WO2017055689A1

WO2017055689A1 - Method and apparatus for detecting moisture content of building structures

Info

Publication number: WO2017055689A1
Application number: PCT/FI2016/050682
Authority: WO
Inventors: Juhani KORKEALAAKSO; Arto Hujanen; Ismo Huhtinen
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2015-10-02
Filing date: 2016-09-30
Publication date: 2017-04-06
Also published as: FI20155696A

Abstract

According to an example aspect of the present invention, there is provided a method for non-destructive detection of moisture content in building structures, comprising: generating a wide band radar signal, sending the radar signal towards a building structure so that a signal is reflected, receiving the reflected signal, processing the reflected signal for obtaining a reflection response and comparing the obtained reflection response to a predetermined reflection response of a comparable dry building material.

Description

METHOD AND APPARATUS FOR DETECTING MOISTURE CONTENT OF

BUILDING STRUCTURES

FIELD [0001] The invention relates to method and apparatus for examining moisture content in building structures by using non-destructive technologies.

BACKGROUND

[0002] Unwanted moisture in building structures may lead to growth of mould or microbial growth in buildings. This may lead to health risks for people and animals occupying the building space and even endangerment of the structural strength of the building. Assessing the risk requires measuring the moisture content level of the structures. As usable non-destructive methods are not available, dismantling of structures has been needed for reliable evaluation of the risks and the need of repair work. [0003] At present there are several surface moisture meters available. The main drawback of these is that they indicate the moisture of the surface material only. Another widely used method is to make probe holes on the structures and measure the moisture content of the structures at these points. The information thus obtained is difficult to gather and requires lot of work whereby the information is not usable for guiding decision making and usually leads to unnecessary large and at same time insufficient repairs.

[0004] Radar technology has been used for detecting water and moisture in air and ground. Also radar techniques for detecting moisture in building structures have been developed. One method and apparatus is disclosed in US 20050179578. This system includes a transmitter, generating ultra- wideband radio pulses. An ultra- wideband antenna array directs the pulses toward a building wall so that the pulses are reflected, and a receiver receives and processes the reflected pulses. A controller controls the transmitter, receiver and the antenna array and analyses an output of the receiver to detect the presence of moisture inside the wall. Other relating technologies are described in: M. Solla, et al, "Non-destructive testing for the analysis of moisture in the masonry arch bridge of Lubians (Spain)", J. Struct. Control Health Monit. 20 (2013) 1366-1376, CN 101832952 (15.9.2010), which discloses a "Concrete structure moisture content distribution detecting method, involves obtaining moisture content distribution of concrete, and obtaining moisture content of layers of concrete according to propagation speed of radar wave in each layer" and JP 2008076382. These methods are suitable for detecting free water in structures, but are not suitable for effective study of health of structures or early detection of increasing moisture.

SUMMARY OF THE INVENTION

[0005] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims. [0006] According to a first aspect of the present invention, there is provided a method for non-destructive detection of moisture content in building structures, comprising generating a wideband radar signal, sending the radar signal towards a building structure so that signal is reflected, receiving the reflected signal, processing the reflected signal for obtaining a reflection response and comparing the obtained reflection response to a predetermined reflection response of a comparable dry building material.

[0007] According to a second aspect of the present invention, an apparatus is provided comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to perform a method for non-destructive detection of moisture content in building structures, comprising generating a wideband radar signal, sending the radar signal towards a building structure so that signal is reflected, receiving the reflected signal, processing the reflected signal for obtaining a reflection response and comparing the obtained reflection response to a predetermined reflection response of a comparable dry building material. [0008] According to third aspect of the invention, the method includes measuring a dry part of the building structure for obtaining the predetermined reflection response of dry material.

[0009] According to fourth aspect of the invention, the method comprises providing a map of reflection responses of at least a selection of dry building materials for making the comparison to the measured response. [0010] According to fifth aspect of the invention, the method comprises obtaining the moisture content by analysing the frequency spectrum of the response measurement in relation to a frequency spectrum of a comparable dry material.

[0011] According to sixth aspect of the invention, the method comprises measuring the dielectric constant ε of a building material and comparing it to dielectric constant of a comparable dry material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGURE 1 illustrates schematically measurement of moisture over a wall of a building;

[0013] FIGURES 2 - 5 illustrate examples of moisture damages that may be found in building structures.

[0014] FIGURE 6 illustrates the relation between thickness of a water molecule layer to the relative dielectric constant ε_Γ. [0015] FIGURE 7 illustrates water saturation of materials.

[0016] FIGURE 8 describes the relation between the water content and the relative humidity.

[0017] FIGURE 9 describes the relation between the water content and the relative humidity for different materials. [0018] FIGURE 10 describes the change of amplitude and transition of a reflection spectre on moist board in relation to the moisture content.

