well as analytical data identifying critical bridge foundation elevations raising structural concerns. Trusted NDT Technologies Since the 1960s, transportation assets have been evaluated by divers using a combination of NDT devices that migrated inland largely from the offshore oil and gas fields, combined with modified “topside” (above water) NDT testing equipment and a few improvised pieces of gear. In addition to VT using ubiquitous handheld or helmet-mounted high-definition photographic and video cameras, measuring devices (usually wooden folding rulers) and clear water boxes (used to aid still photography in murky waters), inland diver inspectors conduct underwater structural inspec- tions using a small variety of hand-held NDT devices. Ultrasonic Testing (UT) Other than VT, select UT methods are by far the most common NDT tech- nology employed during underwater bridge inspections to identify, locate, and size discontinuities in steel, timber, and concrete members. The equip- ment used underwater comes in two varieties: modified topside ultrasonic equipment, typically consisting of a transducer connected with a long cable to a conventional topside UT scope or a self-contained, water-tight unit handheld by the diver (see Figure 1). In the former instance, the diver manipulates a straight-beam or angle-beam transducer underwater while readings are taken from a topside technician who controls the UT instrument in a clear, benign environment. More common today is self-contained equipment built specifi- cally for use underwater (see Figure 2). Ultrasonic thickness testing (UTT) is the most common UT technique employed during underwater bridge inspections. It is commonly used to obtain thickness measurements in steel members. The member to be tested is cleaned to establish a clean, smooth surface, and measurements are obtained in predetermined locations. For sub- structure supports using steel piles or columns, section measurements are typ- ically obtained in the splash zone, at the mudline, and near the midpoint of the water column. The results are typically archived in a matrix format, for ease of comparison to future measurements taken in the same areas. From this, deterioration rates may be established. Section thickness data of concern is evaluated analytically and appropriate actions are taken, which might include corrosion mitigation measures, structural repairs, or, in extreme instances, mod- ifications to loading of the element(s) until repair or replacement actions are effected. UTT equipment used in the field is almost exclusively of the self-contained variety. Ultrasonic angle-beam testing using shear waves (such as shear wave and phased array ultrasonic testing) is con- ducted on a very limited scale (primarily during in-depth, Level III inspections) on submerged bridge members for the inspection of welds, crack detection, and for sizing of discontinuities. While both single-element and multi-element phased array systems have been adapted for use underwater, the latter equipment type is seldom used during underwater bridge inspections. Unlike UTT, angle- beam testing in the inland environment is usually conducted using a topside scope, and the diver is merely manipu- lating the transducer, due to limitations introduced by water turbidity. Magnetic Particle Testing (MT) MT plays a diminished role in underwa- ter bridge inspection as compared to its use in the offshore underwater inspec- tion industry. Diver-manipulated yoke systems are at times used, but princi- pally only during Level III inspections in clear, calm waters, where the powdered metallic filings can be applied and indi- cations can be seen by the diver (see Figure 3). Unfortunately, these condi- tions are not often experienced in the inland environment. Other NDT Methods Other NDT methods and techniques are available to the underwater bridge inspection team for testing of steel members. Techniques such as acoustic emission testing (AE), time of flight dif- fraction (TOFD), and underwater pulsed eddy current (PEC), while in use in the offshore inspection arena, have not readily advanced into the inland bridge inspection industry. Underwater PEC in particular holds promise, consider- ing that the technology is specifically designed to detect corrosion hidden under marine growth or coatings. Steel wall thickness can thus be measured Figure 1. Underwater-capable UTT unit with long transducer cable. Figure 2. Diver held UTT unit. Note display visible to diver. Figure 3. Underwater MT system, with diver- held yoke and lamp. 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 37 2301 ME Jan New.indd 37 12/20/22 8:15 AM

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