ReACT-SHM proposes a next-generation structural health monitoring system that moves beyond passive damage detection toward active mitigation. By integrating embedded Fiber Bragg Grating sensors, acoustic emission monitoring, and stress-responsive nanoscale agents, the system detects early microcrack formation and locally reinforces high-risk regions within concrete. This hierarchical, physics-guided approach aims to slow damage progression, reduce maintenance costs, enhance safety, and extend infrastructure lifespan.