**Dominik Logo ´n \* and Krzysztof Schabowicz**

Faculty of Civil Engineering, Wrocław University of Science and Technology, Wybrze ˙ze Wyspia ´nskiego 27, 50-370 Wrocław, Poland; krzysztof.schabowicz@pwr.edu.pl

**\*** Correspondence: dominik.logon@pwr.edu.pl

Received: 8 June 2020; Accepted: 1 July 2020; Published: 4 July 2020

**Abstract:** This paper presents the recognition of micro-events and their concentration in quasi-brittle cement composites and the identification of the destruction process based on acoustic emission and sound spectrum. The tests were conducted on a quasi-brittle composite of a cement paste reinforced with a high volume of dispersed polypropylene fibers. The possibility of identifying the destruction process based on acoustic emission and sound spectrum was confirmed. This paper focused on the identification of micro-events using the 3D spectrum. It was shown that the identification of the concentration of micro-events precedes the occurrence of critical crack fcr, ending the Hooke's law range. The ability to recognize this phenomenon with the use of the 3D spectrum makes it possible to predict the structure destruction process and subsequently to assess the structure destruction (micro and macro-cracks) and the reinforcement destruction (pull-off, breaking). It was confirmed that the three-dimensional spectrum provided additional information, enabling a better recognition of micro and macro-changes in the structure of the samples based on the analysis of sound intensity, amplitudes, and frequencies.

**Keywords:** quasi-brittle cement composites; acoustic emission; acoustic spectrum; micro events
