A comparison of the capacitance method and the microwave impulse method for the determination of moisture profiles in three typical porous building materials is presented in this paper. The basic principles of the measuring methods are given. The calibration process is described in detail. On the basis of the measured results, it can be concluded that the capacitance method provides better accuracy in the range of lower moisture content than to the microwave impulse method, which is more accurate for the highest values of moisture content.

Pavlík, J.

Tesárek, P.

Černý, R.

Acta Polytechnica

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

1 IntroductionMeasuring transient moisture profiles is considered asa common and effective tool for determination of the liquidmoisture diffusivity of porous building materials. In classicalexperimental setups rod specimens that are water and vaporproof insulated on all lateral sides (parallel to the direction ofmoisture transport) are placed in either horizontal or verticalposition. Then, they are brought into contact with thewater byone of their face sides. Beginning from this time, the moisturecontents are measured at different positions and time inter-vals during the experiment, producing moisture content pro-files versus time. Finally, the measured moisture profiles areanalyzed using the methods of inverse analysis, and the mois-ture diffusivity vs. moisture content function is determined.There are a variety of methods for determination of mois-ture content. The most frequently used in building physicsare the -ray attenuation technique [1–3] and the NMRtechnique [4–5]. Other commonly used techniques includethe capacitance method [6], positron emission tomography[7], neutron radiography [8] and the microwave method [9].The TDR technique [10] originally used in soil science andX-ray radiography, evolved from the well-known medicaltechnique [11-12] can also be applied to building materials.In this paper, we use the capacitance method and the mi-crowave impulse method to determine the moisture profilesin porous building materials.2 Materials and samplesThree typical building materials, namely cement paste,ceramic brick and autoclaved aerated concrete, were tested.The cement paste was prepared using Portland cement CEMI 32.5 R (ENV 197-1) (Horní Srní, CZ) and water. The waterto cement ratio w  0.3 was chosen in our experiments. Thebulk density of the cement paste was 1910 kgm3. The ce-ramic brick was produced by the brick kiln at Nebužely, CZ.The bulk density of the ceramic brick was 1720 kgm3. Theautoclaved aerated concrete (AAC) was produced by Ytong(Laussig, Germany). Its bulk density was 650 kgm3.For the measurements of the moisture profiles we used thefollowing samples: 6 specimens 20×40×280–300 mm foreach measured material and each method. The samples wereinsulated on all lateral sides by water- and vapor-proof plasticfoil. They were stored in laboratory conditions at a tempera-ture of 25 °C and relative humidity of 50 %.42 ©  Czech Technical University Publishing Housee http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 45  No. 1/2005 Czech Technical University in PragueComparison of the Capacitance Methodand the Microwave Impulse Method forDetermination of Moisture Profiles inBuilding MaterialsP. Tesárek, J. Pavlík, R. ČernýA comparison of the capacitance method and the microwave impulse method for the determination of moisture profiles in three typical porousbuilding materials is presented in this paper. The basic principles of the measuring methods are given. The calibration process is described indetail. On the basis of the measured results, it can be concluded that the capacitance method provides better accuracy in the range of lowermoisture content than to the microwave impulse method, which is more accurate for the highest values of moisture content.Keywords: Moisture profiles, capacitance method, microwave impulse method, building materials.Fig. 1: Block diagram of the capacitance device3 Experimentals methods3.1 Capacitance methodThe capacitance device designed in [6] was used for themeasurements. A block diagram of the device is shown inFig. 1. The low-voltage supply drives an oscillator with a 400kHz working frequency, which has constant output voltagefeeding a circuit where the measuring capacitor (with the ana-lyzed sample as a dielectric) is connected in series with a resis-tance. On this resistance, the voltage after rectifying is deter-mined. This depends significantly on the moisture content ofthe dielectric. The relation between the moisture content andthe voltage measured on the resistance can be determined bya calibration. The measured voltage increases with increasingcapacity. Proper choice of the resistance can ensure that thedependence of the measured voltage on the capacity is linearin the range of approximately one or two orders ofmagnitudeof the capacity. The voltage is recorded at specified time inter-vals by a data logger.The described capacitance moisture meter was equippedwith electrodes in the form of parallel plates with dimensions20×40 mm. Moisture