nondestructive identification of freezing-induced cracks in concrete sleepers of high-speed railway in korea

초록

The construction of a high-speed railway in Korea was recently impeded by the unexpected problem of cracks in sleepers. These were presumably induced by the volume expansion of freezing water entrapped in fastening assemblies during the winter. A total of 306,000 sleepers had to be investigated urgently for hair cracks and possible latent cracks. In the context of these findings, an appropriate NDT technique for inspecting concrete sleepers should be designed to meet two requirements: speed and reliability in testing. We performed research to identify an optimal NDT technique, using sleepers with three different crack conditions: without any cracks, with a surface-opening crack, and with a hair crack and possible latent cracks. Flexural rigidity was chosen as a key parameter to assess the structural integrity of a sleeper, rather than material stiffness or depth to multiple reflections. Our results allowed us to design a two-stage testing procedure: the Flexural-Rigidity Assessment of Concrete Tracks by Anti-Symmetric Lamb Waves (FRACTAL) technique for preliminary scanning, and SASW tests for in-depth investigation. A total of 18,177 suspicious sleepers were examined by FRACTAL tests, and 387 of these sleepers were double-checked by SASW tests. Prior to the overall examination of sleepers, a series of statistical analyses were performed to establish the criteria for defining the quality of a sleeper. The FRACTAL results agreed with SASW results in terms of identifying sleepers with hair cracks or possible latent cracks.