Porosity and electrical resistivity-based empirical calculation of the oxygen diffusion coefficient in concrete

Abstract

Concrete carbonation induced by gas transport has an adverse effect on the service life of the steel-reinforced material. Gas tightness is crucial where concrete is used to contain radioactive materials and of utmost importance where acting as a radon barrier. Since as a rule gas travel inside concrete is governed either by diffusion or permeability, the material’s performance in that respect is assessed by analysing those two parameters.

The gas diffusion coefficient in concrete is not readily found, however, due to the practical difficulties involved in preventing gas from leaking out of standard diffusion cells. An alternative approach is to measure permeability at high gas concentration and pressure. The existence of a relationship between the oxygen diffusion coefficient and permeability in OPC concrete was established by the authors in a previous paper.

Diffusion has also been shown to be related to porosity and, in solutions, to electrical resistivity. Little is known, however, about the relationship between gas diffusion and resistivity where the pores may be filled not with a liquid, but with air. Inasmuch as resistivity is a non-destructive technique and porosity a property that can be readily measured, these two parameters could be used to directly assess concrete durability, its performance in nuclear facilities and its suitability as a radon barrier.

This paper describes a study conducted to formulate equations from which to analytically determine the oxygen diffusion coefficient. These equations are derived from empirical measurements of oxygen diffusion coefficient, concrete porosity and electrical resistivity. The findings, which corroborate the existence of such relationships, were used to formulate an equation to calculate the diffusion coefficient directly from the experimental values of concrete porosity and resistivity.

Since porosity depends primarily on concrete batching, curing and moisture content, two OPC concrete mixes were prepared using different water/cement ratios and two curing conditions. The combination resulted in four types of concrete. Pre-conditioning at three values of relative humidity was subsequently deployed.