Nondestructive Evaluation and Structural Health Monitoring
- Nondestructive Evaluation (NDE)
- Structural Health Monitoring (SHM)
Each NDE technology has a specific physical principle and can detect certain type(s) of defect(s). To improve the quality and reliability of information provided by NDE data, collecting data with two or more NDE tools and technologies is recommended. The Long-Term Bridge Performance (LTBP) Program has identified the following five NDE technologies as appropriate technologies to assess the condition of bridge concrete decks:
- Half-cell potential (HCP) to assess corrosion activity.
- Electrical resistivity (ER) to assess corrosion activity.
- Impact echo (IE) to detect and characterize delamination.
- Ultrasonic surface waves (USW) to assess concrete quality by measuring concrete modulus.
- Ground penetrating radar (GPR) to provide a qualitative assessment of the deck condition, detect and locate corrosion induced damages, and to measure the concrete cover.
The HCP method can be used to identify corrosion activity of steel reinforcement in reinforced concrete structures. HCP measurement is based on the coexistence of corroding areas (or anodic half-cells) and noncorroding areas (or cathodic half-cells) on rebar. The measurement is calculated as the potential differences, or voltages, between the reference electrode and the steel-concrete interface, which can be attributed to the corrosion activity at the surface of the steel. (Visit the NDE Web Manual for more about HCP.)
Electrical resistivity can be used to detect a corrosive environment and determine reinforced concrete's susceptibility to corrosion. Resistivity measurement can be used in tandem with other corrosion assessment techniques such as HCP to provide a more comprehensive assessment of corrosion. Damaged and cracked areas, due to increased porosity, will form preferential paths for fluid and ion flow. This will lead to higher moisture and chloride concentrations and higher concrete electrical conductivity, manifesting as a lower ER. The lower the ER of the concrete, the higher the current passing between anodic and cathodic areas of the reinforcement steel will be. (Visit the NDE Web Manual for more about ER.)
The IE method is a stress-wave-based method used to detect defects in concrete, primarily delamination. The objective of the IE survey is to detect and characterize wave reflectors or resonators in a concrete bridge deck or other structural elements. This testing method is conducted using an impact source and a nearby receiver. The impact generates waves that propagate within the solid material. Waves are reflected by internal defects (difference in acoustic impedance) or external boundaries. When the reflected waves, or echoes, return to the surface, they produce displacements. The transient response time of the solid structure is measured with a sensor mounted on the surface close to the impact source. The multiple reflected waves monitored by the sensor manifest as resonance conditions in the frequency domain analyses. (Visit the NDE Web Manual for more about IE.)
The USW method is used to evaluate material properties (elastic moduli) in the very near surface zone. The surface wave velocity can be converted to the material modulus, or the concrete modulus in the case of bridge decks, using either the measured or assumed mass density or Poisson’s ratio of the material. A USW test consists of recording the response of the deck at two receiver locations due to an impact on the surface of the deck. (Visit the NDE Web Manual for more about USW.)
GPR is a rapid, nondestructive testing method that can be used to identify and map areas with a high likelihood of corrosion-based deterioration, assess construction quality, determine structural reinforcement layout, and estimate the thickness of the deck, overlays, or reinforcement cover. A GPR antenna transmits high frequency electromagnetic waves into the structure and detects the reflected signals. (Visit the NDE Web Manual for more about GPR.)
The FHWA LTBP Program developed a multifunctional NDE platform to enhance the assessment of bridge decks. The RABIT™ bridge deck assessment tool was developed to deploy a suite of NDE technologies simultaneously. The technologies included on the RABIT™ bridge deck assessment tool are ER, IE, USW, GPR, and a panoramic camera to allow for high-definition imaging of the bridge deck.
More information about RABIT™ bridge deck assessment tool can be found on the Products Page.
To assist with the day-to-day challenges that practitioners face when deciding which NDE technology will be the best option for their specific investigation needs, FHWA developed the NDE Web Manual. The NDE Web Manual is a web application that consolidates information on an array of technologies and informs users about: how a technology works, for which applications it works best, and the best practices for use. The user can choose to browse the technologies or they can input the known variables they’re working with (infrastructure type, material, problem type, etc.), and the NDE Web Manual will provide a list of technologies that are best suited for that situation.
More information about the NDE Web Manual can be found at Nondestructive Evaluation (NDE) Web Manual, Version 1.0.
Eight protocols related to NDE have been included in Report FHWA-HRT-16-007, Long-Term Bridge Performance (LTBP) Program Protocols, Version 1. The following is a list of the NDE protocols in version 1 and for what purpose the collected data are used:
- FLD-DC-NDE-001, Electrical Resistivity Testing: Indication of a concrete member’s ability to protect reinforcing steel from corrosion.
- FLD-DC-NDE-002, Ground Penetrating Radar Testing for Bridge Decks: Detection and characterization of concrete deterioration in bridge decks, characterize presence, pattern, depth of structural steel reinforcement in the deck, estimate deck thickness, and identify anomalies or locating construction elements such as posttensioning conduits.
- FL-DC-NDE-003, Half-Cell Potential Testing: Assessment of the probability of active steel corrosion in a reinforced concrete member.
- FL-DC-NDE-004, Impact Echo Testing: Estimation of deck thickness and detection and characterization of the presence of delamination in bridge decks or other reinforced concrete elements, material evaluation, vertical crack characterization, detection of anomalies, concrete overlay debonding, etc.
