In order to study the splitting tensile failure characteristics of ultra-high performance concrete (UHPC), three different loading rates (0.1, 0.01, 0.001 mm/s) were used for the splitting tensile loading. During the loading process, the digital image correlation (DIC) method was used to observe the crack evolution by non-contact deformation. At the same time, the acoustic emission (AE) technology was used to dynamically monitor the entire failure process of UHPC, and the failure characteristics of the specimen were analyzed based on the DIC strain cloud map and AE parameters. The results show that due to the incorporation of steel fibers, the brittleness of concrete is improved, and the post-peak load-displacement curve of UHPC decreases more gently without a sharp decline. The higher the loading rate, the higher the splitting tensile strength measured by UHPC, which conforms to the law of concrete rate effect. When the loading rate is increased from 0.001 mm/s to 0.01 mm/s and from 0.01 mm/s to 0.1 mm/s, the splitting tensile peak load is increased by 27.9 % and 28.5 % respectively. The digital image correlation method is used to measure the deformation of UHPC splitting tensile experiment, and continuous deformation data can be obtained, which can completely capture the development process of cracks. It is proved that DIC can well reflect the evolution process of cracks on the surface of specimens. The analysis based on RA-AF value can quickly and effectively determine the type of crack development in concrete. According to the analysis results, the crack types of UHPC splitting tensile experiment are mostly tensile cracks, and the proportion of tensile cracks in the total cracks increases with the increase of loading rate.

The development of cohesionless cracks can be regarded as a sign of macroscopic crack development. An identification model of cohesionless cracks was adpoted with results of crack development by digital image correlation (DIC) method. Three-point bending tests were carried out on fully-graded concrete specimens with different loading rates (0.0001 mm/s, 0.0010 mm/s, 0.0100 mm/s), considering the rate correlation of crack propagation of fully-graded concrete. The results show that for the fully-graded concrete specimens with different loading time intervals, cohesionless cracks will occur throughout the peak load, indicating that the prefabricated cracks have cracked when the peak load is reached. The cracks of the specimen is mainly concentrated after the load is loaded to 50% of the peak load. Under the same conditions, the slower the loading rate is, the earlier the unbonded cracks appear. With the increase of the loading rate, the cracks are thin and long.

The bonding strength of epoxy grout-concrete composite was investigated by axial tensile test to reveal the bonding damage evolution mechanism between epoxy grout and concrete under dry and wet interface condition. The ringing count, b value and the spatial position of acoustic emission (AE) source during the tensile process of composite were obtained by AE technique, and the spatial evolution process of bonding damage of composite was analyzed. The evolution characteristics of strain and displacement damage on the fracture surface of the composite under wet and dry interface condition were obtained by a digital image correlation method. The results show that the bonding strength under dry interface condition is 60% higher than that under wet interface condition. Under wet interface condition, the AE source of epoxy grout-concrete is concentrated at the interface, and a regular strain concentration zone is formed at the interface, showing the characteristics of interface failure. Under dry interface condition, however, there are a lot of AE sources in the concrete, and there are several irregular strain concentration zones on the fracture surface, indicating the failure mode of the concrete matrix as a whole.