AbstractThe initiation and development of microcracks (with width generally less than 100–150 μm) introduced by heating or fire plays a critical role in the stability and durability of concrete structures, especially for large dams, nuclear power plant containments, or structures under fatigue loading. However, the concealment of microcracks and the nonlinearity of concrete materials make it difficult to evaluate appropriately the extent of microcracks and the effect on the compressive strength of concrete after thermal damage. This paper used broadband frequency excitation instead of traditional dual-frequency excitation to excite thermally damaged concrete, and the microdamage state was reflected in the generated ultrasonic modulated signal. The variation characteristics of modulated signals was described using the concept of a damage index (DI) based on the sideband peak count (SPC). The results showed that the peak value of DI based on broadband frequency coupling of the nonlinear ultrasonic modulation method is more stable and accurate than the resonance method. The development state of microcracks under the influence of different parameters (water-to-cement ratio, fine-to-coarse aggregate ratio, and heating temperature) was measured sensitively using the peak value of DI. The established statistical relationship of the peak value of DI provides a method and reference for the evaluation of the state of microcracks and the loss of compressive strength in actual structures.