For the sake of understanding the deterioration behavior of concrete in actual railway tunnel structures subjected to aggressive sulfate medium in practice,detailed field investigations and tested analysis on sprayed concrete linings of approximately 40-year-old railway tunnels in environments containing sulfate ion were carried out,respectively.The results show that the deterioration of concretes in the investigated area is serious,which involves complicated physicochemical process between the sulfate salt and concrete.Among them,the secondary sulfateminerals such as gypsum formation under very high concentration sulfate ion condition by accumulating and evaporation process dominate,followed by the crystallization of sulfate salt and formation of thaumasite.
To investigate the durability,especially the long-term stability of cement-based materials with very low w/b,the air permeability test,carbonation test,capillary absorption rate test and dilation potential test were adopted under long-term heat treatment condition.Microstructure of these materials is also analyzed by scanning electronic microscopy(SEM)and mercury intrusion porosimeter(MIP)in order to further unveil its mechanism and interrelation between microstructure and its properties.The results indicate that in the area investigated,cement-based material with w/b 0.17,like RPC,possesses low porosity and excellent durability. Moreover,its porosity will further decrease under long-term heat treatment compared with normal heat treatment.Its long-term durability is much superior to that of other cement-based materials with w/b 0.25 or 0.35 as high strength concrete(HSC).
