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Study of the mechanical properties of mortars based on organic binders used in the reproduction of outdoor artworks

Alba Cerezo, Xavier Mas-Barber et Stéphane Kröner

Résumés

Cette œuvre porte principalement sur la caractérisation de mortiers reliés en résine organique, utilisés lors de la reproduction de sculptures et ornements en pierre exposés à l'extérieur. Les échantillons ont été préparés avec deux agglomérants organiques souvent utilisés dans le cadre de la conservation et la préservation du patrimoine culturel, des agrégats naturels de roche calcaire Tosca Rocafort broyée, ainsi que de trois additifs différents. Tous les échantillons de mortier préparés ont été soumis à des tests de vieillissement accéléré effectués avec des techniques de radiation UV, puis au testeur de flexion 2kn avec Deben Microtest et SEM, afin d'évaluer quantitativement la résistance ainsi que la fissuration sous contrainte.

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Texte intégral

This research was funded through the Project HAR2011-29538 of the Spanish Ministry of Science and Innovation. We also acknowledge the assistance of the Laboratory of Analysis of Chemical/Stone Materials of the IRP (Instituto Universitario de Restauración del Patrimonio) of the UPV (Universitat Politècnica de València). authors wish to thank the group of researchers currently involved in the design and development of the experimental testing of materials: T. Mastroiacovo and G. Medda (Freelance, Italy), J. Orozco (Instituto Tecnología de los Materiales, UPV, Spain), M.Planes and J.L Moya López (Electron Microscopy Service at UPV), J. Grafiá (Instituto de Restauración del Patrimonio, UPV), L. Pérez (Physic, Universidad Complutense de Madrid) and V. Mira (Bateig Novelda Stone, Alicante).

Introduction

1In recent years there is an increasing trend based on maintaining and/or minimizing agents and mechanisms that spoil artworks exposed outdoors. In most cases, the environmental control becomes complex and practically unavoidable. In this sense, the replacement of the original copy is a preventive conservation measure that allows safeguarding the artwork without altering its relationship with its original environment. These are situations of irreparable losses in sculptures and decorative stone material, which sole premise is to act under criteria of sustainability, efficacy and affinity with the materials of the monument. The most widespread method is the reproduction of sculptural works by ornamental organic matrices, through the technique of molding/bilayer system, resulting imprints faithful to the original ones.
Currently, the organic matrix composite mortars offer the possibility of creating parts of artworks that will be resistant to the most common environmental deterioration agents, enabling an improvement in their mechanical and structural characteristics, with notable saving of time and economic resources, while complying with the criteria of reversibility, compatibility, discernibility and respect for the original. This practice is intended to allow placing originals in indoor spaces with controlled environmental parameters, while exposing reproductions outdoors. Therefore, this physical- mechanical characterization of mortars will allow professionals choosing the most suitable for each case, and determine the amount of material needed to create the replacement part. Thus, sculptural copies will be made being aware of the strength of the material chosen and of how sunlight affects its mechanical properties over time. Similarly, this trial will allow the professional deciding whether the additive to be added to the mixture will decrease or increase the strength of the mortar. On the other hand, the professional knowing the minimum amount of material that will be necessary to achieve the desired strength will result in better management of the resources.

Methodology

2Organic compounds mortars consist of a matrix of thermosetting resin that binds inorganic filler, being the aggregate of Tosca de Rocafort limestone the selected one for this case.

Organic matrix. Binders

3We studied two types of synthetic thermohardening resins to be used as binders for aggregates, specifically a two-component epoxy resin ALY554 ® with a TETA hardening agent, and an unsaturated polyester resin Sintolit orthophalic Tritone ® with a MEKP catalyst agent were used, both provided by the CTS Spain SA company.

Aggregates. Micronized stone

4Some mineral origin loads were chosen, as they were able to provide colour and texture according to the characteristics of the stone material under study. An inorganic aggregate –chemically inert-, was selected from the crushing of raw material Tosca de Rocafort (TR) limestone, specifically a micronized powdered calcite grain size 0,25-0,06mm.

Additives

5In this research three additives were studied, capable of improving performance against biological deterioration and enlightenment material. We used the Biotin® R biocide, from the CTS company, added in percentages of 3% and 5% as regards the binder phase of the mortar. The UV inhibitor Tinuvin® B75 from the Basf Company was added to the mortar binder phase in percentages of 0.1% and 0.3%, and the additive Hindered Amine Light Stabilizer (HALS) Tinuvin® 292, from the Kremer Company, added in percentages of 1% and 2% in the binder phase of the mortar.

