NDT METHODS FOR THE ASSESSMENT OF STRUCTURAL TIMBER: REPORT ON THE RESEARCHES CARRIED OUT AT THE UNIVERSITY OF TRENTO (ITALY)

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1 NDT METHODS FOR THE ASSESSMENT OF STRUCTURAL TIMBER: REPORT ON THE RESEARCHES CARRIED OUT AT THE UNIVERSITY OF TRENTO (ITALY) Maurizio Piazza and Mariapaola Raggio University of Trento, Dpt. of Mech. and Structural Eng. (DIMS), Italy Abstract A throughout experimental campaign, carried out at the Department of Mechanical and Structural Engineering (DIMS) of the University of Trento (Italy), aims at evaluating the effectiveness and reliability of some non-destructive testing methods for the in situ assessment of structural timber. During the experimental campaign, different types of specimens have been tested, with regard to their dimensions, species, geographic origin, age, condition and moisture content. The analyzed parameters are longitudinal stiffness and density: the former estimated by means of sonic and ultrasonic methods, hardness tests and modal analyses, the latter by means of the Pilodyn and the Resistograph tests. ND data have been compared with the values obtained from the direct measurement of the analysed property. Keywords Timber, non-destructive tests 1. INTRODUCTION In this paper a review of the research activity on ND testing of timber elements, carried out at the DIMS laboratory of the University of Trento is reported. The results of different testing methodologies, aiming at estimating two fundamental grading parameters of timber, namely density and longitudinal stiffness, are presented and discussed. Although correlations between these properties and different NDT-parameters have been studied in several research projects, there are only few reports in which several methods are applied to the same sample of sawn timber. The latter approach has been adopted in the researches reported in this paper, in order to better compare the different techniques analysed. Indeed, results of studies made with different samples of timber are difficult to interpret and 1039

2 compare, because the differences may be due to either the sample used or the measurement methodology. 2. THE EXPERIMENTAL ACTIVITY 2.1 The test material The variety of presented case studies permits to consider different parameters affecting the behaviour of the material and the outcomes of the tests. In the researches reported in this paper, different types of specimens have been analysed, with regards to their dimensions, species, geographic origin, age, condition and moisture content. Full-scale elements, coming from disassembled timber roofs, were tested, in order to have information on the material characteristics and the global behaviour of the original structure. On the other hand, tests were also carried out on samples at smaller scale, obtained from portions of old beams, which were sawn in order to investigate, in a convenient time lapse, the behaviour of the sample at different moisture contents, or in order to eliminate highly decayed parts or notches, whose presence could affect the feasibility and reliability of the test. A total of 46 elements were investigated, among which the 65% in structural size. All tests were carried out on old timber, recovered from disassembled structures, whose age globally spans five centuries. The species analysed are three softwoods, namely fir (Abies Pectinate) (6%), spruce (Picea Abies Karst) (52%) and larch (Larix Europea) (20%), and a hardwood, chestnut (Castanea sativa) (22%), that are typically present in traditional timber structures in Italy. Test material has been labelled, in order to recognize its species and origin. The first letter in the sample s name (F, S, L, C) means the species, namely fir, spruce, larch and chestnut respectively. The second letter indicates the origin: elements labelled TN and VE are roof beams, coming from Trento and Venice respectively, both dating back to the fifteenth century. Elements labelled R come from the old roof the Ai Caduti dell Adamello refuge, on the Adamello glacier in Lobbia Alta (Italy); they are second or even third hand timber and their age spans the first four decades of the 20th century [1]. Elements labelled T are members of two queen-post trusses of a theatre roof in Trento, that dates from the second decade of the 19th century. Timber in structural size was tested at the EMC, while smaller scale elements were tested at both EMC and fiber saturation point (FSP). EMC was measured using the electrical resistance moisture meter [2]. 2.2 Testing equipment and testing procedures During the different research programs carried out at the DIMS laboratory, several techniques have been used for the determination of the modulus of elasticity. Among the global ND testing, the following measures have been taken: (NDT1) fundamental frequency of longitudinal vibration with the element centrally supported (24% of the samples); for this test the RION SA-77 device was used; (NDT2) fundamental frequency of longitudinal vibration with the element centrally restrained (20% of the samples); for this test an accelerometer, an instrumented hammer and a FFT analyser (ONO SOKI, CF-350) were used; (NDT3) fundamental frequency of transversal vibration with the element simply supported at the ends (8% of the samples); 1040

