Non-destructive assessment of gluing in cross-laminated timber panels

Cross Laminated Timber (CLT) panels are wooden structural products consisting of finger jointed boards glued together and arranged in orthogonal layers. The lamination process reduces the scatter of the board properties so that minor strength reducing features such as knots and other defects are dispersed in the product. CLT panels have very high axial load-carrying capacity and bending strength, due to their large resisting areas, and high shear strength. However, the structural performance of CLT panels depends on the proper application of the glue between the layers. The bond set up at the interface between longitudinal and orthogonal layers can be seriously compromised by the lack of gluing occurring during the production process. For this reason, quality control is an essential tool for verifying the correct gluing process of CLT panels. This paper illustrates the preliminary results of an experimental programme aimed to test the effectiveness of Ultrasonic Testing (UT) in detecting adhesion anomalies and defects in CLT panels. UT was carried out on several CLT panels having different configurations (length, thickness of the layers, number of layers, timber species). For each panel, the velocity of longitudinal stress wave V was measured along several paths perpendicular to the plane of the CLT panels. V data was numerically processed and then for each panel the distribution of V on a horizontal plane was graphically represented implementing the iso-velocity curves. The match between the low-velocity areas resulting from data processing and the unglued areas visually assessed was evaluated and critically discussed.