Thermo-Mechanical Interaction and Time-Dependent Cracking Risk Assessment of Early-Age Fly Ash High-Performance Concrete

Fly ash (FA) is widely used in high-performance concrete (HPC) to mitigate hydration heat in massive bridge structures; however, its influence on early-age thermal cracking risk remains governed by the interaction between thermal stress and strength development rather than temperature reduction alone. This study investigates the effect of FA replacement on early-age thermo-mechanical behavior through adiabatic temperature rise (ATR) tests, strength development experiments, and numerical simulation of a representative 2.0 m × 3.0 m bridge pier cross section. Increasing FA replacement reduced the maximum adiabatic temperature rise from 58.1 °C to 47.9 °C, while delaying early-age tensile strength development. The ultimate hydration degree increased approximately linearly with FA content, indicating the need to adjust hydration models for HPC with high binder content. Thermo-mechanical analysis showed that mixtures with 0–20% FA maintained η < 1.0 during the first 7 days, whereas 30% FA exhibited a distinct cracking risk window between 39–68 h. Notably, the maximum cracking potential did not coincide with peak core temperature but occurred within a critical thermo-mechanical period of 42–65 h. The results demonstrate that early-age cracking risk is governed by the time-dependent stress-to-strength ratio rather than temperature magnitude alone. For the investigated structural dimensions and boundary conditions, FA replacement levels of 10–20% provide the most favorable balance between hydration heat reduction and mechanical performance.