| Summary: | High-speed railway or rail (HSR) is a hot topic all over the contemporary world. And there is a growing tendency towards the application of nonballasted or slab tracks in HSR. Cement-asphalt mortar (CAM), a composite of Portland cement and asphalt emulsion, is widely used as a cushion layer in the two prevailing prefabricated concrete slab tracks of HSR in China, namely CRTS I and CRTS II. After a few years’ operation and service, however, premature cracking has been identified in the CAM layer along part of CRTS I and CRTS II. This is mainly caused by the fatigue of CAMs under repetitive traffic loading, that is, mechanical fatigue. In this research work, therefore, static, dynamic mechanical and most importantly, fatigue properties of the two typical CAMs, namely CAM-I and CAM-II, were investigated. Using a 4-point bending (4PB) test method, static or quasi-static mechanical properties of these two CAMs were studied. Results indicated that the 4PB test method was suitable for characterising their static bending properties in the laboratory, and more reliable results could be obtained, especially on modulus of elasticity, compared to the compression test method which was usually used for formulation design and quality evaluation. However, irrespective of the test methods used, CAM-I and CAM-II were found to be distinctively different in their static mechanical properties and behaviour at room temperature, due to the changes in the microstructures of their binding materials, cement-asphalt binder (CABs), used in CAMs at different A/Cs. The primary functions of CAMs as the cushion layer in CRTS I and II, especially damping, had been demonstrated to be in close relation to their viscoelasticity. Based on the DMA method, the temperature spectra of dynamical modulus and loss factors of CAMs or CABs were obtained to characterise their temperature susceptibility and viscoelasticity, respectively. On the temperature spectrum of CAM-I, the two characteristic temperatures of the asphalt binder, Tg implied to be around -20 °C and TR&B measured to be 48 °C, could be determined, andthese two not only defined the viscoelastic zone for CAM-I and the immediate temperature range for the fatigue tests of CAM-I, but also the two boundaries of service temperatures for CAM-I and CAM-II under traffic loads. The higher A/C caused a decrease in dynamic modulus of CAMs but an increase in their loss factors and temperature susceptibility, and a balance should be considered between them in the future design of new CAMs. For the first time, 4PB fatigue of CAM-I and CAM-II were investigated using two different fatigue test schemes, and the results indicated that, in terms of the fatigue behaviour, the CAM-I can be considered as a cement-modified asphalt mortar whilst CAM-II an asphalt-modified cement mortar. Therefore, it is preferable to use the asphalt mixture-based fatigue test configuration for CAM-I and this fatigue test scheme was an ideal one to be used. Additionally, it was found that low temperature was beneficial to the fatigue life of CAM-I whereas high temperature was detrimental to its fatigue life. On the other hand, the fatigue test configuration of cement-based materials including plain concretes is favourable to use for CAM-II but might not suitable for CAM-II, especially when the influence of the reversal stress, R, or the test temperature was separately considered. Different from plain concrete materials, the reversal stress or the test temperature had a significant impact on the fatigue life of CAM-II. Higher temperature would greatly reduce its fatigue life, and this temperature should not be higher than TR&B of the asphalt binder used. Much longer fatigue life of CAM-II was observed under low temperatures if the same stress, instead of normalised stress level, was applied, and CAM-II was more like plain concretes when temperature fell to around Tg of the asphalt binder.
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