This study investigated the mechanical properties and the effects of the volume fraction for carbon fibers (CF-VF), polypropylene fibers (PPF-VF) and aramid fibers (AF-VF) with a fixed aspect ratio of 650 for the carbon fibers, 400 for the polypropylene fibers and 900 for the aramid fibers in hybrid fiber-reinforced concrete (HFRC). Furthermore, compressive, splitting tensile and flexural tensile tests were carried out to obtain the optimal total volume fraction for the three types of fibers, as well as the optimal ratio between the CF-VF, PPF-VF and AF-VF. In addition, stress–strain curves of normal concrete and HFRC were examined to explore the whole mechanical process. The results indicated the CF-VF, PPF-VF and AF-VF have a significant effect on the tensile and flexural strengths of HFRC. The HFRC with a fiber additional ratio of 25:50:25 had the best hybrid effect. Moreover, a calculation method based on the compressive strength of normal concrete and HFRC and the volume fraction is proposed to calculate the strength of HFRC in engineering as a reference. Besides, a uniaxial compression constitutive mathematical model of normal concrete and HFRC is established.

A novel method was proposed to improve the bond behavior of new‐to‐old concrete interface, which was beneficial to introduce the fiber‐reinforced concrete only at the old concrete interface. This study investigated the effect of the fiber addition, strength grade of new concrete, interfacial angle, and surface treatment types on the bond behavior in terms of the new‐to‐old concrete through the axial tensile tests. The three‐dimensional digital image correlation technique (3D‐DIC) and scanning electron microscope were adopted to evaluate the variation of specimen surface strain distributions and microstructure of fiber‐reinforced concrete and bond interface between new‐to‐old concrete. The experimental results indicated that interfacial angle and surface treatment type were significantly promoted bond behaviors, while the specimen cooperating with steel fibers had the highest bond strength. Besides, the maximum strain locations obtained from 3D‐DIC method were the same as the location of the specimen failure, which indicated the 3D‐DIC method can be adopted to forecast the structural failure. The microcrack strain located in the major crack was decreased with the development of the major crack. Ample crystals and Ca(OH)² were generated in the interface between the new‐to‐old concrete to weaken the bond strength. Moreover, this paper provided the mechanics‐driven and machine learning method to predict the bond strength. This study provides a new interface bonding method for the fabricated and large span structure to effectively avoid cracking of new‐to‐old concrete.