**1. Introduction**

The ultrasonic technique is used for many purposes, but the most common purpose is diagnostic [1–4]. Ultrasounds are employed by many branches of the industry; they are a crucial tool for electronic engineering, telecommunications or material engineering. Generally, the application of ultrasounds is broad and covers active and passive uses. The passive use includes ultrasonic spectroscopy and defectoscopy, ultrasonic diagnostic for medical purposes, and hydrolocation. Ultrasonic waves are more and more often used to test kinetics of hardening of different types of substances. The active use includes ultrasonic coagulation and dispergation, ultrasound therapy, cavitation, development of sonoluminescence, or chemical reactions. Other active applications are: crushing and forming hard media, bonding, soldering, washing, extracting, and drying of substances. They are also used quite commonly to measure stresses in metal constructions. These methods have been elaborated to measure stresses caused by thermal treatment of rolled profiles or during welding. Ultrasonic stress measurements are based on the acoustoelastic effect (AE), that is, on the dependence of acoustic wave velocity on stress. Measurements of stress in bolts are the oldest application of the

AE method. Stress is determined on the basis of measured change in the times of flight of ultrasonic waves propagating along the stressed bolt. New tests and applications related to measurements of stresses in rails, train wheels and unit shafts, have been presented in [4].

Non-destructive techniques are used for other popular materials, such as concrete or ceramics, to determine time of set, and changes in the modulus of elasticity. Ultrasonic testing using minor-destructive techniques (MDT) can determine compressive strength [5,6]. Classic methods of damage detection have been intensively developed [7–10]. No attempts have been so far made to determine stress state in materials with porous structure, such as concrete or rock. These materials behave advantageously under compression, and the complex structure of ordinary concrete cause difficulties in interpreting the results. This aspect is completely different for autoclaved aerated concrete (AAC), whose compressive strength is relatively low and at the same time this material is more homogeneous than concrete despite its porous structure.

Autoclaved aerated concrete (AAC) contains cement, calcium, and lime as binding material, sand used as a filler and tiny quantities of aluminium powder (or paste) which is used as a blowing agent. Density of this type of concrete ranges from 300 to 1000 kg/m3, and its compressive strength varies from 1.5 to 10 N/mm2. AAC has been commonly used since the middle of the 1950s. This material (>40% of the construction segment in Europe) is used for masonry structures, precast wall or floor elements, and intels [11]. The open-pore structure explains why AAC is sensitive to direct exposure to moisture, which results in worse insulating and strength properties. The available articles, apart from general relationships specified in standards, do not contain detailed references expressed as empirical relations to determine strength properties of AAC using NDT and semi-NDT techniques. This work describes the practical application of selected issues on ultrasonic testing presented in the papers [12,13].

This paper is an attempt to evaluate changes in stress state of masonry units made of autoclaved aerated concrete built into small fragments of the masonry wall under axial compression. The masonry wall was also made of autoclaved aerated concrete with considerably more porous structure than ordinary concrete. The aim of the tests was to define empirical relationships concerning values of vertical stresses in the AAC masonry wall including the acoustoelastic (AE) effect well-known in practice [14]. This paper is divided into the theoretical part containing detailed bases of the AE method, and the research part consisting of stages I and II In stage I of the tests, experiments were performed on 24 small cube specimens (100 × 100 × 100 mm) of autoclaved aerated concrete with nominal densities of 400, 500, 600, and 700 kg/m3. The elastooptic constant β<sup>111</sup> was determined that showed the longitudinal wave *c*p0 depended on stress σ33. In stage II, nine small models made of autoclaved aerated concrete with nominal density of 500 kg/m3 were prepared and tested in the second phase of tests. They were used to measure velocity of the ultrasonic wave cp. Relationships determined in the first phase were used to identify the stress state in the masonry wall and validate σ33—*c*<sup>p</sup> relationship.
