**4.2 Impedance variations due to curing process**

The strength of the concrete results from the hydration process of the concrete. During hydration, the mechanical properties of the concrete, such as strength, impedance etc., changed. The impedance technique for monitoring the strength development of concrete employs the change in the mechanical impedance during the hydration process. Figs. 11 and 12 show the measured impedance signals from the wired and wireless systems at six different curing ages. In addition, each dataset was normalized to the maximum value. First, the results from the 60MPa are reported. The resonant frequencies in the impedance signals shifted gradually to the right side with increasing curing age (Fig. 11) due to strength development of the concrete. This confirmed that the impedance technique can be used to monitor the strength development of concrete. In Fig. 12, the impedance data from the 100MPa specimens showed a similar pattern to that obtained from the 60MPa specimens. Although wireless data has some noises, the quantity of the shift in the resonant frequency measured using the wired and wireless system was similar. The noises of wireless data are caused by the resolution problem of wireless sensor node. The frequency resolution can be fixed at a certain level (in this study, that is 1Hz) when NI PXI equipment is used. However, the wireless sensor node can sample with maximum 512 points. In this study, the frequency band of the measured signal is 5kHz with 500 sampling points. Hence, the frequency resolution is 10Hz when the wireless sensor node is used. However, these bumps can be negligible because these cannot affect to the patterns from the curing process. Therefore, the applicability of a wireless impedance measuring system to monitor the curing process of concrete was established.

Ubiquitous Piezoelectric Sensor Network

(UPSN)-Based Concrete Curing Monitoring for u-Construction 89

The resonant frequency increased in both cases 60MPa and 100MPa. All the resonant frequency shift data was normalized to the maximum value. As the curing process progressed, the strength of the cylinder increased during the hydration process. Since the resonant frequency is associated with the strength of a concrete cylinder, the resonant frequency in the impedance signals of the cylinder increased with increasing cylinder strength. In addition, the change in resonant frequency measured using the wired system and wireless system were similar in 60MPa and 100MPa. Fig. 1 shows a typical strength development curve of 30MPa at a curing temperature of 21.1 ºC to compare these results with the typical strength development of curing concrete. The changing patterns between the increasing resonant frequency and the development of the compressive strength were similar. Also the RFS of wired and wireless represent similar pattern. Therefore, the RFS of

the impedance can be used to monitor the strength development of the concrete.

(a) 60MPa Wired Data (b) 60MPa Wireless Data

(c) 100MPa Wired Data (d) 100MPa Wireless Data

In addition to the RFS, the cross-correlation coefficient index (1-CC) was calculated to provide

(Re( ) Re( ))(Re( ) Re( )) <sup>1</sup> 1 1

0 1

*Z Z*

*i i*

σ σ

,0 0 ,1 1

(5)

*Z ZZ Z*

− −

quantitative information. The 1-CC values were derived using the following equation:

Fig. 13. Resonant frequency shift-based estimate of strength development

1

*i*

=

*N*

1

*N*

**4.3.2 Cross-correlation coefficient** 

*CC*

− =− <sup>−</sup>

Fig. 11. Impedance variation measured at 60MPa concrete cylinder

Fig. 12. Impedance variation measured at 100MPa concrete cylinder

#### **4.3 Signal processing for the impedance variation**

Two methods, resonant frequency and cross-correlation coefficient, were applied to examine the trend of the impedance variations more precisely:

#### **4.3.1 Resonant frequency shift**

To visualize the curing process of the concrete, the resonant frequency shift (RFS), derived as Eq. (4), at each curing age was plotted, as shown in Fig. 13.

$$RFS = \frac{f\_i - f\_o}{f\_o} \tag{4}$$

where fi is the current resonant frequency of the impedance data at each measurement day, and fo is the resonant frequency of the 3rd day measured impedance data as a baseline.

(a) Wired data (b) Wireless data

(a) Wired data (b) Wireless data

Two methods, resonant frequency and cross-correlation coefficient, were applied to examine

To visualize the curing process of the concrete, the resonant frequency shift (RFS), derived

*<sup>f</sup> <sup>f</sup> RFS f*

where fi is the current resonant frequency of the impedance data at each measurement day, and fo is the resonant frequency of the 3rd day measured impedance data as a baseline.

*i o o*

<sup>−</sup> <sup>=</sup> (4)

Fig. 11. Impedance variation measured at 60MPa concrete cylinder

Fig. 12. Impedance variation measured at 100MPa concrete cylinder

**4.3 Signal processing for the impedance variation** 

the trend of the impedance variations more precisely:

as Eq. (4), at each curing age was plotted, as shown in Fig. 13.

**4.3.1 Resonant frequency shift** 

The resonant frequency increased in both cases 60MPa and 100MPa. All the resonant frequency shift data was normalized to the maximum value. As the curing process progressed, the strength of the cylinder increased during the hydration process. Since the resonant frequency is associated with the strength of a concrete cylinder, the resonant frequency in the impedance signals of the cylinder increased with increasing cylinder strength. In addition, the change in resonant frequency measured using the wired system and wireless system were similar in 60MPa and 100MPa. Fig. 1 shows a typical strength development curve of 30MPa at a curing temperature of 21.1 ºC to compare these results with the typical strength development of curing concrete. The changing patterns between the increasing resonant frequency and the development of the compressive strength were similar. Also the RFS of wired and wireless represent similar pattern. Therefore, the RFS of the impedance can be used to monitor the strength development of the concrete.

