Correlations between destructive and four NDT techniques tests on historic timber elements

The paper presents results from a wide experimental campaign by NDT techniques carried out both in situ and in the laboratory using several different techniques. On site, ancient timber elements from the roof of Palace Malvezzi (Bologna) have been undergone to visual inspection, ultrasound tests, moisture content determination, drilling penetration resistance, spectral analysis, ground penetrating radar. In the laboratory comparative work has been carried out on ancient timber beams and specimens cut from the same original beam dismantled from a historic roof, of the same botanical species (spruce) and dated from the same period as Palace Malvezzi. Research in the laboratory has foreseen in addition to the above mentioned NDT techniques, also mechanical testing either in the elastic field of material or destructive. The mechanical testing has regarded, from the same beam, 4 full-scale beams 13/14 x 256 cm3, in different health state and with on-purpose varied moisture content to check if influence on mechanical properties was correctly detectable by the proposed procedure, 19 specimens of dimensions 25 x 25 x 400 mm3 and 4 specimens of 25 x 25 x 100 mm3. Six smaller beams (150 cm in span), cut from a similar historic beam were also tested. The modulus of elasticity was obtained on full-scale beams by 4-point bending test but also by ultrasound testing and by spectral analyser. Compression strength was obtained on 25x25x100 mm3 samples and bending strength was obtained from the prisms of 25x25x400 mm3. Local weak points (decay, knots, splits, joints) were first determined on site and in the laboratory by visual inspection and by testing via penetration drilling resistance and ground penetrating radar, both parallel and perpendicular to grain. On site, afterwards, the modulus of elasticity was determined by ultrasonic device and moisture content was measured. A correlation between NDTs parameter and results from destructive tests of compression, tensile and bending strength is presented in order to propose a methodology for obtaining accurate mechanical properties of the material, residual bearing capacity, stiffness and stability of timber elements. Experimental campaign has been designed, developed and financed inside the EU FP7 project SMooHS (Smart Monitoring of Historic Structures) (2008-2011) by partners from University of Zagreb and University of Bologna.