**Ubiquitous Piezoelectric Sensor Network (UPSN)-Based Concrete Curing Monitoring for u-Construction**

Seunghee Park and Dong-Jin Kim

*Department of Civil and Environmental Engineering/u-City Design and EngineeringSungkyunkwan University Cheoncheon-dong Jangan-gu Suwon Republic of Korea* 

#### **1. Introduction**

74 Modern Telemetry

Suster, M.; Chaimanonart,; N.; Guo,J.; Ko, W. H. & Young, D. J. (January 2005). Remote-

Florida, January 2005, pp.255-258.

Powered high-performance strain sensing microsystem, Technical Digest, the 18th IEEE International Conference on Micro Electro Mechanical Systems, Miami,

> Recently, there has been increasing demand for high-rise buildings or wide-span bridges. These structures are constructed with a mount of mass concrete. However, the concrete might be susceptible to brittle fracture if the curing process is inadequate. Therefore, to prevent this drawback, it is essential to predict the strength development of concrete during the curing process. In addition, real-time monitoring of the curing strength is important for reducing the construction time and cost because it can determine the appropriate curing time to achieve sufficient strength to progress to the next phase safely. The in-situ strength of concrete structures can be determined with a high precision by performing the strength testing and/or material analysis on core samples removed from the structure (Irie et al., 2008). However, this method might destroy the concrete structure. Therefore, a range of methods based on the thermal, acoustical, electrical, magnetic, optical, radiographic, and mechanical properties of the test materials have been developed to monitor the strength development without damaging the host structures (ACI Committee 228, 2003; Lamind and Pielert, 2006; Metha and Monterio, 2005). These methods typically measure certain properties of concrete from which the strength and/or elastic constants can be estimated. Among these techniques, several methods using a Schmidt hammer or an integrated temperature have been normally used. However, these are unsuitable for use at construction sites because they do not allow real-time monitoring of the curing process of concrete structures at inaccessible places.

> The recent advent of smart materials, particularly piezoelectric materials, can provide a solution for the real-time monitoring for strength development. Electromechanical impedance techniques that employ piezoelectric materials have emerged as a potential tool for the implementation of a built-in monitoring system for civil infrastructures (Park G. et al., 2000, 2003; Park S. et al. 2005, 2006, 2011). This technique utilizes high-frequency structural excitation, which is typically > 20 kHz from surface-bonded PZT (Lead-Zirconate-Titanate) patches, to sensitively monitor the changes in the mechanical impedance of the test structures. Furthermore, the recent advances in online monitoring, including actuation and sensing, on-board computing, and radio-frequency (RF) telemetry, have improved the

Ubiquitous Piezoelectric Sensor Network

strength development curve (Shariq et al., 2010).

Fig. 1. Typical Strength Development Curve

patch, Za(ω), in most applications as

ω

 ω

**2.2 Electromechanical impedance modeling** 

(UPSN)-Based Concrete Curing Monitoring for u-Construction 77

as well as throughout the ensuing period of approximately 28 days. Fig. 1 shows a typical

The electromechanical impedance-based SHM techniques have been developed as a promising tool for real-time structural damage assessment on critical members of large structural systems (Park G. et al., 2003, Koo et al., 2009, Taylor et al., 2009a, 2009b, Mascarenas et al., 2009). They make use of piezoelectric sensors such as piezoceramic (PZT) and macro-fiber composite (MFC) patches, which form a collocated sensor and actuator, often referred to as a self-sensing actuator (Giurgiutiu, 2007). The basis of this active sensing technology is the energy transfer between the actuator and the host mechanical system. If a PZT attached on a structure is driven with a sinusoidal voltage, it causes the local area of the structure to vibrate (the converse piezoelectric effect). And the structural response causes an electrical response in the PZT (the direct piezoelectric effect). Liang et al. (1996) first proposed a one-dimensional analytical model of this setup as in Fig. 2, and showed that the electrical admittance (inverse of the electrical impedance), Y(ω), of a PZT is directly correlated to the local mechanical impedance of the host structure, Zs(ω), and that of a PZT

> 2 31

 κ

*s s a*

ω

+

ω

 ω (1)

( ) () () () 1 () ()

where G is the conductance (real part); B is the susceptance (imaginary part); C is the zeroload capacitance of a PZT; and κ31 is the electromechanical coupling coefficient of a PZT. Given that the mechanical impedance and the material properties of the PZT stay constant, the equation shows that a change in the structure's mechanical impedance directly results in a change in the electrical impedance measured by the PZT. Since damages cause a change in the structure's local mass, stiffness, or damping properties and consequently its mechanical impedance, the structure's mechanical integrity can be assessed by monitoring the PZT's electrical impedance. It should be noted that the admittance function, Y(ω), is a complex number. Bhalla et al. (2002) demonstrated that the real part of the measured admittance is more sensitively changed due to the structural damage condition as compared to the

=+ = −

*<sup>Z</sup> Y G jB j C Z Z*

 ωω

accessibility of the impedance method for in-field measurements. Lynch et al. (2004) designed a wireless active sensing unit to monitor civil structures, which was constructed of off-the-shelf components and had the ability to command active sensors and actuators from a computational core combined with wireless transmission and sensing circuits, embedded algorithm to process the acquired data, and structural status broadcasting. Grisso and Inman (2005) designed a DSP (Digital Signal Processor) based prototype to provide wireless assessment of thermal protection systems. It was able to directly detect damages by analyzing variations of the electrical impedance of PZT sensors bonded to the structure. The obtained impedance signals were compared with the pre-stored baseline and a statistical damage index was calculated. Mascarenas et al. (2007) proposed a wireless sensor node which consists of a miniaturized impedance measuring chip, a microprocessor, and a radiofrequency identification (RFID) module. Low cost impedance measuring chip actuated the structure through a PZT and measured the structural impedance response, and RFID module delivered the diagnostic result to a base station. Park S. et al. (2009) improved the wireless sensor node of Mascarenas et al. (2007) by adding two multiplexer IC chips for 16 channels for the cases of multiple sensors in a small region and by embedding signal processing algorithms in the microcontroller unit (MCU) for both structural damage identification and sensor self-diagnosis. Research groups at Los Alamos National Lab recently developed a compact impedance-based wireless sensing device (WID3) for low power operation (Overly et al. 2007, 2008), which requires around 60 mW of power to operate. Here, a wake-up capability was combined for low power operation. Taylor et al. (2009a, b) extended the capability of the WID3 by implementing a module with lowfrequency A/D and D/A converters to measure low-frequency vibration data for multiple SHM techniques. It is wirelessly triggered by a mobile agent for use in the mobile-hostbased wireless sensing network. Min et al. (2010) developed a multi-functional wireless sensor node integrating piezoelectric actuating, sensing, signal processing, temperature compensating, and energy harvesting modules.

In this context, this study presents a series of efforts to confirm the applicability of the electromechanical impedance technique using both wired and wireless systems for online monitoring on the strength-development during the curing process of concrete.
