**Radosław Jasi ´nski**

Department of Building Structures and Laboratory of Civil Engineering Faculty, Silesian University of Technology, ul. Akademicka 5, 44-100 Gliwice, Poland; radoslaw.jasinski@polsl.pl; Tel.: +48-32-237-1127

Received: 21 May 2020; Accepted: 22 June 2020; Published: 25 June 2020

**Abstract:** The structure safety can be assessed, but only indirectly, by identifying material properties, geometry of structures, and values of loads. The complete and comprehensive assessment can be done only after determining internal forces acting inside structures. Ultrasonic extensometry using an acoustoelastic effect (AE) is among the most common non-destructive techniques (NDT) of determining true stresses in structures. Theoretical bases of the method were described in the mid 20th century. They were founded on the correlation between ultrasonic waves and the value and direction of stresses. This method is commonly used to determine stresses mainly in homogeneous materials without any inherent internal defects. This method is rarely applied to porous or composite materials, such as concrete or rock due to a high dispersion of results. Autoclaved aerated concrete (AAC), characterized by high homogeneity and porosity, is the popular material in the construction sector, used to produce masonry units. The discussed tests involved the acoustoelastic effect to determine stresses in the masonry wall made of AAC. This paper presents a widely theoretical background for the AE method, and then describes the author's own research on AAC divided into two stages. At first, the empirical relationships between compressive stress and velocity of longitudinal ultrasonic wave, including humidity, were determined. In stage II, nine masonry walls were tested in axial compression. Mean compressive stresses in the masonry wall determined with the proposed method were found to produce a satisfactory confidence level up to ca. 50% of failure stresses. Results were significantly understated for stresses of the order of 75% of failure stresses.

**Keywords:** masonry structures; autoclaved aerated concrete masonry units (AAC); compressive strength; minor-destructive (MDT) techniques; non-destructive techniques (NDT); ultrasonic testing; acoustoelastic effect (AE)
