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

Abstract

a concrete slab track system with various thicknesses, ranging from 0.11 to 0.13 m. Recently, some newly constructed sleepers have exhibited both surface-opening and latent cracks, as shown in Figure 1. A total of 332 sleepers with surface-opening cracks were discovered among the 306,000 sleepers installed for the second-stage construction of the KTX railway from Taegu to Gyeongju, South Korea. A team of investigators organized by the South Korean government concluded that conical cracks had developed in the sleepers because of volume expansion of the freezing water entrapped in the embedded fastening assemblies during the winter. Among the 306,000 sleepers, 18,177 were identified to be suspicious or damaged by freezing. The problem was then to identify sleepers with internal damage for repair. Sleepers with surface-opening cracks are easily detected by visual inspection. However, sleepers with hairline cracks or latent cracks are hard to detect. To make matters worse, the KTX railway construction schedule required the inspections to be finished in a couple of months.The goal of the research described here was to design an optimal nondestructive testing (NDT) method for the identification of sleepers with hairline cracks or latent cracks. The impact-echo method (1, 2), the spectral analysis of surface waves (SASW) method (3), and the flexural rigidity assessment of concrete tracks by antisymmetric Lamb waves (FRACTAL) method (4) were used to investigate three typical sleepers: an undamaged sleeper, a sleeper with surface-opening cracks, and a sleeper with hairline cracks and possible latent cracks. The proposed optimal procedure involves two stages of testing: FRACTAL tests for preliminary scanning and SASW tests for in-depth investigation. The procedure was applied to all 18,177 of the suspicious sleepers, and some typical results are presented in this paper.In the impact-echo test, stress waves from an impulse source are applied to the top surface of a platelike structure and are then reflected back from the bottom of the surface or from internal anomalies within the structure. The multiple reflections between the two plate boundaries are analyzed to determine the thickness of the structure or the 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 ma...

Investigation of Material Improvements to Mitigate the Effects of the Abrasion Mechanism of Concrete Crosstie Rail Seat Deterioration

“…Industry experts have questioned the use of air entrainment in concrete crossties citing the possible adverse effect on the abrasion resistance of the rail seat. According to the published literature related to concrete materials, the abrasion resistance of concrete is directly related to its compressive strength (Witte and Backstrom 1995;Hadchti and Carrasquillo 1988). Also, concrete compressive strength is inversely related to the air content (Mindess et al 2003).…”

“…Through previous research on RSD, the University of Illinois at Urbana-Champaign (UIUC) has identified five possible mechanisms having the potential to contribute to RSD. The feasible mechanisms are abrasion, crushing, freeze-thaw cracking, hydraulic-pressure cracking, and hydro-abrasive erosion (Joh et al 2010;Zeman et al 2009). Of these mechanisms, hydraulic-pressure cracking and hydro-abrasive erosion were investigated at UIUC and found to be feasible mechanisms resulting in RSD (Zeman 2010;Choros et al 2007;Bakharev 1994).…”