Bridging the Gap in NDT-E Deck Inspection

By SHANE D. BOONE, Ph.D., Chair of ASNT Research Council Infrastructure Committee

For generations, bridges have served to link communities across the country. They carry hundreds of millions of people over rivers, railroads, and valleys to their work, school, home, and other important destinations.  In addition, the US transportation infrastructure, particularly highways and associated bridges, carry the goods and services needed for our society to properly function. Bridges are a critical part of the nation’s infrastructure, worthy of our attention and preservation efforts.

Approximately 2.7% of the bridges in the United States have been classified as fracture critical as they have nonredundant elements that are susceptible to an increased risk of failure. This threat highlights an urgent need to inspect and preserve older bridges and associated infrastructure.  Specifically, nonredundant and associated fracture critical bridges may be susceptible to sudden collapse due to vessel impact or brittle failure of their members.  FHWA’s focus on T-1 steel over the last several years and recent bridge impacts and associated failures are reminders of this vital need.

The attention focused on the vulnerability of critical structures highlights the importance of continually assessing the structural integrity of bridges. While not every bridge is susceptible to catastrophic failure due to a vessel strike or brittle failure, other structural impairments such as corrosion of rebar in structural concrete can be found in a wide array of environments all of which can impact a bridge’s serviceability.

Our infrastructure is aging, with many bridges approaching their original design life.  For this reason, the FHWA mandates bridge inspection schedules and protocols. There is a large variety of bridges in the country, each one a crucial element of transportation infrastructure. The I-10 Bonnet Carré Spillway Bridge, a four-lane highway in southeastern Louisiana with a minimum clearance of only 5.21 meters, spans 8 miles over predominantly marshy terrain. Although it is unlikely to collapse from external forces, it remains susceptible to impact from degradation from consistent flooding of the spillway and associated environmental conditions. 

While most bridge inspections focus on evaluating the integrity of the support structures, it’s equally important for inspectors to assess the bridge deck. The bridge deck is a primary failure mechanism and often the biggest cost for bridge owners and operators. These frequently traversed roads are typically not considered as critical to a bridge’s structural integrity, but rather an element of structural serviceability.  What is less well understood, however, is that serviceability and structural integrity are directly and inextricably linked.

A Key to Bridge Longevity: Maintaining Serviceability

Serviceability is an important consideration for bridge inspectors, as it can be a leading indicator of potential safety hazards. Deterioration in bridge decks is also typically associated with corrosion of the steel reinforcement embedded within the substructure elements as deicing salts and other chloride containing elements can leak through joints, scuppers, and other drainage pathways onto pile caps, piles, and other structural concrete bridge supports. When reinforcing steel corrodes, it expands and creates a crack in the surface of structural concrete. Corrosion weakens the bridge structure over time and can lead to collapse if left unattended. Implementing a risk-based inspection frequency for bridge deck inspection provides an improved method for the asset management of this structural element. 

Fortunately, inspectors have well established and proven testing methods to check for material and structural flaws to help anticipate and address serviceability issues before they become safety issues. One method to improve the efficiency, reliability, and repeatability of this inspection is through the frequent application of nondestructive testing and evaluation (NDT-E).

To prevent bridge failure, engineers and owners are required to conduct continuous inspections. NDT-E helps confirm the serviceability and structural integrity of structural concrete bridges, protecting the infrastructure and human safety.

NDT-E in Bridge Inspection

NDT-E is used to inspect and assess the integrity of materials and structures without affecting their serviceability. By using the appropriate NDT-E method to examine a bridge deck, for example, inspection personnel can locate and quantify discrepancies in the structure and recommend to engineers where material repair, preservation or replacement is needed. 

As the only inspection process that can be conducted at highway speeds, NDT-E also is an ideal process for state, county, and municipal crews seeking to avoid lane closures and traffic delays. This makes it especially well-suited for routine inspection of bridges that run several miles over difficult terrain, such as the Bonnet Carré Spillway Bridge.

For bridge deck inspection, the most frequently applied NDT-E techniques are ground-penetrating radar (GPR), infrared thermography (IR), and impact echo (IE).

GPR operates by transmitting electromagnetic energy into an elastic material using an antenna attached to a survey vehicle. This energy is reflected to the antenna with an arrival time and amplitude that is related to the location and nature of dielectric discontinuities in the material (air/asphalt or asphalt/concrete, reinforcing steel, etc.). A vacuum theoretically has a relative permittivity or dielectric constant of one and would allow a complete transfer of these waves, and a perfect conductor would have an infinite dielectric permittivity and cause a perfect reflection of the waves. Steel and other metallic elements, therefore, have a relatively infinite dielectric permittivity and act as nearly perfect reflectors. These differences in dielectric constants create dielectric discontinuities that reflect energy and allow for detection of voids, air gaps, and embedded reinforcing elements such as reinforcing steel as well as areas of moisture, chlorides, and precursors to steel degradation.

