Study On Mechanical Properties And Damage Evolution Of Dredged Sand Concrete Under Freeze-thaw Cycle

Frost resistance has always been a difficult problem for concrete to overcome in cold areas.Pore crack initiation in concrete structure under freezing-thawing cycle affects the mechanical properties of concrete.In order to study the mechanical properties and frost resistance of dredged sand concrete,this paper designed five kinds of concrete with different dredged sand replacement rates(0%,12.5%,25%,37.5%and 50%),and carried out freeze-thaw cycle tests on dredged sand concrete with different times of freeze-thaw(0,25,50,75 and 100).Combined with acoustic emission technology,three-point bending fracture test was carried out on concrete specimens.At the same time,CT scanning test,mercury intrusion porosimetry(MIP),scanning electron microscope test(SEM)and X-ray diffraction test(XRD)were carried out on dredged sand concrete under different freeze-thaw cycles.The mechanism of mechanical properties and frost resistance damage and deterioration of dredged sand concrete under the action of freeze-thaw cycle is studied from macroscopic and microscopic perspectives.The conclusions are as follows:(1)As the number of freeze-thaw cycles increases,the apparent appearance of dredged sand concrete deteriorates gradually.There is also an increase in mass loss,and a decrease in the relative dynamic elastic modulus and compressive strength.With the addition of dredged sand,the loss rate of concrete compressive strength decreases obviously,which indicates that adding proper dredged sand can enhance the frost resistance of concrete.To predict the freeze-thaw durability of concrete with varying levels of dredged sand replacement,a freeze-thaw damage model was developed based on the relative dynamic elastic modulus and the rate of mass loss.The freeze-thaw life of concrete with dredged sand replacement rate of 0%,12.5%,25%,37.5%and 50%is predicted to be 128,92,134,147 and 163 times,respectively.(2)The 3D model of dredged sand concrete was reconstructed by CT technology,and the evolution law of pore structure of dredged sand concrete under the action of freeze-thaw cycle was studied.The addition of dredged sand can reduce the internal porosity of concrete.The freeze-thaw deterioration of dredged sand concrete has regional characteristics and gradually deepens from outside to inside.According to the results of mercury injection test,the addition of 50%dredged sand increased the number of harmless pores by 3.60%and reduced the number of multiple harmful pores by 3.42%,indicating that the addition of dredged sand improved the structure of concrete pores,increased the compactness and increased the strength of concrete.The pore structure of dredged sand concrete has fractal characteristics.With the growth of dredged sand replacement rate,the fractal dimension increases,and with the growth of freeze-thaw cycles,the fractal dimension decreases.There is a negative correlation between the fractal dimension and the structural characteristic parameters of dredged sand concrete.The freeze-thaw damage evolution model was established based on porosity,and the predicted values of compressive strength under different freeze-thaw cycles were predicted.The morphology and phase of dredged sand concrete were analyzed by SEM and XRD.The results show that dredged sand mainly affects the performance of concrete in a physical way.(3)With the increase of dredged sand content,the fracture property and frost resistance of concrete are improved.The fractal dimension of fracture surface decreases with the growth of concrete dredged sand replacement rate.As the number of freeze-thaw cycles increases,the fractal dimension of the fault surface increases and the fault surface becomes coarser.The higher freeze-thaw cycles,the lower the peak load,fracture toughness and fracture energy of dredged sand concrete.The higher the replacement rate of dredged sand,the greater the fracture energy of dredged sand concrete.The cumulative acoustic emission ringing count,acoustic emission energy and acoustic emission b value can reflect the three-stage fracture process of dredged sand concrete.The damage model of dredged concrete based on acoustic emission events follows Weibull distribution and can better reflect the influence of freeze-thaw cycle on damage.