Study On Mechanical Properties And Chloride Ion Transport Of Steam-cured Concrete In Salt Spray

A large number of key track structures such as track slabs for high-speed railways in China adopt steam-cured concrete prefabricated components,which often have defects such as surface cracking and internal pore coarsening in their production practice,which have adverse effects on the safety and durability of track structures.For track structures serving in coastal areas,these defects can provide channels for the intrusion of corrosive media such as chloride ions in the marine environment,posing more severe challenges to the operation safety and service life of high-speed railways.Given this,this paper takes steam-cured concrete for high-speed railways as the main research object,uses various testing methods and characterization methods,systematically studies the hydration products,microstructure characteristics,compressive strength,and other basic physical and mechanical properties of steam-cured concrete and its chloride ion transport performance in salt spray environment.The chloride ion transport model of steam-cured concrete in a salt spray environment is modified,and based on the test results,the service life of steam-cured concrete prefabricated components for the high-speed railway is predicted.The main research results are as follows:(1)The effects of curing system,age,and cementitious system composition on concrete cube compressive strength,axial compressive strength,and elastic modulus were compared and studied,and the effect of steam-curing on cementitious material hydration was discussed by combining X-ray diffraction and thermogravimetric analysis.The results show that steam curing improved the 7d compressive strength and elastic modulus of concrete,where the increase of cube compressive strength at 7d age was between 8%and 25%,and the elastic modulus only increased by about 3%,but steam curing hurt the long-term mechanical properties of concrete.The concrete(F20S10 group)has the least calcium hydroxide content,which indicates that the multi-component composite effect of fly ash+mineral powder can improve the secondary hydration degree of cementitious system and improve the physical and mechanical properties of concrete in a later stage.(2)The pore structure of standard-cured and steam-cured concrete was characterized by multi-scale joint characterization using various characterization methods such as Mercury intrusion porosimetry,Nuclear Magnetic Resonance,super-depth-of-field microscope imaging,capillary water absorption characteristics,etc.,and the paste-aggregate interface of concrete was scientifically characterized by scanning electron microscope.The results show that steam curing increases the proportion of large capillary pores in concrete,and increases the pore size and looseness degree of the paste-aggregate interface.The addition of fly ash and mineral powder does not reduce the porosity of concrete but can optimize the pore structure of steam-cured concrete,reduce the proportion of harmful pores(>200nm),and reduce the capillary water absorption coefficient.The reduction of multi-harm pores can reach 4%at 90d,and the capillary water absorption coefficient is reduced by more than 11.5%.The paste-aggregate interface of cement-fly ash-mineral powder ternary cementitious system is relatively dense,with low porosity and excellent pore structure distribution.(3)The exposure test of steam-cured concrete in marine atmosphere and indoor accelerated salt spray test were carried out,and the effects of curing conditions,gelling system composition,temperature conditions and erosion time on chloride ion transmission were analyzed.The results show that the chloride ion erosion depth of steam-cured concrete and standard-cured concrete is almost the same after 13 months of real sea exposure,both around 4mm,but the chloride ion content and diffusion coefficient of steam-cured concrete are higher.The results obtained from the real sea exposure test and indoor salt spray test have similar rules,that is,F20S10 group concrete has good steam curing applicability,and the free chloride ions migrating to the matrix interior under a salt spray environment are relatively less.The chloride ion diffusion coefficient of steam-cured concrete increases with increasing temperature and decreases gradually with increasing erosion time.The diffusion coefficient of steam-cured concrete has a power function relationship with the number of wet-dry cycles,and the surface chloride ion content has an optimal logarithmic time-varying model relationship with the number of wet-dry cycles;The diffusion coefficient is roughly linear with temperature.(4)Based on the test results of the chloride ion transport test of steam-cured concrete,the chloride ion transport model of steam-cured concrete under a salt spray environment was modified based on chloride ion binding,exposure time,and test temperature.The conclusions are as follows:The chloride ion binding of steam-cured concrete conforms to linear binding theory.The chloride ion binding coefficient of F20S10 group steam-cured concrete in a salt spray environment is 0.365.The time decay coefficient can be taken as 0.646.The activation energy of chloride ions during diffusion is 2.2×10~4 J/mol.Based on the above parameters,the service life prediction of steam-cured concrete prefabricated components in a salt spray environment was carried out,and the service life was 70 years.