meter readings along the rod specimenwere taken every 5 mm in order to achieve certain space aver-aging of the results and reduce the effect of inhomogeneitiesof the material.The experimental setup is shown in Fig. 2. The specimenis fixed in horizontal position in order to eliminate the effectof gravity on the moisture transport. The lateral sides of thespecimen are water and vapor-proof insulated in order tosimulate 1-D water transport. A viscous sponge ensuring agood contact of the surface of the specimen with the water isput into a Perspex water-filling chamber and applied to oneface side of the specimen. The sponge sucks water from a freesurface, being about 1 cm below the lower side of the speci-men. The water in the chamber is maintained on constantlevel using a float. If the water level in the filling chamber de-creases due to water suction by the specimen, the water levelin the float chamber decreases in the same way. The needle ofthe float opens the hole in the cover of the float chamber andwater from a burette flows through the hole into the floatchamber until the needle closes the hole again due to theincrease in the water level. In this way, a continuous water sup-ply to the measured specimen is achieved.For a particular material, the calibration curve of the ca-pacitance moisture meter is usually determined in advanceusing the gravimetric method. In this case, we chose anothermethod. The calibration was done after the last moisture me-ter reading, when the moisture penetration front was at aboutone half of the length of the specimen, using this last readingand the results of the standard gravimetric method measure-ments after cutting the specimen into pieces 1 cm in width.The final calibration curve for each material was constructedusing the data for 3–6 samples. In the regression analysis, alogarithmic function was found as the best approximationto the measured data. An example of the calibration curve ispresented in Fig. 3.3.2 Microwave impulse methodThe measuring system designed in [13] was used for theexperiments. It is relatively compact and consists of threebasic components (see Fig. 4), namely an impulse generator,an applicator and a sampling oscilloscope.The GPSI-1a generator (Radan, Ltd.) produces triangularimpulses 250 ps in width and with an amplitude of 2 V. Theapparatus consists of the impulse generator itself, its feedcircuits, controlling, auxiliary and protecting circuits. Theenergy output is realized by three SMA coaxial connectors.These signals make it possible to determine the reference andmeasured position of the impulse and to synchronize the sam-pling oscilloscope.The applicator connected to the generator output ensuresthe necessary exposure of both measured and reference speci-mens. It consists of two pairs of transmitting and receivingantennas formed by coaxial/waveguide reducers and horns.The pairs of antennas are fixed parallel in separate holdersensuring a defined position, and therefore also stability andreproducibility of measurements. The specimens of the testedmaterials are put into the applicator between the measuringantennas. The sample thickness is limited mechanically toabout 100 mm. From the electric point of view it is limited bythe attenuation in the measured material and the sensitivityof the oscilloscope. The dynamics of the signal is over 20 dB.The Tektronix 7603 sampling oscilloscope analyses theimpulse signals. It has a 7T11A sampling sweep unit and two©  Czech Technical University Publishing Housee http://ctn.cvut.cz/ap/ 43Czech Technical University in Prague Acta Polytechnica Vol. 45  No. 1/2005Fig. 2: Experimental setup of the capacitance methody = 0.0752Ln(x) - 0.229R2 = 0.99040.000.020.040.060.080.100.120.140.160.180 50 100 150 200Moisture meter readingMoisturecontent[kgkg-1]Fig. 3: Calibration curve of the capacitance method for cementpaste7S11 sampling units with a S-4 sampling head. The timeresolution of the oscilloscope is about 10 ps, and the sensitiv-ity is 2 mV. The frequency range is up to 14 GHz. The signalfrom the oscilloscope display is recorded by a digital cameraand analyzed by a PC.The experimental setup was very similar to that for the ca-pacitance method. The details of the setup are shown inFig. 5. Scanning by the microwave impulse moisture meteralong the specimen was done every 10 mm.Similarly as with the capacitance method, the calibrationcurve was determined using the results of the last scan and thedata obtained by the standard gravimetric method after cut-ting the specimen into pieces 10mm inwidth. The calibrationcurve was constructed as the dependence of the moisturecontent on the permittivity of the measured material. Thepermittivity of thematerial was calculated on the basis ofmea-suring the time difference   t t t21 2 1  , where t2 is thetravel time of the impulse to pass the thickness of a measuredspecimen and t1 is the respective travel time in the air (see[13] for details of the calculation procedure). As in the case ofthe capacitance method, here too a logarithmic function wasfound to be the best approximation of the measured data. Anexample of the calibration curve is shown in Fig. 6.4 Experimental results and discussionFig. 