without time-consuming surface prepa- ration in piles, caissons, and the like. With respect to the underwater inspection of concrete, several commer- cially available testing instruments have successfully been modified and tested for underwater use. Conventional rebar locators, operating to detect a magnetic flux disturbance caused by an embedded ferrous object, may be used under- water to locate and size rebar, as well as to measure the amount of concrete cover. Rebound devices (also known as the “Schmidt hammer”) that evaluate the compressive strength of concrete have also been successfully modified to operate underwater. UT methods are also available to estimate compressive strength and detect hidden discontinu- ities in concrete members. While easily operated by the diver, working in concert with trained above water testing techni- cians, each of these technologies has to date seen limited use in the underwater inspection of in-service bridge assets. A More Innovative Approach The historical VT-biased, “hands-on” approach to conducting underwater bridge inspection described above has been developed to detect discontinuities and evaluate conditions with relative celerity and reasonable levels of inspec- tion quality. In practical application, however, this approach is often found to be suboptimal, due to the multiple chal- lenges inherent in conducting inspec- tions in the underwater environment. As with inspection work conducted in dry environments, underwater VT examines only the exterior surface of the asset’s elements. Additionally, while diver-operated NDT test methods are helpful, the risk factors lying within the underwater environment—minimal visi- bility, extreme cold, submerged drift and debris, stubborn biofouling, chemical and biological pollution, vessel traffic, dangerous aquatic animals, inspection time limitations imposed upon the dive team by physiologic restrictions to diving at depth, coupled with a nationwide lack of divers themselves—introduce physical barriers and psychological restrictions precluding a thorough inspection of the asset. This being the case, the NBIS in 23CFR650.305 (“Definitions”) defines an underwater inspection as one involving wading, diving, or “other appropriate techniques.” Consequently, modern underwa- ter bridge inspections are increasingly reliant upon marrying new technologies and NDT methods with conventional, crewed diving inspection to gain a broader overall picture of the asset and its condition, increasing efficiency while lowering risk in the process. Underwater engineering inspectors today utilize tra- ditional VT and handheld UT and MT techniques, in concert with acoustic imaging techniques as well as NDT- capable ROVs, to obtain more detailed information about the asset’s elements as well as the bridge site at a more mac- roscopic level. Acoustic Imaging (Sonar) Inspection Sonar technology plays an ever-increasing role in today’s underwater bridge inspection procedures, both to increase inspector safety and improve inspection quality. Sidescan, sector-scan, multibeam, and real-time volumetric imaging sonar systems each play a role, and all have the ability to “see” underwater when little or no visibility exists to the human eye. While a sonar unit cannot remove marine growth, reliably detect hairline cracks, or thoroughly measure the penetration depth of foundation undermining at a bridge pier, it can outperform a diver in many other areas, including the mea- surement of local scour holes (in clear water conditions), in assessing opening widths and heights of scour voids under bridge foundation elements, in visualizing debris accumulations and other hazards adjacent to a bridge pier, and in the ability to assess the sizeable areas of streambed situated out and away from the pier face. Considering that the number-one cause of bridge failure worldwide is scour (and not those superficial, hairline cracks), and considering that most bridges span waterways exhibiting adverse conditions (deep, turbid, debris-laden, and/or swift water), one can make a strong case for mandating sonar assessments in certain inspection scenarios. To assist in the assessment of sonar technology during underwater bridge inspections, on 14 June 2018, the FHWA released Technical Report FHWA-HIF- 18-049, Underwater Inspection of Bridge Substructures Using Imaging Technology, which evaluated various sonar imaging technologies and compared their perfor- mance in real-world bridge inspection tests to conventional data collection using divers. As per the report’s abstract, the study determined that: Ñ Sonar technology is capable of iden- tifying larger-scale characteristics of interest such as scour holes, debris, and moderate to large size voids and protrusions. It is more limited in iden- tifying small-scale cracks or features hidden by marine growth. Ñ Sonar is particularly effective in adverse conditions where limitations on diver bottom time exist, such as deep water, and conditions where diver safety or mobility is of particular concern including swift currents or turbid water. In all environments, including those with adverse condi- tions, sonar technology is useful for identifying macro features quickly. Ñ Sonar technologies can be used to inspect underwater structural elements where divers cannot work effectively, to guide divers for a closer look, or to provide independent inspection insights. Sonar can also be effective in covering large areas quickly. The study reported two overall conclusions: Ñ “Sonar inspections have not demon- strated the ability to identify some smaller scale elements of substructure condition that may be important in assessing the bridge and recom- mending maintenance.” Ñ “Sonar technologies offer significant opportunities for improving under- water bridge inspections, especially in adverse environments or to inspect extensive areas.” In summary, the study found that sonar cannot replace diver inspectors, but can yield an improved process when FEATURE | UNDERWATERINSPECTION 38 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 38 12/20/22 8:15 AM

conducted in combination with tradi- tional diving inspection. Depending upon the technology of choice, sonar equipment uses a remotely controlled sonar head that is either hard-mounted, pole-mounted in a boat, towed, or even affixed to an autonomous watercraft. The newest technologies even provide “live” data capture, offering a 3D display in real time. Each sonar technology has its own advantages, as highlighted in the following. Sidescan Sonar A tried-and-true technology with offshore origins, sidescan sonar employs a towed or otherwise transported “fish” that continually moves through the water while scanning the channel bottom off to both of its sides. This technology yields processed data images with excellent resolution, readily revealing areas of local scour, pier foundation exposure, and accumulated drift and debris better than a diver can envision. One recent and unique adaptation of sidescan has been its integration into small autono- mous watercraft, which can be deployed to gather bridge scour data adjacent to bridge piers during flooding conditions, before divers can safely access the bridge site (see Figure 4). Such systems are currently in use with several state DOT agencies and other service providers to help engineers make public safety deci- sions during emergency situations. (As an example, Figure 13 displays a typical data output screen from an autonomous vehicle system using sidescan sonar.) Sector-Scan Sonar Another established and valuable tech- nology, sector scan employs a rotating head containing one or more sonar transducers, which is rotated in staccato fashion via a stepper motor, to gather image data of up to 360° from a single static deployment. Mounting of the scanning sonar head is quite flexible, as it may be rigidly mounted, attached to a pole for boat use, or deployed hanging vertically from a tripod which is lowered to the channel bottom. Tripod deployment of a sector-scan sonar head is illustrated in Figure 5. Individual scans can typically be com- pleted in a few seconds, and output data of the site is displayed in either Figure 4. Autonomous vessel equipped with sidescan sonar evaluates a bridge. Figure 5. Sector scan sonar head deployed in a tripod on channel bottom. Figure 6. Post-processed sector scan image, showing elevation view of pier. 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 39 2301 ME Jan New.indd 39 12/20/22 8:15 AM COURTESY: BRIAN ABBOTT COURTESY: HYDRONALIX COURTESY: BRIAN ABBOTT