Monitoring Moisture Content in Autoclaved Aerated Concrete as a Mean to Achieve Higher Durability

Transcription

1 Monitoring Moisture Content in Autoclaved Aerated Concrete as a Mean to Achieve Higher Durability Zbyšek Pavlík 1 Lukáš Fiala 2 Zbigniew Suchorab 3 Robert Černý 4 T 11 ABSTRACT Autoclaved aerated concrete (AAC) is a structural material which is commonly used around Europe, particularly as it combines ease of construction with excellent combination of its mechanical and thermal properties. However, the empirical principles employed in construction design until now have led in many countries to a series of failures which are beginning to have serious consequences for the practical applications of the material. High moisture content in AAC blocks in the construction phase belongs to most serious flaws because it can lead to hygric shrinkage related problems in the structure. In this paper, the time domain reflectometry (TDR) method is presented as a universal method for monitoring moisture content in AAC both during the production phase and in situ. TDR can be generally classified as a dielectric method, based on an analysis of the behavior of dielectrics in a time-varying electric field, and consists in the measurement of permittivity of moist porous media. In contrast to most methods used for determination of moisture content, TDR method does not require calibration for every material. Another advantage of the TDR method is that it is well applicable for the materials with higher salinity, where an application of methods such as the resistance method or the capacitance method is impaired by a significant loss of accuracy. TDR also enables continuous long-term non-destructive monitoring of moisture content in constructions. KEYWORDS Autoclaved aerated concrete, Moisture content, Time-domain reflectometry method Czech Technical University, Faculty of Civil Engineering, Department of Materials Engineering and Chemistry, Thákurova 7, Prague 6, Czech Republic, Phone: , Fax: , pavlikz@fsv.cvut.cz Czech Technical University, Faculty of Civil Engineering, Department of Materials Engineering and Chemistry, Thákurova 7, Prague 6, Czech Republic, Phone: , Fax: , fialal@fsv.cvut.cz Lublin University of Technology, Faculty of Civil and Sanitary Engineering, Department of Environmental Protection Engineering, Nadbystrzycka 40b, Lublin, Poland, Phone: , zibi@fenix.pol.lublin.pl Czech Technical University, Faculty of Civil Engineering, Department of Materials Engineering and Chemistry, Thákurova 7, Prague 6, Czech Republic, Phone: , Fax: , cernyr@fsv.cvut.cz

2 1 INTRODUCTION In spite of the autoclaved aerated concrete (AAC) is a structural material commonly used around Europe, high frequency of AAC structures defects has become well known among the potential investors what leads in consequence to the weakening of the position of AAC on the European building materials market which is unfortunate because the material can be considered as environmentally friendly and has considerable potential for future applications. The new precise walling technologies (thin joint systems) lead to much better thermal properties of the envelopes because the thermal bridges characteristic for classical brick-like walling with AAC are practically excluded, and this results in significant savings of both heat energy and the amount of material necessary for construction. The main problem with the current applications of AAC is that the major European AAC producers (with their whole corporate chain in different European countries including Czech Republic) restrict themselves just to the advertisement of their products and neglect further research work. They often prescribe unrealistic conditions of walling for instance concerning the initial moisture content in AAC blocks. Unsuccessful applications of user-unfriendly technologies offered by the AAC producers logically lead to a negative response of both designers and investors, and if the producers would insist on these improper technologies, the development could aim to a dead end. In the current building practice one can find different walling technologies of AAC structures based on classical walling principles or on precise-walling technology (thin joint system). The differences are as follows. In the classical way of walling the AAC blocks are built using standard mortar joint of 10 mm nominal size. The AAC walls are built on site in standard masonry construction and productivity can often be low. Problems such as the use of overstrong mortar or missing movement joints can lead to cracking of AAC walls. However, problems are generally not found with classical AAC masonry providing that good practice is followed. Precise walling requires the use of mortar joints that are not greater than 3 mm thick, typically 1-2 mm. The blocks are often larger than those used in classical walling. The potential benefits of precise walling are improved productivity and better thermal performance of the completed building. The system can be constructed faster than classical systems and it does not rely so heavily on traditional masonry methods. The precise walling also generates less waste from both blocks and the mortar on site and the amount of mortar used is much less than in classical walling. These precise-walling systems are often referred to as thin joint AAC blockwork. The precise-walling technologies are being employed over Europe with an increasing frequency but they are still considered as innovative. Compared to the classical walling technologies (large joints), precise walling technologies lead to much better thermal properties of the envelopes because the thermal bridges characteristic for classical brick-like walling with AAC are practically excluded. Therefore, the use of precise-walling technologies should be encouraged and a growth in the use of the technology should be expected. However, good design parameters are required for precise-walling AAC with render systems. There are clearly benefits to be gained from precise-walling AAC but problems have become evident and this could diminish the growth and use of the technology. When problems do occur they are often due to the differences between AAC and the traditional renders that are used. In particular, the moisture conditions and transfer between the materials often determines the suitability of the AAC system with a particular render finish. The design of AAC building envelopes has mostly been done by empirical rules for construction until now. As a result, approaches to design have differed case by case and failures have resulted as a result of incorrect specifications and errors in construction. Typical failure examples are cracking of both

