Microstructural characteristics and their impact on mechanical properties of steel-PVA fiber reinforced concrete

Hybrid fiber reinforced concrete (HFRC) has been proposed to improve the mechanical properties of concrete , especially the tensile behavior and ductility. To better understand the mechanisms of such enhancements, a series of macro-scale and micro-scale tests have been performed and combined to analyze the relationships between microstructural characteristics and mechanical properties of HFRC. The X-ray CT test and machine learning were used to reconstruct detailed microstructures of HFRC specimens. The morphological and statistical analyses were performed to observe the fiber distribution and pore structure . For the mechanical properties, a series of systematic mechanical tests, including the compressive test , the tensile test , and the four-point bending test, were carried out to identify the mechanics and toughness of HFRC. All results reveal that (i) increasing steel fibers could significantly increase the steel fiber density around the edge regions of the specimen, make a higher fraction of steel fibers distributed along the 90° of Phi angle, and result in higher fiber connectivity; (ii) adding more PVA fibers could reduce the fraction of steel fibers distributed along the 90° of Phi angle and significantly increase the porosity and pore connectivity of HFRC; (iii) these different effects on the microstructure result in different macro mechanical responses: steel fibers have a main and positive influence on the mechanical properties of HFRC, especially the strength indices, while introducing PVA fibers could improve the toughness of HFRC but the more addition of PVA fiber could reduce strength indices.