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|Federal Highway Administration > Publications > Public Roads > Vol. 73 · No. 2 > NDE Showcase for Bridge Inspectors|
Publication Number: FHWA-HRT-09-006
NDE Showcase for Bridge Inspectors
by Frank Jalinoos
FHWA offers a new 1-day seminar on nondestructive evaluation methodologies to help State DOTs improve routine and special testing of steel and concrete bridge components.
Approximately 600,000 bridges support, carry, and connect public roadways in the United States. These structures are critical components in a surface transportation system that is expected to support a thriving economy, provide for safe and efficient mobility, and ensure a high level of national security by facilitating rapid responses to emergencies, attacks, and natural disasters. These bridges and roads also serve as a conduit for a variety of personal travel needs, helping to ensure a satisfactory quality of life in the United States.
The Federal Highway Administration (FHWA) and State departments of transportation (DOTs) need a focus on improving the condition of the Nation's bridges through enhancement of current asset management practices, bridge inspection programs, and preservation strategies. Needed are improved quality of bridge inspection data, bridge management system implementation, and movement toward a more risk-based, data-driven method of oversight. These efforts are important in enhancing the Nation's bridge safety.
The requirements for periodic inspection of all bridges on public roads are well established and codified in regulations within the National Bridge Inspection Standards (NBIS). The 2004 legislation was in part a response to a series of major bridge failures over four decades. According to the NBIS, all publicly owned highway bridges (including culverts) located on public roads in the United States that are longer than 20 feet (6 meters) must be inspected at least once every 24 months.
The standards also describe the necessary qualifications of the persons who serve as program managers and team leaders performing the onsite inspections. For both positions, the NBIS require some appropriate form of professional accreditation or a minimum number of years of experience inspecting bridges. In addition, inspectors need to complete Safety Inspection of In-Service Bridges (FHWA-NHI-130055), a comprehensive, 2-week training on bridge inspection offered through FHWA's National Highway Institute (NHI).
In addition to the NHI course, FHWA continues to develop other training to help ensure that bridge inspectors have the knowledge to perform both routine bridge inspections and assessments of structures with critical features or components of concern, such as fracture-critical members and bridges needing underwater inspections for substructure components.
In 2008, the FHWA Office of Infrastructure Research and Development (R&D), in cooperation with FHWA field offices and State DOTs, identified a need for training on nondestructive evaluation (NDE) testing to improve the quality and accuracy of bridge inspections at the State level.
A 2008 white paper, published by an ad hoc group consisting of the American Society of Civil Engineers' Structural Engineering Institute and the American Association of State Highway and Transportation Officials, echoed this conclusion. Specifically, the paper identifies the education of bridge owners and inspectors on readily available NDE technologies and their applications as a short-term research need.
"DOTs are always looking for more efficient and cost-effective tools and techniques to augment their current bridge inspection and evaluation," says Dr. Sreenivas Alampalli, P.E., director of the New York State DOT's Bridge Evaluation Services Bureau, chair of the ad hoc group, and a participant in FHWA's new seminar for bridge inspectors. "With improved inspection and evaluation tools, the overarching goal is safer bridges at reasonable costs resulting from better bridge management."
In terms of training, State bridge inspectors typically face three key problems. First, opportunities for training are sometimes limited due to budgets for training and associated travel. Second, offsite training programs commonly used by State DOTs can result in fragmented training of inspectors. Third, sending groups of inspectors to different training sessions at different times can result in wide variations in the information and experiences gained.
To overcome these challenges and meet the expressed need for NDE training, FHWA developed the Bridge Inspectors NDE Showcase (BINS) program. BINS is an informal, 1-day, demonstration-based seminar designed to expose State DOT bridge inspection staff to basic NDE tools. The purpose of the showcase is to familiarize bridge inspectors with various NDE methodologies; provide them with the knowledge of how, when, and where to apply NDE tests during a bridge inspection; and acquaint them with the capabilities and limitations of each methodology.
Types of Bridge Inspections
The typical bridge inspector, either a State DOT employee or a private consultant, is trained and qualified to perform the initial and routine inspections required by the NBIS. The NBIS describe five types of bridge inspections. The initial inspection provides the structure inventory and appraisal data, establishes baseline structural conditions, and identifies and lists existing problems or locations that might have potential problems.
