Investigation of long-term tension stiffening mechanism for ultra-high-performance fiber reinforced concrete (UHPFRC)

Abstract

Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) is promptly emerging concrete that has ultra-high compressive strength, and its ductile properties are ensured in the addition of steel fibers. Steel fibers in UHPFRC feature strain-hardening characteristics and exert the damage pattern as intimately spaced narrow cracks. The tensile performances of reinforced concrete elements are closely interrelated with the bonding interactions between the reinforcing rebars and concrete. Since the UHPFRC exhibits different tensile behavior and cracking behavior from traditional strength concrete, it is crucial to analyze the tension stiffening mechanism and cracking propagation of UHPFRC elements in long-term serviceability limit. This study presents the time-dependent tensile behavior of reinforced UHPFRC, especially on the instantaneous and sustained tension stiffening mechanism and post-cracking behavior for the long term. A newly designed testing rig was adopted to introduce the novel testing method. The testing rig was designed in a way to overcome the limitations in applying the sustained tensile loads found in the critical literature review. This novel testing method includes the design of the testing frame for intensive sustained tensile loads application, specimen preparations, and installations. To investigate the long-term tension stiffening mechanism, different types of sustained tensile loads, i.e., 20 kN, 40 kN, 60 kN, and 80 kN, were considered. The applied sustained tensile loads were ascertained from different percentages of cracking load (40 kN) of reinforced UHPFRC derived from the 28-day instantaneous tension stiffening test. Crack width and crack spacing of reinforced UHPFRC specimens were determined for both instantaneous and sustained tensile loads. Notably, crack propagation was considerably affected by the long-term sustained tensile loads compared to instantaneous tensile loads. The autogenous and drying shrinkage strains were 392 micro-strains (µɛ), and 170 µɛ, respectively, at 180-day, where the total shrinkage strain of 562 µɛ significantly influenced the time-dependent tension stiffening behaviors of UHPFRC. A maximum total average concrete strain of 3500 µɛ was found for the sustained tensile load of 80 kN at 180-day. In addition, the material properties of UHPFRC, such as compressive strength, splitting tensile strength, and modulus of elasticity tests, were also conducted, and the maximum compressive strength of 160 MPa was reported at 120-day.