015, 2018 and 2023 covering the Lebna watershed. These images were processed using Agisoft Metashape Pro 1.6.3. based on the Time-SIFT photogrammetric approach introduced by Feurer & Vinatier (2018) to derive digital elevation models (DEMs). The DEM of differences (DoD), calculated by subtracting the 2023 DEM from the 2015 DEM, using QGIS, allows detecting geomorphological changes.

A total of 154 Sentinel-1 images, covering the study area, from 2017 to 2022 were processed using the advanced DInSAR technique known as Parallel-Small BAseline Subset (P-SBAS) approach (Casu et al., 2014). This approach uses small temporal and spatial baselines to limit the decorrelation phenomena. The Sentinel-1 images were processed using P-SBAS InSAR S-1 service implemented in Geohazard Exploitation Platform (GEP).

Results

The derived DEMs for the satellite images have a spatial resolution of 1m, showing different landforms. The DoD between 2023 and 2015 shows some 3D changes such as eroded areas, excavations, newly constructed buildings … The InSAR imagery products were used to compare/validate the results derived from the photogrammetric approach.

In this study, three coherence images, following the flooding event that happened on mid October 2018, were used.

Conclusion and perspectives

The interpretation of the coherence variation should be approached carefully. Several components can contribute to its value, such as the viewing geometry, evolution of vegetation cover, soil roughness and soil moisture changes.

Further investigation on the decomposition of the coherence according to the above mentioned decorrelation factors is required to explore the changes related to environmental processes such as erosion, by refining the extraction of the erosion information from other signals. To complement erosion detection from coherence images, integrating precipitation dataset could helps in understanding the environmental conditions that contribute to soil erosion. The change of coherence due to temporal baseline change appears to be relatively progressive within the study area. The 12-days coherence pairs are expected to be sensitive to detect the potential changes in the earth surface, with coherence values between 0.1 and 0.9. Using longer temporal baseline pairs could potentially lead to a decrease of sensitivity, with most values remaining between 0.1 and 0.3. Although the temporal baseline was within 12 days, the mean coherence was around 0.36.

Material and methods

For an identified eroded area on the DoD (2023-2015), the mean coherence values are gradually decreasing indicating that a ground variation occurs between the SAR acquisitions modifying the structure of scattering targets.

  (a) Zoom-in of the DoD, the eroded area is framed in black (b) Coherence variation for the eroded area (c) Field observation of the eroded area (framed with dashed lines in red): bare ground and potential signs of surface changes. The position and view angle the photo (c) is marked in (a) with a camera icon and an arrow (b) (c) Casu, F., Elefante, S., Imperatore, P., Zinno, I., Manunta, M., De Luca, C., & Lanari, R. (2014). SBAS-DInSAR parallel processing for deformation time-series computation. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 7(8), 3285-3296. Feurer, D., & Vinatier, F. (2018). Joining multi-epoch archival aerial images in a single SfM block allows 3-D change detection with almost exclusively image information. ISPRS Journal of Photogrammetry and Remote Sensing, 146(July), 495-506. Mekki, I., Bailly, J. S., Jacob, F., Chebbi, H., Ajmi, T., Blanca, Y., Zairi, A., & Biarnès, A. (2018). Impact of farmland fragmentation on rainfed crop allocation in Mediterranean landscapes: A case study of the Lebna watershed in Cap Bon, Tunisia. Land Use Policy, 75(January), 772-783.


