Study On Anti-carbonization And Salt-freezing Resistance Performance Of UHTCC Under Bending Load

The ultra-high toughness cement-based composite material is a new type of high-performance fiber reinforced cement-based composite material that uses PVA fiber volume of not more than 2%to formulate a ultimate tensile strain of more than 3%and a limit crack width of less than 100?m.The composite material has great application prospects in improving the durability,safety and sustainability of reinforced concrete structures.This paper conducts experimental research and analysis on the anti-carbonization and salt-freezing properties under the bending load.The main research contents of the thesis are as follows:(1)Anti-carbonization test under bending load.The effect of carbonization time,water-cement ratio,load rate and bending moment on the carbonation depth of UHTCC under continuous bending load was studied by phenolphthalein titration.The results indicate that the carbonation depth of UHTCC increases with the increasing carbonization time,and the carbonization depth increases rapidly in the first 7 days,and the growth is relatively slow after 7 days.And,increasing the water-cement ratio will accelerate the carbonization process of UHTCC,and the carbonization depth of UHTCC increases with the increasing load rate.What is more,the carbonization depth at each position of the UHTCC specimen increases with the increase of the bending moment,and the carbonization depth increases with the bending moment in logarithmic form.(2)Study on the mechanism of carbonation test.It is the Microhardness,TGA,XRD and MIP that can reflect the carbonation depth and carbonization mechanism of UHTCC materials.The results show that the carbonization depth value reflected by the microhardness is slightly higher than the carbonization depth value measured by the phenolphthalein titration method,but the difference is not much difference.And,the porosity of the carbonized zone is smaller than that of the non-carbonized zone,and the pore structure of the carbonized zone is significantly better than that of the non-carbonized zone.Combined with XRD analysis,the reason is that CaC03formed by carbonization fills the internal voids of UHTCC material,resulting in a significant reduction in porosity and optimization of pore structure.It is also found that the TGA method can be used to divide the carbonation test area into three parts:a fully carbonized zone,a partially carbonized zone,and a non-carbonized zone.(3)Salt-free freezing test under bending load.The influence of deflection,bending moment and salt solution concentration on the relative dynamic elastic modulus was studied by measuring the ultrasonic propagation time,which reflected the damage degree of salt freeze on UTHCC material.The results show that the 3%concentration of salt solution has the greatest damage to the material,and the introduction of the bending load will increase the damage degree of the material,and the damage degree of UHTCC will increase with the increase of the deflection.(4)Chloride ion penetration test.The effects of deflection,bending moment and salt solution concentration on chloride ion erosion depth were studied by silver nitrate titration method.The effects of deflection,bending moment and salt solution concentration on free chloride ion content were studied by chloride ion content rapid analyzer.The results show that with the increase of deflection,bending moment and salt solution concentration,the greater the erosion depth of chloride ion,the weaker the resistance of UTHCC to chloride ion erosion;At the same time,as the deflection,bending moment and salt solution concentration increase,the content of free chloride ions also increases.(5)Four-point bending test before and after salt freezing.The study compared the four-point bending strength of UTHCC specimens with different salt solution concentrations before and after salt freezing test.The result shows that after 28 freeze-thaw cycles,the UHTCC specimens have reduced ultimate bending strength and ultimate mid-span deflection,but still exhibit good strain hardening characteristics,indicating that UHTCC specimens have excellent salt-freezing properties.And the ultimate bending strength and mid-span deflection of 3%concentration of salt solution decreased most obviously.