Experiment Study On The Durability Of Concrete With Nano-matterials Under The Combined Action Of CO₂ Erosion And AAR

Urban metro is located in underground space and vulnerable to the erosion of aggressive CO2 existed in groundwater.And because of intensive passenger,the durability of urban metro concrete is also affected by the carbonation due to the high CO2 concentration in metro atmosphere.Erosion caused by CO2,either from the groundwater or metro atmosphere,will cause the neutrality of concrete,which will lead to the rust of steel bar,and affect the normal service of metro tunnel.Alkali aggregate reaction(AAR),another serious durability problems for concrete,will cause expansion and cracking of concrete.Its occurrence is difficult to prevent and the damage caused by AAR is difficult to repair.Meanwhile,the cracks caused by AAR will accelerate other corrosive effects.Therefore,it is very important to study the durability of metro concrete under the combined action of CO2 erosion and AAR.Due to the characteristics of small particle size,large specific surface area and extremely high surface energy,nanomaterials possess many different specific physical and chemical properties,such as size effect,small bulk effect,surface effect and macroscopic quantum tunneling effect.These characteristics are widely used in many fields,including cement-based materials.Therefore,the research of nanomaterials has attracted wide attention in academic fields.In this paper,suitable amounts of nanomaterials(nano-SiO2,nano-silica sol and nano-Fe2O3 particles)were added to metro concrete to prepare nano-concrete to study the durability of metro concrete under the combined action of CO2 erosion and AAR.The main research contents and achievements are listed as follows:(1)The 28d compressive strength test of nano-concrete is carried out to analyze the effects of different kinds and contents of nanomaterials on 28d compressive strength of concrete.The results show that,When the contents of nano-SiO2,nano-silica sol and nano-Fe2O3 are 2%,2%and 1%respectively,the concretes have higher compressive strength.(2)The durability tests of nano-concrete under the combined action of aggressive CO2 and AAR are carried out to analyze the effects of different kinds and contents of nanomaterials on the durability of concretes,and reveal the improvement mechanism.The test results show that the durability of concretes is improved obviously after mixed with nanomaterials,and the optimum contents of nano-SiO2 and nano-Fe2O3 are 2%and 1%,respectively.Then the interaction between aggressive CO2 and AAR is discussed in detail and the improvement mechanism of concrete durability is revealed by combining the concrete microstructure and chemical composition of concrete pore solution.(3)The durability tests of nano-concrete under the combined action of carbonation and AAR,as well as durability tests of AAR are carried out under the same condition to analyze the effects of different kinds and contents of nanomaterials on concrete durability,and reveal the improvement mechanism.It can be concluded from the test results that the optimum contents of nano-SiO2 and nano-Fe2O3 are 2%and 1%,respectively.The interactions between carbonation and AAR are deeply discussed and the improvement mechanism of nanomaterials is revealed.(4)the effects of nano-SiO2 and nano-silica sol on the durability of concrete under AAR are analyzed,and the results show that the improvement effect of nano-SiO2 is better than that of nano-silica sol.Additionally,the similarities and differences between carbonation and the erosion of aggressive CO2 are analyzed.