Practice report

Prototype application of photo-optical moisture sensors

This article presents a photo-optical method for measuring the moisture content of porous building materials. A series of moisture diagnostics tests was performed for porous building materials using photo-optical sensors. Such testing is currently unique in the construction sector in the German state of Thuringia. Measurements were carried out during the setting and hardening of various screeds.

The construction sector increasingly relies on the monitoring of the condition of buildings and structures. Both for new projects and existing buildings, the collection of building information is crucial for the purposes of quality assurance, repair and maintenance, and the assessment of the condition of buildings. In the construction industry, emphasis needs to be put not only on latest technological advancements but also on changes in quality assurance, safety and related monitoring measurements.

A new laboratory-scale measuring method was developed at the chair for construction chemistry at Bauhaus University of Weimar. This method makes it possible to determine the moisture content of the pores and their volume without resorting to abstract mathematical models. In addition, this method provides a number of advantages compared to other state-of-the-art porosimetry and moisture measurement procedures. The innovative method is particularly well-suited to meeting the measurement requirements in various revitalization settings (such as determining the most appropriate time for refurbishment or the sustainability/effectiveness of technical measures). In this field, the moisture content needs to be measured at relatively long intervals in defined locations within structural components without significant distortions caused by the interference of, for example, dissolved salts, temperature and surface drying, and corrosive sensor degradation. The photo-optical moisture sensor makes it possible to monitor the moisture content and distribution within the structural component for the purpose of tracking the condition of the building irrespective of the salt content of the brickwork or its temperature. This method is primarily characterized by the previously unknown combination of long-term moisture measurement performed for both original building materials found on-site and reference materials in the actual condition of the component, using an infrared transmission principle implemented in the sensors. In summary, measuring the degree of moisture penetration by means of determining the light transmission of the material provides long-term benefits compared to electrical methods. These benefits are associated with a number of key characteristics that favor prototype application in construction practice:

 

1. Independence of salt content (no color variations caused by salts with a deleterious effect on buildings)

2. Independence of the temperature of the pore solution (no relevant temperature influence)

3. Chemically “neutral” building material measurements (no electrode reaction)

4. Long-term monitoring option

5. Cost-efficient measuring method requiring no maintenance and causing only minimal destruction of the material, requiring little time for preparation and no sophisticated measuring equipment.

 

As part of the transfer of knowledge and technology, the Chamber of Crafts and Trades for eastern Thuringia initiated a project in 2010 that was funded by the Thuringian Ministry for the Economy, Labor and Technology. This project was to provide a measuring method for the long-term monitoring of the moisture content in various building sections. The research concentrates on developing the theoretical basis for utilizing the measuring elements whilst ensuring the possible application in a wide range of settings and materials and considering simple and easy handling, low costs and the provision of parameters that enable quality assurance in the construction trades. The project is being carried out jointly by the Chamber of Crafts and Trades for eastern Thuringia, with the Environmental Center of the Thuringian crafts and trades, and Bauhaus University of Weimar.

 

Photo-optical moisture sensor

The finding that porous building materials used in construction practice show a degree of light transmission that can be determined quantitatively by the moisture content of their pores made it possible to analyze such materials up to a thickness of 9 mm [2]. The new photo-optical sensor provides the advantage of integrating both light source and light detector in a compact, heavy-duty enclosure whilst working independently of the salt content of the pore solution and the temperature of the material. As previously found, electrical conductivity measurements are unable to quantify the degree of moisture present in the material at greater salt contents because the salt concentration significantly influences the resistance of the solution. By contrast, the optical measuring method does not require metal electrodes susceptible to corrosion whilst also excluding electrolysis. Fig. 1 shows the photo-optical moisture sensor setup, which includes the following components:

 

1. Porous building material (PBM)

2. Light source (LED)

3. Photodiode (PD)

4. Contact layer (porous)

5. Mains connection

6. Insulation wall segment

 

The light emitted by the infrared LED is transmitted by the moisture present in the pores; the light intensity is captured by the photodiode. The higher the moisture content, the greater the photodiode‘s potential difference.

