Methods to determine wood moisture content and their applicability in monitoring concepts

Abstract

The reaction of wood to moisture forms an integral part of any task in connection with this natural and renewable building material. Changes in wood moisture content lead to changes of virtually all physical and mechanical properties (e.g. strength and stiffness properties) of wood. Another effect of changes of the wood moisture content is the associated shrinkage or swelling of the material. The existence of high moisture content can initiate decay or growth of fungi. The correct estimation of timber moisture content and the subsequent initiation of potentially necessary measures are therefore essential tasks during the planning, execution and maintenance of buildings built with wood or woodbased products. This is supported by the high amount of damages on structural timber elements that can be attributed to ignorance of the reaction of wood to moisture. Also supported by this fact, the in-situ monitoring of moisture content of structural timber elements has recently received considerable interest and growth. This paper describes common methods to determine wood moisture content and evaluates them with respect to their applicability for monitoring concepts.

Assessing the integrity of glued-laminated timber elements

“…The temperature is measured by a thermometer; the relative humidity is commonly measured by capacitative hygrometers [27]. The measurement of wood MC can be realized by several methods.…”

“…Wood changes its dielectric properties in proportion to its MC; for measurements, a condensator is placed on the wood surface. The method is standardized [28] and delivers an acceptable accuracy for MCs from 2% up to the fiber-saturation point, although temperature considerably affects the dielectric properties of wood [27]. Only measurements of the average wood MC near the surface (<35 mm) are possible, and a moisture gradient cannot be obtained with this technique, see Fig.…”

“…6 (left). Since water has a much higher conductivity than wood, the electrical resistance decreases with increasing MC [27]. In practice, two electrodes are rammed into the wood or attached to the surface with a defined distance of usually 30 mm and the resistance to electrical current between them is measured.…”

“…Commercially available capacitative MC measurement device[27] (left) and schematic of areas measured by capacitative method (right).…”

Resistance Measurements to Find High Moisture Content Inclusions Adapted for Large Timber Bridge Cross-Sections

“…For the sake of bridge monitoring, it is sufficient to know whether the MC is sound, elevated, or over the FSP. The sensor used to monitor moisture content (MC) is usually placed on the underside of the bridge deck to measure the MC at a depth of a few centimeters (Dietsch et al 2015;Björngrim et al 2016). These sensors would not be able to detect high levels of MC close to the top of the deck or in the middle of the deck close to the pre-stressing rods, nor would an inspector be able to evaluate those spots.…”