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

that these technologies are equally applicable to other concrete members with metal reinforcement. Table 1 highlights NDE techniques that have applicability for the detection of defects and dete- rioration in bridge decks. To successfully track the early-age corrosion progression, NDE techniques such as electrical resistivity (ER), half-cell potential (HCP), and ground-penetrating radar (GPR) are typically used. For later phases of corrosion to end of service life physical damage, ultrasonic surface waves (USWs), impact echo (IE), and thermal/ infrared (IR) imaging are commonly used. To assess the service life of the bridge deck, it will be of interest to be able to assess the condition of a bridge deck at all stages of deterioration. With recent advancements in robotic technol- ogy, however, there has been a shift toward the development of automated bridge data collection systems which include automation in data pro- cessing for near real-time results. Some of these multi-sensor platforms can be deployed at traffic speeds such as GPR, IRT, and sounding/chain drag systems for deck scanning. For higher-resolution, more in-depth evaluation, the deck is closed for traffic and other systems such as ER, HCP, USW, and IE surveys are used. The robotic systems allow for several times faster data collection time than deploying individual NDE tools—thus decreas- ing lane closure time and traffic congestion. The multi-sensor systems also allow for the acquisi- tion of large physically independent datasets to increase the reliability of the results and reduce false calls. DEVELOPMENT OF NDE STACK Given the complex nature of the corrosion process, it is imperative to collect data from a suite of NDE technologies in order to ensure consistency among different technologies. To accomplish this, the use of “NDE Stack” plot is described herein, where multiple NDE contour plots are organized in a sequential manner. As shown in Figure 7, initially a set of contour maps is used to study the deck’s corrosive environment starting with rebar concrete cover depth (obtained from the GPR data) ER and HCP to assess corrosion rate and activity, respec- tively and GPR for the corrosive environment. This group is followed by contour maps indicative of concrete physical damage, starting with USW indicating changes in concrete elastic moduli and concrete quality and IE, which is indicative of delaminated areas. The two corrosion and damage stack groups are summarized as follows: Ñ Corrosion Stack: The two principal methods for corrosion assessment include ER and HCP, with ER evaluating the corrosive environment and HCP assessing the probability of active corrosion. The GPR (both cover depth and depth-corrected amplitude) can identify the corrosive environ- ment and the presence of moisture. Areas where GPR indicates low concrete cover depth are good candidates for increased corrosion activity. The Surface crack map GPR cover depth Electrical resistivity Half-cell potential GPR amplitude Ultrasonic surface waves Impact echo IR/ chain drag Corrosion stack Damage stack Figure 7. Proposed NDE stack template. FEATURE | BRIDGEINSPECTION TA B L E 1 NDE techniques and their application in bridge deck deterioration detection Corrosion State NDE Method Defect/Deterioration Early-Stage Corrosion Half-Cell Potential (HCP) Probability of active rebar corrosion Electrical Resistivity (ER) Likelihood and severity of corrosive environment Ground Penetrating Radar (GPR) Detection of deterioration caused by corrosion and moisture, and rebar cover depth/thickness Late-Stage Damage - Delamination Ultrasonic Surface Waves (USW) Measurement of degradation of elastic moduli Impact Echo (IE) Deck delamination detection and characterization Thermal/Infrared Testing (IR) Shallow delamination detection 30 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 30 12/20/22 8:15 AM COURTESY: FRANK JALINOOS, FHWA