The migration of chloride ions in concrete

The chloride penetration resistance of concrete—an important durability parameter—is routinely evaluated using tests involving the application of an electrical voltage, for example ASTM C1202—“Standard Test Method for Electrical Indication of a Concrete's Ability to Resist Chloride.” To properly interpret the results of these tests, it is essential to have a theory that accurately describes the migration (movement due to an electrical potential gradient) of ions in concrete. In this thesis, a theory is developed for the prediction of chloride migration in concrete, including the effect of the concrete matrix. The theory is based upon a lognormally distributed penetrability caused by the characteristics of the pore structure—pore size, tortuosity, connectivity—and due to chloride binding. This theory requires two material parameters to describe the movement of chloride ions, the mean penetrability (λ) and variation in penetrability (ζ) of the pore structure.; To investigate and validate this theory, a series of experiments were conducted. First, the pore structure of a variety of concretes was investigated using both mercury intrusion porosimetry and image analysis. Parameters were developed which characterize both the fineness of the pore structure and its range of pore sizes. In addition, two types of migration experiments were conducted: the rapid migration test where migration is allowed to occur for a given time and the chloride profile in the concrete determined and migration tests where the mass of chloride which has passed completely through the concrete is monitored with time. The theory developed was able to predict both of these test results. The fitted parameters were not a function of the test conditions. However they were slightly different between the two test procedures. This is attributed to the difference in binding characteristics in the two tests. A procedure was also established to predict diffusion coefficients from the results of a Rapid Migration Test.