Anne-Laure Amilhat Szary
email: laure.amilhat@ujf-grenoble.fr

Walls and border art: the politics of art display

Keywords: border, wall, barrier, art, performance, landscape, artscape, USA, Mexico MOTS CLES : frontière, mur, barrière, art, performance, paysage, « artscape », Etats-Unis, Mexique

This detour through art aims at demonstrating the performative function of contemporary walls and barriers, designed to impose a geopolitical vision through landscape changes. The text assesses the link between art and borders by formulating the hypothesis that a "border art" (art on the border, art born from the border, art against the border, etc.) is emerging. It tries to understand how the closing up of a border not only reactivates cultural production on an international border, but also transforms the latter's meaning. On the USA / Mexico border for instance, the building up of the security fence sin 2006 seems to have been accompanied by a strong artistic upsurge. This can be nuanced by analyzing the changes of the nature of artistic production, with more mobile works, marked by a strong presence of videos and performances, as is the fixity imposed by the line inferred a fluid creative answer.

RESUME

Ce "détour" par l'art vise à démontrer la fonction performative des murs et barrières contemporains, destinés à imposer un discours géopolitique dans le paysage. Le texte se confronte avec le lien art-frontières en posant l'hypothèse de l'émergence d'un « art de la frontière » (dont l'expression anglaise border art rend mieux compte de l'ambivalence entre lieu et causalité : art sur la frontière, art né de la frontière, contre la frontière, etc.). Il cherche à comprendre comment la fermeture d'une frontière non seulement réactive la production culturelle sur la frontière internationale, mais en transforme également le sens. Sur la frontière USA / Mexique par exemple, il semble que, depuis 2006, l'érection de la barrière de sécurité ait été accompagnée d'une recrudescence artistique. Celle-ci peut être nuancée par une évolution de la nature de la production artistique, avec plus d'oeuvres mobiles marquées notamment par la vidéo, et la performance, comme ci la fixité imposée par la ligne inférait une réponse créative fluide.

heritage valorization of borders have been assessed, one example is the "Two banks garden"of Kehl, Germany and Strasburg, France1 , which includes a cross border footbridge over a river which ownership was the case for three wars. Indeed, this points to the fact that in contexts of political integration, and open limits, planning development projects root themselves in the border, valorizing or "instrumentalizing" the borderland: they bring its visibility out along with meaning and value, and actually contradict its predicted disappearance [START_REF] Fall | Drawing the line. Boundaries, identity and cooperation in trans-boundary protected areas[END_REF][START_REF] Charlot | Traces, mémoire et reterritorialisation de l'ancienne frontière interallemande". Après les frontières, avec la frontière[END_REF][START_REF] Fourny | De la frontière naturelle à la nature comme lien transfrontalier. Du rôle et de la place de l'environnement et du milieu dans les coopérations transfrontalières[END_REF][START_REF] Reitel | Ville et frontières[END_REF]Reitel à paraître). The announced "borderless world" seems on the contrary paved with what one could call "walls" on the one hand, or "bridges" on the other, or both. The materiality and visibility of both, question the spatial structures of borders. Indeed they seem to contradict the virtual character of surveillance mechanisms, the reticular aspect of control devices (mobile customs patrols, airports), and the fluidity of commercial and financial flows. These manifestations of what we could call the re-location and signification of the border functions seem to spark off a certain process of re-territorialization of the border line itself, through its "inscription" in the ground. The analysis of this process is central to this paper: this apparent contradiction between meaning and shape implies that the new tagging of borders has multiples finalities that go way beyond their primary governmental functional appearance.

Why do governments take the decision to materialize the closing of their national space? It is often asserted that this is done in order to stop -or to control at best -flows (of migrants, terrorists, etc.), but this type of answers rapidly appears to be very unsatisfactory. Recent research on walls can grossly be divided between those who offer to understand them either through the analytical lens of security paradigms changes [START_REF] Anderson | The Transformation of Border Controls : A European Precedent ?[END_REF][START_REF] Zurek | Global Surveillance and Policing: Borders, Security, Identity[END_REF][START_REF] Walters | Border/Control[END_REF][START_REF] Brunet-Jailly | Borderlands: Comparing Border Security in North America and Europe (Governance)[END_REF][START_REF] Van Houtum | The European Union as a Gated Community: The Two-faced Border and Immigration Regime of the EU[END_REF] or through that of a critical approach of globalization [START_REF] Ballif | Le défi des « teichopolitiques ». Analyser la fermeture contemporaine des territoires[END_REF], or even through the denunciation of the encirclement of the Western world [START_REF] Andreas | The wall around the West: state borders and immigration controls in North America and Europe[END_REF]Andreas et Biersteker 2003Van Houtum et Pijpers 2007). These authors all draw our attention on the fact that this pretention to control and differentiate flow may seem illusory when opposed to the possible means of technically skirting the obstacle (traffickers satellite guidance in the deserts of the USA / Mexico border for example) or of paying more to by-pass the barrier through other routes. All this encompasses us to go further and formulate another question : what does one want to signify when thus making the limits of his territory more visible ? "The temptation of the wall is not new" tell us the poets : "Each time that a culture or a civilization has not been able to think the other, to think itself with the other, to think the other within itself, these stiffs preserves of stones, iron, barbed-wire or closed ideologies have raised, have fallen, and yet come back to us in new stridences." (Glissant et Chamoiseau 2007 2 . Behind a sign, the academic questions the semantic depth: in spite of the apparent proximity conveyed by mediatic images, walls are not all equivalent. A few tentative balances exist, which try to contextualize walls amongst other contemporary borders, which take either a cartographic [START_REF] Foucher | L'obsession des frontières[END_REF][START_REF] Ballif | Le défi des « teichopolitiques ». Analyser la fermeture contemporaine des territoires[END_REF] or a chronological [START_REF] Vallet | The (re)building of the wall in international relations. Introduction to a special issue on Walls and Security barriers[END_REF] aspect. But their results appear as very heterogeneous since these authors do not submit to one unified definition of the security barrier or closed border. While assessing three aspects of security politics (military, migratory and social security), walls answer diverse -sometimes complementary functions : protection, consolidation of territorial gains, separation. Rather than to stop enemy armies, these new walls seem to be justified by a preventive narrative: by freezing the conditions of conflict, they are supposed to reduce it when it is often quite the opposite that happens. If walls appear as the paragon of hard barriers, further analysis reveals that they are not as linear as they may seems in the first place and E. Weizman qualifies the Israeli / West bank « security fence » of a "moving border" [START_REF] Weizman | Hollow land: Israel's Architecture of Occupation[END_REF][START_REF] Brown | Murs. Les murs de séparation et le déclin de la souveraineté étatique[END_REF], Parizot 2010).

Our aim is to determine what form of promethean representation of the world can bring the instigators of such projects to believe that an artifact, however tall and technological may it be, will be able to contain global flows. Understanding walls as narratives lead us to detect that a certain conception of power and politics is at stake when, paradoxically, most walls are built by democracies. The closed border plays the role of a new symptom (an original assemblage of old political modalities) revealing the evolution of the relationship of our societies to their space. The need for spatial border demarcation that walls express participates to a need of showing the intangible: the issue is no longer to reassure the community on its coherence but on its mere existence, opposing an inside to an outside by tracing a wall. Both the existence of the State and that of the nation on which it can build are at stake through this kind of representation of power. Only they can justify the possibility of investing so much material and symbolic energy. This text offers to understand the wall as projection of political intentionality which amounts to imposing a transformation of spatial markers, concerning both those who live within the concerned landscape, but also (and above all?) those who will receive its images. This implies to consider closed borders as material artifact which performative power is extremely powerful. This complex proposal can be evidenced by the interventions which artists undertake on walled borders, thus trying to transform the imposed image. If the work of art can first of all appear as a king of symbolic crossing of the border, it also gives food for thought on the closing of borders defined as a landscape intervention. To the violence of the wall can only be opposed its diversion. By becoming the back-up of artistic productions which embody various figures of contestation, the wall allows the population which had been deprived of expression to seize the word. The Berlin wall is often quoted as the most characteristic example, even if it was only painted on one size. The research on which the text is built upon is based on the report that a growing number of artistic projects are located on the US-Mexico borderland. Some are referenced and analyzed, and it seems that a geopolitical focus is emerging [START_REF] Berelowitz | Marcos Ramirez Erre: Border Art 'From This Side[END_REF][START_REF] Prieto | Le mur à la frontière entre le Mexique et les États-Unis : flux, contrôle et créativité de l'esthétique géopolitique[END_REF]. What this paper will endeavor to understand is that fencing a border not only activates but re-activates a cultural production on the significances and meanings of international limits but also transform the original meaning of the fence itself. On the USA / Mexican border for instance, it seems that since 2006, building the security-fence building has been accompanied by an artistic recrudescence, which is nuanced by a shift in the nature of the production, with more mobile artworks, notably performances and cinematic projects. Everything happens as if the fixity imposed the line inferred a fluid response. If it can easily be understood that "Where social scientists describe discursively, artists create symbolic models that succinctly capture intense personal experiences in the social environment and structure" [START_REF] Berelowitz | Marcos Ramirez Erre: Border Art 'From This Side[END_REF]), but the political weight of these artistic displays has yet to be discussed more extensively.

