Characterization of acoustic emission signals in recycled axial compression-damaged polypropylene fiber-reinforced concrete

To investigate the damage evolution characteristics of polypropylene fiber reinforced concrete (PFRC) under dynamic compressive loading, a series of reciprocating axial compression tests on prismatic specimens were conducted. The test variables included polypropylene fiber volume content (0.05%, 0.10%, and 0.15%) and aspect ratio (100, 200, 300). The complete stress-strain curves were obtained, and key mechanical parameters such as peak stress, ductility, and dissipated energy were extracted. Meanwhile, with the aid of acoustic emission (AE) technology, the dynamic propagation paths of internal cracks and the evolution characteristics of fracture types in PFRC under reciprocating axial compression were analyzed through AE signal characteristic parameters including ring count, AE energy, rise angle−average frequency (RA‒AF), and center frequency. The results show that under reciprocating axial compression, the envelope of the complete stress-strain curve of PFRC specimens presents a trend of first rising and then falling, and the overall position of the envelope is slightly lower than the stress-strain curve under monotonic axial compression. It also exhibits the typical stage characteristic and can be divided into six stages: linear elasticity, elastoplasticity, microcrack development, plastic deformation, unstable crack propagation, and failure. Different from the brittle failure of normal concrete (NC), the descending section of the PFRC specimen curve is gentler and the area enclosed by the loading-unloading branch curves is larger, indicating that the ductility and toughness of the latter are significantly improved. The incorporation of an appropriate amount of polypropylene fibers can significantly increase the peak stress and energy dissipation capacity of the specimens, and when the fiber content is constant, the larger the aspect ratio, the further increase in peak stress and energy dissipation capacity. The AE characteristic parameters are affected by the coupling of fiber volume content and aspect ratio. When the aspect ratio is 200, compared with NC, the cumulative ring count and cumulative AE energy of PFRC specimens with volume contents of 0.05%, 0.10%, and 0.15% increase by 41.5%, 91.5%, 64.3% and 41.5%, 187.2%, 135.2% respectively, that is, the AE characteristic parameters first increase and then decrease with the volume content, while showing a positive correlation with the aspect ratio. The RA‒AF correlation analysis shows that the incorporation of an appropriate amount of polypropylene fibers can significantly increase the proportion of shear cracks in PFRC under reciprocating axial compression, with the maximum proportion reaching 69.8%. The center frequency of NC is mainly concentrated in the high-frequency band (300‒400 kHz), while that of PFRC is mainly located in the low-frequency band (100‒150 kHz). With the incorporation of polypropylene fibers, the center frequency shifts from the high-frequency band to the low-frequency band, which means that the damage deterioration of PFRC under reciprocating axial compression gradually transitions from being dominated by tensile cracks to being dominated by shear cracks, which is mutually confirmed by the conclusions obtained from the RA‒AF analysis method. The research results can provide experimental basis and theoretical support for the damage evolution and failure early warning technology of polypropylene fiber reinforced concrete structures under dynamic compressive loads.