| Most of the concrete durability problems are due to harmful substances,which pass through concrete pore network and causes erosion of concrete.Therefore,the concrete-penetration ability of external materials,that is permeability,is of great significance for the assessment of the durability of concrete.In this research,the influences of water-cement ratio and fly ash content on gas permeability and carbonation resistance of concrete were analyzed by testing concrete samples with different mix.Test factors of gas permeability of concrete were analyzed and the influences of porosity,pore area,pore size on gas permeability of concrete were also discussed.Then the relationship between carbonation resistance and gas permeability was analyzed.And the effects of high temperature and stress on gas permeability of concrete were studied.The main conclusions of the research are as follows:The measured gas permeability of concrete is between 5.78×10-18-1.90×10-17m2 and thus the concrete belongs to low permeability medium.When the fly ash content of concrete is in the range of 0-30%,the gas permeability of concrete declines with the adding of fly ash content.And as the water-cement ratio of concrete varies in the range of 0.35-0.55,the gas permeability of concrete decreases with the reduction of water-cement ratio.The porosity of concrete increases with the increase of water-cement ratio,and decreases with the increase of fly ash content.BET surface area increases slightly and then decreases with the increase of water-cement ratio.And it increases with the increase of fly ash content.However,the water-cement ratio and fly ash content have no significant effect on the most probable diameter(MPD)of concrete.The gas permeability of concrete is little effected by porosity and MPD,whereas it is largely effected by pore path deflection.The carbonization depth of concrete increases,but slows down with the time of carbonization.For concrete with 30% fly ash content,the smaller the water-cement ratio,the smaller the carbonization depth of concrete.For concrete with water-cement ratio of 0.45,the higher the content of fly ash,the greater the carbonation depth of concrete.Compared with the permeability of concrete,the carbonation performance of concrete is more affected by the content of alkaline substances.Compared to gas permeability of concrete at 60℃,it is 3.8 times at 200℃,11.7 times at 350℃ and 72.6 times at 500℃.High temperature makes the gas permeability of concrete more sensitive to confining pressure and the K12MPa/K3 MPa declines from 0.87 to 0.41.As the axial load is less than 0.6P,gas permeability of concrete stay stable or slightly decreases along with the increase of axial load.Once the axial load exceeds 0.7P,the gas permeability begins to increase with the increase of axial load and at the top of 0.8P,the gas permeability accelerates to increase. |
