**1. Introduction**

The vast presence of aging infrastructure throughout the nation including transportation and energy-related infrastructure has raised concerns regarding the level of service, reliability, and vulnerability to natural disasters. The American Society of Civil Engineers (ASCE) 2013 Report Card stated a grade of "D+" for US infrastructure and an estimated investment of \$3.6 trillion needed by 2020 for upkeep. One of the major challenges facing decision makers is resource allocation, which is dependent on available information related to the current state of each structure. Reliable monitoring techniques that can effectively assess structural conditions are needed to evaluate the robustness of structures and the urgency of any repair, replacement, or maintenance activities.

Monitoring nuclear facilities, in particular, is of special interest due to safety considerations and the relatively long half-life of nuclear waste products. Reinforced concrete elements are used to construct several portions of nuclear facilities. Potential degradation mechanisms of reinforced concrete [1] include corrosion of reinforcement [2–5], alkali-silica reaction [5–7], freeze-thaw cycling,

sulfate attack, deformation mechanisms including creep and shrinkage, stresses due to structural constraint combined with seasonal effects such as thermal cycling and precipitation, and extreme events [8–10].

Advances in computing and data transfer over the last several decades have allowed for the development of wireless systems and remote monitoring. Acoustic emission (AE) is one emerging monitoring method that has proven to have the potential for early damage detection through laboratory and field applications [11,12]. As a passive piezoelectric sensing technique, acoustic emission is able to detect stress waves (in the kHz range) emitted from sudden releases of energy such as cracking of the concrete matrix [13,14]. The method is suitable for real-time monitoring over the long term and its high sensitivity enables it to detect active cracks long before they become visible (micro-cracking).

Corrosion of reinforcing steel is a degradation mechanism that affects the durability of concrete structures. The cracking of the concrete matrix associated with corrosion damage makes acoustic emission a well-suited method for monitoring its progression. Early investigations related to acoustic emission monitoring of corrosion damage in reinforced concrete date back to the 1980s [15–17]. Several investigations demonstrated the potential of utilizing AE for this degradation mechanism [18–20]. However, the quantification of damage was not fully resolved. Quantification of corrosion damage in reinforced concrete structures has been more recently addressed in a series of publications using accelerated corrosion results in laboratory settings [2,3,21] as well as natural corrosion tests [3,22,23].

This study investigates the applicability of deploying acoustic emission for the remote monitoring of selected areas at the Savannah River Site (SRS) 105-C Reactor Facility in Aiken, South Carolina (Figure 1). This is an inactive nuclear facility under surveillance and maintenance operations as well as deactivation and decommissioning operations. AE monitoring was conducted at areas known to have active corrosion damage and/or visible cracking. This allows the examination of the applicability of previously developed AE methods for corrosion damage detection and classification.

To aid in the development of damage algorithms and to provide a more controlled study, an aged reinforced concrete block specimen cut from a similar reactor facility was maintained and monitored in the University of South Carolina Structures and Materials Laboratory for the majority of the project duration. This specimen was subjected to wet/dry cycling to accelerate the corrosion process. Electrochemical measurements were periodically recorded while acoustic emission was monitored continuously.

The activities undertaken and reported in this study represent a step toward the development of an acoustic emission based approach for the assessment of reinforced concrete structural systems through remote monitoring. To the authors' knowledge, this research is the first of its kind to study the suitability of acoustic emission to monitor corrosion damage in the field while comparing the results to a test block from a similar structure tested in a controlled laboratory environment.

**Figure 1.** Reactor building 105-C at the Savannah River Site.

The study is divided into two main activities: (1) Remote acoustic emission monitoring and analysis of data collected at the 105-C Reactor Facility and (2) Accelerated corrosion testing to assess corrosion damage within an aged and reinforced concrete block supplied by SRNS at the University of South Carolina Structures and Materials laboratory.
