Cement asphalt mortar for high-speed rail

Abstract

During the past two decades, the high-speed rail technology has been developed rapidly in China. At present, China has the most complete technology system, the most integrated capacity, the longest operating mileage and the highest operating speed. Many new structures, technologies and materials have been developed and employed in the construction of the infrastructure of high-speed rail. Ballastless track systems have been used as a key component for the high-speed rail line. Among which, three types of ballastless slab track, i.e., CRTS I, II and III slab tracks, are implemented in the backbone lines in China, such as Beijing-Shanghai line, Haerbin-Dalian line. The ballastless slab track systems are different in structural forms and materials employed from each other. Cement asphalt (CA) mortar is the key engineering material for CRTS I and II slab tracks of high-speed rail, which has significant effects on the track dynamics, durability and riding comfort of slab ballastless track. In order to meet the needs for massive construction of high-speed rail in China, intensive studies on CA mortar have been carried out, and complete set of technology with independent intellectual property rights have been established. This gives solid support for the construction of 7×103 km ballastless slab track in China. Approximately, 1.5×106 m3 CA mortar has been used in the construction of high-speed rail. In practice, fresh CA mortar is cast into the narrow chamber enclosed by prefabricated slab and base concrete plate to form a “sandwich” structure. The cushion layer of CA mortar is required to have strong bonding or close contact with the slab situated above. The construction quality of CA mortar cushion layer is crucial to the quality of track system, and thus high-speed rail lines. This paper briefly introduces the development history of CA mortar in China, mainly gives a comprehensive overview on the rheology, early deformation, microstructure, mechanical properties and durability of CA mortar, from the perspective of material science and practical engineering. China carried out preliminary study on CA mortar for railway at 1970s, and intensive studies since 2006. CA mortar is an organic-inorganic composite, which is either cement-based or asphalt-based material, depending on cement-to-asphalt ratio. Cement hydrates provide rigid framework, while asphalt agglomerates provide damping ability for the material. The rheological properties of fresh CA mortar are similar to those of fresh cement mortar. In order to fill up the narrow and sealed space, CA mortar should have relatively high viscosity, low yield stress and appropriate thixotropy. Aluminum powder is used in combination with expansive agent to compensate the shrinkage of CA mortar. The pore structure of CA mortar is similar to that of cement mortar. Due to the low strength of asphalt, mercury intrusion porosity (MIP) test is not suitable for CA mortar. Nuclear magnetic resonance (NMR) techniques have been developed to characterize the pore structure of CA mortar. The mechanical properties of CA mortar mainly depend on cement-to-asphalt ratio, and barely depend on water-to-cement ratio. Viscoelastic properties of CA mortar are temperature-dependent, and can be well characterized by dynamic mechanical thermal analysis (DMTA). The durability of CA mortar is mainly associated with water which causes calcium leaching and softening. The different deformation between slab and CA mortar layer due to temperature and aging of asphalt may also cause the durability problem of CA mortar. Based on the established models, CA mortar cushion layer can meet the designed service life.