## 5 Conclusions  
This paper presents an experimental investigation of detecting crack-induced damage using active sensing approach with embedded SAs through two RC beams with NSM pre-stressed CFRP plates. Actuators and sensors for damage detection can be formed by embedding SAs in the test specimens. The embedded SA-induced stress wave signal is highly sensitive to the debonding crack condition between the CFRP and concrete. Based on the experimental results, the following conclusions can be drawn:  
1. The debonding cracks only appear in the CFRP reinforcement region. The non-pre-stressed bonding section can prolong the bond stress-transmission length of the CFRP-concrete interface. The increase in the contact area between the CFRP and the concrete improves the stress distribution of the concrete. The analysis results show that there is an effective non-pre-stressed CFRP bond  
Figure 19: Damage index of OS2 in the beam 1.  
Figure 20: Damage index of OS1 in the beam 2.  
Figure 21: Damage index of OS2 in the beam 2.  
length (300 mm), which can improve the load characteristics of the test specimen and enhance the bearing capacity and deformation performance of the test specimen.
2. Wavelet packet theory has been successfully applied to the real time monitoring of the debonding damage process of the RC beams with NSM pre-stressed CFRP plates. It can be seen from the time domain and frequency domain analysis that the tension end region cracks earlier than the bond end region with the increase of load. Stress concentration at the end of CFRP leads to debonding damage at the early loading stage. This indicates the starting point of structural damage. It is shown that increasing the length of the non-pre-stressed CFRP bond can slow down the appearance and expansion of the crack. It can be judged from the damage index that increasing the contact area between CFRP and concrete affects the damage degree and bearing capacity of the non-pre-stressed CFRP bonding section; the existence of the non-pre-stressed CFRP bonding section affects the damage degree and bearing capacity of the tension end and the bond end region. Wavelet packet damage index has the ability to detect debonding crack initiation. After the occurrence of debonding cracks, the value of damage index increasing continuously monitors the development of debonding cracks.
3. The active sensing approach based on SAs can monitor the occurrence and development of cracks for RC beams with NSM pre-stressed CFRP plates in real time. The experimental results show that it is feasible to use the active sensing method based on stress wave to monitor the debonding crack damage of the CFRP-concrete in real time.  
In addition, the effectiveness of the proposed active sensing approach was experimentally verified by the debonding performance of different lengths of non-pre-stressed CFRP bonded. The method is of significance to monitor the initial installation quality and the long-term efficiency of the CFRP-concrete. The predicted structural damage occurred earlier than the real damage. Meantime, the piezoceramic SAs are economical with low manufacturing cost and they can be easily integrated with concrete structures. The low sampling rate requirement for the data analysis reduces the cost of the data acquisition system. It has the potential to be applied to the health monitoring at a very economical cost without using additional bulky equipment. Therefore, the proposed active sensing approach based on SAs has great potential to be applied in practice for inaccessible damage detection of RC beams with NSM pre-stressed CFRP plates.