Efficiency of the machine grading of chestnut structural timber: prediction of strength classes by dry and wet measurements

Sweet chestnut is a timber species used for construction since many centuries thanks to its good mechanical performances and its natural durability. The use as solid timber is historical in many Mediterranean countries, but today it needs to be strength graded if applied as structural product. Visual grading have been recently developed, but machine grading could allow a more efficient use of the material and could be a first step also for the implementation of structural engineered products. Therefore, this paper analyse the machine grading of chestnut timber, deriving the machine settings according the European regulations, highlighting weaknesses and strengths, and verifying the feasibility of the strength grading on wet timber. Both the dynamic modulus of elasticity and the merely frequency of vibration were compared as different machine indicating properties. There were 781 timber pieces collected and tested, with different cross sections and these were divided into four sub-samples originating from the southern, central and northern regions of Italy. Both D and C strength classes (EN 338, Structural timber--strength classes, 2009) were included in the calculations and the causes of wrong upgrading were analysed. Machine grading performed satisfactory, several class combinations were possible and higher strength classes were attainable when compared with the visual grading process, which allows sorting of only one strength grade. The assignment to C-classes allowed for higher yields in upper classes, while the D-upper classes were unpredictable by the only frequency measurement. Machine grading performed directly on wet timber achieved comparable yields to dry grading, but practically could be carried out only by means of dynamic modulus measurement.