Non-destructive methodologies for the evaluation of moisture content in sawn timber structures: ground-penetrating radar and ultrasound techniques

Abstract

The moisture content (MC) is a critical parameter for most physical and chemical pathologies of timber and, in the case of structural wood, a MC > 20% can be dangerous for any load-bearing construction. The complexity of evaluating MC while timber is in use by means of the current methods (oven-drying and resistance wood meter) led us to test non-destructive techniques to evaluate this parameter on site. With this in mind, measurements with two non-destructive techniques, ground-penetrating radar (GPR) and ultrasound, were carried out on joists of Pinus pinaster Ait. from their initial green state until the point of hygroscopic equilibrium moisture content. In particular, the analysis presented in this paper focuses on the capacity of each technique to register the velocity variations of their waves during the timber drying process. Prior to the GPR analysis, it was necessary to distinguish between differences in the propagation velocity of electromagnetic waves (vEM) attributable to the wood anisotropy and those due to variations in MC. The propagation velocity of the electromagnetic waves was always found to be lower when the electrical field was parallel to the grain of the wood than when it was perpendicular to it. However, when the field was perpendicular, its direction whether radial or tangential, did not significantly affect the vEM. The direct measurements illustrate the ability of the GPR technique to characterize the MC of timber as a clear decrease in the MC resulted in an increase in the vEM. A strong correlation was obtained between the two parameters with coefficients of determination, R2 > 90%. Longitudinal elastic wave velocities (vP) were assessed by means of a ultrasound technique during the timber drying process. Despite the fact that the vP increased with the decreasing MC of each joist, the determination coefficient between these two variables was very low. The analysis presented in this paper is a successful application of the GPR technique to the study of wood s physical properties and has a promising future for the non-destructive, on-site analysis of timber MC.