Following the initial inspection, most of the 24-month inspections are routine visual inspections to determine a bridge's physical and functional condition. During visual inspections, inspectors typically use tools for cleaning, probing, sounding, measuring, and recording notable conditions. They use ladders or power lifts for access and small hand tools such as wire brushes and scrapers, picks and small hammers, flashlights and mirrors, magnifying glasses, and measuring tools.
To identify and characterize a small, previously observed crack in a steel member, inspectors will use dye penetrant testing or magnetic particle testing. In the first test, the inspector applies a high-visual-contrast dye penetrant to locate surface defects. Magnetic particle testing involves applying an external magnetic field or electric current, spraying iron or magnetic iron oxide particles over the magnetized specimen, and then detecting the particles under ultraviolet light. Attracted by the surface field, the magnetic particles hold on to the edges of the defect and reveal its location.
The other three types of inspections are indepth, special, and damage inspections. An indepth inspection is a closeup, hands-on inspection of one or more members above or below the water level to identify deficiencies not readily detectable using routine procedures. Inspectors also employ this method during inspection of a fracture-critical member.
The special inspection is used to monitor a known or suspected deficiency. An example is monitoring a steel member with a known fatigue crack until a repair can be completed or the bridge can be replaced.
Finally, a damage inspection is an unscheduled inspection to determine structural damage resulting from extraordinary loads (for example, from a hurricane, earthquake, or storm) or human actions such as vehicular collision.
In most cases, unscheduled inspections require application of methods that are more advanced than those used during visual inspections. Inspectors might employ NDE testing in situations where visual assessment might be inadequate to determine the severity and extent of defects or deterioration. In some situations, the persons who conduct the routine 24-month inspections required by the NBIS are not trained in the application of other NDE testing methods.
NDE Tests for Steel Bridges
Many kinds of NDE methodologies are available for inspectors to apply in specific circumstances to enhance their ability to identify, locate, measure, and characterize deterioration, localized damage, or a flaw in a bridge member or component. Familiarity with the capabilities and limitations of each type of NDE test, plus the appropriate times when they can be applied, could enhance the effectiveness of DOT inspectors.
Certain types of problems are common to bridges that are of similar material and design, or that have similar details. Bridges that have steel superstructures (steel girders and girder connection members and details) can be susceptible to cracking caused by flaws in weld zones, high-stress locations, and components prone to damage through fatigue. In the early stages, these cracks might be difficult or impossible to detect using only visual inspection. Characterizing the cracks in terms of size, depth, and orientation might be difficult as well. Two well-known NDE technologies are available to identify, locate, and assess these cracks.
Ultrasonic testing (UT) uses high-frequency sound energy (ultrasonic waves) to assess cracks. UT is capable of detecting and sizing both surface and internal cracks. Inspectors apply UT on the surface of steel components, normally using it only after cleaning the steel surface of all old paint and corrosion. The ultrasonic wave travels through the steel at a velocity dependent upon the properties of the steel until a discontinuity (that is, a crack) reflects the signal back through the material to a receiver. The inspector then interprets the distance to the discontinuity after processing the signal.
Eddy current (EC) uses electromagnetic induction to assess surface flaws, material thickness, and coating thickness. EC is a current induced on a conductive material (such as a steel bridge member or component) through induction from a magnetic field. The primary goal usually is to find flaws in the material at or just below the surface. Inspectors can use EC on steel that is painted or on cleaned surfaces.
NDE Tests for Concrete Bridge Members
For concrete bridges, the most common form of deterioration and damage occurs in reinforced components that are subjected to an environment where chlorides in solution are in contact with the surface of the concrete. This contact usually happens as a result of applying deicing salts to reinforced concrete bridge decks. It also occurs when spray from vehicles on a roadway passing beneath a bridge contacts its concrete substructure or superstructure or in marine environments where saltwater or spray comes in contact with the concrete substructure or superstructure.
Another common problem is runoff from decks that passes through leaking bridge joints, contacts the tops of abutments and pier caps, and penetrates the concrete. When the chloride ions reach the level of the reinforcing steel, corrosion occurs, the corrosion products expand, and the resulting tensile forces crack the concrete. Eventually, the cracking produces a complete delamination and separation of the concrete from the component. Two well-known technologies are available to identify, locate, and assess subsurface conditions in concrete members even if no apparent distress is detectable during visual inspection of the surface.
Ground penetrating radar (GPR) utilizes electromagnetic waves to assess subsurface flaws and to image embedded reinforcement or tendons. Typically used in concrete, masonry, and timber structures, GPR technology can locate the reinforcing steel in a concrete member and infer corroded sections. The technology also can identify and characterize cracks, voids, and moisture zones. The most common application in bridges today is for surveys and inspection of the condition of bridge decks.
