In-situ emissivity measurement of construction materials

Abstract

Infrared thermography is a non-destructive diagnostic technique used in the field of constructions as a method for reading of the existing heritage, and represents a model of knowledge of high reliability based on the measurement of physical parameters (1,2,3,4,5). Quantitative information can be achieved once the target emissivity e and the reflected temperature Tr are known. In this paper, a method to evaluate in-situ the emissivity of building materials is proposed. The measurement procedure is performed in two different steps: firstly, the emissivity values of reference labels are assessed in the laboratory by a dynamic test. Subsequently, the measurement of emissivity is done in situ, under controlled boundary conditions, using the reference labels to characterize the various building materials.The experimental results, in addition to providing information about the tested construction materials, confirm the need for a simple a reliable method to measure emissivity in-situ. The present research refers to three technical elements: the structure of vertical elevation, which is part of the Technology Unit Class Structure, the plaster which is part of Technology Unit Class External Partition , and the vertical window frames which is part of Technology Unit Class Vertical Closures. These three technical elements, have the functions of bearing wall and finishing and protection of the building and they are made with building materials exposed to the problems of degradation and alteration related to durability and life cycle of the technical element. The instrumental measuring is a control strategy of the obsolescence conditions of buildings and so an innovative process control of the technical elements is structured, through a process of data acquisition and organization of information for the reconstruction of a knowledge unified framework of the built environment. In this scenario, the diagnostic concept of the performance and diseases of building systems extends to the general evaluation of all the physical performance of the built environment, in order to bring out aspects based on more objective evidence and clarity. With that comes the need to use the diagnostic tools to detect the physical properties of the materials to have a comprehensive framework; the establishment of such quantitative characteristics of the object requires the use of specific techniques to survey data in the infrared. The acquisition of experimental data is done with infrared thermography (6). The infrared technique, and in particular the IR thermography, has been applied for many years to evaluate the performance of the building and has been used to identify defects in insulation, air leaks, heat loss through the windows, humidity (7). Other applications have been made to evaluate the physical parameters of building materials (8,9). The quantitative approach and the temperature measurement with the IR thermography require the knowledge of two important parameters: emissivity of the surface and reflected temperature. While the reflected temperature may be approximately estimated using low emissive targets, emissivity values can be found in the literature for construction materials, or measured in the laboratory on samples, or estimated in-situ. (10,11) It is worth recalling that the emissivity depends strongly on the spectral response of the IR camera used for inspection, and therefore each reference material should be characterized for the specific IR sensor. In this work, two IR cameras with different spectral bands are used: FLIR SC6000 (InSb detector with a 3-5 µm spectral band) and FLIR SC640 (uncooled microbolometer detector with 7.5-13 µm spectral band). A method for wall emissivity measurement in-situ is proposed. The test is easy to be performed and it takes only a few minutes.