In this context, a geographer's conceptual toolbox can be very useful to analyse and explain the relationship of the artwork with the space/place it is located onto. Landscape theory appears as one of those tools, when used as a geopolitical narrative. Hence, I will present the geo-political conditions of the genesis of the art productions that lead to border displays, and on the conditions of reception of such creations. The USA / Mexico border appears as a very fertile fieldwork to improve our understanding and knowledge on the politics of art display.

1/ A state of art: from landscape to artscape, via land art.

Traditionally the way geographical thinking would be able to deal with art production would be for the analysis to focus on the relationship between artists and space. This would first base on landscape analysis, but also take into account the artistic manifestations of geographical documents (i.e. drawn cartouches on old maps) but also the map itself, which for a long time balanced aesthetics and scientific priorities [START_REF] Vanci | Atlas et les territoires du regard. Le géographique de l'histoire de l'art (XIXe et XXe siècles)[END_REF][START_REF] Cosgrove | Geographical imagination and the authority of images[END_REF]. Geography can thus resort to art to analyze space. One of the first attempts in this field was by Dennis Cosgrove who showed how, during the modern period, landscape was naturalizing power. According to Cosgrove's readings of Flemish and English paintings, landscape featured the territorialization of the national communities, and in the making in various European places [START_REF] Cosgrove | The Iconography of Landscape. Essays on the Symbolic Representation, Design and Use of Past Environments Cambridge[END_REF][START_REF] Cosgrove | Social formation and symbolic landscape[END_REF]) What I am suggesting here is we can understand our post-modernity at the light of the artistic production resulting from the closing down of the USA / Mexican border. One cannot but acknowledge that landscape also results from power because « together imposes and represents a visual order » [START_REF] Zukin | Landscapes of Power : from Detroit to Disney World[END_REF][START_REF] Mitchell | Landscape and Power[END_REF]). On the USA / Mexican border, the landscape dimension is well documented: the boundary definition was largely set on a landscape estimate, according to the drawings and reports from the scientific member of the « US-Mexico Boundary Survey » commission and from the artists that went with them [START_REF] Hall | Drawing the borderline: Artist-explorers of the U.S.-Mexico Boundary Survey[END_REF]. Border landscapes are most often scrutinized to determine the conditions of environmental bifurcations that follow the political separation [START_REF] Rumley | The Geography of Border Landscapes[END_REF]. However, if stress is often put on the fact that the border impact leads to land use differentiation (such as ranching and farm sizes for example) it can also be evidenced that the multiplicity of exchanges in the borderlands lead to some cultural sharing (such as the architectural rapprochements built on the hybridization of traditional and post-modern elements in the cities of the USA / Mexico border, cf. . [START_REF] Arreola | The Mexican Border Cities: Landscape Anatomy and Place Personality[END_REF][START_REF] Herzog | From Aztec to High tech : architecture and landscape to the Mexico-United States border[END_REF]). If landscape illustrates the consubstantiality of power and the control of the material reality, its questioning can help us to understand what the gesture of commanding means: making landscapes, whether by picturing it or by building a wall through it, consists in projecting one's intention over a portion of space that has been defined by the sight of it. The aim of this research is not to work on the traditional border impact, but on the landscape disruptions that the building of the fence generates. I will work on the wall as defined as a landscape intervention. The following descriptive images illustrate this idea: the present-day security fence crossing the Arizona desert is compared to the 1976 Running Fence erected by Christo and Jeanne Claude a little further north in the Sonoma and Marin Counties, California. The visual effect of both "installations" can be compared, but the the suppleness of the materials used by Christo convey the absurdity of the enclosure of the desert whereas the contemporary fence is not put up for debate. The fences the USA are currently building on their southern border constitute a material object worth observing. Indeed, the concrete (and barbed-wire, and second-hand gulf war TOLES) artefact is of at least two interesting for two reasons:

-First it expresses a relationship concerned with the politics of space control (based on the supposed capacity of the fence to hold back flows thanks to the existence of an obstacle -a very promethean project). It is also marking the desire to write a discontinuity in the southern space, assuming that it is not or no long is perceptible)

-Because of its material dimension, the fence become the original medium for analysis : on one hand the closing up of the landscape arouses the irrepressible wish to know the other side, to which the wall is supposed to make impossible, on the other hand, the strength of this intervention on landscape stimulates a multitude of landscape reactions. Words and colors plated on its surface subvert the wall and stress its symbolic as well as functional roles. The link between art and territory is not new, and the first artists who experimented out-of-place creation and invented land art in the 1960's took two directions. For some, getting away from the museums and the traditional creation / display places meant leaving the city and working on and in nature (cf. Robert Smithson and inaccessible places, Jean Verame and the deserts, Frans Kracjberg and the Amazonian forest). For others, the reflexion on territory led to the exploration and contestation of borders (the oil barrels that Christo set up to cut Visconti street in 1961, for instance, directly address the absurdity of the Berlin wall; the underwater dam built by Peter Hutchinson in 1969, mimics boundary making; whereas, the same year Lawrence Weiner stages border transgression in Terminal Boundaries). Minimal art reformulated the question of display. In this sense, the contemporary artistic wave can first appear as a similar response to comparable political stimuli. However, if one goes beyond land art, it appears that not only space is omnipresent in the contemporary art production, but so is the questioning of space itself. It is known that many visual artists use the term "geography" to explain their work. The exhibitions "Geography and the Politics of Mobility" (2003, Generali Foundation in Vienna) by Ursula Biemann and the "Mobile Transborder Archive" (a scenario project for inSite 2005 between San Diego and Tijuana) illustrate this trend. Both work on the concept of territory and borders, originating in the intensification of cross-boundary mobility and the growing restrictions put to movements across.. From an analytical point of view, everything is under construction. For geographers, the relationship to images has long appeared as self-evident, photography appear as mere illustrations and yet their biases were only questioned recently [START_REF] Rogoff | Terra infirma: Geography's visual culture[END_REF][START_REF] Sanders | Developing Geographers through Photography: Enlarging Concepts[END_REF]). Geographers only recently started to explore this relationship between territory and art(cf. [START_REF] Foster | Cultural geographies in practice: Some reflections on artgeography as collaboration[END_REF], with a special emphasis on re-visiting land art [START_REF] Volvey | Fabrique d'espaces : trois installations de Christo et Jeanne-Claude[END_REF][START_REF] Volvey | Christo et le Land Art dans la carte du territoire. La monumentalité cristolienne[END_REF][START_REF] Volvey | Christo et le Land Art dans la carte du territoire. La monumentalité cristolienne[END_REF][START_REF] Volvey | Spatialités de l'art[END_REF]). The stress on the aesthetics of representation is even newer(cf. Savary 2005 who demonstrates how style builds urban landscape representation). My contention is that it is useful to draw upon other disciplinary literatures to get over these obstacles. In particular, the recent breakthroughs in visual anthropology are very promising as the discipline distances itself from the ethnographic interpretation of images [START_REF] Evans | Visual Culture: The Reader[END_REF][START_REF] Van Leeuwen | Handbook of Visual Analysis[END_REF]2007). This complex work on images which has proven very fertile in the studying of cultural miscegenation on the American continent [START_REF] Gruzinski | La colonisation de l'imaginaire : sociétés indigènes et occidentalisation dans le Mexique espagnol[END_REF][START_REF] Segre | Intersected Identities: Strategies of Visualisation in Nineteenth-and Twentiethcentury Mexico (Remapping Cultural History[END_REF]) could stimulate border analysis. What we offer to do in this text is to set up the basis for a visual geography that will question the power of the eye through the analysis on an emergent artistic focus on the hardening of the border. Based on the idea of artscapes [START_REF] Galofaro | Artscapes El Arte Como Aproximacion Al Paisaje Contemporaneo / Art as an Approach to Contemporary Landscape[END_REF], we propose the study art on the border as a "borderscape".

2/ Is there any such thing as border art?