3 external and internal finishes, detachment of renders from the AAC block (sometimes with the AAC itself), cracks around windows and doors, frost failure of external render. Experience has shown that reasons of defects were due to the use of methods for brick structures being employed by the designers for AAC blockwork. However, the brick-based technologies which were proven by their successful applications over centuries cannot be applied to anything other than brick structures. The properties of AAC differ from those of bricks to such a degree that any analogy between systems based on AAC and on bricks has always to be doubtful. The current standards cannot help to the designers in preventing failure. Moisture analysis in both Czech and European codes is restricted to water vapor transport in steady-state conditions. This presents a risk for a designer as water suction, water vapor convection, and the cross effects between heat and moisture transport are not considered and this can often lead to the underestimation of the amount of liquid water in the envelope or the condensation zone can appear in another place than predicted by the standard calculation. Hygric and thermal movements are also not considered in standards and the designer is not required to evaluate the thermo- and hygro-mechanical response of the envelope. On this account, there is very difficult to perform durability and service live assessment of AAC-based envelope systems because complex view on AAC material performance is missing and a precise and serious analysis of hygro-thermo-mechanical performance of the new AAC technologies based on sound scientific knowledge is a very actual problem. From the above given information results the necessity to determine moisture transport and storage parameters of AAC that will enable to estimate its hygric performance. Since the experimental assessment of hygric material parameters requires measurement of liquid moisture content, the suitable moisture measurement method for application for AAC has to be chosen and calibrated. Because water possesses many anomalous properties that also affect the properties of materials, various methods of determination of moisture content were devised and various moisture meters constructed which can take advantage of it. Basically, the measuring methods can be divided into two main groups; absolute and relative methods. The absolute methods (or direct methods), which are based on the removal of water from the specimen (by drying, extraction, etc.) are usually used as reference methods for calibration of relative methods. The disadvantage of these methods consists in fact that they are time consuming and they are not applicable for instantaneous determination of moisture profiles. Therefore, relative methods which determine the amount of water in material on the basis of measuring another physical quantity (permittivity, electrical conductivity, absorption of radiation energy etc.) are widely used in technical practice. However, the application of particular methods is limited for prescribed types of materials and specific conditions of measurement. For instance, frequently used resistance moisture meters are practically inapplicable for materials with a higher amount of salts, because the measuring errors rapidly increase with the increasing moisture content. The same problem can be found for capacitance moisture meters working on lower frequencies. On the basis of the theoretical analysis of the currently used moisture measurement method we have chosen for the moisture measurement in AAC the time domain reflectometry (TDR) method that has proved to be very reliable for various applications and materials. Already in 1930s it became a recognized technique in cable testing. Other applications were focused on investigation of electrical properties of liquids. A fast development of the TDR technology was initiated in 1980s in soil science where the method found an increasing use in soil moisture measurement (see e.g. [Topp et al., 1980], [Dalton & van Genuchten, 1986]). Nowadays is the TDR technique frequently used in many fields for instance for moisture measurement in building materials and structures [Kupfer & Trinks, 2005], [Kupfer et al., 2007], [Aghaei et al., 2005], for monitoring of quality of food products [Schimmer et al., 2007], etc.

