Potential use of recycled asphalt pavement and crushed concrete as backfill for mechanically stabilized earth walls

Abstract

Increasing demands for construction aggregates have raised concerns within the highway-related government agencies and the industrial community for the past twenty years. Although yearly production of recycled asphalt pavement (RAP) and crushed concrete (CC) has significantly increased over the past few decades, many state agencies and jurisdictions have limited experience using these materials as an MSE wall backfill. The use of more locally available RAP or CC not only minimizes the cost of transporting select materials from remote areas but also alleviates the cost of disposing these materials in a commercial landfill. This dissertation presents results from comprehensive experimental studies that focus primarily on the intrinsic geotechnical properties of RAP and CC. The preliminary testing involves determining the general index properties of RAP and CC. Subsequent testing includes laboratory and field investigations to evaluate compaction characteristics, shear strength properties, hydraulic conductivity, and collapse potential of the study materials. Test results obtained on RAP and CC specimens are analyzed and compared with the results obtained from the conventional fill material (CFM) to further evaluate comparatively their suitability as MSE wall backfill. The last part of this study covers two detailed investigations on the most significant potential problems for RAP and CC backfill. Creep becomes a major concern for RAP because of its viscoelastic properties. The creep potential of RAP is investigated through a series of constant stress, drained triaxial creep tests. The experimental data were fit with an empirical-based model developed by Singh and Mitchell (1968) to describe the creep behavior of RAP with respect to confining pressure and stress level. Empirical expressions to predict creep rupture were investigated. The laboratory data were also used to identify the upper yield strength, below which creep rupture does not occur. For CC, the most significant concern is the potential for calcium carbonate (CaCO3) precipitation. In this study, laboratory measurement of calcium ion concentration of CC-water mixtures were conducted on a wide range of CC grain sizes to determine the threshold particle size that may exacerbate the CaCO3 formation of a CC backfill.