Moisture Measurements in Masonry Materials by Time Domain Reflectometry Luigi Mollo 1 and Roberto Greco 2 Abstract: Time domain reflectometry (TDR) has been evaluated as a tool for the measurement of the moisture content of masonry materials. To this aim, the relationship between relative dielectric permittivity, measured by TDR, and volumetric water content, measured gravimetrically, has been experimentally determined over samples of three cement mortars and a clay brick. The obtained experimental points do not fit the calibration curves available in literature for other porous materials, such as soils and rocks. Instead, specific calibration relationships have been determined. The obtained results show that in all cases dielectric permittivity is highly sensitive to moisture variations, indicating TDR as a promising tool for moisture measurement of building materials. Owing to the feasibility of the proposed experimental procedure for moisture measurements of real masonry walls, the technique appears to be suitable for field monitoring applications. DOI: 10.1061/(ASCE)MT.1943-5533.0000188. © 2011 American Society of Civil Engineers. CE Database subject headings: Bricks; Masonry; Construction materials; Mortars; Moisture; Material properties; Measurement. Author keywords: Brick masonry; Construction materials; Mortars; Bricks; Moisture; Material properties; Measurement. Introduction Moisture in porous building materials may derive from several causes, such as absorption of humidity from air, capillary rise, rain penetration, and condensation phenomena. In many cases, it may lead to material deterioration because of salt concentration after wetting-drying cycles (Selvarajah and Johnston 1995; Fassina et al. 2002), mainly affecting the rising damp zone (Lubelli et al. 2004). Thus far, microstructural studies do not provide correlations between microporosity of materials and hygrometric properties (Raimondo et al. 2007). Direct measurement of the water content of building materials is thus needed, but gravimetric techniques are destructive and time-consuming (Lubelli et al. 2004; Cultrone et al. 2007). Several studies were carried out to develop noninvasive meas- urement techniques for moisture determination of building materi- als (Adan et al. 2004; Roels et al. 2004). Experiments with concrete and sandstone blocks indicated time domain reflectometry (TDR) as the most promising technique for moisture measurement in building materials, provided a specific calibration for the test material (Phillipson et al. 2008). In recent years, TDR has been widely used for measuring soil volumetric water content, because it provides easy water content estimation with little disturbance (Robinson et al. 2003). The TDR measurement is based on the correlation between bulk dielec- tric permittivity ε r of a wet porous material and its volumetric water content θ (Topp et al. 1980). TDR has also been successfully applied to water content measurements in rocks (Sass 2005; Sakaki and Rajaram 2006). Thus far, some experimental applications of TDR to water con- tent determination of building materials have been carried out over specimens of concrete (Korhonen et al. 1997; Pavlík et al. 2002; Phillipson et al. 2008), insulating materials (Roels et al. 2004; Pavlík et al. 2006; Plagge et al. 2006), and masonry materials, such as bricks or mortars (Korhonen et al. 1997; Hauschild and Menke 1998; Pavlík et al. 2002; Pavlík and Černý 2004, 2008). Although the dielectric properties of artificial materials may differ signifi- cantly from those of natural rocks or soils (Korhonen et al. 1997), in most cases TDR measurements of the water content of building materials were carried out using θðε r Þ relationships originally developed for soils (Pavlík and Černý 2004; Pavlík et al. 2006; Plagge et al. 2006). For TDR water content measurements in masonry materials, the relationship between ε r and θ for the test materials is needed. In this study, the relationship has been experimentally determined for clay bricks and three different kinds of mortar, defining an exper- imental procedure feasible for moisture measurements in real masonry walls. Porous Media Water Content Measurements by TDR A typical TDR setup for porous media water content measurement consists of a pulse generator and a transmission line, constituted by a coaxial cable and a metallic probe of known length. A steep electromagnetic pulse is transmitted via the coaxial cable from the generator to the probe inserted into the porous material. The pulse moves along the probe, with velocity of propagation depend- ing on the relative dielectric permittivity of the porous medium. At the end of the probe, the pulse is reflected back to the origin of the line. From the measurement of the travel time of the electromag- netic pulse along the probe, the mean velocity of propagation is calculated, in turn providing the mean dielectric permittivity of the material. To link dielectric permittivity to volumetric water content, the θðε r Þ relationship is needed. This relationship has been deeply 1 Seconda Università di Napoli, Dipartimento di Ingegneria Civile, via Roma 29 81031 Aversa (CE), Italy. E-mail: luigi.mollo@unina2.it 2 Seconda Università di Napoli, Dipartimento di Ingegneria Civile, via Roma 29 81031 Aversa (CE), Italy (corresponding author). E-mail: roberto .greco@unina2.it Note. This manuscript was submitted on October 30, 2009; approved on September 29, 2010; published online on October 1, 2010. Discussion per- iod open until September 1, 2011; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, Vol. 23, No. 4, April 1, 2011. ©ASCE, ISSN 0899-1561/ 2011/4-441–444/$25.00. JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / APRIL 2011 / 441 Downloaded 13 Apr 2011 to 143.225.62.236. Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org