Just as a wall provokes, a movement of the eye bounces against it. Just like on the movie screen the wall tears the frame, the walled border constitutes an object that deflects scientific landscape approaches. That is why I suggest looking at walls through the artistic production that it arouses and which represents a kind of subversion of the violence imposed on landscape by the ruling power. This also means that I think the artistic representation is so strong that it influences the way those receiving the art work will then relate to the landscape. Political landscape can therefore be as much transformed as highlighted by what we can tentatively call an "artscape" or "borderscape". Borders can of course constitute good settings and have been very many times given a cinematographic existence [START_REF] Torrans | The magic curtain : the Mexican-American border in fiction, film, and song[END_REF][START_REF] Malagamba | Una visión del arte fronterizo. El poder del lugar y las geografías recordadas". Por las fronteras del Norte : una aproximación cultural a la frontera México-Estados Unidos[END_REF]. This kind of featuring has led to studies with exhaustive claims and typology: Elena D'ell'Agnese's research team has thus viewed a good quantity of Mexican and American movies to define the types of "loci" that the border endorses, according to the way in which it can (or not) be crossed [START_REF] Dell'agnese | The US-Mexico Border in North-American Movies: a Political Geography perspective[END_REF]. In these fiction works however, as well as in the analysis which has been drawn from them, the aesthetic part of the place is not considered. And what we want to do here is to go beyond an analysis that considers the border as the context of the imaginary creation, to try and understand why the boundary allows the opening up of the creative field, contributing to an aesthetic renewal. From this point of view, the USA / Mexico border represents a fertile artistic mould : zone of cultural exchange, it has seen the rapid development varied forms of hybrid and popular art [START_REF] Valenzuela Arce | Nuestros piensos. Culturas populares en la frontera México-Estados Unidos[END_REF][START_REF] Herrera-Sobek | Santa Barraza, artist of the borderlands[END_REF][START_REF] Torrans | The magic curtain : the Mexican-American border in fiction, film, and song[END_REF][START_REF] Saldívar | The borderlands of culture : Américo Paredes and the transnational imaginary[END_REF]). We will concentrate on the recent trends of this cultural production, as much as, from the 1970's, an art that builds up to address politics surges, rooting in the border. At a time when visual arts were committing to a very special link with concept, different artistic events accompanied the contestation movements. Rupert García's painted silk screen called « ¡Cesen Deportación ! », shown in 1972 to denounce the inhumanity of treatment to which the migrant bracero were submitted in now acknowledged for its pioneering role; it reflects direct influences from conceptual and pop arts. In literature, the expression goes through a vast diversification process and theater becomes a place of rejection of the violence that the border embodies for the migrants. But it is certainly from the 1980's onward that this border becomes, as such, an issue and a central theme for numerous artists living under the influence of the area of flows and exchanges between Mexico and the United States. A first performance by Sara-Jo Berman and Guillermo Gómez-Peñaleads to their initiating of the Poyesis Genetica performance troup in 1980, thanks to the support of the Casa de la Cultura de Tijuana, creating the "first border performance art milieu (1983-4)" (Gómez-Peña 1991). The foundation of the Border Arts Workshop / Taller de Arte Fronterizo (BAW / TAF) 3 roots into the history of the Chicano movement and its demand for civil rights for the Hispanic community of the United States. This group of artists has initiated a series of exhibitions that have marked the spirits of the border area inhabitants who were able to see them or at least here from them. An regular publication launched in 1986, La Linea Qsuebrada/ The Broken Line,was thought as a "bilingual/experimental magazine that was instrumental to the definition of a border aesthetics" (Gómez-Peña 1991). The group also participated in focusing on a very special mode of artistic expression, that of performances [START_REF] Gómez-Peña | The New World (B)order[END_REF][START_REF] Fusco | Corpus Delecti: Performance Art of the Americas[END_REF][START_REF] Gómez-Peña | Dangerous border crossers: The artist talks back[END_REF]. One of the founding members of the Group, Guillermo Gómez-Peña, has specialized in this kind of work: his textual production often goes with the artistic event in order to insist on the objectives given to his work : the questioning of art and its display (« what lies behind and under the realization of a performance art, especially in the crossing of very volatile geographical and cultural borders », Gómez-Peña 2000) as well as the reassessment of place as such 4 . This artistic production has known a first growth period which coincided with the perspective of border opening that the NAFTA seemed to be promoting. The first bi-national art fair on that border, InSITE, was held in 1994. This creative wave has not remained unknown, of course (Yúdice 2003a;[START_REF] Yúdice | Producing the Cultural Economy: The Collaborative Art of INSITE[END_REF], and an analysis corpus is being produced [START_REF] Fox | The fence and the River. Culture and Politics at the U.S. -Mexico Border[END_REF]. Facing this expression which touches literature (theater, novels, poetry, but also comics), music, plastic arts and cinematography,we propose to restrain this discussion to the analysis of these two last creative types, because of their visual nature. We will not study cinema as such but video production as part of visual installations: it very much accompanies performances, with a similar ability to counter balance fixed aspect of the wall by its fluidity. This conjunction of imaginaries tends to prove that the artistic phenomenon goes way past the individuality of each work of art and fits into a socio-spatial including relationship The specific statement of the Border Art Workshop could not be enough to testify of the existence of such a thing a "border art". The growing success of the InSITE fairs which repeated themselves in 1997, 2001 and 2005, attracting every time more diversified artists could also be a testimony to that assessment. Studying the interaction between the fencing of the border and artistic activity at the USA / Mexico border seems indeed to suggest that as the walling process expanded inland, so did the art production. Another border art workshop has emerged in Nogales, Taller Yonke 5 . Their interventions go from murals to wall interventions. Their proposal is to "experiment visual art in spaces that are of everyday use for human beings, art for the masses, in which the main elements of the creation process are the relation to place or site, and to the people who use it on a daily basis" 6 . Such a declaration sounds very much like the situationist proclamations on the political impact of art display. In that context, it was not as much about bringing art to the streets, that about constructing the urban experience as art. In this case, I think that border art is being strengthened by the harshening of border politics and that this factual coincidence lays the ground for a geopolitical analysis of a renewed display of art.

3/ Far from disappearing, borders are more visible

The acknowledgment of border art allows me to reformulate a geopolitical debate, that of the political visibility of borders. I do suggest that walls constitute a form of "visibilization" of political action, such as the connection between the two photos of Christo's Running Fence and the Security Fence induces. One can wonder on what kind of promethean representation of the world7 lead the instigators of such projects to believe that such artifacts, as high and technological as they may be, will be able to contain a planetary flow? How can they make abstraction of the vanity of such enterprises (cf. Duras 1978) ? Why do they chose this kind of spatial solution and what 4 "Border culture is a project of "redefinition" that conceives of the border not only as the limits of two countries, but also as a cardinal intersection of many realities. In this sense, the border is not an abyss that will have to save us from threatening otherness, but a place where the so-called otherness yields, becomes us, and therefore comprehensible."

1986/ cf. the border art section of their website : http://www.muralesfrontera.org/Arte-Fronterizo.html , and their works : Border Dynamics, winter 2003, Paseo de Humanidad, winter 2004, Mural "Taniperla", spring 2005, Muro Invisible, winter 2009. 6 "Su propuesta es la de experimentar con el arte visual en los espacios de uso cotidiano del ser humano, arte para las masas, donde los elementos principales del proceso de creación es la relación del espacio o sitio, y las personas que cotidianamente lo usan. Su relación en el proceso de elaboración, y después cuando se convierte en un punto de referencia." http://www.muralesfrontera.org/quienessomos.html meaning do they give it in such paradoxical situations? If some research has been led to explain the setting of these walls around the western world [START_REF] Andreas | The wall around the West: state borders and immigration controls in North America and Europe[END_REF][START_REF] Andreas | The rebordering of North America : integration and exclusion in a new security context[END_REF], none really tackles the anthropological issues of these political projects. To my knowledge, political anthropology or geography have not concentrated on the question of artefact. What seems to be new is the need for inscription, demarcation of intention into space. For its interpretation, it seems we can relay on stimulating work on the appearance of politics: from the 1980's especially (going along with the "theatrical" treatment of political behaviors), some scholars have submitted the occidental political systems to the analysis grid initially built to understand the clannish functioning of indigenous societies. It then belongs to Georges Balandier to have first insisted on the possibility of reading the political arena as a true theatrical "stage" [START_REF] Balandier | Le pouvoir sur scènes[END_REF]. This approach got interested in the forms through which the political power was getting conscious of its intervention power on the representation it gives of itself. This concerns the possibility for power to reflect towards the community which the it represents (being elected by her, in democracy) an image of "coherence and cohesion" and of the "efficiency of the link" [START_REF] Abélès | Anthropologie de l'Etat[END_REF][START_REF] Abélès | Anthropologie du politique[END_REF]. The field is then open to research on all the elements that accompany the « staging of politics »: the recurring symbols, but also the rituals and the forms of dramatic art made possible by the technological progress of the media and their growing participation in the formation of public opinion. This paper does make the hypothesis that the will of spatial inscription of the border stands out for the same type of socio-anthropological functioning, this need to show the intangible8 . However, in this case what is needed is not to reassure the community on its coherence but on its very existence by distinguishing the inside from the outside thanks to a wall. It would be the very existence of the state, and that of the nation on which the state can rely upon which could be at stake through this king of representation of power. Therefore the justification of such material and symbolic investments: locking oneself up allows oneself to prove his own existence as well as the fact that the community can last along. Getting into border landscapes in a period of profound change of boundary regimes which makes them less visible : when « boundaries cease to be purely external realities, they become also, and maybe before anything, what Fichte, in his Reden an die deutsche Nation, had superbly called the « internal boundaries », innere Grenzen, that is to say, as he himself writes, located 'every and nowhere' "9 (cited by Balibar 1996b). If "borders are no longer on the edge of politics but […] objects, or more exactly things in the very space of politics"10 (Balibar 1996a), the landscape where they are set will appear as spaces of visibilization of politics. The materialization of a concrete wall does not only symbolise the barrier function performed by any border, but it also prevents from seeing what is happening on the other side. The more the « other side » becomes invisible and unknown , the more it gets frightening [START_REF] Newman | On Borders and Power: A Theoretical Framework[END_REF])… and thus fatally desirable and attractive. The movie by Chantal Akerman [START_REF] Akerman | De l'autre côté[END_REF] illustrates well the way the wall induces distinction in a landscape that had always previously been felt as homogeneous, thus conferring a profound meaning to transgression, to the crossing of the barrier artificially imposed to the eye and to the will. In this perspective, we sense that not only walls do express this phenomenon, but also all sorts of efforts to materialize open borders. The multiplication of objects that are entitled to materially demarcate the boundary while authorizing cross-relations could be called "bridges". We have mentioned them in our introduction but our purpose is not to treat them here. The reflexion on the materiality and visibility of contemporary borders help us to put up hypothesis on the implications of art display in such places. In a context where the relationship to the spectator is part of the art production itself [START_REF] O'doherty | Inside the White Cube. The Ideology of the Gallery Space[END_REF]O'Doherty, Architecture et Project 2007), the politics of display and their geo-graphic need to be clarified. This perspective is suggested in a few texts which art my constitute a dimension of politics : "the inevitable difference between the represented and its representation is the very location of politics" [START_REF] Bleiker | The aesthetic turn in international political theory[END_REF].