[0019] FIGURE 11 illustrates schematically an example apparatus capable of supporting at least some embodiments of the present invention.

EMBODIMENTS [0020] DEFINITIONS

[0021] In the present context, the term building depicts any man-made structures in general. [0022] Building materials are any common construction materials including but not limited to wooden boards and planks, veneer, wood fibre board, bricks, mortar, concrete or gypsum plaster.

[0023] The invention uses radar technology and wideband antenna for measuring reflection of radar signal from a building structure in order to obtain indication of moisture content of the structure. The measured signal is compared to a reference signal obtained by measuring a dry spot of the structure or a reference signal or signals measured beforehand. The main indication is dielectric constant and variations of it in a three dimensional structure.

[0024] FIGURE 1 illustrates a self-locating radar scanner in accordance with at least some embodiments of the present invention. The scanner 1 comprises a frame 2 allowing transferring the scanner unit 3 along the wall surface in two dimensions (x, y). The third dimension for the measurement is formed by the beam of the radar and measurements made from the reflected signal. The scanner 1 should preferably be able to transilluminate the whole depth of the wall, floor, roof or other structure that is measured. The invention is not dependent on how the transfer of the scanner unit is performed or how the locating the scanner unit is performed. The frame structure as illustrated in FIGURE 1 and laser locating system can be mentioned for these purposes. The location data is needed for forming an accurate map of the moisture content of the structure in three dimensions.

[0025] FIGURE 2 is an example of moisture damage in the inner surface of a room at the depth of 0 - 20 mm. A rather limited damage area 4 is located close to the corner. FIGURE 3 illustrates the structure at a depth of 50 - 70mm. The wall as well as the heat insulation of the floor show harmful amounts of moisture. The frame posts of the back wall and the insulation of the floor have moisture that did not show on the inner surfaces. FIGURE 4 shows the structures at the depth of 150 - 179 mm. The moisture in insulation reaches to outer insulation layers and has spread along a lower floor beam along at the wall. FIGURE 5 shows the structure at depth of 250 - 270 mm. The sources of moisture can be found on the wooden panelling of the outer wall surface. The moisture has spread from local leaking points to insulation, vertical frame posts and load bearing wooden structures of the floor etc. This is a typical damage and shows that even extensive damages may be very difficult to spot without taking apart the structures of the building. [0026] Radar sends electromagnetic wave that penetrates structures and reflects from boundary surfaces of the structural parts if their electrical properties are different. The propagation speed of the wave in material is c (in vacuum c= 300 000 km/s), and the distance resolution of the radar ΔΓ depends on bandwith B used for the measurement.

In order to obtain sufficient resolution, wide bandwidth is needed (20 mm vs 7,5 GHz). Good resolution is needed for locating the structures and locating the measured values of moisture in crosswise direction of the structure.

[0027] Dielectric coefficient is used to quantify the ability of a material to polarize and weaken the electrical field penetrating it. Relative dielectric constant is dielectric constant of the material in relation to coefficient of vacuum. In general the dielectric constant is complex number where the real part of represent the wave lenght in material and imaginary part losses in material (ε_η, relative dielectric constant; vacuum e_r =1, wood sr =2,5...8, glass ε_η =5...10, concrete ε_η =4...8 , water ε_η =80,). The relative dielectric constant of wet material depends on thickness of the water layer in material. The dielectricity increases when the mobility of the water molecules increases as the thickness of the water molecule layer increases. When the water molecules are engaged to a solid material, the engaged molecules are strongly bonded to the material and as the number of the superposed molecules increases, the mobility of the molecules increases as the relative dielectric coefficient (FIGURE 6). For the strongly bonded molecules of the first layer it is about ε_η =3,2 and already for fifth layer it is ε_η =80,2. Thus, a change in dielectric constant shows rapidly presence of water as the dielectric constant of the material and the material borderline increases.

[0028] The ability of porous material like wood, insulation materials, concrete, bricks) varies, but the relationship between porosity, water content and water bonding follows unlinear behavior shown in FIGURE 7. As the thickness of the water layer increases, increases the relative humidity of a porous material, water content and usability of the water for microbes, molds and decaying agents. The circumstances beneficial for emergence of microbes are known in building industry by temperature, water content or relative moisture. For example, for wood relative moisture content (relative humidity, RH) of 70% is considered critical and at 90%> risks for decay damages are increased. This corresponds water content of 20 - 24 weight-%> in wood.

[0029] The relation between water content and relative humidity is material dependent. Figure 8 shows the principle. At low moisture levels (A) water forms a single layer of molecules. When the relative humidity and moisture content increases, multiple layers of molecules is formed (B) and as the moisture content further increases, water starts to form interconnected layers (C) and pores start to fill by internal capillary condensation. When water starts to fill pores, area (D) of absorbed water begins and when the pores are filled, starts the supersaturated regime where pores are full and free water exists. The amount of free water has a strong relationship for the relative dielectric constant and changes in the dielectric constant can be seen in radar measurements as moisture content increases.