4 Since the possibilities of using TDR method for measurement of moisture content were proven for wide range of materials, in this paper we present the TDR method as a universal method for monitoring moisture content in AAC both during the production phase and in situ. 2 TIME-DOMAIN REFLECTOMETRY METHOD TDR method can be generally classified as a dielectric method, based on an analysis of the behavior of dielectrics in a time-varying electric field, and consists in the measurement of permittivity of moist porous media. The determination of moisture content using the permittivity measurements is based on the fact that the static relative permittivity of pure water is equal to approximately 80 at 20 C, while for most dry building materials it ranges from 2 to 6 [Katze, 1989], [Owen et al., 1961]. The permittivity of materials is strongly affected by the orientation of molecules in the electric field. This characteristic is high for water in gaseous and liquid phase, but is significantly lower for water bound to a material by various sorption forces, which makes the orientation of water molecules more difficult. This feature makes it possible to distinguish between the particular types of bond of water to the material using the permittivity but on the other hand, it results in the dependence of the sensitivity of moisture measurements to the amount of water in the material. The relative permittivity of water bonded in a monomolecular layer is approximately 3.1, but for further layers it increases relatively fast. Therefore, the dependence of relative permittivity on moisture content is generally characterized by a more or less gradual change at the transition from a monomolecular to a polymolecular layer. Consequently, the methods of moisture measurements based on the determination of changes of relative permittivity have lower sensitivity in the range of low moistures where their application is rather limited. The principle of TDR device consists in launching of electromagnetic waves and the amplitude measurement of the reflections of waves together with the time intervals between launching the waves and detecting the reflections. The fundamental element in any TDR equipment used for the determination of moisture content in porous materials is a device to observe the electromagnetic pulse echo in time domain. The method application originates from the application of electric cable tester. The measuring device usually consists of four main components: a step or needle pulse generator, a coaxial cable wave guide, a sampler and an oscilloscope to register or visualize the trace of echo. The pulse generator produces the electromagnetic wave that propagates through the measured medium. The Fourier transform of an electrical pulse consists of sine waves covering a large frequency range but dependent on the shape of pulse. The highest frequency present in the pulse depends on its slew rate. This means that step pulse and needle pulse can be used equivalently if their rise time is comparable. A very important part of TDR equipment is the probes. Rods of the probes are the signal conductors. There are a lot of probe constructions available for TDR measurements. They generally differ in shape, material and number of rods but general idea is that TDR probe is an extension of the coaxial cable with specified geometry. The TDR probe itself is conductively connected to the coaxial cable in such a way that the cable is open ended and the probe forms this open end. In principle, the coaxial cable and the probe differ not only in the shape but also in a type of dielectric material. While the cable has usually polyethylene as a dielectric, the measured material serves as a dielectric of the probe. Thus, the cable dielectric is nearly ideal but the measured moist material usually contains dissolved salts and therefore conducting current appears. This is, however, not disturbing the measurement because of the high frequency of the pulse. The sampler detects the electromagnetic waves launched by the pulse generator and transmitted through the coaxial cable and TDR probe system. TDR meter consists of two main components, a high precision timing device and a high precision voltmeter. When the electromagnetic waves launched by

