Effect of rebar ratio, rebar diameter, cover thickness, and fiber shape on flexural and cracking behavior of rebar-reinforced UHPC beams

This study investigated the flexural and cracking behavior of seven ultra-high-performance concrete (UHPC) rectangular beams, reinforced with steel rebars and 2 % hooked-end steel fibers (by volume), under four-point bending. Their failure modes, load-displacement responses, crack spacings, and load-crack width relationships were analyzed and compared with five previously studied UHPC beams reinforced with steel rebars and 2 % straight steel fibers. Key experimental variables included rebar ratio ranging from 1.21 % to 1.99 %, rebar diameters of 16 mm and 20 mm, and concrete cover thicknesses of 20 mm, 30 mm, and 40 mm. The results revealed that all UHPC beams exhibited typical flexural failure, characterized by yielding of the steel reinforcement, multiple cracking, the formation of one localized main crack, and mild crushing of the UHPC. Increasing the rebar ratio significantly enhanced the post-cracking stiffness and load-carrying capacity, while having minimal effect on their cracking load. Rebar diameter, concrete cover thickness, and fiber shape showed negligible influence on flexural performance during the elastic and crack development phases but had varied effects during the yielding phase. Furthermore, a higher rebar ratio, reduced concrete cover thickness, and the use of hooked-end steel fibers improved the beams' resistance to cracking, while the rebar diameter had a limited impact. A unified average crack spacing model for UHPC beams reinforced with steel rebars and with straight fibers or hook-end fibers was proposed, and analytical models were developed to predict crack width. These predictions aligned well with experimental results from this study and other prior research.