Mesoscopic Mechanism And Mechanical Model Of Damage And Debonding Of Sandstone-concrete Interface Induced By Freeze-thaw

The good bonding performance of rock-concrete is the important premise to guarantee the protective effect of concrete engineering.For engineering protection in cold regions,interface damage and debonding are prone to occur under the influence of freeze-thaw effect,which brings many challenges to engineering protection safety in cold regions.For the cognitive of rock-concrete interface damage and debonding mechanism induced by freeze-thaw,it is necessary to systematically analyze based on the microscopic characteristics of interface damage.Therefore,this paper relies on the National Natural Science Foundation of China"Damage and debonding mechanism of the interface between rock and concrete due to the freeze-thaw cycles"(No.41772333).Nuclear magnetic resonance(NMR)stratification microanalysis during freeze-thaw process was carried out on sandstone-concrete composites.The evolution law of freezing-thawing damage within the range of interfacial influence zone(IIZ)is focused on to reveal the microscopic characteristics and mechanism of interface damage and debonding of sandstone-concrete induced by freeze-thaw.According to the theory of damage fracture mechanics,the formation and interfacial debonding fracture of IIZ are studied.The research results have important practical significance for bolt-shotcrete support of tunnel in cold regions and protection safety of slope shoterete.The main research results include:(1)NMR SE-SPI sequence has good applicability to rock or concrete under different spatial resolutions and can provide guidance for pore analysis of porous materials such as rock.With the increase of freeze-thaw cycles,the water content at the interface of the sandstone-concrete composites rises slowly at first and then increases rapidly;the number of micropores of sandstone side adds,and the mesoporous content decreases;however,both micropores and mesoporous of concrete side aggrandize.(2)Based on the SE-SPI sequence analysis method of NMR,the intensity of NMR signals at different spatial positions is obtained to determine the range of IIZ.Specifically,at-10??10?,-15??15?,and-20??20?,the range of IIZ is 13.6mm,9.1mm,and 4.5mm,respectively.At the same time,relying on the basic theories of frost heave hydrostatic pressure,water migration,freezing temperature,matrix permeability and matrix fracture toughness,the microscopic characteristics and evolution mechanism of IIZ are revealed.(3)NMR test results are used to obtain the damage variable characterization results of sandstone side,concrete side,and IIZ respectively.The crack propagation of IIZ is analyzed according to the fracture mechanics theory,and then an interface crack deflection criterion on account of M value is proposed.Then,in view of the theoretical analysis of interface fatigue damage and debonding under variable-amplitude load,the relationship between IIZ fatigue damage and the amplitude of effective stress intensity factor under freeze-thaw cycles is discussed.Moreover,the time-history model of sandstone-concrete interface debonding evolution under freeze-thaw is established.(4)Based on the experimental tests and basic theoretical analysis,the debonding process of sandstone-concrete interface under freeze-thaw action is divided into four stages:? the initiation of antecedence cracks induced by frost force;? the fatigue damage and expansion of antecedence cracks under repeated freezing-thawing action;? the penetration of cracks and debonding of interface within the IIZ range;? the whole debonding and fracture of the composites.