In the laboratory, the values captured by each sensor are calibrated using various degrees of moisture penetration. A diamond circular saw was used to cut the building material samples into parallel panels with thicknesses of only a few millimeters. Calibration was carried out using de-ionized water. In the course of the evaporation process, the PBM was set to a certain moisture content at room temperature, and monitored gravimetrically.

The compact size of the photo-optical moisture sensor makes it possible to easily apply these sensors to the measuring points during the erection and assembly of the component, or to insert them subsequently into an existing component through a small coring [1, 2, 4]. Thereafter, the moisture content equalizes between the porous building material of the sensor and the building material to be tested, which makes it possible to determine the moisture content of the latter. The design and technology used for the photo-optical moisture sensor may vary. Single- and multi-segment moisture sensors or a geometrically defined arrangement of several sensors may be used. Fig. 2shows the application of several photo-optical moisture sensors placed in brickwork. Such an arrangement enables not only moisture measurement as such but also the capture of moisture distribution across the entire depth of the component to be tested.

 

Preliminary tests of building materials

Several building materials with varying properties were used for the purpose of characterizing their pore systems by photo-optical measurements:

» CAF-C25-F5 calcium sulfate float finish (Maxit)

» CT-F4 cement screed, 1:4 mixing ratio, CEM I 42.5 N Portland cement

» Lime mortar (MGII)

The effective porosity of these materials lies between 16 and 30%. In addition, the porous building materials show distinctive different distributions of pore radii. Table 1 contains the parameters of the tested building materials.

The effective porosity of the specimens PVol. (measured in percent by volume) was calculated using Equation 1; the degree of moisture penetration (Dfg) measured during the tests was calculated using Equation 2.

 

m specimen mass in kg (t – trocken = dry, fg – feucht-vollgesättigt = moist, fully saturated)

VPr specimen volume in m³

rW specific density of water

PVol. = rW · (mfg – mt) · 100%

VPr.(1)

 

Dfg = m – mt · 100%

mfg – mt(2)

 

Table 1 Material parameters

SpecimensDry bulk density r [g/cm³]Effective porosity PVol. [%]Cement screed2.1716.1CaSO4 float finish 1.9221.6Lime mortar1.9329.3

  

Measurements

This type of photo-optical testing carried out for several screed finishes is unprecedented because no proof has been established to date that the moisture content of porous building materials, such as screed or lime mortar, was determined by means of photo-optical measuring methods.

Prior to utilizing the photo-optical sensor under real-life conditions, the behavior of the test materials as a result of water evaporation over time was to be determined. In the course of the hardening process of the individual screed finishes, the change in the mass of wet specimens over time was measured in the laboratory using identical vessels (10 x 10 x 3 cm³). Fig. 3 shows the loss of mass as a function of time for three specimens. The diagram shows that the evaporation process is progressing much more slowly than the optical recording times during the test, which lie between two and three seconds. This means that the mass difference cannot be detected during the measurement.

Both ambient temperature and humidity were simultaneously measured during the testing of the photo-optical moisture sensor under real-life conditions.

Measurements were carried out at room temperatures of 17.5 ± 2 °C. Fig. 4 reveals that temperature fluctuations are not periodic, and that an increase was observed only for a relatively short midday period. Relative humidity was also measured continuously (Fig. 5). It initially amounted to about 60% and decreased to under 40% in the course of the measurements. Such a trend in the measured values shows that the drying processes documented for the two screed samples and the brickwork influence the degree of indoor humidity.

With a view to expanding the range of application, it was necessary to measure the values captured by the photo-optical sensor in fresh screed (prior to the formation of pores) during its setting and hardening in order to analyze the sensor for the first stage of screed hardening. The light absorption coefficient increases as the degree of moisture penetration decreases, and the photodiode voltage changes (the photodiode detects the light emitted by the LED). Fig. 6a shows several photo-optical sensors, Fig. 6b illustrates the test setup used for the measurements. Fig. 7b shows a cement screed in the front and a calcium sulfate float finish in the rear part of the image.  

