Research On Fracture Performance Of Cement Asphalt Mortars Based On SCB Method

High-speed railway(HSR)is a new trend in the development and application of modern railway technology.The main track-form in HSR is divided into traditional ballasted track and new ballastless track,and slab track is the most widely used.In China’s HSR network,there are mainly two slab tracks,namely,China Railway Track System(CRTS)I and CRTS II.These two tracks are similar in structure and both contain a new organic-inorganic composite material,also known as cement asphalt mortar(CAM).CAM is filled in the middle gap between the top plate and the bottom trackbed and hardens to form an elastic cushion layer.However,in the operation of HSR for only more than 10 years,it is identified that pre-cracking has occurred in this cushion layer.To address this issue,therefore,based on the semi-circular bending test method(SCB)and three fracture mechanics characterization parameters(critical stress intensity factor,fracture energy and critical strain energy release rate),fracture properties and failure mechanisms of two typical CAMs used in CRTSΙand CRTSΙΙ,namely CAM-Ⅰand CAM-II,are studied in this dissertation,with such influencing factors as the specimen thickness,notch length,loading rate and temperature considered,in an attempt to provide a basis for evaluating the cracking of the cushion layer and looking for an ideal repair or replacement cushion material in the future.Preliminary results indicated that critical strain energy release rate,J_c,is the most suitable fracture mechanics characterization parameter to compare the fracture properties of two different CAMs.The critical strain energy release rate of two CAMs is influenced by loading rate and temperature.With the increase of loading rate,the critical strain energy release rate of CAM-Ⅰdecreased,whilst the critical strain energy release rate of CAM-II increased.As temperature effect is concerned,for CAM-I,its critical strain energy release rate at a low or high temperature is lower than that at ambient temperature.But for CAM-II,its critical strain energy release rate trends to decrease as the temperature rises.It was observed that,furthermore,the fracture morphology of two CAMs is different.The crack of CAM-Ⅰis curving and the fracture surface is rough,whereas the crack of CAM-II is straight and the fracture surface is smooth.It may reveal the different failure mechanisms of the two CAMs.In the binding material of CAM-Ⅰ,asphalt is the main phase and cement-based materials is the dispersed phase.Asphalt is a typical viscoelastic material,and its modulus of elasticity is hugely different from that of the cement-based material or the fine aggregate.In this case,under loading,holes will be formed in the asphalt-cement interface or the asphalt-fine aggregate interface and ductile fracture will occur.In contrast,in the binding material of CAM-II,cement-based material is the main phase and the asphalt phase is the dispersed phase.The cement-based material is a porous material.Under loading,the cement-based material starts to crack first.Due to stress concentration,the pores in the cement-based materials will extend until they penetrate each other,and lead to quasi-brittle fracture.