3 (NDT4) propagation time of stress waves along the entire length of the beam (17% of the samples); the test setup was an instrumented hammer, an accelerometer as receiving transducer and a FFT analyser; ultrasonic waves velocities; the Sylvatest [3] or, alternatively, another instrument, with a source of 20 khz were used. Longitudinal measures were made for the total of the samples, both with the transducers placed at the ends of the element (propagation parallel to the grain) (NDT5) and with the transducers equally spaced at a fixed distance, on the same element face, for local piecewise readings (NDT6). Among the local ND methods the hardness test invented by Piazza and Turrini [4] was used to estimate the longitudinal modulus of elasticity (NDT7), by measuring the load force R required to embed 5 mm a 10 mm diameter steel hemispherical bit. The measures were made in five positions, equally spaced along the element, for each lateral side, each measure is the mean of five different tests. In this paper only results of the MoE calculated from NDT1, NDT5 and NDT7 are reported, in order to compare the most of the samples data. Measures on saturated samples were taken using NDT5 and NDT7. For the indirect measure of the density of the wood, the following tests were chosen: (NDT8) measure of the penetration of a steel pin (Pilodyn ), driven into the wood with a constant energy (NDT8); measurements were taken close to the ends and at the mid-span of each element. (NDT9) measure of the drilling resistance [5] during the penetration into the wood of a drill bit (Resistograph ); tests were carried out on three sections equally spaced along the element; for each section two tests were performed along the apparently tangential direction and one along the apparently radial direction. In order to verify the ND methods, MoE was directly measured in bending, in accordance with the four-point loading method [6, 7], and compared with the NDT data. The data for density are derived from the ratio between mass and volume of the entire element. Because, most of the elements were characterized by the presence of geometrical irregularities, notches, wane and spilts, measures have been taken at every 100 cm along the element axe, for the volume determination. Density values obtained by direct weighting have been compared with the mean values resulting from indirect measurements. A visual inspection on each tested element was preliminary performed, searching for main natural defects (knots, fissures, resin pockets, etc.) and biological degradation [8]. 3. ANALYSIS OF THE RESULTS In the following Tables a synthesis of the NDT campaigns is shown. Values of MoE obtained from NDT tests are reported in Tables 1-3 and compared with the values obtained from the four-point bending tests. The difference between the predicted values and the bending property is also shown. For conditioned wood, the mean differences in percentage between the MoE obtained from static bending tests and the adopted NDT methods are 15% for the dynamic tests (NDT1), 30% for the ultrasonic tests (NDT5) and 2% for the hardness tests (NDT7). For saturated wood the mean error is 21% for (NDT5) and -7% for (NDT7). The mean error of the NDT methods in predicting the stiffness of full-scale elements is: 13%, 23% and 4%, for NDT1, NDT5 and NDT7 respectively. 1041

4 Table 1: MoE data from NDT of full-scale elements at the EMC MoE Bending test NDT 1 NDT 5 NDT 7 Element N. (MPa) (MPa) % (MPa) % (MPa) % S-T S-T S-T S-T S-T S-T S-T S-T S-T S-T S-T S-T S-T S-T S-T S-R \ \ S-R \ \ S-R \ \ S-R \ \ S-R \ \ S-R \ \ S-TN S-TN S-TN L-VE L-VE \ \ \ \ F-TN \ \ \ \ F-TN \ \ \ \ F-TN \ \ \ \ L-TN \ \ \ \ 1042

5 Table 2: MoE data from NDT of small-scale elements at the EMC MoE Bending test NDT 5 NDT 7 Element N. (MPa) (MPa) % (MPa) % L-s L-s L-s L-s L-s L-s C-s C-s C-s C-s C-s C-s C-s C-s C-s C-s Table 3: MoE data from NDT of small-scale elements at the FSP MoE Bending test NDT 5 NDT 7 Element N. MC % (MPa) (MPa) % (MPa) % L-s L-s L-s C-s C-s C-s C-s C-s C-s C-s C-s C-s C-s

6 Figure 1: Static bending MoE versus MoE given by NDT (test material at the EMC) Figure 2: Static bending MoE versus MoE given by NDT (test material at the FSP) 1044

7 As a further treatment of the gathered data, statistical regression analyses between parameters obtained from all individual grading-methods and the bending stiffness resulting from the static tests were made. For conditioned timber, the higher correlation is given by the longitudinal vibrational tests (R 2 =0.55), which is, however, comparable with results of the hardness tests (R 2 =0.45), while a modest correlation is exhibited by the longitudinal ultrasonic tests (R 2 =0.22) (Figure 1). Poor correlations were found for both ultrasonic and hardness tests in case of saturated wood (Figure 2). Figure 3: Density values (kg/m 3 ) vs. Pilodyn data (mm) Figure 4: Density values (kg/m 3 ) vs. Resistograph data 1045

