Seminar Series - 2002-2003 Seminars

Application of Diffuse Ultrasonic Methods For Cement-Based Materials

Sathish Ramamoorthy - M.S. Thesis Defense
Advisor:  Dr. Joseph A. Turner

Date:  Tuesday, August 5, 2003
Time:  10:00 a.m.
Place:  W183 Nebraska Hall


The propagation and scattering of ultrasound in concrete is used to study damage in concrete when the wavelengths are comparable to the microstructure. At these frequencies, ultrasound is scattered considerably by the heterogeneities in the concrete. In the limit of many scattering events, the evolution of energy may be modeled as a diffusion process. The study of the application of diffusion process in cement-based materials is organized in two parts.
 
In the first part the determination of the depth of surface-breaking cracks in concrete specimens using an ultrasound diffusion technique is discussed. Experiments were carried out on pre-cracked concrete specimens of varying crack depth (0 - 40% of the specimen thickness). Tone burst excitations over a frequency range of 400-600 kHz were used. The scattering coefficients were determined by measuring the diffuse energy at a zone away from the crack using contact transducers. After recovering the diffusivity and dissipation transducers were placed at the specimen surface with source and receiver separated by the crack. The arrival of the peak diffuse energy at the receiver is delayed due to the presence of crack. This delay is the prime indicator used for determining crack depth. Numerical and analytical analyses were also used for comparison. These results are in basic agreement with the experiments. In addition, these analyses are used to study the limits of this technique. In particular, it is shown that this technique is applicable to cracks greater than the scattering mean free path, whichis estimated at about 1 cm for these specimens. Results obtained from real cracked specimens are also discussed.
 
In the second part, the use of diffuse ultrasound to measure the dissipation in cement mortars to study the hardening behavior and correlate the strength gain of the cement mortars to the measured dissipation is discussed. The rate of strength gain determines the time required for opening the structure for use. Current monitoring techniques include thermal monitoring and nuclear magnetic resonance. Relevant experiments were conducted using several specimens of cement cubes 50.8 mm on a side. Ultrasonic dissipation was measured at regular intervals over a period of 28 days over a frequency range of 200-1000 kHz. Compressive strength tests were also conducted on these specimens at 1, 3, 7, 14 and 28 days. The results show that the ultrasonic dissipation decreases as a function of curing time, asymptotic to the 28 day value. This change in dissipation can be correlated to the gain in strength. [Work supported by NSF]


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