Engineered Semi-Flexible Composite Mixture Design and Its Implementation Method at Railroad Bridge Approach

Considerable variation in the vertical displacement can cause railway tracks’ transition problems at the bridge approach. The vertical displacement gaps can result in amplification of the dynamic force and frequency, and gradually degrade the serviceability of the railway track. Many strategies, focusing on either modifying the track component or making changes to the entire structure, were used to mitigate transition problems. In particular, asphalt concrete underlayment as a structural adjustment method provides additional support to the ballast and protects the subgrade. However, its effect of reducing dynamic impact at the bridge approach is limited because asphalt mixture has a limited range of modulus and cannot make enough adjustments to the entire structure. Therefore, this paper aims to develop an engineered semi-flexible composite mixture (SFCM) design to mitigate the transition problem. The experiment showed that SFCM is a viscoelastic material with a wider modulus range, and its modulus can adjust with its air voids and the concrete slurry content. Track analysis using a 2.5D sandwich model was conducted to simulate the effects of the structure and material on the responses of the railway track under the dynamic loads and determine the arrangement of the transition zone. A four-segment transition zone design was eventually proposed for a special case of bridge approach. This method can be used to develop transition zones for achieving a smooth transition at the bridge approaches.