4/ Where is border art displayed?

From our first research, we are able to produce an attempt list and typology of the places or border art display. On the first hand, it appears that some galleries and cultural centers have played a polarizing role in the expansion of border art between San Diego and Tijuana. We have already mentioned the Casa de la Cultura, en Tijuana, but the Centro de la Raza, home to the Chicano community in San Diego has gained a growing legitimacy in that field over the years. Noticeably enough, these two exhibition halls are located in the biggest twin towns of the border. In that sense, the geography of art display would not differ much from that of many other social phenomena of the area. Our hypothesis that the walling of the border deports artistic activity towards the interior twin towns, following the demarcation process. The bursting out of creative focis in Arizona / Sonora seems to be an indirect result of the progression of the Security Fence. The localization of display can also be questioned in its complex interaction between space and time. Our first analysis reveals that there are seldom occurrences of permanent border art display. It is mainly expressed through performances, temporary exhibitions and exceptional art fairs, the InSITE editions. This ephemeral character is important for the interpretation of the spatial construction at stake. Finally, a large part of the work is done on the wall itself. Many interesting events took place on the beach, notably the carved mirrors installed on the railings by Valeskia Soares, in the Parque de la Amistad / Friendship Park, before it was closed to public access in December 2008. A lot of photographs of ornamented spontaneous altars erected to the migrants who died in the crossing of the wall are generally constitute the image that common sense builds of "border art". A very interesting installation can be mentioned to suggest the power of this type of display, although it can already appear as a little old. Silvia Gruner's work called The Middle of the Road/La mitad del camino was dedicated to the installation of 111 gypsum little statues of an Aztec goddess, Tlatzoltéotl, directly hung on the metal grid. The display was made with the help of the inhabitants of the Colonia Libertad, in the suburbs of Tijuana. The artist's intention was to underline the crossing status of the area and to make a comforting gesture towards the illegal migrants, since this goddess is traditionally associated to motherhood and baby delivery, also wishing to show those leaving the country that they were not abandoning their culture behind in their villages. The success of the work was confirmed of the report of illegal crossings on behalf of the American immigration officers who came to rob some statuettes whereas some migrants actually ran away with one of them. The illustration taken from the InSITE 2005 webpage illustrates an unseen shape of the border : by insisting on the exhibition sites rather than on the boundary, one gets an image of a reticular border. Not only does the "exhibition [appear] as communicative space" (to use the seminar title on the architectures of discourse of Ute Meta Bauer's -2005 InSITE curator), but display helps to evidence new territorialities.

5/ What are the contents of border art?

My first analysis, although based on secondary sources because of the distance, has let me to propose a first typology of the artistic production on borders. When figurative, the plastic works two appear to scenarize two main thematics: barbed wire and body appear as two main figures 11 . The works of the Border Art Workshop, and mainly that of Terry Allen or David Abalos work on various possibilities incorporating barbed wire into their work wich includes a good many installations [START_REF] Fox | The fence and the River. Culture and Politics at the U.S. -Mexico Border[END_REF]). One of the realizations which I find most inspiring on that theme is Michael Schnorr's Burning Fence realized in 1991, where the artist reproduced a fence in wood covered by cloth, which he moistened with gasoline and to which he then set fire.

The second recurrent theme is that of the body and its suffering. The embodiment of the border by the implied artists certainly appeals to Foucauldian biopolitics and should have to be questioned further into that direction. This perspective brings together the figurative representation of the body in a variety of visual arts, including photo and video, but also a strong accent put on performances. The works by Guillermo Gomez-Peña are the most famous, but he is joined in that dynamic by a number of others. Performance is quite development in Latin America, with a number of famous Colombian performers, with a desire to address political and social violence in a different way. It is now expanding to Central America, with a group of active women performers12 , amongst whom Maria Adela Diaz has directly addressed the border problematics (in Caution, or Borderline, where she shuts herself into a wooden case thrown to sea, to address the issue of illegal migration).

One interesting example of that perspective is an event presented to the InSITE fair in its 2005 edition. Javier Téllez's work entitled One Flew Over The Void/Bala perdida concluded with a show inspired by Dave Smith, the canon ball man. The show figured mental patients with whom the artist worked to question all kinds of borders together, and the artist launched himself over a reproduction of the barrier at Tijuana, evidencing in his way the tensions that grow within a border zone.

The body work also includes the spectator's participation, especially calling on the power of the bouncing eye. In a work for InSITE 2001, Valeskia Soares used carved mirrors that he set on the barrier fence at the Parque de la Amistad / Friendship park in San Diego -Tijuana. Looking at the other, one bumped into his own image, but quotations from Italo Calvino's text The invisible cities, were written so that only people from the other side could actually read them. Another illustration of this orientation is Cristina Fernandez's installation, called Llegadas y Salidas (Insite 97). She set up the reproduction of an old brass marine telescope in front of the fence in Colonia Libertad, to suggest that migrants, like seamen in search of new lands, could use the tool to discover new territories and estimate the difficulty in conquering them. These works raise the question of the impact of art border: who receives it and with what consequences? We are aware that there is a sense of "Quixotic gesture" in this kind of cultural production, as the Australian artist Ian Howard puts it, comparing his border rubbings to Don Quixote's conquests. The question of the reception of border art and its impact will constitute an important component of our field work.

Concluding perspectives

The artistic production on the USA / Mexico borderland area can be seen as a resistance scene, where shifting narratives have followed one another, from Chicano Civil Rights claims (Border Arts Workshop) to open borders hope (cf. Fluid Border Performances by Guillermo de la Peña when NAFTA was launched). We have thrived to characterize what we could see as a paradoxical barrier times artistic explosion. It is based on the sense that artistic work is strong enough not only to renew the perspectives of landscape perception but also ensure it with a strong capacity of political influence. "Thousands of artists in the U.S. and other countries are currently crossing different kinds of borders. And as they do it, they are making a new kind of art, an art of fusion and displacement that shatters the distorting mirrors of the "Western avant-garde" wrote one the border artists (Gómez-Peña 1991): we hope to have been able to demonstrate how the contemporary border art scene renews the debate on the display of art but also how, by

Figures

  Figures 1-2: The border crossing the Californian deserts : the Secure Border Initiative, 2006 vs from Christo's Running Fence,1976The fences the USA are currently building on their southern border constitute a material object worth observing. Indeed, the concrete (and barbed-wire, and second-hand gulf war TOLES) artefact is of at least two interesting for two reasons:-First it expresses a relationship concerned with the politics of space control (based on the supposed capacity of the fence to hold back flows thanks to the existence of an obstacle -a very promethean project). It is also marking the desire to write a discontinuity in the southern space, assuming that it is not or no long is perceptible)

Figure 3 :

 3 Figure 3 : Another vision of the border: the reticular spaces of art display, Insite exhibition places on both sides of the USA / Mexico border (http://www.insite05.org/)

San Diego, CA: Border Art Workshop / Taller de Arte Fronterizo, 1988 / catalogue

  de l'exposition Vidas Perdidas / Lost Lives de 1989 La Frontera / The Border: Art about the Mexico / United States border experience.

	3 Cf. leur site internet (http://www.borderartworkshop.com/index.html) et deux beaux catalogues :
	-The Border Art Workshop (BAW/TAF) 1984-1989: A documentation of 5 years of interdisciplinary art projects dealing
	with U.S.-Mexico border issues (a binational perspective). Taller de Arte Fronterizo (BAW/TAF) 1984-1989:
	Documentación de 5 años de proyectos de arte interdisciplinario sobre asuntos de la frontera de Estados Unidos con
	México (una perspectiva binacional).

San Diego: Centro Cultural de la Raza, Museum of Contemporary Art, 1993, Chávez, Patricio and Madeleine Grynsztejn, curators, catalogue

  d'une exposition de 1993, projet Dos Ciudades / Two Cities issu d'une collaboration entre le Centro Cultural de la Raza et le Museum of Contemporary Art de San Diego.

The " Jardin des Deux Rives / Garten der Zwei Ufer", opened in

The park is designed by the German Rüdiger Brosk, the footbridge by the French Marc Mimram.2 « La tentation du mur n'est pas nouvelle (…). Chaque fois qu'une culture ou qu'une civilisation n'a pas réussi à penser l'autre, à se penser avec l'autre, à penser l'autre en soi, ces raides préservations de pierres, de fer, de barbelés, ou d'idéologies closes, se sont élevées, effondrées, et nous reviennent encore dans de nouvelles stridences. »

"The Secure Fence Act Builds On Progress Securing The Border", Fact Sheet: The Secure Fence Act of 2006, http://www.whitehouse.gov/news/releases/2006/10/20061026-1.htmln

L'utilité politique du paysage commence à être étudiée au niveau local, les politiques paysagères pouvant être mises en oeuvre pour « empaysager » un projet, cf.[START_REF] Debarbieux | Actualité politique du paysage[END_REF]

« les frontières cessent d'être des réalités purement extérieures, elles deviennent aussi et peut-être avant tout ce que Fichte dans ses Reden an die deutsche Nation avait superbement appelé les 'frontières intérieures' : innere Grenzen, c'est-à-dire, il le dit lui-même, invisibles, situées 'partout et nulle part' » Balibar 1996b quoting Fichte : Speech to the German Nation, 1807.

