Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components

Abstract

The mechanical and microstructural properties as well as durability of more than century-old steel railway bridges that are still widely in use, among others in Poland, have not been well recognized. Ignoring the effect of material and mechanical properties on the operation of an old-steel bridge may eventually lead to the structure’s failure. This study investigates the reliability of a 100-year-old bridge made of wrought (puddled) iron. The aim of this study is to identify the microstructural and mechanical properties of over century-old steel bridge components in relation to the requirements for modern mild steel grades. The paper presents an analysis of the material and mechanical behaviour of a truss railway bridge that had been in operation for over a century. Laboratory tests were performed on samples taken from the component parts of a dismantled railway bridge, i.e. stringers, crossbars, lower belts, stringer bracings and wind bracing. Fatigue, fracture, hardness and tensile tests are performed to estimate the mechanical properties of the investigated steel bridge component parts. Spectroscopy is used to examine the chemical composition of every element. Light optical microscopy and scanning electron microscopy are employed to examine the microstructure of the steel component parts and its non-uniformities. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) are used to obtain data about the fracture mechanism and the chemical composition of non-metallic inclusions. Microstructural and mechanical results are discussed in relation to modern mild steel grades S355 and S235, which are widely used structural ferrous alloys. Based on the chemical composition analysis, the possibilities of increasing the strength of the bridge by welding methods and the application of fibre-reinforced polymer (FRP) composite materials are discussed. Obtained results add to the body of knowledge about the maintenance, reinforcement and demolition of bridge structures, which is crucial for preventing failure of over 100-year-old steel structures. This research is an introduction to the study on reinforcing old steel structures by FRP composite materials.