Viscoelastic deformation behavior of cement and emulsified asphalt mortar in China railway track system I prefabricated slab track

Under repeated train-induced loads, cement and emulsified asphalt mortar(CA mortar) as a viscoelastic material has a time-dependent deformation, part of which is irreversible. This could lead to debonding between the mortar layer and the track slab. Based on the theory of viscoelasticity and the analytical method of the time hardening law(THL), the viscoelastic deformation behavior of CA mortar was studied. Using ABAQUS, we established a solid model of China railway track system(CRTS) Ⅰ prefabricated slab track, with CA mortar at different initial Young’s moduli under cyclic loading corresponding to the influence of actual train loads. The results reveal that the fitted parameters of the THL for CA mortar are suitable for describing its viscoelastic deformation. As the initial Young’s modulus increases, the strain difference before and after cyclic loading gradually decreases, and the displacement difference increases from 0.2 mm to 0.6 mm. The deformation mainly occurs at the end of a mortar layer with longitudinal distribution of about 2.5 times the fasteners’ spacing. It follows that the viscoelastic performance of CA mortar is one of the most important reasons that cause debonding underneath the track slab. Therefore, we suggest that the adverse effects of viscoelastic behavior of CA mortar should be considered when researching such deformation and damage.

Effects of supercritical CO_(2) on viscoelastic properties of shales

Laboratory uniaxial compression creep tests,with differential stress of 30 MPa hold for 3 h,were performed on Chang-7,Longmaxi(LMX)and Barnett shales to study the influence of SC-CO_(2)on short-term viscoelastic properties.To this end,the wet shale samples were treated with SC-CO_(2)with a pressure of30 MPa and a temperature of 110℃for 14 days.We analyzed the creep data using the fractional Maxwell model.To investigate microscopic structural alterations,the surface morphology of the same location,before and after SC-CO_(2)-water exposure,was examined by SEM images.Compared with dry shales,dynamic and static elastic moduli decreased by up to 25.02%and 55.83%,respectively,but the creep deformation increased by 200%for LMX and Chang-7 shales,and 500%for the Barnett shale treated by SC-CO_(2).Compared to dry sample,there is an increase in calculated fractional orders of 0.02,0.07,0.22 for SC-CO_(2)treated samples,indicating that SC-CO_(2)treatment is likely to enhance shale creep.SEM investigation confirmed physicochemical mechanisms responsible for the observed elastic damage and creep enhancement,including mineral dissolution and swelling caused by SC-CO_(2).This work would further improve our current understanding of the time-dependent deformation of shale under chemicalmechanical coupling effects during CO_(2)capture utilization and storage.

Addressing the permanent deformation behavior of hot mix asphalt by triaxial cyclic compression testing with cyclic confining pressure

Rutting or permanent deformation is one of the major distress modes of hot mix asphalt in the field. Triaxial cycle compression testing （TCCT） is a standardized and scientifically accepted test method to address this distress mode in the lab and to characterize the resistance to permanent deformation. In most labs and according to EN 12697-25, standard TGCTs are carried out with cyclic axial loading and a constant confining pressure. In road pavements on the other hand, dynamic traffic loading due to passing wheels leads to cyclic confining pressure. In order to bring the TCCT closer to reality, the radial reaction and its phase lag to axial loading in standard TCCTs are analyzed and an enhanced TCCT with cyclic confining pressure is introduced. The cyclic confining pressure takes into account the viscoelastic material response by the radial phase lag to axial phase loading. In a subsequent test program, TCCTs with different confining pressure amplitudes were carried out on two hot mix asphalts. Results from standard and enhanced TCCTs were analyzed, compared and discussed. It is shown that the resistance to permanent deformation in- creases significantly when the viscoelastic material response is taken into account in the TCCT by introducing cyclic confining pressure.