Effect of Size of Inhomogeneity on the Surface Wave Attenuation in Cementitious Media D. G. Aggelis 1 and T. Shiotani 2 Abstract: The present study focuses on surface wave propagation in cementitious material with inhomogeneity. Thin, flakey inclusions were added in contents of up to 10% by volume and different sizes inside the matrix to realistically simulate cracking. The results are focused on the attenuation created by the inhomogeneity. As damage is added up in content, the level of attenuation strongly increases for the whole exam- ined frequency band. However, the size and the population of the inclusions present equally a strong influence on the attenuation. The distortion of the frequency content is also evaluated by spectral density functions, showing that simple waveform analysis can enhance the characterization of damage in addition to the valuable assessment based on wave velocity. DOI: 10.1061/(ASCE)MT.1943-5533 .0000487. © 2012 American Society of Civil Engineers. CE Database subject headings: Rayleigh waves; Frequency; Damage; Ultrasonic methods; Cement; Wave attenuation. Author keywords: Rayleigh waves; Frequency; Damage; Coherence; Ultrasound. Introduction Concrete structures are exposed to deterioration factors like weath- ering, corrosive agents, thermal expansion, and contraction or even freezing and thawing. Furthermore, they support operation loads, the own weight of the structure, and possibly dynamic overloading, like the one created by earthquakes. Most of the above factors affect primarily the surface of the structures, which is directly exposed to the atmospheric conditions and sustain maximum flexural loads. Deterioration is, therefore, bound to start from the surface in most cases. This deterioration may be manifested in the form of large cracks breaking on the surface, and microcracking propagating on the surface layer of the material. Inspection techniques based on the propagation of elastic waves have long been used for the esti- mation of the quality, and general condition of the material (Kaplan 1959, Popovics et al. 1990, Boyd and Ferraro 2005). The most common measurement used is the “pulse velocity”. Considering that the material is homogeneous, pulse velocity is directly related to the modulus of elasticity and correlated with the strength of the material through empirical relations. Pulse velocity is measured by the first disturbance detected from the waveform. This measure- ment is heavily dependent on the strength of the signal relatively to the noise level, which could be induced due to environmental conditions and equipment components. If the initial arrival of the wave is weaker than or similar to the noise level, pulse velocity is underestimated. This could certainly be the case in actual struc- tures, where long propagation distances through damaged materials are applied. Rayleigh waves can also be excited in a concrete sur- face; they propagate within a penetration depth of approximately one wave length and carry more of the excitation energy (Gudra and Stawinski 2000, Qixian and Bungey 1996). Their velocity is also related to elasticity and Poisson’ s ratio (Sansalone and Carino 2004). The measurement of Rayleigh velocity is usually conducted by a reference peak point, therefore it is not directly in- fluenced by the noise level. However, for cases of severe damage or long propagation, the strong reference cycle used for the measure- ment is severely distorted and makes the selection of reference points troublesome (Aggelis and Shiotani 2007). Frequency do- main techniques like phase difference calculation between signals recorded at specific distances may provide solution for velocity measurement revealing also the dependence of velocity on fre- quency [Sachse and Pao 1978, Aggelis and Shiotani 2008a]. In addition to the wave velocity, attenuation has also been widely used for detection of microstructural changes or existence of damage (Landis and Shah 1995, Kim et al. 1991). It is calculated by the reduction of the wave amplitude between two measurement points. Attenuation has been shown more sensitive to damage or void content than wave velocity, as has been revealed in several studies (Shah et al. 2000, Aggelis and Shiotani 2007). It has also been correlated to the size of the aggregates, as well as air void size and content in hardened and fresh cementitious materials (Punurai et al. 2006, Philippidis and Aggelis 2005). Energy related param- eters show strong correlation to the curing and strength of hydrating concrete (Voigt et al. 2003) as well. The sensitivity of attenuation to the microstructure is such, that the content of “heterogeneity” is not the only dominating factor; the typical size and shape of the inclu- sions play an equivalently important role and therefore, Rayleigh wave attenuation has been related to parameters like aggregate size, and damage content (Jacobs and Owino 2000, Owino and Jacobs 1999, Aggelis and Shiotani 2007). This sensitivity to the microstructure may complicate the assessment, but, on the other hand offers possibilities for more accurate characterization. Accu- rate characterization would require determination of several dam- age parameters like the number (or equivalent damage content) of the cracks, their typical size, as well as their orientation. Though this is a nearly impossible task, especially in situ, advanced features that are sensitive to the above damage parameters should be con- tinuously sought for. 1 Dept. of Materials Science and Engineering, Univ. of Ioannina, 45110 Ioannina, Greece (corresponding author). E-mail: daggelis@cc.uoi.gr 2 Graduate School of Engineering, Kyoto Univ., Nishikyo-ku, Kyoto 615-8540, Japan. Note. This manuscript was submitted on June 13, 2011; approved on January 24, 2012; published online on January 26, 2012. Discussion period open until January 1, 2013; separate discussions must be submitted for in- dividual papers. This paper is part of the Journal of Materials in Civil Engineering, Vol. 24, No. 8, August 1, 2012. ©ASCE, ISSN 0899- 1561/2012/8-0–0/$25.00. 1 JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / AUGUST 2012 / 1