08319 - Long Term Corrosion Performance of Soil Reinforcement in Mechanically Stabilized Earth Walls

Mechanically stabilized earth walls, MSEW, around Florida were instrumented through their concrete covers to survey corrosion rates of galvanized strip soil reinforcement by polarization measurements. The first broad survey was made 10 years ago as a baseline and a new survey is currently underway to assess long term corrosion rate trends. Initial results of an ongoing survey confirm low rates consistent with controlled backfill composition. BACKGROUND An MSEW is more advanced form of a retaining wall, often larger and being able to hold back more backfill. This is achieved by reinforcing strips or meshes (in the present case of galvanized steel) placed into the soil holding it in place by friction. The strips mechanically stabilize the earth while undergoing tension. The wall is covered with concrete medallions that connect to the reinforcements. The medallions have only a secondary structural role in holding up the wall but provide cover that protects the soil from washing away. This configuration exposes metal to the soil environment with associated corrosion risk. If the reinforcement were to degrade prematurely by corrosion the soil could shift resulting in potential structural risk. Present durability criteria by Florida Department of Transportation (FDOT) establish a design service life of 75 years. To that end, FDOT specifies that the backfill in the walls meets electrochemical criteria (resistivity > 3 000 Ω•cm; 5 < pH < 10; soluble chloride < 100ppm; sulfate < 200ppm) chosen to minimize corrosion deterioration. Based on prior experience 1,2 those criteria are expected to limit corrosion rates of less than 1⁄2 mpy (13 μm/year) for the galvanized layers, which are consistent with tolerable residual reinforcement strength throughout the design life given usual reinforcement dimensions .