Gradation Control of Bottom Ash Aggregate in Superpave Bituminous Mixes

Dwindling sources of traditional aggregates, ever increasing haulage distance, and diminishing landfills are the primary factors that favor the reuse of construction-quality waste materials in highway pavements. Although numerous studies have been conducted on the performance of municipal solid waste incineration bottom ash (MSWI-BA) in cement concrete, little is known about its characteristics in bituminous concrete pavements, especially in Superpave design hot mix asphalt (HMA). By comparing the air voids and voids filled with asphalt of 20% MSWI-BA HMA samples aged for 2 and $4h$ for a trial mix, all the blends prepared for this research were aged for $4h$ prior to compaction. Varying the slope/curvature, and the proportions of stone sand and stone dust in trial mixes showed that slope/curvature is more significant than aggregate type in increasing voids in mineral aggregates (VMA). The design binder contents in mixes were found to increase from 6.2% for 0% BA content to 7.4% for 20% BA, a difference of 1.2% (a 19% increase). Also, as bottom ash content is increased to 20%, the effective asphalt decreases from 5.1 to 4.4%, while the absorbed asphalt increases from 1.3 to 3.2%. These findings are consistent with the high absorptivities (i.e., 9.2% water) found for the bottom ash and its high internal porosity resulting in total surface area increase of aggregate structures containing MSWI-BA. Although the 13.0% minimum VMA specified by Superpave was met for all MSWI-BA contents of the design mix used for this study, a marked decrease from 15.3% VMA for 0% BA to 13.4% VMA for 20% BA was measured. Results of the $\%G_{\mathrm{mm}}$ at $N_{\mathrm{initial}}$ for 0–20% MSWI-BA HMA are essentially the same, falling between 85 and 86% compaction. Based solely on the results of the Superpave densification values ($\%G_{\mathrm{mm}}$ at $N_{\mathrm{initital}}$ and $N_{\max }$) for this study, substitution of up to 20% MSWI-BA for virgin aggregates yield mixes having aggregates structures that are well developed to resist compaction, rutting, and tenderness in agreement with the findings of Wang et al. in 2000. Obviously, rutting potential, fatigue characteristics, resilient modulus, and field studies should be evaluated to thoroughly investigate the performance of MSWI-BA amended HMA.