Abstract: To investigate the basic mechanical properties of hybrid fiber-reinforced recycled coarse aggregate concrete (HFR-RCAC), 45 sets of specimens were prepared for compressive, split tensile, and flexural tests, with the volume of steel-polypropylene hybrid fibers (HF) (0%, 0.5%, and 1.0%) and the mass substitution rate of recycled coarse aggregate (0%, 25%, 50%, 75%, and 100%) as the main test parameters. The full-field displacement and strain of specimen surfaces were monitored with the help of digital image correlation (DIC) method, and the microstructure of the interfacial transition zone between polypropylene fibers and recycled coarse aggregates was observed by scanning electron microscope (SEM). The test results show that the mechanical properties of HFR-RCAC decreases with the increase of the replacement rate of recycled coarse aggregate, and the incorporation of fibers can compensate for the loss of strength caused by the recycled coarse aggregate, which can significantly improve its mechanical properties. The full-field strain evolution of the compressive damage of specimen was obtained by DIC method, and the dynamic evolution characteristics of macro- and microscopic cracks on the surface of specimen were analyzed based on the evolutionary process of maximum principal strains, and the enhancement of concrete by HF is mainly manifested in the form of hindering the expansion of macro cracks. The microstructural changes in the transition zone of polypropylene fiber-cement matrix interface were characterized by SEM, which explains changes in the mechanical properties of HFR-RCAC as well as the fiber-modified reinforcing mechanism from a microscopic point of view.

Key words: hybrid fiber, recycled concrete, digital image correlation, full-field strain evolution, mechanical property