Study On The Evolution Of Meso-structure Of Asphalt Concrete Waterproof Layer On High-speed Railway

The existing fiber-reinforced concrete waterproof layers on high-speed railway subgrade can't met the demand of waterproofing and drainage in cold regions due to existing crack-free,pulverized and other problems which caused by freezing and thawing in China.For the abovementioned reasons,the research team relied on the China Railway Corporation " Experimental study on comprehensive prevention and control technology of high-speed railway subgrade frost-expansion in cold region" to take the lead in the study of the new cold zone waterproof sealing layer structure based on asphalt concrete,but the current asphalt mixture waterproof sealing layer.The design system is not yet mature,and the steady state of the performance decay of the waterproof sealing layer during the 60 years of use is not fully considered.In this paper,based on the test section of the waterproof sealing layer of the asphalt mixture of BeijingZhangjiakou high-speed railway,the meso-structure and mechanical properties of the waterproof sealing layer are studied by freeze-thaw cycles,aiming to improve the durability and extend the service life of the asphalt concrete waterproof sealing layer in cold regions.Firstly,through the investigation and analysis of the existing test conditions of freezing and thawing cycle,a laboratorial freezing and thawing cycle procedure based on the historical climate data of Zhangjiakou section of Beijing-Zhangjiakou high-speed railway as well as the existing highway specification was proposed in this paper.The freezing and thawing cycle procedure contains proportional 18 hours freezing under three different temperatures(-5? for 12 hours,-15? for 4 hours and-25? for 2 hours)and 6 hours thawing at 25?.Afterwards,through the X-Ray CT combined with image processing technology,the threedimensional voids figures of asphalt mixture specimen with different initial air voids designs before and after freezing and thawing cycle are reconstruct.From the perspective of visualization,the following three failure modes exist in the internal structure of asphalt concrete under the action of freezing and thawing cycle: the void volume increases because the water in the void expands as it freezes;new voids appear in some places because of the impact of freezing and thawing;the original voids are connected after the formation of new voids or the increase of volume.Moreover,the three failure modes have nothing to do with the voids in the initial design of the specimen.At the same time,combined with the parameters of threedimensional void ratio,void number,equivalent void diameter and volume fractal dimension of the specimen obtained from digital image processing program,the proportion of different failure forms with the increase of freeze-thaw cycle number in damage process was studied.Results show that in the initial stage of freezing and thawing cycle,voids generation dominates.While with the increase of freeze-thaw cycles,voids generation and voids link tend to balance.In addition,through comparing the parameters of specimens with different initial void ratio,it is found that the degree of freeze-thaw damage is closely related to the initial void of the specimen.And when the voids are large,the specimen is easy to speed up freeze-thaw damage because water can expand through the surface pores into the interior of the specimen.The dynamic modulus of uniaxial compression test are used to study the mechanical properties of the asphalt mixture specimen after freeze-thaw testing.The dynamic modulus at 15? at 10 Hz and dynamic modulus master curve are adopted in this paper to study the freezethaw damage in different initial void design specimen and the evolution of the whole temperature performance.The results show that the dynamic modulus decreases with the increase of freeze-thaw cycle,and the damage degree is related to the initial design void.In addition,by contrasting the dynamic modulus master curve of the specimens before and after freezing and thawing cycle,it can be found that the curve shape will change as the freeze-thaw cycles increasing.The master curves of asphalt mixture under the freeze-thaw cycle are quite different at low frequency(high temperature).In addition,the correlation between the change rule and the initial design void is stronger.The master curve of 7% initial void specimen changes greater than that of 3% and 5% initial void specimen.Finally,the gray correlation analysis method was used to analyze the similarity of the mesoscopic parameters with the 15?,10 Hz dynamic modulus before and after the freeze-thaw cycle.The correlation degree of gray correlation analysis was from large to small: interface shear strength,Pore number,Pore volume fractal dimension,Slip amount,Pore equivalent diameter,Three-dimensional void ratio.