Determination of asphalt-aggregate ratio in steel slag asphalt mixtures through optimal asphalt film thickness design and comprehensive performance validation

Steel slag has great potential as an aggregate for asphalt pavements, but its high porosity and specific gravity make conventional asphalt-aggregate ratio design methods less accurate and less efficient. To address this issue, this study proposes an asphalt-film-thickness-based design method for steel slag asphalt mixtures. Four gradations each of AC-13 and SMA-13 mixtures were firstly selected, and five asphalt-aggregate ratios ( P a ) were evaluated for each gradation to determine the optimal P a in accordance with current specifications. The asphalt film thickness ( DA ) on mineral surfaces in both steel slag asphalt mixtures (SSAM) and basalt asphalt mixtures (BAAM) was measured and compared using backscattered electron microscopy and fluorescence microscopy. Optimal DA values corresponding to peak mixture performance were identified through comprehensive evaluations, including Marshall stability tests, water immersion Marshall tests, and splitting tests. Based on these results, the surface area coefficient of steel slag was calculated. Subsequently, the optimal P a values for two additional SSAM gradations were determined using the derived optimal DA , and their temperature stability and moisture resistance were verified through rutting tests, freeze-thaw rutting tests, and low-temperature bending beam tests. The results indicate that the surface area coefficient of steel slag is 0.00145, and the optimal DA values for SSAM and BAAM used in the surface layer are determined to be 6.75 μm and 6.15 μm, respectively. Compared to conventional design methods, the dynamic stability of SSAM and BAAM designed using the DA method increased by 19.04% and 3.62%, respectively, while their post-freeze-thaw dynamic stability exhibited approaching. The maximum flexural strain of the SSAM increased by 3.82%, whereas that of the BAAM decreased by 4.36%. Overall, the proposed method provides a more convenient and performance-oriented approach for determining the suitable P a of SSAM, supporting their efficient application in pavement engineering.