Tung Duc

Ech Dao

De Larrard Mohsen

François

Mai Lan Nguyen

Thierry Sedran

Pierre Hornych

INNOVATIVE ROAD COATING: AN ORIGINAL SOLUTION TO IMPROVE SAFETY AND DURABILITY OF CONCRETE PAVEMENT

This paper deals with an innovative and sustainable pavement structure offering safety, comfort and durability while ensuring an attractive initial and life cycle cost. This solution is a result of a 3 years extensive research and development undertaken by the LafargeHolcim Research Center since 2014. The new structure is composed of a conventional concrete layer offering required structural capacity and an ultra-thin wearing course providing high standards in terms of safety and riding comfort in addition to aesthetic possibilities (color choices). This new structure will allow road management authorities to save structural maintenance costs over pavement service life. In terms of composition, the innovative wearing course is made of very thin semi-porous polymer modified cement concrete layer placed (wet on wet) on an optimized concrete layer. The top layer has showed very promising results in terms of mechanical performance and durability, i.e. flexural strength, scaling resistance and resistance to raveling. In terms of riding comfort, this innovative wearing course has shown to be efficient in terms of noise absorption, skid resistance and drainage. The whole structure was found to be cost competitive when compared to equivalent rigid and flexible structures under same traffic conditions. The accelerated pavement testing (FABAC), carried out with Ifsttar Nantes, has shown a very good performance of this solution. At the time being, the mechanical performance and the surface quality are still maintained in perfect condition after 5 millions of cycles of 6.5 tons semi-axle, which corresponds to more than 20 years of traffic (500 heavy trucks passing per day).

INTRODUCTION

Context

Actual wearing course in asphalt offers flexibility, reactivity, and comfort for road users. Asphalt industry has been progressing over the last 20 years and offers now a huge range of solutions addressing noise, roughness, drainage, porosity, visual comfort and safety distances. While the trend in modified bitumen is clearly in favour of elasticity with all PMB (polymer modified bitumen) applications, conventional rigid pavements are not offering a lot of varieties. Some progress has been done recently, by American and European research programs as (FHWA, 2006), (ACPA, 2009), (Silvia, 2005). The results are promising, in terms of skid resistance, evenness improvements and also in terms of noise and polluting air emission reducing, but their application are still limited as the use of concrete pavement is still behind asphalt offers.

We can list some main historical obstacles to concrete pavements developments as:

 high initial price of the investment,  perception of the end users in term of comfort, safety due to the problem of jointing and quality of the surface,  hardening time and lack of flexibility increase the obsession on traffic organization for the road authorities,  industry awareness on the concrete manufacturing, jobsite preparation and concrete placing then curing.

The concrete pavement may definitely become competitive with asphalt pavement with a lower cost for the maintenance during its service life. In terms of economic, the initial construction cost will no longer be the major factor but the maintenance costs including the economic loss of traffic jams due to maintenance work need to be considered (FEHRL, 2008). The impact on road maintenance budget is massive. In general, mature countries are spending in maintenance yearly amounts between 0,2 % to 0,4 % of their GDP. Those massive amounts are not strictly maintenance but come also form the need to reinforce gradually the structure, as it gets damaged. This raises the time life, and involves road management authorities into permanent asphalt dependence. Furthermore, the bitumen prices raise for a long time drove asphalt roads to be now often more expensive at construction costs than concrete ones. The recent fall of bitumen prices makes road authorities difficult to take a long terms decision for their future construction.

Objectives

The aim of this 3 year project (2014-2017) is to address the development of an innovative coating, in order to offer:  a durable complete solution with concrete structural capacity,  expected standard in terms of comfort and safety,  technical innovations as color choices, easy placing.

This new wearing course aims to offer an interface between the structural function and the user perception. It will allow road management authorities to benefit from concrete economics of a rigid pavement:

 equivalent costs at construction level  no structural maintenance over the life time, (only surface maintenance, to be assessed).

