Study On The Properties And Evolution Of Highly Ductile Cementitious Composites(PVA-ECC) After Freeze-Thaw Cycle

In the cold regions of northern China,concrete is constantly subjected to the effects of freeze-thaw cycles,which greatly affects the service life and safety of concrete.In order to effectively solve the problems of concrete cracking,poor durability,and low tensile strength,this article selected a high-ductility cement-based composite material（Engineered cementitious composites,ECC）reinforced with polyvinyl alcohol fibers（PVA）.PVA-ECC is an engineering composite material with a uniaxial tensile strain of up to 3%.Therefore,this article used PVA-ECC as the experimental material and freeze-thaw environment as the research background.Through freeze-thaw cycle tests,with different freeze-thaw cycle times（0、100、200、300）and environmental temperatures for testing performance after freeze-thaw cycles（20°C、0°C、-10°C、-20°C、-30°C）as experimental parameters,the porosity structure,ice formation characteristics,and mechanical properties of PVA-ECC were studied using nuclear magnetic resonance（NMR）and digital image correlation（DIC）technologies.The specific research contents are as follows:（1）Through freeze-thaw cycle tests,the relative dynamic elastic modulus and mass loss rate of PVA-ECC specimens were tested to obtain the freeze-thaw damage degree.Combining with the Weibull probability distribution model,the freeze-thaw damage was analyzed.The test results showed that with the increase of freeze-thaw cycles,the relative dynamic elastic modulus gradually decreased and the freeze-thaw damage degree gradually increased.In addition,the relationship between the damage variable and freeze-thaw cycle times was established by the Weibull probability distribution function’s freeze-thaw damage evolution model,and the life of PVA-ECC was predicted based on the model.（2）The pore structure evolution characteristics of different freeze-thaw cycles after freeze-thaw cycles were tested by nuclear magnetic resonance（NMR）technology.Combined with NMR₂ spectrum and pore size distribution,the evolution law of pore structure of PVA-ECC with freeze-thaw cycles was analyzed.The experimental results showed that the peak area of₂ spectrum of PVA-ECC in the melting state increased gradually with the increase of freeze-thaw times,the curve distribution of pore size showed an obvious upward trend,and the porosity increased gradually.The overall fractal dimension Dt of pores was distributed in 2.6102～2.6344,and the fractal characteristics were obvious.（3）The evolution characteristics of pore structure at different temperatures after freeze-thaw cycles were tested by nuclear magnetic resonance（NMR）technology.Combined with NMR₂ spectrum and pore size distribution,the evolution law of pore icing of PVA-ECC at different temperatures was analyzed.The experimental results showed that the unfrozen water in the pores gradually decreased with the decrease of temperature in the range of0°C～-30°C for PVA-ECC samples under freezing state after freeze-thaw cycles.The amount of ice in the pores increased fastest in the range of 0°C～-10°C.This was because the larger the pore diameter,the higher the minimum temperature required for the freezing of unfrozen water in the pore.（4）The changes in splitting tensile strength and flexural strength of PVA-ECC under different temperature ranges could be divided into three stages:slow increase stage（20℃～0℃）,rapid increase stage（0℃～-10℃）,and steady increase stage（-10℃～-30℃）.For specimens with different numbers of freeze-thaw cycles,their splitting tensile strength and flexural strength at-30℃temperature have increased by about 1.22 times and 1.8 times,respectively,compared to normal temperature.（5）The strain changes during the whole process of splitting tensile test were tracked using digital image correlation（DIC）technique,and horizontal and vertical strain maps were compared under different freeze-thaw cycles and temperatures.The experimental results showed that the variation of horizontal and vertical strain maps of the disc was basically similar.With the increase in freeze-thaw cycles and decrease in temperature,the digital image correlation analysis of the central position deformation of PVA-ECC samples showed better results for the strain maps under different freeze-thaw cycles and temperatures.This validated the effectiveness of obtaining full-field deformation of PVA-ECC specimens based on digital image correlation method.