A damage constitutive model of polypropylene fiber reinforced recycled aggregate concrete based on AE amplitude-frequency extremum

Adding polypropylene fiber into recycled aggregate concrete (RAC) can not only improve the performance, but also increase economic benefits. To study the single-blend and double-blend polypropylene micro and macro fibers and their effect on the compressive strength of RAC specimens, polypropylene micro fibers of two sizes and polypropylene macro fibers of two sizes were selected to design and produced 30 groups of polypropylene fiber reinforced RAC test specimens with 0%, 25% and 50% coarse aggregate substitution rates by controlling the fiber mixing proportion and the stress-strain curves, elastic modulus, peak strength, peak strain and acoustic emission amplitude-frequency extremum of each group of test specimen were obtained. According to the test results, the elastic modulus and peak stress of test specimens without polypropylene fibers decrease gradually with the increase of the coarse aggregate substitution rate. However, there is a certain increase in elastic modulus and peak stress after polypropylene fibers are added. A damage constitutive model for polypropylene fiber reinforced RAC was established, and by fitting with this model, it is found that although the elastic modulus and peak stress of RAC test specimens are increased by a certain extent, the fitting parameters α_c of RAC are greater than those of ordinary concrete, and its post-peak strength is lower than that of ordinary concrete. The evolution law of acoustic emission amplitude-frequency extremum of polypropylene fiber reinforced RAC was studied, and it is found that the cumulative amplitude-frequency extremum N_p of RAC is larger than that of the test specimens without polypropylene fibers, indicating that the addition of polypropylene fibers limits the crack propagation and increases the cumulative amplitude-frequency extremum representing fracture energy.