Assessment of Corrosion Potential of Coarse Backfill Aggregates for Mechanically Stabilized Earth Walls

The service life of mechanically stabilized earth walls depends on the rate of corrosion of the metallic reinforcements used in their construction. The assessment of corrosion potential requires an accurate evaluation of pH, resistivity, and sulfate and chloride concentrations of aqueous solutions in contact with the surrounding aggregate. Highway agencies tend to use larger aggregates that contain only a small amount of fine material (passing the Number 40 sieve) in the backfill. Evaluation of the electrochemical parameters of coarse aggregates is challenging because traditional evaluation methods call for the use of fine material. In this study, the suitability of traditional soil characterization techniques for use with coarse aggregates was assessed through leaching experiments performed on coarse limestone and dolomite aggregates from six quarries in Texas. Chemical differences were isolated from size-related kinetic leaching effects by comparing the results from same-sized material collected in the field with material derived from the crushing of larger (≥⅜ in.) aggregates in the laboratory. The testing demonstrated that the fines collected from the field were enriched in chemicals that, when exposed to water, decreased pH and resistivity and increased sulfate concentrations compared with the bulk rock. This was likely the result of sulfur compounds in the atmosphere reacting with carbonate rocks to produce reactive surface layers that were mechanically abraded into the fines. This phenomenon could bias traditional soil testing results and, therefore, the assessment of corrosion potential. This study demonstrated that a more accurate assessment of the electrochemical parameters can be obtained by crushing the coarse material to meet testing size specifications.