Superpave Mixture Design Method

Chapter 11 discusses the Superpave mixture design method with all the steps required in the design process. The Superpave gyratory compaction discussed in the previous chapter is an important step in the Superpave mixture design. The selection of aggregate, asphalt binder, and aggregate gradation is done according to the Superpave criteria prior to mixture design. Throughout the mixture design method, typically three trial aggregate blends are used to check whether any of the blends meets the Superpave criteria. Volumetric properties including air voids (V_a), voids in mineral aggregate (VMA), voids filled with asphalt (VFA), and dust proportion (DP), and compaction properties including %G_mm at N_initial, %G_mm at N_design, %G_mm at N_maximum, are determined after the compaction process. If the air-void content is 4%, the values of the volumetric and compaction properties for each of the trial mixtures are checked against the Superpave criteria. If the criteria are met, the blend is acceptable; otherwise, a redesign of the aggregate blend is required. If the air-void content is altered from 4%, the asphalt binder content is reestimated to correct for the air-void content using a special formula. Afterwards, the other volumetric and compaction properties are recalculated accordingly and compared with the Superpave criteria to check whether they meet the criteria. If the criteria are met, the blend is acceptable; otherwise, a redesign of the aggregate blend is needed. The next step after the selection of the design aggregate blend, asphalt binder contents of ±0.5% and ±1.0% from the estimated asphalt binder content are used to compact at least two specimens at each binder content using the Superpave gyratory compactor (SGC). The volumetric and compaction properties are calculated again after compaction for each asphalt binder content and plotted versus the asphalt binder content. The asphalt binder content at 4% air voids is selected as the design asphalt binder content. The volumetric and compaction properties at the design asphalt binder content are then determined for the mixture and compared with the Superpave criteria. The moisture sensitivity test is also conducted on gyratory-compacted specimens of the mixture following a standard procedure to determine the indirect tensile strength ratio (TSR) of the specimens that should be equal or higher than 0.80.