Parameter Study to Obtain the Best Image in Reassigned Spectrogram for Reinforced Concrete Plate

• Main Authors: WANG,HUNG-HUA, 王宏華
• Other Authors: CHENG,CHIA-CHI
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/2a9hy4

碩士 === 朝陽科技大學 === 營建工程系 === 106 === In the research, the transmission characteristics of Lamb waves excited by an impact on surface of concrete slab are mainly used to search for the parameters producing the optimal image of dispersion curve for concrete plate-like structure and a preliminary study on the influence of the reinforcement layer in the slab.This method features configuration of one excitation source and one receiver and application of Short Time Fourier Transform(STFT) with its energy density spectrum reassigned in order to obtain dispersion curves of Lamb waves. Using finite elements to simulate the reaction of receivers at different distance form a impact source ,one can extract the A0 antisymmetric modes from the numerical slowness spectrogram which are compared with the theoretical solutions obtaining from the program Disperse. The numerical simulation results show that the optimum window length and overlap in STFT are change with the ratio between the concrete slab (d) and the thickness (Th) of the concrete slab When the taping distance is between 0.2m and 0.6m, the overlap of the windows will cause the signal to be scattered in the slowness direction, which will affect the interpretation of the result. When the window is too long, no matter how the distance between the impactor and receiver will lead to the overall signal wavy, affecting the results of MATLAB program capture. The major signals in the spectrum are along A0 theoretical dispersion curve except for the following circumstances: when the tapping distance (d) to plate thickness (Th) ratio d-/-Th is equal to 0.5,the signal is closer to the non-dispersive Rayleigh wave signal; When d-/-Th is equal to 2.5, the trend of the low frequency signal is deviated from the A0 anti-symmetric mode, and the main signal is along the S0 mode. When d-/-Th is equal to 7.5, the A0 signal with the frequency of 6-10kHz will disappear, S0 Mode would be dominating. For the study of the effect from steel reinforcing layer,numerical simulation is performed-for three-layered concrete slab (concrete + steel + concrete) and plate embedded steel bar with different spacing. When the steel sheet is embedded, the S1 modal effect of the three-layer concrete-steel theoretical solution is very significant. When the thickness of the steel is 3cm and 6cm for 0.2m-thick-plate, the A0 modal signal frequency will disappear above 15 kHz., The velocity-wavelength can only have the signal for a wavelength of 0.16 m or more., The reinforcement has little effect when the spacing of the steel bars is 12 cm. When protective layer is 13cm, single concrete slab, three concrete slabs, steel spacing 6cm, spacing 12cm steel are not affecting signal. The experimental part is the pier and the reinforced concrete slab. Both of them are arranged with dense steel reinforced. However, except for the vertical test line of the piers which has little influence on the reinforcement signal due to deeper location of steel bars,the test results from the horizontal and oblique directions (at arbitrary angle) shows heavy influence from the reinforced bars.The signal can be removed by filtering the signal with slowness lower than 0.35 ms/m.