Study of the rust penetration and circumferential stresses in reinforced concrete at early stages of an accelerated corrosion test by means of combined SEM, EDS and strain gauges

Abstract

Corrosion of steel bars for reinforcement leads to expansive pressure on the surrounding concrete that causes internal cracking and, eventually, spalling and delamination. The numerical and analytical modelling of cover cracking due to the corrosion of concrete usually involves a delay in such corrosion as a consequence of accepting a propagation time, after the beginning of the process, in which rust products penetrate the porous network of the concrete and pressure is mitigated. However, the assessment of this delay is based on empirical data. Regarding such a time, there is limited published research that focuses on the initial stage of the propagation of rust products in the porous structure of the surrounding concrete.This work offers a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and strain gauges to study the phenomena involved at the beginning of corrosion. It focuses on the analysis of penetration of rust products into pores and measurement of strain and stresses caused in the concrete, near to the rebars, at the beginning of corrosion. It also involves use of an analytical model to calculate the width of the crack that could be used to estimate the time of cracking. The method has been applied to two concretes with a different porous network and with two distinct corrosion current densities to examine how the technique could monitor the reinforcement corrosion process and detect cracking of the surrounding concrete. The experimental work was performed by using an accelerated corrosion test on a conventional concrete (CC) and a concrete with silica fume (SFC) by submitting them, respectively, to a current density of 50 μA/cm2 and 100 μA/cm2. Part of the concrete specimens had embedded strain gauges, placed as closely as possible to the rebar, used to monitor the strain. The penetration of the rust layer was observed and measured by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). A good relationship between the velocity of the penetration of the rust products in the porous network and the delay of the cracking pressure in concrete was observed by introducing a reduction factor.