1. Introduction
As a building material wood, under certain circumstances, presents decay problems and structural damage, being the decay process also an irreversible one [
1]. These problems are caused, among other reasons, by fungi and insects, and it is habitual to find them at the same time in the same construction [
2].
Besides other environmental factors one of the causes that favour the growth of xylophagous fungi is moisture [
3,
4]. Some of these factors, which act together, are temperature, pH value and the amount of O
2 available. This means that they affect each other [
5]. Wood has a critical moisture level, and if it is exceeded, there is a risk of fungal development [
6]. This moisture level is called the Fibre Saturation Point (FSP). Below this point, water is contained in cell walls, and above this point, water is accumulated in the cellular lumen. This critical moisture value stands at around 30% in wood samples is temperate regions [
5]. Some authors place at 18%–20% the minimum percentage of wood moisture needed to suffer the attack of xylophagous fungi, being 25%–55% the optimum interval [
7].
Within xylophagous fungi, white rot fungi degrade significantly the cell wall, affecting their physicochemical properties [
7]. The types of rot we can find in wood are: white rot, where fungi attack preferably lignin; brown rot, the most harmful, where fungi focus their attack on cellulose; and soft rot, where fungi mostly attack the cellulose of the secondary wall [
8].
Regarding xylophagous insects, their growth is influenced by various parameters: wood species, wood moisture, environmental temperature and the presence of rot fungi in the wood. The moisture content of wood for xylophagous insect attack covers the entire range, this means that there are insects which attack dry wood, some others attack wet wood, and there are also insects which attack in intermediate moisture ranges [
8].
In addition to what has been said above, changes in moisture content can cause changes in the mechanical properties of wood [
9,
10]. If we talk about foundations, controlling moisture can be especially important. Klaasen [
11] showed in his article a relationship between moisture content of a wood foundation and its compression strength loss. This compression strength loss derives from the degree of degradation that wood suffers as a consequence of moisture. These foundations can have a high cultural significance, in addition to their structural value, in cities as Venice or Amsterdam [
12]. In these cities, monitoring moisture control on an ongoing basis could be interesting since foundations are in many cases in direct contact with water, which puts foundations at risk of suffering some kind of degradation.
Some authors have studied damages in buildings of historical value [
13,
14,
15]. Others have studied the possibility of regulating moisture to preserve this kind of structures [
16] since wood moisture can influence the durability of a construction [
17].
To restore buildings with wooden frameworks, optimizing and improving the procedures to perform, knowing as much as possible the moisture of wood, is necessary. The use of non-destructive techniques allows the substitution of part of the damaged structure instead of whole sections, thus avoiding unnecessary economic and environmental costs [
13].
Morales
et al. [
13] used an ultrasound technique as a non-destructive method for the evaluation of wooden structures considering that the variation of ultrasonic propagation speed provides information about the loss in density in deteriorated wood. Hervé
et al. [
18] used another non-destructive technique to evaluate deterioration in wood based on its density study, developing a map created from X-ray tomography. Another interesting technique is the infrared tomography technique. It allows obtaining information about the condition of the structure but it has a big disadvantage, it only detects defects in the surface and in not much depth (approximately 1 mm) [
19].
As regards the moisture content in wood, Papez
et al. [
17] measured moisture by three methods. The first one is the gravimetric method. It is a direct method, but it cannot be employed in a real structure because it would lead to the damage to the structure. The other two are indirect methods and are performed by local resistive and capacitive sensors with a measurement range of 7%–30% and 0%–50%, respectively. For example, the FH A636-MF resistive sensor used by Papez
et al. has a distance between electrodes of just 7 mm. This operating distance is very small if the sensor is used to determine the moisture of a beam.
Rodriguez-Abad
et al. [
20] proposed a non-intrusive method to measure moisture content of wooden beams using the ground penetrating radar (GPR) technique. For this purpose, a direct wave and a reflected wave are propagated by a transmitting antenna and collected by a receiving antenna. The direct wave travels by the beam’s surface to the receiving antenna. The reflected wave passes through the beam and is reflected in a metal reflector and then, it travels to the receiving antenna. Differences in velocity, arrival times and amplitudes of these two kinds of waves are studied. The behaviour of these waves changes depending on the dielectric properties of the material. The permittivity of water is higher than that of wood so the wave propagation speed increases in inverse proportion to the moisture content of the beam. Despite the fact that this is a very interesting technique, the equipment needed to perform the measurements makes it practically impossible to carry out
in situ and for certain height measurements. In the case of [
21] and applying this technique, the coefficient of correlation for wood moisture was 0.86 in the best case.
Schajer
et al. [
22] used a measurement system based on microwaves. This system, non-intrusive like the previous technique, is based on the propagation of a microwave through a beam and measuring its depolarization, attenuation and phase shifting. With this data, the moisture content, the density and the orientation of the wood grain in the beam can be determined simultaneously. For their part, Denzler
et al. [
23] described in their work a prototype to measure moisture below the FSP and density in wood. It is based on microwave transmission. This prototype does not require making physical contact with the wood sample to be measured. Despite the fact these two works [
22,
23] present very good results, the moisture range of the wood samples was below the FSP. In addition, as in the previous case, the infrastructure needed to take measures hinders its use in a building.
Ludwig
et al. [
24] used the infrared thermography technique to estimate moisture content in a wooden structure comparing the temperature increase that exists in different points of the structure before and after applying heat homogenously. This technique is known as the active infrared thermography technique, since it involves the intentional application of heat from an artificial heat source. The main disadvantage of this system is that it only provides moisture variations, but not a specific moisture value.
For their part, Kandemir-Yucel
et al. [
25] combined infrared thermography technique with the ultrasonic velocity measurement technique to check the condition of a mosque with a wooden structure. Both techniques are non-destructive. In this work, temperature differences greater than 2 °C between different points of the structure indicated differences in their moisture content; and lower moisture contents cause an increase in ultrasonic velocity. This latter technique is very sensitive to changes in environmental conditions, therefore it is very important to know and to control these conditions for a correct interpretation of the results.
Another non-intrusive and interesting technique is time domain reflectometry (TDR), where a signal is propagated and the behaviour of the reflected waves is observed. This technique was originally used for detection of defects in wiring, but today this technique can be applied to measuring the moisture content in porous materials [
26]. Dahlen
et al. [
27] used this technique in their work to measure the moisture content in wood, but these measures were only made in a moisture range above the FSP.
Nevertheless, in spite of the aforementioned works, none of them has reached the required specification and sensitivity to be able to confront restoration in the building field in a fast, effective and economic way. Therefore, in this work we have developed a non-intrusive transportable and inexpensive capacitive sensor, which is able to measure in situ the moisture of a wooden beam on site, where the wood will act as a dielectric. In this way, we can establish the moisture content of the wood sample and act accordingly to restore the building.