Preliminary Insights into the Mechanical and Environmental Performance of Nonconventional Granular Sub-Bases Incorporating Crumb Rubber and Recycled Concrete Aggregate

This study evaluates four granular sub-bases for pavement applications under Colombian conditions. A natural granular sub-base with a nominal maximum aggregate size of approximately 38 mm (i.e., GSB38) served as the control. Three modified mixtures comprised (i) 90% GSB38 with 5% Recycled Concrete Aggregate (RCA) and 5% Crumb Rubber (CR), (ii) 80% GSB38 with 10% RCA and 10% CR, and (iii) 70% GSB38 with 15% RCA and 15% CR. Thus, these formulations represented total recycled contents of 10%, 20%, and 30%, respectively, calculated on a dry-mass basis. Particle size distribution, standard Proctor compaction, Los Angeles abrasion, and four-day soaked California Bearing Ratio (CBR) tests were performed, and results were evaluated against Colombian specifications. An attributional cradle-to-gate Life-Cycle Assessment (LCA) with a one-tonne functional unit used SimaPro 9.4, Ecoinvent 3.9, and TRACI 2.1. All formulations complied with gradation, compaction, and abrasion requirements. As recycled content increased, optimum moisture content rose from 7.695% to 9.001%, maximum dry density fell from 2.388 to 2.072 g/cm3, and abrasion loss rose from 42.0% to 45.5%. Design CBR responded nonmonotonically, with design CBR values of 44.4%, 24.9%, 59.3%, and 90.1% for formulations with 0, 10, 20, and 30% total recycled material content, respectively. The 10% formulation failed the minimum CBR criterion, whereas the other mixtures met the high-traffic requirement. All recycled formulations reduced all considered environmental indicators; for example, the 30% recycled-content mixture decreased ozone depletion by 20.199% and global warming potential by 3.501%. Overall, the sub-base formulations with 20% and 30% of total recycled content showed strong technical and environmental potential, although replicated testing and field validation remain necessary.