Electrical Resistance Changes as an Alternate Method for Monitoring the Corrosion of Steel in Concrete and Mortar

A very sensitive, unambiguous, and simple procedure was introduced for the assessment of corrosion in concrete. This method consists of making resistance change measurements (RCM) on small diameter (228.6 μm) 1040 steel (UNS G10400) wires embedded in concrete (or mortar) prisms and correcting the measurements for temperature by use of a noncorroding control wire. Metal loss caused by corrosion lead to a loss in cross-sectional area and, hence, an increase in resistance. Through proper temperature correction, these methods lead to a sensitivity of 1 μm diameter loss. The wires were heat-treated at 538°C for 1 h to attain an oxide chemistry similar to bare rebar, as determined by x-ray photoelectron spectroscopy. Although this method was designed for the evaluation of inhibitive admixtures, it was used in the present study to examine supplementary cementitious materials. The rate of steel corrosion in these was assessed by RCM using mortar prisms (25.4 cm by 5 cm by 5 cm) of the following composition: plain mortar, 50% slag, 25% fly ash, and 6% silica fume. These prisms were ponded with 0.6 M sodium chloride (NaCl) for 140 days and subsequently wet/dry cycled for an additional 140 days. Wires in blocks containing slag, fly ash, and silica fume showed fewer instances of corrosion and lower average rates of corrosion than wires in plain mortar. Slag additions were shown to be more effective than fly ash additions at mitigating corrosion, while silica fume was effective only at deeper cover depths. The RCM method also was compared to electrochemical methods: open-circuit potential measure ments (half-cell potentials), linear polarization, and electrochemical impedance spectroscopy. In all cases, RCM was more sensitive and definitive on the steel corrosion rates within the mortar. The simplicity and low cost of this approach make it ideally suited as a standard method to assess corrosion prevention methods in concrete.