Sandstone-concrete Interface Transition Zone(ITZ)Damage And Debonding Micro Mechanism Analysis Under Freeze-thaw

The sufficient bond between concrete and rock is an important prerequisite to ensure the effect of shotcrete support.However,in the cold region engineering protection system,the rock-concrete binary interface is used as the traditional weak surface,and the bonding between the concrete and the rock surface is easily affected by the freezing and thawing environment in the cold area,resulting in interface damage and debonding,which causes support failure,affecting the safety of engineering protection.Scientific cognition of the micro mechanism of the damage and debonding of the rock-concrete interface is the basic work of improving the interface protection.In this paper,the sandstone-concrete binary sample is taken as the object to carry out the experimental study on the microstructure,micro mechanics and debond evolution of the sandstone-concrete interface transition zone(ITZ)under the freeze-thaw conditions.The major research activities include:?The microstructure,bonding state and hydration products of the sandstone-concrete ITZ binary were analyzed by the scanning electron microscope microscope test;?Through scanning electron microscopy and stereomicroscope test micro-testing methods,this paper respectively carry out freeze-thaw cycle tests of-5??5?,-10??10?,-15??15?,-20??20?,and analyze sandstone-concrete The whole process of ITZ cracking,expansion and debonding of binary ITZ at different temperature ranges and cycle times;?Through the nano-indentation test under freeze-thaw cycles,the influence of freeze-thaw parameters on the micromechanics and width of sandstone,concrete and ITZ was analyzed.According to the test results,the following main conclusions are obtained:(1)The main hydration products of sandstone-concrete ITZ are C-S-H gel,AFt crystal and CH crystal.Among them,C-S-H gel is the main source of the bonding strength of binary ITZ.As the number of freeze-thaw cycles increases,The hydration products will gradually break,reduce and disappear.(2)As the number of freeze-thaw cycles increases,the sandstone-concrete binary ITZ debonding process can be divided into four stages:?freeze-thaw induced micro-crack initiation;?the crack is continuously subjected to cyclic loading of freeze-expansion force,and the crack gradually expansion;?crack penetration in some areas;?separation of sandstone and concrete,that is,debonding of the whole binary body(3)The number of freeze-thaw cycles has a significant effect on the micro-mechanical properties of sandstone-concrete.The main conclusions include:?The hardness and elastic modulus values of the sample show a“U”shape distribution from the sandstone interface to the concrete;?Different freeze-thaw times have a certain effect on the width of the ITZ;?During different freeze-thaw cycles Under the condition of the number of times,the hardness value and elastic modulus value of the concrete cement matrix and ITZ showed a decreasing trend.With the increase of the freezing and thawing times,the hardness value and the elastic modulus value of the same freezing and thawing times decreased and showed an increasing trend.(4)According to material theory and fracture force theory,the sandstone-concrete ITZ debonding is analyzed,and then the microscopic debonding evolution mechanism and freeze-thaw induced ITZ micromechanical evolution model are proposed.