Charles G. Carll

Decay of Wood and Wood-Based Products Above Ground in Buildings

This paper is an overview of what we know about occurrence of wood decay above ground within buildings. It presents information concerning under what conditions decay may become established. In laboratory tests involving optimum moisture and temperature conditions for decay fungi, and direct contact with large quantities of specific well-developed decay fungi, substantial decay in small specimens of untreated wood of nondurable species can occur in a few weeks. The simultaneous occurrence of optimum conditions for decay and high degree of inoculation with mature decay fungi is probably very rare in buildings. However, spore germination also proceeds rapidly at optimum moisture and temperature conditions. For most decay fungi, optimum moisture conditions mean moisture contents above fiber saturation (usually around 25 to 30% mc,) but well below the waterlogged condition. Optimal temperatures for most decay fungi are in the range of 2 1 to 32°C, Untreated wood and wood-based products will not decay if intermittently wetted for short periods to moisture contents above fiber saturation or if wetted to such levels for periods of a few months when temperature is low. However, little is known in quantitative terms about decay development under fluctuating conditions. Moisture and temperature conditions are not expected to fluctuate appreciably behind external insulation and finish system (EIFS) claddings. Given this, we can find nothing in the research literature that would contradict the 20% wood moisture content rule for this application.

Accuracy of Wood Resistance Sensors for Measurement of Humidity

This work was undertaken to evaluate the accuracy of wood resistance sensors for measurement of relative humidity and to identify sources of error in this use. Relative humidity can be expressed as a function of the logarithm of the sensors electrical resistance and of its temperature. We found that single-point calibration of each sensor compensates for most between-sensor variation, although care must be exercised during calibration. With careful calibration readings, error in relative humidity readings made with these sensors can be limited to ± 10% relative humidity under most conditions. The literature indicates that a lower degree of error than this is anticipated when electrical resistance is used to estimate moisture content measurements. Our data suggest that sorption hysteresis and sensor memory are significant contributors to this (± 10%) relative humidity error.

Moisture Meter Calibrations for Untreated and ACQ-Treated Southern Yellow Pine Lumber and Plywood

This study investigates the effects of alkaline copper quaternary (ACQ) preservative treatment and of plywood glue lines on resistance-based moisture content (MC) measurements. Moisture meter readings using stainless steel screws as electrodes were acquired over a range of moisture conditions in Southern Yellow Pine (SYP) lumber and plywood. Calibration equations are presented for predicting gravimetric MC from meter readings taken in SYP lumber and SYP plywood with or without ACQ treatment. These corrections assume that the meter has been set for SYP. Correlation equations are also presented for directly relating resistance to gravimetric MC, which may be useful for automated data collection systems employed in monitoring moisture levels in buildings. The conductance of SYP lumber was raised by treatment with ACQ, particularly at higher moisture contents, but was unaffected by vacuum-pressure treatment with water. The conductance of untreated SYP plywood, measured with electrodes penetrating the glue lines, exceeded that of untreated SYP lumber. The conductance of SYP plywood was lowered by treatment with ACQ, by vacuum-pressure soaking with water, and by exposure to rain. We suggest that electrolytes in the plywood glue lines increase the conductance of untreated plywood relative to that of untreated lumber, and that the concentration of these electrolytes is lowered by the (aqueous) preservative treatment process, thereby lowering the conductance of these high-conductance pathways.

How variability in OSB mechanical properties affects biological durability testing

Summary Loss in bending strength of wood has been shown to be a more sensitive measure of decay than is weight loss. Using modulus of rupture as the decay criterion is problematic for oriented strandboard (OSB) because of variation in mechanical properties due to particle orientation and size. Moreover, the small specimen size required for such tests increases the variance in mechanical properties. This study compared the variance in bending strength of ASTM D1037 standard-sized specimens and small specimens from two samples of commercial OSB. The small specimens were found to have a significantly higher level of variance in bending strength than the standard-sized specimens. A simple method of sorting the specimens based on strand orientation on the tensile surface significantly reduced the level of variance measured. The effects of differing levels of variance on the size, design and limitations of the experimental study are presented.