Determination of chloride diffusion constants for concretes of differing water to cement ratios and admixtures

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Virginia Tech


Reinforced concrete exposed to chlorides is subject to rapid deterioration once the concentration of the chloride ion in the concrete reaches a critical level to cause corrosion of the reinforcing steel. The chloride ion diffuses through concrete according to Fick's Law, which is a function of time, a driving concentration, and a diffusion constant. The diffusion constant varies with temperature and the variety of concrete .

The research included determination of diffusion constants for six types of concretes after 8 weeks and 16 weeks of ponding with an NaC1 solution. In addition, one set of these samples was subjected to the outside environment while the other was indoors under controlled conditions. The mixes included water to cement ratios of 0.35, 0.40, 0.45, and 0.50, and two water to cement ratios of 0.45 with 15% cement replacement with pozblan admixtures (silica fume and type'F flY ash). Thus, the effects of temperature, water to cement ratio, and pozzolanic admixtures in regard to the diffusion constant for concrete were found.

The method by,which a diffusion constant was found is as 1)X3% solution of-NaC1 is ponded on top of a four inch thick 'specimen of concrete. 2) Powder samples of the paste are taken at specified time at depths of -0.5, 1.0 1,:5, and 2-.5 inches. 3) These samples are analyzed for chloride content by potentiometric titration. 4) The data is fit to Fick's Law by nonlinear regression and an effective diffusion constant determined.