A Study of the Main Fracture Precursor Characteristics of Polypropylene Fiber-Reinforced Recycled Concrete with Varying Coarse Aggregate Ratios

This study investigates the cracking scale and main fracture precursor characteristics of polypropylene fiber (PPF)-reinforced recycled aggregate concrete (RAC). A total of 30 test groups were prepared, utilizing coarse aggregate substitution rates of 0% and 25% across varying coarse aggregate ratios. The energy and dominant frequency of acoustic emission (AE) signals from the PPF-reinforced RAC were recorded during uniaxial compression testing. The K-means clustering method was employed for two-dimensional clustering analysis to differentiate between large-scale and small-scale cracking. Subsequently, a support vector machine (SVM) was utilized to delineate the boundary between these two types of cracking. The effectiveness of single-blend and double-blend micro- and macro-PPF in mitigating cracking was examined by analyzing the frequency of large-scale cracking signals and the evolution characteristics of the AE b-value. An assessment of the evolution of AE signals during the rupture process of PPF-reinforced RAC revealed significant trends: a decrease in the AE b-value, an increase in large-scale cracking signals, and the disappearance of low-energy small-scale cracking signals. A precursor response coefficient was introduced, highlighting that the precursor response ability concerning the disappearance of low-energy small-scale cracking signals proved to be a significant characteristic, thereby offering insights for predicting main fractures in PPF-reinforced RAC.