Filtern
Dokumenttyp
Sprache
- Englisch (11)
Schlagworte
- Impact echo (11) (entfernen)
Organisationseinheit der BAM
This study investigates the non-destructive detection of delaminations in concrete plates using non-contact laser ablation, instead of the conventional hammer excitation, as part of the impact echo method. We performed tests on five concrete specimens of different sizes, two of which contained artificial delaminations. A range of steel ball hammers was used as reference impulse sources, the responses of which were compared with wave excitation generated by a 7 ns pulsed 1064 nm Nd:YAG laser with 150 mJ pulse energy. Signals were recorded by surface-mounted accelerometers and two contactless methods: microphones and a laser Doppler vibrometer. The laser generates frequencies across a broad range of frequencies (0 to 150 kHz) but with much less energy than the hammers' narrower frequency spectra; the laser pulse energy transferred into the specimen is 0.07 mJ, corresponding to about 0.5 ‰ of the impulse source energy. Because of this, the thick intact plates' characteristic thickness stretch resonance frequency can be reliably detected by the hammer excitations but not when using laser excitation. However, the laser can excite low-frequency flexural vibration modes over a shallow delamination at 3 cm depth. The low-frequency flexural vibration results are verified by numerical natural frequency analysis.
Tunnel lining evaluation is performed in the Hanging Lake Tunnel near Glenwood Springs, Colorado, using variations of four nondestructive evaluation (NDE) methods: air- and ground-coupled ground penetrating radar (GPR), ultrasonic echo (US echo), ultrasonic tomography (UST), and impact echo (IE). Potential regions of interest are identified using high-speed air-coupled GPR and visual inspection. A robotic frame with pneumatically-operated vacuum plates was used to automate the ground-coupled GPR, US echo, and IE tests, while the UST technique was used manually. This study shows that a particular combination of NDE techniques is a powerful tool for assessing the condition of tunnel linings and can detect potential anomalies such as delamination, depth of surface cracks, reinforcement depth and layout, and lining thickness. By mapping the phase shift of the ultrasonic pulses at detected interface, the potential bonding of layer interfaces and reinforcement is assessed.
An in-depth investigation of tunnel linings is performed at Eisenhower Tunnel, 60 miles west of Denver, Colorado, using a combination of four nondestructive testing methods: air- and ground-coupled ground penetrating radar (GPR), ultrasonic echo (US echo), ultrasonic tomography (UST), and impact echo (IE). A robotic frame with pneumatically operated vacuum plates is used to automate GPR, US echo, and IE. UST was performed manually. These techniques are used to identify and locate the reinforcement mesh and structural steel ribs. Local changes in ultrasonic phase are calculated by phase analysis processing techniques and apparent relationships between positive and negative phase shifts and steel and air interfaces are observed. According to previous laboratory research, these relationships indicate the bonded state between the concrete and the embedded steel, but without ground truth verification this can only be suggested. It is shown that this combination of methods can be used synergistically to provide tunnel owners with the layout and possible condition of tunnel lining reinforcement and structural components.
The delamination of concrete slabs is the separation along a plane roughly parallel to, and generally near, the surface. Corrosion-induced delamination is a common problem in old concrete bridge decks. If undetected, delaminations could expand, reach the surface, and result in spalling. Early detection of delamination is necessary for planning timely repairs that prevent costly deck replacement projects. Most bridge owners rely on routine visual and traditional surveys of bridge deck conditions. These surveys are highly subjective and can locate only large shallow delaminated zones. Several nondestructive testing (NDT) techniques have recently been employed for bridge deck evaluation to obtain more objective and comprehensive assessment. The reliability of applicable methods needs to be established before a greater role for NDT in routine inspections can be encouraged. This paper presents a validation study aimed at evaluating the effectiveness of three NDT techniques, namely impact echo, ultrasound (US) echo, and US linear array, in detection of delamination. This study is unique because the subject test specimens were deteriorated bridge deck segments preserved from the demolition of a prestressed box girder bridge. The results of the tests conducted on one of the specimens are presented and discussed here: impact echo provided satisfactory overall assessment, but the individual results were often difficult to interpret; US echo detected deep delaminations but not shallow ones; and US linear array located the extent of deep delaminations and provided indications of shallow ones.
The impact-echo method has been successfully applied to identify defects inside concrete. In addition, to detect ungrouted tendon ducts in a large concrete slab, a scanning impact-echo technique is developed. However, since resonant frequencies in the spectrum responsible for the travel paths via defects are only taken into account, the method could lead to erroneous results due to complicated spectra obtained in the tests. Consequently, Stack Imaging of spectral amplitudes Based on Impact-Echo (SIBIE) procedure has been developed to improve the data interpretation. Conventionally, SIBIE is applied to a single measurement data and a point information of defects is obtained at the area, where the impact test is performed. In this study, SIBIE is applied to scanning impact-echo data. Locations of ungrouted tendon ducts embedded in a large concrete specimen are investigated. In order to visualize the whole cross-section tested, the SIBIE analysis is modified, introducing an elliptical integration mode. It is demonstrated that ungrouted tendon ducts are successfully located by the modified SIBIE analysis, whereas results of the conventional B-scan analysis are not so good as the modified SIBIE analysis.
Impact echo is an acoustic method based on the use of transient stress waves generated by an elastic impact; it is used for nondestructive testing of concrete structures. In practical applications, the signals obtained often are superimposed by further mechanical vibrations and the so-called geometry effects, which are caused mainly by surface waves. Because of attenuation in the concrete as well as the divergence of the acoustical waves, impact echo signals are transient. As a result, the frequency content changes over time. Normally the analysis is carried out on the Fourier power spectrum of the signal. However, the Fourier spectrum is still affected by the mentioned effects and has well-known deficiencies for short transient signals within longer time sweeps. Application of the Hilbert-Huang transform is presented as a refined method for the time-frequency analysis of nonstationary impact echo data. The basic properties of the method and its practical application for time-frequency analysis of impact echo data, signal filtering, and pattern identification are presented.