Abstract
Infrared thermography is a growing technology in the engineering geological field both for the remote survey of rock masses and as a laboratory tool for the non-destructive characterization of intact rock. In this latter case, its utility can be found either from a qualitative point of view, highlighting thermal contrasts on the rock surface, or from a quantitative point of view, involving the study of the surface temperature variations. Since the surface temperature of an object is proportional to its emissivity, the knowledge of this last value is crucial for the correct calibration of the instrument and for the achievement of reliable thermal outcomes. Although rock emissivity can be measured according to specific procedures, there is not always the time or possibility to carry out such measurements. Therefore, referring to reliable literature values is useful. In this frame, this paper aims at providing reference emissivity values belonging to 15 rock types among sedimentary, igneous and metamorphic categories, which underwent laboratory emissivity estimation by employing a high-sensitivity thermal camera. The results show that rocks can be defined as “emitters”, with emissivity generally ranging from 0.89 to 0.99. Such variability arises from both their intrinsic properties, such as the presence of pores and the different thermal behavior of minerals, and the surface conditions, such as polishing treatments for ornamental stones. The resulting emissivity values are reported and commented on herein for each different studied lithology, thus providing not only a reference dataset for practical use, but also laying the foundation for further scientific studies, also aimed at widening the rock aspects to investigate through IRT.
Highlights
Infrared thermography (IRT) is a scientific application for the acquisition and analysis of thermal information, based on the detection of the thermal radiation emitted by an object
The achieved results show that tested rocks are characterized by high emissivity values, generally greater than 0.89, with the exception of polished rock granite surfaces, which show a slightly lower value due to the reflection effect brought by the polishing treatment (Figure 7)
Each rock type is not characterized by a univocal emissivity value, but rather by narrow ranges of variability arising from the heterogeneity of the rock surface
Summary
Infrared thermography (IRT) is a scientific application for the acquisition and analysis of thermal information, based on the detection of the thermal radiation emitted by an object. The feature controlling the relation between the emitted radiation and the surface temperature of the investigated material is known as emissivity (ε) [3] This surface property, characterizing all forms of matter capable of emitting thermal radiation, ranges between 0 (referred to as a perfect emitter—mirror) and 1 (referred to as a theoretical perfect absorber—blackbody). IRT has experienced a growing diffusion across a wide range of scientific fields; a literature review [4] and references therein highlighted that IRT is employed in medicine or biology, chemistry, engineering and material science, life and sport sciences, meteorology and agriculture, the most predominant fields involving IRT applications are applied science and engineering In the latter cases, the use of IRT is focused on building materials and structures characterization: Al-Hadhrami et al [5] studied the thermal behavior of concrete specimens aiming at assessing their density, while Huh et al [6] and Janku et al [7]
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