A simple thermodynamic model on the cracking of concrete due to rust formed after casting

The purpose of this study is to use thermodynamic data to estimate the pressure exerted by the crystallization of iron oxyhydroxides following the equation proposed by Correns and Steinborn.

Standard free energy and molar volume data have been considered for goethite, lepidocrocite, magnetite and hematite, which are described in the literature as the most commonly found mineral phase rust constituents.

The studied mineral phases generate higher to lower crystallization pressure values in the following order: goethite > lepidocrocite > hematite > magnetite. The crystallization pressures calculated for these phases are in the 32-350 MPa range, which is higher than the tensile strength of concrete (of the order of 0.2-10 MPa) and thus leads to failure of the cover concrete.

The aim of this paper is to shed light on this issue by calculating the stresses generated by the crystallization of iron oxide from a supersaturated solution using thermodynamic data. A deliberately simplistic method was proposed, taking as reference the Correns–Steinborn model (Correns and Steinborn, 1939; Correns, 1949). The crystalline phases considered in this paper are those most commonly found in the literature as rust constituents, that is, goethite (α-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe₃O₄) and hematite (α-Fe₂O₃). The FeO synthetic phase was also included as a reference.