Evaluation of the Effect of Aggregate Gradation on the Mechanical Properties and Moisture Susceptibility of Cold Recycled Asphalt Mixtures

Asphalt pavements deteriorate over time due to heavy traffic and environmental factors, necessitating frequent maintenance or rehabilitation. Cold recycled asphalt mixtures (CRAM) provide a sustainable approach for reusing reclaimed asphalt pavement (RAP), reducing construction costs, conserving natural resources, and mitigating environmental impacts. However, the mechanical performance and durability of CRAM are highly dependent on aggregate gradation. This study investigates the effect of variations in aggregate gradation, particularly deviations from permissible limits caused by plant production errors or construction-related issues, on the mechanical properties of CRAM. In this research, RAP was used as the coarse aggregate (95% by weight), while virgin aggregate served as filler (5% by weight). Five gradation curves were evaluated, including four outside the permissible range and one base mix within the permissible range, based on the gradation type specified in Code 339. These five mix designs were as follows: (1) Mix A: with the reduction of filler below the permissible range specified in Code 339. (2) Mix B: with the increase of filler beyond the permissible range specified in Code 339. (3) Mix C: with the reduction of coarse aggregates below the permissible range specified in Code 339. Mix (4) D: with the increase of coarse aggregates beyond the permissible range specified in Code 339. (5) Base Mix: with an aggregate gradation within the permissible range of Code 339. Marshall specimens were prepared under ambient conditions and cured for 7 days before testing, while indirect tensile strength (ITS) specimens were tested after 7 and 28 days of curing in both dry and moisture-conditioned states. The results indicate that increasing the coarse aggregate proportion or reducing filler content led to a decrease in Marshall stability by 18% and 7%, respectively, compared to the base mix. Conversely, increasing fine aggregate content and reducing coarse aggregates enhanced Marshall stability by 30% and 22%, respectively. In terms of tensile behavior, reducing fine aggregates decreased Dry ITS by 20%, whereas increasing fine aggregates or adjusting coarse aggregate proportions resulted in Dry ITS increases of 11%, 16%, and 1.5% relative to the base mix. Although some mixtures achieved the minimum Dry ITS requirement after 28 days, certain designs exhibited poor visual quality and insufficient cohesion, emphasizing the importance of precise gradation control during plant production. The base mix failed to meet moisture durability requirements in terms of Dry ITS and tensile strength ratio (TSR), highlighting the need for gradation optimization or the use of suitable additives. Additionally, increasing the emulsified bitumen content in mix B* to 5.5% improved Dry ITS by 21% relative to the base mix, demonstrating the synergistic effect of proper gradation and emulsified bitumen content on moisture resistance. However, the base mix failed to meet moisture durability requirements in terms of Dry ITS and TSR, emphasizing the need for gradation optimization or the use of suitable additives. Additionally, increasing the emulsified bitumen content in mix design B* to 5.5% improved Dry ITS by 21% relative to the base mix, demonstrating the synergistic effect of proper gradation and emulsified bitumen content on moisture resistance. Overall, these findings confirm that aggregate gradation has a significant influence on both the mechanical performance and moisture durability of CRAM. Maintaining optimal proportions of fine aggregate and filler is particularly crucial, as deviations from their ideal levels can substantially compromise the mixture’s strength, cohesion, and durability. Therefore, careful monitoring and control of aggregate gradation and emulsified bitumen content during production are essential to ensure acceptable Marshall stability, ITS, and TSR, providing practical guidance for the sustainable rehabilitation of asphalt pavements using cold recycled mixtures.