Nondestructive Identification of Freezing-Induced Cracks in Concrete Sleepers of High-Speed Railways in South Korea

Abstract

The construction of a high-speed railway in South 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 the fastening assemblies during the winter. A total of 306,000 sleepers had to be investigated on an urgent basis for hairline cracks and possible latent cracks. These findings suggested that an appropriate nondestructive testing (NDT) technique for the inspection of concrete sleepers should be designed to meet two requirements: speed and reliability in testing. Research was performed to identify an optimal NDT technique by using sleepers with three different crack conditions: without any cracks, with a surface-opening crack, and with a hairline crack and possible latent cracks. Flexural rigidity rather than material stiffness or depth to multiple reflections was chosen as a key parameter for assessment of the structural integrity of a sleeper. The results allowed the design of a two-stage testing procedure: the flexural rigidity assessment of concrete tracks by antisymmetric Lamb (FRACTAL) waves technique for preliminary scanning and spectral analysis of surface waves (SASW) tests for in-depth investigation. A total of 18,177 sleepers that required testing were examined using FRACTAL tests, and 387 of these sleepers were double-checked using SASW tests. Before the overall examination of sleepers, a series of statistical analyses were performed to establish the criteria for defining the quality of a sleeper. The results of the FRACTAL test agreed with those of the SASW test in identifying sleepers with hairline cracks or possible latent cracks.