Study On The Transmission Mechanism Of Chloride In Concrete Under Hydrostatic Pressure

Chloride penetration is one of main reasons for durability degradation of concretestructure. In order to reduce the economic loss caused by chloride penetration, a largenumber of experiments have been carried out on the transportation mechanism ofchloride in concrete structure at home and abroad. Fick’s second law is considered asthe best model which can be used to describe the transportation of chloride in concrete.However, the application of Fick’s second law is limited. This approach would beapproximately valid if diffusion is the only or at least the dominating transportmechanism for chloride ions in concrete. For underground structures, which withstandlarge hydrostatic pressure, how will the large hydrostatic pressure influence thetransportation of chlorides in the concrete? Is Fick’s second law still applicable? Basedon these problems, a lot of experiment was carried out on the following aspects:(1)Characteristic of chloride ions’ transmission and distribution in concrete wereinvestigated under different hydrostatic pressure，different time period and differentage. Results indicated that the hydrostatic pressure would not affect the transmissionof chloride ions significantly if the depth of the underground structures is small, orthe ground water level is low(less than0.1Mpa). The penetration depth of chloridewill increase with increase of water pressure and time，but there is not a simplelinear relationship among them.(2)Nowadays, penetration depth of water is used to evaluate permeability of concrete.But it is not totally right. Based on this, the relationship between penetration of waterand penetration of chloride in concrete was investigated by determining penetrationdepth of water and chloride. Results indicated that the transportation of water andchloride is not concurrence simultaneity. But there is a simple linear relationshipbetween them. Then advice can be given on how to propose the transportation model ofchloride under hydrostatic pressure.(3)Study on the feasibility of evaluation by using existing equipments. Resultsindicated that the existing method can satisfy the need of durability evaluation. Thenadvice can be given on how to evaluate the antichloride erosion impact of concrete. (4)By now, most of service life prediction models of concrete structures underchloride environment are based on Fick’s second law. Based on such condition, researchon the application condition of the two kinds of transportations of chloride (diffusionand penetration) was carried out by comparing the data obtained through experiment.Results indicated that the non-steady state diffusion was applicable to evaluate thechloride resistance of concrete without mineral additives, but for concrete with mineraladditives, results can be over-evaluated. For underground structures with deep burydepth and high request for the resistance to penetration, the impermeability of concreteshould be firstly considered in order to guarantee the structures’ service life in chlorideenvironment. Besides, the manners to evaluate the concrete structures under chlorideenvironment were given in details, which can provide theoretical basis for theunderground engineering in chloride environment.