The ultrasonic wave velocities in wood impregnated with a fire-retardant chemical were measured via a non-contact method. An air-coupled ultrasonic wave was made to propagate along the radial direction of the wood. The wave velocities in the wood samples after chemical impregnation were larger than those before impregnation. With increased chemical concentration, the relative changes in the velocities increased to a maximum of 16.3%, and these velocity changes exhibited a strong correlation with the chemical retention. These findings suggest that it is possible to evaluate the uniformity distributions of chemicals in fire-retardant-treated wood via a non-contact and nondestructive method based on air-coupled ultrasonics.
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