Transportation Of Chloride Ions In Damaged And Cracked Concrete And Its Action

Chloride ions have a tremendous influence on the durability of marine concrete structures damaged by environments and loads. It is important to study the chloride ions transportation in damaged and cracked concrete in order to have a better understanding on the ion transmission mechanism and to predict the reasonable service life of concrete structures. In this thesis, the following studies have been carried out.Modified dual porous media model and two-dimensional model are introduced separately to discuss the chloride ions transport in saturated and cracked concretes on basis of the stationary or flowing solutions. The change of time-dependent surface chloride concentration, the decrease of apparent diffusion coefficient, the linear bonding of chloride ions in concrete porous, and the self healing are considered in these two models. The calculated results are agreed very well with the experimental results.The dual porous media model is modified to analyze the chloride ions transport in unsaturated and cracked concrete. Water and chloride ions are expressed as diffusion and advection-diffusion equations respectively. The diffusion coefficients are expressed as saturation's equation with the Rayleigh-Ritz distribution of concrete porous diameter and crack width. A cracked concrete specimen periodicity exposed to chloride solutions is calculated by finite element and finite difference methods, which results are agreed very well with the experimental results.Three different types of concrete materials, including OPC (ordinary performance concrete), PFC (polypropylene fiber concrete) and SFC (steel fiber concrete), were cured in different humilities for 28 days. To reflect the influence of high temperatures on the durability of concrete, parts of them were baked in oven. Some of these specimens were immersed into chloride solutions for 60 and the others for 180 days. Then the chloride in concrete was tested by RCT method. The results show that both the reduction of curing humidity and the increase of baking temperature can increase the diffusion coefficient. But this process can be slowed by polypropylene and steel fibers.After freeze-thaw cycles, the dynamic elastic modulus, flexural strength and compressive strength of former three types of concretes were tested firstly. Then some of the tested specimens were exposed to chloride solutions for 120 days. Results show that the dynamic elastic modulus, flexural and compression strengths decrease with the increase of freeze-thaw cycles. Chloride concentration at the same depth and diffusion coefficient increase with the freeze-thaw cycle times.The specimens with or after the compression stresses were exposed to chloride solution for 180 days. The experimental results show that when the stress level is less than 70%, chloride ions present little variations, the diffusion coefficient decrease a little. But when the stress level increase to 95%. both the chloride concentration and diffusion coefficient increase with the increase of stress levels. The chloride diffusion coefficient with stress is a little smaller than that after stress. A polynomial equation is proposed to predict this regulation according to the test results.Concrete beams were subjected to three-point bending by coupling in pairs, and cracks appeared in the mid-span of these beams. The influences of cracks and tensile tress on chloride concentration and diffusion coefficient were studied. Results show that the chloride concentration in cracked zone is much larger than that in uncracked section. The chloride diffusion coefficient in tensile zone increases 1.1～2.2 times. Chloride concentration increases with the increase of water to cement ratio and stress level. Varied load accelerates the chloride transportation. Tensile stress also speeds up the reinforcement corrosion process. The steels in tensile zone corrode much more when the water to cement ratio increases and the cover depth decreases, but vary a little in beams with different stress levels and crack widths in this thesis.The service life of damaged concrete is predicted on basis of the probabilistic framework of Monte Carlo Simulation when regards steel corrosion starting as limit state. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from previous experimental or statistical studies, and the damage ratios are getting from test results. The predicted life reduces with the decreases of curing saturation, and with the increase of temperature, freeze-thaw cycle times and tensile stress. Compressive stress below 80% of the ultimate compressive strength increases the life, but decreases the life above that level.