Corrosion rates and the time to cracking of chloride contaminated reinforced concrete bridge components

In order to predict the future needs of existing bridges, Bridge Management Systems use models to predict the time when damage will reach a level to cause repair, rehabilitation, or replacement of the structure. One such model is the deterioration model, which has three distinct phases. The second phase of the model, the corrosion phase, is the focus of this study.

During the corrosion phase, chloride ion concentration reaches a threshold level at the depth of the reinforcing steel which initiates corrosion. The corrosion continues until sufficient pressure is exerted on the surrounding concrete to cause cracking.

This study is a continuation of a study implemented in the Materials Division at Va Tech. The study includes the monitoring of the corrosion rate of steel reinforcing bars placed in simulated bridge decks. The corrosion rates were varied by placing between 0 - 9.6 Ibs/yd³ of chloride ions in the concrete to produce six different series. Also, the depth of concrete cover, bar spacing, bar size, and exposure conditions were varied.

The specimens were monitored until the time that the cracking of the concrete was observed. At that time, samples of the steel reinforcing bars were removed and the actual amount of corrosion which had occurred was determined as the weight loss of the steel. The actual weight loss of the steel reinforcing bars was then compared to the predicted weight loss from the corrosion rate measurement devices.

The time to cracking and the mode of cracking was compared to Bazant's equations for cracking which are the basis for the corrosion phase of the deterioration model. Although only one series cracked during the study, corrections in the use of Bazant's equations were proposed.