8 Also density parameters, obtained by both direct weighting and indirect methods have been compared. In particular, the mean values derived from Pilodyn tests (depth of penetration in mm) and Resistographic tests (the integral of the drill resistance function divided by the penetration length) have been used as NDT parameters for the regression analysis (Figures 3 and 4). A higher correlation is given by the Resistograph (R 2 =0.51), while Pylodin s correlation coefficient is R 2 = CONCLUDING REMARKS The large number of tests for each ND method allows to estimate its sensitivity and to evaluate the scatter of the results concerning the investigated properties. Global methods are useful to have information on the entire element; nevertheless they present some operative limits on site, such as the practical impossibility of longitudinal sensing, because of the difficult approachability of the transversal faces of the element. Vibrational methods need an accurate definition of the boundary conditions, in order to properly calculate the natural frequency of the tested element. Moreover, correlations of the dynamic modulus of elasticity to the NDT parameters, measured by means of vibrational, sonic or ultrasonic methods, depend also on density. While real values of density are determined in the laboratory, NDT should be used on site, to estimate mean density values. A good alternative to global methods seems to be the hardness test proposed by Piazza and Turrini; however, a reduction factor depending of the defectiveness of the element is needed. Moreover, in order to obtain a sufficiently reliable mean value of the investigated parameter, a large number of tests along the whole element must be carried out. The MoE of 1/3 of central part of the beam measured with the four-point bending test has been compared to local readings of 1/3 of central part of the elements (NdT 6 and 7); the obtained results, however, are comparable with results of the cited NdT on the entire length of the element. For the chestnut elements, the adopted NDT techniques showed lower correlations than for the tested softwood species. This is due to the fact that the most of the NDT methods were conceived and tested especially for species that are more widely used in modern constructions, while hardwood species, such as chestnut, are common in traditional structures, especially in the Mediterranean area. Moisture content affects different parameters involved in the adopted NDT technique, such as the internal friction, the density and the hardness of the material. In general, poor correlations have been found for saturated specimens. A deeper knowledge of their influence is required, in order to calibrate the method for specimens at different moisture contents. Pylodin data, obtained averaging results on the whole element, are not able to predict global parameters (i.e. mean density). On the contrary, single Pilodyn data permit to map the condition of the surface decay and highlight the regions of the element that need further investigation, with more sophisticated ND methods. A correlation between Resistograph data and density can be found only when the element shows similar Resistographic curves in the different tested sections. This means that the method is not able to predict density (or related mechanical properties), in case of elements with local macroscopic heterogeneities (i.e. checks, local decay). 1046

9 Finally, different methods have to be combined and the obtained data have to be critically analysed, in order to predict reliable values of the investigated parameters. ACNOWLEDGEMENTS This research has been partially supported by the Italian Department of University and Research, within the research project (Years 2007/08) Diagnosis techniques and totally removable low invasive strengthening methods for the structural rehabilitation and the seismic improvement of historical timber structures. The writers would like to acknowledge the precious help of their former students Massimo Valzolgher, Werner Schlagenauf, Giovanni Brentari, Alberto Zamatteo Gerosa, Alessandro Foresti. REFERENCES [1] Pezzo, M.I. and Zamatteo Gerosa, A., Analisi dendrocronologica di alcune di travi del rifugio Ai Caduti dell Adamello, Lobbia Alta, Trentino, Annali del Museo civico di Rovereto (2006). [2] CEN, EN Moisture content of a piece of sawn timber - Part 2: Estimation by electrical resistance method, European Committee for Standardization, Brussels (2002). [3] Sandoz, J.L., Grading of construction timber by ultrasound, Wood Science and Technology 23 (1989) [4] Turrini, G. and Piazza, M., Il recupero dei solai in legno. Esperienze e realizzazioni, Recuperare 7 (1983). [5] Rinn, F., Chancen und Grenzen bei der Untersuchung von Konstruktionshölzern mit der Bohrwiderstandsmethode, Bauen mit Holz 9 (1992). [6] CEN, EN 408. Timber structures. Structural timber and glued laminated timber. Determination of some physical and mechanical properties, European Committee for Standardization, Brussels (2003). [7] CEN, EN Structural round timber - Test methods, European Committee for Standardization, Brussels (2004). [8] UNI , Legno strutturale - Classificazione a vista di legnami italiani secondo la resistenza meccanica: terminologia e misurazione delle caratteristiche caratteristiche (Structural timber Visual strength grading: terminology and measurement of features), Milano, Ente Nazionale Italiano di Unificazione (2003). 1047