Determination of Micro-Events and Microcracks in the Compressive Strength of Concrete Using the 3D Acoustic Spectrum in the Low Frequency and Infrasound

Acoustic emission (AE) measurements are commonly used in studies of cementitious composites subjected to various types of loading. Recording macrocracks that result in a decrease in stress is not relatively difficult. This paper focuses on the possibility of identifying micro-events and microcracks during the compression of concrete with dispersed reinforcement. Most AE studies on cement composites correspond to a reduction in stress exceeding the elastic range defined by Hooke’s law, typically associated with the formation of the first crack and detected in the medium- and high-frequency ranges. However, identifying micro-events which do not reduce stress beyond the elastic range is difficult. This study demonstrates that such micro-events can be detected using low-frequency sound and infrasound. In many papers, medium- and high-frequency acoustic signals are effective for recording macrocracks or reinforcement damage. In this work, a 3D acoustic spectrum was used to analyze recorded data in the infrasound range in a concrete compressive test. This approach proved to be the most effective method for determining the critical point fcr (the end of the elastic range) regarding low-intensity micro-events and microcracks. This type of micro-damage has no significant influence on the linear stress–strain correlation at fcr. The results indicate that identifying micro-events and low-intensity microcracks using medium- and high-frequency acoustic signals is not possible and that infrasound should be considered for the detection. Significant differences in stress and displacement corresponding to fcr and fmax were confirmed in concrete compressive tests. The results indicate that accurately determining fcr is required for correctly assessing the durability of cementitious composites.