Technically, by comparison to existing solutions for both flexible and rigid pavements, the characteristics that the innovative wearing course might have are as follows:

 since this course will not play the structural rule, when it is well bonded to the subbase, the thinner it is, the less stress appears in the course. Furthermore, with the economic reason, the thickness of this course must be from 2 to 4 cm,  to respect the thickness, the Dmax of aggregate cannot be bigger than 10 mm,  the concrete has to have a good mechanical strength (at least 8 MPa at 28 days as flexural strength…) to ensure the service life of the course versus the loading of the traffic (ravelling, cracking in the case where the bonding strength between two layers decrease with the time),  this course must have a skid resistance comparable to the existing solution, as the very thin asphalt concrete (BBTM in French). It is mostly influenced by the texture (micro and macro) of concrete, which is governed by the aggregate skeleton (in quality and grading)  in order to maintain the longevity of the course, it is preferred that this layer will work in compressive mode under the loading of vehicles because the tensile strength of concrete, even though it can be improved by the special binders, is much lower than the compressive strength. Thus, this wearing course has to have a good bonding strength to concrete subbase, at least 0.5 MPa by pull-off test, recommended by (NCPTC, May 2014),  this innovative course must be environment friendly and gives a large aesthetic choices to applicators,  the noise reduction must be comparable to that of very thin asphalt concrete BBTM. This is related to the texture and structure (including open porosity and matrix stiffness) of the concrete,

 it must be competitive with the others existing solution such as asphalt pavement with a very thin asphalt concrete,  the innovative material should be more durable than that of BBTM without ravelling, rutting or scaling during its service life.

METHODOLOGY

The properties of the polymer modified cement concrete (PMCC), used for the wearing course, were studied in the first stage, as mechanical behaviour as 3-point flexural strength, compressive strength and the durability (bonding strength with the support, scaling). Then, some properties concerning road application as noise absorption was also investigated.

In the second stage, the mixed structure of 2-lift pavement (polymer modified concrete on conventional concrete) was tested under the real traffic. The Accelerated Pavement Testing FABAC of Ifsttar Nantes (The French institute of science and technology for transport, spatial planning, development and networks) was chosen. After certain number of loading cycle, the levelness, skid resistance and cracks were measured in order to survey the durability of the road under the traffic.

RESULT AND DISCUSSION

3.1. Laboratory tests 3.1.1.

Mechanical strength

The strength of concrete containing different families of polymer was measured by the 3-point flexural test and compressive test on the prisms of 4 cm x 4 cm x 16 cm. The surface used in the mechanical tests was the top surface which was not in contact to the mold surface.

The Figure 1 shows that the mechanical strength of concretes, at a constant rheology, is so different, depending on the type and dosage of polymer. In general, polymer improves the flexural strength of concrete (up to 40 % compared to the control concrete without polymer). Otherwise, the influence of polymer on the compressive strength needs to be studied further, in the context that it can enhance or reduce the latter.

Scaling resistance

Figure 1-influence of polymer type on the mechanical strength of concrete

The influence of polymer on the resistance to scaling of concrete was also studied with 2 different families: an acrylic acid ester copolymer which showed very efficient to improve the flexural strength and classically recommended polymer for concrete, Styrene Butadiene emulsion.

The scaling test used the most severe method, developed by LCR, in which the pervious concrete is submerged in salty water containing 3% of NaCl and suffers the freeze-thaw cycles (cf. Figure 2) until the complete failure.

Figure 2 -a 24 hour freeze-thaw cycle used for scaling testing.

The polymer pervious concretes performance is compared to that of classical pervious concrete which was being tested at the same time.

The classical pervious concrete used a crushed aggregate 6/10 mm within cement paste volume of 160 liters for a Water/Cement ratio of 0,34.

Figure 3 -scaling results and comparison with a classical pervious concrete.

The figure shows that polymer can help to improve the scaling resistance of pervious concrete. In the case that we need the Innovative Coating resist to the scaling, we shoud add the polymer in the concrete.

Bonding strength

In order to maintain the good function of each layers in the double lift pavement system, the bonding strength of the PMCC layer on the top to the conventional concrete has to be high enough, a direct tensile strength from 0.5 MPa to 2.5 MPa, obtained by the pull-off test (NF EN 1542 standard), is considered good enough as recommended by (NCPTC, May 2014).

In this study, the concretes (with and without polymer) were batched on a fresh RCC. The minislabs of 30x40 cm were casted with 5 cm of compacted RCC and 4 cm PMCC on the top. The concretes were then compacted by the vibrating hammer.