« les frontières ne sont plus le bord du politique mais […] des objets, disons plus exactement des choses dans l'espace même du politique ».

Mundane art seems to follow the same paths: we found trace, on the No More Deaths website (No More Deaths -No Mas Muertes -Tempe Student Auctions Art to Benefit NMD.mht), of a Student Art Auctions organized in Tempe to benefit NMD. The winner's drawing, a painting by Gabe Kaplan, a seventh grade student at Dobson Montessori School in Mesa, figures barbed wire crossing the Statue of Liberty, thus evoking the contractions he feels his country is going through. Kaplan's narrative that accompanies the piece reads: Deporting illegal immigrants solves nothing. We're a nation of immigrants. Most of our ancestors emigrated here from other countries, yet many of us have little tolerance for Mexican immigrants. Not all immigrants are bad. Countless have jobs and have been contributing to society for quite awhile. Sometimes they were brought over when they were children and have grown up here as Americans. They might not even speak Spanish. We've come to depend on the work of migrants in agriculture and construction. By deporting illegal immigrants we are making the people who are a vital part of our economy leave. Some of the workers came across a huge expanse of desert to get into America. The number of border deaths is going up every year. We need to change the process of becoming an American citizen to help prevent so many people from dying. Unless we change some of our laws people will keep being sent back to Mexico or dying in the desert.

They significantly appear in the "Elles@centrepompidou" exhibition organized by the French Pompidou Museum between May 2000 and May 2010 dedicated to women artists in the collections of the modern art museum.


Rosalie Vandromme

Nicolas Desramaut

Audrey Baills

Mélanie Fontaine

Audrey Hohmann

Gilles Grandjean

Olivier Sedan

Anne Puissant

Jean-Philippe Malet

Integration of landslide hazard maps into probabilistic risk assessment in context of global changes: an alpine test site

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.

Integration of landslide hazard maps into probabilistic risk assessment in context of global changes: an alpine test site VANDROMME R. (1) , DESRAMAUT N. (1) , BAILLS A. (1) , FONTAINE M. (1) ,HOHMANN A. (1) , GRANDJEAN G. (1) , SEDAN O. (1) , PUISSANT A. (2) , MALET J.P. (3) (1) BRGM, Risks division, 3 avenue Claude Guillemin, BP36009, 45060 Orléans Cedex 2, France. +33 2 38 64 34 82 (2) LIVE (3) IPGS The aim of this work is to develop a methodology to integrateglobal changes scenarios into quantitative risk assessment. This paper describes a methodology to take into account effects of changing climate on landslides activity and impacts of social changes on exposure to provide a complete evaluation of risk for given scenarios. This approach isapplied for demonstration purpose on a southern alpine test site.

Mechanical approaches represent a solution to quantify landslide susceptibility and to model hazard on unprecedented conditions, as it is likely to occur. However, as the quantity and the quality of data are generally very heterogeneous at a regional scale, it is necessary to take into account their uncertainty in the analysis. In this perspective, a new hazard modeling method has been developed and integrated in a GIS-based software called ALICE ® .To go further, climate change scenarios have been computed for the alpine test site (Barcelonnette area, France) using the REMO-COSMO-LM.From the precipitation time series, a daily index of the soil water content has been computed thanks to a reservoir-based model (GARDENIA ® ). Hence, the program classifies hazard zones depending on the several spatial data (lithological, DEM, etc…) and different hydrological contexts varying in time. The probabilistically initiated landslides are then propagated thank to a semi-empirical model (BORA) to provide real hazard maps.

Different scenarios of land-use have been developed using an automate cellular model to cover the probable range of development of potential elements at risks in the future. These exposure maps are then combined with the aforementioned hazard maps to obtain risk maps for the different periods and the different land-use development scenarios. Potential evolutions of landslide risks are then evaluated, with a general increase in the 7 communes. This methodology also allows the analysis of the contributions of both considered global changes (climate and land-use) to the evolution of risk. This communication, realized within the framework of Safeland project, is supported by the European Commission under the 7th Framework Programme for Research and Technological Development, Area "Environment", Activity 1.3.3.1 "Prediction of triggering and risk assessment for landslides"(grant agreement n°226479).


Simon Rouchier

Geneviève Foray

Nathalie Godin

Monika Woloszyn

Jean-Jacques Roux

Damage monitoring in fibre reinforced mortar by combined digital image correlation and acoustic emission

Keywords: damage, mortar, non destructive testing, digital image correlation, acoustic emission × ÑÓÒºÖÓÙ Ö Ò× ¹ÐÝÓÒº Ö

Introduction

The ageing of building materials and components has many repercussions on their mechanical, hygric and thermal properties: damaged materials are more permeable to moisture which may increase degradation, cause mould problems, bring health problems or form thermal bridges, sinking the overall energetic performance of the building. In order to properly estimate the overall performance of existing buildings in the prospects of energetic renovation, one must dispose of a reliable way to characterize damage and fractures in such materials. There is a high interest in extending the use of non destructive damage observation to the case of building materials, in order to help decision processes for maintenance. Moreover, because of environmental concerns, building facades tend to include multi-layered components such as outer thermal insulation, of which durability must be assessed. In such cases, damage monitoring aims at preventing the degradation of hygrothermal properties.

Imaging techniques for crack detection include X-ray radiography and tomography [START_REF] Slate | X-rays for study of internal structure and microcracking of concrete[END_REF] for three-dimensional fracture observation [START_REF] Landis | Microstructure and fracture in three dimensions[END_REF] or propagation monitoring [START_REF] Landis | Cracking, damage and fracture in four dimensions[END_REF]. The detection of pre-existing cracks can be facilitated 1 by impregnation techniques [START_REF] Hornain | Microscopic observation of cracks in concrete -a new sample preparation technique using dye impregnation[END_REF] allowing their automatic detection and quantification [START_REF] Ammouche | A new image analysis technique for the quantitative assessment of microcracks in cement-based materials[END_REF] by microscopic examinations. While these techniques may have a good resolution and precision, they are limited in terms of specimen size and thus not applicable on the building scale. Another possible method for such large scale applications is the recording of elastic wave velocities, which can for instance be applied to durability studies or to repair work assessments [START_REF] Shiotani | Elastic wave validation of large concrete structures repaired by means of cement grouting[END_REF].

Displacement mapping techniques are applicable to damage monitoring, and consist in measuring local displacements of a sample during its deformation. Electronic speckle pattern interferometry [START_REF] Jia | Two-dimensional electronic speckle pattern interferometry and concrete fracture processes[END_REF][START_REF] Shah | Nondestructive techniques for studying fracture processes in concrete[END_REF] and digital image correlation (DIC) [START_REF] Sutton | Determination of displacements using an improved digital correlation method[END_REF] are optical non-destructive techniques and can be used for the observation of the progressive crack development, or more generally of two-dimensional strain mapping. Optical methods have also been recently extended to three-dimensional displacement and strain fields [START_REF] Orteu | 3-D computer vision in experimental mechanics[END_REF][START_REF] Chalencon | Etude des intéractions rhéologie, fissuration et microstructure pour le développement d'un outil de formulation : application aux mortiers poreux minces fibrés dédiés à l[END_REF].

A third category of damage monitoring methods includes the recording of acoustic emissions (AE). In a material under loading, elastic waves are emitted as a consequence of crack initiation and propagation. Recording and analysis of acoustic activity is of great importance in the fields of civil engineering and material sciences. Among others, it has been used for damage estimation of concrete [START_REF] Ohtsu | Quantitative damage estimation of concrete by acoustic emission[END_REF][START_REF] Colombo | Assessing damage of reinforced concrete beam using "b-value" analysis of acoustic emission signals[END_REF][START_REF] Suzuki | Quantitative damage evaluation of structural concrete by a compression test based on AE rate process analysis[END_REF], identification of damage mechanisms [START_REF] Philippidis | Damage characterization of carbon/carbon laminates using neural network techniques on AE signals[END_REF][START_REF] Godin | Influence of hydrolytic ageing on the acoustic emission signatures of damage mechanisms occuring during tensile tests on unidirectionnal glass/polyester composites: application of a Kohonen's map[END_REF][START_REF] Elaqra | Damage evolution analysis in mortar, during compressive loading using acoustic emission and X-ray tomography: effects of the sand/cement ratio[END_REF][START_REF] Ohno | Crack classification in concrete based on acoustic emission[END_REF][START_REF] Momon | Unsupervised and supervised classification of AE data collected during fatigue test on CMC at high temperature[END_REF], estimation of the fracture energy [START_REF] Muralidhara | Fracture process zone size and true fracture energy of concrete using acoustic emission[END_REF] or failure prediction [START_REF] Shiotani | Evaluation of progressive failure using AE sources and improved B-value on slope model tests[END_REF][START_REF] Momon | Acoustic emission and lifetime prediction during static fatigue tests on ceramic-matrix composite at high temperature under air[END_REF][START_REF] Maillet | Analysis of acoustic emission energy release during static fatigue tests at intermediate temperatures on ceramic matrix composites: towards rupture time prediction[END_REF].

Acoustic emission and other wave propagation measurement techniques are the most preferable methods for large scale damage monitoring, but the interpretation of recorded waveforms requires preliminary studies. Digital image correlation and acoustic emission recording were simultaneously performed during tensile loading tests of fibre reinforced mortar samples. The methodology aims at correcting the downsides of two techniques by combining them: the applicability of optical techniques hardly extends to field studies, while the interpretation of AE measurements is difficult without a view of the crack evolution. A methodology is presented, that allows interpreting acoustic signals on the basis of a previous characterization. Mortar samples were loaded by uniaxial tension while the local displacements of their surface were monitored by a camera, and the acoustic activity was recorded by sensors placed around the damaged area. By correlating the data gathered by the two techniques during this lab experiment, it is possible to interpret in field AE measurements with more clarity. The main target is therefore to calibrate the AE technique, so that it can be autonomously used for damage monitoring of building components.