Method of preparation of test specimens

6To prepare the mortar specimens, different phases were followed as described: 1) Preparation of aggregates in the type and dosage corresponding to the stone material studied; 2) Preparation of the binder phase required for the production of the specimens to be performed; 3) Gradual pouring of the aggregate in the matrix (TR epoxy organic mortar 2:1 and TR polyester organic mortar 4:1 –two or four parts of the arid and one of the binder), kneading continuously; 4) Addition of the catalyst agent; 5) Pouring of the mixture obtained in the mold and maintenance and the exhaust hood until sufficiently hardened and 6) Unmolding and acronym signing of each specimen for recognition and classification according to the test performed. The specimens were prepared using a RTV silicone mold size (55 ±2 x 10 ±2 x 4 ±0.2)mm, taken from the UNE-EN ISO 3673-2:2013 for the epoxy resin and UNE-EN ISO 3672-2:2002 for polyester resin. 70 specimens were prepared in all.

Instrumentation

Accelerated aging with UV radiation chamber

7Three specimens of each type were subjected to accelerated aging with ultraviolet irradiation. The ultraviolet irradiation test light is particularly important in the field of organic materials, because this class of polymers is disclosed to be particularly sensitive to photo-light deterioration. It aims to subject the materials studied to accelerated aging, according to the UNE-EN ISO 4892-3:2006, determining, approximately, the intensity of damage and alterations suffered by them under stress conditions. For this research we have used an OSRAM L36/37 fluorescent lamp that emits UV light with the power of 36 W, with a wavelength of 313nm, at a distance of 10 cm, for an exposure time of 1000 hours.

Flexion and traction microtest

8The aged specimens were then subjected to a flexural cracking test. The flexural test compares the tensile strength of the mortar, obtaining comparable data. These data were obtained as graphs of force (N) and elongation (mm). For this purpose, the TENSILE 2KN DEBEN tester Microtest (Gafan) with software for data DEBEN Microtest V5.3.53 was used.  

Scanning Electronic Microscope (SEM)

9SEM was used for morphological examinations of mortars allows observing imperceptible features with the binocular magnifier. In this study, a scanning electronic microscope Jeol JSM 6300 with an x-ray Link-Oxford-Isis microanalysis system was used, with analytical conditions of filament tension of 10-20kV, with the power of 2x10-9 A and a working distance of 15mm.

Results and discussion

Microtest

10The figure 1 show Force/strain curves of the mortar specimens before and after being subjected to the UV test. The mortars behave in different ways according to the type of resin employed, so that those made of polyester turn into more rigid and frail, while those of resin epoxy acquire ductility and absorb mechanical efforts in a better way. It is observed that the amount and size of the aggregates influence strength, as well as the fracture types (Figure 2. A-F). In all the cases, the stress of the materials is favored by the presence of microspores that are developed during the preparation of the specimens.

Fig. 1 Force (MPa)/Strain (%) curves of the mortar specimens

Fig. 1 Force (MPa)/Strain (%) curves of the mortar specimens

Force (MPa)/Strain (%) curves of the mortar specimens TRI-TR, TRI-TR-T292 1%, ALY-TR, ALY-TR-BIO R 5%, ALY-TR-T292 1% before and after being subjected to the UV test.

Credits missing

Fig. 2 Observation of the fracture

Fig. 2 Observation of the fracture

It is observe the fracture types during the influence of the tensile strength (A. TRI-TR, B. TRI-TR-T292 1%, C. TRI-TR-T292 1% (after uv), D. ALY-TR, E. ALY-TR-BIO R 5% and F. ALY-TR-BIO R 5% (after uv).

Credits missing

SEM

11The specimen TRI-TR-T292 1% (Figure 3. A, before aging and B, after aging) does not offer changes on texture after UV aging, however, the specimen ALY-TR-BIO R 5% (C, before aging and D, after aging) show the presence of microcrackings and fissures as a consequence of the degradation.

Fig. 3 Sight of photochemical degradation

Fig. 3 Sight of photochemical degradation

(a-d) Sight of photochemical degradation of the surface and cracking area of the resins and mortars specimens through SEM.

