Investigation on the attenuation trend of Acoustic Emission in reinforced concrete structures

Abstract

Considered as an effective real-time monitoring tool, Acoustic Emission (AE) measurements is a promising technology for reinforced concrete (RC) structures. However, its application on real RC structures is still limited. Due to the lack of knowledge on the crack induced acoustic emission in large scale structures.

The aim of this study is to explore the relationship between the fracture energy and the energy of AE signals at structural level. This serves as a basis for the quantification and localization of cracking activities at structural level.

This study is based on the AE and crack propagation measurement of a series of large scale RC specimen tests. To avoid the influence of existing cracks, the first part of study focuses on the development of the first flexural crack. It was assumed that the amount of energy required for unit length of crack opening is linearly proportional to the energy of the AE signals that are generated upon the opening of this segment of the crack. These signals can only be monitored AE sensors at given locations. By then, they have travelled through the bulk concrete and possibly already existing cracks, thus their energy has attenuated due to the geometric spreading and the damping property of the material. When these effects are taken into account, the total energy of the AE signals that were obtained by the AE sensors at given location (defined as cumulative signal strength CSS) has a potential of reflecting the fracture energy of the corresponding crack. In the part of the study, this process is theoretically studied first. The theoretical result was further validated by the AE measurement obtained from experiments.

Theoretical investigation is carried out based on a simplified model considering only length of the crack and the horizontal distance between crack and AE sensor. Theoretical result shows that both crack length and horizontal distance would affect CSS. However, when the horizontal distance is big enough, crack length is no longer the dominant factor, and CSS drops significantly with the increase of horizontal distance. The CSS of different sensors in a row in the experiments are used to validate this attenuation phenomenon. Exponential curve fitting is carried out to describe the attenuation of experimental results in different tests. Finally, a comparison of attenuation in percentage terms between curve fitting results and theoretical results is carried out. In the uncracked specimens, the results fit each other well.

Furthermore, the effect of the existing cracks to the attenuation of the CSS is studied as well. In that case, a dramatic drop of CSS is observed compared to the uncracked structures.

The study shows CSS detected by AE sensors could partly indicate the cracking behavior of RC structures. The attenuation tendency gives a guidance for sensor installation in future tests.