4.0 Introduction
Dazu Rock Carvings can be perceived as an example of the highest level of Chinese cave temple art from the 9th to 13th centuries. The United Nations Educational, Scientific and Cultural Organization (UNESCO) designed them as the world cultural heritage in December 1999. In particular, the Niche of Sakyamuni Entering Nirvana constructed of sandstone is the largest bust of Sakyamuni in the world, and also an important part of Dazu Rock Carvings (Tong, 2003). It has suffered from extensive surface weathering over the past 1000 years. The surface weathering diseases include efflorescence, granular disintegration, biological colonization, cracks, seepage, scaling, blistering and missing parts, and granular disintegration in total dominates that. Therefore, the formation and mechanism of granular disintegration is the research object of the studied statues.
Until lately, the study on granular disintegration adopts on-site investigation and description as the key study approach. Traditionally, the development of granular disintegration on rock surfaces can be attributed to physical weathering such as dry-wet cycle; freeze-thaw cycle and salt crystallization due to water absorption and temperature (Rodriguez-Navarro, Linares-Fernandeza, Doehneb, & Sebastian, 2002; Cardell et al., 2003; Oguchi & Yuasa, 2010; Labus & Bochen, 2012); and chemical action including dissolution, oxidation, reduction and hydrolysis (Prieto, Silva, Rivas, Wierzchos, & Ascaso, 1997; Pera, Arribas, Critelli, & Tortosa, 2001; Herrera, Arroyave, Guiamet, de Saravia, & Videla, 2004; Lee, Jo, & Kim, 2011). However, Ilieş, Irimuş, and Rus (2015) pointed out that over-reliance on field observations limited our understanding of the granular disintegration formation. Recognition of this limitation is implicit in many studies which sought to determine physical and mechanical parameters from the macro level, including, e.g., mechanical strength, porosity, ultrasonic velocity, mass and water absorption (Jeng, Lin, & Huang, 2000; Tan, Chen, Yang, & Cao, 2011; Özbek, 2014). Nonetheless, few micro level studies were conducted using indirect methods, e.g., XRD, SEM and chemical analyses. They can be usefully complemented by (1) providing direct evidence of cracks and pores in rocks and their constituent grains; and (2) analyzing mineral and chemical composition to evaluate their potential contributions towards rock deterioration (Clément & Kimpe, 1977; Pye & Cambridge, 1985; Sassoni & Franzoni, 2014).
Since the 1990s, some studies have been conducted to understand the weathering effects on Niche of Sakyamuni Entering Nirvana. These studies include preserving environmental and meteorological characteristics (Zhou & Deng, 2002; Chen, Jiang, & Xi, 2004); effect of the water seepage (Wang & Zhang, 1992; Zhang & Jiang, 2016); hydraulic parameters and seepage model of rock (Wang, Wang, & Zhang, 1999; Wang, Wen, Wang, & Zhang, 2001); prevention of water hazard and destruction mechanism of Na2SO4 crystals on sandstone by laboratory accelerating ageing experiments (Tan, 2013); surface weathering characteristics and degree of sandstone (Zhang et al., 2018). However, the relation between granular disintegration and sandstone weathering mechanism has not been revealed experimentally.
In this study, we conducted three weathering experiments to examine different influences on sandstone weathering of Niche of Sakyamuni Entering Nirvana. From these experiments, we analyze the changes in physical and mechanical properties of sandstone and the sandstone weathering mechanism. The research outcomes could provide some physical and mechanical parameters and basis for the protection of the studied statues.