Research On Evaluation Method Of Concrete Damage Degree Based On Gas Permeability

In actual engineering,the concrete structure will be damaged by external environmental factors and loads,causing changes in the internal microstructure and affecting its permeability.We have paid more attention to the changes in the mechanical properties of materials in the past,but the research on the transmission properties of concrete materials is not enough,and this may have a certain impact on the normal use of the structure.Since the durability of concrete materials is largely determined by permeability,this paper studies the effects of temperature damage caused by different heating rates and load damage caused by different axial stresses on the gas permeability of concrete materials based on the law of gas flow in porous materials.At the same time,through the pore feature analysis test,in-depth study of the relationship between the macro-permeability of cement-based materials and micro-structure changes,and then to make a reference for the scientific assessment of the deterioration of the durability of concrete materials.In this paper,concrete and mortar samples with different degrees of damage(temperature damage and mechanical damage)are taken as the research object,and the gas penetration test system introduced from France by Hubei University of Technology is used to conduct gas penetration under different confining pressures and intake pressures.Sexual testing.On the one hand,by setting different heating rates(5?/min,10?/min,15?/min),the concrete and mortar samples are caused to different degrees of temperature damage,and then the micro pore structure analysis and macro gas permeability test In order to establish the relationship between the changes in the internal structure of the cement-based material and the evolution of gas permeability.On the other hand,by applying axial loads of different degrees(30%,50%,70%,90%of the ultimate stress)to the sample,the variation law of gas permeability of cement-based materials under different stress levels was studied.Finally,under the conditions of temperature damage and mechanical damage,the damage index is defined by the change of gas permeability,and the durability of concrete materials is evaluated by the size of the damage index.Finally,the main conclusions are as follows:(1)Through the porosity measurement and the BJH method pore volume analysis test,it is found that with the increase of the heating rate,the microscopic pore characteristic value BJH method cumulative pore volume and average pore diameter are continuously increasing,while the macroscopic pore characteristic value porosity It is also increasing,and the two show a good linear correlation.(2)Through the BET specific surface area and BJH pore size analysis test,it is found that the microscopic pore characteristic value BET specific surface area increases with the increase of the heating rate,the pores with a pore diameter greater than 100 nm increase significantly,and the probability of the most probable pore diameter also increases Large,indicating that the increase in heating rate leads to the continuous appearance of new fine pores in the concrete material,and the original pore volume is also expanding.(3)Through the gas permeability test under different temperature damage conditions,the analysis shows that the overall gas permeability of cement-based materials shows an upward trend.When the heating rate is small,the gas permeabilitydoes not change much,but when the heating rate reaches a certain threshold level,the gas permeability of the sample increases sharply,the threshold is about 10?/min.(4)Through gas permeability tests under different confining pressures and intake pressures,the gas permeability of cement-based materials shows a decreasing trend with increasing confining pressures,and concrete materials are more sensitive to changes in confining pressures.The essential permeability of cement-based materials is not affected by the inlet pressure,but is a reflection of the permeability of the material itself.(5)Through the gas permeability test under different load damage conditions,the analysis shows that when the load level is small,the gas permeability does not change much,or even slightly decreases,but when the load level reaches a certain threshold level,try This kind of gas permeability increases sharply.The threshold is about 50% of the ultimate stress.(6)The damage index is defined by the measured gas permeability change value,and the temperature damage index and the mechanical damage index are used as the evaluation criteria,and the degree of deterioration of the durability of the concrete material is divided into three categories: basically intact,deteriorated,and severely damaged.