Experimental Study On Gas Permeability Of Ultra-high Performance Concrete

Permeability is an important index to evaluate the durability of cement-based materials.According to different transmission media,it can be divided into water permeability,chloride ion permeability,gas permeability and so on.Among them,gas molecules are easier to pass through the pore structure and interface transition zone of concrete materials,and its seepage process and test results can more accurately reflect the durability of concrete materials,so gas permeability is an ideal physical parameter to measure the durability of UHPC.In addition,the gas permeability index is also strictly stipulated in the design specifications of nuclear power field,protection engineering and other fields,and the gas permeability performance of UHPC must be determined before it is applied in these fields.At present,there is still a lack of domestic specifications and testing methods related to the gas permeability of UHPC.Therefore,this paper uses the improved CEMBUREAU method to research the effects of water-binder ratio,silica fume content,steel fiber,PP fiber,coarse aggregate size and content,and nanomaterials on UHPC gas permeability coefficient.The main conclusions are as follows:（1）The UHPC matrix with different water-binder ratio and silica fume content showed good gas permeability resistance,and the gas permeability coefficient was 10^-19 orders of magnitude or below.With the increase of water-binder ratio,the gas permeability coefficient of UHPC matrix also increases significantly.The addition of Micro silica can reduce the gas permeability coefficient of UHPC,and the decrease is more significant when the content is more than 20%.The change of gas permeability coefficient caused by the change of water-binder ratio gradually decreases with the increase of Micro silica content.（2）The gas permeability coefficient of UHPC increased greatly after the incorporation of fiber and aggregate.The addition of long steel fiber,short steel fiber and polypropylene fiber will lead to the deterioration of the anti-gas permeability performance of UHPC,and it is more obvious with the increase of the content.Under the same dosage,the long steel fiber group has worse gas permeability resistance than the short steel fiber group.In the UHPC with steel fiber,the permeability coefficient of matrix gas increases with the increase of the content of coarse aggregate.At the same dosage,the group with large particle size coarse aggregate has higher gas permeability coefficient than the group with small particle size coarse aggregate.The incorporation of nano-Si O2 or nano-Ca CO3 in UHPC can compensate for the deterioration caused by the incorporation of steel fiber.（3）Under standard curing conditions,extending curing age can improve the air permeability of UHPC.The gas permeability coefficient of UHPC decreases with the increase of curing age,but the rate of change decreases with the increase of curing age.Hot water curing at 90℃has better improvement effect on the air permeability of UHPC.（4）Under standard and hot water curing conditions,there is a positive linear correlation between water absorption porosity and gas permeability coefficient of UHPC matrix.However,when UHPC is incorporated into the fiber,its water absorption porosity and gas permeability coefficient generally show a positive correlation trend,but the degree of data dispersion is large.When UHPC is mixed with coarse aggregate,there is no linear correlation between its water absorption porosity and gas permeability coefficient.（5）The results of mercury injection test showed that the content of capillary pores in the matrix of UHPC decreased when silica fume was added,and the pore size became smaller and porosity decreased at most.However,after the addition of steel fiber or PP fiber,a large number of pores and pores are produced in the matrix,and the porosity increases.There are also a lot of pores and pores in the UHPC matrix with coarse aggregate,but the porosity is related to the particle size and dosage of coarse aggregate.（6）The addition of silica fume will accelerate the hydration reaction of UHPC matrix,and the pores and cracks in the matrix are greatly reduced.Compared with the UHPC without silica fume at different ages,the microstructure of UHPC is denser.Under the standard curing conditions,with the increase of curing age,the pores and cracks in the fiber-slurry interface transition zone and the coarse aggregate-slurry interface transition zone also decrease gradually,and their thickness gradually thinns or even disappears.As the distance between the matrix and the fiber or aggregate increases,the porosity decreases.