Relating Corrosion of Mechanically Stabilized Earth Reinforcements with Fluid Conductivity of Backfill Soils

The service life of mechanically stabilized earth (MSE) walls depends on the rate of corrosion of the metallic reinforcements used in their construction. Several highway agencies in the US are more frequently specifying coarse (with hardly any aggregates passing No. 40 sieve) backfills to construct their MSE walls. The traditional assessment of corrosion potential of backfills requires the accurate evaluation of pH, resistivity, and sulfate and chlorine concentrations of aqueous solutions in contact with the surrounding aggregate. Evaluation of the electrochemical parameters of coarse backfills is challenging because traditional methods (e.g., AASHTO T-288, T-290 and T-291) utilize only fine materials. To address this concern in a practical manner, a new procedure that evaluates the corrosively of the backfill materials from the electrochemical properties of liquid extracted from a leach test on the specified gradation of the backfill has been proposed recently. This paradigm shift required an evaluation of the relevance of these electrochemical properties to the rate of corrosion of metallic reinforcements embedded within the backfill. A methodology was devised to monitor and estimate the corrosion rate of galvanized steel in MSE walls with a conductivity sensor coupled with laboratory electrochemical techniques. The fluid conductivity of six coarse-grained backfill soils undergoing wet/dry cycles and continuously saturated conditions were measured for up to 120 weeks. The corrosion rates of galvanized steel were determined indirectly by using the conductivities of the solutions obtained from leaching the six backfills. The conductivity of the leach-liquor appears promising to monitor the corrosion rate even though the predicted corroded thickness was slightly less than the measured corroded thickness.