Methodology

Experimental setup

The studied material is a commercial formulation (Lafarge, MAITE monocomposant) used for external thermal insulation composite systems. It is a Portland cement mortar including dry redispersable polymer systems with a water to dry material weight ratio of 0.16 and reinforced with 1% weight of glass fibres. The purpose of the fibers is to improve the ductility of an originally fairly brittle material. A more complete explanation of the elaboration procedure was presented in [START_REF] Rouchier | Influence of diffuse damage on the water vapour permeability of fibre-reinforced mortar[END_REF]. The material was cast into prismatic samples of 500 grams and dimensions 300×100×10 mm, kept two days at a 90% relative humidity and 21 days at 50% RH. Granulometric analysis of this material has been presented by [START_REF] Chalencon | Lubricated compression and X-ray microtomography to analyse the rheology of a fibre-reinforced mortar[END_REF], along with a thorough description of its behaviour during hydration. It has already shown an ability for multi-cracking [START_REF] Chalencon | Etude des intéractions rhéologie, fissuration et microstructure pour le développement d'un outil de formulation : application aux mortiers poreux minces fibrés dédiés à l[END_REF], and is thus expected to facilitate the progressive visualisation of crack patterns.

The prismatic samples were notched as to ensure stress concentration on a relatively small area during mechanical loading, on which the observation was focused. Tensile loading was applied along the vertical axis, using a 5 kN force cell imposing a constant displacement speed of 1 mm/min. The tests were carried out until complete failure of the specimen. The setup of the samples and of the observation equipment is displayed on Fig. 1.

The experimental setup includes a CCD camera with a fixed 2.8 focal ratio, focused on the surroundings of the notch. Because of the shallow depth of field of the camera in this configuration, the focus is manually fixed as to prevent an automatic adjustment of the lens during the tests. The camera was positioned as to cover an observation zone of 101×67 mm with a spatial resolution of 22 µm/pixel. A non-uniform speckle pattern was applied on the specimen surface with a black paint spray as to facilitate the procedure of the correlation algorithm: this ensures that the initial gray-scale level distribution. Pictures were taken at time intervals of 5 or 10 seconds in order to allow computation of the displacement mappings during successive stages of loading. The camera Figure 1: Experimental setup for tensile loading and damage monitoring was used as an optical extensometer to monitor the macroscopic strain in the direction of loading. The material presents some ductility before the peak of loading [START_REF] Rouchier | Influence of diffuse damage on the water vapour permeability of fibre-reinforced mortar[END_REF], followed by a strain-softening behaviour during which crack propagation is slowed by the presence of fibres. This behaviour facilitated the process of progressive damage monitoring by acoustic and optical measurements.

In addition to the optical monitoring apparatus, four Micro-80 acoustic sensors were placed around the monitored area of the specimen, continuously recording the AE activity during damage and fracture propagation. The sensors form a 80 × 70 mm (height×width) area and are connected via pre-amplifiers to a MISTRAS data acquisition system. They have a diameter of 8 mm, and are characterised by the position of their center. The settings of the AE recording setup are detailed below.

The methodology has been applied on a series of samples, 6 of which showed suitable for data interpretation (i.e. stress concentration and fracture occured in the observation area). Results are presented in two parts: first (Sec. 3), the ability of digital image correlation for the observation of fracture patterns is assessed. Strain mappings are calculated by the DIC algorithm and a procedure is explained for observing the evolution of crack size distributions during tensile tests. In the second part (Sec. 4), these optical measurements serve as a basis for the interpretation of AE recordings. Measurements performed on all samples were used to establish the ability of AE to quantify, locate and identifty damage and fractures.

Digital image correlation

Optical techniques such as DIC are non-destructive and therefore do not disturb eventual further testing on the samples. DIC provides a full map of the deformations at the surface of a specimen and allows following the fracture development without restriction of number or size of the cracks. It also presents the advantage of being easily implemented into most experimental setups, requiring no strict operating conditions or time consuming preparation, nor does it require gauges in contact with the specimen which might interfere with the experiment. The technique enables full field measurements of the local displacements of a sample's surface. It has been used for the estimation of stress intensity factors near crack tips [START_REF] Réthoré | Estimation of mixed-mode stress intensity factors using digital image correlation and an interaction integral[END_REF][START_REF] Roux | Stress intensity factor measurements from digital image correlation: post-processing and integrated approaches[END_REF] or the identification of elastic properties [START_REF] Hild | Digital image correlation: from displacement measurement to identification of elastic properties -a review[END_REF] or damage laws [START_REF] Leplay | Damage law identification of a quasi brittle ceramic from a bending test using Digital Image Correlation[END_REF], among other uses. The technique has been proven suitable for the observation of local displacements of brittle building materials such as concrete [START_REF] Choi | Measurement of deformations on concrete subjected to compression using image correlation[END_REF][START_REF] Corr | Digital image correlation analysis of interfacial debonding properties and fracture behavior in concrete[END_REF][START_REF] Rouchier | Characterization of fracture patterns and hygric properties for moisture flow modelling in cracked concrete[END_REF]. The principle of DIC is briefly summed up below. For a more complete view of its theory and applications, one can refer to a recent book on the subject [START_REF] Sutton | Image correlation for shape, motion and deformation measurements: basic concepts, theory and applications[END_REF].

The principle of digital image correlation is the conservation of the optical flow between two pictures of a specimen, taken at different stages of its deformation [START_REF] Sutton | Determination of displacements using an improved digital correlation method[END_REF]: it lies on the assumption that two consecutive images contain the same total amount of each gray-scale level, and are only distinguished by their spatial distributions.

The planar displacement u (x) at each pixel of coordinates x of a surface is defined as:

g (x + u) = f (x) ( 1
)
where f is the distribution of gray-scale level, or the texture, of a reference image, typically taken at the start of the mechanical loading, and g is that of a deformed image. Eq. 1 means that the gray-scale level of a pixel of coordinates x on the initial image f is equal to that of a pixel of coordinates x + u on the final image g , i.e. that this pixel has been displaced by a vector u. The exact displacement field u(x) can generally not be explicitely calculated without additional assumptions of regularity. In the prospects of the numerical resolution of Eq. 1, the functional φ, operating on displacement fields, is defined:

φ (v) = g (v + x) -f (x) 2 dx ( 2
)
where v is an approximation of the solution u, and is constructed as a linear combination of functions v i :

v (x) = N i (x) v i ( 3
)
where N i are chosen basis functions (typically bilinear functions of x). The target is to determine the best possible approximation by minimizing the value of φ on all elements of the grid. Assuming a certain smoothness of the investigated displacement field, g is replaced by its first-order Taylor expansion in Eq. 2, resulting in:

∇g (v + x) ⊗ ∇g (v + x) : N j ⊗ N i dx v i = f (x) -g (x) ∇g (x) N i (x) dx (4)
Since f , g and N i (x) are known, Eq. 4 is a linear system of unknowns v i , from which the approximate displacement field v can be obtained. Understandably, the choice of the basis functions N i (x) is important for a good approximation of the displacement field, and several options have been presented in the literature for different types of mechanical tests. In the present case, the computation of the displacements is achieved with the Icasoft software 1 , which uses bilinear finite element shape functions. The displacement field is analysed by dividing the image into grid elements, each of which is assigned two displacement components in the decomposition of Eq. 3. The result of Eq. 4 is a rough estimate of the real solution, since it only computes an integer number of pixels for the displacement fields. Subpixel displacements can be determined by interpolating the displacement fields between elements of the grid with bilinear or spline functions [START_REF] Hild | Digital image correlation: from displacement measurement to identification of elastic properties -a review[END_REF]. The result of the algorithm is an approximation of the displacement field u of each point of the surface. While gradients of u define the strain fields, a local displacement jump indicates a discontinuity that appeared after the reference image was taken: in quasi-brittle materials, it translates as a crack of measurable aperture.

Acoustic emission

Principle

In a material under loading, elastic waves are emitted as a consequence of microstructural dislocations or crack propagation suddenly releasing stored energy. An AE hit occurs as a signal captured by a sensor exceeds a pre-set threshold value of amplitude. The result of the measurement procedure is a set of temporally separated AE hits being captured during the recording process. In the present study, the interpretation of these measurements is threefold: we investigated how AE signals can be used for damage quantification, localisation and identification.

Quantitative damage estimations consist in measuring the mechanical degradation state of a material sample on the basis of the number of recorded AE signals, or on their generation rate. Ohtsu and Watanabe [START_REF] Ohtsu | Quantitative damage estimation of concrete by acoustic emission[END_REF] performed such a diagnostic procedure on concrete samples and established a relationship between the AE emission rate and damage evolution. Similar analyses were later carried out by Suzuki and Ohtsu [START_REF] Suzuki | Quantitative damage evaluation of structural concrete by a compression test based on AE rate process analysis[END_REF], among others.