Fig. 13. Resonant frequency shift-based estimate of strength development

#### **4.3.2 Cross-correlation coefficient**

In addition to the RFS, the cross-correlation coefficient index (1-CC) was calculated to provide quantitative information. The 1-CC values were derived using the following equation:

$$1 - \text{CC} = 1 - \frac{1}{N - 1} \frac{\sum\_{i=1}^{N} (\text{Re}(\mathbf{Z}\_{i,0}) - \text{Re}(\overline{\mathbf{Z}\_0}))(\text{Re}(\mathbf{Z}\_{i,1}) - \text{Re}(\overline{\mathbf{Z}\_1}))}{\sigma\_{\mathbf{Z}\_0} \sigma\_{\mathbf{Z}\_1}} \tag{5}$$

Ubiquitous Piezoelectric Sensor Network

**6. Acknowledgment** 

**7. References** 

(UPSN)-Based Concrete Curing Monitoring for u-Construction 91

impedance system. Therefore, a wireless system that can improve the applicability to a construction site can be used to monitor the strength development of concrete. Consequently, the wireless strength development monitoring system for concrete can be employed comfortably in construction sites. Furthermore, piezoelectric sensors that monitor the strength development can be used for structural health monitoring (SHM) after construction. In addition, embedded curing monitoring and a SHM system for high strength

This study was supported by National Nuclear R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0025889) and u-City Master and Doctor Support Project funded by Ministry of Land,

ACI Committee 228. (Nov 1, 2003)). *In-place methods to estimate concrete strength report,*

Bhalla, S., Naidu, A.S.K. and Soh, C.K. (2002). Influence of structure-actuator interactions

Giurgiutiu, V. (July 1, 2007*). Structural health monitoring: with piezoelectric wafer active sensors*,

Grisso, B.L. and Inman, D.J. (2005). Developing an autonomous on-orbit impedance-based

Irie, H., Yoshida, Y., Sakurada, Y., and Ito, T. (2008). Non-destructive-testing Methods for Concrete Structures, *NTT Technical Review*. Vol. 6, No. 8, (May 2008), pp. 1-8 Koo, K.Y., Park, S., Lee, J.J. and Yun, C.B. (2009). Automated impedance-based structural

Lamond, J. F. and Pielert, J. H. (2006). Significance of tests and properties of concrete and concrete-making materials, *ASTM International*, Vol. 169, pp. 667, ISSN 00660558 Lee, S.J., and Sohn, H. (2006). Active self-sensing scheme development for structural health

Liang, C., Sun, F.P. and Rogers, C.A. (1996). Electro-mechanical impedance modeling of

Lynch, J.P., Sundararajan, A., Law, K.H., Sohn, H. and Farrar, C.R. (2004). Design of a

Mascarenas, D.L., Todd, M.D., Park, G. and Farrar, C.R. (2007). Development of an

*Materials and Structures*, Vol. 16, No. 6, pp. 2137-2145, ISSN 09641726

Elsevier/Academic Press, ISBN 9780120887606, Amsterdam

*Structural Health Monitoring*, Stanford, CA, September.

and temperature on piezoelectric mechatronic signatures for NDE, *Proceedings of the ISSS-SPIE International Conferences on Smart Materials Structures and Systems*, ISSN

SHM system for thermal protection systems, *Proceedings of the 5th Int'l Workshop on* 

health monitoring incorporating effective frequency shift for compensating temperature effects, *Journal of Intelligent Material Systems and Structures*, Vol. 20, No.

monitoring, *Smart Materials and Structures*, Vol. 15, No. 6, pp. 1734-1746, ISSN

active material systems, *Smart Materials and Structures*, Vol. 5, No. 2, pp. 171-186,

wireless active sensing unit for structural health monitoring, *Proceedings of the SPIE Annual Int'l Symposium on Smart Structures and Materials*, ISSN 0277786X, San Diego,

impedance-based wireless sensor node for structural health monitoring*, Smart* 

concrete can be developed to improve the applicability and efficiency of this system.

Transport and Maritime Affairs (MLTMA). This all-out support is greatly appreciated.

American Concrete Institute, MI, USA

0277786X, Bangalore, December 2002.

4, pp.367- 377, ISSN 1045389X

09641726

ISSN 09641726

CA, March.

where Zi,0 is the impedance function at the baseline (the impedance data of 3rd day), Zi,1 is the current impedance at each measured day, and 0 1 , σ σ *Z Z* are the standard deviations of each dataset, respectively. The data was normalized to the maximum value. Fig. 14 shows the 1-CC data of 60MPa and 100MPa respectively. The 1-CC data shows the same pattern with a commercial strength development curve (Fig. 1). Also, the wired data and wireless data has similar pattern. Therefore, the 1-CC value can provide more reliable quantitative information on strength development.

Fig. 14. 1-CC-based estimate of strength development
