Measurement of deformation of the concrete sleepers under different support conditions using non-contact laser speckle imaging sensor

Prestressed concrete sleepers play a significant role in distributing wheel loads from the rails to the ballast layer. Cyclic wheel loads may cause local deterioration of the ballast layer which would affect the support conditions beneath the sleepers. Therefore, understanding the influence of different support conditions on the deformation of sleepers is necessary for maintaining safe railroad operations. In this study, a non-contact and non-destructive laser speckle imaging sensor (LSIS) was developed and successfully applied to investigate the flexural behaviour of concrete sleepers under various support conditions, including full support, partial support and centre binding. The results show that LSIS was in agreement with foil strain gauges of achieving high-resolution strain measurements with a maximum mean absolute error of 5.41 +/- 3.12 mu epsilon (equivalent to a mean absolute error of 7.15%) as the detection threshold (20 mu epsilon) is exceeded. More than three times change in the negative bending moment was observed at the mid-span of the sleeper as the support condition was changed from full support to centre binding, which indicates the potential risks of voided ballast support and importance of ballast tamping in railway maintenance.