The Effect Of Chloride Corrosion On Interfacial Bonding Properties Of GFRP Cement-based Materials

Concrete filled steel tubular(CFST)members are composed of steel and concrete.The service life of these two materials in chloride corrosion environment is limited,and they need high maintenance cost in service.GFRP pipe-cement-based material component is a new type of composite component formed by placing cement-based material in GFRP pipe.It can replace some concrete-filled steel tubular components which need to work under chloride corrosion environment.Because of the significant difference between GFRP and cement-based materials,it is necessary to study the interfacial bond properties between GFRP and cement-based materials under chloride corrosion.Aiming at the effect of chloride corrosion on the interfacial bonding between GFRP pipe and cement-based material,the research project was carried out with the support of the National Natural Science Foundation of China(51578236).By means of indoor simulation test,the GFRP pipe-cement-based material was analyzed under the conditions of full immersion,dry-wet cycle at room temperature and dry-wet cycle at high temperature.The law of internal chloride ion erosion and the change rule of interfacial bond properties are as follows:(1)The chloride corrosion tests were carried out under the conditions of full immersion,dry-wet cycle at room temperature and dry-wet cycle at high temperature to study the penetration depth and chloride concentration at different depths of chloride ions under different conditions.The results show that the depth of chloride ion invasion and the chloride ion concentration at the same depth will be increased by the single factor of dry-wet cycle and high temperature.When the double coupling of dry-wet cycle and high temperature,the depth of chloride ion invasion and the chloride ion concentration at the same depth will increase more than the single effect.The microstructure changes of cement-based materials under three kinds of erosion environments were studied and qualitatively observed by SEM.The results show that the microstructure of the samples in the full immersion group is more compact than that in the earlier stage,and the microstructure of the samples in the room temperature dry-wet cycle group and the high temperature dry-wet cycle group is looser than that in the earlier stage.(2)Distribution of chloride ions in longitudinal sections of full-immersed and dry-wet cycling specimens was measured by silver nitrate colorimetry,and the difference of chloride ion migration and erosion mechanism under the two environments was analyzed.The results show that the chloride ion intrusion in the full-immersed specimens is mainly diffusion;the dry-wet cycle specimens includedrying stage and immersion stage;in the immersion stage,the chloride ion intrusion is mainly capillary absorption,and the diffusion effect is relatively weak.Because of the different corrosion mechanism,the chloride ion color region of the whole immersion group is rectangular,and the chloride ion color region of the room temperature dry-wet cycle group and the high temperature dry-wet cycle group is parabolic.(3)The bond stress-slip curves of 60 specimens of GFRP tube-cement-based materials were compared and analyzed.It was found that the main reason for the loss of interfacial bond strength was the decrease of interfacial chemical bond force.The results show that the interfacial bond loss will be increased when the dry-wet cycle and high temperature are combined,and the interfacial bond loss will be greater when the dry-wet cycle and high temperature are coupled.The loss mechanism is as follows:the microstructure of cement at the interface becomes loose after chloride attack,accompanied by Friedel salt crystals and sodium chloride crystals,resulting in the formation of a relatively loose interlayer between the core cement and the inner wall of GFRP pipe,making the chemical bonding force decrease.(4)By measuring the Micro-index of cement paste in the core area and interface area of high-temperature dry-wet cycling specimens,combining qualitative observation of SEM with quantitative analysis of MIP mercury intrusion data,further research was carried out.The mechanism of chloride migration and interfacial bond loss in dry and wet cycling specimens were investigated.The results show that the microstructure of cement in the core area is denser,the microstructure of cement in the interface area is looser,and there are more pores,and a large number of sodium chloride crystals are formed.The porosity of cement in the interface area is larger than that in the core area,and the pore volume is larger.With the increase of porosity and the formation of a large number of NaCl crystals,the interfacial chemical bonding force decreases and the interfacial bonding property decreases.