Research On Evaluation Index Of Durability Of Concrete

Durability design of concrete has been the precondition to ensure design working life of concrete structure, and it is quite desiderate for durability design to determine evaluation index of durability of concrete properly. According to environment types classified by the standard document named Temporary Regulations on Durability Design of Railway Concrete Structure, with the guidance of the integrity theory, an experimental research on evaluation index of durability of concrete under different types of environment, including chloride environment, carbonation environment, freezing and thawing environment, and sulfate physical attack environment, has been carried out from two paths, i.e. penetration of corrosive medium and the ability of concrete to resist destroy, in this paper.Research results show that, for concrete with W/C ratio no higher than 0.55, both charge passed and diffusion coefficient of chloride of concrete decrease with the decrease of W/C, the decrease of cement content, the increase of furnace slag content and the addition of silicon fume, and when fly ash is mixed into concrete, both 28d-charge passed and 56d-charge passed decrease, while 28d-diffusion coefficient of chloride increases and 56d-diffusion coefficient of chloride decrease. There is a good correlation between charge passed and diffusion coefficient of chloride of concrete; furthermore, correlative coefficient of 56d is higher than that of 28d. For fly ash concrete, the penetrability to resist chloride ion will be exaggerated if the same evaluation value of charge passed as that of other kinds concrete is selected to evaluate the penetrability of concrete to resist chloride ion. The strength and charge passed all have good relativity with carbonization depth of concrete, and when the strength of concrete is more than 50MPa, the speed of carbonization of concrete is very low. The influencing factors of concrete to resist freezing and thawing include W/C ratio, cement content, fly ash content and air content, among these factors, air content is the most important factor. The concrete with air content between 4.0% and 9.0% have good performance of resisting freezing and thawing. The factors influencing the performance of mortar to resist sulfate physical attack are mainly the strength and penetrability; moreover, there is a good negative relativity between strength and weigh loss ratio of mortar specimen under sulfate physical attack.Based on the aforementioned, the evaluation index system of durability of concrete under different environment has been established. Under the chloride environment, diffusion coefficient of chloride ion can be chosen to evaluate durability of concrete; when W/C ratio of concrete is not higher than 0.55. 56d-charge passed can also be chosen to evaluate durability of concrete, but for fly ash concrete, the standard value of evaluation index should be lowered. Under the carbonation environment, both strength and charge passed are put forward to be the evaluation indices of durability of concrete. Under the freezing and thawing environment, air content and air-void spacing factor are selected as evaluation indices of durability of concrete, and when chloride ions exist in this environment, an index of penetrability of concrete to resist chloride ion should be added. Under the sulfate physical attack environment, strength and charge passed are selected to evaluate the durability of concrete.