Diagnosis Complementary Methods Non Destructive Testing

4.1 Introduction

Nowadays there are many instruments that allow a fast and non-intrusive evaluation of the physical state of timber elements and, at the same time, the estimation of the physical and mechanical properties of the material. A brief description of equipment used in non destructive testing campaigns (NDT) carried out by NCREP is made.

4.2 Hammer, Chisel and Hygrometer

The hammer and the chisel are basic instruments that allow a quick evaluation of the damages of timber

Figure 8. Use of the videoscope for the visualization of a timber floor. Belomonte Palace, XVIII century, Porto.

elements. Throughout the comparison between the sound originated by the impact of a hammer in a healthy and in a deteriorated piece of wood, it is possible to distinguish the existence of rotten parts (cavities, cracks, etc.). On the other hand, with the chisel it is possible to evaluate the superficial hardness of the wood and its integrity.

The Hygrometer allows the estimation of the moisture content of the wood, contributing to the perception of the potential for biotic attacks. High moisture contents may reveal deficiencies in fa├žades or, in particular, on roof structures. This instrument allows technicians to act preventively in order to eliminate water infiltrations.

On the inspection of a XIX century building timber roof, a moisture content of about 20% indicated the existence of water infiltration responsible for the presence of biotic attacks (termite). On an early XX century building, the 18% moisture content found on a timber floor, together with poor ventilation conditions, was responsible for the appearance of the dry rot fungus Serpula Lacrymans.

4.3 Videoscope

The Videoscope is an instrument that allows the observation of hidden areas or of difficult access, Figure 8. This instrument allows us to observe timber floor structures through small holes or gaps. For instance, in Belomonte Palace, an imposing XVIII century building that needed urgent rehabilitation measures, the Videoscope was a precious help, enabling the observation of some floor frameworks and the detection of metallic reinforcements and the presence of biotic attacks without the need to remove the floorboard. With this visualization it was possible to conclude, without any damage for the structure, that it wasn't necessary to reinforce the beams, but only to provide a curative treatment against insects.

4.4 Resistograph

In several timber floors and roofs it has been used the Resistograph (model E300), an instrument that relates

Figure 9. Use of the resistograph on wood stairs. Sao Domingos building. XIX century (1856), Porto.



Penetration depth (section diameter): 0.30m.

Figure 10. Results from the resistograph on a tie beam of a wood truss of Belomonte Palace, XVIII century, Porto.

Figure 9. Use of the resistograph on wood stairs. Sao Domingos building. XIX century (1856), Porto.

the energy spent by the penetration of a needle in a wood element with the material resistance to drilling, Figure 9. The fact of carrying out barely imperceptible drills, without any influence in the mechanical resistance of the piece, turns this instrument into one of the most used in timber structures survey. In order to guarantee the adaptation to the majority of timber structures, it has been used a 300 mm length needle and adopted a speed of penetration of 5 cm/minute.

With Resistograph results it is possible to detect density variations and internal defects (voids, cracks, etc.) along the cross sections of the structural elements. Results can be easily understood and provide interesting qualitative information, as they give draw registries of the variation of wood drilling resistance. However, there are some significant limitations in the use of this instrument to achieve quantitative values, since there are experimental studies only for some species and neither all correlations between the results obtained by the Resistograph and the mechanical or physical characteristics of the wood are acceptable. Nevertheless, the characteristic which presents better correlations is the density and it's possible to obtain a profile of its radial variation along the drilling, namely, the differences of density between the initial lumber (Spring) and the final lumber (Fall), as well as the referred losses of density due to degradations/voids (Botelho, 2006).

As a result of its use, it was possible, for instance, to verify that the timber floors and stairs of a XIX century (1856) building, located in Sao Domingos square, were in excellent state of conservation, therefore not demanding reinforcement measures. On other hand, in the timber roof of Belomonte Palace, a building around 300 years old, some interior voids were discovered in the supports of tie beams (diameter of 0,30 m) with around half of its cross-section. The extension of the degradation along the elements was evaluated, allowing estimating the length of a future prosthesis to install (Faria, 2002). In Figure 10 illustrates the results given by the Resistograph, where a quite long void on the radial profile of one of the tested elements was

Figure 11. Pilodyn used in a timber floor. Antonio Carneiro building, XX century (1916), Porto.

detected. Once again, the qualitative information is very helpful, since it estimates the extension of existent degraded material along the cross section.

