Bond behavior of staggered lap and anchored reinforcement in ultra-high performance concrete

Abstract

To investigate the bond behavior between ultra-high-performance concrete (UHPC) and reinforcement, 13 groups of pull-out tests on staggered lap rebars in UHPC and 10 groups of direct pull-out tests on anchored rebars were conducted. The variables studied included development length, cover thickness, reinforcement diameter, and reinforcement spacing. Most specimens exhibited a splitting pull-out failure mode, characterized by ductile behavior. However, when the reinforcement diameter or spacing was large, the specimens showed brittle failure characteristics, transitioning from splitting pull-out failure to UHPC splitting failure. Compared to anchored specimens, the bond strength of lap splice specimens was less influenced by development length but more sensitive to cover thickness. Additionally, refined finite element models for both types of specimens were developed and validated, and the stress state in the lap splice region and the characteristics of bond stress distribution were investigated. The results revealed that as the spacing increased, the compressive stress struts formed between the lapped reinforcements became discontinuous. At peak load, the bond stress in lap splice specimens has a bimodal distribution, whereas in anchored specimens, the bond stress has a single distinct peak near the loading end and gradually decays with increasing distance from the loading end. Based on the observed and inferred non-uniform bond stress distributions, a model for the reinforcement stress in UHPC was derived, which offered high consistency with experimental results reported in prior studies.