Thermo-Mechanical Compatibility of Viscoelastic Mortars for Stone Repair

materials Article Thermo-Mechanical Compatibility of Viscoelastic Mortars for Stone Repair Thibault Demoulin 1 , George W. Scherer 2 , Fred Girardet 3 and Robert J. Flatt 1, * Received: 18 November 2015; Accepted: 5 January 2016; Published: 18 January 2016 Academic Editor: George Papanicolaou 1 2 3 * Physical Chemistry of Building Materials, Institute for Building Materials, HIF, ETH Zurich, Zurich 8093, Switzerland; Department of Civil and Environmental Engineering, Eng. Quad. E-319, Princeton University, Princeton, NJ 08544, USA; RINO Sarl, Blonay 1807, Switzerland; Correspondence: ; Tel.: +41-44-633-2890; Fax: +41-44-633-1087 Abstract: The magnitude of the thermal stresses that originate in an acrylic-based repair material used for the reprofiling of natural sandstone is analyzed. This kind of artificial stone was developed in the late 1970s for its peculiar property of reversibility in an organic solvent. However, it displays a high thermal expansion coefficient, which can be a matter of concern for the durability either of the repair or of the underlying original stone. To evaluate this risk we propose an analytical solution that considers the viscoelasticity of the repair layer. The temperature profile used in the numerical evaluation has been measured in a church where artificial stone has been used in a recent restoration campaign. The viscoelasticity of the artificial stone has been characterized by stress relaxation experiments. The numerical analysis shows that the relaxation time of the repair mortar, originating from a low Tg , allows relief of most of the thermal stresses. It explains the good durability of this particular repair material, as observed by the practitioners, and provides a solid scientific basis for considering that the problem of thermal expansion mismatch is not an issue for this type of stone under any possible conditions of natural exposure. Keywords: reprofiling; filling; patch; acrylic polymer; repair; artificial stone; mortar; thermal stress; viscoelasticity; on-site measurement 1. Introduction Many dimension stones used for the construction of historical buildings show, after a certain time, superficial degradations in the first centimeters that do however not affect the stone below this depth. That is the case, for instance, during the formation of scales in a sandstone during a spalling process. These alterations are formed parallel to the outer surface and are independant of the stone bedding orientation, suggesting that a combination of transport properties and environmental exposure causes the stress from a degradation process to reach critical levels only at a certain depth. An example of such alteration is presented in Figure 1a. When decisions are made to restore these stones, the question is how to remediate these alterations: should the degraded part of the stone be replaced by another stone, or by an appropriate mortar? Since the alteration is generally superficial, a reinstatement of natural stones would imply a removal of potentially sound original material to a depth of at least 10 cm to ensure a good placement [1], while the use of a plastic mortar that can be substituted for the lost parts would result in a minimal loss of historical material and, in addition, extend its lifetime. The latter practice is called “reprofiling” or “filling”, while the piece itself is called “plastic repair” [2], “piecing-in” [1], “fill” or “patch”. This strategy complies with modern building conservation principles that favour a Materials 2016, 9, 56; doi:10.3390/ma9010056 www.mdpi.com/journal/materials Materials 2016, 9, 56 2 of 26 minimal intervention and the retaining of as much historical material as possible [2]. It is one of the reasons that explains the increasing use of repair mortar in conservation, as pointed out by Torney in Scotland [2], and the increasing research undertaken, in both the philosophical side of the repair [3–5] and the material science side, with the development of compatibility criteria, upon which Isebaert wrote a recent review [6], as well as new mortar formulations [7] sometimes based on polymers [8]. Figure 1. (a) Common dimension of a flake in a historical building molasse sandstone; (b) Old acrylic-based mortar used in Lausanne, after more than 30 years; (c) Reversibility of the old acrylic-based mortar. The present work was initiated by the curiosity and questioning of stone carvers who used an acrylic-based mortar for the repair of calcareous sandstones in the late seventies in Switzerland. This mortar was developed at that time by Professor Furlan’s team in the Ecole Polytechnique Fédérale de Lausanne, and applied in the townhall of Lausanne, in locations protected from the direct sun but washed weekly by a high-pressure water hose to clean the remnants of the market. More than thirty years later, the mortar is in most of the locations still in place, and the repairs have been judged durable and successful by the stone carvers from the point of view of the integrity of the natural stone it aimed to repair, as shown in Figure 1b. The stone carvers thus proposed this mortar for the restoration of the Catholic Church of Notre-Dame de Vevey, Vevey, Switzerland, where it was used in 2011. However, owing to the very different natures of the original stone and the repair mortar, a more comprehensive understanding of the interactions between them would provide the foundation to decide in which situations this material could be beneficial or not. The development of an acrylic-based repair mortar by Professor Furlan’s team originated in the confluence of three factors: the particular mode of alteration of the local stone, the resurgence of interest towards repair mortars due to the spreading of the minimal-intervention approach, and the increasing use of polymers in conservation. Indeed, many of the Swiss historical buildings have been erected with the soft stone present in the Swiss Plateau, a sandstone called molasse, mainly composed of quartz and felspars cemented by calcite and clays [9]. This stone is sensitive to wetting and drying cycles that commonly lead to granular desintegration and spalling of flakes of 0.5 to 3 cm [10], but the stone is often in good conditions above this limit, as illustrated by Figure 1a. Eventually, the properties of reversibility, transparency and stability generally attributed to acrylic polymers [1,11] oriented the team towards devising a mortar with an acrylic binder. These properties contributed to the large use of acrylic polymers in the conservation of heritage materials. Since the early 1930’ s, where they were used as picture varnishes [12], they have been applied on glass pigments, paper, silver, iron, wood and stone [13]. In the particular field of stone conservation, they have mainly been used, alone or mixed with other polymers, as consolidants or protective agents [1,11]. The search for better and more stable polymers led to the development of many (...truncated)