To obtain quantitative values it's necessary to perform a statistical handling of the results, obtaining a medium value, called Value of Resistograph (VR). After that, and if applicable to the wood specie of the element, it can be used one of the several studies that correlates this value (VR) with physical or mechanical parameters of that wood specie, such as the density (p), the strength (fc,90) and the modulus of elasticity (Ec90), obtained in laboratory tests. Despite the poor correlations that were usually found, in particular with the mechanical parameters (fc 90 and Ec 90), there are some acceptable correlations. For instance, Botelho (2006) reached a correlation R2 = 70,09% between wood density and VR for Pinus radiate.

4.5 Pilodyn

Another commonly used instrument in the survey of timber floors and roofs is called Pilodyn and has a functioning similar to the concrete Shmidt Hammer, Figure 11. With this instrument it's possible to achieve physical parameters of the wood, such as density, from correlations with the superficial hardness or, by other words, with wood superficial penetration resistance. It's used in the detection of illness manifestations through periodic measurements, in the establishment of resistance classes and categories, and productivity parameters in relation to the density between different wood species (Feio, 2005).

In the surveyed structures, the Pilodyn allowed finding wood defects through the reduction of the wood resistance to the penetration of the needle.

Measurements are made on different points of structural elements (middle spam and supports) and on different structural elements with the objective of doing, with a large number of surveyed structures, comparative studies about the elements integrity.

It is important to refer that, due to the short number of existing correlations for the different species, it isn't possible to reach quantitative values, but only qualitative, but even so valuable information about the conservation state of timber elements.

Most of the correlations for the values obtained by this equipment relates the superficial hardness only with the wood density and not with mechanical properties, which ends up being a disadvantage. On the other hand, the majority of the experimental tests showed that the moisture content considerably affects the depth of penetration (Bonamini et al., 2001) and, so, it's important to evaluate correctly the wood moisture content. Gorlacher (1987) obtained good correlations (R2 = 0,74-0,92) between the density and the depth of penetration of the Pilodyn for an high number of measurements. Furthermore, Feio (2005) found some correlation not only for the density, but also for the modulus of elasticity and the strength of the chestnut wood (Castanea Sativa, Mill). However, and although for the density the correlation is acceptable, for the modulus of elasticity, Ec,90, as for the strength, fc,90, the estimated correlations are poor, and the author does not advise its use as a quantitative measure. Turrini and Piazza (1983) proposed empirical relations between the impact force and the modulus of elasticity, adopting a factor of reduction for the modulus of elasticity based on the visual classification of the elements: 0,8 for elements without defects and 0,5 for elements with some knots and small degraded areas.

4.6 Seismographs

The seismographs are instruments used frequently by NCREP in the inspection of timber floors. In particular, two 18 bits resolution seismographs have been used, Figure 12. They include tri-axial accelerome-ters, and an autonomous memory. The transference of the information to a computer allows afterwards sign analysis. Readings are done in different locations on a floor, supplying its natural frequencies. This evaluation permits estimating the floor stiffness and, consequently, the efficiency of its structure or its physical and (or) material state.

4.7 Load tests

The load tests allow an in-situ realistic evaluation of the stiffness of a structure and, in particular, of the capacity of a structure to resist a certain load. These are expensive tests that are used by NCREP whenever considered convenient. The tests are normally done for loads of about 1,5x the service load and consists on the

Figure 12. Seismograph used by NCREP.

application of controlled loading and unloading cycles with continuous force and displacement monitoring. It is important to refer that after unloading no important residual deformations should exist on the structure. Thus, before carrying out such a test, it's important to estimate, numerically or not, its behaviour, trying to avoid surpassing the structure elastic range. In chapter 5 a case of a building in which a load test was performed is presented.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook

Post a comment