PSI - Issue 5

Baris Arslan et al. / Procedia Structural Integrity 5 (2017) 171–178 Baris Arslan et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Structural health monitoring (SHM) is employed by researchers in order to provide information regarding to the quantification of structural identities in terms of durability. Due to their fast response time and simple structure, piezoelectric ceramics (PZT) have been widely used in SHM applications recently. Depending on the advances in PZT sensor technology, electromechanical impedance spectroscopy (EMIS) has emerged. EMIS utilizes the electromechanical coupling property of PZT actuator-sensor, which is based on actuating high frequency vibrations and sensing the local dynamic responses of the affixed structure simultaneously. In 1994, Liang et. al. [1,2] analyzed electromechanical coupling relationship between PZT patches and the body structures. After this study, many researchers in the field of civil engineering have showed their interests on EMIS. In 1995, Sun et. al. [3], applied this technology in order to monitor health conditions and structural reliability of truss structures. Between the years 1999 and 2001, Park et. al. [4 – 6] affixed PZT patches to the reinforced brickwork, the joint part of steel bridge and the pipe network in order to obtain relative changes in the EMIS results of these structures. Subsequently, in 2002, Bhalla et. al. and Naidu et. al. [7,8] monitored the initial damage in concrete and the increased strength of the concrete during the curing process by employing EMIS and obtained the correlation between the concrete stiffness and the impedance peaks. Afterwards, several researchers [9,10] studied the quantification of the strength development in concrete by using EMIS. So far, all of the mentioned studies above, focused on affixing PZT patches on the surface of the structures by using appropriate adhesives. Both in theory and practice, this method resulted in many problems, like uncontrolled quality of the adhesive layers and the environmental conditions [11]. While EMIS technology has been developing, embedded PZT-concrete structures has been emerged [12 – 14]. In this case, embedded PZT patches are employed to investigate the vibration characteristics of the host structures. When the PZT actuator, which is embedded into the host structure, is imposed with AC voltage, high frequency vibrations are generated by the structure. These vibrations are sensed conversely and the electrical impedance of the PZT patch is affected. By investigating these changes in EMIS, structural properties and the stiffness of the host structures can be theoretically evaluated [15]. Concrete is a composite material, whereas approximately 75% of the volume of the concrete is occupied by the aggregates [16]. Many researchers concluded that the mechanical properties and the compressive strength of concretes are influenced by the varying sizes of the aggregates [16 – 22]. However, these investigations are performed by employing destructive methods such as stress-strain failure tests. These tests require long time periods until concrete is settled. As a result, early-age developments cannot be monitored. In order to investigate the early-age developments, non-destructive methods (NDM), such as radiography, acoustic emission, magnetic field, thermal field and ultrasonic techniques are employed by researchers to estimate the quality of the concrete [12,15]. Most of these techniques are referred to vibration propagations and the change-in-stiffness methods which are closely related to the mechanical properties and directly related to the change in modulus of elasticity and density [23]. Due to the varying sizes of the aggregates; resistance, stiffness and the mass of the concrete are identified. These are all present in one single object, mechanical impedance [12]. Mechanical impedance is the measure of how much a structure resisted against the motion when subjected to a harmonic force; namely defined as the complex ratio of force vector to velocity vector ( / ) M Z F u  . The mechanical impedance is also a function of the frequency ω of the applied force and could change the frequency greatly. At the resonance frequencies, the mechanical impedance would be lower, in such a way that less force is needed to cause a structure to move at a given velocity. Thus, the changes in mechanical properties can be explored by monitoring the changes in the mechanical impedance spectroscopy (MIS) [9]. However, especially in a bulk material like concrete, MIS is currently difficult to obtain experimentally at high frequencies unless computational FEM analyses are done. In this study, in order to predict the effects of the varying aggregate sizes on the compressive strength of the concrete models, consequently, the PZT patches and the hosted concrete cubes are modelled by using the mechanical material properties that are reported in the literature previously [18,24]. Direct dynamic harmonic analyses are done on both the PZT patches and the host concrete cubes to investigate MIS and EMIS results. The relative MIS and EMIS results are obtained due to the interactions between the embedded PZT patches and the host concrete models. Eventually, correlations between MIS, EMIS and the compressive strength of the concrete models are interpreted.