[0030] FIGURE 9 shows some examples of relationship between water content and relative humidity RH. Moisture related problems start usually when relative humidity reaches a level of about 90% RH at which point the amount of free water starts to notable increase. The safe level for most construction materials is about 70% RH.

[0031] FIGURE 10 shows the effect of moisture to the reflected response of the radar excitation. As can be seen from the FIGURE 10, the amplitude of the response increases and the frequency spectrum of the same moves towards lower frequencies. This enables detecting the amount of moisture by analyzing the changes in the reflected power spectrum. If power spectrum for different materials and different moisture contents are available, these can be compared to measured spectrum. The comparison gives the moisture content.

[0032] According to the invention a radar reflection of a structure of a building is measured by a radar and a wide band antenna. The measurement can be performed either in frequency domain or in time domain. If the measurements are performed in frequency domain, the bandwidth must be sufficiently large, for example 2 - 13 GHz. Increase in bandwidth increases the resolution and the minimum bandwidth is thus defined by the resolution needed. If the measurements are performed in time domain, the sent pulse must be sufficiently short (for example 1 ns). A frequency analyze is performed on the measured reflected signal as a function of reflection time. The measurement made in frequency domain can be transformed to time domain by using Inverse Fast Fourier Transform (IFFT) for obtaining information of reflections on different moments of time The original measured frequency response contain sum of all reflections from different reflection points. On the other hand the time response gives reflections on different positions (moments of time), but not the frequency response of individual refelection positions. The Short Time Fourier Transform (STFT) is a technique where one gets information of frequency responses of reflections on different time positions. STFT is done to time domain signal. In order to define the amount of moisture, mixing formulas modelling the dependence of electrical parameters of a material in relation to the amount of moisture in the material can be used. An example of such formula is is presented by Lasri, et al, "Free space moisture measurements of the cellular concrete, Third workshop on electromagnetic wave interaction with water and moist substances, Athens, GA, pp 184- 188.

[0033] The reflected response is used for interpreting the amount of water by comparing it to a response of a dry material. By defining the position of the antenna in relation to the structure to be measured, a three dimensional reflection map of the structure can be obtained. A more thorough analyze can be performed on each point of the map for defining the moisture content more accurately. The amount of moisture can be defined by several methods, for example by analyzing the changes in the reflection spectrum of a resolution element. When moisture enters the structure, its frequency spectrum is transferred on lower frequency. The changes of the reflection spectrum can be depicted in the resolution element by simple numbers, whereby changes can be shown as simple three dimensional map. The invention enables examining local or periodical changes in the structure. Possible problem points can be then detected from the changes.

[0034] The apparatus for implementing at least some embodiments described above include and antenna 8 for sending a wide band radar signal and receiving a response thereto from a target structure 12. The radar signal is produced by a transmitter 10 and the reflected signal is detected by a receiver. The operation of the apparatus is performed by a control unit 11, which may include at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to perform the operational and computational functions needed for implementing at least one embodiment of the invention.

[0035] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0036] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed. [0037] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0038] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0039] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0040] The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0041] At least some embodiments of the present invention find industrial application in examining moisture damages or structural health of buildings and for measurement of moisture in building industry.

Claims

CLAIMS:

1. A method for non-destructive detection of moisture content in building structures, comprising: generating a wideband radar signal, sending the radar signal towards a building structure so that a signal is reflected, receiving the reflected signal, processing the reflected signal for obtaining a reflection response and comparing the obtained reflection response to a predetermined reflection response of a comparable dry building material.

2. A method according to claim 1, comprising measuring a part of the building structure for obtaining the predetermined reflection response of dry material.

3 . A method according to claim 1, comprising providing a map of reflection responses of at least a selection of dry building materials for making the comparison to the measured response.

4. A method according to one of the claims 1 - 3, comprising obtaining the moisture content by analysing the frequency spectrum of the response measurement in relation to a frequency spectrum of a comparable dry material.

5. A method according to the claim 4, comprising obtaining the moisture content by defining the changes in frequency spectrum of the response.

6. A method according to the claim 4, comprising obtaining the moisture content by defining how much the amplitude of the response is changed.

7. A method according to claims 5 and 6, wherein the moisture content is defined by transfer of the spectrum and change of the amplitude.

8. An apparatus for non-destructive detection of moisture content in building structures, comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to perform a method for nondestructive detection of moisture content in building structures, the method comprising generating a wideband radar signal, sending the radar signal towards a building structure so that signal is reflected, receiving the reflected signal, processing the reflected signal for obtaining a reflection response and comparing the obtained reflection response to a predetermined reflection response of a comparable dry building material.