**1. Introduction**

Structural condition diagnostics and monitoring are two important issues in the economic and social context. Aging infrastructure, deteriorating environmental conditions, and increasing operational loads are the primary stimuli for fast-progressing research on a new interdisciplinary field of technical knowledge called "Structural Health Monitoring (SHM)", closely connected to the safe service life of structures [1,2].

Two different problems are involved in the durability assessment, one associated with the analysis of existing structures designed according to the standards previously in force, and the other associated with ensuring the anticipated useful life, taking into account the mechanical and strength-related characteristics of the newly designed structures.

In this work, the author's attention will be focused on assessing the structural integrity of existing facilities and its impact on their durability.

Durability, safety, and usability are the three foundations of structural reliability. The durability factor is, unlike in the past, equally important and has to include the evaluation of environmental impacts (moisture, frost, CO2, de-icing agents, etc.) on the present and future health of the structure. Ability to assess the environmental effects on the degradation level is critical for the durability and resistance of structures. Research programs conducted by the international ISO standardization organization [3], the American Concrete Institute [4], or those implemented within the Basic Research in Industrial Technologies/ European Research in Advanced Materials (BRITE/EURAM) framework [5]

by the European Union countries, revealed that environmental changes—on a local, regional, and global scale—were faster than initially assumed, thereby making the environmental assessment an urgent issue.

It is particularly important to determine the onset of the damaging process in the materials and structural elements, because attempts to stop once-initiated processes are often inefficient, leading to the failure or catastrophic collapse of the entire structure. The Eurocodes propose four approaches to design for safety, thus providing a certain amount of autonomy to the designers. Niriaki in [6] eliminates this autonomy by proposing his original approach independent of structural safety but based on the durability and resistance analysis.

The demand for the method capable of detecting the onset of the deterioration process and monitoring its progress within the entire volume of the structure, not only in the subjectively chosen parts, is fulfilled by the technique of acoustic emission (AE). The method involves performing comparative analysis of the acoustic emission signals recorded during the tests and those collected in the database of reference signals corresponding to particular destructive processes. The results are used to identify and locate active destructive processes in reinforced concrete structures (IADP—Identification of Active Damage Processes), in prestressed concrete structure (RPD—Recognition of Destructive Processes), and in steel structures, thereby allowing the global monitoring across the whole member and recording only active deterioration processes developing under actual loading conditions [7–17]. The AE method is being increasingly developed for use in the construction of gas pipelines.

This article aims to discuss examples of AE monitoring of different engineering structures, including the AE-based Structural Health Monitoring System (SHMS) on the My Thuan Bridge.
