Reclaimed Asphalt Pavement from BBTM Mixes as a Component in Soil–Cement: Preliminary Effects on Density, Moisture and Unconfined Compressive Strength

Current Spanish road specifications, in accordance with circular economy strategies, allow reclaimed asphalt pavement (RAP) to be used in hot-mix asphalt production. RAP is produced by milling asphalt layers during pavement rehabilitation activities. The percentage of permitted RAP depends on the pavement layer and expected traffic conditions. However, current regulations do not authorize its use in the base or sub-base layers of flexible pavements with unbound materials or in semi-rigid pavements, such as soil–cement or gravel–cement layers. This research evaluates the influence of replacing natural aggregates with RAP obtained from a BBTM surface mixture (gap-graded asphalt mixture) in the manufacture of soil–cement for semi-rigid pavements. This study focuses on the effects of RAP incorporation on the maximum dry density and optimum moisture content determined through the modified Proctor test, as well as on the unconfined compressive strength (UCS). Four mixtures were analyzed: a reference mixture containing only natural aggregates and three additional mixtures in which 25%, 50%, and 75% of the aggregate was substituted with BBTM-RAP. In total, 3.5% of cement was used in each mixture. Modified Proctor and 7-day UCS tests were subsequently performed to assess the mechanical behavior of the mixtures. The results showed that increasing the RAP content reduced the maximum dry density while increasing the water demand required for compaction. Higher RAP percentages also led to lower compressive strength values. Nevertheless, the average unconfined compressive strength (UCS) at 7 days for all mixture formulations exceeded 2.5 MPa. These findings support the ongoing investigation into the potential application of BBTM-RAP as a more sustainable alternative material for semi-rigid pavement base layers.