Nondestructive Testing Methods for Civil Infrastructure

The dynamic responses of a wide-flange steel beam with artificially introduced cracks were studied analytically and experimentally. Frequencies, displacement mode shapes (DMS), and strain mode shapes (SMS) are determined in both the analytical and experimental analyses. Modal damping ratios are also extracted in the experimental study. The sensitivities of the change of the modal parameters due to the damages are studied. The absolute changes in mode shapes were used to determine damage locations. Results show that the damage of a beam can be detected and located by studying the changes in its dynamic characteristics. SMS shows higher sensitivity to local damage than DMS does.

A full-scale highway bridge, in the process of being demolished and replaced, was monitored using vibrational techniques. The bottom of the flange and web of one fascia girder were incrementally cut. Vibrational monitoring during the passage of a test vehicle was used to demonstrate that the vibrational signature of a bridge will change when a major defect occurs. Included in this paper is a discussion of how vibrational studies have been used in the evaluation of bridges.

For many years the Federal Highway Administration, and before that the Bureau of Public Roads, had focused its nondestructive evaluation program on development of better tools to inspect components of steel bridges. In the early 70s the emphasis was changed and attention was given to the need for an inspection device that would detect and assess the condition of the steel elements of concrete bridge structures. This gave rise to the development of the Magnetic Field Disturbance (MFD) method and the ensuing development of the Magnetic Perturbation for Cables (MPC) inspection system. At present the Magnetic Flux Leakage Inspection System (MFLIS) for both main cables of suspension bridges and steel elements of concrete structures is under development. This paper summarizes the development of the aforementioned systems and field and laboratory applications.

Quantitative acoustic emission (AE) techniques can be used to monitor crack growth and to deduce microfracture mechanisms in quasi-brittle materials. Multichannel data acquisition systems are used to record the AE waveforms. The results of a quantitative AE analysis are quite sensitive to the interpretation of the recorded waveforms. Several techniques are presented here which facilitate AE data interpretation. Included are noise reduction and signal enhancement methods and time domain deconvolution methods which are used to recover microcrack fracture parameters.

Performance evaluation of composite rods used as reinforcing bars in concrete structures has been undertaken using acoustic emission (AE) technique. The AE technique has shown promise as a method to establish response characteristics of Aramid Fiber Reinforced Plastic (AFRP) bars under different axial tension levels. This paper discusses the experimental gripping mechanisms used to test AFRP specimens for direct tension, the AE source location method, and the procedure used to eliminate extraneous noises. The behavior of AFRP bars under different tension levels is also discussed in terms of AE parameters, such as number of events and event duration. Finally, this paper discusses a methodology to predict stress levels of AFRP bars using AE parameters, so that this technique can be applied to monitor concrete structures post-tensioned with FRP tendons.

The state-of-the-art in nondestructive testing (NDT) technology has advanced significantly over the past 15 years and has reached a stage where continuous monitoring of structures can be successfully achieved. The need for the adoption of advanced NDT technology for Maglev is absolutely essential because of the Maglev system safety and performance considerations and high cost of construction. In this paper, several monitoring methods such as Acoustic Emission, Ultrasonics, Ground Penetrating Radar, Infrared Thermography, and Fiber Optic Sensors have been evaluated in terms of their possible use in a Maglev system. Also, monitoring of guideway alignment and detection of debris on the guideway using Lasers and Fiber Optics have been investigated.

Due to geometry and traffic safety considerations, the I-40 bridges over the Rio Grande were scheduled to be demolished in the fall of 1993. This created a unique opportunity for testing. The bridges are classified as .fracture critical,. two girder steel bridges. The bridges were tested to determine the effect of defects on the redistribution of loads, on the load capacity, and on the potential for collapse. Two nondestructive techniques, resonant ultrasound and modal analysis, were tested on the bridge. Base line data was accumulated for the undamaged bridge. Then four different levels of damage were introduced in the middle span of one of the girders. Data was accumulated at every stage. The final stage was a near full depth crack in the girder. Static tests and forced vibration tests were performed on the structure. This is an on-going project. This paper describes the preliminary stages of the project. Full results of the testing, i.e., data obtained from the testing as well as evaluation and assessment of the results will be published at a later date.

From 1701 to 1993, most suspension bridges performed satisfactorily under wind loads. Among those that did not, a few were destroyed, some partially damaged, and others experienced unacceptable vibrations. Suspension bridges with unacceptable vibrations were fitted with various stiffening devices to improve performance. Twentieth century suspension bridges performed better than 19th century bridges since fewer bridges were destroyed or damaged in the present century. However, performance data, where available, varies from brief descriptions to detailed explanations. Data on performance is being collected for a Computer-Aided Information System, currently under development. The performance of a suspension bridge, under wind loads, is recorded by listing the vibrational response characteristics and the damage, if any. The performance is evaluated for the bridge as a .whole. and for the individual .parts. during the .construction. and .service. stages. While this paper evaluates the performance of full-scale bridges only, the Computer-Aided Information System will include the performance of scaled laboratory models and models in wind tunnels. This paper is a work-in-progress report on the performance of 26 full-scale suspension bridges. Five of these bridges were destroyed by wind and six were partially damaged. Seven bridges that were not damaged had stiffening devices added to improve performance. One bridge was damaged after the devices were added. Another bridge, with a record of satisfactory performance in the unstiffened single-deck state, had bottom-chord lateral bracing added to improve its performance in the stiffened double-deck state. Six bridges experienced, according to their designers, acceptable vibrations.

This study seeks improved prediction accuracy for dynamic parameters of long-span bridges obtained from experimental measurements. Ambient vibration tests of a long-span bridge were conducted to estimate natural frequencies, damping, and mode shapes. These parameters were used to evaluate and modify a computational dynamic model for the bridge. Transient vibration tests were conducted to obtain the bridge response under the effects of two measured trucks traversing the bridge with near steady speed. With the input and output of the bridge known, a system identification method for experimental determination of the modal parameters was developed and applied to investigate the bridge's dynamic properties.