Damage-fracture mechanism analysis of steel-polypropylene fiber reinforced recycled concrete based on acoustic emission and digital image correlation.

• Recycled aggregate has a negative effect on the mechanical properties of SPRAC, whereas hybrid fiber has a positive effect. • The cracks in SPRAC specimens all extend gradually from the bottom of the center to the top, and cracks are mainly tensile and shear cracks. • The b-values can be used to calculate early warning parameters for predicting SPRAC conditions. • The deformation of SPRAC under loading is analyzed using XTDIC software to investigate the crack initiation and extension. • The primary factor causing the SPRAC's mechanical qualities to deteriorate under bending load is the destruction of these zones. The engineering application of recycled aggregate concrete is significant for resource conservation and environmental protection. However, the applied range of recycled aggregate concrete is greatly limited due to microscopic defects such as microcracks in recycled aggregate. Steel-polypropylene fiber reinforced recycled concrete (SPRAC) makes up for the defects brought by recycled aggregate to a certain extent due to the action of hybrid fiber. To further promote the practical application of SPRAC, it is crucial to evaluate its damage-fracture mechanic's response mechanism under different hybrid fiber admixtures and recycled aggregate replacement rates. For this purpose, the following work is carried out in this paper: firstly, SPRAC beam specimens with different substitution rates of hybrid fiber and recycled aggregate were prepared; and the damage-fracture process of specimens under four-point bending load was monitored in real-time by using acoustic emission and digital image correlation techniques to analyze the influence of hybrid fiber and recycled aggregate on the flexural properties and damage-fracture characteristics of SPRAC; the inherent relationship between the acoustic emission rise time/amplitude (RA) and the average frequency (AF) is utilized to classify the crack type of SPRAC in the course of the data analysis; the b-value is also analyzed to assess the crack evolution process in SPRAC, and the corresponding warning thresholds are given.; strain and displacement fields on the specimen surface are analyzed using the digital image correlation; finally, the microscopic results of SPRAC are also given to better understand the damage-fracture evolution mechanism of SPRAC. Results show that the flexural strength and fracture toughness of SPRAC decreased nonlinearly with the increase of recycled aggregate content; on the contrary, the addition of hybrid fiber improved the mechanical properties of SPRAC. The study in this paper provides a new method to characterize the damage-fracture mechanism of SPRAC under bending loads quantitatively. It provides a reference for the engineering application of SPRAC. [ABSTRACT FROM AUTHOR]