ABSTRACT
The number of aging concrete structures is continuously increasing worldwide. Maintenance of large structures requires the determination of damage parameters as well as appropriate methods for repair. In both of these tasks, non destructive testing can play a crucial role. Stress wave parameters, as pulse velocity or acoustic emission indices, can be correlated with damage (Kaplan 1959, Anderson & Seals 1981, Shiotani et al. 2005) helping the engineers to focus on specific parts of the structure and select an adequate repair technique. Additionally, after the repair, the evaluation of improvement of structural integrity is an important question that can be handled only by NDT. In the present paper, some aspects of the stress wave monitoring of an old concrete dam in a cold area of Japan are discussed. The repair technique selected was the injection of cement (grouting) through boreholes in order to eliminate the defects (Binda et al. 1997, Williamson 1988, Thanoon et al. 2005). Cement injection enhances the integrity, something that is expected to increase also the velocity of stress waves. However, in the present case, the pulse velocity was substantially reduced, raising doubts about the repair process. In order to explain this behavior, an investigation with scattering theory (Waterman & Truell 1961) was undertaken, as well as laboratory tests on grouted concrete specimens. It is revealed that the initial property mis-match (stiff concrete matrix and soft fresh
grout) as well as the low temperature, influence the propagation velocity of the whole structure and an increase of velocity should not always be expected.
