Moisture monitoring in glulam timber structures with embedded resistive sensors: study of influence parameters

Abstract

Today, a strong expansion in the construction of timber bridges is being promoted by the use of glulam. However, durability issues related to wetting/drying cycles limit their development. Timber bridge supervisors have therefore expressed their needs concerning continuous moisture content (MC) monitoring in timber structures. Checks on the MC of timber bridges are commonly based on the resistive method. A recent study carried out in the laboratory has shown the great potential of surface-type electrodes for local MC measurement in the laminated timber of glulam. Nevertheless, many parameters influence MC measurements and no information exists concerning surface electrodes. The novelty of this paper is to make the MC measurements with surface-type electrodes more reliable and accurate. In this study, the influence of electrode size and boundary conditions was therefore investigated. The influence of embedded sensors on the bending strength and elastic modulus of glulam beams was also evaluated in order to validate the electrodes instrumentation. Then, the influence of temperature and MC on the electrical resistance was studied, allowing a calibration curve to be established. The results show that small electrodes reduce the measurement uncertainties and therefore permit a more reliable estimation of MC. The boundary conditions also influence the measurements and should be considered. The embedded electrodes have no significant influence on the bending strength and elastic modulus of glulam beams. Finally, based on the literature review, a model for estimating wood MC was established, considering temperature variation.