Analysis Of Carbonation Of Tunnel Lining Concrete Under Variable Pressure

As the country with the most construction of tunnels in the world,our country has gradually taken the leading position in term of the construction scale and speed of tunnels.In this paper,according to the characteristics of air pressure generated by train passing through tunnel,the influence of air pressure on the carbonation of concrete tunnel lining is analyzed.The construction of tunnel structures is complex and its maintenance cost is high.The design life of tunnels is generally 100 years or more.Thus,it is urgent to carry out the research on the carbonation performance of the high speed railway tunnel concrete,in order to ensure the normal service life of tunnel structures and their normal use of the function.And the study has certain reference value for the design and life prediction of high speed railway tunnel structures.The factors affecting the carbonation of concrete have been analyzed in this paper.Based on the main influence factors of concrete carbonation,theoretical and experimental analysis methods were combined to investigate effects of the different air pressure on concrete carbonation with the different water-to-cement ratio by the means of the improved pressure carbonation test chamber.The main contents of this paper were as follows:(1)In theory,based on the Darcy's law and the Fick's law of diffusion,the carbonation depth of concrete under constant air pressure was calculated Then the results were used to analyze the influence of constant air pressure on carbonation.The carbonation depth of concrete under different air pressure was also gained in this paper.(2)The distribution of pressure in internal structure under non-constant air pressure was analyzed The influence of non-constant pressure on carbonation was considered and carbonation depth of concrete under non-constant pressure was also analyzed.Considering the characters of concrete and air,the impact of many factors on carbonation was studied,including the persistence of air,different water-to-cement ratio and different admixture content.(3)The pressure carbonation test chamber was used to investigate effects of the different air pressure on concrete carbonation with the different water-to-cement ratio.(4)The results of experiment are analyzed,which present a reasonable agreement with the results of theory(W/C=0.50).The experimental results show that the air pressure has great influence on the early carbonation of concrete.Under the action of air pressure,the carbonation depth of concrete specimens at 1D carbonation age is as high as 866.7% of the carbonation depth of concrete specimens in ordinary carbonation test chamber.The carbonation depth of concrete with water cement ratio of 0.50 under the action of 0.01 MPa,0.03 MPa and 0.05 MPa air pressure is 8.14 mm,10.99 mm,12.96 mm,indicate the air pressure is directly proportional to the carbonation depth of concrete under different air pressure.In addition,The carbonation depth of concrete with water cement ratio of 0.36,0.42 and 0.50 under the action of 0.01 MPa air pressure is 4.03 mm,6.02 mm,8.14 mm,indicate the water cement ratio is proportional to the carbonation depth of concrete.