Influence of the Distance from the Pith in Resistance Drilling Tests on Timber Beams

Resistance drilling test is commonly employed to evaluate the state of preservation of historic timber. Results are mainly qualitative, owing to the heterogeneity of the material and the localized inspection. Notwithstanding, a quantitative parameter (RM) can be adopted, so that different tested areas can be compared. Many factors can modify the test results, e.g., wood species, drilling settings, defects, and degradation. In the paper, the results of an experimental campaign on timber beams, aimed at evaluating the influence of the test-to-pith distance on the outcomes of the resistance drilling test, both from qualitative and quantitative points of view, are discussed. Firstly, a spruce new sawn timber element was tested; 55 tests were carried out with constant settings of the drill and following a predefined grid. The specimen was then sliced along the rows of the grid. The drill needle tended to deviate from the orthogonal direction as it crossed the rings tangentially, and the distance from the pith increased. The highest values of RM were obtained for those tests closer to the pith. On average, the feed RMs for the central ward were 21% higher than those on the extreme wards of the cross-section, whereas the drilling RMs resulted 10.4% higher. This study was then extended on a sample of 24 old and new timber beams, for a total of 440 tests. Results were aggregated as a function of the distance to the pith. On average, tests carried out close to the pith yielded RM values 9% higher than those on the neighboring ward, and 22% higher than those on the outer wards.