Acoustic Emission and Damage Evolution Characteristics of Carbon Nanotube Concrete Three-Point Bending Beam

To study the impact of the content of carbon nanotubes (CNTs) on the damage evolution and crack propagation in concrete beams, concrete three-point bending beam specimens containing 0, 0.1%, 0.3%, and 0.5% CNTs were prepared. The three-point bending tests were conducted by using the YNS300 electro-hydraulic servo universal testing machine equipped with an acoustic emission (AE) testing system. Acoustic information such as AE energy, ringing count, and amplitude were obtained. The crack type and damage evolution process were analyzed based on AE parameters. The results show that the AE energy of carbon nanotube concrete (CNTC) three-point bending beam is significantly higher than that of plain concrete beams. However, as the content of carbon nanotubes increases, the AE energy generated during failure gradually decreases. The addition of CNTs can improve the bearing capacity of concrete beam. However, after exceeding a certain threshold, the load-bearing capacity decreases as the CNTs increases. The incorporation of carbon nanotubes advances the time of the first amplitude mutation in the concrete three-point bending beams. Prior to the failure of the specimens, the ringing count and cumulative ringing count increase slowly, approximating a straight line. However, at the instant of specimen fracture, both of them rise sharply. The AE signal sources belong to the tensile crack. The fracture of carbon nanotube-reinforced concrete three-point bending beams is classified as Mode Ⅰ fracture. There is no significant relationship between the proportions of tensile-shear cracks and the CNTs content. However, the number of AE average frequency and angle of rise of signal sources is significantly correlated with the CNTs content. The damage evolution law of concrete beam samples with different CNTs is basically consistent. In the early stage of the test, the damage curve is basically horizontal, and the damage variable in the failure stage is in a rapid growth stage at first, and then abruptly increases.