Experimental study on nonlinear ultrasonic characterization of concrete during crack propagation

Damage and failure of concrete structures are essentially caused by crack initialization and expansion. Conventional ultrasonic tests cannot detect changes in the micro-cracks in concrete. Second harmonic generation (SHG) is effective at characterizing the micro-damage. However, research on the influence of micro-cracks on the nonlinear parameter β is still insufficient. Three cubic concrete specimens 150 mm × 150 mm× 150 mm were fabricated, and the design and average compressive strength were 30 MPa and 39.24 MPa, respectively. Before compressive loading, the integrity of concrete specimens was verified using scanning electron microscopy. The load rate is set to 0.6 kN/s during the test. A simultaneous test system for stress σ, strain ε, and nonlinear parameter β was designed, and the regularity of the nonlinear parameter β during compressive loading was investigated. The nonlinear parameter β increases significantly at the beginning of the compressive load. As the load continues to increase, the nonlinear parameter β decreases rapidly and then fluctuates during the subsequent loading. Furthermore, attenuation was also analyzed in this experiment based on the head-wave amplitudes. At the beginning of the compressive loading, the attenuation is weak as no obvious damage exist in concrete. The non-linear parameter β is less affected by the attenuation when the integrity of the concrete specimens is good. When the cracks expand and penetrate, strong attenuation can be observed at the fundamental and second harmonic waves. The results of the experiment have considerable importance with respect to the method of SHG for micro- and macro-scale damage in concrete.