Aging concrete infrastructure demands affordable structural health monitoring (SHM) solutions capable of detecting damage at its earliest stage. This study develops and experimentally validates a fully low-cost ultrasonic SHM system (∼US$150) for incipient and progressive damage detection in reinforced concrete beams. The system integrates a microcontroller-based data acquisition module, a piezoelectric transducer array, and a real-time MATLAB interface for multi-domain signal tracking. A comprehensive comparison between time-domain envelope features and frequency-domain spectral features shows that spectral attributes exhibit markedly superior sensitivity to microcracking and load-induced degradation. A data-driven feature ranking and fusion strategy was used to construct a weighted composite damage index (DI). Across three beam experiments, the normalized DI displayed strong monotonic correlation with applied load (Spearman ρ > 0.90) and consistently identified crack initiation loads. Notably, in one specimen the DI detected sub-visual microcracking 20 kN before surface crack formation. The results demonstrate that high-sensitivity ultrasonic damage detection can be achieved without laboratory-grade instrumentation, offering a practical pathway toward scalable and economically accessible SHM for the concrete infrastructure.
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