| Summary: | 碩士 === 國立臺灣科技大學 === 營建工程系 === 106 === The setting and hardening process of cement paste are considered as an important property for the application of concrete in the field, which can affect the time and cost during the framework removal and the other related works. The conventional setting time measurement by the means of Vicat needle is found having many disadvantages such as manually operated, time consuming, fluctuating data, irreproducible, and non-continuous. To mitigate these problems, the new method to observe the stiffening process of cement paste is developed using two ultrasonic-based nondestructive methods: the ultrasonic wave reflection method (UWRM) and the ultrasonic wave transmission method (UWTM). Both techniques are based on monitoring the waveform signals of longitudinal wave (P-wave) and shear wave (S-wave) that either travel through the paste or reflect at the interface between two different media. The signals were analyzed using Fast Fourier Transform (FFT) and Short-time Fourier Transform (STFT) comprehensively. The cement paste mixes were prepared with three different water-to-cement ratios of 0.3, 0.4 and 0.5.
The experimental results indicated that both techniques could estimate the setting time of cement paste and the evolutions of engineering properties like modulus elasticity and Poisson’s ratio credibly. The final setting time of cement paste was determined by the arrival time of the first reflected waveform based on the P-wave reflection from the boundary of paste and air, which has the compatibility of more than 95.62% as compared with that from the traditional setting time measurement. However, the initial setting time could not be defined due to the waveform still being remained in the regime of noise spectrum. In the other hand, by using the acrylic plate as buffer material to reflect the waveform signal from the paste, the S-wave reflection provides the better measurement than that of the P-wave reflection. The minimum point of amplitude in the S-waveform reflection response exhibited the occurrence of final setting time with the difference in value less than 6% as compared with that of the traditional setting time measurement. Nevertheless, the P-waveform reflection response did not demonstrate the reliable result related with final setting time showing the difference in value up to 21.30% as compared with that of the traditional setting time measurement.
Moreover, the ratio of longitudinal wave velocity to shear wave velocity for the hardened cement paste at ages from 1 to 28 days was found in the range of 1.621.67 which equivalent to the Poisson’s ratio about 0.190.25. The correlation between compressive strength and ultrasonic wave velocity shows the exponential relationship with the coefficient of determination values of 0.878 and 0.910 for P-wave and S-wave, respectively. Both strength and ultrasonic velocity of hardened cement paste decreased with the increase of water content due to the slow hydration reaction.
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