Subway concrete is easy to be attacked by aggressive CO2 in underground space

and alkali aggregate reaction (AAR) is a severe durability problem which is difficult to find and repair. Durability decrease of subway concrete caused by these two problems seriously affects the normal use of the subway tunnel. To investigate the impact of nano-particles on the durability of subway concrete under the combined action of aggressive CO2 and AAR

two kinds of nano-particles (nano-SiO2 and nano-Fe2O3) were respectively mixed into plain concrete to prepare concrete with nano-particles. Durability test was carried out by using a self-developed equipment

and erosion depth

expansion

and sonic velocity were selected as indices to evaluate the durability of the concrete with nano-particles under the combined action of aggressive CO2 and AAR. Test results show that after mixed with nano-particles

the erosion depth and expansion of the concrete were decreased obviously

while the sonic velocity was increased significantly

which demonstrates that the durability of the concrete with nano-particles was better than that of plain concrete. At the age of 182 d

the concrete with 2% of nano-SiO2 showed the best durability performance that the erosion depth and expansion were the smallest and the sonic velocity was the largest with least decrease

followed by the concrete with 1% of nano-Fe2O3. The enhancement of durability was due to the special physical and chemical properties of the nano-particles which improved the microstructure of the concrete and the chemical composition in pore solution.