Mechanical characterisation of wooden structures by means of an in situ penetration test

The problem of evaluating wood decay and related residual strength capacity in existing wooden structures in historical buildings is particularly important when deciding on the possibility of restoration and the kind of strengthening intervention needed. The technique of the penetration test consists in the penetration of a steel pin into wood with repeated and energy constant blows transmitted by a rebound hammer. This test, based on the penetration measurement layer after layer, makes it possible to distinguish between different degrees of decay in each layer as a function of the number of blows necessary for 1-cm penetration. Due to the anisotropy and the different behaviour of wood with respect to moisture content, the results of the penetration test may be very spread. This article presents the experimental procedures, the tests and the results obtained to point out the significance of these parameters and to find an indication of a possible correlation between the results obtained with the penetration test and the wood ultimate strength obtained from a usual three point bending test on small, clear specimens. The results of tests on spruce wood show good correlation among these data and therefore a curve - number of blows vs. bending resistance - is proposed, taking into account moisture and penetration direction as leader parameters. (C) 1998 Elsevier Science Ltd. All rights reserved.