## 2 Materials and Methods
### Methods  
Basic acoustic emission monitoring is carried out during the routine operation of the facility under load (in exceptional cases under the test load). The tests performed during the routine operation of the facility aim to:  
* look for active damage processes (progressing under service conditions) in the structure;
* identify and locate the damage;
* estimate the risk potential of the damage;
* determine the extent of the risk posed to the safety of the facility under routine operation. The uSamos system (PAC MISTRAS Corp., 195 Clarksville Rd, Princeton Jct, NJ 085, USA) with three PCI-8 cards was used to develop the reference signal databases (24 measurement channels) used in subsequent stages to assess the condition of steel and concrete structures. Flat type sensors  
Figure 5: (**a**) Arrangement of measuring points on the gas pipeline. (**b**) Measuring scheme. operating in the frequency range of 30-80 kHz was used for all structures. A 55 kHz sensor was also used in the concrete structures. Signal amplification was 40 dB. AEWin software was used for the measurements. To analyze the measurement data, Noesis 5.8--ADVANCED DATA ANALYSIS PATTERN RECOGNITION SOFTWARE (PAC MISTRAS Corp., Princeton, NJ, USA) was applied. The reference signal database was created using 13 parameters of acoustic emission: rise time (\(\upmu\)s), counts to peak amplitude, counts, energy (EC), amplitude (dB), average frequency (kHz), root-mean square (RMS) (V), reverberation frequency (kHz), initiation frequency (kHz), absolute energy (aJ), signal strength (pVs), duration (\(\upmu\)s), and average signal level (ASL) (dB). The Identification of Active Damage Processes (IADP) method is based on the analysis of acoustic waves generated by active destructive processes developing under operational loads in buildings and engineering structures. The signals received by acoustic sensors mounted on the object are compared to the database of reference signals created earlier for defined destructive processes. The identified destructive processes are then localized by analyzing the difference in time of signal arrival to individual sensors. The identification and the location of active damage processes make it possible to assess the technical condition of structures, and thus can be the basis for the diagnosis and management strategies. The big advantage of the IADP method is that the AE sensors can be arranged to cover the entire structure being measured and that the measurements can be performed under actual operational loads. In its original version, the IADP method was developed to analyze destructive processes in prestressed concrete elements. It was then expanded and applied in the analysis of steel elements for which a new database was created with pattern signals of destructive processes resulting from cyclic and monotonic loads. Patterns of AE signals were obtained as a result of tests carried out at temperatures ranging from \(-60\)\({}^{\circ}\)C to \(+60\)\({}^{\circ}\)C on monotonically loaded V-notch specimens made with St3S and 18G2A grade steels and with the steel recovered from an old bridge, on bent models of V-notch elements under monotonic and cyclic loading at \(+20\)\({}^{\circ}\)C, and models of welded, riveted, and bolted nodes made of the same grade steel. The following destructive processes being the AE signal sources can be distinguished in prestressed concrete structures:  
* microcracks;
* friction between crack surfaces;
* formation and development of cracks in concrete;
* cracking at the concrete-reinforcement interface;
* concrete crumbling;
* friction at the concrete-reinforcement interface;
* corrosion;
* plastic deformation and cracking of cables and other reinforcements. The values of the AE signal parameters allow the classification of the signals into classes, each of which is characterized by dominant destructive processes and different risk levels.