Testing Calibration Issues in Resistance Drilling Applied to Timber Elements

Resistance drilling devices are commonly used for the onsite inspection of wood and timber structural components in existing buildings. Although they provide a measure related to the density variation along a section of an element, results are used mainly qualitatively, due to several parameters that affect the computed values. In this paper, several new and old timber elements, taken from the dismantling of original roofs from a large existing historical building in northern Italy, are tested through a series of ND (non-destructive), SD (semi-destructive) and destructive testing procedures. Various wood species (spruce, fir, larch, oak, elm, pine) are taken into consideration. Among the applied testing methods (visual inspection, resistance drill, ultrasonic, needle penetration, radar, lab tests bending/compression), not all reported here, a focus on the resistance drill tests results is addressed, to evaluate the influence of consumption of the needle tip on the amplitude output. This was done by correlating the drillings to a reference sample on each test position to obtain data with an enhanced quantitative content. From the visual old timber elements grading, SD tests execution and successive element strength characterization through laboratory tests, it finally emerged the need for proper calibration of assessment methodologies through the application of a combined approach, in order to achieve more reliable results.