Corrosion initiation of reinforcing steel induced by combined penetration of chlorides and carbon dioxide in concrete with construction cold joints

Abstract

Corrosion of reinforcing steel in concrete is regarded as the greatest threat to the durability and integrity of concrete structures in many regions worldwide. The safety and performance of structures is at risk due to a reduction of the carrying capacity of the steel and bond strength to concrete. The most important causes of corrosion are the ingress of chlorides or carbon dioxide, which is reflected in the exposure classes of standards. Most of the corrosion research has involved the isolated exposure to each, however, in many structures, corrosion occurs due to the combined exposure. When carbonation occurs in concrete with chloride, or when chloride penetrates into carbonated concrete, these mechanisms interact to increase the corrosion rate. The corrosion rate is increased under this condition due to an increase in the chloride content beyond the carbonation depth.

The corrosion rate may also increase when the transport of gasses, moisture and ions occurs at defects such as cracks and cold joints. Many types of defects can increase the chloride content at the concrete surface, which can be diffused faster to the steel level if the carbonation depth is sufficient. An increase in the chloride content at defects may thus be considered as a severe case for corrosion and has not been addressed yet in the literature. A cold joint was selected for this study as it is formed over the entire cross-section of concrete elements, unlike defects that form at the concrete surface. This corrosion case may result in a reduction of the corrosion initiation time and occurrence of cracking during the propagation phase. It is practical to consider such cases of corrosion as these may be present more frequently due to urbanisation and population growth in marine environments. The aim of this study was to determine if the corrosion rate could be increased due to an increase in the chloride content at a cold joint in carbonating concrete.

The experimental programme was set out to determine the corrosion state of reinforcing steel, chloride penetration and carbonation depths, transport of gasses, moisture and ions, and corrosion damages to the steel and concrete. It is useful to investigate any corrosion cases by supplementing the corrosion rate with other parameters such as chloride penetration, carbonation depth, etc. The concretes that were used to cast the test specimens included either fly ash or metakaolin, for the comparisons to ordinary concrete. The exposure period of this study lasted 294 days, which was intended to achieve a carbonation depth that was influential to the chloride penetration and corrosion rate. A cold joint was included in selected specimens for comparison to the monolithically-cast specimens, with a time delay of 24 hours between the casting. The exposure conditions were either combined exposure to chloride and carbon dioxide, or chloride exposure in isolation.

The results found an agreement from the comparisons between the experimental parameters which validated the hypothesis of this study. The chloride content was increased beyond the carbonation depth at a cold joint in all concretes within the timeframe of the exposure. The corrosion rate of the steel was increased due to the increase in the chloride content beyond the carbonation depth in the vicinity of a cold joint. The benefit of fly ash and metakaolin was limited to the test specimens with a larger cover depth where the corrosion rate was lower than the ordinary concrete. The oxygen permeability was lower than the ordinary concrete, which is the main factor to the corrosion rate in saturated or partially-saturated concrete. The concrete resistivity was higher than the ordinary concrete, in that there was a lower conductivity to allow for a passage of current in the corrosion reactions.