Investigation Of Concrete Pore Structure And Damage Evolution Under Freeze-thaw Cycles

Due to the expansion of human living space and the diversification of living environments,numerous concrete structures have been exposed to rigorous special conditions.Freeze-thaw(F-T)cycles influenced the mechanical properties and durability of concrete in cold regions.As a typical artificial composite,the complex meso-and micro-internal structure of concrete imparts its macro-properties,so the investigation of internal meso-and micro-structure has an important meaning to the further research on the damage mechanism and improvement of mix proportion and manufacturing techniques.The pore structure is an important part of concrete internal structure which influences the frost resistance directly.Most research is focused on the qualitative analysis and quantitative analysis based on the porosity.However,this method can only characterize the pore structure partly.Introducing more characters to the investigation of concrete pore structure and fully consideration of pore structure characteristics are significantly important.Firstly,based on the traditional pore structure test,mercury intrusion method,the computed tomography(CT)was introduced to this research.The testing range of pore diameter extended to 10nm-1mm.Based on the results of CT scanning,the average CT number and box-dimension were used to characterize the damage process of concrete under F-T cycles.The distribution of pore diameter was included to cover the shortage of simple parameter.The testing results indicated that the proportion of harmful pore increased after F-T cycles and the 100~thh F-T cycles was a key point.Secondly,the relationship of meso-scale porosity and macro-mechanical property was built up.Besides,the damage theory was introduced into the study and the relationship pf F-T cycles and different damage parameters based on porosity,average CT number and standard deviation of CT number were established.The damage parameters of porosity and standard deviation of CT number could reflect the damage process accurately,but the damage parameter of average CT number failed at the high degree of damage.Thirdly,3D printed specimens were fabricated based on the digital models of CT data and tested by uniaxial compression tests.3D printed specimens can accurately repeat the internal meso-pore structure of concrete.Particularly,the stereo lithograpghy technique can produced 3D printed specimens with three phase structures.The experimental results showed that the printing techniques and materials impact pore structure-strength relation directly.In this research,3D printed specimens fabricated by three-dimension jet-ink printer have similar meso-pore structure-strength relation with the cement concrete specimens.