Study On Fatigue Property And Damage Of Alkali-activated Slag Concrete In Cold Area

In traffic infrastructure projects such as roads and bridges in cold regions,the deterioration service performance of concrete material has become an important problem affecting its frost resistance and fatigue resistance.Therefore,the experimental study on the effects of alkali-activated slag concrete freeze-thaw cycle and flexural fatigue load is carried out.It provides a basis for the design of frost resistance and fatigue resistance of alkaliactivated slag concrete and the qualitative safety evaluation and decision making of the damaged structure.As a new green building material for environmental protection and recycling of industrial solid waste,the properties of alkali-activated cementitious material are one of the hot topics in the research of building materials.In view of the defects of alkaliactivated cementitious materials,such as short setting time,high large shrinkage rate and alkali corrosive damage in the preparation process,which seriously limit the application in practical engineering,NaAlO2 and Na2SiO3 are proposed as composite solid alkali activators to improve the working performance,shrinkage performance and compressive strength of alkali-activated cementing materials.It is great significance to promote the application and popularization of alkali-activated slag concrete in cold area traffic engineering.Therefore,this paper mainly carries out the following research work:(1)The effects of NaAlO2/Na2SiO3 mass ratio and Na2O content on the setting time,fluidity,drying shrinkage,autoshrinkage and compressive strength of alkali-activated slag slurry were studied.It was found that NaAlO2 and Na2SiO3 as composite solid alkali activators could effectively prolong the setting time,inhibit the shrinkage and improve the compressive strength of alkali-activated slag slurry.The initial setting time and final setting time of NaAlO2/Na2SiO3 alkali-activated slag with the mass ratio of 100/0 were 8 times and 6 times of those with the mass ratio of 0/100,respectively.The drying shrinkage and autoshrinkage of NaAlO2/Na2SiO3 specimens with 80/20 mass ratio were 0.54 and 0.73 times of those with 0/100 mass ratio at 7d,respectively.When the mass ratio of NaAlO2/Na2SiO3 was 20/80,the compressive strength of the specimen at 3d,7d and 28d reached 46.6MPa,63.7MPa and 77.3MPa,respectively.Combined with the XRD,FT-IR,SEM-EDS and BET,the reaction mechanism of NaAlO2-Na2SiO3 activated slag was discussed.Research shows that alkali activated reacted early phase active Al3+ ion wrapped in slag particles surface,which was the main reason for the delay setting time,the tetranatrolite zeolite generated could effectively inhibit contraction,slag cement materials base excitation late reaction activity of Al3+ ions easy to replace the bridge[SiO4]4+ tetrahedron Si4+ ions,generates more highly crosslinked gel structure,improve mechanical properties of cementitious materials.(2)Taking ordinary cement concrete as the control group,the freezing-thawing test was carried out on alkali-activated slag concrete with cementing material content and water-cement ratio as the changing parameters,and the influence law of different freeze-thaw cycles on mass,compressive strength and dynamic elastic modulus was obtained.The experimental results show that the frost resistance of alkali-activated slag concrete was obviously better than that of ordinary cement concrete,and the damage range of compressive strength and dynamic elastic modulus loss rate was obvious during freeze-thaw cycle.Combined with SEM-EDS and BET,the freeze-thaw damage mechanism of alkali activated slag concrete.Based on the damage mechanics theory,the freeze-thaw damage attenuation model and cumulative damage model of alkali-activated slag concrete with compressive strength and relative dynamic elastic modulus as damage variables were established,and the model analysis was given.(3)The influence of freeze-thaw cycle number,freeze-thaw medium and stress level on the flexural fatigue life of alkali-activated slag concrete is obtained through the flexural fatigue test of alkali-activated slag concrete after freeze-thaw cycle.The results show that the freezethaw medium and the number of freeze-thaw cycles are important factors affecting the flexural fatigue life of alkali-activated slag concrete,and the stress level is positively correlated with the flexural fatigue life.Probabilistic limit design method is used to analyze the flexural fatigue life of alkali-activated slag concrete.It has been proved that the flexural fatigue life basically conforms to two-parameter Weibull distribution and three-parameter Weibull distribution.Combined with K-S test,it could be seen that the three-parameter Weibull distribution is more suitable for describing the distribution law of flexural fatigue life of alkali-activated slag concrete.Therefore,the S-N,two-parameter Weibull distribution and three-parameter Weibull distribution flexural fatigue equations of alkali-activated slag concrete were established.The flexural fatigue equations of two-parameter Weibull distribution and three-parameter Weibull distribution with failure probability and the corresponding P-S-N flexural fatigue equations were obtained and analyzed accordingly.(4)The influence of the number of freeze-thaw cycles,freeze-thaw medium and stress level on the flexural fatigue deformation of alkali-activated slag concrete is analyzed.The ?-N curve and E-N curve of alkali activated slag concrete were established.The results show that?-N curve and E-N curve have three-stage damage characteristics independent of the number of freeze-thaw cycles,freeze-thaw media and stress level.The critical values of flexural fatigue strain and fatigue elastic modulus are used to judge the flexural fatigue failure of alkaliactivated slag concrete.Based on the energy release theory of fracture mechanics and continuum damage mechanics,the flexural fatigue damage mechanism of alkali-activated slag concrete was explored.The maximum flexural fatigue strain,minimum flexural fatigue strain and fatigue elastic modulus were defined as the fatigue damage variables.The flexural fatigue damage evolution curve of alkali-activated slag concrete was studied and the flexural fatigue damage evolution model of alkali-activated slag concrete was established.Taking the flexural fatigue residual strain as the fatigue damage variable,the flexural fatigue damage evolution equation of alkali-activated slag concrete was established.The expressions of residual flexural strength under different loading cycle ratios under different stress levels were derived from fatigue damage variables.