Study On Mechanical Response And Fracture Law Of Rock-Concrete Interface Under Impact Load

Rock-concrete interface widely exists in tunnel and other projects,and rock-concrete interface is the weak part of structural mechanics of tunnel lining,which is more likely to fracture under external loads(such as construction blasting).Once the rock-concrete interface is damaged,it will pose a serious threat to the safety and durability of the tunnel.Therefore,it is particularly important to conduct in-depth research on the mechanical response and fracture law of the rock-concrete interface.This paper uses MTS and SHPB testing equipment to conduct three-point bending tests on rock-concrete bi-material semi-circular disk specimens under static and dynamic load conditions,taking into account the effects of loading rate and age on the mechanical response and fracture law of the rock-concrete interface.At the same time,Brazilian splitting tests were conducted on the rock-concrete bi-material semi-circular disk specimens to study the directional effect of the mechanical response of the rock-concrete interface.The stress state of the rock-concrete interface was analyzed using the displacement trend line method by combining high-speed photography with digital image correlation technology,revealing the mechanical response and fracture law of the rock-concrete interface under dynamic load.The main innovative achievements of this paper are as follows:(1)The study on the fracture toughness and fracture energy of the rock-concrete interface under different loading rates found that the dynamic fracture toughness and fracture energy of the rock-concrete interface have a significant loading rate effect.The fracture toughness and fracture energy of the rock-concrete interface increase with the increase of loading rate.Moreover,there are significant differences in the loading rate effect of rock concrete interface fracture toughness with different sand/cement ratios,indicating that the concrete sand/cement ratio can control the loading rate effect of rock-concrete interface fracture toughness.(2)Research on the fracture toughness of the rock-concrete interface at different ages has found that the fracture toughness of the rock-concrete interface increases with the increase of concrete age.Moreover,the smaller the age,the more significant the loading rate effect on the fracture toughness of the rock-concrete interface,indicating that the smaller the age,the greater the impact of dynamic load on the interface.In addition,the influence of the sand/cement ratio on the fracture toughness of the rock-concrete interface is significantly different in different age ranges.(3)Research on the fracture energy of rock-concrete interfaces at different ages has found that the fracture energy of the rock-concrete interface increases with the increase of concrete age(except for the sandstone-concrete interface).For the sandstone-concrete interface,the maximum age of fracture energy is 7 days,and the fracture energy at 28 days is 35.21%lower than that at 7 days.The variation of fracture energy at the rock-concrete interface with age varies depending on the type of rock.In addition,by comparing the fracture energy of rock-concrete interfaces with different sand/cement ratios at different ages,it was found that before the age of 7 days,the fracture energy of the interface first increased and then decreased with the increase of sand/cement ratio.However,at the age of 28 days,the fracture energy of the interface increased with the increase of sand/cement ratio.This indicates that the influence of sand/cement ratio on the fracture energy of the interface varies at different ages.(4)Research on the failure modes of rock-concrete interfaces at different ages has found that the older the age,the more obvious the phenomenon of fracture of the rock or concrete base material at the interface.Moreover,at different ages,the rock type and concrete sand/cement ratio have a significant impact on the failure characteristics of the interface.It is worth noting that for the limestone-concrete interface,after 7 days of age,the rock surface is completely covered by the fractured concrete material.For the marble concrete interface,the concrete surface is completely covered by the fractured marble material at 28 days.After a specific age,the rock-concrete interface may not necessarily be the weakest point in the structure’s mechanical properties,and the rock type plays a decisive role.(5)A study on the failure modes of rock-concrete interfaces at different inclinations revealed three typical failure modes in the bi-material disk specimen.As the inclination angle of the interface increases,the failure mode of the rock-concrete disk specimen changes from interfacial tensile failure to tensile shear coupling failure,and finally to tensile failure of the base material.In addition,when the inclination angle of the interface is 15°,the fracture mode of the rock concrete interface is significantly influenced by the interface morphology.When the width of the rock concrete interface groove is small,the concrete serrations at the interface break.However,when the width of the interface groove is large,the rock serrations at the interface break.(6)Analysis of the peak load of bi-material disk specimens with different interface inclination angles found that the peak load of bi-material disk specimens increased with the increase of interface inclination angle,both under static and dynamic loads.The inclination angle of the interface is sufficient to have a significant impact on the bearing capacity of the rock-concrete composite structure.In addition,if 0°≤θ≤30°,the peak load of the sample first increases and then decreases as the width of the interface groove increases.At this point,the optimal groove width on the rock surface is 5 mm.If 45°≤θ≤90°,the peak load of the sample tends to stabilize with the increase of the interface groove width.It indicates that grooving the rock surface under a small inclination angle can significantly improve the bearing capacity of the rock-concrete composite structure.Therefore,in order to improve the load-bearing capacity of the structure,different treatment methods should be adopted for the rock surface at different interface inclinations.