Developing a Torque-Based Criterion for Estimating Mixing and Compaction Temperatures of Asphalt Mixtures

Accurate determination of mixing and compaction temperatures is essential for producing durable asphalt pavements while minimizing energy consumption and binder aging. Conventional temperature selection is based on asphalt binder viscosity, which does not adequately represent the workability of modern asphalt mixtures containing polymer modifiers or alternative mineral fillers. This study proposes a torque-based criterion as a performance-oriented alternative for estimating the required production temperatures of conventional and polymer-modified asphalt mixtures. A high-capacity laboratory workability device was developed to measure the mixing torque of 15 kg asphalt mixtures at temperatures ranging from 120 to 160 °C. The experimental program included surface, binder, and base mixtures incorporating hydrated lime, limestone, and cement fillers, as well as asphalt modified with 4% styrene-butadiene-styrene (SBS) and 5% polyvinyl chloride (PVC). Based on the experimental results, torque criteria were established for conventional hot mix asphalt, and regression models were developed to estimate the production temperatures of polymer-modified mixtures. The proposed torque-based approach predicted mixing temperatures of 163–192 °C and compaction temperatures of 147–176 °C for SBS-modified asphalt, while the corresponding ranges for PVC-modified asphalt were 131–170 °C and 110–149 °C, respectively. The lower PVC compaction prediction of 110 °C represents an extrapolated model value outside the experimentally investigated temperature range and requires experimental verification before practical application. Compared with the conventional viscosity-based method, the torque criterion reduced the required mixing and compaction temperatures by 13–57 °C (6–30%) and 17–65 °C (9–37%), respectively. Validation using a Superpave gyratory compactor demonstrated nearly identical volumetric properties for the two methods, with %Gmm at Ndesign of 95.90% and 95.88%, and air voids of 4.10% and 4.12%, despite approximately 40 °C lower processing temperatures using the torque-based method. These findings indicate that torque-based workability can provide a practical basis for estimating asphalt production temperatures, while its potential for reducing energy demand, limiting binder aging, and supporting more sustainable pavement construction requires further validation.