7 presents typical moisture profiles in cement pastespecimens measured by the capacitance method and the mi-crowave impulse method. A comparison of the two sets of44 ©  Czech Technical University Publishing Housee http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 45  No. 1/2005 Czech Technical University in PragueFig. 4: Block diagram of the microwave impulse methodFig. 5: Experimental setup of the microwave impulse methody = 0.1251Ln(x) - 0.1929R2 = 0.9450.000.020.040.060.080.100.120.140.160.180 5 10 15 20PermittivityMoisturecontent[kgkg-1]Fig. 6: Calibration curve of the microwave impulse method forcement pastecurves shows three basic features. First, the agreement of theresults obtained by the two methods is reasonably good forlonger times, but for short times from the beginning of the ex-periment it is less good. Second, the agreement is better forhigh moisture content than in the low moisture range. Third,the data scattering seems to be higher for the microwave im-pulse method than for the capacitance method.However, an exact direct comparison of the measuredmoisture profiles is difficult in general. Due to the characteris-tic features of each technique, the experimental data may beobtained for different positions and different time steps.Therefore, making an exact comparison of a profile at a cer-tain time step would only be possible by an interpolation inthe time domain. To overcome this problem the Boltzmanntransformation was applied to the experimental data. Foreach technique the obtained moisture content versus distanceprofiles was replotted as a moisture content versus  profile,with xt 1 2.If the Boltzmann conditions are fulfilled (a constantboundary condition applied to a semi-infinite homogeneous©  Czech Technical University Publishing Housee http://ctn.cvut.cz/ap/ 45Czech Technical University in Prague Acta Polytechnica Vol. 45  No. 1/20050.000.020.040.060.080.100.120.140.160.180.00 0.05 0.10 0.15 0.20Position [m]Moisturecontent[kgkg-1]... t=3600 st=9000 st=76800 st=97800 st=160800 st=177000 st=183600 st=246900 st=263400 s0.000.020.040.060.080.100.120.140.160.180.00 0.05 0.10 0.15 0.20Position [m]Moisturecontent[kgkg-1]t=76920 st=161100 st=183720 st=247140 st=261420 sa)b)Fig. 7: Typical moisture profiles in cement paste specimens measured by a) the capacitance method, b) the microwave impulse methodmedium that is initially at a uniform moisture content), all themeasured moisture profiles should fall on a single -profile.Figures 8–10 compare the Boltzmann transformed exper-imental data of the capacitance method, the microwave im-pulse method and the gravimetric method for all three mate-rials. In all cases, the Boltzmann transformation seems tohold very well. However, a systematic difference between thecapacitance method and the microwave impulse method incomparison to the gravimetric technique can be observed.The capacitance method shows a better agreement with thegravimetric method for lower values of moisture content,where the samples are almost dry (naturally wet). On theother hand, the microwave impulse method gives resultscloser to the gravimetric measurements in the range of high-est moisture content.The reasons for these differences lie in the physical back-ground of the two methods. The accuracy of the capacitancemethod depends mainly on achieving an ideal contact of thesample surface with the probe, because any appearance of airbubbles on the surface can damage the accuracy substantially(two capacitors in series, one of them with very low permit-tivity of the dielectric). Once a good contact is achieved, theaccuracy of the method depends just on the accuracy of volt-age measurement, which is only seldom a limiting factor.Therefore, the accuracy should be comparable for both highand low moisture content.The microwave impulse method in the presented setupis a transmission-based technique. Its accuracy dependsmainly on the precision of measuring the time difference  t t t21 2 1  (see above). The permittivity of boundwater,which prevails in the material in the range of low moisturecontent, is about 20 times lower than the permittivity of freewater. Therefore, for low moisture content the time differenceis much lower than for high moisture content, and the sensi-tivity and accuracy of the method is also substantially lower.5 ConclusionsBoth the capacitance method and the microwave impulsemethod were shown to be well applicable for regular determi-nation of moisture profiles in porous building materials.However, taking into account the particular advantages ofeach method, in order to achieve the highest possible ac-curacy the capacitance method can be recommended formeasurements where lower moisture content is expected,while the microwave impulse technique is better for highermoisture content measurements.AcknowledgmentsThis research has been supported by the Czech ScienceFoundation, under grant No. 103/03/0006.References[1] Nielsen, A.: “Gamma-ray Attenuation Used forMeasuring the Moisture Content and Homogeneity ofPorous concrete.” Building Science, Vol. 7, (1972),p. 257–263.