Characterisation of thermomechanical properties of bituminous mixtures used for railway infrastructures

The research presented in this document was carried out in collaboration between the National School of Public Works (Ecole Nationale des Travaux Publics de l’Etat – ENTPE) of the University of Lyon and the French National Railway Company (Societe Nationale des Chemins de Fer francais – SNCF). The objective of this study is the characterisation of the thermomechanical properties of road base-course bituminous mixtures commonly used in France for their use in railway track structures. Linear Viscoelastic (LVE) and fatigue resistance properties have been investigated. The influence of moisture damage including freeze-thaw cycles on the properties of the studied mixtures was also studied. LVE and fatigue resistance properties were obtained by means of sinusoidal tension-compression tests on cylindrical samples at the ENTPE/LTDS laboratory. A protocol for moisture conditioning of bituminous mixtures samples was developed based on the French and American standard test methods for moisture susceptibility and on the literature review on the subject. Moisture damage was assessed using complex modulus and fatigue tests. The LVE behaviour of the materials was described using the 2S2P1D (2 Springs, 2 Parabolic elements and 1 Dashpot) model developed at the ENTPE/LTDS laboratory. Three road base-course mixtures available in the French market were studied: a GB3, a GB4 and a mixture called GB PMB (for Polymer-Modified Bitumen). The GB3 mixture stands as the reference material since it is commonly used for the base-courses of conventional roads in France. The GB4 mixture stands as a better performing material than the GB3. The specific studied GB4 formulation corresponds to that used as sub-ballast of the Brittany-Loire high-speed line (HSL) in western France. The GB PMB stands as an improved version of the GB3 in which the base bitumen is replaced by polymer-modified bitumen. The interest of the study of this third mixture is to assess the relevance of using PMB’s in bituminous mixtures intended for railway platforms. The test zone of the East-European HSL served as case study to identify the loading conditions and the behaviour of a bituminous sub-ballast layer in a French track. The several advantages of using bituminous mixtures as sub-ballast material identified in the literature are confirmed by the feedback from this test zone. The results obtained show that the studied bituminous mixtures present excellent bearing capacities (stiffness) for their use as sub-ballast layers in France. The used moisture conditioning protocol did not alter the LVE behaviour of the materials. The good performance of the materials is then expected to be perennial regarding the effect of moisture on the LVE behaviour. With respect to fatigue resistance, the results show that the use of a PMB provides an important increase of the fatigue life as well as a reduction of the susceptibility to moisture damage. However, given the identified loading levels of the bituminous sub-ballast in a common French track structure, all three mixtures present adequate fatigue resistance properties.