**4. Experimental verification**

In order to verify the feasibility of the proposed electromechanical impedance technique for online monitoring of the strength developed during the curing process of the concrete structures, a series of experimental studies have been carried out using both wired and wireless systems.

### **4.1 Experimental setup and test procedure**

Two types of concrete cylinders with design strength of 60MPa and 100MPa were prepared to measure the impedance signals during the curing process of concrete, as shown in Fig. 8. The cylinders were developed by isothermal air curing. PZT sensors, 20 mm × 20 mm × 0.508 mm in size, were attached to the concrete cylinders. The PZT sensors were installed on the cylinders in the first 24 hours after casting. Since concrete is a non-conducting material, a conducting copper paste was applied to the specimen before bonding the PZT sensor to the host structure. The PZT patches were bonded to the top center of the cylinder surface, as shown in Fig. 8. The experimental setup for the wired impedance measurement system consisted of cylinders with the PZT sensors, a self-sensing circuit board and a DAQ system (PXI 1042Q, National Instruments Inc.). The DAQ system consisted of an Arbitrary Waveform Generator (AWG), a Digitizer (DIG), embedded controller and data acquisition software (LabVIEW). The wireless system was comprised of the cylinders with the PZT sensors, a wireless sensor node, a RF receiver (KETI), and a laptop computer equipped with data acquisition software (MATLAP), as shown in Fig. 9, 10.

Ubiquitous Piezoelectric Sensor Network

Fig. 10. Wireless impedance measuring system

**4.2 Impedance variations due to curing process** 

concrete was established.

signal to noise ratio, the signals were acquired 3 times and averaged.

(UPSN)-Based Concrete Curing Monitoring for u-Construction 87

 (a) Wireless impednace sensor node (b) RF reciever

The frequency ranges so the shift in the resonant frequencies could be observed clearly in the measured impedance signals were determined to be 45 kHz ~ 50 kHz for the 60MPa cylinder and 35 kHz ~ 40 kHz for the 100MPa cylinder. The first test was carried out 3 days after mixing because before 3 days, the piezoelectric sensors could not be attached completely. Subsequent tests were performed at 5, 7, 14, 21 and 28 days. In particular, days 3, 7, 14, and 28 are important days in evaluating the in-place compressive strength in the construction codes of many countries. Three cylinders for each group were tested using the wired and wireless systems simultaneously to compare their performance. To improve the

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


(c) PZT attached concrete specimen

Fig. 8. Test specimen: High Strength Concrete Cylinders

(a) NI-PXI DAQ system (b) Self-sensing circuit

Fig. 9. Wired impedance measuring system

(a) 60MPa Concrete specimen (b) 100MPa Concrete specimen

(c) PZT attached concrete specimen

(a) NI-PXI DAQ system (b) Self-sensing circuit

Fig. 8. Test specimen: High Strength Concrete Cylinders

Fig. 9. Wired impedance measuring system

(a) Wireless impednace sensor node (b) RF reciever

Fig. 10. Wireless impedance measuring system

The frequency ranges so the shift in the resonant frequencies could be observed clearly in the measured impedance signals were determined to be 45 kHz ~ 50 kHz for the 60MPa cylinder and 35 kHz ~ 40 kHz for the 100MPa cylinder. The first test was carried out 3 days after mixing because before 3 days, the piezoelectric sensors could not be attached completely. Subsequent tests were performed at 5, 7, 14, 21 and 28 days. In particular, days 3, 7, 14, and 28 are important days in evaluating the in-place compressive strength in the construction codes of many countries. Three cylinders for each group were tested using the wired and wireless systems simultaneously to compare their performance. To improve the signal to noise ratio, the signals were acquired 3 times and averaged.