Credits missing

Conclusions

12All the resins and organic mortars specimens, subjected to accelerated aging tests through 1080h (45 cycles) of ultraviolet light irradiation, show significant differences on tensile test trials. The results of the specimens have been compared to natural stones specimens, showing that those of resin without additives or aggregates are the most resistant, followed by mortar specimens and, finally, by those of natural stone. Moreover, it has been demonstrated that flexion strength varies considerably depending on the type of additive used, showing those prepared with the inhibitor UV Tinuvin 292 on 1% the higher stability.

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Document annexe

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Table des illustrations

Titre Fig. 1 Force (MPa)/Strain (%) curves of the mortar specimens
Légende Force (MPa)/Strain (%) curves of the mortar specimens TRI-TR, TRI-TR-T292 1%, ALY-TR, ALY-TR-BIO R 5%, ALY-TR-T292 1% before and after being subjected to the UV test.
Crédits Credits missing
URL http://ceroart.revues.org/docannexe/image/4262/img-1.jpg
Fichier image/jpeg, 336k
Titre Fig. 2 Observation of the fracture
Légende It is observe the fracture types during the influence of the tensile strength (A. TRI-TR, B. TRI-TR-T292 1%, C. TRI-TR-T292 1% (after uv), D. ALY-TR, E. ALY-TR-BIO R 5% and F. ALY-TR-BIO R 5% (after uv).
Crédits Credits missing
URL http://ceroart.revues.org/docannexe/image/4262/img-2.jpg
Fichier image/jpeg, 644k
Titre Fig. 3 Sight of photochemical degradation
Légende (a-d) Sight of photochemical degradation of the surface and cracking area of the resins and mortars specimens through SEM.
Crédits Credits missing
URL http://ceroart.revues.org/docannexe/image/4262/img-3.jpg
Fichier image/jpeg, 1,1M
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Pour citer cet article

Référence électronique

Alba Cerezo, Xavier Mas-Barber et Stéphane Kröner, « Study of the mechanical properties of mortars based on organic binders used in the reproduction of outdoor artworks », CeROArt [En ligne],  | 2014, mis en ligne le 05 septembre 2014, consulté le 29 mars 2017. URL : http://ceroart.revues.org/4262

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Auteurs

Alba Cerezo

Alba Cerezo holds both a Bachelor's Degree in Fine Arts from the University of Murcia, Spain, and a Master's Degree in Conservation and Restoration of Cultural Property from the Universitat Politècnica de Valencia, where she carried out her research on flexural strength of organic matrix mortars for the project "Prevention in Sculpture and Ornaments in Cultural Heritage. Implementation of the bilayer system in the molding process and preparation of organic mortars in reproduction"(HAR2011-29538) granted by the Spanish Ministry of Science and Innovation, under the direction of Xavier Mas-Barbera.

Xavier Mas-Barber

Xavier Mas-Barberà is a PhD in Conservation and Restoration of Cultural Heritage (Universitat Politècnica de València, Valencia, Spain, 2006). He is a professor of Sculpture and Ornaments Conservation and Restoration at the Faculty of Fine Arts and his research interests are the study of alterations and interventions of stone materials at the Institute for Cultural Heritage Conservation (Universitat Politècnica de València). Since 2007, he is responsible for the stone material area with a focus on the development of new materials and methods (cleaning, desalination, consolidation, molding, copy and protection processes) for treatment of sculptures and ornaments carved on stone. Address: Instituto Universitario de Restauración del Patrimonio, Universitat Politècnica de València, Camino de vera s/n 46022 Valencia, Spain; e-mail: jamasbar@upvnet.upv.es

Stéphane Kröner

Stephan Kröner is a Ph.D. in Geology (Johannes Gutenberg University Mainz, 2005). He completed his undergraduate degree at the Karlsruhe Institute of Technology (Germany) in 1998 and moved to the University of Lausanne (Switzerland), where he obtained his degree in geology (2000). During his diploma and doctorate, he specialized in the field of structural geology, geochronology and isotope geology in order to understand mountain building processes in the Alps and the Damara Orogen (Namibia). In 2007, he entered the Institute for Cultural Heritage Conservation (IRP) (Universitat Politècnica de València Valencia, Spain) and is responsible for the characterization of inorganic materials (stones, ceramics, pigments, mortars, etc.).

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Droits d’auteur

Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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