*3.4. My Thuan Bridge*

Examples of localized concrete cracks within compression cables are shown in Figure 16.

**Figure 16.** Examples of localized concrete cracks within the compression cables on My Thuan Bridge.

Described below are two complete measurement systems being the elements of the condition assessment system that helps decision makers plan maintenance and repairs [30].

The vertical displacement measurement system (Figure 17) measures the condition of and long-term changes in the span, and determines support settlements and their effect on the span vertical alignment. The static measurements are performed every hour or every 30 min, whereas the dynamic measurements are made when the accelerometer reading is higher than the value determined in the design calculations or on demand. The system comprises seven measurement points along the span and, if needed, a hydro profile meter and a hydro level (GEOKON) can be mounted on the pylons and the Global Navigation Satellite System (GNSS) can be used both along the span and on the pylons.

**Figure 17.** Arrangement of measurement points in the vertical displacements system.

The strain measurement system (Figure 18) measures long-term strain variations in the span and pylons. The sensors are mounted on the rebar, first uncovered and subsequently secured again. The system comprises seven measurement points with two sensors at each point to reduce the risk of error. The location of the measurement points should be established to the accuracy of ±1.0 cm. As the bridge is large, the wind profile and its effective velocity need to be determined.

The angle measurement system measures torsion in the spans generated by all types of loads, span rheology, and workmanship quality. The system comprises seven measurement points located along the span length. At each point, torsion and bending are measured.

**Figure 18.** Arrangement of measurement points in the strain measurement system.

Prior to the design of the indicated system, for a period of 1.5 years, every 6 months AE was measured at selected points, which confirmed the development of microcracks and cracks in the concrete elements. This supported the idea of applying the AE method for monitoring the facility using the proposed prototype. Measurements were carried out in March 2016, October 2016, and March 2017. The measurement in each case lasted an hour and was conducted in the afternoon of the traffic peak. Below are sample graphs of signal energy versus time for each measurement period and one graph showing the duration of signals against time for one measurement period, because it was similar in each research period.

Analysis of the graphs (Figures 19–22) indicates that in each case the signal energy is not very high and reaches the values of 5000 ec and 6000 ec in the first and the following two measurement periods, respectively. Also, the values of the signal duration are low and reach a level of approximately 1500 μs in each analyzed period. While analyzing the graphs, it can be also observed that the data indicates generation of Class 3–6 signals (Table 4). The duration of the signals is relatively short—it is assumed that the signals are induced by atmospheric conditions and by vehicle traffic. Further test results are dependent on the implementation of the prototype installation. Currently, the research team is working on creating the reference base.

**Figure 19.** Energy versus time for the first measurement period—March 2016.

**Figure 20.** Energy versus time for the second measurement period—October 2016.

**Figure 21.** Energy versus time for the third measurement period—March 2017.

**Figure 22.** Signal duration versus time for three measurements.

#### **4. Conclusions**

The examples of acoustic emission evaluation of various types of engineering structures and the proposed global monitoring system based on the measurement of acoustic emission signals accompanying destructive processes, covers the entire volume of the element under test or its selected part, and allows locating and identifying active destructive processes and their dynamics in real time. The data collected can be the basis for determining the structural condition of the structure [33–39].

The system is a useful tool for:


The findings presented in this article indicate that the use of the acoustic emission method with the systems of the global structural health monitoring ensure insight into the condition of structures.

#### **5. Patents**

"The system for detecting and locating active destructive processes in communication road infrastructure structural" Wiesław Tr ˛ampczy ´nski, Grzegorz Swit, Barbara Goszczy ´ ´ nska.

"Method for testing and/or monitoring of destructive processes in steel structures subjected to loads" Leszek Gołaski, Barbara Goszczy ´nska, Wiesław Tr ˛ampczy ´nski, Grzegorz Swit. ´

"Method for diagnosis and/or monitoring of technical condition of reinforced concrete and prestressed concrete structures and a system for diagnosing the condition of reinforced concrete and prestressed concrete structures" Wiesław Tr ˛ampczy ´nski, Grzegorz Swit, Leszek Gołaski, Barbara ´ Goszczy ´nska, Kanji Ono.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.