[2] Kumaran, M. K., Bomberg, M.: “A Gamma-spectrome-ter for Detemination of Density Distribution and Mois-ture Distribution in Building Materials.” Proceedings ofthe International Symposium on moisture and humid-ity, Washington DC, 1985, p. 485–490.[3] Descamps, F.: “Continuum and Discrete Modelling ofIsothermal Water and Air Transfer in Porous Media.”PhD thesis, KU Leuven, 1997. Leuven, Belgium.[4] Gummerson, F. J., Hall, C., Hoff, W. D., Hawkes, R.,Holland, G. N., Moore, W. S.: “Unsaturated Water Flow46 ©  Czech Technical University Publishing Housee http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 45  No. 1/2005 Czech Technical University in Prague0.000.020.040.060.080.100.120.140.160.180.200 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006xt-1/2[ms-1/2]Moisturecontent[kgkg-1]capacitance methodmicrowave methodgravimetric methodFig. 8: Boltzmann transformed experimental data of the capaci-tance method, the microwave impulse method and thegravimetric method for cement paste0.000.020.040.060.080.100.120.140.160.180.200 0.0005 0.001 0.0015 0.002xt-1/2[ms-1/2]Moisturecontent[kgkg-1] capacitance methodmicrowave methodgravimetric methodFig. 9: Boltzmann transformed experimental data of the capaci-tance method, the microwave impulse method and thegravimetric method for ceramic brick00.10.20.30.40.50.60.70.80.90 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006xt-1/2[ms-1/2]Moisturecontent[kgkg-1] capacitance methodmicrowave methodgravimetric methodFig. 10: Boltzmann transformed experimental data of the capaci-tance method, the microwave impulse method and thegravimetric method for autoclaved aerated concretewithin Porous Materials Observed by NMR Imaging.”Nature, Vol. 281 (1979), p. 56–57.[5] Pel, L.: “Moisture Transport in Porous Building Mate-rials.” PhD thesis, TU/e, Eindhoven, 1995, Netherlands.[6] Semerák P., Černý R.: “A Capacitance Method for Mea-suring the Moisture Content of Building Materials.”Stavební obzor, Vol. 6 (1997), p. 102–103 (in Czech).[7] Hoff, W. D., Wilson, M. A., Benton, D. M., Hawkesworth,M. R., Parker, D., Fowles, P.: “The Use of Positron Emis-sion Tomograhy to Monitor Unsaturated Flow withinPorous Construction Materials”. Journal of Materials Sci-ence Letters, Vol. 5 (1996), p. 1101–1104.[8] Pel, L., Ketelaars, A. A. J., Odan, O. C. G., Well, A. A.:“Determination of Moisture Diffusivity in Porous MediaUsing Scanning Neutron Radiography.” InternationalJournal of Heat and Mass Transfer, Vol. 36, (1993),p. 1261–1267.[9] Hasted, J. B., Shah, M. A.: “Microwave Absorption byWater in Building Materials.” Brit. J. Appl. Phys., Vol. 15,(1964), p. 825–836.[10] Plagge, R., Grunewald, J., Haüpl, P.: “Application ofTime Domain Reflectometry to Determine Water Con-tent and Electrical Conductivity of Capillary PorousMedia.” In: Proceedings of the 5th symposium on Build-ing Physics in the Nordic Countries: Göteborg, Sweden.1999, p. 337–344.[11] Van Besien, T.: “Experimentele bepaling van de vocht-diffusiviteit met behulp van X-straal radiografie.” (Ex-perimental Determination of Moisture Diffusivity UsingX-ray Radiography). MSc Thesis, KU Leuven, 2001,Belgium.[12] Roels, S., van Besien, T., Carmeliet, J., Wevers, M.:“X-ray Attenuation Technique for the Analysis of Mois-ture Flow in Porous Building Materials.” In: Second In-ternational Conference on Research in Building Physics,J. Carmeliet, H. Hens, G. Vermeir (eds.), A.A. BalkemaPublishers, Lisse: 2003. The Netherlands, p. 151–157.[13] Pavlík, J., Tydlitát, V., Černý, R., Klečka, T., Bouška, P.,Rovnaníková, P.: “Application of a Microwave ImpulseTechnique to the Measurement of Free Water Contentin Early Hydration Stages of Cement Paste.” Cement andConcrete Research, Vol. 33, (2003), p. 93–102.Ing. Pavel Tesárekphone:+420 2 2435 5436e-mail: tesarek@fsv.cvut.czIng. Jaroslav Pavlíkphone:+420 2 2435 5436e-mail: pavlikj@fsv.cvut.czProf. Ing. Robert Černý, DrSc.phone:+420 2 2435 4429Email: cernyr@fsv.cvut.czDepartment of Structural MechanicsCzech Technical University in PragueFaculty of Civil EngineeringThákurova 7166 29 Prague, Czech Republic©  Czech Technical University Publishing Housee http://ctn.cvut.cz/ap/ 47Czech Technical University in Prague Acta Polytechnica Vol. 45  No. 1/2005

Comparison of the Capacitance Method and the Microwave Impulse Method for Determination of Moisture Profiles in Building Materials

A comparison of the capacitance method and the microwave impulse method for the determination of moisture profiles in three typical porous building materials is presented in this paper. The basic principles of the measuring methods are given. The calibration process is described in detail. On the basis of the measured results, it can be concluded that the capacitance method provides better accuracy in the range of lower moisture content than to the microwave impulse method, which is more accurate for the highest values of moisture content

Comparison of the Capacitance Method and the Microwave Impulse Method for Determination of Moisture Profiles in Building Materials

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