Figure 4 -bonding strength with bottom concrete.

The results from the Figure 4 shows that:  in the wet/ wet mode, the concretes have a very good bonding strength in every configuration of placing (more than 2.5 MPa). It does not need a tack coat because the strongest bonding obtained is without the tack coat. It seems that the tack coat addition creates a supplementary link in the composite system which increases the possibility of failure during the testing,  the polymer added into the concrete has no significant improvement in the bonding strength compared to the conventional concrete.

Noise absorption

The noise absorption of PMCC was measured by an impedance tube according to ISO 10534-2, realised by Matelys laboratory (Matelys, 2016). This device (see Figure 5) is usually used to measure the acoustical absorption in a frequency range between 250 Hz to 2500 Hz * of the construction material including road material as asphalt concretes.

Figure 5 -impedance tube used for acoustical absorption measurement, developped by ENTPE, France.

In this study, 2 gradations (a 2/4 mm and a 4/6 mm from Rivolet quarry) were used to design 4 types of PMCC with 2 different level of voids content. More information about the concretes' recipes and their mechanical strength are described in the The samples used in the test are presented in the following pictures : * PERS is a pervious mixing mixture of more than 20% of voids, obtained by mixing the rubber aggregate with a polymer binder as the epoxy. The noise absorption efficiency of this material has been proved during the 6 year European research project PERSUADE.

F1 F2 F3 F4

Figure 8 -noise absorption of PMCC comparison with those of PERS and of porous asphalt concrete.

Some remarks are raised from this figure:

 the concretes made with coarser aggregate are better in terms of noise absorption, which may be comparable to the most efficient PERS or porous asphalt concrete,  the F3 is the most efficient in the "road" frequency range with a very good mechanical strength (at 28 days). However, its bonding strength testing is necessary since the concrete has generally less contact points to the concrete support than the finer aggregate concrete. Its skid resistance due to a bigger macro texture and its resistance to ravelling need to be further studied as well,  the results of F4 are very scattered due to the segregation of the cement paste, result of a high cement paste volume (210 liter) at high W/C ratio (0.47),  as a result, an empirical model was built in order to predict this property of PMCC from the parameters measured carried out on the samples. This model can be used to explain the mechanism of dissipation of noise in the concrete. However, the micro structure of concretes, which is linked to the mix design parameters, need to be investigated in order to have a more accurate model,  however, if reducing of noise generation is needed, both the macro texture (negative or positive surface) and the elasticity of the wearing course would be taken into account (FHWA-IF-08-004, 2007) where is necessary to do a real measurement with passing vehicles (type CPX test).

Accelerated Pavement Testing with FABAC

The Accelerated Pavement Testing machine of Ifsttar, called FABAC, was used to test the behavoir of pavement made of innovative coating under the real loading condition. The pavement will receive cyclic load of a 65 kN haft-axle of twin wheels with loading speed of 1 m/s. The structure composed of 20 cm of conventional concrete C30/37 type S1, overlaid by 3 cm of PMCC, all placed on an unbound aggregate layer 0/31.5 of 10 cm. In the loaded area, two undoweled transverse joints espacing 50 cm were created. Filled by an elastomer product, these specific areas allows studying the degradation of concrete arround the joints under the vehicle passing.

The test was ended at 5 million load cycles, which corresponds to 21 years of traffic within 500 heavy lorry per day per lane, according to French Pavement Design method.

Visual aspect with transverse profile measurement

The photos were taken after 1 and 5 million of loading cycles. They show that there was no damage around the joints caused after 5 million passing of vehicles. The levelness of the pavement was also followed, by the transverse profil measurement. The results taken from 25 cm before the first joint showed that the pavement and also the PMCC have no rutting problem after a long period charged by heavy trucks.

