ER evaluates the corrosive environment of concrete by measuring the resistivity of concrete, which is significantly influenced by the moisture content (Nguyen et al. 2017), chloride concentration (Rupnow and Icenogle 2012), and the presence of cracks. The resistance can be related to the normally observed corrosion rates (Gowers and Millard 1999 Hornbostel et al. 2013). The most commonly used probe for field evaluation is a four-point Wenner probe, shown in Figure 1, which conforms with AASHTO TP 95-11 specifica- tions. On the other hand, HCP provides information about the probability of active corrosion of reinforcement (ASTM C876-15). The method can be used at any time during the life of a concrete structure provided the temperature is above 2 °C (Elsener et al. 2003). HCP has a maximum penetration depth of around 200 mm (Bien et al. 2007). There are different types of HCP probes, also called reference electrodes. A rolling HCP probe is shown in Figures 1 and 2. Because the probe needs to be connected to the steel reinforcement, and the reinforce- ment needs to be connected to provide continuity throughout the survey area, the method is not appropriate for inclusion in automated or robotic systems. Delamination detection and characterization are primar- ily done using acoustic methods. Among acoustic methods, impact echo is the most commonly used. IE examines the transient response of a plate-like structure subjected to an acoustic pulse. The objective of the IE test is to detect resonant modes (Sansalone and Carino 1989). The peaks in the response spectrum represent either the thickness stretch Lamb wave modes (Gibson and Popovics 2005) or flexural modes (Kee and Gucunski 2016). The delamination depth can be obtained from the resonant frequency. In the case of shallow delamination (shallower than about 50 mm), where the resonant frequency is high, a more reliable approach is using a constant-phase IE approach (Almallah and Gucunski 2019). Air-coupled IE, in which contact sensors are replaced by microphones (Zhu and Popovics 2007) or MEMS (Sun et al. 2018), opens opportunities for rapid data collection. Another acoustic method that will be discussed later, ultrasonic tomography (UST), provides more detailed imaging of delaminated elements. Finally, shallow delamination can be rapidly detected using infrared thermog- raphy (IRT) (Maser and Roddis 1990 Maierhofer et al. 2001). However, delamination detection significantly depends on environmental conditions (Washer et al. 2009). The quantitative assessment of concrete quality is most frequently assessed by the measurement of concrete elastic modulus using the USW method (Nazarian et al. 1994). The USW modulus evaluation relies on the measurement of the velocity of propagation of surface waves of wavelengths shorter than the deck thickness. The presence of shallow delamination often leads to the measurement of an apparent low concrete modulus (Azari et al. 2012). Typical USW devices consist of a source and two receivers. One of the devices, the portable seismic property analyzer (PSPA), is shown in Figures 1 and 2. GPR provides a qualitative assessment of the deck condition. The condition assessment is based on the evaluation of the amplitude of electromagnetic waves reflected from the top rebar layer (Barnes and Trottier 2000 Maser and Bernhardt 2000 Tarussov et al. 2013). Using certain thresholds, most often related to the results from other NDE technologies, areas with high signal attenuation in the obtained map are described as deteriorated concrete (Barnes et al. 2008). Consideration of the variability of concrete cover thickness and concrete conduc- tivity is essential for an accurate assessment of signal attenua- tion (Dinh et al. 2016). Bridge deck evaluation by GPR can be conducted using ground-coupled (Figure 1) and air-coupled antennas. Typical productivity for ER, HCP, and IE data col- lection using the shown devices is about 100 m2 (1000 ft2) on a 60 × 60 cm (2 × 2 ft) test grid. It is about 25 m2 (250 ft2) for ME | NDEOFBRIDGES IE ER HCP GPR USW Figure 2. Manual NDE data collection on bridge piers and girders: (a) GPR (b) ER (c) USW (d) IE (e) HCP. 58 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 58 12/20/22 8:15 AM

USW and about 500 m2 (5000 ft2) for GPR on survey lines with a 60 cm (2 ft) spacing. More information about the perfor- mance of NDE technologies for the condition assessment of bridge decks can be found in an SHRP 2 report (Gucunski et al. 2013). Condition assessment of other bridge components can be done using the same NDE technologies and devices as for concrete bridge decks. The biggest challenge in the NDE tech- nology deployment is access to the element to be evaluated. The application of the previously described five NDE technol- ogies in the condition assessment of concrete bridge piers and prestressed girders is shown in Figure 2. Automation of NDE Data Collection Comprehensive condition assessment of concrete bridges can be done using manual NDE technologies. However, and as illustrated in the previous section, manual multi-NDE tech- nology data collection requires significant time and effort. In addition, in bridge deck evaluation, and in cases when super- structure and substructure evaluation is done using snoopers, work zones for inspection cause traffic interruptions. All are adding to the cost of inspections, slowing or interrupting the traffic flow, and increasing risks for the inspectors and the traveling public. Considering that there are more than 620 000 bridges in the US, manual inspection and condition monitoring of the bridge network using NDE is not feasible. Automation of NDE data collection is, therefore, critical for their wide adoption as a tool for accurate condition assessment and monitoring and, thus, economic bridge management. The following sections describe some efforts in the automation of NDE data collection in the inspection of bridge decks and other bridge components. Automation of Bridge Deck NDE One of the earliest attempts for automated multi-NDE of concrete bridge decks was the robotic system BETOSCAN developed at the German Federal Institute for Material Research and Testing (BAM) (Raupach et al. 2009). BETOSCAN enabled the deployment of multiple NDE devices: ultra- sonic, potential mapping, microwaves, and cover meters. While the system had significant capabilities, it could be deployed in the evaluation of smaller areas only because of a single NDE sensor installation. The RABIT (Robotics Assisted Bridge Inspection Tool) platform (Gucunski et al. 2017) brings elements of the previous efforts and implements them in a much bigger robotic platform with multiple NDE devices or sensor arrays (Figure 3). RABIT integrates four NDE technologies: ER, GPR, USW, and IE. There are four ER probes and two acoustic arrays on the front end of the platform. The two acoustic arrays are equivalent to 16 IE devices and 12 USW devices. Two GPR arrays are mounted on the rear end of the platform, each having eight antenna pairs of dual polarization. Finally, two high-resolution cameras for deck surface imaging are mounted on the front end. RABIT is a fully autonomous system, whose navigation was achieved through an integration of the differ- ential global positioning system (DGPS), inertial measurement unit (IMU), and the wheel odometry. The data collection path is preprogrammed in terms of the deck sections to be evalu- ated and increments in the data collection. All the NDE data are streamed and monitored in real-time in the command van (Figure 3). The data collection production rate depends on the length of the bridge and data collection increment. For bridge decks approximately 100 m (330 ft) long, and data collection increment of 0.6 m (2 ft), RABIT can evaluate about 350 to 400 m2 (3500 to 4000 ft2) per hour. A sample of results from the RABIT includes the condition maps of a bridge shown in Figure 4, the ER map describing the severity of the corrosive environment, the IE delamination map, and GPR and USW maps of qualitative and quantitative concrete quality assessment. The condition maps are comple- mented by a high-resolution image of the deck surface which is obtained from stitched camera images. A section of the bridge deck with visible cracks is shown in the same figure. Such images become permanent records of the deck surface that can be reviewed at any time. GPR USW-IE ER ER USW-IE Camera Camera ER ER GPR Figure 3. Robotic platform RABIT: (a) front view (b) back view (c) unloading from the command van (d) screens in the command van. 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 59 2301 ME Jan New.indd 59 12/20/22 8:15 AM