Testing Aggregate Backfill for Corrosion Potential

Abstract

The Kansas Department of Transportation (KDOT) has designed and constructed numerous mechanically stabilized earth (MSE) walls to support new and expanded highway projects throughout Kansas. MSE walls often contain galvanized steel strips as mechanical reinforcement within layers of specified backfill material. Inclusion of these strips creates a stronger composite material connected to a visually appealing wall facing, however galvanized steel reinforcement is potentially vulnerable to corrosion. Corrosivity of MSE backfill is typically characterized using electrical resistivity among other properties. KDOT currently uses the American Association of State Highway and Transportation Officials (AASHTO) standard T 288 to calculate the resistivity of MSE backfill. There is concern that this method may not reflect field conditions well, and thus may mischaracterize the corrosivity of backfill. AASHTO T 288 tests were conducted as a part of this research, and the condition of these samples during testing was not consistent with expected field conditions. A new procedure has been proposed to ASTM that appears to more accurately simulate field conditions behind MSE walls. This ASTM C XXX-XX (the New ASTM) has been extensively tested and compared with AASHTO T 288 in this experimental study. The New ASTM simulated expected field conditions more accurately than the AASHTO test. Results also appear to indicate the need for a larger resistivity box to accurately characterize the corrosivity of larger aggregates. Preliminary recommendations for box geometry are 8:1 minimum electrode spacing to maximum particle size and 3:1 minimum height to maximum particle size. It was also observed that increasing the number of soak/drain cycles of the material resulted in a substantial increase in resistivity values that plateaued at a higher value than observed for both the AASHTO and proposed ASTM methods.