Phase composition of the interface area between steel reinforcement and cement matrix in reinforced concrete

Chemical and phase composition of the interface between the cement matrix and steel in reinforced concrete, depending on its age, was studied. It was found that, contrary to traditional views, steel does react with cement matrix, and this process begins with the diffusion of iron ions. They react with calcium ions, forming hydrated calcium ferrite in the cement matrix. By about 30 years of age, an iron oxide film forms on the surface of the steel. It is fragile and completely disappears after prolonged use of reinforced concrete. After a while, calcium ions also begin to diffuse into the oxide film and even into the steel. In general, there are three ways of hydrated calcium ferrite formation: 1) through the reaction of iron ions with calcium ions; 2) transformation of the C–S–H phase as a result of incorporation of iron ions replacing silicon in tetrahedra; 3) diffusion of calcium ions and their reaction with iron ions. Due to the gradual diffusion of calcium ions into steel and formation of hydrated calcium ferrite in its pores, microcracks are formed, and particles of steel are carried out even into the cement matrix. All these processes do not reduce the strength of steel adhesion to the cement matrix, which even in very old reinforced concrete (79 years) is 3 MPa. In this case, even traces of rust are missing.

Author: W. Kurdowski, B. Trybalska

Section: Concrete

Keywords: transition zone, reaction of iron with cement matrix, calcium hydroferrite