MICROCRACKS DETECTION OF THERMAL DAMAGED CONCRETE WITH NONLINEAR ULTRASONIC MODULATION BASED ON BROAD BAND FREQUENCY COUPLING

The initiation and development of microcracks introduced by heating or fire plays a critical role in the stability and durability of concrete structure. However, the concealment of microcracks and the nonlinearity of concrete materials make it difficult to appropriately evaluate the size and extension state of microcracks inside the thermal damaged concrete. In this paper, broadband frequency excitation instead of traditional dual-frequency excitation is utilized to excite the thermal damaged concrete, and the generated ultrasonic modulated signal reflects the micro damage state. The concept of damage index (DI) based on the sideband peak count (SPC) is proposed to quantitatively describe the variation characteristics of modulated signals. The results show that the peak value of DI based on broadband frequency coupling of nonlinear ultrasonic modulation method reflects the generation and development of microcracks in thermal damaged concrete. The peak value of DI increases sensitively with the increasing of water-cement ratio, fine-coarse aggregate ratio, and the heating temperature. Meanwhile, the statistical relationships of the peak value of DI with the residual strength and the area ratio of microcracks in thermal damaged concrete are established respectively.