The strength of concrete near the surface would be greatly reduced subjected to high temperature, usually accompanied by surface cracking and spalling. In this study, two concrete slabs heated to 600°C and 800°C on one surface were tested using a small hammer and a displacement receiver placed on the two ends of a test line. The dispersion curve was obtained by performing Short-Time Fourier Transform and amplitude reassignment technique on a received displacement waveform. Combining 10 dispersion curves obtained from multiple grid lines, a three-dimensional surface wave velocity contour map is constructed. Concrete cores were taken after NDT testing and compared with the contour map of the location where the core was taken. The test results show that both the concrete strength and the crack depth affect the surface wave velocity. This technique allows rapid assessment of the cross-sectional wave velocity distribution of a surface, even on rough, spalled surfaces.
The present study is based on developing a novel in-house BRISK-DIC methodology to identify and correlate natural patterns to perform 3D vibration studies of large structures such as wind turbines. The method includes identifying natural patterns using the BRISK (Binary Robust Invariant Scalable Keypoints) algorithm and its integration with an in-house 3D-DIC method for the measurement of in-plane and out-of-plane displacements. The stereo-calibration is performed using a novel calibration technique that uses an IMU sensor and Laser Measure for extrinsic parameters and a representative computational stereovision system for intrinsic parameters. In a preliminary field experiment, the developed technique is used to study the vibrations of a light tower of 10m height. The BRISK algorithm is applied to the selected reference images from both of the cameras to detect natural patterns. The in-house 3D-DIC program uses the identified natural patterns as well as calibration parameters for correlation and reconstruction purposes. The developed BRISK-DIC method is able to measure 3D displacements as well as accurately obtain the natural frequencies of the light tower. The vibration results are validated using an accelerometer. Finally, in another field experiment, the technique is successfully implemented for 3D vibration study of a utility-scale wind turbine. Natural patterns are identified at the nacelle and correlated to obtain the vibrational parameters of the wind turbine tower. The measured natural frequencies are validated with the measurement carried out in the past using a ground-based microwave interferometer.
The present study is based on performing 3D vibrations studies of large structures using digital image correlation (DIC) technique. A 3D-DIC setup is developed for measuring in-plane and out-of-plane 3D displacements as well as natural frequencies of a vibrating structures (small to large scale). The DIC setup is first validated with laboratory experiments. The first indoor lab experiment is based on the application of the developed DIC setup in measuring pure in-plane and out-of-plane displacements. The in-plane and out-of-plane translations have been carried using a micrometer and a Vernier caliper, respectively. The developed DIC setup is able to accurately measure these in-plane and out-of-plane static displacements. With this success, in another indoor experiment, the developed DIC setup is used for performing vibration study of a PVC pipe (length 1m, inner diameter 56.6mm, outer diameter 60.6mm). The DIC setup is able to measure the dynamic displacements of the pipe in all three axes. Apart from this, the natural frequency measurement is also accurate. The first fundamental frequency of the pipe is 4.167 Hz. After successful validation and application in the indoor experiments, the developed DIC method is used for a field experiment. In the field experiment, the DIC technique is applied to perform vibration study of a light pole (length 3m, diameter 75.5mm, made of iron). The cameras are placed at a distance of 14.6m from the pole, whereas the distance between the cameras is 8.5m. A large size calibration board is fabricated and used for calibrating the stereovision system. The developed DIC method is able to produce 3D displacements of the vibrating pole as well as accurately measure its natural frequency. The fundamental natural frequency of the light pole is 4.9Hz. An accelerometer was also used during the field experiment for the validation of the developed DIC-based non-contact 3D vibration testing method.
A new flaw detection method for concrete plate-like structure is realized using the dispersion profile of the group velocity of surface waves obtained by a sensor with proper distance from the transient impacting load. The waveform obtained by the sensor is analyzed using STFT and reassigned method to obtain a group velocity spectrogram. The delaminating crack or honeycomb which locates underneath the test line between the impactor and the receiver as well as the low-density layer on top of sound concrete are proved to be detectable in both numerical and experimental studies. The velocity turning point in the wavelength-velocity profile is about 1.6 to 2.2 times of the depths of the flaws or the low-density layer wavelength. As the proposed method is easy to operate, inexpensive and effective on solving many problems of concrete deterioration, one essential question to be concerned is the effect of dense reinforcing rebar to the stress wave propagation. In this preliminary study, the theoretical modal dispersion curves for a plain concrete plate and a concrete plate containing a thin steel layer are compared. A 2D numerical model with concrete and steel layers was constructed. The images of slowness spectrograms obtained by placing impactor and receiver at variant distances are compared with theoretical modal dispersion curve. Experiments are performed on a heavy lattice arranged bridge pier. The results show that the response of the rebar layers is near 0.3 ms/m in slowness spectrogram instead of around 0.5 ms/m plain concrete. The steel rebar layer affects the results more severely when the test line is parallel to the direction of shallower rebars. For more clearly observing the condition of concrete, one can filter the response in the waveform with the time less than 0.4 ms/m multiplying the impactor-receiver distance.
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