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- 2010 (8) (entfernen)
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- Englisch (8) (entfernen)
Schlagworte
- Concrete (4)
- Microcracking (3)
- Beton (2)
- Load (2)
- Nondestructive testing (NDT) (2)
- Sonic wave velocity (2)
- Stress (2)
- Zerstörungsfreie Prüfung (2)
- Acoustoelasticity (1)
- Dispersion (1)
The changes in the sonic surface wave velocity of concrete under stress were investigated in this paper. Surface wave velocities at
sonic frequency range were measured on a prismatic concrete specimen undergoing several cycles of uniaxial compression. The
loading was applied (or removed) gradually in predefined small steps (stress-controlled). The surface wave velocity was measured
at every load step during both loading and unloading phases. Acoustic Emission (AE) test was conducted simultaneously to
monitor the microcracking activities at different levels of loading. It was found that the sonic surface wave velocity is highly stress
dependent and the velocity-stress relationship follows a particular trend. The observed trend could be explained by a combination
of acoustoelasticity and microcracking theories, each valid over a certain range of applied stresses. Having measured the velocities
while unloading, when the material suffers no further damage, the effect of stress and damage could be differentiated. The slope
of the velocity-stress curves over the elastic region was calculated for different load cycles. This quantity was normalized to yield a
dimensionless nonlinear parameter. This parameter generally increases with the level of induced damage in concrete.
Nondestructive testing (NDT) of concrete structures plays an important role in civil engineering. CSIR-SERC, India, for the past three decades involved in the conditon assessment of civil infrastructures based on the expertise and the methodology developed in the area of NDT. Prof., Wiggenhauser, BAM, Berlin visited SERC, under the CSIR- Humboldt Reciprocity Research Award for 2006. During his visit, scientists of SERC have gained knowledge in the area of advanced NDT methods. A unique large scale two storied reinforced concrete specimen was cast at SERC, with columns and beams of different sizes, cross section having different percentages of reinforcement. The defects suc has honeycombs, cracks, delamination, presence od conduits, ducts, etc. were also incorporated. Facilities were created at SERC in the area of advanced NDT methods. BAM, Germany, has offered assistance to three scientists of SERC to work at BAM and to have further exposure in the area of advanced NDT methods. One of the scientists is pursuing Ph.D., in the application of advanced NDT methods for identification of damage in concrete structures. Activities have been carried out on the NDT-CE (Non-Destructive Testing in Civil Engineering) Compendium which is a compilation of the descriptions of 115 methods. It is freely accessible on the Internet. This paper highlights the collaborative research work carried out by SERC and BAM in the area of advanced NDT methods for the evaluation of defects/ damage in reinforced concrete structures and also as a quality assurance tool. The future programme of work is also discussed.
Surface wave velocities over the sonic frequency range (<20 KHz) were measured on concrete specimens undergoing various cycles of loading and unloading. Acoustic Emission test (AE) was conducted simultaneously to monitor the microcracking activities. The sonic surface wave velocity was found to be highly stress-dependent. The observed changes in surface wavespeed are repeatable and follow a particular trend. By measuring the wave velocities in both loading and unloading phases, the effects of stress and stress-induced damages could be distinguished. The observed trend could be explained by a combination of acoustoelasticity and microcracking theories.
Ground penetrating radar (GPR) was used to characterize the frequency-dependent dielectric relaxation phenomena in ordinary Portland cement (OPC) hydration in concrete changing from fresh to hardened state. The study was experimented by measuring the changes of GPR A-scan waveforms over a period of 90 days, and processed the waveforms with short-time Fourier transform (STFT) in joint time-frequency analysis (JTFA) domain rather than a conventional time or frequency domain alone. The signals of the direct wave traveled at the concrete surface and the reflected wave from an embedded steel bar were transformed with STFT, in which the changes of peak frequency over ages were tracked. The peak frequencies were found to increase with ages and the patterns were found to match closely with primarily the well-known OPC hydration process and secondarily, the evaporation effect. The close match is contributed to the simultaneous effects converting free to bound water over time, on both conventional OPC hydration and dielectric relaxation mechanisms.
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.