It is also possible to locate the source of a signal by placing several sensors in contact of a specimen. When the propagation speed of AE waves in the given material is known, the different arrival times of a same hit at each sensor enables the localisation. The source can be located on a 1D axis with two sensors, on a plane with three and in 3D with four. Examples of 2D or 3D crack and damage localisation include [START_REF] Granger | Experimental characterization of the selfhealing of cracks in an ultra high performance cementitious material: Mechanical tests and acoustic emission analysis[END_REF][START_REF] Ohno | Crack classification in concrete based on acoustic emission[END_REF] and generally employ more sensors than strictly necessary, for accuracy purposes. In the following, the term of AE event denotes a signal received by all sensors placed on the sample.

Settings

The propagation speed of acoustic waves in each material sample was measured prior to the tests with the lead breaking procedure: a repeatable AE wave is generated on the specimen surface by breaking pencil leads and measuring the difference in arrival times between sensors. This method also allows measuring the attenuation of the material, i.e. the amplitude decrease of a wave per unit length, and setting the timing parameters of the acquisition apparatus (peak definition time, hit definition time and hit lockout time), which are set for distinguishing signals from an uninterrupted AE stream. The average measured values of the propagation speed and attenuation coefficient are shown on Tab. 1, along with the settings of the AE recording system. During mechanical tests, four AE sensors were placed on the specimen as shown on Fig. 1, forming a 80 × 70 mm area. The acquisition system records the waveform of all AE hits received by all sensors. When a signal is captured by three or more sensors within a small enough time frame, a localisation algorithm is run as to calculate the 2-dimensional coordinates of its source. In case of a signal captured by four sensors, the calculation is repeated for all combinations of three, and the final result is the averaged value of all combinations. The following usual assumptions are made for simplifications purposes: the specimen thickness is neglected, AE propagation speed is uniform and constant (not influenced by damage). Because of these assumptions, and of the heterogeneity of the material, uncertainties are expected in the results of the localisation process.

Accuracy assessment of the 2D localisation

An accuracy test of the signal localisation algorithm was performed prior to the mechanical loading tests. First, the isotropy of the material in regards to the propagation speed was confirmed. Then, sensors were placed on a specimen surface and pencil leads were broken on a set of test points. Five lead breaks were performed on each of the five test points. Pencil breaks have a high energy release and are easily distinguishable from background noise: weaker events were filtered out, and stronger ones were localised. The results are shown on Fig. 2.

The accuracy of the localisation algorithm is estimated by comparing the exact origins of the signals (the test points) with the computed ones: the overall precision of the results is satisfactory but irregular. Acoustic waves generated in the center of the monitored area are the most accurately located. Inversely, signals originating from the sides of the monitored area are found with less precision. The material is indeed heterogeneous and the propagation speed may exhibit local variations due to variable pore and grain sizes [START_REF] Rouchier | Influence of diffuse damage on the water vapour permeability of fibre-reinforced mortar[END_REF]. The accuracy of the algorithm is however considered sufficient for the purpose of the study.

Imaging results

All material samples were loaded as described above and pictures of their surfaces were taken at time intervals of 5 or 10 seconds during loading. For purposes of clarity, the procedure for extracting the crack size distributions from the optical measurements is illustrated with one of the samples (later labeled as sample 1). The results are then displayed for all samples on Fig. 6.

The correlation algorithm works by applying Eq. 4 to the reference image f (taken at the beginning of the loading process) and the deformed image g (taken during loading), in order to compute the displacement field over a grid on the specimen surface. Fig. 3 shows the image of the sample surface from the CCD camera shortly after crack initiation, before its input into the correlation algorithm. The calculation area of the DIC algorithm is delimited by the notches and shown by the red dotted line.

The influence of the grid size on the accuracy of the subpixel interpolation algorithm has been studied by many authors (see for instance [START_REF] Besnard | Finite-Element" displacement fields analysis from digital images: application to Portevin-Le Châtelier bands[END_REF][START_REF] Hild | Digital image correlation: from displacement measurement to identification of elastic properties -a review[END_REF]). With the settings of the present work, i.e. a grid composed of 8×8 pixel elements, performance estimations show a possible accuracy of 10 -1 to 10 -2 pixel for the computation of the displacement fields: in the present case, the displacement uncertainty is under 1 µm. An example of strain mapping resulting from the correlation procedure is shown on Fig. 4, on which the gray scale levels indicate the local value of the longitudinal strain in the direction of loading.

This local strain value is the gradient of displacement between adjacent pixels. Since the material is quasibrittle, locally high values indicate the presence of cracks. These cracks originate from the tips of the notches (y = 0 mm, x = 0 or 70 mm) and spread on a relatively large area of over 40 mm height. The present example shows the state of the sample surface shortly after the initiation of a macroscopic fracture (see Fig. 4 On the basis of these full field strain mappings, a methodology is proposed and implemented to measure the aperture and total length of all cracks appearing on the sample surface: successive thresholds were applied on each strain mapping, as to only display cracks exceeding certain values of aperture. Image processing of the resulting profiles allowed measuring the total developed crack length in each width interval. An example of this procedure is shown on Fig. 5.

Fig. 5(a) and 5(b) show the strain mapping 4 once it has been thresholded as to only display discontinuities of over 2 µm and 5 µm, respectively. The appearing patterns were processed by topological skeletonisation in order to reveal cracks as tortuous linear paths: the skeleton of a shape is a thin version that is equidistant to its boundaries, and preserves its main geometrical properties, such as its length, connectivity and direction. The ImageJ software was used for this process. As an example, its applicaton to the distributions showed on Fig. 5(a) and 5(b) are respectively on Fig. 5(c) and 5(d). The resulting paths are measurable, and the substraction of the total visible crack length of Fig. 5(c) by that of Fig. 5(d) gives the amount of cracks of which aperture w is in this interval. The procedure was performed on each successive strain mapping of each specimen, for a set of threshold values allowing separating microcracks from larger fractures: 3, 5, 10, and 50 µm. The results are shown on Fig. 6.

These results are displayed as total visible crack length in each aperture interval, per square meter of specimen surface, as a function of the progression of the mechanical loading process. This progression is quantified by / peak , where peak indicates the value of peak strain. The observation was limited to crack apertures exceeding 3 µm, as smaller strain values may be difficult to distinguish from the background noise of the mappings. Two observations can be made on the basis of these profiles:

• Smaller crack ranges always appear first during loading. The development of successive ranges of aperture follows the same order, from smaller to larger cracks. While this observation was to be expected, the clear visualisation of this succession indicates the validity of the methodology for quantifying cracks by ranges of size.

• In most samples, the smaller ranges of fissures reach a maximum around / peak = 3 to 4, before decreasing. This phenomenon is caused by the presence of a macroscopic fracture, that initiates around = peak . The opening of this fracture causes neighboring cracks to close, decreasing the total length of the smaller scales.

In addition to these results, a specific fracture area A frac was defined as such: where the subscript i ranges over the number of aperture subsystems measured by the image processing procedure. w i is the mean crack width of the corresponding subsystem and l i its specific length, i.e. the ordinate of Fig. 6. The specific fracture area is therefore a summation of the visible lengths of all intervals, weighted by their respective openings. For the purpose of this calculation, the amount of aperture subsystems is greater than that displayed on Fig. 6: ranges were defined between values of 2, 3, 5, 10, 50 and 100 µm. A frac is non-dimensional and represents the ratio of visible sample surface occupied by cracks. The purpose of defining this quantity is the comparison of DIC and AE results, as explained below.

A frac (t ) = i w i l i (t ) (5)
These measurements show that digital image correlation allows measuring all geometrical characteristics of cracks that may influence fluid flow in a construction material. With a relatively simple experimental setup, and the appropriate set of tools for image processing, the progressive opening and closing of discontinuities can be quantified without restriction of number or density. Although it is limited to 2-dimensional observations in the present case, the technique is therefore suitable to serve as a basis for the understanding of acoustic signals recordings. The following section shows how DIC can be used for the calibration of AE measurements, so that this technique can be autonomously used for damage monitoring of building components.

Interpretation of acoustic emission 4.1 Damage quantification

First, an attempt of global damage quantification was made on the basis of the total AE activity. This activity is represented by the number of localised AE events, i.e. signals received by three or more sensors. Noise effects are therefore expected to be reduced, as low-energy, non-localisable AE hits are not accounted for. Fig. 7 depicts the cumulative number of AE events and the loading profile of each sample.

Acoustic signals start emitting before the peak of loading at a slow rate. The recorded acoustic energy in this interval is low, and few signals can be captured as the material's acoustic attenuation is relatively high (approximately 0.25 dB.mm -1 ). These signals are emitted as a consequence of the previously observed microcracking. The profiles of cumulative AE events then increase significantly after peak strain: on all tests, high AE rates can be seen to occur simultaneously with sudden stress drops caused by crack propagation.

As a consequence of this observation, an attempt was made to correlate the total amount of visible fractures, previously observed by optical measurements, with the cumulated AE activity at each time of loading. For all available data points, the value of A frac was compared with the cumulated AE activity at the corresponding time of the test. Fig. 8 shows this comparison over all strain mappings captured during loading of all samples, representing a total of 83 data points.

It has already been found [START_REF] Ohtsu | Quantitative damage estimation of concrete by acoustic emission[END_REF][START_REF] Suzuki | Quantitative damage evaluation of structural concrete by a compression test based on AE rate process analysis[END_REF] that the AE rate can give a quantitative indication on the damage level of construction materials. This is confirmed in the present case: the profile of fracture specific surface A frac versus cumulated AE events N can be roughly approximated by a power law (see Fig. 8), resulting in a correlation coefficient of R 2 = 0.9236 between the results of both techniques (DIC and AE). Such an agreement is quite satisfactory, considering that the two techniques rely on very different physical principles. A connection was established between the amount of cracks appearing on the surface of the specimen and the rate at which elastic waves are released in the medium under tension. This observation confirms that AE can be used for damage quantification of building components with fair accuracy, once it has been calibrated by a method such as digital image correlation.

