Optical reflectance apparatus for moisture content determination in porous media

Highlights

• Test of a new portable measurements device for laboratory and in situ moisture evaluation.

• Same sensitivity on different kind of materials.

• Limits of spectroscopic measurement on both softwood and hardwood raw surfaces.

• Calibration with recommended measurement method (gravimetry).

• Use of APD (avalanche photon diode) in very low spectroscopic signal.

Abstract

Monitoring surface moisture content (MC)is a required activity and a topic of current interest in conservation studies. The present work examines the possibility of using the spectral reflectance factor at 970 nm (R_λ970) for the noninvasive quantitative determination of surface MC in building materials. Indeed, the quantification of surface liquid water by optical measurements is poorly exploited on porous media despite it is well documented in many remote sensing technologies such as in agriculture, soil science and some industrial application. The measurements apparatus is tested in a laboratory experiment on different specimens of building materials. The custom-made detector is a Si-core Avalanche Photo Diode (APD) operating in Geiger mode which satisfy the requirements of non-invasiveness and ease of use with the additional opportunity for developing economic, automated and reliable solutions. The results obtained for six representative cultural heritage building materials clearly show the dependence between R_λ970 and surface SMC: R_λ970 increases as surface MC decrease following the drying process of an unsaturated porous matrix i.e. evaporative flux supported by capillary pumping. This work reports the possibility to use the reflectance value at a specific wavelength for an alternative and non-invasive measurements of MC.

Introduction

The deterioration and degradation of materials surface in historic construction is often linked to the presence of water [1]. Indeed, many forms of decay are related to the content of water inside a material, e.g. freeze-thaw cycles and salts crystallization cycles. For this reason, the accurate assessment of moisture content (MC) of porous materials is needed. Most of the deterioration processes involve the transport of gases, liquids and ions and the moisture level plays a crucial role in all these processes and their comprehension is of the utmost importance in any methodology of moisture content measurement [2]. This study examines the possibility of using a characteristic absorption band of liquid water in the near infrared at 970 nm (R_λ970) for a quantitative determination of the surface moisture content (MC) in porous media. The definition of a standard procedure for the determination of surface MC in historical materials is currently an open challenge in conservation studies [3]. Non-invasive and non-destructive measurements are preferable in cultural heritage applications due to the high relevance of the objects. To extend the measurements on the largest number of objects, the methodology should also ideally be cheap, easy to perform, reproducible and non-invasive. In addition, it would be desirable that the measure methodology is able to quantitatively and continuously measure MC. Indeed, the establishment of thermohygrometric cycles or abrupt changes in MC must be monitored, since they are extremely harmful for the materials conservation. It is also necessary to understand the advantages and disadvantages of this specific methodology since it provides a superficial information i.e. surface moisture content. Actually, the moisture distribution along the thickness of a wall is spatially heterogeneous [4]. The study of the external layers of the material is however crucial in evaluating the decay processes due to water transport phenomena in between material microstructure and environment [5]. If the increase of moisture is detected before the visible effects occur, then early conservation procedures can be started. It must be remarked that the quantitative determination of MC in common porous geomaterials is strictly connected with the determination of the material's porosity and water pathways [6]. Indeed, the study of porosity, and hygric properties of building materials is a fundamental step for evaluating their state of conservation and planning correct restoration projects [7].