Micro-drilling resistance measurements of dense hardwoods for hydraulic structures

Hydraulic timber structures such as mooring poles, fenders, quay walls, lock gates and jetties are widely used in inner and marine harbours all over the world. In general, these structures give satisfactory performance with regard to the technical requirements. Because of the harsh exposure conditions such as brackish and contaminated water, drying-out and mechanical impact, the deterioration processes have to be monitored with regard to the service life performance. Visual inspection is limited to defects at the surface. Internal 'defects' such as cracks, juvenile and intermediate wood as well as biological decay influence the mechanical performance and are difficult to detect. Until now, micro-drilling technique has been successfully applied to softwoods and some medium-dense hardwoods. In this paper it has been investigated what kind of defects through the cross section of hardwood beams can be detected with the corresponding accuracy and reliability. It could be concluded that drilling resistant measurements on eucalypt and azobé allows detecting of defects such as decay and cracks, but also high and low density zones due to slow growth rates and juvenile wood areas. For this it is necessary that the micro-drilling is calibrated especially with regard to the drilling depth, the wood cross section and the wood species.