Determination of anisotropy to enhance the durability of natural stone

Abstract

Anisotropy is a petrophysical property of natural stone and other construction materials that determines their quality and resistance to decay caused by a variety of agents, such as water. A study was conducted on nine types of stone widely used in Spain's built heritage, using six previously defined anisotropy indices. This paper proposes a new index (dMm) related to the two indices, dM and dm, that best reflect anisotropy in materials. These indices can be used to determine the degree of anisotropy, which helps explain the differential decay observed in stone materials quarried in the very same bed and used to build the same structure. The conclusion reached is that anisotropy should be determined in the natural stone used both to restore the architectural heritage and in new construction, for the appropriate choice of material quality ensures greater resistance to decay and, therefore, increased durability. Materials with the lowest possible anisotropy should be selected, as this property governs their hydraulic behaviour: the lower the anisotropy in a material, the better its behaviour in relation to water and the longer its durability. 1-Introduction All kinds of natural stone used in construction have certain properties that vary according to the direction in which they are measured: in other words, they are anisotropic. A variety of causes of anisotropy in rock can be identified, including crystal and grain orientation, the accumulation of minerals, stratification and lamination, stress orientation and the number and orientation of pores and cracks. Anisotropy, then, which depends on composition, texture and structure (Takemura et al 2003, Guéguen and Schubnel 2003), is directly related to the origin of rocks and any subsequent alteration. Every type of rock (granite, limestone, slate, marble) has its own level of anisotropy. Ascertaining anisotropy in natural stone and how it develops is of utmost importance, for this information is essential to determining: a) how it should be quarried and most efficiently processed; b) the quality of the stone products and their suitability for use as ashlars or ornamental elements; and c) its placement on buildings. This is why the use of natural stone in construction is influenced by its anisotropy index. Likewise, the directions of anisotropy have a bearing on stone material decay and consequently on its quality and durability (Fort et al 2008, Fernandez-Revuelta et al 2008, Sheremeti-Kabasti and Snethlage 2000). These issues must be considered when planning restoration work on the architectural heritage, or when a certain type of natural stone is to be used for structural, aesthetic or insulation purposes in new construction, whether of buildings or civil works.

An urban geomonumental route focusing on the petrological and decay features of traditional building stones used in Madrid, Spain

“…In this route the palace is viewed, on the one hand, as an outdoor geological museum in terms of both formation and petrological properties of the traditional stone varieties used to build it (Fort et al 1996(Fort et al , 2011Fort and Gomez-Heras 2007). On the other, it is seen as an on-site urban laboratory that allow to understand the interaction between building stones, decay forms and causes of alteration (Fort et al 2000(Fort et al , 2003Varas et al 2007).…”

“…The brown stains, located mostly on the limestone below cornices and imposts, can be attributed to the filtration of rainwater leaking out of downpipes onto the joints (Figure 2d), which clay fill showed significant loss of cohesion (Varas et al 2007), and favored by the rusting of some of the metal clamps as well (Fort et al 2004b The berroqueña stone covers monzogranites, granodiorites and leucogranites with varying petrographic and petrophysical characteristics (Fort et al 2011) and comprise several plutons (Villaseca et al 1998(Villaseca et al , 2000. Their outcrops cross the region of Madrid from north to northeast along the Guadarrama Mountain Range (Central System, Variscan Orogeny).…”

“…Alpedrete monzogranite is equigranular, fine-to medium-grained, with scattered cordierite and a 10% biotite content, while the Zarzalejo monzogranite is somewhat less equigranular, with a medium to coarse grain and a higher biotite content (Fort et al 2011). Porphyritic varieties may also occur locally in the Alpedrete pluton (ITGE 1990).…”

“…Plagioclase appears as idiomorphic and subidiomorphic, zonated and macled crystals (Fort et al 1996(Fort et al , 2011). …”

“…The table in Figure 3 gives the values of some of the petrophysical properties of the biotitic monzogranite seen from this stop, both taken from the quarry (Fort et al 2011) and the façades´s palace (Fort et al 1996). The high ultrasonic velocity and low percentages of porosity accessible to water and water absorption indicate that it is a building material highly suitable for dimensioning very strong and water-resistant structures.…”