Effects of Sewage Sludge Ash on Fatigue Performance of Asphalt Mixtures Exposed to Runoff with Varying pH Levels

Municipal wastewater treatment generates large volumes of organic sludge (a byproduct classified as industrial waste). The disposal of the sludge creates significant economic and environmental challenges. Incinerating this sludge to produce sewage sludge ash (SSA) offers a sustainable recycling solution, enabling its repurposing as a construction material. SSA demonstrates potential as a substitute for fine mineral aggregates in hot mix asphalt (HMA), providing benefits such as mitigating sludge disposal issues, reducing natural resource consumption, and enhancing asphalt performance. Given the detrimental effects of runoff infiltration on pavement structure and the critical role of fine aggregates in fatigue resistance of HMA, this study investigates the impact of aggregates (passing sieve No. 50) replacement with SSA on the fatigue performance of mastics and HMAs under moisture conditions at varying pH levels (5, 6, 7, 8, and 9). Dynamic shear rheometer and linear amplitude sweep tests were conducted to evaluate the rheological properties and fatigue life of mastics at intermediate temperatures under different moisture conditions. Indirect tensile fatigue tests were also performed to assess the fatigue life of HMAs under moisture exposure. Results showed that altering the pH of infiltrating water to the HMA significantly increases the mastic’s fatigue parameter (G*×sinδ) value and reduces the fatigue life of both mastic and HMAs. In other words, exposing HMA to acidic/alkaline moisture conditions demonstrated the impaired performance of the mastic and HMA due to changes in the pH level of infiltrating water, weakening its resistance to fatigue cracking. Acidic conditions (pH=5 and pH=6) exhibited the most pronounced adverse effects, highlighting the vulnerability of HMAs to pH-induced fatigue cracking. Using SSA yielded an average 10% reduction in the mastic’s G*×sinδ value under varying moisture conditions, with the most significant decrease (13%) observed in acidic environments. The replacement of mineral aggregates with SSA underscored SSA’s effectiveness in counteracting the detrimental impacts of acidic water on the mastic’s fatigue performance. Also, SSA increased the fatigue life of HMAs by 41% on average under both dry and moisture-exposed conditions due to the increasing fatigue life of mastics. SSA enhances the fatigue cracking resistance of the HMA and mastic in dry conditions and also mitigates the degrading effects of acidic, alkaline, and neutral moisture exposure on fatigue performance. In addition, SSA reduced the sensitivity of mastics and HMAs to changes in strain and stress levels, respectively, indicating improved resistance to increased loading levels. Statistical validation showed that replacing mineral fine aggregates with SSA significantly enhanced the rheological properties of mastics and fatigue life of HMAs. The results of this study underscore the potential of waste-derived materials, such as SSA, in advancing sustainable infrastructure solutions.