Title: Constant Chloride Flux Model to Predict Airborne-Chloride Penetration in Concrete
Author(s): Jeanette Visser
Publication: Symposium Paper
Appears on pages(s): 89.1-89.10
Keywords: airborne chloride; diffusion, constant flux, durability, variable surface concentration
The lack of resistance models to determine the penetration of airborne chlorides into the concrete makes service-life prediction hardly possible for concrete structures exposed to chlorides, something badly needed as airborne chlorides can lead to severe bar corrosion even at 10 km (6 miles) from the sea. Predicting chloride penetration currently is performed by solving the diffusion equation assuming submerged conditions and trying to remedy the different load condition for airborne chlorides by empirical extensions. This has resulted in many different models, none of which satisfactory explained the observed chloride penetration. A different solution of the diffusion equation is provided in this paper, introducing a constant chloride flux into the boundary conditions. This constant chloride flux model relates also the diffusion coefficient to the surface concentration, a missing relation up to now. The proposed model has been shown to satisfactorily fit test results obtained in the wind tunnel, especially with respect to the amount of the chlorides taken up by the concrete and the total amount of chlorides deposited on the surface. The model can, therefore, appropriately describe airborne-chloride penetration, as long as chloride content at concrete surface does not reach its maximum. Furthermore, the constant chloride flux model describes quite well the differences observed between the chloride profiles resulting from a direct exposure to a marine environment for similar concrete mixes. Especially the low surface concentration founds for concretes with a high diffusion coefficient and high surface concentrations found for concrete with a low diffusion coefficient while exposed at the same location, are now well described.