Damage localisation

The previous section addressed the matter of global damage quantification, and showed a connection between the cumulated number of AE events and the total opening of fractures. The resulting empiric evolution law, however, does not separate or identify cracks of different sizes, nor does it allow their localisation. The present section addresses the latter question by applying the localisation procedure, of which accuracy was previously assessed. In addition to the set of variables describing its waveform, each AE signal is assigned one temporal and two spatial coordinates. All localised sources were then superimposed with the strain mappings computed by the DIC algorithm. Fig. 9 shows the example of five successive stages of a sample, indicated on its loading profile 9(a). The first image (Fig. 9(b)) depicts the sample surface at the stress peak, at the time of initiation of a macroscopic crack. The red dots depict all AE sources that have been located from the beginning of the test. As can be seen on Fig. 7, the cumulative number of AE events is still low at this point. A moderate concentration of signals originates from a relatively large area around the fracture process zone, and occurs mostly ahead of the crack tip. After this point of the loading process, a higher emission rate is recorded during macroscopic crack expansion. Fig. 9(c) to 9(f) show the state of the sample surface during its propagation. This propagation generates significantly higher concentrations of AE signals, that follow the crack trajectory.

The spatial correlation of both techniques is quite satisfying : most AE sources were located in areas of high strain, and their progress through the width of the sample is coordinated with the optically recorded crack propagation. The localisation procedure is however not flawless, as showed in the previous paragraph. This can explain the imperfect match between spots of high AE concentration and crack patterns. An other possibility is the fact that DIC only captures discontinuities that appear on the specimen surface, while AE sources originate from the entire volume. This matter is however minimised by the small thickness of the sample.

These measurements were carried out on all tested samples, and led to similar results. An important outcome of this procedure is the observation that AE signals seem to occur in two phases during the entire damage and fracture process. First, a moderate number of spatially scattered waves can be recorded as the material undergoes microcracking, distributed over a relatively large area. Then, during the propagation of a macroscopic crack, AE signals are captured at a much higher rate. Although the signal concentration is higher, their spatial distribution is still scattered, possibly because of the local variation of the AE wave propagation speed, which were not accounted for. Distributed signals continue to be emitted ahead of the crack tip. It must be recalled that most crack patterns appearing on the optical measurements have an aperture in the order of a few micrometers: AE monitoring allows the observation of crack far below the visible range.

Fracture identification

Signal localisation has added geometrical informations to the quantitative estimation of damage. In addition to informing on the global degradation state of the material, it has been shown that the AE technique may indicate where damage occurs, and give an estimate of the trajectory of the main cracks. However, it does not provide a complete understanding of damage mechanisms so far. The separation between microcracks and larger fractures has been graphically showed and translates into different concentrations of signals, but these observations do not allow their systematic separation.

AE amplitude is associated with the magnitude of the fracture, since the emitted energy depends on the crack opening displacement. Microcracks that develop before stress concentration and material failure are therefore expected to generate AE waves of smaller amplitude than larger fractures. This assumption can be made to study the development of the fracture process of concrete and rock-like materials.

In the field of seismology, the b-value serves as a predictive tool for material failure and is defined as the ratio of AE events of small to large amplitude:

log 10 N = a -b A (6)
where N is the number of signals, among a number of recently detected AE waves, exceeding a given amplitude A. At any time during the development of damage, the parameters a and b are defined by linear approximation of the current N versus A profile. High b-values therefore indicate a prevalence of microcracks, while lower values indicate their coalescence into macrocracks. An improved value of this indicator, or I b-value, was proposed by [START_REF] Shiotani | Evaluation of progressive failure using AE sources and improved B-value on slope model tests[END_REF] and applied by [START_REF] Colombo | Assessing damage of reinforced concrete beam using "b-value" analysis of acoustic emission signals[END_REF][START_REF] Aggelis | Acoustic emission characterization of the fracture process in fibre reinforced concrete[END_REF], among others, to study the fracture behaviour of concrete:

I b = log 10 N µ -α 1 σ -log 10 N µ + α 2 σ (α 1 + α 2 ) σ (7)
where µ and σ are the averaged value and standard deviation of the AE amplitude among the group of recent hits, α 1 and α 2 are filtering parameters (both were set to 0.5 in the present study). At any given time, N µα 1 σ indicates the number of recent events, of which amplitude exceeds µα 1 σ (similarly, N µ + α 2 σ indicates the number of recent events of which amplitude exceeds µ + α 2 σ). The definition of the I b-value is similar to that of the b-value, with an additional filtering of non-representative signals of very high or low amplitude that might disturb the results. At all times of a test, the I b-value is calculated from the amplitude distribution of the 200 latest consecutive signals. In order not to filter out waves of smaller amplitude, the procedure was applied on the basis of all recorded AE hits rather than of the localised events. Fig. 10 shows the I b profiles superimposed with the loading curves of the fibre reinforced mortar samples.

As already mentioned, and seen by other authors [START_REF] Shiotani | Evaluation of progressive failure using AE sources and improved B-value on slope model tests[END_REF][START_REF] Shiotani | Detection and evaluation of AE waves due to rock deformation[END_REF], the evolution I b is strongly related to the type of fracture. A common observation [START_REF] Colombo | Assessing damage of reinforced concrete beam using "b-value" analysis of acoustic emission signals[END_REF][START_REF] Aggelis | Acoustic emission characterization of the fracture process in fibre reinforced concrete[END_REF] is that it starts at higher values as the first signs of damage are in the form of diffuse microcracking, generating mostly AE waves of low amplitude. Shortly before the initiation of a macroscopic fracture, these cracks aggregate: the energy release increases, decreasing the slope of the amplitude distribution. This results in a sudden drop of the I b-value shortly before the maximum stress, that acts as a warning for the specimen failure. Such a drop can be clearly seen on most samples of which the I b profiles are shown on Fig. 10. In the case of the present material, a decrease of the value under 0.03 to 0.04 is a fair indicator for crack initiation. This trend is however not always clear: indeed, the material has a high acoustic attenuation and the number of low-energy signals, detected before macro-fracturing, may be low.

As can be seen on Fig. 9, the time of drop of the I b indicator corresponds to the time at which AE sources start concentrating in a narrow band close to the point of macro-crack initiation. This observation indicates that, in addition to a quantitative estimation of damage, AE allows the separation of fissures according to their size, as well as the time and position of failure.

Conclusion

In the purpose of extending the use of non destructive damage observation to building materials, the present study consisted in the simultaneous use of digital image correlation and acoustic emission for the monitoring of progressive damage development in a formulation of fibre reinforced mortar under tensile loading. The aim was to assess the ability of AE to provide quantitative measurements of the crack networks, and to calibrate it with the support of an optical technique. In the course of the presented methodology, the following results were obtained:

• Digital image correlation, associated with an automated image processing procedure, allowed a precise twodimensional mapping of damage patterns, from micro-cracks to macroscopic fractures.

• The cumulative number of captured AE signals is a fair quantitative indicator of the total developed crack area.

• Before propagation of a macroscopic fracture, a moderate number of AE signals are emitted in a large area ahead of the crack tip. High concentrations of AE signals are then emitted along the crack trajectory, in fair accordance with damage mappings performed by digital image correlation.

• The time and position of initiation of a macro-crack can be anticipated: analysing the amplitude distribution of received signals allows visualising the transition between a regime of micro-cracks and the apparition of a larger fracture emitting waves of higher amplitude.

The methodology can however be improved: a better localisation can be reached by accounting for the variations of the AE propagation speed in the damaged material. This is partially possible with preliminary measurements performed in a diffusely damaged medium, but may be more challenging after fracture initiation and a highly nonuniform distribution of cracks. Furthermore, a more thorough interpretation of the AE waveforms may provide a better interpretation of the signals and a better understanding of the degradation mechanisms of the material.

Figure 2 :Figure 3 :

 23 Figure 2: Accuracy testing of the localisation procedure

Figure 4 :

 4 Figure 4: Example of a longitudinal strain mapping

Figure 5 :Figure 6 :

 56 Figure 5: Image processing procedure for the measurement of crack size distributions: thresholds of 2 µm (left) and 5 µm (right)

Figure 7 :

 7 Figure 7: Stress-strain profiles and cumulative AE signal distributions

Figure 8 :

 8 Figure 8: Fracture specific surface versus number of recorded acoustic events, and power law approximation

5 Figure 9 :

 59 Figure 9: AE sources localisation (dots) and strain mapping (black and white patterns)

Table 1 :

 1 Settings of the AE acquisition system

	Preamplifier gain	40 dB
	Threshold of detection 32 dB
	Peak definition time	50 µs
	Hit definition time	100 µs
	Hit lockout time	1000 µs
	Propagation speed	2313440 mm.s -1
	Attenuation	0.25 dB.mm -1

Icasoft Digital Image Correlation Software -http://icasoft.insa-lyon.fr/

Acknowledgements

The results presented in this paper have been obtained during the Renovbat project for the study of flow in fractured building components. Funding is provided by the Energy research cluster of the Rhône-Alpes region, which the authors would like to thank for their financial support through the project.