- FL-DC-NDE-005, Linear Polarization Resistance Testing, Evaluation of the instantaneous corrosion rate, as compared to other methods on which metal loss is measured over a finite period.
- FL-DC-NDE-006, Dye Penetrant Testing: Examination of materials for detecting discontinuities that are open to the surface, such as cracks, seams, laps, cold shuts, laminations, through leaks, or lack of fusion.
- FL-DC-NDE-007, Ultrasonic Surface Wave Testing—Concrete: Variation of elastic modulus of over the concrete deck.
- FL-DC-NDE-008, Ultrasonic Testing—Steel Fatigue Cracking: Location and measurements of cracks or discontinuities in steel members.
Protocols addressing data reduction and processing for NDE are planned for future versions of the protocols. Additional information about the NDE protocols developed as part of version 1, released in January 2016, can be found in Report FHWA-HRT-16-007, Long-Term Bridge Performance (LTBP) Program Protocols, Version 1. More information on the LTBP Program Protocols in general can be found on the Product page.
The FHWA Advanced Sensing Technology (FAST) NDE Laboratory is a world-class laboratory and is the keystone of FHWA’s research and testing efforts related to the application of nondestructive testing technologies for condition assessment of highway infrastructure. The mission of the FHWA FAST NDE Laboratory is to conduct state-of-the-art research, development, and implementation of nondestructive testing systems and technologies to improve the Nation’s highway infrastructure assets.
The laboratory was originally established in 1998 to centralize and better coordinate research related to NDE of steel and concrete material and improve the research and practice related to nondestructive testing. Since its establishment, the FAST NDE Laboratory has acted as a resource for the FHWA, State transportation departments, industry, and academia concerned with the development, deployment, and evaluation of innovative NDE technologies for condition assessment of highway infrastructure assets, which includes bridges, tunnels, pavements, and ancillary features, such as sign posts and high-mast lighting. In addition, the laboratory has been providing forensic investigation services to the National Transportation Safety Board (NTSB), among other agencies.
The FAST NDE Laboratory houses state-of-the-art NDE equipment to support FHWA’s strategic vision to provide training to stakeholders who maintain and manage transportation infrastructure such as bridges, pavements, tunnels, and ancillary structures. Visit the FAST NDE Laboratory website to find more information about some of the equipment available in the lab.
Protocols addressing SHM are planned for future versions of the protocols. Additional information about the 51 protocols developed as part of Version 1 released in January 2016 can be found in Report FHWA-HRT-16-007, Long-Term Bridge Performance (LTBP) Program Protocols, Version 1
More information on the LTBP Program Protocols in general can be found on the Product page.
FHWA convened a diverse group of experts in June 2016 to develop, discuss, and debate future research directions for existing and emerging assessment technologies related to NDE and SHM.
The motivation and objectives of this workshop were to foster meaningful interactions between bridge owners and individuals engaged in technology development and refinement activities and to bring the various research communities together to promote and identify opportunities for the integration of NDE and SHM.
In addition the following goals were identified to ensure tangible products that may make near-term impacts on the field:
- Identify guidance needs, strategies, and delivery mechanisms to effectively promote and enable the adoption of SHM approaches by end users.
- Develop an NDE research roadmap for transportation infrastructure (inclusive of integration opportunities with SHM).
Nondestructive Evaluation and Structural Health Monitoring Workshop key outcomes include:
- Foster Ongoing Government-Industry-Academia Collaboration—Integrating end user and research perspectives together with collaboration between NDE and SHM researchers is essential to moving the use of assessment technologies forward. The workshop strongly recommended that FHWA develop a diverse government-industry-academia working group that meets regularly to continue to debate and discuss the issues surrounding NDE and SHM research and implementation.
- SHM Guidance—The following are the top three recommended forms of guidance:
- Web Manual—Expand the current NDE Web Manual to include SHM technologies and techniques.
- Case Studies—Provide detailed documentation of complete SHM case studies that would present tangible examples of successful and unsuccessful SHM implementation.
- Synthesis Report—Develop a synthesis report that documents both domestic and international practices and policies related to SHM.
- NDE Research Roadmap—Recommendations for specific NDE research topics were developed by participants in the workshop and the following are the top six research topics:
- Data Fusion, Analysis, and Visualization—Data have become easier and less expensive to collect, but the ability to fuse disparate data sources, efficiently process data (without extensive expertise), and develop visualizations that permit intuitive interpretation remain elusive.
- Measurement of Intrinsic Stress—In many structures, intrinsic stresses (e.g., dead load, manufacturing/fabrication, etc.) overwhelm transient, live load stresses, and there is currently no reliable and nondestructive approach to estimating them.
- Quantification of NDE Reliability—Using NDE approaches to effectively inform decisions requires the reliability of such techniques (e.g., probability of false positives and false negatives) be established.
- Establishing the Cost Benefit of NDE/SHM—The inability to accurately estimate the return on investment associated with NDE and SHM was viewed by the workshop participants as the most significant barrier to their implementation.
- Estimation of Remaining Service Life—The translation of NDE and SHM data into reliable estimates of the remaining service life is critical for making informed decisions related to repair, retrofit, and replacement.
- Monitoring Corrosion, Corrosion Rate, and Section Loss—Although corrosion is responsible for a large portion of the degradation of transportation infrastructure, there are currently no direct measurement methods available to identify and track its progression.