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

corrosion stack is particularly useful for character- izing cyclic or condition-based bridge preserva- tion activity—especially useful for bridges with a National Bridge Inventory (NBI) rating between 7 and 9 (bridges in “good” condition). Ñ Damage Stack: IE is the primary NDE technique for the detection and characterization of deck delamination. USW provides a quantitative assessment through the measurement of concrete modulus and is an indicator of concrete quality. It sometimes indicates delaminated areas as well. The damage stack is useful for characterizing the extent of condition-based preservation activity— especially useful for bridges with an NBI rating of 5 or 6 (“fair” bridges). Figures 8 and 9 demonstrate a sample of an NDE stack from a bridge in the Mid-Atlantic cluster obtained through the FHWA Long-Term Bridge Performance (LTBP) program (Gucunski et al. 2016 Kim et al. 2019). As a part of the QC/QA process, it is important to ensure all NDE results are consistent and follow reasonable correlation across different 30 0 10 20 30 40 50 60 70 80 90 100 110 120 0 0.5 1 1.5 2 2.5 in. 3 3.5 4 4.5 kΩ × cm 100 (mV) Attenuation at top bar level (dB) 150 –7 Sound Fair Poor Serious –9–1 1 –13 –15 –16 –17 –18 –19 –20 –25 –30 –43 50 90 % probability of no corrosion activity 90 % probability of no corrosion activity Moderate High Low Very low Corrosion activity uncertain –50 –150–250–350–450–550–650–750 908070605040302010 0 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 20 10 30 0 20 40 60 80 100 120 140 160 180 200 220 240 260 20 10 30 0 20 40 60 80 100 120 140 160 180 200 220 240 260 20 10 30 0 20 40 60 80 100 120 140 160 180 200 220 240 260 20 10 Figure 8. Comparative assessment of NDE technologies on corrosion stack: (a) GPR cover depth (b) ER (c) HCP (d) GPR amplitude. 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 31 2301 ME Jan New.indd 31 12/20/22 8:15 AM Co ve r ER HC P GPR COURTESY: DR. NENAD GUCUNSKI, RUTGERS UNIVERSITY