The dynamic behavior of a reinforced concrete beam was studied theoretically and experimentally. A relationship between the resonance frequency of the beam and the applied bending moment was established. A frequency criterion corresponding to the crack control criterion in the ACI Building Code was developed. It was found that resonance frequencies and damping will change rapidly when cracks are initialized and growing. The resonance frequency shift could be as large as 20 to 25% of the original value, and the decay rate could change by a factor of 4. The applied bending moment not only determines the number and the size of cracks but also determines the crack opening or closing condition that affects both resonance frequency and damping. To accurately estimate the frequency and damping of a cracked-reinforced concrete beam, empirical formulas are proposed based on a small number of experiments. The technique has promise for global nondestructive evaluation of reinforced concrete structures such as highway bridges.
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