Filling tree cavities with polyurethane foam adversely affects the accuracy of sonic tomography

Abstract

ABSTRACT Cavity filling has been frequently applied by arborists to urban trees over many decades. Polyurethane rigid foam was introduced to the arboricultural industry as a filling material to substitute for the concrete popularly used until the 1970s. However, cavity filling causes difficulties for tree inspectors in assessing and diagnosing internal tree defects using standard decay detection techniques. This study assessed the performance of two advanced wood decay detection techniques, sonic tomography, and resistance micro-drilling, by carrying out trials in detecting internal tree defects before and after they were filled with polyurethane foam. Ten hollow cross-sections cut from 10 separate trees were used in this study. Test results from resistance micro-drilling showed its performance was not significantly affected by the foam filling. In contrast, the sonic tomography exhibited a significantly weakened performance, consistently underestimating the extent of decay present in the cross-sections in the presence of the polyurethane foam filling, with acoustic wave travelling times reduced by 11% to 55% due to the treatment. We conclude that for foam-filled cavities, sonic tomography supplies a less reliable assessment of the internal decay present and that current best practice would be to rely on the accuracy of micro-drilling results instead.