Nondestructive Characterization of Dry Heat-Treated Fir (Abies Alba Mill.) Timber in View of Possible Structural Use

Aleš Straže,Gorazd Fajdiga,Bojan Gospodarič

doi:10.3390/f9120776

Abstract

The use of heat-treated timber for building with wood is of increasing interest. Heat treatment improves the durability and dimensional stability of wood; however, it needs to be optimized to keep wood’s mechanical properties in view of the possible structural use of timber. Therefore, dry vacuum heat treatment varying the maximum temperature between 170 °C and 230 °C was used on fir (Abies alba Mill.) structural timber, visually top graded according to EN 338, to analyze its final weight loss, hygroscopicity, CIELAB color, and dynamic elastomechanical properties. It turned out that weight loss and total color difference of wood positively correlates with the increasing intensity of the heat treatment. The maximum 40% reduction of the hygroscopicity of wood was already reached at 210 °C treatment temperature. The moduli of elasticity in longitudinal and radial direction of wood, determined by ultrasound velocity, increased initially up to the treatment temperature of 210 °C, and decreased at higher treatment temperature. Equally, the Euler-Bernoulli modulus of elasticity from free-free flexural vibration of boards in all five vibration modes increased with the rising treatment temperature up to 190 °C, and decreased under more intensive treatment conditions. The Euler-Bernoulli model was found to be valid only in the 1st vibration mode of heat-treated structural timber due to the unsteady decrease in the evaluated moduli of elasticity related to the increasing mode number.

Highlights

Thermal treatment at high temperature, i.e., between 160 ◦ C to 260 ◦ C, is one of the eco-friendly methods for the enhancement of the biological durability of wood and lignocellulosic composites.Heat treatment processes vary in terms of furnace design, type and condition of heating medium, and treatment schedules, and mostly depend on final usage of heat-treated material
The rising of the heat treatment temperature induced a significant increase in the weight loss of fir wood (Abies alba Mill.; ANOVA, p = 1.42 × 10−12 )
This caused a drop in the mean density of wood after the heat treatment of 2.5% at a temperature of 170 ◦ C (ρ170 = 415 kg/m3 ) and up to 10.3% at the heat treatment temperature of 230 ◦ C (ρ230 = 392 kg/m3 ; ANOVA, p = 0.07)

Summary

Introduction

Thermal treatment at high temperature, i.e., between 160 ◦ C to 260 ◦ C, is one of the eco-friendly methods for the enhancement of the biological durability of wood and lignocellulosic composites.Heat treatment processes vary in terms of furnace design, type and condition of heating medium, and treatment schedules, and mostly depend on final usage of heat-treated material. The common factor of these processes is a modification of the chemical structure of timber, which has consequences on the physical and mechanical properties of wood [1,2,3,4,5,6,7]. The important aspects in a case of thermally treated wood are strength reduction and stiffness alteration, which vary with the anatomical direction of wood, testing method, and wood species. Many studies have shown a reduction in the bending stiffness and strength of heat-treated wood, combined with the reduced wood density [8,9,10,11,12,13,14], since the latter is the main influencing factor in the mechanical properties of wood [15]

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