for similar purposes. A futuristic view of using UAS-based NDE for elevated structures (bridges, buildings, dams, power plants, tunnels, etc.) is discussed in detail by Chen et al. (2023). These systems, including human-robot systems, have the potential to transform bridge inspections in the future to minimize impacts to traffic at bridge sites (see Figure 3). Ñ Similarly, modern underwater inspections are increasingly reliant upon new technologies and nondestructive testing methods beyond visual inspection (see Figure 4). These technologies are used in conjunction with conventional diving inspection to gain a broader overall picture of the asset and its condition, increasing efficiency while lowering risk in the process. Severns (2023) discusses the more modern underwater inspec- tion approach, emphasizing the modern NDT technologies utilized as well as their benefit to the process. Ñ Improving the accuracy in the detection and characterization of deterioration and defects using NDE methods is essential for the condition assessment of reinforced concrete bridge elements. At the same time, improving the speed of NDE data collection and interpretation— allowing economical periodical evaluation—will enable capturing of deterioration processes and defect formation leading to the development of more realistic deterioration, predictive, and life cycle cost models. Ultimately, those will lead to better bridge management. Gucunski et al. (2023) provide an overview of the current practice of bridge evaluation by NDE methods, recent efforts to improve the speed of NDE data collection through automation and robotics, and improved condition interpretation through advanced visu- alization and combined analysis of results of multiple NDE technologies. Ñ Deployment of large-scale wired and wireless sensor networks for bridge structural monitoring (SM) is also being accepted by more and more owners. Augmented by use of artificial intelli- gence (AI) and deep learning for data analysis, Figure 4. Underwater ultrasonic inspection of steel using remotely operated vehicle. FEATURE | BRIDGEINSPECTION Figure 3. Climbing robot with NDE capabilities assisting with bridge inspection. 28 M AT E R I A L S E V A L U AT I O N • J A N U A R Y 2 0 2 3 2301 ME Jan New.indd 28 12/20/22 8:15 AM COURTESY: RICH ARRIETA, NAVAL INFORMATION WARFARE CENTER COURTESY: DR. GENDA CHEN, INSPIRE UNIVERSITY TRANSPORTATION CENTER

these technologies can be useful, especially for complex structures. Remote sensing using satellite-based technologies for system-wide monitoring of the structural assets as a first level of monitoring is also perceived to play a major role as the cost associated with satellites becomes affordable (see Figure 5). Early-Age Preservation Activities A bridge preservation program consists of per- forming cost-effective cyclical and condition-based preventive maintenance (PM) activities that seek to prolong the service life of bridges and delay the need for rehabilitation or replacement. PM activ- ities, as part of the bridge preservation program, can extend the service life of a bridge when it is in good or fair condition. In Figure 6, the leftmost line represents a service life of a bridge without PM and the rightmost line represents the same bridge with both early-age and NDE-based cyclical and condition-based (blue vertical line) PM activities applied when the bridge elements are in good and fair condition. The comparison shows the longev- ity of a bridge’s service life in a bridge preservation program. The need for collecting early-stage NDE data using advanced multi-sensor systems is discussed for bridge preservation decision-making (Jalinoos et al. 2010a, 2010b). In the next section, field NDE data examples from a reinforced concrete bridge, specif- ically bridge decks, are included to demonstrate a data-driven tool that can lead to bridge preservation decision-making. Corrosion of Bridges Steel-reinforced concrete bridges endure exposure to corrosive materials and environments such as deicing salts and salt water. Unmitigated, these materials accelerate corrosion and significantly reduce service life. The corrosion process begins once the flow of ions from anodes to cathodes has reached a threshold as to destroy the protective oxide film on the rebar. The expansion of corrosive products leads to micro- and macro-cracking of the concrete, delamination, and ultimately bridge deck spalling. Clearly, the ability to characterize a cor- rosive environment, measure metal corrosion and broken strands, and detect delamination/deterio- ration is essential in describing concrete member conditions. Due to significant resources spent fixing bridge decks and the value in preserving the superstructure members under them, there have been considerable NDE research and implementa- tion efforts regarding bridge decks during the last decade. Hence, this section describes some of these efforts related to bridge decks. It should be noted Figure 5. Processed historical InSar Satellite data (20 708 distinct radar scatters) detecting no settlement at Merrimac Memorial Bridge Tunnel. –5 +5 Velocity (mm/year) North approach South approach Condition-based maintenance Legend Solid-colored lines = with preservation (cyclical and condition-based maintenance) Dashed-colored lines = without preservation Increased service life Severe Poor Fair Good Time Figure 6. A comparison of bridge condition over time with and without bridge preservation (FHWA 2018). J A N U A R Y 2 0 2 3 • M AT E R I A L S E V A L U AT I O N 29 2301 ME Jan New.indd 29 12/20/22 8:15 AM Condition COURTESY: EDWARD HOPPE, VIRGINIA DOT