5 the generator are detected by the sampler, the sampler measures the voltage between the shield and the conductor at a certain time interval. The set of data obtained consists of voltage as a function of time when the transmitted pulse echo comes back to the device. The coaxial cable connects the step-pulse generator and the sampler. The shield of the coaxial cable is connected to earth and its electric potential is 0 V. The electromagnetic waves produced by the step- or needle-pulse generator are launched into the coaxial cable with a voltage drop of several tenths of a volt between the conductor and the shield. The evaluation of data obtained by a tester is based on the following basic principles. Any change of impedance in the cable-probe system causes a partial or total reflection of the pulse. Therefore, one reflection will be on the cable/probe interface, where the dielectric is suddenly changed, and therefore the impedance must also be changed, while the second reflection is on the open end of the probe, where the impedance tends towards free space impedance and the wave is reflected in phase. Reflected pulses can be either in phase with the incident pulse, which happens in the case when the electromagnetic waves pass an increase in impedance, or in counter phase, when a decrease of impedance is met. Time/velocity of pulse propagation depends on the apparent relative permittivity of the porous material, which can be expressed using the formula 2 ct p ε r =, (1) 2L where ε is the complex relative permittivity of the porous medium, c the velocity of light ( m/s), t p the time of pulse propagation along the probe rods measured by TDR meter and L the length of the sensor s rod inserted into a measured porous medium. Complex relative permittivity of porous medium consists of the real part ε r and imaginary part ε r, ε = ε + ε. (2) r r r Both parts of the complex relative permittivity depend on applied measuring frequency. From Figure 1 it is evident that imaginary part of the complex relative permittivity can be in case of TDR measurement neglected and the real part is more or less constant. Knowing the relative permittivity of the studied material we can estimate water content in a medium in a few of possible ways for example using empirical calibration, homogenization techniques and empirical conversion functions. Complex relative permittivity exhibits also temperature dependence which can be described according to [CRC, 2002] by equation 2 ε = T T (273 < T (K) < 373). (3) Therefore, in practical application of TDR technique for moisture measurement especially in situ temperature compensation has to be done.

6 Figure 1. Illustration of complex relative permittivity dependence on frequency. 3 MEASURING TECHNOLOGY, MATERIALS AND SAMPLES For the TDR measurements in this paper, the cable tester LOM/RS/6/mps produced by Easy Test which is based on the TDR technology with sin 2 -like needle pulse having rise-time of about 200 ps, was employed. It is computer aided instrument [Malicki & Skierucha, 1989] originally designed for measurements of soil moisture. The built-in computer serves for controlling TDR needle-pulse circuitry action, recording TDR voltage versus time traces, and calculating the pulse propagation time along particular TDR probe rods and the relative permittivity of measured material. A two-rod miniprobe LP/ms (Easy Test) was used for the determination of moisture content that was designed by Malicki et al. [Malicki et al., 1992]. The sensor is made of two 53 mm long parallel stainless steel rods, having 0.8 mm in diameter and separated by 5 mm. The sphere of sensor s influence was determined with the help of a simple experiment. The probe was fixed in the beaker and during the measurement, there was added water step by step. From the measured data (relative permittivity in dependence on water level) there was found out that the sphere of influence creates the cylinder having diameter about 7 mm and height about 60 mm, circumference around the rods of sensor. The accuracy of moisture content reading given by producer is ± 2% of displayed water content. The measuring technology can be divided into three basic steps; probe calibration, sample arrangement and probes placing, data evaluation and determination of moisture content. The probe calibration was done for every probe using the known dielectric constants of water and benzene (see Pavlík et al., 2006, for details). The moisture measurements were done on the AAC samples provided by Polish producer PPH FAELBED Inc.. Three different types of AAC based materials were studied. Nominally, AAC 500,

