Low-cost ultrasonic monitoring for incipient and progressive damage in reinforced concrete beam

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.