Study On Freeze-thaw Damage Characteristics Of Cemented Sand And Gravel Material

In recent years,as a new type of dam,Cemented sand and gravel(CSG)dam have applied more and more widely in water conservancy project due to their environmental protection and energy-saving advantages.Many CSG dams have been built in the central and western regions in China.In order to further promote CSG dam in the cold,frost resistance will be an important property for many designers and researchers in this field.This article,through the design of typical CSG materials mixture ratio test,freeze-thaw cycle test,compressive strength test,the nuclear magnetic resonance experiment(NMR),scanning electron microscope(SEM),etc.select macro-mechanical indexes(mass loss rate,relative dynamic elastic modulus and compressive strength loss rate)and micro indexes(phase structure,porosity,pore size distribution,fractal dimension)to analyze the damage and deterioration of CSG material after freeze-thaw cycles and finally analyze the correlation between the macro and micro indexes by mathematical statistics method.The main research contents are as follows:(1)According to the macro-performance tests,with the increase of fly ash content,the frost resistance of CSG materials is significantly improved;For different ages,the frost resistance of this material increases obviously at the age of 28 days to 90 days,and tends to be almost stable after 120 days.In addition,the CSG materials in the test group will be destroyed after 20 freeze-thaw cycles.(2)SEM results show that there is a large amount of unhydrated fly ash in the CSG materials at the age of 28 days compared with 90 days.Therefore,the continuous hydration of cementified material at the age of 90 days has a significant enhancing effect on the frost resistance of CSG materials.With the increase of the number of freeze-thaw cycles and of the sand percentage,the internal structure of the material changes obviously,which is manifested in the gradual thinning and fracture of a large number of fibrous structures;the columnar gel gradually becomes fibrous and pores form.(3)According to the analysis of meso-pore structure characteristics by NMR test,the freeze-thaw cycles is essentially a process of continuous loading and unloading of the pore wall.In this process,the tiny pores inside the CSG materials gradually increase,and some pores turn into macropores,which are mostly filled with gravity water.Therefore,the destruction takes place first,causing the frost heave to increase gradually.(4)After the freeze-thaw cycles,there is a high correlation between the macro and micro indexes of CSG materials,The larger the porosity is,the worse the macro indexes are;the larger the pore size distribution is above 50 nm,the worse the macro indexes are.