Fig. 7 shows the voltage of the photodiode recorded over time for the setting and hardening of the cement screed whereas Fig. 8 shows this voltage over time for the calcium sulfate float finish.

The tests clearly demonstrated that the photo-optical sensor is very sensitive. The drying processes of the screed specimens were captured by the photo-optical sensor under identical conditions. The experiments show that the equalization of moisture between the sensor material and the tested building material occurs relatively quickly. At the same time, the experiments revealed that the photo-optical moisture sensors were capable of capturing the required moisture content in the screed specimens, for example 1.9 m.-%. In the second part of the test, a specimen to which a photo-optical sensor was attached was cut out of the cement screed and moistened thoroughly again. Then the specimen could dry on all sides. Fig. 9 shows the specimen (a) and the dimensions of the specimen with both sensors (b).

Such measurements make it possible to calibrate the photo-optical sensor. The calibration curve includes the values recorded during the drying process. The measurements enabled a photo-optical sensor calibration curve to be established that ranged from 100 to 10 percent by volume of moisture in the screed.  

Another test was carried out at a brick wall using two photo-optical moisture sensors. The sensors were placed in the cement mortar joints vertically on top of each other (Fig. 10). The brickwork segment was placed in a tub filled with water so that moisture could penetrate from the bottom to the top. The tests were repeated four times and showed that the humidity sensor responded reproducibly measured data (Fig. 11)

 

Summary

A series of tests was carried out using a photo-optical moisture sensor that was tested, for the first time, under real-life conditions on construction sites in Thuringia. The tests reveal that the photo-optical moisture sensors are capable of measuring the moisture content also at levels below 1.9 m.-%. The sensors may be placed at the measuring points during the on-site erection of the component or subsequently inserted through a small coring. The degree of moisture penetration measured as a function of light transmission provides several advantages compared to electrical methods:

 

1. Independence of salt content (no color variations caused by salts with a deleterious effect on buildings)

2. No relevant temperature influence of the pore solution

3. Chemically “neutral” measurements in the building material

4. Long-term monitoring option

5. Cost-efficient measuring method requiring no maintenance and causing only minimal destruction of the material, requiring only little time for preparation and no sophisticated measuring equipment.

 

The design and technology used for the photo-optical moisture sensor may vary. In future, it will be possible to use more sensitive photo-optical sensors in order to expand the detectable range of moisture penetration. ¢

O. Bakhramov, P. Höhn, I. Hohle, Ch. Kaps

References/Literatur

[1] Bakhramov, O.: Lichtoptische und impedanzspektroskopische Charakterisierung von offenporigen, feuchte- und salzhaltigen Bauwerkstoffen, Dissertation, Bauhaus-Universität Weimar, 2009, Weimar, (Eigen-)Verlag, ISBN: 978-3000-335778

[2] Bakhramov, O., Goretzki, L., Kaps, Ch.: Neues Verfahren zur Feuchtemessung in Baustoffen, WTA-Blatt, Band 2, September 2007

[3] Bakhramov, O.: Untersuchungen des Porenvolumens von Bauwerkstoffen mittels lichtoptischer Methode, Europäisches Sanierungsbuch 2010, Februar 2010, 153-165, ISBN: 978-3410-175216

[4] Bakhramov, O., Kaps, Ch., Samigov, N.: Lichtoptischer Feuchte-Sensor und seine Anwendung, IBAUSIL – 17. Internationale Baustofftagung, Weimar, September 2009, 2.  Band, 1121-1126↓

Acknowledgements

We would like to thank the Thuringian Ministry for the Economy, Labor and Technology (Department of Crafts and Trades) and the Chamber of Crafts and Trades for eastern Thuringia for the project funding and support provided. Also we thanks the staff of the educational network Bau- u. Ausbau gGmbH Gera.

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