Based on this operating principle, the depth and location of steel reinforcement can be measured from the GPR signal. The signal contains a record of the travel time and thicknesses of the layers within the structural member. By combining a series of sampled signals into a single image (B-Scan), features within the structural member can be identified.  It is most often used to identify placement of structural elements in material including concrete and asphalt of road surfaces and bridge decks.

Impact-echo is an ultrasonic method that involves impacting the structure and measuring the dynamic response. For concrete slabs and walls, the elastic wave travels from the front surface to the back surface, reflects off the back surface back to the front surface, and continues this reverberation pattern. The equipment used to make this measurement measures the resonant frequency.  This resonant frequency, referred to as the thickness resonance, is directly related to the concrete velocity and thickness in accordance with ASTM C1383-04.

The impact-echo test provides a resonant frequency associated with the thickness of the deck slab. If the slab is intact, the thickness data is clear, and should correspond with the expected slab thickness. If the slab is delaminated, the thickness data is unclear and generally does not correspond to the thickness of the slab. If the slab is thicker than attenuation capability of the dynamic forced induced on the system, the data collected can be used in a relative sense to identify areas that have a relative lack of stiffness when compared to the rest of the structure.

IR is a remote sensing technique used to measure surface temperatures based on the infrared radiation emitting from an object with thermal application. As the sun typically heats a bridge deck over the course of the day, areas that have been decoupled from the bridge deck (delaminations or debonded areas) absorb less energy and reflect it more.  By detecting this emissivity, inspectors are able to detect subsurface delaminations or debonding between layers. 

These three technologies can be used in conjunction to detect various stages of the structural concrete degradation cycle with GPR identifying the presence of chlorides, moisture, and those precursors to degradation that may begin the corrosion process, IE detecting the coalescing of microcracks into larger cracks and subsequent delaminations, and IR identifying large delaminations and debonding of overlaid layers.  

Emerging Technologies and the Evolution of NDT-E

The utilization of a multi-technological approach to bridge deck inspection allows for the detection of varying indications throughout the degradation cycle. This approach can be utilized to strategically implement a maintenance or preservation strategy with the understanding of differing degradation levels across a broad range of bridge decks.  While not necessarily related to the application of any one technology, the combination and fusion of these data sets is an emerging capability related to computing power and better understanding of the analysis techniques used to determine the location and severity of degradation detected.

Artificial intelligence (AI) and other machine learning tools have improved the data collection and evaluation process across inspection methods. AI can be used to identify automated flight paths for drones collecting both IR and high-resolution images, analyze the data and images captured to stitch them into orthomosaic images and associated 3D models, and automatically identify delaminations, cracks, patches, and spalls.  These techniques are particularly helpful when applying these types of inspection techniques in difficult terrain, such as an airboat when inspecting bridges in marshy environments like the Bonnet Carré Spillway in Louisiana. 

Although the industry has begun to embrace such innovative tools, the implementation by all states has not yet been realized.  Many states still rely on traditional methods of deck inspection such as manual visual inspection or chain drag.  While these methods are found to be tried and true, many studies have found them to be less repeatable and reliable than the quantitative capabilities provided by NDT-E.  As more states successfully implement NDT-E and make these data collection, analysis, data storage, and reporting processes more consistent, the comparison of data from agency to agency as well as the understanding of bridge performance as it relates to degradation and subsequent preservation activities will be better understood. 

Rules and Regulations for NDT-E Bridge Deck Application

These NDT-E methods only scratch the surface of inspection techniques, each of which sometimes requires its own certification and qualification. Depending on the methodology implemented, inspectors are required to complete varying levels of training and  certification via third party organizations such as the American Society for Nondestructive Testing (ASNT). In addition to obtaining certification recognized industry-wide, inspectors also must adhere to testing standards and guidelines. The American Society for Testing Materials (ASTM) International Standards are widely considered the highest level of national regulation, with different guidelines clearly describing how to perform the appropriate testing method. Each standard precisely explains the method and outlines the procedure and data processing aspects of the inspection method. The Federal Highway Administration (FHWA) also instructs bridge engineers and owners to conduct regular inspections through the National Highway Institute (NHI) and provides a specific course on the theory and application of NDE.

Bridge deck inspection is necessary for bridges everywhere. The vulnerability of fracture critical bridges has recently sparked conversation about bridge safety and highlighted the importance of structural integrity and the need for consistent bridge inspection, again. NDT-E provides a tool to ensure owners of these structures have improved data to best manage these assets with regards to maintenance and preservation for both bridge safety and serviceability. With quality routine inspection, bridges like the Bonnet Carré Spillway and thousands more like it will continue to serve travelers for decades to come.