Investigation of tensile creep and tension stiffening behaviour for Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC)

Abstract

Ultra-high-performance fiber reinforced concrete (UHPFRC) has improved properties over conventional concrete, such as high tensile strength, greater compressive strength and enhanced post cracking characteristics. The steel fibers in UHPFRC are recognized as providing resistance to crack widening in tension zones because of the fibers bridge adjacent cracks, which consequently enhances the tensile performance. Although, UHPFRC is capable of resisting the induced tensile stresses, it has still limitations under sustained tensile loads. It is also not well understood whether these characteristics would resist the induced tensile stress over a longer period or if they would leave the serviceability of the structure at risk. Therefore, the research presented in this study is concerned with the time-dependent tensile behaviour of UHPFRC. The present study comprises of an experimental program based on the application of newly developed test rigs, preparation of the test specimens and investigations into the test results. The aims seek to provide an understanding of the instantaneous and time-dependent tensile behaviour of unreinforced and reinforced UHPFRC prisms. Instantaneous tensile tests were involved, applying axial tensile loads to UHPFRC prisms for both aged and unaged concrete. The time-dependent tensile behaviour of UHPFRC was investigated in terms of tensile creep and tension stiffening mechanisms under sustained tensile loads. The sustained tensile loads were considered as different percentages of cracking loads, such as 50% and 75% of the cracking loads of unreinforced UHPFRC specimens for the tensile creep test and 75%, 100%, 150%, and 200% of the cracking loads of reinforced UHPFRC specimens for the tension stiffening test. The cracking loads were determined from 28th day instantaneous tensile responses for both reinforced and unreinforced UHPFRC prisms. Two different test rigs were used to conduct the tensile creep and tension stiffening tests under sustained tensile loads. The rigs were modified to overcome the limitations identified through the critical literature review. The experimental results demonstrate that the tensile creep strain and tension stiffening mechanisms are greatly influenced by the shrinkage strain of UHPFRC. A significant portion of the measured total shrinkage was caused by autogenous shrinkage rather than drying shrinkage. The results demonstrate that higher sustained stress leads to higher tensile creep strain for the first 13 days, at a higher creep rate. Afterwards, the shrinkage strain dominates over the tensile creep strain. The extent of crack propagation and the deterioration of the bonds between the steel fibers and the cement matrix are also significantly affected by the sustained tensile loads.