7 AAC 600 and AAC 700 having different densities and porosities were tested. The experiment was done on samples having dimensions of 40 x 40 x 100 mm. At first, two parallel holes having the same dimensions as the sensor rods were bored into each sample. Then, the sensors were placed into the samples and sealed by silicon gel. The samples were partially saturated by water and insulated to prevent water evaporation. The relative permittivity of wet samples was then continuously monitored until the measured values reached the constant value. Then, the experiment was interrupted, sensors removed from the samples and moisture content in the samples was determined using gravimetric method. In this way, the empirical calibration curve of particular measured materials was accessed that in future work can be used for evaluation of measured relative permittivity data. 4 EXPERIMENTAL RESULTS The obtained results (see Figure 2) clearly document the dependence of relative permittivity of the studied materials on moisture content. The empirical calibration curves have more or less exponential shape for all studied materials. We can observe relatively similar results in the range of lower moisture contents whereas for higher moisture contents (typically from 30%) the effect of different densities plays an important role. However, for rough estimate of moisture content of AAC the results can be considered similar and no extra calibration for every AAC material is supposed to be done. 25,00 measured data AAC500 relative permitivity є [-] 20,00 15,00 10,00 measured data AAC600 measured data AAC700 5,00 0,00 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 volumetric moisture content w [m 3 /m 3 ] Figure 2. Empirical calibration curves of the measured AAC materials. 5 CONCLUSIONS From the point of view of the applicability of TDR methodology for moisture measurement of building materials on AAC basis, the experiment presented in this paper has clearly shown a good perspective of the TDR method for such type of measurements. The obtained results can find use for example in long term monitoring of moisture content in AAC structures which can contribute to the accurate assessment of hygric function of building structures and in consequence to the higher durability and service life of AAC structures. The results can also be used for the determination of hygric parameters of tested materials, particularly moisture diffusivity and water absorption coefficient. It should also be noted that the TDR method has a high potential not only in laboratory

8 measurements, but it also makes possible long term monitoring of moisture content in situ, directly on building site. ACKNOWLEDGMENTS This research has been supported by Czech Ministry of Education, Youth and Sports of Czech Republic, under project No MSM: REFERENCES Topp G. C., Davis J. L. & Annan A.P. 1980, Electromagnetic determination of soil water content: measurements in coaxial transmission lines, Water Resources Research, vol. 3, pp Dalton F. N. & van Genuchten M. T. 1986, The time domain reflectometry for measuring soil water content and salinity, Geoderma, vol. 38, pp Kupfer K. & Trinks E. 2005, Simulations and Experiments for Detection of Moisture Profiles with TDR in a Saline Environment, Electromagnetic Aquametry, Springer Verlag Heidelberg Berlin, pp Kupfer K., Trinks E., Wagner N. & Hübner Ch. 2007, TDR Measurements and Simulations in High Losy Bentonite Materials, Measurement Science ad Technology, London. Aghaei A., van Der Zanden A. J. & Hendriks N. A. 2005, TDR technique for measuring moisture content in brick, Proceedings of the Sixth Conference on Electromagnetic Wave Interaction with Water and Moist Substances, Weimar, Germany. Schimmer O., Oberheitmann B., Baumann F. & Knöchel R. 2007, Instantaneous Distinction Between Double and Single Frozen Fish Using a New Handheld Time Domain Reflectometer, Proceedings of the 7 th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances, Hamamatsu, Japan, pp Katze U. 1989, Complex permittivity of water as a function of frequency and temperature, J. Chem. Eng. Data, 34, pp Owen B. B., Miller R. C., Milner C. F. & Cojan H. L. 1961, The dielectric constant of water as a function of temperature and pressure, J. Phys. Chem., 65, pp CRC Handbook of Chemistry and Physics, 83 rd Edition, D. R. Lide, CRC, Malicki M. A. & Skierucha W. M. 1989, A manually controlled TDR soil moisture meter operating with 300 ps rise-time needle pulse, Irrigation Science, vol. 10, pp Malicki M. A., Plagge R., Renger M. & Walczak R. T. 1992, Application of time-domain reflectometry (TDR) soil moisture miniprobe for the determination of unsaturated soil water characteristics from undisturbed soil cores, Irrigation Science, vol. 13, pp Pavlík Z., Jiřičková M., Černý R., Sobczuk H. & Suchorab Z. 2006, Determination of Moisture Diffusivity Using the Time Domain Reflectometry (TDR) Method, Journal of Building Physics, vol. 30, no. 1, pp