Impact echo (IE), or ultrasonic pulse echo, utilizes impact-generated stress waves to assess subsurface flaws and material thickness. Inspectors typically use IE in concrete and masonry structures, particularly in concrete bridge decks. The most common methods for evaluating bridge decks today are chain dragging and hammer sounding. However, these techniques have significant limitations because of their inability to detect early signs of delamination. IE can overcome the limitations of these traditional methods.
Another technology, infrared thermography (IR), measures the amount of infrared energy emitted by an object to calculate temperature. The radiation emitted by an object increases with its temperature, and the presence of a defect impedes heat flow. Inspectors use IR in all bridge types to assess deterioration, flaws, and moisture intrusion.
BINS Addresses an Acknowledged Need
In March 2008, FHWA's NDE Validation Center at the Turner-Fairbank Highway Research Center in McLean, VA, conducted a BINS presentation on these NDE testing methods. Prior to that pilot presentation, FHWA established the following goals and objectives for BINS:
Expose bridge inspectors to some of the latest technologies related to nondestructive testing and evaluation of bridge components.
Provide them with formal training in the basic principles and general operational procedures of five of the latest portable, technician-driven NDE technologies (ultrasonic testing, eddy current, ground penetrating radar, impact echo, and infrared thermography).
Reinforce the importance of continuous training in the most up-to-date NDE tools.
The overall objective is a new strategy in bridge inspection that integrates basic NDE measurements into the inspection process. The purpose is to use NDE in a spot-checking capacity to supplement standard visual inspection practice. The BINS introduces technician- driven systems with which inspectors can obtain results rapidly in the field and in a format that can be incorporated directly into the inspection report. This approach improves upon traditional NDE surveying methods that require acquisition of large datasets, post-processing, and interpretation performed by experts.
The pilot BINS workshop enabled FHWA to prove the viability of delivering effective training related to NDE inspections through an informal program, demonstrating commercially available NDE tools. The next step will be to expand the workshop into an NHI seminar, which is expected to be available by late 2009.
Benefits of a BINS Seminar
FHWA developed BINS to allow presentation during a 1-day event with morning and afternoon sessions. Alternatively, instructors can present the showcase material over 2 days, with the first session in the afternoon of the first day and the second session in the morning of the second day.
The seminar includes static displays for each of the five nondestructive tests that are highlighted. The displays include functional test equipment, test samples, and tabletop posters describing setup procedures and sample testing procedures. In addition, the displays include educational posters describing operation of the equipment and examples of when to use them.
BINS addresses three typical problems faced by State bridge inspectors: (1) opportunities for training are sometimes limited due to budgets for training and associated travel; (2) offsite training programs commonly used by State DOTs can result in fragmented training of inspectors; and (3) sending groups of inspectors to different training sessions at different times can result in wide variations in the information and experiences gained.
The BINS seminar offers a number of additional benefits, including exposing inspection staff to the latest commercially available NDE tools, enabling them to see the NDE tools in operation, and providing a forum for discussion about which NDE systems may be best suited for use in a given situation and in specific States.
"The BINS workshop makes this important training more accessible to a larger number of bridge inspectors in the States, while also producing real cost savings and improved productivity in terms of reduced expense and travel time," says Barton Newton, State bridge maintenance engineer for the California Department of Transportation (Caltrans). "Caltrans currently utilizes many NDE techniques and will be exploring the possibility of this training to provide our bridge inspectors with the latest state-of-the-art uses of NDE techniques."
The BINS workshop instructors travel to the host State to make the presentations. Up to 100 people can participate in each workshop. Interested State DOTs should call NHI for costs and to schedule the training. The inspectors at the New York State DOT are already using the NDE tools that they encountered in the BINS program in their everyday practice.
"Conducting the BINS seminar at a DOT's headquarters enables the States and Federal agencies to maximize participation and thus improve their bridge inspections," adds Gary Jakovich, bridge engineer for the FHWA Eastern Federal Lands Highway Division.
Frank Jalinoos is program manager for FHWA's NDE Validation Center. He oversees a broad program in NDE of both concrete and steel bridges. His other research areas include foundation and pavement NDE and structural health monitoring. Jalinoos holds a master's degree in geophysical engineering from the Colorado School of Mines.
For more information, contact Frank Jalinoos at 202-493-3082 or firstname.lastname@example.org.
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