Use of Non-Destructive tests in the evaluation of the state of repair of consolidation interventions on wooden structures
Politecnico di Torino, Department of Architecture
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This paper describes an investigation into possible non-destructive methods for the assessment of the state of preservation of previous interventions on wooden structures conducted within the framework of a research project focusing on the restoration of existing buildings. The initial results obtained from laboratory tests with the aid of different types of instruments, are discussed
together with the methodology developed on the basis of the testing program.
Since the second half of the Seventies, numerous interventions have been performed with the aim of consolidating deteriorated wooden structures. Damaged parts, no longer able to fulfil their structural role, were replaced with prostheses of epoxy micro-concrete, combined with fibreglass-reinforced plastic or steel bars.
The connection between the existing structure and the prosthesis was obtained by means of bars which were first clamped to the wood and fastened to it by means of epoxy based adhesive and then embedded in the micro-concrete.
The effectiveness of the intervention essentially depends on the degree of mutual collaboration that is created between the original wooden element and the newly added prosthesis. It is indispensable, in fact, to ensure an effective and possibly homogenous distribution of stresses over the various elements making up the structure.
In some instances, the lower degree of deformability of the epoxy micro-concrete compared to the wood, as well as the different behaviour of the two materials as a function of humidity and temperature conditions, are seen to result in the detachment of the prosthesis from the wooden element. In actual fact, however, this phenomenon will not impair the static effectiveness of the consolidation intervention as long as the connection between the wood and the prosthesis continues to be ensured by the bars.
On the basis of these considerations, it was deemed worthwhile to develop a non-destructive testing method to be used in situ in order to assess the long-term effects of such interventions and to check for the presence of discontinuities in the bar/wood connection.
After an analysis of the literature concerning the non-destructive tests used for this type of investigation, it was deemed advisable to carry out a specific laboratory testing campaign.
Small-scale specimens were prepared, consisting of small oak beams in which Ø14 mm bars (of either fibreglass-reinforced plastic or steel) were introduced and made integral with the wooden element by means of an epoxy-based adhesive (Resisystem 310 TX) (Table 1).
||Specimen with fibreglass-reinforced plastic bar bonded over half its length
||Specimen with fibreglass-reinforced plastic bar bonded over its entire length
||Specimen with fibreglass-reinforced plastic bar bonded over some portions
||Specimen with (Ø16 mm) through hole only
||Specimen with steel bar bonded over half its length
||Specimen with steel bar bonded over its entire length
The testing program was conducted by means of equipment for:
- Pulse wave velocity tests
- X-ray tests
- Thermographic tests.
Pulse wave velocity tests
The tests were performed by means of a C.M.E. US 02/84 testing system, in accordance with RILEM Recommendations and Standard UNI 9524.
The testing equipment consisted of:
Direct transmission measurements were performed, by applying the transducers to two opposite faces of the wooden elements being tested (Fig. 1).
- a high voltage (1000 volt) pulse generator, which is applied to the two faces of a piezoelectric crystal contained in the probes in order to excite the crystal, resulting in the emission of intermittent ultrasonic waves;
- two (receiver and transmitter) probes, with oscillation frequency of 40 Khz. The cross-section of the probes may vary depending on specific testing requirements: in this case, the probes had a (Ø 50) cylindrical cross-section or a truncated cone cross-section.
- an oscilloscope;
- a pulse amplifier;
- an instrument for the measurement of the propagation time of the longitudinal pressure wave, from the triggering (pulse emission) time to the time the first wave front is received.
Fig 1: Pulse wave velocity testing equipment
Only a limited number of tests were conducted with this method, still it was possible to conclude the results obtained through the measurement of pulse wave propagation velocity were not accurate enough to gain reliable evidence as to the presence of bars inside the wooden structures, not to mention the possible presence of discontinuities.
The variations in pulse wave propagation velocity, in fact, turned out to be greatly affected by the density of the wood, the presence of surface discontinuities, the arrangement of the fibres, the intervention of the operator.
Furthermore, the wavelength of the ultrasonic signal - at the frequency used for the tests on wooden materials - is much higher than the size of the discontinuities that may form between the bar and the wooden parts.
X ray tests
This investigation technique is generally adopted on other materials, such as for instance steel, in order to check for defects or assess the quality of welds. The first step therefore consisted of calibrating the equipment for use on wooden materials, so as to define the necessary parameters, such as radiation intensity and radiation time. Once fine tuned, the testing equipment was used to test the specimens described above (1).
Figs. 2-3 show the X-rays of specimens 1v and 1a, characterised by the presence of bars bonded over half their length, of fibreglass-reinforced plastic and steel, respectively (see Table 1).
From an examination of these photos it can be seen that this type of instrumentation makes it possible to identify possible discontinuities between the bars and the wood. The presence of bars inside the specimens is clearly visible, in fact, and so are the bar/wood discontinuities, as revealed by a darker line separating the two elements, owing to the lower radiation absorbing potential of air compared to other materials.
Fig 2: X-ray of specimen 1v
Fig 3: X-ray of specimen 1a
An analysis of the literature prompted the need to explore the applicability of thermographic tests in the assessment of discontinuities inside wooden structures.
The tests conducted so far will have to be supplemented by further studies; at all events, an initial series of tests performed on the specimens containing steel bars (fig. 4) failed to supply encouraging results. Though the specimens were subjected to a prolonged heating process (in view of the poor heat conductivity of wood), it proved impossible to identify any discontinuity in the bar/wood connection, or even to discern with sufficient certainty the very presence of the bars.
Fig 4: The thermographic test
Based on the totality of the tests conducted so far, the X-ray method seems to be the only one that proves effective in the evaluation of the state of preservation and execution quality of previous repair interventions.
In this connection, further tests will be developed to acquire new experience and to explore the possibility of performing in-situ tests, directly on previously repaired structures.
(1) The tests were performed at the laboratory of the C.M.E. company of Albignasego (Padua). The technical data of the instruments employed (produced by the Gilardoni company) are given below:
Automatic development was performed at a temperature of 27°C. The time duration of the cycle was 8 min.
- voltage applied to the tube: 125 KV;
- current going through the tube: 6 mA;
- focal distance: 70 cm;
- size of focal stain 2*2 mm.
The author wishes to express her gratitude to Ing. Lucca, Ing. Lugnani of the C.M.E. company of Albignasego (Padua) and Dott. E. Grinzato of the C.N.R. of Padua who made it possible to carry out the testing program.
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