Non-Corrosivity of Expanded Shale, Clay and Slate Aggregate in Mechanically Stabilized Earth Fills

by Dr. Fariborz M. Tehrani, Ph.D., PE, ENV SP, PMP, SAP, F.ASCE

The recently published Research Report 958 [NASEM 2021] by the Transportation Research Board (TRB) National Cooperative Highway Research Program (NCHRP) addresses improvements in electrochemical test methods and practices for assessing the corrosivity of steel reinforcement in mechanically stabilized earth (MSE) fills. This report reflects on the NCHRP 21-11 project conducted by practitioners and researchers from McMahon & Mann Consulting Engineering and Geology, P. C. and The University of Texas at El Paso from 2016 to 2020. The study incorporated laboratory measurements and field observations and collected 27 samples from 25 sites including two well-graded sand samples of expanded clay lightweight aggregates from South Carolina (field, medium) and Louisiana (producer, crushed coarse) (p. 18).

Researchers evaluated existing standards by ASTM, AASHTO, and DOTs as well as alternative testing methods; and assessed their precision, repeatability, and correlation with potential and rate of corrosion. The producer sample of expanded clay was recognized as “extremely low chloride and sulfate content” (p. 21). Further, results categorized expanded clay field sample as non-corrosive (p. 48) with substantially lower corrosion rate for measured resistivity than natural materials (p. 47). Authors cited absorption and chemical composition as reasons for this major difference, which could not be captured in existing regression analyses that are meant for conventional fills (p. 41). Conclusions of this report includes a proposed test method to evaluate the corrosion potential of earthen materials (p. 70) with notes on significant differences between lightweight materials and conventional fills (p. 66).

National Academies of Sciences, Engineering, and Medicine 2021. Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials. Washington, DC: The National Academies Press.