Influence of Changes in Water-to-Cement Ratio, Alkalinity, Concrete Fluidity, Voids, and Type of Reinforcing Steel on the Corrosion Potential of Steel in Concrete

About the report:

Research on steel corrosion has demonstrated that the concentrations of chloride and hydroxide ion at the concrete/steel interface influence the susceptibility of the steel to corrosive attack. This study used electrochemical means and changes in mix design to increase the alkalinity and improve consolidation of the concrete against the steel to determine if this would increase the corrosion resistance of the reinforcing steel without compromising the properties of the concrete. To understand these effects better, the following concrete properties were evaluated: compressive strength, splitting tensile strength, modulus of elasticity, and length change (shrinkage).

The evaluation involved the casting of different types of concrete samples, all of which contained artificial voids, for corrosion testing. These samples were composed of one of two concrete mixtures: (1) the Virginia Department of Transportation’s (VDOT) Class A4 General Bridge Deck Concrete with Straight Portland Cement (A4), or (2) a self-consolidating concrete (SCC). Selected A4 and SCC mixtures were electrochemically treated 135 days after samples were cast. Some samples were left untreated and functioned as control samples.

There were no significant differences in the concrete properties between the short-term treatment (less than 7 hr) and control samples tested in this study. It was determined that alkalinity, water-to-cement ratio, and steel type all influence the corrosion potential of reinforcing steel when reinforced concrete is subjected to saltwater. In addition, introducing small voids adjacent to the steel created a greater spread between the 25th and 75th percentile of the half-cell potential measurements when compared to locations without voids; however, these same voids have little influence on the average value measured. A study is underway to perform cyclical saltwater ponding on selected samples to allow for future evaluation of corrosion resistance.

This study showed that alkalinity, water-to-cement ratio, concrete fluidity during placement, and steel type all influence the corrosion potential of the reinforcing steel when reinforced concrete is subjected to saltwater. The study recommended that the Virginia Center for Transportation Innovation and Research (VCTIR) continue performing saltwater exposure on these specimens until corrosion becomes evident and then autopsy them and characterize the corrosion of the steel reinforcement. In addition, VCTIR should work with VDOT’s Materials Division to compare various concrete mix designs that would be considered acceptable to VDOT and assess how each influences the corrosion potential on embedded steel. Further, more emphasis should be placed on mix designs, such as SCC mixtures, that could provide VDOT with greater corrosion resistance and reduced labor during placement.

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