Mechanisms Of Chloride Penetration Into Concrete And Development Of Chloride Profile In Marine Environment

One of the most frequently observed deterioration processes in reinforced concrete structures is penetration of chloride and subsequent initiation of steel corrosion. To avoid early steel corrosion and expensive repair measures within the intended service life of reinforced concrete structures several concepts for durability design have been developed in recent years. In most of these recommendations and codes chloride penetration is simulated by means of Fick's second law. This approach would be approximately valid if diffusion would be the only or at least the dominating transport mechanism for chloride ions in concrete. Therefore prediction of service life of structural elements, which are in permanent contact with seawater or any other salt solution, by means of the diffusion equation may be justified.In reality, however, Many reinforced concrete structures are exposed to variable environmental conditions such as splash zone or tidal zone in marine environment or they are temporarily in contact with water containing deicing salt during winter periods. In order to predict chloride penetration under most common circumstances it is necessary, however, to take into consideration the combined action of different transport mechanisms such as capillary suction, diffusion, and convection. The diffusion coefficient depends strongly on the humidity content of concrete,curing and environmental conditions in the process of chloride penetration into concrete. Based on these problems, the investigation and conclusions can be shown as follows.1. The change of chloride profile in concrete can be obtained for different mechanism of chloride penetration into concrete,such as capillary suction,diffusion and convection. Experimental results show that capillary suction is a most powerfula transport mechanism of water or salt solution into concrete. If the surface of concrete comes in contact with salt water up to a certain depth nearly the entire pore space will be filled in a short time. After 7 days,relative water content of concrete can reach to 88.6-96.7%. Chloride can be transported into concrete within one or two weeks up to 20 mm in this way.Then chloride depends on slow diffusion which can continue invading in deeper depth. Part of the penetrated chloride can be washed out again by convection during subsequent drying. Chloride, which had penetrated deep into the concrete, may move closer to the surface by convection during a long lasting drying period.2. Effects of initial moisture content on capillary suction in concrete should be studied.Experimental results show that for concrete with a given mix proportion the moisture content is of utmost importance with respect to chloride penetration. The lower the initial moisture content is, the higher will be the coefficient of capillary suction and hence the absorption of chloride.3. Effects of curing condition and the element in solution on chloride diffusion coefficient.Chloride diffusion coefficient can be measured by steady state diffusion test and non-steady state diffusion test.Experimental results show that diffusion coefficient with curing for 28 days is 1.6-4 times as much as diffusion coefficient with curing 112 days. Optimum curing conditions and long enough curing are necessary to obtain a sufficient degree of hydration. The mature concrete will have less coarse capillary pores and therefore it will absorb less chloride containing water per unit of time; The chloride diffusion coefficient in cement solution is much closer to the diffusion coefficient in the pore solution, as compared to the chloride diffusion coefficient in saturated calcium hydroxide solution.4. Chloride concentration in concrete structures can be compared in different exposured environment.Experimental results show that concrete placed permanently in sea water absorbed most chloride, followed by concrete placed in the tidal zone, while absorption of salt solution in the laboratory was minimal.