Skid resistance

The surface quality of the pavement was characterised by the Skid Resistance Tester (English Pendulum Test -EN 13036-4) and the Dynamic Friction Test (ASTM E 1911). The results show that, after 5 million load cycled, the surface was polished but still in acceptable condition for the traffic, by maintaining a good adherence for the vehicles.

CONCLUSIONS AND PERSPECTIVES

A PMCC was developped for the concrete pavement coating. The laboratory studies confirmed that the innovative material can be used as the wearing course of rigid pavement without any problem.

The experimental slab with PMCC has undergone 5 million heavy trucks axle passing and its final characteristics was still in good condition both in riding safety and mechnical behavior criteria. Indeed, the various tests carried out throughout the loading cycles did not show any significant damage. The macrotexture has not evolved with 5 million cycles with the Fabac machine. The microtexture has decreased normally and is at a satisfactory level after 5 million cycles. The pendulum test gave relatively stable results between 3 and 5 million cycles whereas the DFT friction test showed a more continuous wear of the microtexture. The measurements of transverse profiles do not show any significant deformation between the initial profile and the one with five million passage. The seals at the joints did not change and the edge of the seals was also not impacted by the passage of the wheels.

The debonding was also studied by two different techniques: frequency hopping (IFSTTAR) and impulse radar measurements. In conclusion, the radar measurements do not make it possible to detect an evolution of the state of degradation of this interface studied at this stage of the experiment. This result confirms the good state of the studied interface up to 5 Mcycles. Some pulloff test should be carried out to confirm the results given by the radar measurements.

As perspective, full scale jobsites would be launched in the near futur with the real pavement machines (paver, slipform) in order to validate the wet-on-wet placement technology of the composite structure.

Figure 6

 6 Figure 6 : 2/4 mm Rivolet PMCC samples.

Figure 7

 7 Figure 7 -4/6 mm Rivolet PMCC samples. The results are capitalized in the Figure 8, in which the data obtained of different very-efficientnoise-absorption concretes PERS * (porous elastic road surface, (PERSUADE, 2015)) and of porous asphalt were added for the comparison.

Figure 9 -

 9 Figure 9operation principle of FABAC.

Figure 10 -

 10 Figure 10 -1 st joint after one (left hand picture) and after five million cycle (right hand picture).

Figure 11

 11 Figure 11 -2 nd joint after one (left hand picture) and after five million cycle (right hand picture).

Figure 12 -Figure 13 -

 1213 Figure 12 -Evolution of friction coefficient of PMCC, measured by SRT.The figure shows that the microtexture is decreased but still at good level and cleaning is needed to maintain a good skid resistance of the pavement.

  2009). Development of the Next Generation Low Maintenance Concrete Surface. FEHRL. (2008). New road constrcution concepts. FHWA-IF-08-004. (2007). The little book of quieter pavements.

Table 1 below : Table 1 :

 1:1 recipies and mechanical strength of concretes used in the noise absorption study.

	F3	Rivolet 4/6	13.0%	22.0%	0.33	160.00	44.72	10.86
	F4	Rivolet 4/6	8.0%	7.0%	0.47	210.00	21.74	5.44
	N°	Gradation	% voids content (designed)	porosity by impedance test (measured)	W/C ratio	Vpaste	Rc 28 days (MPa)	Rf 28 days (MPa)
	F1	Rivolet 2/4	13.0%	14.0%	0.33	200.00	36.14	9.67
	F2	Rivolet 2/4	10.0%	15.0%	0.40	210.00	36.33	9.07

FHWA. (2006). Eavaluation of U.S. and European concrete pavement noise reduction methods. Matelys. (2016). Experimental measurement of noise absorption of LMPC. NCPTC. (May 2014). Guide to Concrete Overlays: Sustainable Solutions for Resurfacing and Rehabilitating Existing Pavements (3rd edition). National Concrete Pavement Technology Center. PERSUADE. (2015). Poro Elastic Road SUrface for Avoiding Damage to the Environment. Silvia. (2005). Sustainable Road Surfaces for traffic Noise Control (SILVIA). European Commission