Utilisation of Reclaimed Asphalt Pavement for Improving Substandard Subbase

This research attempted to improve substandard subbase by mixing it with reclaimed asphalt pavement. Three substandard soils, namely Soils A, B, C, and RAP, were collected from Phangnga, Southern Thailand. Each soil was mixed with the RAP having the soil to RAP by-weight ratios of 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50. A whole RAP, namely sample D, was also studied, resulting in a total of 19 soil-RAP mixtures. Several testing techniques were carried out to obtain the best mixture for being employed as a subbase. Besides basic properties, the compaction test was conducted to obtain optimum water content. Consequently, such water was employed to prepare samples for the CBR test. Considering overall compaction behaviour, it was found that the maximum dry density gradually increases with the increase of RAP content from 0% up to 30%. After that, it decreased steadily with further increases in RAP from 40% to 50%. Strikingly, all soils had the maximum dry densities when mixed with 30% RAP. As such, the CBR results unsurprisingly showed the same trend, i.e., the maximum values occurred when the mixtures contained 30% RAP. The CBR values for the samples A70R30, B70R30, and C70R30 were 11.0%, 15.9%, and 25.1%, respectively. This indicates that only soil C mixed with 30% RAP qualifies for subbase. These results suggest that 30% of RAP is the most appropriate portion to be mixed with substandard subbase soil to achieve a qualified subbase.