Carbonation Resistance Evaluation And Life Prediction Of The Second Lining Concrete Of High-speed Railway Underground Station

Beijing-Zhangjiakou high-speed railway is an intercity high-speed railway connecting Beijing with Zhangjiakou,Hebei Province.It is not only the transportation supporting project for the 2022 Winter Olympics and an economic service line for the coordinated development of the Beijing-Tianjin-Hebei region,but also the first intelligent high-speed railway in China that adopts the domestically developed Beidou navigation satellite system and has a maximum design speed of 350 km/h.Beijing-Zhangjiakou high-speed railway passes through the Badaling Great Wall,a world famous scenic spot,under which the Badaling Great Wall station,which has the largest buried depth（102 m）,the largest construction scale（361,43 m²）and the longest designed life（300 years）,is a"world super project".Badaling Great Wall Station is in a constant temperature and humidity environment.The concentration of chlorine ion in surrounding rocks is extremely low,and the scattered current can also be effectively controlled.However,due to the dense crowd and high CO₂concentration in the station,the carbonation of the concrete cover is the main factor affecting the durability of the reinforced concrete material of the second lining structure.Therefore,it is very important to study the carbonation resistance and life prediction of the second lining concrete.Based on the design requirements and internal environment of Badaling Great Wall Station of Beijing-Zhangjiakou high-speed railway,by simulating the environmental and construction conditions,casting specimens in the same mix proportions and combining with the core samples,the accelerated carbonation process of HPC and NC under different conditions was studied respectively.The carbonation resistance of the second lining concrete was comprehensively evaluated and its life prediction was carried out,by means of phenolphthalein method,p H value testing,phase analysis method,thermal analysis method and micro-morphology analysis,etc.The effect of coating materials on the carbonation resistance of NC was studied,and the maintenance measures of NC during operation were proposed.It is found that the carbonation resistance of HPC using standard curing in the second lining structure of underground station is obviously stronger than that of NC using natural curing.The smaller the water-binder ratio is,the better the curing condition is,the stronger the carbonation resistance is.When the same curing condition is used,the carbonation resistance of NC can be close to that of HPC.In this thesis,the"carbonation frontier"index is used to investigate the carbonation resistance performance of concrete under different mix proportions and curing conditions,and it is found that there is no obvious carbonation boundary in each area of HPC.Secondly,the carbonation depth of HPC and NC is predicted by using the similarity theory method,the semi-theoretical and semi-empirical model method,and the random process method,respectively,and the experimental results are used for verification.The results show that the prediction results based on semi-theoretical and semi-empirical model have a large relative error and are not suitable for predicting the service life of the second lining concrete.The prediction results based on similarity theory are different from the actual ones.The prediction results based on random process method are the closest to the measured values and the reliability is relatively high.According to the carbonation life criterion,the results of the life prediction method based on random process show that the predicted service life of HPC can exceed 300 years and meet the design service life requirements,while the predicted service life of NC is relatively short.Finally,in view of NC of the second lining structure with short carbonation life,the improvement effect of two different types of coating materials on its anti-carbonation performance was studied by defining the"carbonation retardation efficiency".It was found that the carbonation resistance of NC can be improved by using waterbased capillary inorganic waterproofer and epoxy-fluorocarbon coating materials.However,the carbonation retardation efficiency of epoxy-fluorocarbon coating materials is 100%within 28 days of accelerated carbonation of NC,far higher than that of waterbased capillary inorganic waterproofer,which has a better effect on the carbonation resistance of NC.Therefore,in view of the service life design requirements of the second lining structure of Badaling Great Wall station of Beijing-Zhangjiakou high-speed railway,it is suggested to coat the surface of NC with epoxy-fluorocarbon coating materials to retard or prevent the deterioration of NC and extend its service life.At the same time,during the whole operation period,it is necessary to conduct regular tests on the degree of integrity of the coatings and the carbonation depth of NC of the second lining structure,and conduct abnormal inspection and timely maintenance in combination with the on-site monitoring system.