Interface Shear Behavior of Ultrahigh-Performance Fiber-Reinforced Concrete Using Digital Image Correlation Technique

Ensuring good interface shear resistance is essential in structural applications. Hence, understanding the interface shear behavior of ultrahigh-performance fiber-reinforced concrete (UHPFRC) is essential. The interface behavior of UHPFRC will be different from conventional concrete due to the absence of coarse aggregates and the presence of steel fibers. Therefore, this study involves testing 10 monolithically cast UHPFRC Z-shaped interface specimens with different volume fractions and types of fibers. The parameters considered in the study are 1.0% and 2.0% fiber volume and straight and hybrid (a combination of hooked-ended and straight steel) fibers. The interface shear crack load and possible types of failure modes are identified using the digital image correlation (DIC) technique. A direct tensile stress–strain and interface shear stress–strain relationship is also identified to propose a simplified interface shear capacity model. The cracking tensile strain across the shear interface is identified beyond the direct tension localization strain range. Similarly, the crack slip and width at different loading stages are evaluated, and it is identified that the crack-slip response is the same along the interface plane. On the contrary, the crack opening varies along the shear interface plane. Shear cracking and ultimate shear stress increased at higher fiber volume fractions due to the increased tensile strength of UHPFRC. The reduction in interface shear capacity is significant with the addition of hybrid fibers compared to the specimens with only straight steel fibers. This reduction is due to the lesser number of fibers across the shear interface in hybrid fiber-reinforced specimens. A simplified interface shear design model was developed based on experimental and literature data, where concrete tensile strength is a critical parameter in determining interface shear resistance.